Luc Rouppe van der Voort

• Ornig, Sascha; Voort, Luc Rouppe van der; Carlsson, Mats; Baso, Carlos José Díaz; Øyre, Eilif Sommer & Poquet, Ignasi Josep Soler [Show all 7 contributors for this article] (2026). Investigating white-light flare mechanisms via the Paschen jump using high-resolution continuum observations from the Swedish 1-m Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 712. doi: 10.1051/0004-6361/202660421. Full text in Research Archive Show summary

  Context. The continuum is understood to contain a large portion of the energy emitted by a solar flare. The optical continuum, known as white light (WL), is particularly relevant since it may be observed by ground-based instruments. Aims. We measured the WL enhancements short- and longward of the Paschen jump in order to gain insights into the possible mechanism(s) behind the creation of these increases in our two case studies. Methods. We took measurements from the Swedish 1-m Solar Telescope of the pseudo-continuum around the Ca II 8542 Å line as well as the true continuum around the K I 7699 Å and the Fe I 6173 Å line, providing us with observations on both sides of the Paschen jump. Results. We observe WL enhancements of over 40% against the dark (pen-) umbral background in both flares. No WL excess is detectable against the granulation outside the sunspots. The WL excess in flare 1 is co-temporal with the derivative of the GOES soft X-ray and hard X-ray (HXR) measurements from the Advanced Space-based Solar Observatory (ASO-S), and the flare is compatible with the Neupert effect. For the second flare, a preceding smaller flare may be the cause of the temporal discrepancy. Signatures of chromospheric evaporation and condensation are found in the WL area for both flares. The ratio of intensities blueward and redward of the Paschen jump (i.e., the Paschen ratio) in flare 1 is below one for most WL pixels. This is in disagreement with the accepted WL formation mechanisms, which are both of photospheric and chromospheric origin. We believe this is a consequence of the Ca II 8542 Å pseudo-continuum being affected by line wing opacity changes. Conclusions. The co-temporality of WL and HXR enhancements suggests that the WL emission enhancements in flare 1 (and parts of flare 2) are a result of direct electron precipitation. We conclude that more reliable continuum measurements free of any nearby line influence are necessary in order to obtain conclusive evidence for the formation mechanism(s) behind optical continuum enhancements from such analysis as presented in this work.

• Sand, Mats Ola; Noraz, Quentin; Aulanier, Guillaume; Martinez-Sykora, Juan; Carlsson, Mats & Voort, Luc Rouppe van der (2026). Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202557602. Full text in Research Archive Show summary

  Context. The formation mechanism for the dynamic type II spicules has remained elusive for many years. Their dynamical behaviour has long been linked to magnetic reconnection, yet no conclusive evidence has been provided. However, one recent observational study found signs of magnetic reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to magnetic reconnection due to flux emergence and convective motions in the photosphere. Aims. We aim to explore whether we can connect EBs to type II spicules, and determine to what extent we can use EBs as an observational proxy to probe magnetic reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. Methods. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes underlying their formation. To detect EBs and classify the spicules, we synthesised the chromospheric H α spectral line using the multilevel radiative transfer code RH 1.5D. We also traced shocks and current sheets to decipher the origin of EBs and spicules. We selected one type II spicule with a strong EB near its footpoint and studied their formation in detail. Results. A magnetoacoustic shock advects the magnetic field lines towards an oppositely directed ambient field, creating a current sheet. The current sheet accelerates dense plasma via a whiplash effect generated by magnetic reconnection into the inclined ambient field, launching the spicule. Several EB profiles trace shock- and magnetic-reconnection-induced dynamics during this process at the spicule footpoint. Conclusions. We present a new EB triggering mechanism in which a shock-induced current sheet reconnects, triggering an EB in the lower solar atmosphere. The shock-induced current sheet generates the upwards propagation of a type II spicule via reconnection outflows. These results provide a plausible physical origin for the recently observed connection between EBs and spicules.

• Faber, Jonas Thoen; Joshi, Reetika; Voort, Luc Rouppe van der; Wedemeyer, Sven; Øyre, Eilif Sommer & Poquet, Ignasi J. Soler [Show all 7 contributors for this article] (2026). Fine details in solar flare ribbons: Statistical insights from observations with the Swedish 1-m Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556738. Full text in Research Archive Show summary

  Context. Flare ribbons serve as chromospheric footprints of energy deposition resulting from particle acceleration during magnetic reconnection. Their fine-scale structure provides a valuable tool for probing the dynamics of the flare reconnection process. Aims. Our goal is to investigate the fine-scale structure of flare ribbons through multiple observations of flares, utilising data obtained from the Atmospheric Imaging Assembly (AIA) and the Swedish 1-m Solar Telescope (SST). Methods. The aligned AIA and SST datasets for the three solar flares were used to examine their overall morphology. The SST datasets were specifically used to identify fine-scale structures within the flare ribbons. For spectroscopic analysis of these fine-scale structures, we applied machine-learning methods ( k -means clustering) and Gaussian fitting. Results. Using k -means, we identified elongated features in the flare ribbons, termed as ‘riblets’, which are short-lived and jet-like small-scale structures that extend as plasma columns from the flare ribbons. Riblets are more prominent near the solar limb and represent the ribbon front. Riblet widths are consistent across observations, ranging from 110−310 km (0.15−0.41), while vertical lengths span 620−1220 km (0.83−1.66), with a potential maximum of 2000 km (2.67), after accounting for projection effects. Detailed H β spectral analysis reveals that riblets exhibit a single, redshifted emission component, with velocities of 16−21 km s −1 , independent of the viewing angle. Conclusions. Our high-resolution observations of the three flare ribbons show that they are not continuous structures, but rather are composed of vertically extended, fine-scale substructures. These irregular features indicate that the reconnection region is not a smooth, laminar current sheet, but rather a fragmented zone filled with magnetic islands (blobs or riblets), consistent with the theory of patchy reconnection within the coronal current sheet.

• Ashfield, William; Polito, Vanessa; Lörinčík, Juraj; Pontieu, Bart Walter De; Chintzoglou, Georgios & Bose, Souvik [Show all 10 contributors for this article] (2025). Spectroscopic observations of solar flare pulsations driven by oscillatory magnetic reconnection. Nature Astronomy. 10(1), p. 54–63. doi: 10.1038/s41550-025-02706-4. Full text in Research Archive Show summary

  Solar and stellar flares often exhibit oscillations, or quasi-periodic pulsations (QPPs), across the electromagnetic spectrum. While magnetic-field reconnection drives these events, it remains to be determined whether oscillatory reconnection causes the quasi-periodicity, or whether waves drive or mediate this process. Exploiting coordinated observations from NASA’s Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope, here we present spectroscopic observations of QPPs in a solar flare at high-temporal (<1 s) and high-spatial (~60 km) resolution. Downwards velocities in the flare ribbon show synchronized oscillations at different atmospheric layers with a period of ~32 s. These velocities correlate with hard X-ray emissions, indicating a modulated deposition of accelerated electrons in the chromosphere as the driver. By negating magnetohydrodynamic sausage modes as the modulator, we demonstrate that repeated reconnection drives the QPPs. The QPP–reconnection relationship established here provides observational benchmarks for reconnection models and diagnostics for probing energy release across astrophysical environments.

• Gošić, M.; Hansteen, Viggo Haraldson; Dalda, A. Sainz; Pontieu, Bart Walter De & Voort, Luc Rouppe van der (2025). Bifrost Models of the Quiet Sun. I. Comparison with Solar Observations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 990(2). doi: 10.3847/1538-4357/adf4db. Full text in Research Archive Show summary

  Studying the emergence of magnetic fields is essential for understanding the physical mechanisms behind various phenomena in the solar atmosphere. Most importantly, the emerging fields offer valuable insights into how energy and mass are transferred to the upper solar atmosphere. As a result, they have garnered significant attention from both observational and theoretical perspectives. In this article, we present two models of quiet-Sun-like magnetic fields generated by the Bifrost code. We compare these models with observations from the Swedish 1 m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). By tracking the magnetic features in both the SST and Bifrost data, we determine the similarities and differences between the fields identified in the models and those observed. We conduct a quantitative comparison of various properties, such as flux content, flux densities, horizontal and line-of-sight velocities, lifetimes, sizes, and surface interactions. Additionally, we identify and analyze the properties of the largest emerging bipoles in the SST and Bifrost data. Our findings indicate that the magnetic bipoles in the Bifrost simulations are generally stronger than those observed with the SST. However, a qualitative comparison of the chromospheric and transition region responses to the emerging fields in the Bifrost models, SST, and IRIS observations shows similar heating processes occurring above and around the emerging fields. Finally, we outline our plans for future work aimed at studying the emergence of magnetic fields in the quiet Sun, with a particular focus on the chromosphere and upper atmospheric layers.

• Wilde, M. De; Pietrow, A. G. M.; Druett, M. K.; Yabar, A. Pastor; Koza, J. & Kontogiannis, I. [Show all 25 contributors for this article] (2025). Synthesizing Sun-as-a-star flare spectra from high-resolution solar observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 700. doi: 10.1051/0004-6361/202554870. Full text in Research Archive Show summary

  Context. Spatially resolved observations of the Sun and the astronomical sample size of stellar bodies are the respective key strengths of solar and stellar observations. However, the large difference in object brightness between the Sun and other stars has led to distinctly different instrumentation and methodologies between the two fields. Aims. We produced and analyzed synthetic full-disk spectra derived from 19 small area field-of-view optical observations of solar flares acquired by the Swedish 1-m Solar Telescope (SST) between 2011 and 2024. These were used to investigate what can and cannot be inferred about physical processes on the Sun from Sun-as-a-star observations. Methods. The recently released Numerical Empirical Sun-as-a-Star Integrator (NESSI) code provides synthetic full-disk integrated spectral line emission based on smaller field-of-view input while accounting for center-to-limb variations and differential rotation. We used this code to generate pseudo-Sun-as-a-star spectra from the SST observations. Results. We show that limited-area solar observations can be extrapolated to represent the full disk accurately in a manner close to what is achievable with Sun-as-a-star telescopes. Additionally, we identify nine spectral features, four of which are caused by instrumental effects. Most notably, we find a relation between the heliocentric angle of flares and the width of the excess emission left by them as well as a source of false positive coronal mass ejections-like signatures, and we defined an energy scaling law based on chromospheric line intensities that shows that the peak flare contrast roughly scales with the square root of the bolometric energy. Conclusions. The presented method of creating pseudo-Sun-as-a-star observations from limited field-of-view solar observations allows for the accurate comparison of solar flare spectra with their stellar counterparts while allowing for the detection of signals at otherwise unachievable noise levels.

• Poquet, I. J. Soler; Baso, C. J. Díaz; Voort, Luc Rouppe van der & Vissers, G. J. M. (2025). Automatic detection of Ellerman bombs using deep learning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 699. doi: 10.1051/0004-6361/202453524. Full text in Research Archive Show summary

  Context. Ellerman bombs (EBs) are observable signatures of photospheric small-scale magnetic reconnection events. They can be seen as intensity enhancements of the H α 6563 Å line wings and as brightenings in the SDO/AIA 1600 Å and 1700 Å passbands. Reliable automatic and systematic detection of EBs would enable the study of the impact of magnetic reconnection on the Sun’s dynamics. Aims. We aim to develop a method to automatically detect EBs in H α observations from the Swedish 1-m Solar Telescope (SST) and in SDO/AIA observations using the 1600 Å, 1700 Å, 171 Å, and 304 Å passbands. Methods. We trained models based on neural networks (NNs) to perform automatic detection of EBs. Additionally, we used different types of NNs to study how different properties – such as local spatial information, the spectral shape of each pixel, or the center-to-limb variation – contribute to the detection of EBs. Results. We find that for SST observations, the NN-based models are proficient at detecting EBs. With sufficiently high spectral resolution, the spatial context is not required to detect EBs. However, as we degrade the spectral and spatial resolution, the spatial information becomes more important. Models that include both dimensions perform best. For SDO/AIA, the models struggle to reliably distinguish between EBs and bright patches of different origin. Permutation feature importance revealed that the H α line wings (around ± 1 Å from line center) are the most informative features for EB detection. For the SDO/AIA case, the 1600 Å channel is the most relevant one when used in combination with 171 Å and 304 Å. Conclusions. The combination of the four different SDO/AIA passbands is not informative enough to accurately classify EBs. From our analysis of a few sample SDO/AIA 1600 Å and 1700 Å light curves, we conclude that inclusion of the temporal variation may be a significant step towards establishing an effective EB detection method that can be applied to the extensive SDO/AIA database of observations.

• Bhatnagar, Aditi; Prasad, Avijeet; Siverio, Daniel Elias Nóbrega; Voort, Luc Rouppe van der & Joshi, Jayant (2025). Small-scale dynamic phenomena associated with interacting fan-spine topologies: Quiet-Sun Ellerman bombs, UV brightenings, and chromospheric inverted-Y-shaped jets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202453344. Full text in Research Archive Show summary

  Context. Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower solar atmosphere. Sometimes, they exhibit transition region counterparts, known as ultraviolet (UV) brightenings. Magnetic field extrapolations suggest that QSEBs can occur at various locations of a fan-spine topology, with UV brightening occurring at the magnetic null point through a common reconnection process. Aims. We aim to understand how more complex magnetic field configurations such as interacting fan-spine topologies can cause small-scale dynamic phenomena in the lower atmosphere. Methods. QSEBs were detected using k -means clustering on H β observations from the Swedish 1-m Solar Telescope (SST). Further, chromospheric inverted-Y-shaped jets were identified in the H β blue wing. Magnetic field topologies were determined through potential field extrapolations from photospheric magnetograms derived from spectro-polarimetric observations in the Fe  I 6173 Å line. UV brightenings were detected in IRIS 1400 Å slit-jaw images. Results. We identify two distinct magnetic configurations associated with QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets. The first involves a nested fan-spine structure where, due to flux emergence, an inner 3D null forms inside the fan surface of an outer 3D null with some overlap. The QSEBs occur at two footpoints along the shared fan surface, with the UV brightening located near the outer 3D null point. The jet originates close to the two QSEBs and follows the path of high squashing factor, Q . We discuss a comparable scenario using a 2D numerical experiment with the Bifrost code. In the second case, two adjacent fan-spine topologies share fan footpoints at a common positive polarity patch, with the QSEB, along with a chromospheric inverted-Y-shaped jet, occurring at the intersection having high Q values. The width of the jets in our examples is about 0 . 3, and the height varies between 1–2. The width of the cusp measures between 1–2. Conclusions. This study demonstrates through observational and modelling support that small-scale dynamic phenomena, such as associated QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets, share a common origin driven by magnetic reconnection between interacting fan-spine topologies.

• Bhatnagar, Aditi; Prasad, Avijeet; Voort, Luc Rouppe van der; Siverio, Daniel Elias Nóbrega & Joshi, Jayant (2025). Magnetic topology of quiet-Sun Ellerman bombs and associated ultraviolet brightenings. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202452822. Full text in Research Archive Show summary

  Context. Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower atmosphere of the quiet Sun. Recent work has shown that a small percentage of them can occur co-spatially and co-temporally with ultraviolet (UV) brightenings in the transition region. Aims. We aim to understand how the magnetic topologies associated with closely occurring QSEBs and UV brightenings can facilitate energy transport and connect these events. Methods. We used high-resolution Hβ observations from the Swedish 1-m Solar Telescope (SST) and detected QSEBs using k-means clustering. We obtained the magnetic field topology from potential field extrapolations using spectro-polarimetric data in the photospheric Fe I 6173 Å line. To detect UV brightenings, we used coordinated and co-aligned data from the Interface Region Imaging Spectrograph (IRIS) and imposed a threshold of 5σ above the median background on the (IRIS) 1400 Å slit-jaw image channel. Results. We identify four distinct magnetic configurations that associate QSEBs with UV brightenings, including a simple dipole configuration and more complex fan-spine topologies with a 3D magnetic null point. In the fan-spine topology, the UV brightenings occur near the 3D null point, while QSEBs can be found close to the footpoints of the outer spine, the inner spine, and the fan surface. The height of the 3D null varies between 0.2 Mm and 2.6 Mm, depending on the magnetic field strength in the region. Some QSEBs and UV brightenings, though occurring close to each other, are not topologically connected with the same reconnection process. The energy released during QSEBs falls in the range 1023–1024 ergs. Conclusions. This study shows that magnetic connectivity and topological features, such as 3D null points, are crucial in linking QSEBs in the lower atmosphere with UV brightenings in the transition region.

• Baso, Carlos José Díaz; Miliać, I.; Voort, Luc Rouppe van der & Schlichenmaier, R. (2025). Spectral resolution effects on the information content in solar spectra. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202452331. Full text in Research Archive Show summary

  Context. When interpreting spectropolarimetric observations of the solar atmosphere, wavelength variations in the emergent intensity and polarization translate into information on the depth stratification of physical parameters such as the temperature, velocity, and magnetic field. Resolving the fine details in the shapes of the spectral lines and their polarization gives us the capability to resolve small-scale depth variations in these physical parameters. With the advent of large-aperture solar telescopes and the development of state-of-the-art instrumentation, the requirements on spectral resolution have become a prominent question. Aims. We aim to quantify how the information content contained in a representative set of polarized spectra of photospheric spectral lines depends on the spectral resolution and spectral sampling of that spectrum. Methods. We used a state-of-the-art numerical simulation of a sunspot and the neighboring quiet Sun photosphere to synthesize polarized spectra of magnetically sensitive neutral iron lines. We then applied various degrees of spectral degradation to the synthetic spectra and analyzed the impact on its dimensionality using principal component analysis and the wavelength power spectrum using wavelet decomposition. Finally, we applied the Stokes Inversion based on Response functions (SIR) code to the degraded synthetic data to assess the effect of spectral resolution on the inferred parameters. Results. We find that the dimensionality of the Stokes spectra and the power contained in the small spectral scales significantly change with the spectral resolution. We find that regions with strong magnetic fields where convection is suppressed have more homogeneous atmospheres and produce less complex Stokes profiles. On the other hand, regions with strong gradients in the physical quantities give rise to more complex Stokes profiles that are more affected by spectral degradation. The degradation also makes the inversion problem more ill-defined, so inversion models with a larger number of free parameters overfit and give wrong estimates. Conclusions. The impact of spectral degradation in the interpretation of solar spectropolarimetric observations depends on multiple factors, including the spectral resolution, noise level, line spread function (LSF) shape, complexity of the solar atmosphere, and degrees of freedom in our inversion methods. To mitigate this impact, incorporating a good estimation of the LSF into the inversion process is recommended. Having a finely sampled spectrum may be more beneficial than achieving a higher signal-to-noise ratio per wavelength bin. Considering the inclusion of different spectral lines that can counter these effects, and calibrating the effective degrees of freedom in modeling strategies, are also important considerations. These strategies are crucial for the accurate interpretation of such observations and have the potential to offer more cost-effective solutions.

• Baso, Carlos José Díaz; Ramos, A. Asensio; Rodríguez, J. de la Cruz; Santos, J. M. da Silva & Voort, Luc Rouppe van der (2025). Exploring spectropolarimetric inversions using neural fields: Solar chromospheric magnetic field under the weak-field approximation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202452172. Full text in Research Archive Show summary

  Context. Full-Stokes polarimetric datasets, originating from slit-spectrograph or narrow-band filtergrams, are routinely acquired nowadays. The data rate is increasing with the advent of bi-dimensional spectropolarimeters and observing techniques that allow long-time sequences of high-quality observations. There is a clear need to go beyond the traditional pixel-by-pixel strategy in spectropolarimetric inversions by exploiting the spatiotemporal coherence of the inferred physical quantities that contain valuable information about the conditions of the solar atmosphere. Aims. We explore the potential of neural networks as a continuous representation of the physical quantities over time and space (also known as neural fields), for spectropolarimetric inversions. Methods. We have implemented and tested a neural field to perform one of the simplest forms of spectropolarimetric inversions, the inference of the magnetic field vector under the weak-field approximation (WFA). By using a neural field to describe the magnetic field vector, we regularized the solution in the spatial and temporal domain by assuming that the physical quantities are continuous functions of the coordinates. This technique can be trivially generalized to account for more complex inversion methods. Results. We have tested the performance of the neural field to describe the magnetic field of a realistic 3D magnetohydrodynamic (MHD) simulation. We have also tested the neural field as a magnetic field inference tool (approach also known as physics-informed neural networks) using the WFA as our radiative transfer model. We investigated the results in synthetic and real observations of the Ca II 8542 Å line. We also explored the impact of other explicit regularizations, such as using the information of an extrapolated magnetic field, or the orientation of the chromospheric fibrils. Conclusions. Compared to traditional pixel-by-pixel inversions, the neural field approach improves the fidelity of the reconstruction of the magnetic field vector, especially the transverse component. This implicit regularization is a way of increasing the effective signal to noise of the observations. Although it is slower than the pixel-wise WFA estimation, this approach shows a promising potential for depth-stratified inversions, by reducing the number of free parameters and inducing spatiotemporal constraints in the solution.

• Poirier, Nicolas Aurelien Corentin; Danilovic, S.; Kohutova, Petra; Baso, Carlos José Díaz; Voort, Luc Rouppe van der & Calchetti, D. [Show all 7 contributors for this article] (2025). Coronal kink oscillations and photospheric driving: Combining SolO/EUI and SST/CRISP high-resolution observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 696. doi: 10.1051/0004-6361/202453025. Full text in Research Archive Show summary

  Context. The driving and excitation mechanisms of decay-less kink oscillations in coronal loops remain under active debate. The photospheric dynamics may provide the continuous energy supply required to sustain these oscillations. Aims. We aim to quantify and provide simple observational constraints on the photospheric driving of coronal loops in a few typical active region configurations: sunspot, plage, pores, and enhanced-network regions. We then aim to investigate the possible interplay between the photospheric driving and the properties of kink oscillations in the connected coronal loops. Methods. We analysed two unique datasets of the corona and photosphere taken at a high spatial and temporal resolution during the first coordinated observation campaign between Solar Orbiter and the Swedish 1-m Solar Telescope (SST). We applied a local correlation tracking method on the SST/CRISP data to quantify the photospheric motions at the base of coronal loops. The same loops were then analysed in the corona by exploiting data from the Extreme Ultraviolet Imager (EUI) on Solar Orbiter and by using a wavelet analysis to characterise the detected kink oscillations. Results. Each type of photospheric region shows varying dynamics but with an overall increase in strength going from pore to plage to enhanced-network to sunspot regions. Differences can also be seen in the amplitudes of the fundamental kink mode measured in the corresponding coronal loops. This suggests the photosphere is involved in the driving of coronal kink oscillations. However, the few samples available do not allow the excitation mechanism to be further established yet. Conclusions. Despite oscillating coronal loops being anchored in seemingly “static” strong magnetic field regions, as seen from coronal EUV observations, photospheric observations provide evidence for a continuous and significant driving at their base. The precise connection between photospheric driving and coronal kink oscillations remains to be further investigated. Upcoming coordinated observations between Solar Orbiter and ground-based telescopes will provide crucial additional observational constraints, with this pilot study serving as a baseline for future works. Finally, this study provides critical constraints on both the quasi-steady and broadband photospheric driving that can be tested in existing numerical models of coronal loops.

• Sand, Mats Ola; Voort, Luc Rouppe van der; Joshi, Jayant; Bose, Souvik; Siverio, Daniel Elias Nóbrega & Griñón-Marín, Ana Belén (2025). Quiet Sun Ellerman bombs as a possible proxy for reconnection-driven spicules. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202453032. Full text in Research Archive Show summary

  Context. Spicules are elongated, jet-like structures that populate the solar chromosphere and are rooted in the lower solar atmosphere. In recent years, high-resolution observations and advanced numerical simulations have provided insights into their properties, structures, and dynamics. However, the formation mechanism of spicules, particularly the more dynamic type II spicules, which are primarily found in the quiet Sun and coronal holes, remains elusive. Aims. This study explores whether quiet Sun Ellerman bombs (QSEBs), which are ubiquitous small-scale magnetic reconnection events in the lower atmosphere, are linked to the formation of type II spicules. Methods. We analysed a high-quality 40-minute time sequence acquired with the Swedish 1-m Solar Telescope. Hβ data were used to observe QSEBs and spicules, while spectropolarimetric measurements in the photospheric Fe I 6173 Å line provided line-of-sight magnetic field information. We employed k-means clustering to automatically detect QSEBs and explored their potential association with spicules. Results. We identified 80 clear cases in which spicules occurred soon after the QSEB onset and not later than 30 s after the ending of the QSEBs. In all these instances, the events involved type II spicules, rapidly fading from the images. The footpoints of the spicules seemed to be rooted in QSEBs, where the onset of QSEBs often preceded the formation of the associated spicules. In addition to these clear cases, we found around 500 other events that hinted at a connection but with some ambiguities. The combined clear and ambiguous cases constitute 34% of the total detected QSEBs and a smaller percentage of the spicules in our dataset. Conclusions. Our findings suggest that a fraction of the type II spicules originate from QSEBs, supporting magnetic reconnection as a potential driving mechanism. In this context, QSEBs and spicules represent the conversion of magnetic energy into thermal and kinetic energy, respectively. We suggest that an observational programme including multiple Balmer lines would likely detect more unambiguous connections between QSEBs and spicules.

• Bhatnagar, Aditi; Voort, Luc Rouppe van der & Joshi, Jayant (2024). Transition region response to quiet-Sun Ellerman bombs. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202450070. Full text in Research Archive Show summary

  Context. Quiet-Sun Ellerman bombs (QSEBs) are key indicators of small-scale photospheric magnetic reconnection events. Recent high-resolution observations have shown that they are ubiquitous and that large numbers of QSEBs can be found in the quiet Sun. Aims. We aim to understand the impact of QSEBs on the upper solar atmosphere by analyzing their spatial and temporal relationship with the UV brightenings observed in transition region diagnostics. Methods. We analyzed high-resolution Hβ observations from the Swedish 1-m Solar Telescope and utilized k-means clustering to detect 1423 QSEBs in a 51 min time series. We used coordinated and co-aligned observations from the Interface Region Imaging Spectrograph (IRIS) to search for corresponding signatures in the 1400 Å slit-jaw image (SJI) channel and in the Si IV 1394 Å and Mg II 2798.8 Å triplet spectral lines. We identified UV brightenings from SJI 1400 using a threshold of 5σ above the median background. Results. We focused on 453 long-lived QSEBs (> 1 min) and found 67 cases of UV brightenings from SJI 1400 occurring near the QSEBs, both temporally and spatially. Temporal analysis of these events indicates that QSEBs start before UV brightenings in 57% of cases, while UV brightenings lead in 36% of instances. The majority of the UV brightenings occur within 1000 km of the QSEBs in the direction of the solar limb. We also identify 21 QSEBs covered by the IRIS slit, four of which show emissions in the Si IV 1394 Å and/or Mg II 2798.8 Å triplet lines, at distances within 500 km of the QSEBs in the limb direction. Conclusions. We conclude that a small fraction (15%) of the long-lived QSEBs contribute to the localized heating observable in transition region diagnostics, indicating they play a minimal role in the global heating of the upper solar atmosphere.

• Chitta, L.P.; Noort, M. van; Smitha, H.N.; Priest, E.R. & Voort, Luc Rouppe van der (2024). Photospheric Hot Spots at Solar Coronal Loop Footpoints Revealed by Hyperspectral Imaging Observations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 976(1). doi: 10.3847/1538-4357/ad8564. Full text in Research Archive Show summary

  Poynting flux generated by random shuffling of photospheric magnetic footpoints is transferred through the upper atmosphere of the Sun where the plasma is heated to over 1 MK in the corona. High spatiotemporal resolution observations of the lower atmosphere at the base of coronal magnetic loops are crucial to better understand the nature of the footpoint dynamics and the details of magnetic processes that eventually channel energy into the corona. Here, we report high spatial resolution (∼0farcs1) and cadence (1.33 s) hyperspectral imaging of the solar Hα line, acquired by the Microlensed Hyperspectral Imager prototype installed at the Swedish 1-m Solar Telescope, that reveal photospheric hot spots at the base of solar coronal loops. These hot spots manifest themselves as Hα wing enhancements, occurring on small spatial scales of ∼0farcs2, and timescales of less than 100 s. By assuming that the Hα wings and the continuum form under the local thermodynamic equilibrium condition, we inverted the Hα line profiles and found that the hot spots are compatible with a temperature increase of about 1000 K above the ambient quiet-Sun temperature. The Hα wing integrated Stokes V/I maps indicate that hot spots are related to magnetic patches with field strengths comparable to or even stronger that the surrounding network elements. But they do not show the presence of parasitic polarity magnetic field that would support the interpretation that these hot spots are reconnection-driven Ellerman bombs. Therefore, we interpret these features as proxies of locations where convection-driven magnetic field intensification in the photosphere can lead to energy transfer into higher layers. We suggest that such hot spots at coronal loop footpoints may be indicative of the specific locations and onset of energy flux injection into the upper atmosphere.

• Faber, Jonas Thoen; Joshi, Reetika; Voort, Luc Rouppe van der; Wedemeyer, Sven; Fletcher, Lyndsay & Aulanier, Guillaume [Show all 7 contributors for this article] (2024). High-resolution observational analysis of flare ribbon fine structures. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202452370. Full text in Research Archive Show summary

  Context. Since the mechanism of energy release from solar flares is still not fully understood, the study of fine-scale features developing during flares becomes important for progressing towards a consistent picture of the essential physical mechanisms. Aims. Solar flares release most of their luminous energy in the chromosphere. These luminous signatures, known as flare ribbons, act as the footpoints of the released energy and are crucial for the interpretation of reconnection processes causing these events. We aim to probe the fine structures in flare ribbons at the chromospheric level using high-resolution observations with imaging and spectral techniques. Methods. We present a GOES C2.4 class solar flare (SOL2022-06-26T08:12) observed with the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA). Utilising imaging data from SST, IRIS, and AIA, we detail both the global and fine-structure evolution of the flare. The high-resolution SST observations offer spectroscopic data in the Hα, Ca II 8542 Å, and Hβ lines, which we use to analyse the flare ribbon. Results. The flare was associated with a filament eruption. Fibrils and coronal loops were connected from a negative polarity region to two positive polarity regions. Within the eastern flare ribbon, chromospheric bright blobs were detected and analysed in Ca II 8542 Å, Hα, and Hβ wavelengths. A comparison of blobs in Hβ observations and Si IV 1400 Å has also been performed. These blobs are observed as almost circular structures having widths from 140 km–200 km. The intensity profiles of the blobs show a red wing asymmetry. Conclusions. From the high spatial and temporal resolution Hβ observations, we conclude that the periodicity of the blobs in the flare ribbon, which are near-equally spaced in the range 330–550 km, is likely due to fragmented reconnection processes within a flare current sheet. This supports the theory of a direct link between fine-structure flare ribbons and current sheet tearing. We believe our observations represent the highest resolution evidence of fine-structure flare ribbons to date.

• Siverio, Daniel Elias Nóbrega; Cabello, I.; Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Reetika & Froment, C. [Show all 7 contributors for this article] (2024). Small-scale magnetic flux emergence preceding a chain of energetic solar atmospheric events. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 686. doi: 10.1051/0004-6361/202348894. Full text in Research Archive Show summary

  Context. Advancements in instrumentation have revealed a multitude of small-scale extreme-ultraviolet (EUV) events in the solar atmosphere and considerable effort is currently undergoing to unravel them. Aims. Our aim is to employ high-resolution and high-sensitivity magnetograms to gain a detailed understanding of the magnetic origin of such phenomena. Methods. We used coordinated observations from the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO) to analyze an ephemeral magnetic flux emergence episode and the following chain of small-scale energetic events. These unique observations clearly link these phenomena together. Results. The high-resolution (0.057 pixel−1) magnetograms obtained with SST/CRISP allowed us to reliably measure the magnetic field at the photosphere and to detect the emerging bipole that caused the subsequent eruptive atmospheric events. Notably, this small-scale emergence episode remains indiscernible in the lower resolution SDO/HMI magnetograms (0.5 pixel−1). We report the appearance of a dark bubble in Ca II K 3933 Å related to the emerging bipole, a sign of the canonical expanding magnetic dome predicted in flux emergence simulations. Evidence of reconnection are also found, first through an Ellerman bomb and later by the launch of a surge next to a UV burst. The UV burst exhibits a weak EUV counterpart in the coronal SDO/AIA channels. By calculating the differential emission measure (DEM), its plasma is shown to reach a temperature beyond 1 MK and to have densities between the upper chromosphere and transition region. Conclusions. Our study showcases the importance of high-resolution magnetograms in revealing the mechanisms that trigger phenomena such as EBs, UV bursts, and surges. This could hold implications for small-scale events similar to those recently reported in the EUV using Solar Orbiter. The finding of temperatures beyond 1 MK in the UV burst plasma strongly suggests that we are examining analogous features. Therefore, we recommend caution when drawing conclusions from full-disk magnetograms that lack the necessary resolution to reveal their true magnetic origin.

• Joshi, Reetika; Aulanier, Guillaume; Radcliffe, Alice; Voort, Luc Rouppe van der; Pariat, Etienne & Siverio, Daniel Elias Nóbrega [Show all 7 contributors for this article] (2024). Generic low-atmosphere signatures of swirled-anemone jets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449553. Full text in Research Archive Show summary

  Context. Solar jets are collimated plasma flows moving along magnetic field lines and are accelerated at low altitude following magnetic reconnection. Several of them originate from anemone-shaped low-lying arcades, and the most impulsive ones tend to be relatively wider and display untwisting motions. Aims. We aim to establish typical behaviours and observational signatures in the low atmosphere that can occur in response to the coronal development of such impulsive jets. Methods. We analysed an observed solar jet associated with a circular flare ribbon using high-resolution observations from SST coordinated with IRIS and SDO. We related specifically identified features with those developing in a generic 3D line-tied numerical simulation of reconnection-driven jets performed with the ARMS code. Results. We identified three features in the SST observations: the formation of a hook along the circular ribbon, the gradual widening of the jet through the apparent displacement of its kinked edge towards (and not away) from the presumed reconnection site, and the falling back of some of the jet plasma towards a footpoint offset from that of the jet itself. The 3D numerical simulation naturally accounts for these features, which were not imposed a priori. Our analyses allowed us to interpret them in the context of the 3D geometry of the asymmetric swirled-anemone loops and their sequences of reconnection with ambient coronal loops. Conclusions. Given the relatively simple conditions in which the observed jet occurred, together with the generic nature of the simulation that comprised minimum assumptions, we predict that the specific features that we identified and interpreted are probably typical of every impulsive jet.

• Voort, Luc Rouppe van der; Joshi, Jayant & Krikova, Kilian (2024). Observations of magnetic reconnection in the deep solar atmosphere in the Hε line. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 683. doi: 10.1051/0004-6361/202348976. Full text in Research Archive Show summary

  Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon known as Ellerman bombs (EBs). It is most common to observe EBs in the H α and H β spectral lines. High-quality shorter-wavelength Balmer line observations of EBs are rare, but have the potential to provide the most highly resolved view on magnetic reconnection. Aims. We aim to evaluate the H ε 3970 Å line as an EB diagnostic by analyzing high-quality observations in different Balmer lines. Methods. Observations of different targets and viewing angles were acquired with the Swedish 1-m Solar Telescope. These observations sample EBs in different environments: active regions, the quiet Sun, and the penumbra and moat of a sunspot. We employed an automated detection method for quiet-Sun EBs based on k -means clustering. Results. Ellerman bombs in the H ε line show similar characteristics as in the longer-wavelength Balmer lines: a higher intensity than in the surroundings, rapid variability, and a flame-like morphology. In a 24 min quiet-Sun time series, we detected 1674 EBs in the H ε line. This is 1.7 times more EBs than in H β . The quiet-Sun EBs measured in H ε are very similar to those in H β : They have similar lifetimes and a similar area, brightness, and spatial distribution. Most of the EBs detected in H ε are closer to the limb than their H β counterparts because the H ε line core EB emission is formed higher in the atmosphere than the H β EB wing emission. Conclusions. We conclude that the H ε line is well suited for studying EBs, and consequently, for measuring the dynamics of magnetic reconnection in the solar atmosphere at the smallest scales. Our findings suggests that the deep atmosphere in the quiet Sun may host more than 750 000 reconnection events with an EB signature at any time. This is significantly more than what was found in earlier H β observations.

• Moe, Thore Espedal; Pereira, Tiago; Voort, Luc Rouppe van der; Carlsson, Mats; Hansteen, Viggo Haraldson & Calvo, Flavio [Show all 7 contributors for this article] (2024). Comparative clustering analysis of Ca II 854.2 nm spectral profiles from simulations and observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 682. doi: 10.1051/0004-6361/202347328. Full text in Research Archive Show summary

  Context. Synthetic spectra from 3D models of the solar atmosphere have become increasingly successful at reproducing observations, but there are still some outstanding discrepancies for chromospheric spectral lines, such as Ca II and Mg II , particularly regarding the width of the line cores. It has been demonstrated that using sufficiently high spatial resolution in the simulations significantly diminishes the differences in width between the mean spectra in observations and simulations, but a detailed investigation into how this impacts subgroups of individual profiles is currently lacking. Aims. We compare and contrast the typical shapes of synthetic Ca II 854.2 nm spectra found in Bifrost simulations having different magnetic activity with the spectral shapes found in a quiet-Sun observation from the Swedish 1-m Solar Telescope (SST). Methods. We used clustering techniques to extract the typical Ca II 854.2 nm profile shapes synthesized from Bifrost simulations with varying amounts of magnetic activity. We degraded the synthetic profiles to observational conditions and repeated the clustering, and we compared our synthetic results with actual observations. Subsequently, we examined the atmospheric structures in our models for some select sets of clusters, with the intention of uncovering why they do or do not resemble actual observations. Results. While the mean spectra for our high resolution simulations compare reasonably well with the observations, we find that there are considerable differences between the clusters of observed and synthetic intensity profiles, even after the synthetic profiles have been degraded to match observational conditions. The typical absorption profiles from the simulations are both narrower and display a steeper transition from the inner wings to the line core. Furthermore, even in our most quiescent simulation, we find a far larger fraction of profiles with local emission around the core, or other exotic profile shapes, than in the quiet-Sun observations. Looking into the atmospheric structure for a selected set of synthetic clusters, we find distinct differences in the temperature stratification for the clusters most and least similar to the observations. The narrow and steep profiles are associated with either weak gradients in temperature or temperatures rising to a local maximum in the line wing forming region before sinking to a minimum in the line core forming region. The profiles that display less steep transitions show extended temperature gradients that are steeper in the range−3 ≲ log τ 5000 ≲ −1.

• Joshi, Reetika; Voort, Luc Rouppe van der; Schmieder, Birgitte; Moreno-Insertis, Fernando; Prasad, Avijeet & Aulanier, Guillaume [Show all 7 contributors for this article] (2024). High-resolution observations of recurrent jets from an arch filament system. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 691. doi: 10.1051/0004-6361/202449715. Full text in Research Archive Show summary

  Context. Solar jets are collimated plasma ejections along magnetic field lines observed in hot (extreme-ultraviolet (EUV) jets) and cool (chromospheric surges) temperature diagnostics. Their trigger mechanisms and the relationship between hot and cool jets are still not completely understood. Aims. We aim to investigate the generation of a sequence of active-region solar jets and their evolution from the photospheric to the coronal heights using multithermal observations from ground-based and space-borne instruments. Methods. Using the synergy of high-spatial-resolution and high-temporal-resolution observations by the Swedish 1-m Solar Telescope (SST), along with the Solar Dynamics Observatory (SDO), we analyzed a sequence of solar jets originating in a mixed-polarity region between the leading and following sunspots of an active region. We investigated the kinematics of these jets using the spectra from the SST observations. We used a non-force-free field (NFFF) extrapolation technique to derive the magnetic field topology of the active region. Results. A mixed-polarity region is formed over a long period (24 hours) with persistent magnetic flux emergence. This region has been observed as an arch filament system (AFS) in chromospheric SST observations. In this region, negative polarities surrounded by positive polarities create a fan surface with a null point at a height of 6 Mm detected in the NFFF extrapolation. SST observations in the Hβ spectral line reveal a large flux rope over the AFS moving from north to south, causing successive EUV and cool jets to move in the east–west direction and later towards the south along the long open loops. Conclusions. The high-resolution SST observations (0.038 per pixel) resolve the dark area observed at the jet base and reveal the existence of an AFS with an extended cool jet, which may be the result of a peeling-like mechanism of the AFS. Based on the combined analysis of SST and AIA observations along with extrapolated magnetic topology, it is suggested that the magnetic reconnection site may move southward by approximately 20 Mm until it reaches a region where the open magnetic field lines are oriented north–south.

• Krikova, Kilian; Pereira, Tiago & Voort, Luc Rouppe van der (2023). Formation of Hϵ in the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 677. doi: 10.1051/0004-6361/202346796. Full text in Research Archive Show summary

  Context. In the solar spectrum, the Balmer series line H ε is a weak blend on the wing of Ca  II H. Recent high-resolution H ε spectroheliograms reveal a reversed granulation pattern and in some cases, even unique structures. It is apparent that H ε could potentially be a useful diagnostic tool for the lower solar atmosphere. Aims. Our aim is to understand how H ε is formed in the quiet Sun. In particular, we consider the particular physical mechanism that sets its source function and extinction, how it is formed in different solar structures, and why it is sometimes observed in emission. Methods. We used a 3D radiative magnetohydrodynamic (MHD) simulation that accounts for non-equilibrium hydrogen ionization, run with the Bifrost code. To synthesize H ε and Ca  II H spectra, we made use of the RH code, which was modified to take into account the non-equilibrium hydrogen ionization. To determine the dominant terms in the H ε source function, we adopted a multi-level description of the source function. Making use of the synthetic spectra and simulation, we studied the contribution function to the relative line absorption or emission and compared it with atmospheric quantities at different locations. Results. Our multi-level source function description suggests that the H ε source function is dominated by interlocking, with the dominant interlocking transition being through the ground level, populating the upper level of H ε via the Lyman series. This makes the H ε source function partly sensitive to temperature. The H ε extinction is set by Lyman- α . In some cases, this temperature dependence gives rise to H ε emission, indicating heating. The typical absorption profiles show reversed granulation and the H ε line core reflects mostly the Ca  II H background radiation. Conclusions. Synthetic H ε spectra can reproduce quiet Sun observations quite well. High-resolution observations reveal that H ε is not just a weak absorption line. Regions with H ε in emission are especially interesting to detect small-scale heating events in the lower solar atmosphere, such as Ellerman bombs. Thus, H ε can be an important new diagnostic tool for studies of heating in the solar atmosphere, augmenting the diagnostic potential of Ca  II H when observed simultaneously.

• Siverio, Daniel Elias Nóbrega; Moreno-Insertis, F.; Galsgaard, K.; Krikova, Kilian; Voort, Luc Rouppe van der & Joshi, Reetika [Show all 7 contributors for this article] (2023). Deciphering Solar Coronal Heating: Energizing Small-scale Loops through Surface Convection. Astrophysical Journal Letters. ISSN 2041-8205. 958(2). doi: 10.3847/2041-8213/ad0df0. Full text in Research Archive Show summary

  Abstract The solar atmosphere is filled with clusters of hot small-scale loops commonly known as coronal bright points (CBPs). These ubiquitous structures stand out in the Sun by their strong X-ray and/or extreme-ultraviolet (EUV) emission for hours to days, which makes them a crucial piece when solving the solar coronal heating puzzle. In addition, they can be the source of coronal jets and small-scale filament eruptions. Here we present a novel 3D numerical model using the Bifrost code that explains the sustained CBP heating for several hours. We find that stochastic photospheric convective motions alone significantly stress the CBP magnetic field topology, leading to important Joule and viscous heating concentrated around the CBP’s inner spine at a few megameters above the solar surface. We also detect continuous upflows with faint EUV signals resembling observational dark coronal jets and small-scale eruptions when H α fibrils interact with the reconnection site. We validate our model by comparing simultaneous CBP observations from the Solar Dynamics Observatory (SDO) and the Swedish 1‐m Solar Telescope (SST) with observable diagnostics calculated from the numerical results for EUV wavelengths as well as for the H α line using the Multi3D synthesis code. Additionally, we provide synthetic observables to be compared with Hinode, Solar Orbiter, and the Interface Region Imaging Spectrograph (IRIS). Our results constitute a step forward in the understanding of the many different facets of the solar coronal heating problem.

• Murabito, M.; Ermolli, I.; Chatzistergos, T.; Jafarzadeh, Shahin; Giorgi, F. & Voort, Luc Rouppe van der (2023). Investigating the Effect of Solar Ambient and Data Characteristics on Ca II K Observations and Line Profile Measurements. The Astrophysical Journal (ApJ). ISSN 0004-637X. 947(1). doi: 10.3847/1538-4357/acc529. Full text in Research Archive Show summary

  Abstract We analyzed state-of-the-art observations of the solar atmosphere to investigate the dependence of the Ca ii K brightness of several solar features on spectral bandwidth and spatial resolution of the data. In particular, we study data obtained at the Swedish Solar Telescope with the Crisp Imaging Spectropolarimeter and Chromospheric Imaging Spectrometer instruments. The analyzed data, which are characterized by a spectral bandwidth of 0.12 Å and a spatial resolution of 0.078, were acquired close to the disk center by targeting a quiet-Sun area and an active region. We convolved the original observations with Gaussian kernels to degrade their spectral bandwidth and spatial resolution to the instrumental characteristics of the most prominent series of Ca ii K observations available to date. We then studied the effect of data degradation on the observed regions and on parameters derived from Ca ii K line measurements that are largely employed as diagnostics of the solar and stellar chromospheres. We find that the effect of degrading the spectral resolution of Ca ii K observations and line profiles depends on both the employed bandwidth and observed solar region. Besides, we found that the spatial degradation impacts the data characterized by a broad bandwidth to a larger extent compared to those acquired with a narrow band. However, the appearance of the observed solar regions is only slightly affected by the spatial resolution of data with bandwidths up to 1 Å and in the range [3,10] Å. Finally, we derived relationships that can be used to intercalibrate results from observations taken with different instruments in diverse regions of the solar atmosphere.

• Voort, Luc Rouppe van der; Noort, Michiel Van & Rodriguez, Jaime de la Cruz (2023). Ultra-high-resolution observations of plasmoid-mediated magnetic reconnection in the deep solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202345933. Full text in Research Archive Show summary

  Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Aims. To effectively trace magnetic reconnection below the canopy of chromospheric fibrils, we analyzed unique spectroscopic observations of Ellerman bombs in the H α line. Methods. We analyzed a 10 min data set of a young emerging active region observed with the prototype of the Microlensed Hyperspectral Imager (MiHI) at the Swedish 1-m Solar Telescope (SST). The MiHI instrument is an integral field spectrograph that is capable of achieving simultaneous ultra-high resolution in the spatial, temporal, and spectral domains. With the combination of the SST adaptive optics system and image restoration techniques, MiHI can deliver diffraction-limited observations if the atmospheric seeing conditions allow. The data set samples the H α line over 4.5 Å with 10 mÅ pix −1 , with 0.​​065 pix −1 over a field of view of 8.​​6 × 7.​​7, and at a temporal cadence of 1.33 s. This constitutes a hyperspectral data cube that measures 132 × 118 spatial pixels, 456 spectral pixels, and 455 time steps. Results. There were multiple sites with Ellerman bomb activity associated with strong magnetic flux emergence. The Ellerman bomb activity is very dynamic, showing rapid variability and a small-scale substructure. We found a number of plasmoid-like blobs with full-width-half-maximum sizes between 0.​​1 and 0.​​4 and moving with apparent velocities between 14 and 77 km s −1 . Some of these blobs have Ellerman bomb spectral profiles with a single peak at a Doppler offset between 47 and 57 km s −1 . Conclusions. Our observations support the idea that fast magnetic reconnection in Ellerman bombs is mediated by the formation of plasmoids. These MiHI observations demonstrate that a microlens-based integral field spectrograph is capable of probing fundamental physical processes in the solar atmosphere.

• Baso, Carlos José Díaz; Voort, Luc Rouppe van der; Rodriguez, Jaime de la Cruz & Leenaarts, Jorrit (2023). Designing wavelength sampling for Fabry–Pérot observations: Information-based spectral sampling. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202346230. Full text in Research Archive Show summary

  Context. Fabry–Pérot interferometers (FPIs) have become very popular in solar observations because they offer a balance between cadence, spatial resolution, and spectral resolution through a careful design of the spectral sampling scheme according to the observational requirements of a given target. However, an efficient balance requires knowledge of the expected target conditions, the properties of the chosen spectral line, and the instrumental characteristics. Aims. Our aim is to find a method that allows the optimal spectral sampling of FPI observations in a given spectral region to be found. The selected line positions must maximize the information content in the observation with a minimal number of points. Methods. In this study, we propose a technique based on a sequential selection approach in which a neural network is used to predict the spectrum (or physical quantities, if the model is known) from the information at a few points. Only those points that contain relevant information and improve the model prediction are included in the sampling scheme. Results. We have quantified the performance of the new sampling schemes by showing the lower errors in the model parameter reconstructions. The method adapts the separation of the points according to the spectral resolution of the instrument, the typical broadening of the spectral shape, and the typical Doppler velocities. The experiments that use the Ca  II 8542 Å line show that the resulting wavelength scheme naturally places more points in the core than in the wings (by almost a factor of 4), consistent with the sensitivity of the spectral line at each wavelength interval. As a result, observations focused on magnetic field analysis should prioritize a denser grid near the core, while those focused on thermodynamic properties would benefit from a larger coverage. The method can also be used as an accurate interpolator to improve the inference of the magnetic field when using the weak-field approximation. Conclusions. Overall, this method offers an objective approach for designing new instrumentation or observing proposals with customized configurations for specific targets. This is particularly relevant when studying highly dynamic events in the solar atmosphere with a cadence that preserves spectral coherence without sacrificing much information.

• Sowmya, K.; Shapiro, A.I.; Voort, Luc Rouppe van der; Krivova, N.A. & Solanki, S.K. (2023). Modeling Stellar Ca II H and K Emission Variations: Spot Contribution to the S-index. Astrophysical Journal Letters. ISSN 2041-8205. 956(1). doi: 10.3847/2041-8213/acf92a. Full text in Research Archive Show summary

  Abstract The S -index is a measure of emission in the Ca ii H and K lines and is a widely used proxy of stellar magnetic activity. It has been assumed until now that the S -index is mainly affected by bright plage regions in the chromosphere. In particular, the effect of starspots on the S -index has been neglected. In this study, we revisit this assumption. For this, we analyze high-resolution observations of sunspots recorded in the Ca ii H spectral line at the Swedish 1 m Solar Telescope and determine the contrast of spots with respect to the quiet surroundings. We find that the Ca ii H line core averaged over whole sunspots (including superpenumbrae) is brighter than in the quiet surroundings and that the spot contrast in the line core is comparable to the facular contrast. This allows us to get a first estimate of the influence of spots on the S -index. We show that spots increase the S -index. While this increase is quite small for the Sun, it becomes significantly larger for more active stars. Further, we show that the inclusion of the contribution of spots to the S -index strongly affects the relationship between the S -index and stellar disk area coverages by spots and faculae, and present the new relations.

• Testa, Paola; Bakke, Helle; Voort, Luc Rouppe van der & Pontieu, Bart Walter De (2023). High-resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core. The Astrophysical Journal (ApJ). ISSN 0004-637X. 956(2). doi: 10.3847/1538-4357/acf4f1. Full text in Research Archive Show summary

  Abstract High-resolution spectral observations of the lower solar atmosphere (chromosphere and transition region) during coronal heating events, in combination with predictions from models of impulsively heated loops, provide powerful diagnostics of the properties of the heating in active region cores. Here, we analyze the first coordinated observations of such events with the Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging Spectrometer (CHROMIS), at the Swedish 1 m Solar Telescope (SST), which provided extremely high spatial resolution and revealed chromospheric brightenings with spatial dimensions down to ∼150 km. We use machine-learning methods ( k -means clustering) and find significant coherence in the spatial and temporal properties of the chromospheric spectra, suggesting, in turn, coherence in the spatial and temporal distribution of the coronal heating. The comparison of IRIS and CHROMIS spectra with simulations suggests that both nonthermal electrons with low energy (low-energy cutoff ∼5 keV) and direct heating in the corona transported by thermal conduction contribute to the heating of the low atmosphere. This is consistent with growing evidence that nonthermal electrons are not uncommon in small heating events (nanoflare to microflares), and that their properties can be constrained by chromospheric and transition region spectral observations.

• Bakke, Helle; Frogner, Lars; Voort, Luc Rouppe van der; Gudiksen, Boris Vilhelm & Carlsson, Mats (2023). Accelerated particle beams in a 3D simulation of the quiet Sun: Lower atmospheric spectral diagnostics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 675. doi: 10.1051/0004-6361/202346765. Full text in Research Archive Show summary

  Context. Nanoflare heating through small-scale magnetic reconnection events is one of the prime candidates to explain heating of the solar corona. However, direct signatures of nanoflares are difficult to determine, and unambiguous observational evidence is still lacking. Numerical models that include accelerated electrons and can reproduce flaring conditions are essential in understanding how low-energetic events act as a heating mechanism of the corona, and how such events are able to produce signatures in the spectral lines that can be detected through observations. Aims. We investigate the effects of accelerated electrons in synthetic spectra from a 3D radiative magnetohydrodynamics simulation to better understand small-scale heating events and their impact on the solar atmosphere. Methods. We synthesised the chromospheric Ca  II and Mg  II lines and the transition region Si  IV resonance lines from a quiet Sun numerical simulation that includes accelerated electrons. We calculated the contribution function to the intensity to better understand how the lines are formed, and what factors are contributing to the detailed shape of the spectral profiles. Results. The synthetic spectra are highly affected by variations in temperature and vertical velocity. Beam heating exceeds conductive heating at the heights where the spectral lines form, indicating that the electrons should contribute to the heating of the lower atmosphere and hence affect the line profiles. However, we find that it is difficult to determine specific signatures from the non-thermal electrons due to the complexity of the atmospheric response to the heating in combination with the relatively low energy output (∼10 21 erg s −1 ). Even so, our results contribute to understanding small-scale heating events in the solar atmosphere, and give further guidance to future observations.

• Bose, Souvik; Siverio, Daniel Elias Nóbrega; Pontieu, Bart Walter De & Voort, Luc Rouppe van der (2023). The Chromosphere Underneath a Coronal Bright Point. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb544. Full text in Research Archive Show summary

  Abstract Coronal bright points (CBPs) are sets of small-scale coronal loops, connecting opposite magnetic polarities, primarily characterized by their enhanced extreme-ultraviolet (EUV) and X-ray emission. Being ubiquitous, they are thought to play an important role in heating the solar corona. We aim at characterizing the barely explored chromosphere underneath CBPs, focusing on the related spicular activity and on the effects of small-scale magnetic flux emergence on CBPs. We used high-resolution observations of a CBP in H β and Fe i 617.3 nm from the Swedish 1 m Solar Telescope in coordination with the Solar Dynamics Observatory. This work presents the first high-resolution observation of spicules imaged in H β . The spicules were automatically detected using advanced image processing techniques, which were applied to the Dopplergrams derived from H β . Here we report their abundant occurrence close to the CBP “footpoints” and find that the orientation of such spicules is aligned along the EUV loops, indicating that they constitute a fundamental part of the whole CBP magnetic structure. Spatiotemporal analysis across multiple channels indicates that there are coronal propagating disturbances associated with the studied spicules, producing transient EUV intensity variations of the individual CBP loops. Two small-scale flux emergence episodes appearing below the CBP were analyzed, one of them leading to quiet-Sun Ellerman bombs and enhancing the nearby spicular activity. This paper presents unique evidence of the tight coupling between the lower and upper atmosphere of a CBP, thus helping to unravel the dynamic phenomena underneath CBPs and their impact on the latter.

• Bakke, Helle; Carlsson, Mats; Voort, Luc Rouppe van der; Gudiksen, Boris Vilhelm; Polito, V. & Testa, Paola [Show all 7 contributors for this article] (2022). Chromospheric emission from nanoflare heating in RADYN simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 659. doi: 10.1051/0004-6361/202142842. Full text in Research Archive Show summary

  Context. Heating signatures from small-scale magnetic reconnection events in the solar atmosphere have proven to be difficult to detect through observations. Numerical models that reproduce flaring conditions are essential in understanding how nanoflares may act as a heating mechanism of the corona. Aims. We study the effects of non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN simulations of nanoflare heated loops to investigate the diagnostic potential of chromospheric emission from small-scale events. Methods. The Mg  II h and k, Ca  II H and K, Ca  II 854.2 nm, and H α and H β chromospheric lines were synthesised from various RADYN models of coronal loops subject to electron beams of nanoflare energies. The contribution function to the line intensity was computed to better understand how the atmospheric response to the non-thermal electrons affects the formation of spectral lines and the detailed shape of their spectral profiles. Results. The spectral line signatures arising from the electron beams highly depend on the density of the loop and the lower cutoff energy of the electrons. Low-energy (5 keV) electrons deposit their energy in the corona and transition region, producing strong plasma flows that cause both redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons deposit their energy in the lower transition region and chromosphere, resulting in increased emission from local heating. Our results indicate that effects from small-scale events can be observed with ground-based telescopes, expanding the list of possible diagnostics for the presence and properties of nanoflares.

• Mathur, Harsh; Joshi, Jayant; Nagaraju, K.; Voort, Luc Rouppe van der & Bose, Souvik (2022). Properties of shock waves in the quiet-Sun chromosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 668. doi: 10.1051/0004-6361/202244332. Full text in Research Archive Show summary

  Context. Short-lived (100 s or less), sub-arcsec to a couple of arcsec sized features of enhanced brightenings in the narrowband images at the H 2V and K 2V positions of the Ca  II H&amp;K lines in the quiet Sun are known as bright grains. These bright grains are interpreted as manifestations of acoustic shock waves in the chromosphere. Aims. We aim to study time-varying stratified atmospheric properties, such as the temperature, line-of-sight (LOS) velocity, and microturbulence inferred from observations of the bright grains during such acoustic shock events. Methods. With simultaneous observations of a quiet-Sun internetwork region in the Fe  I 6173 Å, Ca  II 8542 Å, and Ca  II K lines acquired by the CRisp Imaging Spectro-Polarimeter and the CHROMospheric Imaging Spectrometer instruments on the Swedish 1-m Solar Telescope, we performed multi-line non-local thermodynamic equilibrium inversions using the STockholm inversion Code to infer the time-varying stratified atmosphere’s physical properties. Results. The Ca  II K profiles of bright grains show enhancement in the K 2V peak intensities with the absence of the K 2R features. At the time of maximum enhancement in the K 2V peak intensities, we found average enhancements in temperature at lower chromospheric layers (at log  τ 500 ≃ −4.2) of about 1.1 kK, with a maximum enhancement of ∼4.5 kK. These temperature enhancements are co-located with upflows, as strong as −6 km s −1 , in the direction of the LOS. The LOS velocities at upper chromospheric layers at log  τ 500 &lt; −4.2 show consistent downflows greater than +8 km s −1 . The retrieved value of microturbulence in the atmosphere of bright grains is negligible at chromospheric layers. Conclusions. This study provides observational evidence to support the interpretation that the bright grains observed in narrowband images at the H 2V and K 2V positions of the Ca  II H&amp;K lines are manifestations of upward propagating acoustic shocks against a background of downflowing atmospheres.

• Ledvina, Vincent E.; Kazachenko, Maria D.; Criscuoli, Serena; Tilipman, Dennis; Ermolli, Ilaria & Falco, Mariachiara [Show all 10 contributors for this article] (2022). Quantifying Properties of Photospheric Magnetic Cancellations in the Quiet Sun Internetwork. The Astrophysical Journal (ApJ). ISSN 0004-637X. 934(1). doi: 10.3847/1538-4357/ac7785. Full text in Research Archive Show summary

  We analyzed spectropolarimetric data from the Swedish 1 m Solar Telescope to investigate the physical properties of small-scale magnetic cancellations in the quiet Sun photosphere. Specifically, we looked at the full Stokes polarization profiles along the Fe i 557.6 nm and of the Fe i 630.1 nm lines measured by the CRisp Imaging SpectroPolarimeter to study the temporal evolution of the line-of-sight magnetic field during 42.5 minutes of quiet Sun evolution. From this magnetogram sequence, we visually identified 38 cancellation events. We then used the Yet Another Feature Tracking Algorithm to characterize the physical properties of these magnetic cancellations. We found on average 1.6 × 1016 Mx of magnetic flux canceled in each event with an average cancellation rate of 3.8 × 1014 Mx s−1. The derived canceled flux is associated with strong downflows, with an average speed of VLOS ≈ 1.1 km s−1. Our results show that the average lifetime of each event is 9.2 minutes with an average of 44.8% of initial magnetic flux being canceled. Our estimates of magnetic fluxes provide a lower limit since studied magnetic cancellation events have magnetic field values that are very close to the instrument noise level. We observed no horizontal magnetic fields at the cancellation sites and therefore cannot conclude whether the events are associated with structures that could cause magnetic reconnection.

• Joshi, Jayant & Voort, Luc Rouppe van der (2022). Properties of ubiquitous magnetic reconnection events in the lower solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 664. doi: 10.1051/0004-6361/202243051. Full text in Research Archive Show summary

  Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high-quality Hβ observations indicate that Ellerman bombs are more common than previously thought, and it was estimated that at any time, about half a million Ellerman bombs are present in the quiet Sun. Aims. We performed an extensive statistical characterization of the quiet-Sun Ellerman bombs (QSEBs) in these new Hβ observations. Methods. We analyzed a 1 h dataset of the quiet Sun observed with the Swedish 1-m Solar Telescope that consists of spectral imaging in the Hβ and Hα lines as well as spectropolarimetric imaging in Fe I 6173 Å. We used the k-means clustering and the 3D connected component labeling techniques to automatically detect QSEBs. Results. We detected a total of 2809 QSEBs. The lifetimes vary between 9 s and 20.5 min, with a median of 1.14 min. The maximum area ranges between 0.0016 and 0.2603 Mm2, with a median of 0.018 Mm2. The maximum brightness in the Hβ wing varies between 1.06 and 2.76 with respect to the average wing intensity. A subset (14%) of the QSEBs displays enhancement of the Hβ line core. On average, the line core brightening appears 0.88 min after the onset of brightening in the wings, and the distance between these brightenings is 243 km. This gives rise to an apparent propagation speed ranging between −14.3 and +23.5 km s−1, with an average that is upward propagating at +4.4 km s−1. The average orientation is nearly parallel to the limbward direction. QSEBs are nearly uniformly distributed over the field of view, but we find empty areas with the size of mesogranulation. QSEBs are located more frequently near the magnetic network, where they are often larger, live longer, and are brighter. Conclusions. We conclude that QSEBs are ubiquitous in the quiet Sun and appear everywhere, except in areas of mesogranular size with the weakest magnetic fields (BLOS ≲ 50 G). Our observations support the interpretation of reconnection along vertically extended current sheets.

• Noda, Carlos Quintero; Schlichenmaier, R.; Rubio, L.R. Bellot; Löfdahl, M.G.; Khomenko, E. & Jurčák, J. [Show all 281 contributors for this article] (2022). The European Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 666. doi: 10.1051/0004-6361/202243867. Full text in Research Archive Show summary

  The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

• Murabito, M.; Guglielmino, S. L.; Ermolli, I.; Romano, Paolo; Jafarzadeh, Shahin & Voort, Luc Rouppe van der (2021). Penumbral decay observed in active region NOAA 12585. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 653. doi: 10.1051/0004-6361/202141034. Full text in Research Archive Show summary

  Context. The physical conditions leading the sunspot penumbra decay are poorly understood so far. Aims. We investigate the photospheric magnetic and velocity properties of a sunspot penumbra during the decay phase to advance the current knowledge of the conditions leading to this process. Methods. A penumbral decay was observed with the CRISP instrument at the Swedish 1 m Solar Telescope on 2016 September 4 and 5 in the active region NOAA 12585. During these days, full-Stokes spectropolarimetric scans along the Fe  I 630 nm line pair were acquired over more than one hour. We inverted these observations with the VFISV code to obtain the evolution of the magnetic and velocity properties. We complement the study with data from instruments on board the Solar Dynamics Observatory and Hinode space missions. Results. The studied penumbra disappears progressively in time and space. The magnetic flux evolution seems to be linked to the presence of moving magnetic features (MMFs). Decreasing Stokes V signals are observed. Evershed flows and horizontal fields were detected even after the disappearance of the penumbral sector. Conclusions. The analyzed penumbral decay seems to result from the interaction between opposite polarity fields in type III MMFs and penumbra, while the presence of overlying canopies regulates the evolution in the different penumbral sectors.

• Bose, Souvik; Joshi, Jayant; Henriques, Vasco Manuel de Jorge & Voort, Luc Rouppe van der (2021). Spicules and downflows in the solar chromosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 647. doi: 10.1051/0004-6361/202040014. Full text in Research Archive Show summary

  Context. High-speed downflows have been observed in the solar transition region (TR) and lower corona for many decades. Despite their abundance, it has been hard to find signatures of such downflows in the solar chromosphere. Aims. In this work, we target an enhanced network region which shows ample occurrences of rapid spicular downflows in the H α spectral line, which could potentially be linked to high-speed TR downflowing counterparts. Methods. We used the k -means algorithm to classify the spectral profiles of on-disk spicules in H α and Ca  II K data observed from the Swedish 1 m Solar Telescope and employed an automated detection method based on advanced morphological image processing operations to detect such downflowing features, in conjunction with rapid blue-shifted and red-shifted excursions (RBEs and RREs). Results. We report the existence of a new category of RREs (termed as downflowing RRE) for the first time that, contrary to earlier interpretation, are associated with chromospheric field aligned downflows moving toward the strong magnetic field regions. Statistical analysis performed on nearly 20 000 RBEs and 15 000 RREs (including the downflowing counterparts), which were detected in our 97 min long dataset, shows that the downflowing RREs are very similar to RBEs and RREs except for their oppositely directed plane-of-sky motion. Furthermore, we also find that RBEs, RREs, and downflowing RREs can be represented by a wide range of spectral profiles with varying Doppler offsets, and H α line core widths, both along and perpendicular to the spicule axis, that causes them to be associated with multiple substructures which evolve together. Conclusions. We speculate that these rapid plasma downflows could well be the chromospheric counterparts of the commonly observed TR downflows.

• Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge & Bose, Souvik (2021). Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 648. doi: 10.1051/0004-6361/202040171. Full text in Research Archive Show summary

  Context. Ellerman bombs are regions with enhanced Balmer line wing emission and mark magnetic reconnection in the deep solar atmosphere in active regions and the quiet Sun. They are often found in regions where opposite magnetic polarities are in close proximity. Recent high-resolution observations suggest that Ellerman bombs are more prevalent than previously thought. Aims. We aim to determine the occurrence of Ellerman bombs in the penumbra of sunspots. Methods. We analyzed high spatial resolution observations of sunspots in the Balmer H α and H β lines as well as auxiliary continuum channels obtained with the Swedish 1-m Solar Telescope and applied the k -means clustering technique to systematically detect and characterize Ellerman Bombs. Results. Features with all the defining characteristics of Ellerman bombs are found in large numbers over the entire penumbra. The true prevalence of these events is only fully appreciated in the H β line due to the highest spatial resolution and lower chromospheric opacity. We find that the penumbra hosts some of the highest Ellerman bomb densities, surpassed only by the moat in the immediate surroundings of the sunspot. Some penumbral Ellerman bombs show flame morphology and rapid dynamical evolution. Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km s −1 and sometimes more than 10 km s −1 . Many penumbral Ellerman bombs migrate from the inner to the outer penumbra over hundreds of km, and some continue moving beyond the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are found in the vicinity of regions with opposite magnetic polarity. Conclusions. We conclude that reconnection is a near continuous process in the low atmosphere of the penumbra of sunspots that manifest in the form of penumbral Ellerman bombs. These are so prevalent that they may be a major sink of sunspot magnetic energy.

• Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge; Siverio, Daniel Elias Nóbrega & Martinez-Sykora, Juan [Show all 7 contributors for this article] (2021). Evidence of the multi-thermal nature of spicular downflows. Impact on solar atmospheric heating. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 654(51). doi: 10.1051/0004-6361/202141404. Full text in Research Archive Show summary

  Context. Spectroscopic observations of the emission lines formed in the solar transition region commonly show persistent downflows on the order of 10−15 km s −1 . The cause of such downflows, however, is still not fully clear and has remained a matter of debate. Aims. We aim to understand the cause of such downflows by studying the coronal and transition region responses to the recently reported chromospheric downflowing rapid redshifted excursions (RREs) and their impact on the heating of the solar atmosphere. Methods. We have used two sets of coordinated data from the Swedish 1 m Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory for analyzing the response of the downflowing RREs in the transition region and corona. To provide theoretical support, we use an already existing 2.5D magnetohydrodynamic simulation of spicules performed with the Bifrost code. Results. We find ample occurrences of downflowing RREs and show several examples of their spatio-temporal evolution, sampling multiple wavelength channels ranging from the cooler chromospheric to the hotter coronal channels. These downflowing features are thought to be likely associated with the returning components of the previously heated spicular plasma. Furthermore, the transition region Doppler shifts associated with them are close to the average redshifts observed in this region, which further implies that these flows could (partly) be responsible for the persistent downflows observed in the transition region. We also propose two mechanisms – (i) a typical upflow followed by a downflow and (ii) downflows along a loop –from the perspective of a numerical simulation that could explain the ubiquitous occurrence of such downflows. A detailed comparison between the synthetic and observed spectral characteristics reveals a distinctive match and further suggests an impact on the heating of the solar atmosphere. Conclusions. We present evidence that suggests that at least some of the downflowing RREs are the chromospheric counterparts of the transition region and lower coronal downflows.

• Ortiz, Ada; Hansteen, Viggo; Siverio, Daniel Elias Nóbrega & Voort, Luc Rouppe van der (2020). Ellerman bombs and UV bursts: reconnection at different atmospheric layers. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 633(A58). doi: 10.1051/0004-6361/201936574. Full text in Research Archive Show summary

  The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst other dynamical phenomena, Ellerman bombs (EBs), which are observed in the wings of Hα and are due to magnetic reconnection in the photosphere below the chromospheric canopy. Signs of magnetic reconnection are also observed in other spectral lines, typical of the chromosphere or the transition region. An example are the ultraviolet (UV) bursts observed in the transition region lines of Si IV and the upper chromospheric lines of Mg II. In this work we analyze high-cadence, high-resolution coordinated observations between the Swedish 1m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα images from the SST provide us with the positions, timings, and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 133 (C II, transition region), 140 (Si IV, transition region), and 279.6 (Mg II k, core, upper chromosphere) nm as well as spectroscopy in the far- and near-ultraviolet from the fast spectrograph raster allow us to study the possible chromospheric and transition region counterparts of those EBs. Our main goal is to study the possible temporal and spatial relationship between several reconnection events at different layers in the atmosphere (namely EBs and UV bursts), the timing history between them, and the connection of these dynamical phenomena to the ejection of surges in the chromosphere. We also investigate the properties of an extended UV burst and their variations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. These results support and expand those obtained from recent numerical simulations of magnetic flux emergence.

• Voort, Luc Rouppe van der (2020). Spicules, The Science of EST. Instituto de Astrofisica de Andalucia. ISSN 9788409200214. Full text in Research Archive
• Drews, Ainar & Voort, Luc Rouppe van der (2020). A multi-diagnostic spectral analysis of penumbral microjets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 638. doi: 10.1051/0004-6361/202037911. Full text in Research Archive Show summary

  Context. Penumbral microjets (PMJs) are short-lived, jet-like objects found in the penumbra of sunspots. They were first discovered in chromospheric lines and have later also been shown to exhibit signals in transition region (TR) lines. Their origin and manner of evolution is not yet settled. Aims. We perform a comprehensive analysis of PMJs through the use of spectral diagnostics that span from photospheric to TR temperatures to constrain PMJ properties. Methods We employed high-spatial-resolution Swedish 1-m Solar Telescope observations in the Ca II 8542 Å and H α lines, IRIS slit-jaw images, and IRIS spectral observations in the Mg II h &amp; k lines, the Mg II 2798.75 Å &amp; 2798.82 Å triplet blend, the C II 1334 Å &amp; 1335 Å lines, and the Si IV 1394 Å &amp; 1403 Å lines. We derived a wide range of spectral diagnostics from these and investigated other secondary phenomena associated with PMJs. Results. We find that PMJs exhibit varying degrees of signal in all of our studied spectral lines. We find low or negligible Doppler velocities and velocity gradients throughout our diagnostics and all layers of the solar atmosphere associated with these. Dark features in the inner wings of H α and Ca II 8542 Å imply that PMJs form along pre-existing fibril structures. We find evidence for upper photospheric heating in a subset of PMJs through emission in the wings of the Mg II triplet lines. There is little evidence for ubiquitous twisting motion in PMJs. There is no marked difference in onset-times for PMJ brightenings in different spectral lines. Conclusions. PMJs most likely exhibit only very modest mass-motions, contrary to earlier suggestions. We posit that PMJs form at upper photospheric or chromospheric heights at pre-existing fibril structures.

• Henriques, Vasco Manuel de Jorge; Nelson, Chris J.; Voort, Luc Rouppe van der & Mathioudakis, Mihalis (2020). Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 642. doi: 10.1051/0004-6361/202038538. Full text in Research Archive Show summary

  Context. The chromosphere of the umbra of sunspots features an assortment of dynamic fine structures that are poorly understood and often studied separately. Small-scale umbral brightenings (SSUBs), umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral dark fibrils are found in any observation of the chromosphere with sufficient spatial resolution performed at the correct umbral flash stage and passband. Understanding these features means understanding the dynamics of the umbral chromosphere. Aims. We aim to fully understand the dynamics of umbral chromosphere through analysis of the relationships between distinct observed fine features and to produce complete models that explain both spectral profiles and the temporal evolution of the features. We seek to relate such understanding to umbral flashes. Methods. We studied the spatial and spectral co-evolution of SDFs, SSUBs, and umbral flashes in Ca  II 8542 Å spectral profiles. We produced models that generate the spectral profiles for all classes of features using non-local thermodynamic equilibrium radiative transfer with a recent version of the NICOLE inversion code. Results. We find that both bright SSUBs and dark SDF structures are described with a continuous feature in the parameter space that is distinct from the surroundings even in pixel-by-pixel inversions. We find a phase difference between such features and umbral flashes in both inverted line-of-sight velocities and timing of the brightenings. For umbral flashes themselves we resolve, for the first time in inversion-based semi-empirical modelling, the pre-flash downflows, post-flash upflows, and the counter-flows present during the umbral flash phase. We further present a simple time-dependent cartoon model that explains the dynamics and spectral profiles of both fine structure, dark and bright, and umbral flashes in umbral chromospheres. Conclusions. The similarity of the profiles between the brightenings and umbral flashes, the pattern of velocities obtained from the inversions, and the phase relationships between the structures all lead us to put forward that all dynamic umbral chromospheric structures observed to this date are a locally delayed or locally early portion of the oscillatory flow pattern that generates flashes, secondary to the steepening large-scale acoustic waves at its source. Essentially, SSUBs are part of the same shock or merely compression front responsible for the spatially larger umbral flash phenomenon, but out of phase with the broader oscillation.

• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Show all 33 contributors for this article] (2020). High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038732. Full text in Research Archive Show summary

  NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe  I lines and spectrally resolved imaging in the chromospheric Ca  II 8542 Å, H α , and Ca  II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.

• Joshi, Jayant; Voort, Luc Rouppe van der & Rodriguez, Jaime de la Cruz (2020). Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038769. Full text in Research Archive Show summary

  Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the lower solar atmosphere. We analyze high spatial resolution observations (0 . 1) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the H β line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the field of view for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier H α observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightenings found in the H β line wings also persist in the line core with a temporal delay and spatial offset toward the nearest solar limb. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the lower solar atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.

• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2019). Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 631. doi: 10.1051/0004-6361/201936617. Full text in Research Archive Show summary

  We characterize, for the first time, type-II spicules in Ca II K 3934 Å using the CHROMIS instrument at the Swedish 1 m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K3 minimum best representing the velocity of the spicules. The K2 features are either suppressed by the Doppler-shifted K3 or enhanced via increased contribution from the lower layers, leading to strongly enhanced but unshifted K2 peaks, with widening towards the line core as consistent with upper-layer opacity removal via Doppler-shift. We identify spicule spectra in concurrent IRIS Mg II k 2796Å observations with very similar properties. Using our interpretation of spicule chromospheric line formation, we produce synthetic profiles that match observations.

• Vissers, Gregal Joan Maria; Rodriguez, Jaime de la Cruz; Libbrecht, Tine; Voort, Luc Rouppe van der; Scharmer, Göran B. & Carlsson, Mats (2019). Dissecting bombs and bursts: Non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 627. doi: 10.1051/0004-6361/201833560. Full text in Research Archive Show summary

  Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. As they are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide useful insight into the overall evolution of active regions. Here we present results from inversions of SST/CRISP and CHROMIS, as well as IRIS data of such transient events. Combining information from the Mg II h &amp; k, Si IV, and Ca II 8542 Å and Ca II H &amp; K lines, we aim to characterise their temperature and velocity stratification, as well as their magnetic field configuration. We find average temperature enhancements of a few thousand kelvin, close to the classical temperature minimum and similar to previous studies, but localised peak temperatures of up to 10 000–15 000 K from Ca II inversions. Including Mg II appears to generally dampen these temperature enhancements to below 8000 K, while Si IV requires temperatures in excess of 10 000 K at low heights, but may also be reproduced with secondary temperature enhancements of 35 000–60 000 K higher up. However, reproducing Si IV comes at the expense of overestimating the Mg II emission. The line-of-sight velocity maps show clear bi-directional jet signatures for some events and strong correlation with substructure in the intensity images in general. Absolute line-of-sight velocities range between 5 and 20 km s−1 on average, with slightly larger velocities towards, rather than away from, the observer. The inverted magnetic field parameters show an enhancement of the horizontal field co-located with the brightenings at heights similar to that of the temperature increase. We are thus able to largely reproduce the observational properties of Ellerman bombs with the UV burst signature (e.g. intensities, profile asymmetries, morphology, and bi-directional jet signatures), with temperature stratifications peaking close to the classical temperature minimum. Correctly modelling the Si IV emission in agreement with all other diagnostics is however an outstanding issue and remains paramount in explaining its apparent coincidence with Hα emission. Fine-tuning the approach (accounting for resolution differences, fitting localised temperature enhancements, and/or performing spatially coupled inversions) is likely necessary in order to obtain better agreement between all considered diagnostics. © ESO 2019

• Pozuelo, Sara Esteban; Rodriguez, Jaime de la Cruz; Drews, Ainar; Voort, Luc Rouppe van der; Scharmer, Göran B. & Carlsson, Mats (2019). Observationally Based Models of Penumbral Microjets. The Astrophysical Journal (ApJ). ISSN 0004-637X. 870(2). doi: 10.3847/1538-4357/aaf28a. Full text in Research Archive
• Froment, Clara; Antolin, Patrick; Henriques, Vasco Manuel de Jorge; Kohutova, Petra & Voort, Luc Rouppe van der (2019). Multi-scale observations of thermal non-equilibrium cycles in coronal loops. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 633. doi: 10.1051/0004-6361/201936717. Full text in Research Archive Show summary

  Context. Thermal non-equilibrium (TNE) is a phenomenon that can occur in solar coronal loops when the heating is quasi-constant and highly-stratified. Under such heating conditions, coronal loops undergo cycles of evaporation and condensation. The recent observations of ubiquitous long-period intensity pulsations in coronal loops and their relationship with coronal rain have demonstrated that understanding the characteristics of TNE cycles is an essential step in constraining the circulation of mass and energy in the corona. Aims. We report unique observations with the Solar Dynamics Observatory (SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured thermal properties across the extreme spatiotemporal scales covered by TNE processes. Methods. Within the same coronal loop bundle, we captured 6 h period coronal intensity pulsations in SDO/AIA and coronal rain observed off-limb in the chromospheric Hα and Ca II K spectral lines with SST/CRISP and SST/CHROMIS. We combined a multi-thermal analysis of the cycles with AIA and an extensive spectral characterisation of the rain clumps with the SST. Results. We find clear evidence of evaporation-condensation cycles in the corona which are linked with periodic coronal rain showers. The high-resolution spectroscopic instruments at the SST reveal the fine-structured rain strands and allow us to probe the cooling phase of one of the cycles down to chromospheric temperatures. Conclusions. These observations reinforce the link between long-period intensity pulsations and coronal rain. They also demonstrate the capability of TNE to shape the dynamics of active regions on the large scales as well as on the smallest scales currently resolvable.

• Rutten, Robert; Voort, Luc Rouppe van der & Pontieu, Bart Walter De (2019). Solar Hα features with hot onsets: IV. Network fibrils. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 632. doi: 10.1051/0004-6361/201936113. Full text in Research Archive Show summary

  Even in quiet areas underneath coronal holes the solar chromosphere contains ubiquitous heating events. They tend to be small scale and short lived, hence difficult to identify. Here we do not address their much-debated contribution to outer-atmosphere heating, but their aftermaths. We performed a statistical analysis of high-resolution observations in the Balmer Hα line to suggest that many slender dark Hα fibrils spreading out from network represent cooling gas that outlines tracks of preceding rapid type II spicule events or smaller similar but as yet unresolved heating agents in which the main gas constituent, hydrogen, ionizes at least partially. Subsequent recombination then causes dark Hα fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the Hα chromosphere around network results from intermittent, frequent small-scale prior heating.

• Voort, Luc Rouppe van der & Drews, Ainar (2019). Penumbral microjets at high spatial and temporal resolution. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 626. doi: 10.1051/0004-6361/201935343. Full text in Research Archive Show summary

  Context. Sunspot observations in chromospheric spectral lines have revealed short-lived linear bright transients that are commonly referred to as penumbral microjets (PMJs). Details on the origin and physical nature of PMJs are to a large extend still unknown. Aims. We aim to characterize the dynamical nature of PMJs to provide guidance for future modeling efforts. Methods. We analyzed high spatial (0.̋1) and temporal resolution (1 s) Ca II H filtergram (0.1 nm bandwidth) observations of a sunspot that were obtained on two consecutive days with the Swedish 1 m Solar Telescope. Results. We find that PMJs appear to be the rapid brightening of an already existing (faint) fibril. The rapid brightening is the fast increase (typically less than 10 s) in intensity over significant length (several hundreds of kilometers) of the existing fibril. For most PMJs, no clear root or source from where the brightening appears to originate can be identified. After the fast onset, about half of the PMJs have tops that move with an apparent velocity of between 5 and 14 km s−1, most of them upward. No significant motion of the top is observed in the other PMJs. About one-third of the PMJs split into two parallel and coevolving linear features during the later phases of their lifetimes. Conclusions. We conclude that mass flows can play only a limited role in the onset phase of PMJs. It is more likely that we see the effect of a fast heating front. ESO 2019

• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Voort, Luc Rouppe van der & Pereira, Tiago Mendes Domingos (2019). Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 627. doi: 10.1051/0004-6361/201935289. Full text in Research Archive Show summary

  Context: The solar chromosphere and the lower transition region are believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution (CRD) in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims: We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods: We used spectropolarimetric observations of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar Telescope and used NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These were used to synthesize Mg II h&k spectra using the RH1.5D code, which we compared with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results: Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot down-flows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions: We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot.

• Vissers, Gregal Joan Maria; Voort, Luc Rouppe van der & Rutten, Robert (2019). Automating Ellerman bomb detection in ultraviolet continua. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 626. doi: 10.1051/0004-6361/201834811. Full text in Research Archive Show summary

  Ellerman bombs are transient brightenings in the wings of Hα 6563 Å that pinpoint photospheric sites of magnetic reconnection in solar active regions. Their partial visibility in the 1600 Å and 1700 Å continua registered routinely by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) offers a unique opportunity to inventory such magnetic-field disruptions throughout the AIA database if a reliable recipe for their detection can be formulated. This is done here. We have improved and applied an Hα Ellerman bomb detection code to ten data sets spanning viewing angles from solar disc centre to the limb. They combine high-quality Hα imaging spectroscopy from the Swedish 1 m Solar Telescope with simultaneous AIA imaging around 1600 Å and 1700 Å. A trial grid of brightness, lifetime and area constraints is imposed on the AIA images to define optimal recovery of the 1735 Ellerman bombs detected in Hα. The best results when optimising simultaneously for recovery fraction and reliability are obtained from 1700 Å images by requiring 5σ brightening above the average 1700 Å nearby quiet-Sun intensity, lifetime above one minute, area of 1–18 AIA pixels. With this recipe 27% of the AIA detections are Hα-detected Ellerman bombs while it recovers 19% of these (of which many are smaller than the AIA resolution). Better yet, among the top 10% AIA 1700 Å detections selected with combined brightness, lifetime and area thresholds as many as 80% are Hα Ellerman bombs. Automated selection of the best 1700 Å candidates therefore opens the entire AIA database for detecting most of the more significant photospheric reconnection events. This proxy is applicable as a flux-dynamics tell-tale in studying any Earth-side solar active region since early 2010 up to the present.

• Chitta, L. P.; Sukarmadji, A. R. C.; Voort, Luc Rouppe van der & Peter, H. (2019). Energetics of magnetic transients in a solar active region plage. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 623. doi: 10.1051/0004-6361/201834548. Full text in Research Archive Show summary

  Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface. Aims. We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations. Methods. We used photospheric Fe I 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code. Results. The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m−2 through the photosphere. Conclusions. We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops.

• Kuridze, D.; Henriques, Vasco Manuel de Jorge; Mathioudakis, M.; Voort, Luc Rouppe van der; Rodriguez, Jaime de la Cruz & Carlsson, Mats (2018). Spectropolarimetric Inversions of the Ca II 8542Å Line in an M-class Solar Flare. The Astrophysical Journal (ApJ). ISSN 0004-637X. 860(1). doi: 10.3847/1538-4357/aac26d. Full text in Research Archive Show summary

  We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using high-resolution full Stokes imaging spectropolarimetry of the Ca ii 8542 Å line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semiempirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and nonflaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post-flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and nonflaring atmospheres shows that during the flare, the Ca ii 8542 Å line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation, which led to results similar to those obtained with the NICOLE inversions.

• Pereira, Tiago Mendes Domingos; Voort, Luc Rouppe van der; Hansteen, Viggo & Pontieu, Bart Walter De (2018). Chromospheric counterparts of solar transition region unresolved fine structure loops. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 611. doi: 10.1051/0004-6361/201832762. Full text in Research Archive Show summary

  Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models. © 2018 ESO

• Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert; Nelson, Chris J. & Huang, Zhenghua [Show all 19 contributors for this article] (2018). Solar Ultraviolet Bursts. Space Science Reviews. ISSN 0038-6308. 214:120(8). doi: 10.1007/s11214-018-0551-0. Full text in Research Archive Show summary

  The term “ultraviolet (UV) burst” is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016–2017.

• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Scharmer, G. B.; Rodriguez, Jaime de la Cruz; Martinez-Sykora, Juan & Nobrega-Siverio, Daniel [Show all 12 contributors for this article] (2017). Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere. Astrophysical Journal Letters. ISSN 2041-8205. 851(1). doi: 10.3847/2041-8213/aa99dd. Full text in Research Archive Show summary

  Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry–Pérot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si iv 1403 Å UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca ii K 3934 Å line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0farcs2 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere. © 2017. The American Astronomical Society. All rights reserved.

• Rutten, Robert & Rouppe, Van Der Voort Luc (2017). Solar Hα features with hot onsets: II. A contrail fibril. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 597. doi: 10.1051/0004-6361/201527560. Full text in Research Archive Show summary

  The solar chromosphere observed in Hα consists mostly of narrow fibrils. The longest typically originate in network or plage and arch far over adjacent internetwork. We use data from multiple telescopes to analyze one well-observed example in a quiet area. It resulted from the earlier passage of an accelerating disturbance in which the gas was heated to high temperature as in the spicule-II phenomenon. After this passage a dark Hα fibril appeared as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset and subsequent visibilities in various diagnostics and conclude that such Hα fibrils can indeed be contrail phenomena, not indicative of the thermodynamic and magnetic environment when they are observed but of more dynamic happenings before. They do not connect across internetwork cells but represent launch tracks of heating events and chart magnetic field during launch, not at present. © ESO, 2017

• Nobrega-Siverio, Daniel; Martinez-Sykora, Juan; Moreno-Insertis, F. & Voort, Luc Rouppe van der (2017). Surges and Si IV Bursts in the Solar Atmosphere: Understanding IRIS and SST Observations through RMHD Experiments. The Astrophysical Journal (ApJ). ISSN 0004-637X. 850(2). doi: 10.3847/1538-4357/aa956c. Full text in Research Archive Show summary

  © 2017. The American Astronomical Society. All rights reserved.

• Drews, Ainar & Voort, Luc Rouppe van der (2017). Microjets in the penumbra of a sunspot. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 602. doi: 10.1051/0004-6361/201630312. Full text in Research Archive Show summary

  Context. Penumbral microjets (PMJs) are short-lived jets found in the penumbra of sunspots, first observed in wide-band Ca ii H line observations as localized brightenings, and are thought to be caused by magnetic reconnection. Earlier work on PMJs has focused on smaller samples of by-eye selected events and case studies. Aims. It is our goal to present an automated study of a large sample of PMJs to place the basic statistics of PMJs on a sure footing and to study the PMJ Ca ii 8542 Å spectral profile in detail. Methods. High spatial resolution and spectrally well-sampled observations in the Ca ii 8542 Å line obtained from the Swedish 1-m Solar Telescope (SST) were reduced by a principle component analysis and subsequently used in the automated detection of PMJs using the simple machine learning algorithm k-nearest neighbour. PMJ detections were verified with co-temporal Ca ii H line observations. Results. We find a total of 453 tracked PMJ events, 4253 PMJs detections tallied over all timeframes, and a detection rate of 21 events per timestep. From these, an average length, width and lifetime of 640 km, 210 km and 90 s are obtained. The average PMJ Ca ii 8542 Å line profile is characterized by enhanced inner wings, often in the form of one or two distinct peaks, and a brighter line core as compared to the quiet-Sun average. Average blue and red peak positions are determined at − 10.4 km s-1 and + 10.2 km s-1 offsets from the Ca ii 8542 Å line core. We find several clusters of PMJ hot-spots within the sunspot penumbra, in which PMJ events occur in the same general area repeatedly over time. Conclusions. Our results indicate smaller average PMJs sizes and longer lifetimes compared to previously published values, but with statistics still in the same orders of magnitude. The investigation and analysis of the PMJ line profiles strengthens the proposed heating of PMJs to transition region temperatures. The presented statistics on PMJs form a solid basis for future investigations and numerical modelling of PMJs. © ESO, 2017

View all works in NVA

• Vogt, Yngve & Voort, Luc Rouppe van der (2026). Astrofysikere avslører stadig nye mysterier på solen | Tu.no. Full text in Research Archive Show summary

  Solen er ikke så stabil som du tror. Hver dag eksploderer det flere hundre tusen bomber på soloverflaten.

• Poquet, Ignasi Josep Soler; Baso, Carlos José Díaz & Voort, Luc Rouppe van der (2026). Multi-wavelength observations of Ellerman bombs. Full text in Research Archive Show summary

  Context. Ellerman bombs (EBs) are observational signatures of small-scale magnetic reconnection, key to understanding the lower solar atmosphere. While their role in active regions has been widely studied using the Hα line, near-ultraviolet (NUV) spectra routinely observed by the Interface Region Imaging Spectrograph (IRIS) offer a promising alternative for EB identification, enabling large-scale studies. Aims. We aim to identify the most important spectral signatures of EBs in the IRIS NUV spectra. With this, we seek to develop a robust criterion for their detection solely using the IRIS NUV spectra. In parallel, we determine the typical atmospheric stratification associated with EBs. Methods. We used four coordinated observations between the Swedish 1-m Solar Telescope (SST) and IRIS. Using the Hα line as a reference, we detected 18 different EBs and studied their associated IRIS NUV spectra. In addition, we used the IRIS2+ inversion tool to infer the temperature, line-of-sight velocity, and non-thermal broadening from the EB spectra. Results. The defining feature of EBs in the IRIS NUV is the enhancement of the wings of the subordinated Mg ii triplet between the Mg ii h&k lines. Inversions reveal that these signatures are produced by a localized temperature increase of ∆T ∼ 1670 K around log(τ)= −3.8. Using only the Mg ii triplet signatures, we found a detection criterion that successfully recovered 14 of 18 Hα-detected EBs. In addition, the shape of the Mg ii h&k lines in relation to the Mg ii triplet can serve as a proxy for the EB formation height. Conclusions. The NUV spectrum observed by IRIS is a good candidate for detecting EBs, opening the doors to large-scale studies across the extensive IRIS database, removing the dependence on Hα observations. This capability bridges the gap between small- and large-scale event studies, which was not possible before due to the lack of coordinated observations.

• Voort, Luc Rouppe van der (2026). SST/CRISP2 observations of an active region. Full text in Research Archive
• Voort, Luc Rouppe van der (2026). Energetic events following magnetic flux emergence into the photosphere. Full text in Research Archive
• Voort, Luc Rouppe van der (2026). SST coordination with Solar Orbiter and other telescopes in space. Full text in Research Archive
• Voort, Luc Rouppe van der (2025). Recent insights from chromospheric observations. Full text in Research Archive
• Voort, Luc Rouppe van der (2025). SST observations in coordination with IRIS, Hinode, and other observatories in space. Full text in Research Archive
• Poirier, Nicolas Aurelien Corentin; Kohutova, Petra; Voort, Luc Rouppe van der & Danilovic, Sanja (2024). About the source of sustained kink oscillations in coronal loops. Full text in Research Archive
• Poirier, Nicolas Aurelien Corentin; Kohutova, Petra; Voort, Luc Rouppe van der & Danilovic, Sanja (2024). Photospheric driving of sustained kink oscillations in coronal loops. Full text in Research Archive
• Poirier, Nicolas Aurelien Corentin; Kohutova, Petra; Voort, Luc Rouppe van der & Danilovic, Sanja (2024). About the source of sustained kink oscillations in coronal loops. Full text in Research Archive
• Voort, Luc Rouppe van der (2024). High-resolution observations of small-scale activity in the low solar atmosphere. Full text in Research Archive
• Voort, Luc Rouppe van der (2024). Impact of small-scale photospheric magnetic reconnection events on the upper atmosphere. Full text in Research Archive
• Poirier, Nicolas Aurelien Corentin; Voort, Luc Rouppe van der; Kohutova, Petra & Danilovic, Sanja (2024). About the nature of sustained kink oscillations in coronal loops: combining coronal and chromospheric diagnostics. Full text in Research Archive
• Voort, Luc Rouppe van der (2024). High-resolution observations of magnetic reconnection in the deep solar atmosphere. Full text in Research Archive
• Voort, Luc Rouppe van der (2023). Den mest energirike strålingen fra Sola. Astronomi. ISSN 0802-7587. 4, p. 9–9. Full text in Research Archive
• Voort, Luc Rouppe van der (2023). High-resolution observations of chromospheric dynamics. Full text in Research Archive
• Poirier, Nicolas Aurelien Corentin; Kohutova, Petra; Joshi, Reetika & Voort, Luc Rouppe van der (2023). About the source of sustained kink oscillations in coronal loops: what do we know and what do we need from future observation campaigns? Full text in Research Archive
• Krikova, Kilian; Pereira, Tiago; Voort, Luc Rouppe van der & Carlsson, Mats (2022). Diagnostic potential of Hε for small-scale energetic phenomena. Full text in Research Archive
• Voort, Luc Rouppe van der (2021). Sola på nært hold. Astronomi. ISSN 0802-7587. p. 32–35. Full text in Research Archive
• Voort, Luc Rouppe van der (2021). Vulkanutbrudd på La Palma. Astronomi. ISSN 0802-7587. p. 55–55. Full text in Research Archive
• Voort, Luc Rouppe van der (2021). Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae. Full text in Research Archive
• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2020). Rapid Downflows in the solar chromosphere. Full text in Research Archive
• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2020). Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines (Corrigendum). Astronomy and Astrophysics (A & A). ISSN 0004-6361. 637. doi: 10.1051/0004-6361/201936617e. Full text in Research Archive
• Jafarzadeh, Shahin; Wedemeyer, Sven; Noda, Carlos Quintero; Wiegelmann, Thomas & Voort, Luc Rouppe van der (2019). Magnetic-field topology throughout the solar chromosphere. Full text in Research Archive
• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2019). Characterization and formation of Ca II K and Mg II k spectra of spicules. Full text in Research Archive
• Henriques, Vasco Manuel de Jorge & Voort, Luc Rouppe van der (2019). Unifying umbral chromospheric dynamics at small scales. Full text in Research Archive
• Voort, Luc Rouppe van der (2019). Penumbral micro-jets at high spatial and temporal resolution. Full text in Research Archive
• Voort, Luc Rouppe van der (2019). Penumbral micro-jets at high spatial and temporal resolution. Full text in Research Archive
• Froment, Clara; Antolin, Patrick; Henriques, V. M. J. & Voort, Luc Rouppe van der (2018). Multi-scale observations of thermal nonequilibrium cycles in coronal loops. Full text in Research Archive
• Froment, Clara; Antolin, Patrick; Henriques, V. M. J. & Voort, Luc Rouppe van der (2018). Multi-scale observations of thermal nonequilibrium cycles in coronal loops. Full text in Research Archive
• Voort, Luc Rouppe van der (2018). Spicules with SST. Full text in Research Archive
• Voort, Luc Rouppe van der (2018). Alignment between IRIS and ground-based data. Full text in Research Archive
• Ornig, Sascha; Carlsson, Mats & Voort, Luc Rouppe van der (2026). Characterization of White-Light Enhancements in Solar Flares: A Combined Observational and Modeling Approach. Universitetet i Oslo. Full text in Research Archive Show summary

  Solar flares are among the most energetic events in the solar system. These sudden brightenings of localized regions in the solar atmosphere drive particle acceleration and heat the ambient plasma to several millions of degrees. The enormous amounts of radiation released in these events, combined with associated phenomena such as coronal mass ejections, have a major influence on space weather around our planet Earth, and it is therefore of vital importance for us to understand the mechanisms behind them. The continuum contains a large portion of the energy released in solar flares, yet its formation mechanisms pose a challenge to the standard flare model. The optical part of the continuum, termed white light (WL), allows us to utilize ground-based observatories for our investigations. With complementary output from numerical simulations, we can gain insights into the dominant processes. However, to date several problems remain. This work investigates solar WL flares (WLFs) using a symbiosis of simulations and observations. It is comprised of three papers, each aimed at answering one of our outstanding questions. Paper I makes use of an ensemble of electron beam-driven solar flare simulations — the F-CHROMA grid — created with the radiation hydrodynamics code RADYN. We investigate how WL enhancements appear in these simulations, what the underlying mechanisms are, and whether we can identify a useful proxy for deducing the dominant mechanism from observations. Our results show intensity enhancements caused primarily by enhanced hydrogen recombination radiation in the optically thin chromosphere, with minor contributions from the radiatively backheated photosphere. The simulated WL enhancements are at least one order of magnitude below observed values. We identify the Paschen jump and its associated intensity ratio as an appropriate proxy for increased optically thin hydrogen recombination radiation. For Paper II, we utilize observations from the Swedish 1-m Solar Telescope (SST) to probe the Paschen jump via continuum measurements redward and blueward of the jump. We find WL enhancements in the dark (pen)umbral regions in sunspots, without any flare-related quiet-Sun intensity increases. The WL increases in one of the flares are co-temporal and (partially) co-spatial with hard X-ray increases, suggesting a common electron-beam cause. Our results indicate that the filter used for observations redward of the jump is unsuitable for measurements of the true continuum intensity due to influences from the nearby spectral line. In Paper III, we attempt to close the gap in amplitude between WL enhancements in simulations and observations by using a background atmosphere modeled after the umbra of a sunspot. Our results show WL increases comparable to observations, and we identify the reduced background intensity as the main reason for this outcome. The mechanisms are identical to those identified in the simulations with a quiet-Sun starting atmosphere used for Paper I. We further find that the electron beam flux is strongly correlated with the resulting WL increases. We conclude that one-dimensional simulations of solar flares can satisfactorily model WLFs of type I. Observationally, we lack adequate measurements of the true continuum longward of the Paschen jump in order to utilize it for the deduction of the formation mechanisms behind WLFs

• Bhatnagar, Aditi & Voort, Luc Rouppe van der (2025). Spectral and topological study of small-scale energetic events in the solar atmosphere. Universitetet i Oslo. Full text in Research Archive Show summary

  Small-scale magnetic reconnection in the quiet Sun contributes to the heating of the solar atmosphere, but the extent of this contribution remains unclear. In this thesis, I investigated the role of quiet-Sun Ellerman bombs (QSEBs) in heating the upper solar atmosphere using high-resolution observations from the Swedish 1-m Solar Telescope (SST) and coordinated observations from the Interface Region Imaging Spectrograph (IRIS). The SST Hβ spectral data and Fe i magnetograms were used along with IRIS Slit-Jaw Images and raster data to detect and analyse QSEBs and their transition region counterparts. The automatic detection of QSEBs using k-means clustering shows that they are abundant on the Sun, but only about 15% exhibit spatially- and temporally- correlated brightenings in ultraviolet diagnostics. This suggests that while QSEBs can locally heat the plasma to upper-atmospheric temperatures, they do not play a dominant role in the global heating of the upper atmosphere. Through potential field extrapolations of the photospheric magnetic field, we further explored three-dimensional magnetic field topologies that link QSEBs to Ultraviolet (UV) brightenings in the transition region. When these events occur in close proximity, the QSEBs are found to occur either at the polarity inversion line of a dipole or at different footpoints of a 3D null having a fan-spine structure. In the fan-spine topology, the QSEBs usually occur at the inner spine, outer spine or fan-surface footpoints based on the strength of the magnetic field at those locations. Meanwhile, the UV brightening is observed close to the 3D null point, whose height can vary based on the magnetic field strength at the inner spine. In addition to identifying QSEBs topologically connected to UV brightenings, we also found a case where a QSEB, while occurring nearby, was not connected to the UV brightening. This suggests that proximity alone is not sufficient to infer causal relationships between QSEBs and UV brightenings, and detailed topological analysis is required to link them to the same magnetic reconnection process. Building upon this analysis, we investigated more complex magnetic field configurations which involved interactions between multiple fan-spine topologies. In one scenario, a smaller fan-spine structure was found nested within a larger one, with QSEBs occurring at the footpoints of the fan-surfaces and a UV brightening near the outer 3D null. The interaction, due to flux emergence, between the emerging fan and the overlying fan surface likely caused the formation of current sheets, facilitating magnetic reconnection. In another case, a similar interaction between fan-surfaces of two adjacent 3D nulls caused the occurrence of a QSEB at their shared footpoint. In both cases, chromospheric inverted-Y-shaped jets were observed originating near the QSEBs, with the first scenario showing the jet closely following regions of high squashing factor. Comparisons with numerical experiment (using the Bifrost code) were performed to validate such a scenario where flux emergence-driven magnetic reconnection can lead to eruption of smallscale jets.

• Krikova, Kilian; Pereira, Tiago & Voort, Luc Rouppe van der (2025). Small-scale energetic phenomena in Hε: line formation and diagnostic potential. Universitetet i Oslo. Full text in Research Archive Show summary

  This dissertation focuses on forward modeling, using state-of-theart 3D radiative magnetohydrodynamic simulations combined with modern non-LTE radiative transfer codes to synthesize spectral lines and analyze their formation. The focus lies on investigating the formation and diagnostic potential of the hydrogen Balmer line Hε in the solar atmosphere, particularly in the context of small-scale energetic phenomena such as Ellerman bombs, UV bursts, and smallscale flares. A deeper understanding of the formation of Hε will help to understand a newly identified phenomenon called quiet Sun Ellerman bombs. Additionally, a modern, in-depth study of the formation of Hε has not yet been performed. By combining state-of-the-art Bifrost 3D radiative magnetohydrodynamic simulations, representing quiet Sun conditions and reproducing small-scale energetic phenomena in combination with the RH1.5D and MULTI3D radiative transfer codes, synthetic spectra were generated to investigate the response of Hε under various solar conditions. We address three core research questions, explored in three papers: 1. How is the Hε spectral line formed under quiet Sun conditions and for different solar features? 2. How can the multi-level source function be calculated, and what role does interlocking play in the line source function? 3. How does Hε respond to small-scale energetic events, such as Ellerman bombs, UV bursts, and small-scale flares? Answering these questions will assist the solar physics community to interpret Hε observations, and also clarify line formation in multi-level atoms, particularly through a deeper understanding of the multi-level source function. Our findings demonstrate that Hε, despite being an unobtrusive spectral line in the solar spectrum, can serve as a valuable diagnostic tool for analyzing and, more importantly, detecting small-scale energetic phenomena. In particular, Hε’s pronounced sensitivity to temperature enhancements in the lower solar atmosphere makes it exceptionally wellsuited for detecting weak heating events in quiet Sun regions, such as quiet Sun Ellerman bombs. This work also underscores the advantages of Hε over other Balmer series lines, such as Hα, in identifying weak energetic events. Unlike Hα or Hβ the line core of Hε is largely unaffected by the solar chromosphere, making it a promising candidate for measuring velocities in the lower chromosphere near the temperature minimum. This dissertation contributes to the field of solar physics by improving the understanding of hydrogen line formation in the solar atmosphere and validating the potential of Hε for upcoming solar missions, such as the Daniel K. Inouye Solar Telescope or the planned European Solar Telescope. Additionally, it provides a detailed investigation of the multi-level source function, a subject rarely addressed in the literature, which will support the interpretation of non-LTE line formation in both solar and stellar atmospheres. The findings especially contribute to the interpretation of high-resolution Hε observations, advancing the exploration of the Sun’s dynamic magnetic processes, and enabling further investigations of a new phenomenon called quiet Sun Ellerman bombs.

• Bakke, Helle; Voort, Luc Rouppe van der & Gudiksen, Boris Vilhelm (2023). Impact of nanoflare heating in the lower solar atmosphere. Universitetet i Oslo. Full text in Research Archive Show summary

  Understanding the mechanisms responsible for heating the solar atmosphere from a few thousand Kelvin in the photosphere to millions of degrees in the corona is a major challenge in solar physics. A number of potential heating mechanisms are continuously studied, and one of the leading candidates is called the nanoflare theory. Nanoflares are small-scale heating events associated with magnetic reconnection, and the heating mechanism is based on the theory that nanoflares occur frequently throughout the solar atmosphere and heat the corona consistently. However, proving this theory has been difficult because the energy released from nanoflares is below the detection threshold of current instrumentation and nanoflares have therefore not been observed yet. So how can we study a phenomenon that is not observable? In this work we utilize numerical simulations in order to investigate the presence and properties of nanoflares in the solar atmosphere. By analysing the atmospheric response to nanoflare heating, it is possible to probe potential heating signatures in simulations to get an idea of what to look for in observations. In our exploration of the nanoflare theory, we have investigated the diagnostic potential of nanoflares by analysing heating signatures in the lower atmosphere and compared our numerical results to an observation of a small-scale coronal heating event.

• Gomez, Juan Camilo Guevara; Wedemeyer, Sven & Voort, Luc Rouppe van der (2022). Observations of the dynamics and structure of the solar atmosphere with ALMA. Universitetet i Oslo. Full text in Research Archive
• Bose, Souvik & Voort, Luc Rouppe van der (2021). On the dynamics of spicules and mass flows in the solar atmosphere. Universitetet i Oslo. Full text in Research Archive Show summary

  The atmosphere of the Sun is envisioned as composed of inherently complex, non-homogeneous, and dynamic layers. A detailed understanding of the physical processes involved in these layers is still lacking. For example, it is largely unknown why the outermost layer of the Sun’s atmosphere (corona) is so much hotter than the photosphere by millions of degrees. Astrophysicists think that the layer sandwiched between the photosphere and the corona, known as the interface region, may hold the key to a better understanding of the nature of this controversy. With the help of coordinated high-resolution ground and space-based observations from the Swedish 1-m Solar Telescope (SST) on La Palma, Spain, and NASA’s Interface Region Imaging Spectrograph (IRIS), I aim to unlock some of the mysteries surrounding the dynamics of the interface region with a focus on small-scale jets, known as “spicules”. Spicules are found almost everywhere on the Sun’s surface and at any given moment there can be as many as 10 million of them rapidly shooting outwards. Because of their “omnipresence”, it is suggested that they play a major role in energizing the outer atmospheric layers of the Sun. This thesis focuses on their physical characteristics, dynamics, and their role in the mass-balance and heating of the solar atmosphere.

• Drews, Ainar; Voort, Luc Rouppe van der & Carlsson, Mats (2021). On microjets in sunspot penumbrae. Universitetet i Oslo. Full text in Research Archive Show summary

  Penumbral microjets (PMJs) are intriguing flashes of light found in the twilight region of sunspots, the penumbra, and they have typical lengths of hundreds of kilometres. Quite some finesse is required to tease out the origins, precise properties, and how PMJs actually move through the Sun’s plasma. PMJs are likely caused by the process of magnetic reconnection, in which “tangled” magnetic field lines suddenly “snap” and realign, releasing energy. This process is typically associated with much larger solar flares. Accordingly, the study of PMJs offers insight into this process at a much smaller scale. The thesis focuses on highly detailed observations from the Swedish 1-m Solar Telescope and NASA’s Interface Region Imaging Spectrograph. A wide range of advanced methods were used, including an automated detection scheme for PMJs involving the machine-learning algorithm k-Nearest Neighbour. Multiple sets of observations were analysed using detailed spectral diagnostics, ranging across many spectral lines. Ultimately, the work accomplished great strides forward in the characterization of PMJs, especially in how they move through the atmosphere and how they evolve in time.

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