Mats Carlsson

• Ornig, Sascha & Carlsson, Mats (2026). Characterization of white-light enhancements under umbral conditions in one-dimensional simulations of solar flares. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202660723. Full text in Research Archive Show summary

  Context. Solar flares with signatures in the optical continuum (white light, WL) pose a challenge not only to the standard flare model, but also to solar flare simulations. In particular, simulations are so far not able to convincingly reproduce observed WL enhancements. Aims. We investigate the effect of different electron beams on an umbral atmosphere and what the differences and similarities to the quiet-Sun response are. Methods. We characterized WL emission in one-dimensional simulations of solar flares using the radiation hydrodynamics code RADYN. We created such simulations with a similar setup as the F-CHROMA grid, but with a starting atmosphere describing umbral conditions. We investigated the influence of different temporal profiles of an electron beam on this umbral atmosphere. Results. Our simulations show maximum WL increases between 40 and 335%, which is comparable to observed values. The reduced umbral background is the main reason for these large increases. We identify hydrogen recombination in an optically thin chromosphere as the dominant process responsible for the increases, with the radiation from the heated photosphere becoming substantial in the later stages (a few seconds after beam heating is turned o ) due to the longer timescale of the cooling of the photosphere compared to hydrogen recombination in the chromosphere. The short timescale of recombination results in very short gradual phases of excess emissions in our simulations. Shorter, more intense beams (i.e., beams with a higher maximum energy flux) lead to a faster and more dramatic atmospheric evolution. Such beams also cause larger WL enhancements due to a higher electron density in the relevant layers. Both the Balmer ratio and the Paschen ratio are substantially higher in our simulations compared to simulations with a quiet-Sun atmosphere. Conclusions. The detectability and amplitude of excess WL emissions depends both on the spectral and temporal structure of the electron beam impacting on the atmosphere as well as the underlying background radiation. The combination of a short, intense beam and an umbral atmospheric structure provides an excellent seed for substantial WL enhancements.

• 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.

• Ornig, Sascha & Carlsson, Mats (2026). Predicted white-light solar flare emission from the F-CHROMA grid of models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556302. Full text in Research Archive Show summary

  Context. Much of a solar flare’s energy is thought to be released in the continuum. The optical continuum, white light (WL), is of special interest since it can be observed from the ground. Aims. We aim to investigate the prevalence of WL emissions in simulations of purely electron beam-driven solar flares, what determines the occurrence of these enhancements, and the underlying causes. Methods. We utilized the F-CHROMA grid of flare simulations created using the radiative hydrodynamics code RADYN. We probed the spectral index, total energy, and low-energy cutoff to draw conclusions about their relationships to the WL intensity. Furthermore, we calculated the 6684 Å continuum intensities as well as the Balmer and the Paschen ratios. Finally, we analyzed two particular cases, one with a high 6684 Å intensity and one with a large Balmer ratio, to determine the dominant mechanisms in these simulations. Results. Of the 84 flares included in the F-CHROMA grid, 33 show WL intensity enhancements that exceed 0.1% relative to the pre-flare level. We conclude that with the parameters presented in the F-CHROMA grid, purely electron beam-driven simulations of solar flares are not able to reproduce observed WL enhancements, as the maximum enhancements in the grid are below 4%, which is significantly lower than observational values. The total energy (which is correlated with the maximum beam flux) is the main factor for deciding whether excess WL emissions will be detectable or not. There is a linear relationship between the Balmer (and Paschen) ratio and the relative continuum increase. Both case studies show that during the time of maximum WL excess, hydrogen ionization and subsequent recombination in an optically thin medium is the dominant mechanism for WL continuum emission enhancements in these electron beam-driven atmospheres. Increased H− emission in the photosphere, as a result of radiative backwarming, becomes dominant during the declining phase of WL emissions in both case studies. We confirm the inability to reproduce the characteristics of type II WL flares, namely, the appearance of photospheric continuum enhancements without prior chromospheric emission increases and the lack of a clear Balmer jump, with the F-CHROMA grid.

• Lamarre, Henri; Charbonneau, Paul; Noraz, Quentin; Strugarek, Antoine; Blaise, Alexis & Brun, Allan Sacha [Show all 8 contributors for this article] (2025). Avalanches in magnetohydrodynamical simulations. Physical Review E (PRE). ISSN 2470-0045. 112(6). doi: 10.1103/28ws-d57z. Full text in Research Archive Show summary

  Scale invariance is a hallmark of many natural systems, including solar flares, where energy release spans a vast range of scales. Recent computational advances, at the level of both algorithmics and hardware, have enabled high-resolution magnetohydrodynamical (MHD) simulations to span multiple scales, offering new insights into magnetic energy dissipation processes. Here, we study the scale invariance of magnetic energy dissipation in two distinct MHD simulations. Current sheets are identified and analyzed over time. Results demonstrate that dissipative events exhibit scale invariance, with power-law distributions characterizing their energy dissipation and lifetimes. Remarkably, these distributions are consistent across the two simulations, despite differing numerical and physical setups, suggesting universality in the process of magnetic energy dissipation. Comparisons between the evolution of dissipation regions reveal distinct growth behaviors in high plasma-𝛽 regions (convective zone) and low plasma-𝛽 regions (atmosphere). The latter display spatiotemporal dynamics similar to those of avalanche models, suggesting self-organized criticality and a common universality class.

• Enerhaug, Elisabeth; Carlsson, M.; Szydlarski, Mikolaj; Gudiksen, Boris Vilhelm & Moortel, Ineke de (2025). Identifying magnetohydrodynamic wave modes in the solar atmosphere: Experiments in 2D. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 701. doi: 10.1051/0004-6361/202554210. Full text in Research Archive Show summary

  Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere. They are thought to play an important role in maintaining the temperature of the solar corona and are important components of coronal seismology. However, research into reliable wave mode identification schemes is still in its early stages. Such a scheme would be a valuable tool for understanding the role of waves in the coronal heating problem. A widely renowned 2003 study in 2D on magnetoacoustic gravity (MAG) waves crossing the β  = 1 layer in the atmosphere provided valuable insights into wave propagation, from the lower to the upper atmosphere. The in-depth analyses of wave propagation through the solar atmosphere offer a valuable reference, which can be used to test other wave mode identifier schemes that could subsequently be expanded to 3D. Aims. We aim to analyse a set of wave mode identification components designed to isolate properties of fast and slow MAG waves. Methods. We recreated the 2003 experiment using the numerical code Bifrost with simplified boundary conditions. We then compared the existing wave analysis to our own scheme. Results. We show that our wave mode identification scheme is equivalent to that of the scheme used in the 2003 study. We show how physical properties such as steepening, maximum, and minimum can be deduced from the scheme. As these wave mode identifiers can be expanded to 3D, the recreation of the piston experiment along with the careful analysis of the components opens the door for further studies into how MAG waves are transmitted across the β  = 1 layer.

• Brchnelova, M.; Gudiksen, Boris Vilhelm; Carlsson, Mats; Lani, A. & Poedts, S. (2025). Constraining the inner boundaries of COCONUT through plasma β and Alfvén speed. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202451772. Full text in Research Archive Show summary

  Context. Space weather modelling has been gaining importance due to our increasing dependency on technology sensitive to space weather effects, such as satellite services, air traffic and power grids. Improving the reliability, accuracy and numerical performance of space weather modelling tools, including global coronal models, is essential to develop timely and accurate forecasts and to help partly mitigate the space weather threat. Global corona models, however, require accurate boundary conditions, for the formulations of which we have very limited observational data. Unsuitable boundary condition prescriptions may lead to inconsistent features in the solution flow field and spoil the code’s accuracy and performance. Aims. In this paper, we develop an adjustment to the inner boundary condition of the COolfluid COrona uNstrUcTured (COCONUT) global corona model to better capture the dynamics over and around the regions of stronger magnetic fields by constraining the plasma β and the Alfvén speed. Methods. Using data from solar observations and solar atmospheric modelling codes such as Bifrost, we find that the baseline homogeneous boundary condition formulations for pressure and density do not capture the plasma conditions physically accurately. We develop a method to adjust these prescribed pressure and density values by placing constraints on the plasma β and the Alfvén speed that act as proxies. Results. We demonstrate that we can remove inexplicable fast streams from the solution by constraining the maximum Alfvén speed and the minimum plasma β on the boundary surface. We also show that the magnetic topology is not significantly affected by this treatment otherwise. Conclusions. The presented technique shows the potential to ease the modelling of solar maxima, especially removing inexplicable features while, at the same time, not significantly affecting the magnetic field topology around the affected regions.

• Kowalski, Adam F.; Allred, Joel C. & Carlsson, Mats (2024). Time-dependent Stellar Flare Models of Deep Atmospheric Heating. The Astrophysical Journal (ApJ). ISSN 0004-637X. 969(2). doi: 10.3847/1538-4357/ad4148. Full text in Research Archive Show summary

  Optical flares have been observed from magnetically active stars for many decades; unsurprisingly, the spectra and temporal evolution are complicated. For example, the shortcomings of optically thin, static slab models have long been recognized when confronted with the observations. A less incorrect—but equally simple—phenomenological T ≈ 9000 K blackbody model has instead been widely adopted in the absence of realistic (i.e., observationally tested) time-dependent, atmospheric models that are readily available. We use the RADYN code to calculate a grid of 1D radiative-hydrodynamic stellar flare models that are driven by short pulses of electron-beam heating. The flare heating rates in the low atmosphere vary over many orders of magnitude in the grid, and we show that the models with high-energy electron beams compare well to the global trends in flux ratios from impulsive-phase stellar flare, optical spectra. The models also match detailed spectral line-shape properties. We find that the pressure broadening and optical depths account for the broad components of the hydrogen Balmer γ lines in a powerful flare with echelle spectra. The self-consistent formation of the wings and nearby continuum level provides insight into how high-energy electron-beam heating evolves from the impulsive to the gradual decay phase in white-light stellar flares. The grid is publicly available, and we discuss possible applications.

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega & Carlsson, Mats (2024). A comparative study of resistivity models for simulations of magnetic reconnection in the solar atmosphere. II. Plasmoid formation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 683. doi: 10.1051/0004-6361/202348046. Full text in Research Archive Show summary

  Context. Plasmoid-mediated reconnection plays a fundamental role in different solar atmospheric phenomena. Numerical reproduction of this process is therefore essential for developing robust solar models. Aims. Our goal is to assess plasmoid-mediated reconnection across various numerical resistivity models in order to investigate how plasmoid numbers and reconnection rates depend on the Lundquist number. Methods. We used the Bifrost code to drive magnetic reconnection in a 2D coronal fan-spine topology, carrying out a parametric study of several experiments with different numerical resolution and resistivity models. We employed three anomalous resistivity models: (1) the original hyper-diffusion from Bifrost, (2) a resistivity proportional to current density, and (3) a resistivity quadratically proportional to electron drift velocity. For comparisons, experiments with uniform resistivity were also run. Results. Plasmoid-mediated reconnection is obtained in most of the experiments. With uniform resistivity, increasing the resolution reveals higher plasmoid frequency with weaker scaling to the Lundquist number, obtaining 7.9–12 plasmoids per minute for S L  ∈ [1.8 × 10 4 , 2.6 × 10 5 ] with a scaling of S L 0.210 in the highest-resolution resistivity cases, transcending into Petschek reconnection in the high- S L limit (where the diffusive effects of the resistivity become small compared to the non-uniform viscosity) and Sweet-Parker reconnection in the low- S L limit. Anomalous resistivity leads to similar results even with lower resolution. The drift-velocity-dependent resistivity excellently reproduces Petschek reconnection for any Lundquist number, and similar results are seen with resistivity proportional to current-density though with slightly lower reconnection rates and plasmoid numbers. Among the different resistivity models applied on the given numerical resolution, the hyper-diffusion model reproduced plasmoid characteristics in closest resemblance to those obtained with uniform resistivity at a significantly higher resolution.

• Varesano, T.; Hassler, D.M.; Prado, N. Zambrana; Plowman, J.; Zanna, G. Del & Parenti, S. [Show all 31 contributors for this article] (2024). SPICE connection mosaics to link the Sun's surface and the heliosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 685. doi: 10.1051/0004-6361/202347637. Full text in Research Archive Show summary

  Aims. We present an analysis of the first connection mosaic made by the SPICE instrument on board the ESA/NASA Solar Orbiter mission on March 2, 2022. The data will be used to map coronal composition that will be compared with in-situ measurements taken by SWA/HIS to establish the coronal origin of the solar wind plasma observed at Solar Orbiter. The SPICE spectral lines were chosen to have varying sensitivity to the first ionization potential (FIP) effect, and therefore the radiances of the spectral lines will vary significantly depending on whether the elemental composition is coronal or photospheric. We investigate the link between the behavior of sulfur and the hypothesis that Alfvén waves drive FIP fractionation above the chromosphere. Methods. We performed temperature diagnostics using line ratios and emission measure (EM) loci, and computed relative FIP biases using three different approaches (two-line ratio (2LR), ratios of linear combinations of spectral lines (LCR), and differential emission measure (DEM) inversion) in order to perform composition diagnostics in the corona. We then compared the SPICE composition analysis and EUI data of the potential solar wind source regions to the SWA/HIS data products. Results. Radiance maps were extracted from SPICE spectral data cubes, with values matching previous observations. We find isothermal plasma of around log T = 5.8 for the AR loops targeted, and that higher FIP-bias values are present at the footpoints of the coronal loops associated with two ARs. Comparing the results with the SWA/HIS data products encourages us to think that Solar Orbiter was connected to a source of slow solar wind during this observation campaign. We demonstrate FIP fractionation in observations of the upper chromosphere and transition region, emphasized by the behavior of the intermediate-FIP element sulfur.

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega; Carlsson, Mats & Martinez-Sykora, Juan (2024). Extreme-ultraviolet (EUV) observables of simulated plasmoid-mediated reconnection in the solar corona. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449357. Full text in Research Archive Show summary

  Context. Understanding the role of magnetic reconnection in the heating and dynamics of the solar atmosphere requires detailed observational data of any observable aspect of the reconnection process, including small-scale features such as plasmoids. Aims. Here, we examine the capability of active and upcoming instruments to detect plasmoids generated by reconnection in the corona including low-density regimes. Methods. We used the Bifrost code to perform simulations of plasmoid-mediated reconnection in the corona with a 2D idealized setup: a fan-spine topology with uniform density including thermal conduction. Through a forward-modeling of extreme-ultraviolet (EUV) observables, we checked whether our simulated plasmoids could be detected with the instruments of Solar Dynamics Observatory (SDO) and Solar Orbiter (SO), as well as the upcoming Multi-Slit Solar Explorer (MUSE) and Solar-C missions. Results. Short-lived (∼10 − 20 s) small-scale (∼0.2 − 0.5 Mm) coronal plasmoids are not resolvable with the Atmospheric Imaging Assembly (AIA) on board SDO. In contrast, they could be captured with the EUV High-Resolution Imager at the Extreme Ultraviolet Imager (EUI-HRIEUV) of SO. The spatial and temporal high-resolution planned for the MUSE spectrograph (SG) is adequate to obtain full spectral information of these plasmoids. To achieve a sufficient signal-to-noise ratio (S/N) for ∼0.8 MK plasmoids in the MUSE/SG 171 Å channel, full-raster images are attainable for regions with electron densities above 109 cm−3, while sit-and-stare observations are recommended for lower-density regions. The future Solar-C mission could also capture these coronal plasmoids using the EUV High-Throughput Spectroscopic Telescope (EUVST), considering rapid changes in Doppler shift and line widths in different EUV lines caused by plasmoid motions along the current sheet. Conclusions. With the combined spectra of MUSE/SG and Solar-C/EUVST in multiple emission lines, along with high-resolution images from SO/EUI-HRIEUV and MUSE/CI, it should be possible to gain new insights about plasmoid formation in the corona.

• 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.

• Pandit, Sneha; Wedemeyer, Sven & Carlsson, Mats (2024). EMISSA (Exploring millimetre indicators of solar-stellar activity): III. Comparison of Ca II indices and millimetre continua in a 3D model atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202347204. Full text in Research Archive Show summary

  Context. Amongst several spectral lines, some of the strongest chromospheric diagnostics are offered by the Ca II H & K lines. These lines can be used to gauge the temperature stratification of the atmosphere since the line core and wings are formed in different regions of the solar atmosphere. Furthermore, the Ca II lines act as tracers for the magnetic structure of the solar atmosphere, as the line cores are formed in the upper chromosphere even though they are formed in non-local thermodynamic equilibrium (NLTE). In contrast, the formation of millimetre (mm) continuum radiation occurs under local thermodynamic equilibrium (LTE) conditions. As a result, the brightness temperatures obtained from observations with the Atacama Large Millimetre/Submillimetre Array (ALMA) offer a complementary perspective on the activity and thermal structure of stellar atmospheres. Aims. The overall aim is to establish more robust solar/stellar activity indicators using ALMA observations in comparison with classical diagnostics, such as the s index and infrared triplet (IRT) index. Methods. We employed the 1.5D radiative transfer codes RH1.5D and advanced radiative transfer (ART) to compute the synthetic spectra for the Ca II lines and the millimetre (mm) continua, respectively. These calculations were performed using an enhanced network atmosphere model, which incorporates non-equilibrium hydrogen ionisation generated by the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) Bifrost code. To account for the limited spatial resolution of ALMA, we simulated the effect using a Gaussian point spread function (PSF). Additionally, we analysed the correlations and slopes of scatter plots between the Ca II indices and mm continuum for the original and degraded resolutions, focusing on the entire simulation box, quiet Sun regions, and enhanced network patches separately. The activity indices generated from these lines could further be used to compare the spectra of Sun-like stars with the solar spectrum. Results. We present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm–8.5 mm) and the Ca II activity indices; namely, the s index and infrared triplet (IRT) index. The Ca II activity indices and mm brightness temperatures are weakly correlated at the high resolution, with the highest correlation observed at a wavelength of 0.3 mm, corresponding to ALMA band 10. As the resolution decreases, the correlation consistently increases. Conversely, the slopes exhibit a decreasing trend with increasing wavelength, while the degradation of resolution does not noticeably affect the calculated slopes. Conclusions. As the spatial resolution decreases, the standard deviations of the Ca II activity indices and brightness temperatures decrease, while the correlations between them increase. However, the slopes do not exhibit significant changes. Consequently, these relationships could be valuable for calibrating the mm continuum maps obtained through ALMA observations.

• Carlsson, Mats & Pontieu, Bart Walter De (2023). An Optically Thin View of the Solar Chromosphere from Observations of the O I 1355Å Spectral Line. The Astrophysical Journal (ApJ). ISSN 0004-637X. 959(2). doi: 10.3847/1538-4357/acf451. Full text in Research Archive Show summary

  Abstract The O i 1355 Å spectral line is one of the only optically thin lines that are both routinely observed and thought to be formed in the chromosphere. We present an analysis of a variety of observations of this line with the Interface Region Imaging Spectrograph (IRIS), and compare it with other IRIS diagnostics as well as diagnostics of the photospheric magnetic field. We utilize special deep exposure modes on IRIS and provide an overview of the statistical properties of this spectral line for several different regions on the Sun. We analyze the spatiotemporal variations of the line intensity and find that it is often significantly enhanced when and where magnetic flux of opposite polarities cancel. Significant emission occurs in association with chromospheric spicules. Because of the optically thin nature of the O i line, the nonthermal broadening can provide insight into unresolved small-scale motions. We find that the nonthermal broadening is modest, with typical values of 5–10 km s −1 , and shows some center-to-limb variation, with a modest increase toward the limb. The dependence with the height of the intensity and line broadening off-limb is compatible with the line broadening being dominated by the superposition of Alfvén waves on different structures. The nonthermal broadening shows a modest but significant enhancement above locations that are in between photospheric magnetic flux concentrations in plage, i.e., where the magnetic field is likely to be more inclined with respect to the line of sight. Our measurements provide strict constraints on future theoretical models of the chromosphere.

• Robinson, Rebecca Anne & Carlsson, Mats (2023). Tracing the signatures of a quiet Sun nanoflare. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 677. doi: 10.1051/0004-6361/202347089. Full text in Research Archive Show summary

  Context. Nanoflare-scale reconnection events are difficult to detect, and even when they are detected, it is tricky to reconstruct the details and trigger mechanisms that power them. However, numerical models of nanoflare-scale reconnection can provide context to observations of small-scale reconnection events via the comparison of synthetic observables to observed signatures of the nanoflare. Aims. We aim to demonstrate how a simulated nanoflare event would look if it were observed by the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA) and the upcoming Multi-slit Solar Explorer (MUSE). The goal is to determine the details (if any) of nanoflare-scale reconnection events that could reasonably be captured by current and future instruments. Methods. We calculated synthetic observables from a quiet Sun simulation of a nanoflare-scale reconnection event, including integrated intensities of Fe  IX at 171.073 Å and Fe  XII at 195.119 Å. Then, we degraded the synthetic observables to SDO/AIA and MUSE resolutions in order to determine whether the instruments are capable of capturing the details of the reconnection event. Results. We determine that even small-scale reconnection events in the quiet Sun can be detected by both SDO/AIA and MUSE. In the 171 channel of each instrument, it is possible to discern details of the two bidirectional jets that emanate from the reconnection site. These two bidirectional jets correspond to two different magnetic features undergoing large-angle reconnection with an overlying horizontal field in the corona. In the 193 channel of SDO/AIA, it is only possible to see one set of bidirectional jets, which corresponds to the most energetic reconnecting feature. However, the calculated count rate for AIA 193 is not sufficient for a reliable observation. Conclusions. Quiet Sun activity is detectable with SDO/AIA and will be detectable with the future MUSE mission. It is possible to detect bidirectional jets with both instruments, which can give context clues as to the mechanisms causing the nanoflare event. The resolution and spectral information of MUSE will give a much more detailed observation of the event, making it much easier to reconstruct a possible trigger mechanism. However, we must be careful in our interpretations of observations when we have limited information, as vastly different physical processes can produce similar observational signatures.

• Huang, Ziwen; Teriaca, L.; Cuadrado, R. Aznar; Chitta, L.P.; Mandal, S. & Peter, H. [Show all 19 contributors for this article] (2023). Imaging and spectroscopic observations of extreme-ultraviolet brightenings using EUI and SPICE on board Solar Orbiter. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202345988. Full text in Research Archive Show summary

  Context. The smallest extreme-ultraviolet (EUV) brightening events that were detected so far, called campfires, have recently been uncovered by the High Resolution EUV telescope (HRI EUV ), which is part of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. HRI EUV has a broad bandpass centered at 17.4 nm that is dominated by Fe  IX and Fe  X emission at about 1 MK. Aims. We study the thermal properties of EUI brightening events by simultaneously observing their responses at different wavelengths using spectral data from the Spectral Imaging of the Coronal Environment (SPICE) also on board Solar Orbiter and imaging data from EUI. Methods. We studied three EUI brightenings that were identified in HRI EUV data that lie within the small areas covered by the slit of the SPICE EUV spectrometer. We obtained the line intensities of the spectral profiles by Gaussian fitting. These diagnostics were used to study the evolution of the EUI brightenings over time at the different line-formation temperatures. Results. We find that (i) the detection of these EUI brightenings is at the limit of the SPICE capabilities. They could not have been independently identified in the data without the aid of HRI EUV observations. (ii) Two of these EUI brightenings with longer lifetimes are observed up to Ne  VIII temperatures (0.6 MK). (iii) All of the events are detectable in O  VI (0.3 MK), and the two longer-lived events are also detected in other transition region (TR) lines. (iv) In one case, we observe two peaks in the intensity light curve of the TR lines that are separated by 2.7 min for C  III and 1.2 min for O  VI . The Ne  VIII intensity shows a single peak between the two peak times of the TR line intensity. Conclusions. Spectral data from SPICE allow us to follow the thermal properties of EUI brightenings. Our results indicate that at least some EUI brightenings barely reach coronal temperatures.

• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2023). Damping of coronal oscillations in self-consistent 3D radiative magnetohydrodynamics simulations of the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 676. doi: 10.1051/0004-6361/202346671. Full text in Research Archive Show summary

  Context. Oscillations are abundant in the solar corona. Coronal loop oscillations are typically studied using highly idealised models of magnetic flux tubes. In order to improve our understanding of coronal oscillations, it is necessary to consider the effect of a realistic magnetic field topology and the density structuring. Aims. We analyse the damping of coronal oscillations using a self-consistent 3D radiation-magnetohydrodynamics simulation of the solar atmosphere spanning from the convection zone into the corona, the associated oscillation dissipation and heating, and finally, the physical processes that cause the damping and dissipation. The simulated corona that forms in this model does not depend on any prior assumptions about the shape of the coronal loops. Methods. We analysed the evolution of a bundle of magnetic loops by tracing the magnetic field. Results. We find that the bundle of magnetic loops shows damped transverse oscillations in response to perturbations in two separate instances, with oscillation periods of 177 s and 191 s, velocity amplitudes of 10 km s −1 and 16 km s −1 , and damping times of 176 s and 198 s. The coronal oscillations lead to the development of velocity shear in the simulated corona, which results in the formation of vortices seen in the velocity field that are caused by the Kelvin-Helmholtz instability. This contributes to the damping and dissipation of the transverse oscillations. Conclusions. The oscillation parameters and evolution we observed are in line with the values that are typically seen in observations of coronal loop oscillations. The dynamic evolution of the coronal loop bundle suggests that the models of monolithic and static coronal loops with constant lengths might need to be re-evaluated by relaxing the assumption of highly idealised wave guides.

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega & Carlsson, Mats (2023). A comparative study of resistivity models for simulations of magnetic reconnection in the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 675. doi: 10.1051/0004-6361/202346447. Full text in Research Archive Show summary

  Context. Magnetic reconnection is a fundamental mechanism in astrophysics. A common challenge in mimicking this process numerically in particular for the Sun is that the solar electrical resistivity is small compared to the diffusive effects caused by the discrete nature of codes. Aims. We aim to study different anomalous resistivity models and their respective effects on simulations related to magnetic reconnection in the Sun. Methods. We used the Bifrost code to perform a 2D numerical reconnection experiment in the corona that is driven by converging opposite polarities at the solar surface. This experiment was run with three different commonly used resistivity models: 1) the hyper-diffusion model originally implemented in Bifrost, 2) a resistivity proportional to the current density, and 3) a resistivity proportional to the square of the electron drift velocity. The study was complemented with a 1D experiment of a Harris current sheet with the same resistivity models. Results. The 2D experiment shows that the three resistivity models are capable of producing results in satisfactory agreement with each other in terms of the current sheet length, inflow velocity, and Poynting influx. Even though Petschek-like reconnection occurred with the current density-proportional resistivity while the other two cases mainly followed plasmoid-mediated reconnection, the large-scale evolution of thermodynamical quantities such as temperature and density are quite similar between the three cases. For the 1D experiment, some recalibration of the diffusion parameters is needed to obtain comparable results. Specifically the hyper-diffusion and the drift velocity-dependent resistivity model needed only minor adjustments, while the current density-proportional model needed a rescaling of several orders of magnitude. Conclusions. The Bifrost hyper-diffusion model is as suitable for simulations of magnetic reconnection as other common resistivity models and has the advantage of being applicable to any region in the solar atmosphere without the need for significant recalibration.

• Pandit, Sneha; Wedemeyer, Sven; Carlsson, Mats & Szydlarski, Mikolaj (2023). Comparison of chromospheric diagnostics in a 3D model atmosphere: Hα linewidth and millimetre continua. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202245412. Full text in Research Archive Show summary

  Context. The H α line, one of the most studied chromospheric diagnostics, is a tracer of magnetic field structures, while the intensity of its line core provides an estimate of the mass density. The interpretation of H α observations is complicated by deviations from local thermodynamic equilibrium (LTE) or instantaneous statistical equilibrium conditions. Meanwhile, millimetre (mm) continuum radiation is formed in LTE, and therefore the brightness temperatures from Atacama Large Millimetre-submillimetre Array (ALMA) observations provide a complementary view of the activity and the thermal structure of stellar atmospheres. These two diagnostics together can provide insights into the physical properties of stellar atmospheres, such as their temperature stratification, magnetic structures, and mass density distribution. Aims. In this paper, we present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm to 8.5 mm) and the width of the H α 6565 Å line. Methods. We used the 3D radiative-transfer codes Multi3D and Advanced Radiative Transfer (ART) to calculate synthetic spectra for the H α line and the mm continua, respectively, from an enhanced network atmosphere model with non-equilibrium hydrogen ionisation generated with the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) code Bifrost. We use a Gaussian point spread function (PSF) to simulate the effect of ALMA’s limited spatial resolution and calculate the H α versus mm continuum correlations and slopes of scatter plots for the original and degraded resolution of the whole box, quiet sun, and enhanced network patches separately. Results. The H α linewidth and mm brightness temperatures are highly correlated and the correlation is highest at a wavelength of 0.8 mm, that is, in ALMA Band 7. The correlation systematically increases with decreasing resolution. On the other hand, the slopes decrease with increasing wavelength. The degradation of resolution does not have a significant impact on the calculated slopes. Conclusions. With decreasing spatial resolution, the standard deviations of the observables, H α linewidth, and brightness temperatures decrease and the correlations between them increase, but the slopes do not change significantly. These relations may therefore prove useful in calibrating the mm continuum maps observed with ALMA.

• 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.

• Janvier, M.; Mzerguat, S.; Young, P.R.; Buchlin, E.; Manou, A. & Pelouze, G. [Show all 59 contributors for this article] (2023). A multiple spacecraft detection of the 2 April 2022 M-class flare and filament eruption during the first close Solar Orbiter perihelion. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 677. doi: 10.1051/0004-6361/202346321. Full text in Research Archive Show summary

  Context. The Solar Orbiter mission completed its first remote-sensing observation windows in the spring of 2022. On 2 April 2022, an M-class flare followed by a filament eruption was seen both by the instruments on board the mission and from several observatories in Earth’s orbit, providing an unprecedented view of a flaring region with a large range of observations. Aims. We aim to understand the nature of the flaring and filament eruption events via the analysis of the available dataset. The complexity of the observed features is compared with the predictions given by the standard flare model in 3D. Methods. In this paper, we use the observations from a multi-view dataset, which includes extreme ultraviolet (EUV) imaging to spectroscopy and magnetic field measurements. These data come from the Interface Region Imaging Spectrograph, the Solar Dynamics Observatory, Hinode, as well as several instruments on Solar Orbiter. Results. The large temporal coverage of the region allows us to analyse the whole sequence of the filament eruption starting with its pre-eruptive state. Information given by spectropolarimetry from SDO/HMI and Solar Orbiter PHI/HRT shows that a parasitic polarity emerging underneath the filament is responsible for bringing the flux rope to an unstable state. As the flux rope erupts, Hinode EIS captures blue-shifted emission in the transition region and coronal lines in the northern leg of the flux rope prior to the flare peak. This may be revealing the unwinding of one of the flux rope legs. At the same time, Solar Orbiter SPICE captures the whole region, complementing the Doppler diagnostics of the filament eruption. Analyses of the formation and evolution of a complex set of flare ribbons and loops, of the hard and soft X-ray emissions with STIX, show that the parasitic emerging bipole plays an important role in the evolution of the flaring region. Conclusions. The extensive dataset covering this M-class flare event demonstrates how important multiple viewpoints and varied observations are in order to understand the complexity of flaring regions. While the analysed data are overall consistent with the standard flare model, the present particular magnetic configuration shows that surrounding magnetic activity such as nearby emergence needs to be taken into account to fully understand the processes at work. This filament eruption is the first to be covered from different angles by spectroscopic instruments, and provides an unprecedented diagnostic of the multi-thermal structures present before and during the flare. This complete dataset of an eruptive event showcases the capabilities of coordinated observations with the Solar Orbiter mission.

• Yardley, Stephanie L.; Owen, Christopher J.; Long, David M.; Baker, Deborah; Brooks, David H. & Polito, Vanessa [Show all 79 contributors for this article] (2023). Slow Solar Wind Connection Science during Solar Orbiter’s First Close Perihelion Passage. Astrophysical Journal Supplement Series. ISSN 0067-0049. 267(1). doi: 10.3847/1538-4365/acd24b. Full text in Research Archive Show summary

  Abstract The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilize the extensive suite of remote-sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote-sensing and in situ measurements of slow wind originating at open–closed magnetic field boundaries. The SOOP ran just prior to Solar Orbiter’s first close perihelion passage during two remote-sensing windows (RSW1 and RSW2) between 2022 March 3–6 and 2022 March 17–22, while Solar Orbiter was at respective heliocentric distances of 0.55–0.51 and 0.38–0.34 au from the Sun. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low-latency in situ data and full-disk remote-sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Postobservation analysis using the magnetic connectivity tool, along with in situ measurements from MAG and SWA/PAS, showed that slow solar wind originating from two out of three of the target regions arrived at the spacecraft with velocities between ∼210 and 600 km s −1 . The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.

• Carlsson, Mats; Fletcher, Lyndsay; Allred, Joel; Heinzel, Petr; Kašparová, Jana & Kowalski, Adam [Show all 9 contributors for this article] (2023). The F-CHROMA grid of 1D RADYN flare models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202346087. Full text in Research Archive Show summary

  Context . Solar flares are the result of the sudden release of magnetic energy in the corona. Much of this energy goes into accelerating charged particles to high velocity. These particles travel along the magnetic field and the energy is dissipated when the density gets high enough, primarily in the solar chromosphere. Modelling this region is difficult because the radiation energy balance is dominated by strong, optically thick spectral lines. Aims . Our aim is to provide the community with realistic simulations of a flaring loop with an emphasis on the detailed treatment of the chromospheric energy balance. This will enable a detailed comparison of existing and upcoming observations with synthetic observables from the simulations, thereby elucidating the complex interactions in a flaring chromosphere. Methods . We used the 1D radiation hydrodynamics code RADYN to perform simulations of the effect of a beam of electrons injected at the apex of a solar coronal loop. A grid of models was produced, varying the total energy input, the steepness, and low-energy cutoff of the beam energy spectrum. Results . The full simulation results for a grid of models are made available online. Some general properties of the simulations are discussed.

• Hansteen, Viggo Haraldson; Martinez-Sykora, Juan; Carlsson, Mats; Pontieu, Bart Walter De; Gošić, Milan & Bose, Souvik (2023). Numerical Simulations and Observations of Mg II in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb33c. Full text in Research Archive Show summary

  The Mg ii h and k lines are among the best diagnostic tools of the upper solar chromosphere. This region of the atmosphere is of particular interest, as it is the lowest region of the Sun's atmosphere where the magnetic field is dominant in the energetics and dynamics, defining its structure. While highly successful in the photosphere and lower to mid-chromosphere, numerical models have produced synthetic Mg ii lines that do not match the observations well. We present a number of large-scale models with magnetic field topologies representative of the quiet Sun, ephemeral flux regions and plage, and also models where the numerical resolution is high and where we go beyond the MHD paradigm. The results of this study show models with a much improved correspondence with IRIS observations in terms of both intensities and widths, especially underscoring the importance of chromospheric mass loading and of capturing the magnetic field topology and evolution in simulations. This comes in addition to the importance of capturing the generation of small-scale velocity fields and including nonequilibrium ionization and ion−neutral interaction effects. However, it should be noted that difficulties in achieving a good correspondence remain, especially when considering the width of Mg ii h and k lines in plage. Understanding and modeling all these effects and their relative importance is necessary in order to reproduce observed spectral features and in isolating the missing pieces necessary to fully comprehend Mg ii formation.

• Robinson, Rebecca Anne; Aulanier, Guillaume & Carlsson, Mats (2023). Quiet Sun flux rope formation via incomplete Taylor relaxation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202346065. Full text in Research Archive Show summary

  Context. Low-altitude nanoflares are among the candidates for atmospheric heating in the quiet Sun’s corona. Low-altitude twisted magnetic fields may be involved in such events, as they are in larger flares. But for nanoflares, the exact role, topology, and formation mechanisms of these twisted fields remain to be studied. Aims. In this paper, we investigate the formation and evolution of a preflare flux rope in a fully stratified, 3D magnetohydrodynamics simulation of the quiet Sun using the Bifrost code. This study focuses on the time period before the rope eventually reconnects with an overlying field, resulting in a nanoflare-scale energy on the order of 10 17 J. One puzzle is that this modeled flux rope does not form by any of the mechanisms usually at work in larger flares, such as flux emergence, flux cancellation, or tether-cutting reconnection. Methods. Using Lagrangian markers to trace representative field lines, we follow the spatiotemporal evolution of the flux rope. By focusing on current volumes (which we call current sheets) between these lines, we identify flux bundles and associated reconnecting field line pairs. We also analyze the time-varying distribution function for the force-free parameter as the flux rope relaxes. Lastly, we compare different seeding methods for tracing magnetic field lines, and discuss their relevance to the analysis. Results. We show that the modeled flux rope is gradually built from the coalescence of numerous current-carrying flux tubes. This occurs through a series of component reconnections that are continuously driven by the complex flows in the underlying convection zone. These reconnections lead to an inverse cascade of helicity from small scales to larger scales. We also find that the system attempts to relax toward a linear force-free field, but that the convective drivers and the nanoflare event prevent full Taylor relaxation. Conclusions. Using a self-consistently driven simulation of a nanoflare event, we show for the first time an inverse helicity cascade tending toward a Taylor relaxation in the Sun’s corona, resulting in a well-ordered flux rope that later reconnects with surrounding fields. This provides context clues toward understanding the buildup of nanoflare events in the quiet Sun through incomplete Taylor relaxations, when no relevant flux emergence or cancellation is observed.

• Robinson, Rebecca Anne; Carlsson, Mats & Aulanier, Guillaume (2022). From incoherent field to coherent reconnection: Understanding convection-driven coronal heating in the quiet Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 668. doi: 10.1051/0004-6361/202244750. Full text in Research Archive Show summary

  Context. Magnetic reconnection in the quiet Sun is a phenomenon that is consistently observed, however, its conditions of occurrence are not as well known as for more energetic events. It has recently become feasible to address this issue with 3D numerical simulations of realistically stratified and convection-driven reconnection. Aims. We aim to illustrate ways by which quiet Sun fields may contribute to solar atmospheric heating via magnetic reconnection that is driven by convective motion. We also aim to compare our complex stratified model to earlier idealized coronal models in terms of reconnection drivers and topological conditions. Methods. We analyzed a simulation of the quiet Sun in which a complex coronal magnetic field is self-consistently driven by the underlying convection. We employed a selection of Lagrangian markers to trace the spatiotemporal behavior of specific magnetic features that are relevant to magnetic reconnection and atmospheric heating. Results. A relatively large-scale reconnection-driven heating event occurs in the simulated corona, in a flattened X-shaped feature characterized by a weak field and high current. It is reminiscent of a hyperbolic flux tube (HFT), which is located at the interface between multiple flux systems. One of these is a smooth overlying horizontal field and the two most relevant others are located below the HFT. They consist of an arcade and a horizontal flux rope which eventually reconnect with the overlying field, raising coronal plasma temperatures up to 1.47 MK. Conclusions. We have identified a reconnection-driven coronal heating event in a quiet Sun simulation. We find that our results are in good phenomenological agreement with idealized coronal flare models, which demonstrates that the same general physical concepts are valid. However, we also find that the reconnecting flux rope and arcade are neither formed by any obvious coherent flux emergence, nor by any ordered photospheric motion or flux cancellation. Instead, they seem to develop merely from the self-consistent convective driving of pre-existing tangled field lines. This gradual and smooth ordering suggests an inverse cascade of magnetic helicity via smaller reconnection events, located at or above slowly-moving photospheric flux concentrations. We suggest that this case is representative of many heating events that may be ubiquitous in the real quiet Sun.

• 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.

• Kowalski, Adam F.; Allred, Joel C.; Carlsson, Mats; Kerr, Graham S.; Tremblay, Pier-Emmanuel & Namekata, Kosuke [Show all 8 contributors for this article] (2022). The Atmospheric Response to High Nonthermal Electron-beam Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope. The Astrophysical Journal (ApJ). ISSN 0004-637X. 928(2). doi: 10.3847/1538-4357/ac5174. Full text in Research Archive Show summary

  Redshifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of the Interface Region Imaging Spectrograph. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam-generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of the hydrogen Balmer series should be readily detected in observations. To accurately interpret the upcoming spectral data of flares with the DKIST, we incorporate nonideal, nonadiabatic line-broadening profiles of hydrogen into the RADYN code. These improvements allow time-dependent predictions for the extreme Balmer line wing enhancements in solar flares. We study two chromospheric condensation models, which cover a range of electron-beam fluxes (1 − 5 × 1011 erg s−1 cm−2) and ambient electron densities (1 − 60 × 1013 cm−3) in the flare chromosphere. Both models produce broadening and redshift variations within 10 s of the onset of beam heating. In the chromospheric condensations, there is enhanced spectral broadening due to large optical depths at Hα, Hβ, and Hγ, while the much lower optical depth of the Balmer series H12−H16 provides a translucent window into the smaller electron densities in the beam-heated layers below the condensation. The wavelength ranges of typical DKIST/ViSP spectra of solar flares will be sufficient to test the predictions of extreme hydrogen wing broadening and accurately constrain large densities in chromospheric condensations.

• Song, Donguk; Ishikawa, Ryohko; Kano, Ryouhei; McKenzie, David E.; Bueno, Javier Trujillo & Auchère, Frédéric [Show all 28 contributors for this article] (2022). Polarization Accuracy Verification of the Chromospheric LAyer SpectroPolarimeter. Solar Physics. ISSN 0038-0938. 297(10). doi: 10.1007/s11207-022-02064-8. Full text in Research Archive
• Rachmeler, Laurel A.; Bueno, Javier Trujillo; McKenzie, D.E.; Ishikawa, R.; Auchère, F. & Kobayashi, K. [Show all 28 contributors for this article] (2022). Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg ii h and k Lines. The Astrophysical Journal (ApJ). ISSN 0004-637X. 936(1). doi: 10.3847/1538-4357/ac83b8. Full text in Research Archive Show summary

  Abstract The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg ii h and k spectral region around 2800 Å along a 200 slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near-ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg ii h and k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semiempirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.

• Cheung, Mark C. M.; Martinez-Sykora, Juan; Testa, Paola; Pontieu, Bart Walter De; Chintzoglou, Georgios & Rempel, Matthias [Show all 27 contributors for this article] (2022). Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). II. Flares and Eruptions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 926(1). doi: 10.3847/1538-4357/ac4223. Full text in Research Archive Show summary

  Current state-of-the-art spectrographs cannot resolve the fundamental spatial (subarcseconds) and temporal (less than a few tens of seconds) scales of the coronal dynamics of solar flares and eruptive phenomena. The highest-resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by the Interface Region Imaging Spectrograph for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), subarcsecond-resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al., which focuses on investigating coronal heating with MUSE.

• Pontieu, Bart Walter De; Testa, Paola; Martinez-Sykora, Juan; Antolin, Patrick; Karampelas, Konstantinos & Hansteen, Viggo Haraldson [Show all 26 contributors for this article] (2022). Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). I. Coronal Heating. The Astrophysical Journal (ApJ). ISSN 0004-637X. 926(1). doi: 10.3847/1538-4357/ac4222. Full text in Research Archive
• Brooks, David H.; Janvier, Miho; Baker, Deborah; Warren, Harry P.; Auchère, Frédéric & Carlsson, Mats [Show all 31 contributors for this article] (2022). Plasma Composition Measurements in an Active Region from Solar Orbiter/SPICE and Hinode/EIS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 940(1). doi: 10.3847/1538-4357/ac9b0b. Full text in Research Archive
• Hong, J.; Carlsson, Mats & Ding, M.D. (2022). An approximate recipe of chromospheric radiative losses for solar flares. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 661. doi: 10.1051/0004-6361/202142839. Full text in Research Archive Show summary

  Context. Radiative losses in the chromosphere are very important for the energy balance of the Sun. There have been efforts to make simple lookup tables for chromospheric radiative losses in the quiet Sun. During solar flares, the atmospheric conditions are quite different, and the currently available recipe is constructed from semi-empirical models. How these recipes work in flare conditions remains to be evaluated. Aims. We aim to construct an approximate recipe of chromospheric radiative losses for solar flares. Methods. We tabulate the optically thin radiative loss, escape probability, and ionization fraction using a grid of flare models from radiative hydrodynamic simulations as our dataset. Results. We provide new lookup tables to calculate chromospheric radiative losses for flares. Compared with previous recipes, our recipe provides a better approximation of the detailed radiative losses for flares.

• Moe, Thore Espedal; Pereira, Tiago & Carlsson, Mats (2022). Effects of spatial resolution on inferences of atmospheric quantities from simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 662. doi: 10.1051/0004-6361/202243192. Full text in Research Archive Show summary

  Context. Small-scale processes are thought to be important for the dynamics of the solar atmosphere. While numerical resolution fundamentally limits their inclusion in magnetohydronamic (MHD) simulations, real observations at the same nominal resolution should still contain imprints of subresolution effects. This means that the synthetic observables from a simulation of a given resolution might not be directly comparable to real observables at the same resolution. It is thus of interest to investigate how inferences based on synthetic spectra from simulations with different numerical resolutions compare, and whether these differences persist after the spectra have been spatially degraded to a common resolution Aims. We aim to compare synthetic spectra obtained from realistic 3D radiative magnetohydrodynamic (rMHD) simulations run at different numerical resolutions from the same initial atmosphere, using very simple methods for inferring line-of-sight velocities and magnetic fields. Additionally we examine how the differing spatial resolution impacts the results retrieved from the STiC inversion code. Methods. We used the RH 1.5D code to synthesize the photospheric Fe  I 617.33 line in local thermodynamic equilibrium (LTE), and the chromospheric Ca  II 854.209 line in non-LTE from three MHD simulation snapshots of differing spatial resolution. The simulations were produced by the Bifrost code, using horizontal grid spacing of 6 km, 12 km, and 23 km, respectively. They were started from the exact same atmosphere, and the snapshots were taken after the same exact elapsed time. The spectra obtained from the high-resolution snapshots were spatially degraded to match the lowest resolution. Simple methods, such as the center-of-gravity approach and the weak field approximation, were then used to estimate line-of-sight velocities and magnetic fields for the three cases after degradation. Finally, the spectra were input into the STiC inversion code and the retrieved line-of-sight velocities and magnetic field strengths, as well as the temperatures, from the inversions were compared. Results. We find that while the simple inferences for all three simulations reveal the same large-scale tendencies, the higher resolutions yield more fine-grained structures and more extreme line-of-sight velocities and magnetic fields in concentrated spots even after spatial smearing. We also see indications that the imprints of subresolution effects on the degraded spectra result in systematic errors in the inversions, and that these errors increase with the amount of subresolution effects included. Fortunately, however, we find that successively including more subresolution yields smaller additional effects; that is to say, there is a clear trend of diminishing importance for progressively finer subresolution effects.

• Finley, Adam J.; Brun, Allan Sacha; Carlsson, Mats; Hansteen, Viggo Haraldson & Shoda, M. (2022). Stirring the base of the solar wind: On heat transfer and vortex formation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 665. doi: 10.1051/0004-6361/202243947. Full text in Research Archive Show summary

  Context. Current models of the solar wind must approximate (or ignore) the small-scale dynamics within the solar atmosphere; however, these are likely important in shaping the emerging wave-turbulence spectrum that ultimately heats and accelerates the coronal plasma. Aims. This study strives to make connections between small-scale vortex motions at the base of the solar wind and the resulting heating and acceleration of the coronal plasma. Methods. The Bifrost code produces realistic simulations of the solar atmosphere which facilitate the analysis of spatial and temporal scales which are currently at, or beyond, the limit of modern solar telescopes. For this study, the Bifrost simulation is configured to represent the solar atmosphere in a coronal hole region, from which the fast solar wind emerges. The simulation extends from the upper-convection zone (2.5 Mm below the photosphere) to the low corona (14.5 Mm above the photosphere), with a horizontal extent of 24 Mm × 24 Mm. The network of magnetic funnels in the computational domain influence the movement of plasma, as well as the propagation of magnetohydrodynamic waves into the low corona. Results. The twisting of the coronal magnetic field by photospheric flows efficiently injects energy into the low corona. Poynting fluxes of up to 2 − 4 kWm −2 are commonly observed inside twisted magnetic structures with diameters in the low corona of 1–5 Mm. Torsional Alfvén waves are favourably transmitted along these structures, and subsequently escape into the solar wind. However, reflections of these waves from the upper boundary condition make it difficult to unambiguously quantify the emerging Alfvén wave-energy flux. Conclusions. This study represents a first step in quantifying the conditions at the base of the solar wind using Bifrost simulations. It is shown that the coronal magnetic field is readily braided and twisted by photospheric flows. Temperature and density contrasts form between regions with active stirring motions and those without. Stronger whirlpool-like flows in the convection, concurrent with magnetic concentrations, launch torsional Alfvén waves up through the magnetic funnel network, which are expected to enhance the turbulent generation of magnetic switchbacks in the solar wind.

• Druett, Malcolm K.; Leenarts, Jorrit; Carlsson, Mats & Szydlarski, Mikolaj (2022). Formation and heating of chromospheric fibrils in a radiation-MHD simulation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 665. doi: 10.1051/0004-6361/202142399. Full text in Research Archive Show summary

  Aims. We examine the movements of mass elements within dense fibrils using passive tracer particles (corks) in order to understand the creation and destruction processes of fibrils. Methods. Simulated fibrils were selected at times when they were visible in a H α image proxy. The corks were selected within fibril H α formation regions. From this set, we selected a cork and constructed the field line passing through it. Other fibrilar corks close to this field line were also selected and pathlines were constructed, revealing the locations of the mass elements forwards and backwards in time. Finally, we analysed the forces acting on these mass elements. Results. The main process of fibrilar loading in the simulation is different to the mass loading scenario in which waves steepen into shocks and push material upwards along the field lines from locations near their footpoints. The twisted, low-lying field lines were destabilised and then they untwisted, lifting the material trapped above their apexes via the Lorentz force. Subsequently, the majority of the mass drained down the field lines towards one or both footpoints under the influence of gravity. Material with large horizontal velocities could also be elevated in rising field lines, creating somewhat parabolic motions, but the material was not generally moving upward along a stationary magnetic field line during loading. Conclusions. The processes observed in the simulation are additional scenarios that are plausible. The criteria for observing such events are described in this work. We note that it is desirable for our simulations to also be able to form more densely packed fibrils from material fed from the base of field footpoints. The experimental parameters required to achieve this are also discussed in this paper.

• 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.

• Pontieu, Bart Walter De; Polito, Vanessa; Hansteen, Viggo Haraldson; Testa, Paola; Reeves, Katharine K. & Antolin, Patrick [Show all 14 contributors for this article] (2021). A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS). Solar Physics. ISSN 0038-0938. 296. doi: 10.1007/s11207-021-01826-0. Full text in Research Archive Show summary

  Abstract The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. IRIS is the highest resolution observatory to provide seamless coverage of spectra and images from the photosphere into the low corona. The unique combination of near- and far-ultraviolet spectra and images at sub-arcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion–neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of waves, the acceleration of non-thermal particles, and various small-scale instabilities. IRIS has provided insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nano-flares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvénic waves, energy release and jet-like dynamics associated with braiding of magnetic-field lines, the role of turbulence and the tearing-mode instability in reconnection, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and various other mechanisms in triggering and driving CMEs. IRIS observations have also been used to elucidate the physical mechanisms driving the solar irradiance that impacts Earth’s upper atmosphere, and the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine-learning techniques have played a key role. With the advent of exciting new instrumentation both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST) and the Atacama Large Millimeter/submillimeter Array (ALMA), and space-based, e.g. the Parker Solar Probe and the Solar Orbiter , we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges.

• Fludra, Andrzej; Caldwell, M.; Giunta, A.; Grundy, T.; Guest, S. & Leeks, S. [Show all 31 contributors for this article] (2021). First observations from the SPICE EUV spectrometer on Solar Orbiter. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 656. doi: 10.1051/0004-6361/202141221. Full text in Research Archive Show summary

  Aims. We present first science observations taken during the commissioning activities of the Spectral Imaging of the Coronal Environment (SPICE) instrument on the ESA/NASA Solar Orbiter mission. SPICE is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths. In this paper we illustrate the possible types of observations to give prospective users a better understanding of the science capabilities of SPICE. Methods. We have reviewed the data obtained by SPICE between April and June 2020 and selected representative results obtained with different slits and a range of exposure times between 5 s and 180 s. Standard instrumental corrections have been applied to the raw data. Results. The paper discusses the first observations of the Sun on different targets and presents an example of the full spectra from the quiet Sun, identifying over 40 spectral lines from neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur, magnesium, and iron. These lines cover the temperature range between 20 000 K and 1 million K (10 MK in flares), providing slices of the Sun’s atmosphere in narrow temperature intervals. We provide a list of count rates for the 23 brightest spectral lines. We show examples of raster images of the quiet Sun in several strong transition region lines, where we have found unusually bright, compact structures in the quiet Sun network, with extreme intensities up to 25 times greater than the average intensity across the image. The lifetimes of these structures can exceed 2.5 hours. We identify them as a transition region signature of coronal bright points and compare their areas and intensity enhancements. We also show the first above-limb measurements with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and far off limb measurements in the equatorial plane in Mg IX, Ne VIII, and O VI lines. We discuss the potential to use abundance diagnostics methods to study the variability of the elemental composition that can be compared with in situ measurements to help confirm the magnetic connection between the spacecraft location and the Sun’s surface, and locate the sources of the solar wind. Conclusions. The SPICE instrument successfully performs measurements of EUV spectra and raster images that will make vital contributions to the scientific success of the Solar Orbiter mission.

• Noda, Carlos Quintero; Barklem, P. S.; Gafeira, Ricardo; Cobo, Basilio Ruiz; Collados, Manuel & Carlsson, Mats [Show all 10 contributors for this article] (2021). Diagnostic capabilities of spectropolarimetric observations for understanding solar phenomena: I. Zeeman-sensitive photospheric lines. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 652. doi: 10.1051/0004-6361/202037735. Full text in Research Archive Show summary

  Future ground-based telescopes will expand our capabilities for simultaneous multi-line polarimetric observations in a wide range of wavelengths, from the near-ultraviolet to the near-infrared. This creates a strong demand to compare candidate spectral lines to establish a guideline of the lines that are most appropriate for each observation target. We focused in this first work on Zeeman-sensitive photospheric lines in the visible and infrared. We first examined their polarisation signals and response functions using a 1D semi-empirical atmosphere. Then we studied the spatial distribution of the line core intensity and linear and circular polarisation signals using a realistic 3D numerical simulation. We ran inversions of synthetic profiles, and we compared the heights at which we obtain a high correlation between the input and the inferred atmosphere. We also used this opportunity to revisit the atomic information we have on these lines and computed the broadening cross-sections due to collisions with neutral hydrogen atoms for all the studied spectral lines. The results reveal that four spectral lines stand out from the rest for quiet-Sun and network conditions: Fe  I 5250.2, 6302, 8468, and 15 648 Å. The first three form higher in the atmosphere, and the last line is mainly sensitive to the atmospheric parameters at the bottom of the photosphere. However, as they reach different heights, we strongly recommend using at least one of the first three candidates together with the Fe  I 15 648 Å line to optimise our capabilities for inferring the thermal and magnetic properties of the lower atmosphere.

• Srivastava, Abhishek Kumar; Ballester, Jose Luis; Cally, Paul Stuart; Carlsson, Mats; Goossens, M. & Jess, David B. [Show all 10 contributors for this article] (2021). Chromospheric Heating by Magnetohydrodynamic Waves and Instabilities. Journal of Geophysical Research (JGR): Space Physics. ISSN 2169-9380. 126(6). doi: 10.1029/2020JA029097. Full text in Research Archive Show summary

  The importance of the chromosphere in the mass and energy transport within the solar atmosphere is now widely recognized. This review discusses the physics of magnetohydrodynamic waves and instabilities in large-scale chromospheric structures as well as in magnetic flux tubes. We highlight a number of key observational aspects that have helped our understanding of the role of the solar chromosphere in various dynamic processes and wave phenomena, and the heating scenario of the solar chromosphere is also discussed. The review focuses on the physics of waves and invokes the basics of plasma instabilities in the context of this important layer of the solar atmosphere. Potential implications, future trends and outstanding questions are also delineated.

• Bergemann, Maria; Hoppe, Richard; Semenova, Ekaterina; Carlsson, Mats; Yakovleva, Svetlana A. & Voronov, Yaroslav V. [Show all 13 contributors for this article] (2021). Solar oxygen abundance. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 508(2), p. 2236–2253. doi: 10.1093/mnras/stab2160. Full text in Research Archive Show summary

  Motivated by the controversy over the surface metallicity of the Sun, we present a re-analysis of the solar photospheric oxygen (O) abundance. New atomic models of O and Ni are used to perform non-local thermodynamic equilibrium (NLTE) calculations with 1D hydrostatic (MARCS) and 3D hydrodynamical (Stagger and Bifrost) models. The Bifrost 3D MHD simulations are used to quantify the influence of the chromosphere. We compare the 3D NLTE line profiles with new high-resolution, R≈700000⁠, spatially resolved spectra of the Sun obtained using the IAG FTS instrument. We find that the O i lines at 777 nm yield the abundance of log A(O) = 8.74 ± 0.03 dex, which depends on the choice of the H-impact collisional data and oscillator strengths. The forbidden [O i] line at 630 nm is less model dependent, as it forms nearly in LTE and is only weakly sensitive to convection. However, the oscillator strength for this transition is more uncertain than for the 777 nm lines. Modelled in 3D NLTE with the Ni i blend, the 630 nm line yields an abundance of log A(O) = 8.77 ± 0.05 dex. We compare our results with previous estimates in the literature and draw a conclusion on the most likely value of the solar photospheric O abundance, which we estimate at log A(O) = 8.75 ± 0.03 dex.

• Rast, Mark P.; González, Nazaret Bello; Rubio, Luis R. Bellot; Cao, Wenda; Cauzzi, Gianna & DeLuca, Edward [Show all 86 contributors for this article] (2021). Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST). Solar Physics. ISSN 0038-0938. 296(4). doi: 10.1007/s11207-021-01789-2. Full text in Research Archive Show summary

  The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.

• Ishikawa, Ryohko; Bueno, Javier Trujillo; Alemán, Tanausú del Pino; Okamoto, Takenori J.; McKenzie, David E. & Auchère, Frédéric [Show all 28 contributors for this article] (2021). Mapping solar magnetic fields from the photosphere to the base of the corona. Science Advances. 7(8). doi: 10.1126/sciadv.abe8406. Full text in Research Archive Show summary

  Routine ultraviolet imaging of the Sun’s upper atmosphere shows the spectacular manifestation of solar activity; yet, we remain blind to its main driver, the magnetic field. Here, we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg ii h & k and Mn i) and visible (Fe i) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere, the field strengths reach more than 300 G, strongly correlated with the Mg ii k line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.

• Gallagher, Andrew J.; Bergemann, Maria; Collet, Remo; Plez, Bertrand; Leenaarts, Jorrit & Carlsson, Mats [Show all 8 contributors for this article] (2020). Observational constraints on the origin of the elements: II. 3D non-LTE formation of Ba II lines in the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 634. doi: 10.1051/0004-6361/201936104. Full text in Research Archive Show summary

  Context. The pursuit of more realistic spectroscopic modelling and consistent abundances has led us to begin a new series of papers designed to improve current solar and stellar abundances of various atomic species. To achieve this, we have begun updating the three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer code, MULTI3D, and the equivalent one-dimensional (1D) non-LTE radiative transfer code, MULTI 2.3. Aims. We examine our improvements to these codes by redetermining the solar barium abundance. Barium was chosen for this test as it is an important diagnostic element of the s-process in the context of galactic chemical evolution. New Ba II + H collisional data for excitation and charge exchange reactions computed from first principles had recently become available and were included in the model atom. The atom also includes the effects of isotopic line shifts and hyperfine splitting. Methods. A grid of 1D LTE barium lines were constructed with MULTI 2.3 and fit to the four Ba II lines available to us in the optical region of the solar spectrum. Abundance corrections were then determined in 1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium abundance was computed from these corrections. Results. We present for the first time the full 3D non-LTE barium abundance of A(Ba) = 2.27 ± 0.02 ± 0.01, which was derived from four individual fully consistent barium lines. Errors here represent the systematic and random errors, respectively.

• Martinez-Sykora, Juan; Leenarts, Jorrit; Pontieu, Bart Walter De; Siverio, Daniel Elias Nóbrega; Hansteen, Viggo & Carlsson, Mats [Show all 7 contributors for this article] (2020). Ion–neutral Interactions and Nonequilibrium Ionization in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 889(2). doi: 10.3847/1538-4357/ab643f. Full text in Research Archive
• Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Jafarzadeh, Shahin; Eklund, Henrik; Gomez, Juan Camilo Guevara & Bastian, Tim [Show all 10 contributors for this article] (2020). The Sun at millimeter wavelengths: I. Introduction to ALMA Band 3 observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 635. doi: 10.1051/0004-6361/201937122. Full text in Research Archive Show summary

  Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started regular observations of the Sun in 2016, first offering receiver Band 3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around 1.25 mm (239 GHz). Aims. Here we present an initial study of one of the first ALMA Band 3 observations of the Sun. Our aim is to characterise the diagnostic potential of brightness temperatures measured with ALMA on the Sun. Methods. The observation covers a duration of 48 min at a cadence of 2 s targeting a quiet Sun region at disc-centre. Corresponding time series of brightness temperature maps are constructed with the first version of the Solar ALMA Pipeline and compared to simultaneous observations with the Solar Dynamics Observatory (SDO). Results. The angular resolution of the observations is set by the synthesised beam, an elliptical Gaussian that is approximately 1.4 × 2.1 in size. The ALMA maps exhibit network patches, internetwork regions, and elongated thin features that are connected to large-scale magnetic loops, as confirmed by a comparison with SDO maps. The ALMA Band 3 maps correlate best with the SDO/AIA 171 Å, 131 Å, and 304 Å channels in that they exhibit network features and, although very weak in the ALMA maps, imprints of large-scale loops. A group of compact magnetic loops is very clearly visible in ALMA Band 3. The brightness temperatures in the loop tops reach values of about 8000−9000 K and in extreme moments up to 10 000 K. Conclusions. ALMA Band 3 interferometric observations from early observing cycles already reveal temperature differences in the solar chromosphere. The weak imprint of magnetic loops and the correlation with the 171, 131, and 304 SDO channels suggests, however, that the radiation mapped in ALMA Band 3 might have contributions from a wider range of atmospheric heights than previously assumed, but the exact formation height of Band 3 needs to be investigated in more detail. The absolute brightness temperature scale as set by total power measurements remains less certain and must be improved in the future. Despite these complications and the limited angular resolution, ALMA Band 3 observations have a large potential for quantitative studies of the small-scale structure and dynamics of the solar chromosphere.

• Hasegawa, Takahiro; Noda, Carlos Quintero; Shimizu, Toshifumi & Carlsson, Mats (2020). On the Formation of Lyman β and the O I 1027 and 1028 Å Spectral Lines. The Astrophysical Journal (ApJ). ISSN 0004-637X. 900(1). doi: 10.3847/1538-4357/aba95c. Full text in Research Archive
• Hudson, Hugh S.; MacKinnon, Alec; Szydlarski, Mikolaj Marcin & Carlsson, Mats (2020). Cosmic ray interactions in the solar atmosphere. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 491(4), p. 4852–4856. doi: 10.1093/mnras/stz3373. Full text in Research Archive Show summary

  High-energy particles enter the solar atmosphere from Galactic or solar coronal sources, and produce ‘albedo’ emission from the quiet Sun that is now observable across a wide range of photon energies. The interaction of high-energy particles in a stellar atmosphere depends essentially upon the joint variation of the magnetic field and plasma density, which heretofore has been characterized parametrically as P ∝ Bα with P the gas pressure and B the magnitude of the magnetic field. We re-examine that parametrization by using a self-consistent 3D MHD model (Bifrost) and show that this relationship tends to P ∝ B3.5 ± 0.1 based on the visible portions of the sample of open-field flux tubes in such a model, but with large variations from point to point. This scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere, which will have a strong effect on the prediction of the emission anisotropy (limb brightening). The simulations show that much of the open flux in coronal holes originates in weak-field regions within the granular pattern of the convective motions seen in the simulations.

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• Kirkaune, Mats; Wedemeyer, Sven; Carlsson, Mats & Brunvoll, Aline Rangøy (2026). Exploring synthetic nanoflare signatures at millimetre wavelengths. Full text in Research Archive Show summary

  Nanoflares have long been considered to be promising candidates for heating the solar corona to millions of degrees. However, their assumed small sizes, short durations, and subtle signals in accessible observables have made direct detection difficult. In the millimetre regime, the Atacama Large (sub-)Millimeter Array (ALMA) offers the capability of measuring chromospheric temperatures at very high temporal and comparatively high spatial resolution. Yet, it is not clear if the currently offered solar observing mode of ALMA is capable of detecting nanoflares. To explore possible millimetre signatures of nanoflares, the F-CHROMA grid of 1D RADYN flare models has been extended to lower total flare energies, into the nanoflare domain. These models simulate the response of a coronal loop to an electron beam inserted at its top. The new simulations were run as a combined effort to characterise nanoflare signatures at different wavelengths. The radiative transfer code RH1.5 was used to compute the millimetre signatures of the modelled nanoflares. These signatures were then degraded to the temporal and spatial resolution achievable with ALMA. The synthetic ALMA light curves indicate that, in principle, ALMA should be able to detect nanoflare signatures, especially when using wider array configurations in combination with a higher frequency band to maximise spatial resolution. This suggests that dedicated ALMA observations could, in principle, place important constraints on atmospheric heating from nanoflares.

• Vogt, Yngve & Carlsson, Mats (2026). Astrofysikere avslører stadig nye mysterier på solen | Tu.no. [Internet]. 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.

• Øyvann, Stig & Carlsson, Mats (2026). Kraftig solstorm ga GPS-forstyrrelser i Nord-Norge. [Internet]. Telecomrevy.no. Full text in Research Archive Show summary

  En kraftig solstorm i januar har ført til betydelige forstyrrelser i satellittnavigasjon og radiosamband, særlig i Nord-Norge. Forskere advarer om at solaktiviteten er inne i en fase der både GPS-tjenester og deler av den digitale infrastrukturen er mer sårbar enn normalt.

• Carlsson, Mats & Tveito, Ole Jon (2026). Nordlys, Naturfenomen | Massivt nordlys over Tinn. Full text in Research Archive Show summary

  Kraftige solstormer som har funnet sted på sola fører nå til massivt nordlys over store deler av Norge. Over Gausta var det tirsdag kveld flott nordlys.

• Carlsson, Mats (2026). Solstorm, Astronomi | Professor varsler solstormer som kan føre til store skader på jorda. [Internet]. Hamar Arbeiderblad. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda.

• Vogt, Yngve & Carlsson, Mats (2026). Eksplosjoner på solen kan lamme det moderne livet på jorden | Tu.no. [Internet]. TU.no. Full text in Research Archive Show summary

  Solaktiviteten er nå på sitt høyeste. Faren er til stede for at en voldsom eksplosjon på solen, i år eller neste år, kan skade alt fra satellitter og telekommunikasjon til strømforsyningen på jorden.

• Carlsson, Mats (2026). Professor varsler solstormer som kan føre til store skader på jorda. [Internet]. MSN. Full text in Research Archive
• Carlsson, Mats (2026). Kan skape kaos på jorda: - – Ville ha vært katastrofalt. [Internet]. Sol.no. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda.

• Carlsson, Mats (2026). Solstormer kan føre til store skader på jorda. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda. – Det er ikke et spørsmål om hvorvidt det skjer igjen, men om når, sier professor Mats Carlsson på Astrofysisk institutt ved UiO.

• Carlsson, Mats (2026). Solstormer, Astronomi | Professor varsler solstormer som kan føre til store skader på jorda. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda.

• Carlsson, Mats (2026). Professor varsler solstormer som kan føre til store skader på jorda. [Internet]. Energiwatch. Full text in Research Archive
• Carlsson, Mats & Vogt, Yngve (2026). Eksplosjoner på solen kan lamme det moderne livet på jorden - Forskningsnytt. Full text in Research Archive
• Carlsson, Mats (2025). SFF hvorfor og hvordan? Full text in Research Archive
• Carlsson, Mats (2025). Striving for Excellence - Experiences from a Long Career. Full text in Research Archive
• Carlsson, Mats (2025). Chromospheric modeling. Full text in Research Archive
• Noraz, Quentin Jean Francois Alain; Carlsson, Mats & Aulanier, Guillaume (2024). What is heating the chromosphere of Bifrost simulations? Full text in Research Archive
• Carlsson, Mats (2024). Understanding the Workings of the Energetic Sun. Full text in Research Archive
• Carlsson, Mats (2024). Sola og nordlyset: eksplosjonene som treffer oss. Full text in Research Archive
• Pandit, Sneha; Wedemeyer, Sven & Carlsson, Mats (2023). Comparison of activity indicators in a 3D model atmosphere. Full text in Research Archive
• Carlsson, Mats (2023). Kunst og astrofysikk møtes: Solen og dens energi. Full text in Research Archive
• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2023). Damping of Coronal Oscillations in Self-Consistent 3D Radiative MHD Simulations of the Solar Atmosphere. Full text in Research Archive
• Pandit, Sneha; Wedemeyer, Sven & Carlsson, Mats (2023). Comparison of activity indicators in a 3D model atmosphere. Full text in Research Archive
• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2023). Damping of coronal oscillations in self-consistent 3D radiative MHD simulations of the solar atmosphere. Full text in Research Archive
• Færder, Øystein Håvard; Nóbrega-Siverio, Daniel & Carlsson, Mats (2023). Magnetic Reconnection Signatures in the Corona through MUSE Forward Modelling. Full text in Research Archive
• Carlsson, Mats (2023). Solar Physics in Norway. Status and Prospects. Full text in Research Archive
• Carlsson, Mats (2023). Understanding the workings of the energetic Sun. Full text in Research Archive
• Carlsson, Mats & Hustadnes, Halldor (2022). Unike bilder av sola. Sola har aldri vært ­«skarpere. [Journal]. Dagbladet. Full text in Research Archive
• Kohutova, Petra; Popovas, Andrius; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2022). Coronal oscillations in the self-consistent 3D MHD simulations of the solar atmosphere. Full text in Research Archive
• Kohutova, Petra; Popovas, Andrius; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2022). Coronal oscillations in the self-consistent 3D MHD simulations of the solar atmosphere. 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
• Carlsson, Mats (2022). Solar atmospheric modelling and diagnostics. Full text in Research Archive
• Carlsson, Mats (2022). Understanding the workings of the energetic Sun. Full text in Research Archive
• Moe, Thore Espedal; Pereira, Tiago & Carlsson, Mats (2022). Effects of spatial resolution on inferences of atmospheric quantities from simulations. Full text in Research Archive
• Moe, Thore Espedal; Pereira, Tiago & Carlsson, Mats (2022). Effects of spatial resolution on inferences of atmospheric quantities from simulations. Full text in Research Archive
• Carlsson, Mats (2021). Vår nærmeste stjerne. Astronomi. ISSN 0802-7587. p. 10–20. Full text in Research Archive
• Moe, Thore Espedal; Pereira, Tiago & Carlsson, Mats (2021). Effects of spatial resolution on Stokes profiles. Full text in Research Archive
• Carlsson, Mats (2021). Modelling of the Solar Chromosphere. Full text in Research Archive
• Carlsson, Mats (2021). Solen, din nærmeste stjerne. Full text in Research Archive
• Carlsson, Mats (2021). Modelling the energy and mass coupling of the chromosphere and corona. Full text in Research Archive
• Carlsson, Mats (2021). Modelling the energy and mass coupling of the solar chromosphere and corona. Full text in Research Archive
• Carlsson, Mats (2021). Solstormer. Full text in Research Archive
• Collet, Mathieu; Gotlieb, Arnaud; Carlsson, Mats; Marijan, Dusica; Mossige, Morten & Simonis, Helmut (2020). RobTest: A CP Approach to Generate Maximal Test Trajectories for Industrial Robots. Full text in Research Archive
• Carlsson, Mats (2020). Radyn. 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

• Enerhaug, Elisabeth; Gudiksen, Boris Vilhelm; Carlsson, Mats; Moortel, Ineke de & Howson, Tom (2025). Identifying magnetohydrodynamic wave modes in the solar atmosphere. Universitetet i Oslo. Full text in Research Archive
• Færder, Øystein Håvard; Carlsson, Mats; Hansteen, Viggo Haraldson & Siverio, Daniel Elias Nóbrega (2024). The role of resistivity in plasmoid-mediated reconnection in the solar corona and its observational imprints. Universitetet i Oslo. Full text in Research Archive Show summary

  Magnetic reconnection is a fundamental mechanism in astrophysics and a promising candidate for solving the coronal heating problem. This process has been shown to produce a variety of different observable phenomena on the Sun, including Ellerman bombs, ultraviolet bursts, coronal bright points, surges, coronal jets, and flares. Achieving a better understanding on the role that reconnection plays in the heating and dynamics of the solar atmosphere requires observational data and numerical reproduction of any aspect related to reconnection, including small-scale features such as plasmoids. A common challenge in mimicking reconnection on the Sun numerically is how to model the electrical resistivity, since this quantity is small compared to the diffusive effects caused by the discrete nature of numerical codes. To address this challenge, we ran a series of magnetohydrodynamic (MHD) simulations using the Bifrost code with its inbuilt hyper-resistivity model along with other resistivity models, for which we developed new modules especially for our research. Our experiments proved that the hyper-resistivity model of Bifrost is at least as suitable for mimicking reconnection phenomena on the Sun as other widely used resistivity models and has the advantage of being applicable in any part of the solar atmosphere without the need for re-calibration. Furthermore, we found that when simulating plasmoid-mediated reconnection with a moderate resolution, another takeaway with the hyper-resistivity model is the capability of reproducing plasmoid characteristics in closer resemblance to those attained with uniform resistivity at a significantly higher resolution. Observing small-scale features of reconnection on the Sun, such as plasmoids, can also be demanding, due to the limited resolution provided by currently-active telescopes and spectrographs. Furthermore, plasmoids in the corona are quite dim because of the low densities, hence often difficult to detect due to an underwhelming signal-to-noise ratio. Through forward-modelling of synthetic extreme-ultraviolet (EUV) observables from simulations of plasmoid-mediated reconnection, we found that combined spectra from the upcoming Multi-slit Solar Explorer (MUSE) and Solar-C missions along with high-resolution images from the MUSE context imager and the recently-launched Solar Orbiter will together contribute strongly in providing new insights into plasmoid formation on the Sun. Those insights can be used to constrain our numerical models in terms of further development of state-of-the-art resistivity models, hence lead us one step forward in unravelling the mysteries of our life-giving star.

• Frogner, Lars; Gudiksen, Boris Vilhelm & Carlsson, Mats (2024). Modelling accelerated particles in the solar atmosphere. Universitetet i Oslo. Full text in Research Archive Show summary

  This thesis explores the Sun’s dynamic atmosphere, focusing on the role of accelerated particles. These particles are propelled to extremely high speeds during solar flares, which are bright, explosive events in the Sun’s atmosphere. Accelerated particle beams can contain much of the energy released during a flare. Traditionally, particle beams have been simulated individually in simplified, one-dimensional atmospheres. By including accelerated particles in an advanced three-dimensional simulation of the Sun’s atmosphere, we could simulate many particle beams in a more realistic setting. Our simulations demonstrate how the Sun’s intricate magnetic field influences how the beams emerge and evolve. These results show how important it is to consider the presence of many beams in a complex magnetic environment when trying to understand the role of accelerated particles. In addition to providing this new perspective on energy transport in the solar atmosphere, our research lays the groundwork for more accurate models in the future.

• Robinson, Rebecca Anne & Carlsson, Mats (2023). Magnetic characteristics of quiet Sun nanoflares. Faculty of Mathematics and Natural Sciences. Full text in Research Archive
• Moe, Thore Espedal; Pereira, Tiago & Carlsson, Mats (2023). Line formation in the magnetized solar chromosphere. Universitetet i Oslo. Full text in Research Archive Show summary

  The theme of this thesis has been the use of forward modeling to study the formation of spectral lines in the solar atmosphere, with a particular focus on the chromospheric Ca II 854.2 nm line which is important for magnetic field diagnostics. Starting from state-of-the-art numerical simulations, modern radiative transfer codes have been used to calculate synthetic spectra, including the effects of Zeeman polarization. Combining the physical state of the model atmospheres with the resulting synthetic spectra has allowed us to investigate how these correlate, thereby guiding the interpretation of real observations. We have used simulations of differing resolutions to estimate the effect of spatially unresolved processes on inferences of atmospheric quantities from spectra. Furthermore, we have examined the use of clustering techniques to classify the shapes of the spectral profiles generated from simulations, and studied how these shapes correlate with the simulations’ atmospheric structure. We have also employed these clustering methods to characterize, contrast and compare the spectra from simulations with the spectra from real observations.

• 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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