SOLDYN : Unravelling the dynamics of the Solar Atmosphere (completed)

The aim of the SOLDYN project is to advance our understanding of the complex dynamics of the solar atmosphere through the combination of high quality observations from both ground-based and space-born instruments.

About the project

The Sun is our star and governs the solar system. There is a long scientific tradition of observing and studying the Sun but many fundamental questions remain unclear. For example, we still lack detailed understanding on why the outer solar atmosphere, the corona, is so hot (millions of degrees) while the solar surface is only few thousands of degrees. Furthermore, it is largely unkown how the corona is filled with mass and what drives the solar wind that supplies our planetary system with plasma and magnetic fields. It has been realised that a small layer in the solar atmosphere, the interface layer between the surface and the corona, may hold the key to answers to these questions. This interface layer, consisting of what is known as the chromosphere and transition region, is very shallow, only a few thousand kilometers but also very dynamic. Its complex dynamics are dominated by magnetic fields and radiative effects and is very challenging to understand. NASA has launched a small explorer spacecraft in the summer of 2013, the Interface Region Imaging Spectrograph (IRIS), which is specifically designed to observe this part of the solar atmosphere with unprecedented capabilities. However, IRIS is mostly sensitive to this small part of the atmosphere. We plan to provide key data that are essentially missing in the IRIS' arsenal: high-quality observations of the atmosphere below. These observations will be acquired through coordinated campaigns with the Swedish 1-m Solar Telescope (SST) on La Palma. In addition, we will use large-scale, state of the art, computer simulations of the solar atmosphere in combination with advanced numerical tools to interpret the complex observational data. The combination of coordinated IRIS and SST observations and advanced numerical simulations will allow to construct a comprehensive picture of the solar atmosphere.

The Swedish 1-metre Solar Telescope (SST) on La Palma in the Canary Islands, one of the prime facilities for high resolution observations of the Sun. The solar physics group at ITA and RoCS has a long tradition of observing with the SST and has an annual quota of more than 40 observing days per year.

Interface Region Imaging Spectrograph — IRIS in the clean room prior to launch in June 2013. The SOLDYN project has conducted a number of successful coordinated observing campaigns with IRIS and SST. During these campaigns, both telescopes are pointing at the same area on the Sun thus providing comprehensive measurements of the solar atmosphere over a extensive height range. (photo courtesy of Jim Dowdall / Lockheed Martin)

Financing

The SOLDYN project is funded by the University of Oslo and the Research Council of Norway and runs from January 2016 to February 2023.

Cooperation

The SOLDYN project is closely integrated in the Rosseland Centre for Solar Physics (RoCS).

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.
Joshi, Jayant & Voort, Luc Rouppe van der (2022). Properties of ubiquitous magnetic reconnection events in the lower solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 664. doi: 10.1051/0004-6361/202243051. Full text in Research Archive Show summary
Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high-quality Hβ observations indicate that Ellerman bombs are more common than previously thought, and it was estimated that at any time, about half a million Ellerman bombs are present in the quiet Sun. Aims. We performed an extensive statistical characterization of the quiet-Sun Ellerman bombs (QSEBs) in these new Hβ observations. Methods. We analyzed a 1 h dataset of the quiet Sun observed with the Swedish 1-m Solar Telescope that consists of spectral imaging in the Hβ and Hα lines as well as spectropolarimetric imaging in Fe I 6173 Å. We used the k-means clustering and the 3D connected component labeling techniques to automatically detect QSEBs. Results. We detected a total of 2809 QSEBs. The lifetimes vary between 9 s and 20.5 min, with a median of 1.14 min. The maximum area ranges between 0.0016 and 0.2603 Mm2, with a median of 0.018 Mm2. The maximum brightness in the Hβ wing varies between 1.06 and 2.76 with respect to the average wing intensity. A subset (14%) of the QSEBs displays enhancement of the Hβ line core. On average, the line core brightening appears 0.88 min after the onset of brightening in the wings, and the distance between these brightenings is 243 km. This gives rise to an apparent propagation speed ranging between −14.3 and +23.5 km s−1, with an average that is upward propagating at +4.4 km s−1. The average orientation is nearly parallel to the limbward direction. QSEBs are nearly uniformly distributed over the field of view, but we find empty areas with the size of mesogranulation. QSEBs are located more frequently near the magnetic network, where they are often larger, live longer, and are brighter. Conclusions. We conclude that QSEBs are ubiquitous in the quiet Sun and appear everywhere, except in areas of mesogranular size with the weakest magnetic fields (BLOS ≲ 50 G). Our observations support the interpretation of reconnection along vertically extended current sheets.
Murabito, M.; Guglielmino, S. L.; Ermolli, I.; Romano, Paolo; Jafarzadeh, Shahin & Voort, Luc Rouppe van der (2021). Penumbral decay observed in active region NOAA 12585. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 653. doi: 10.1051/0004-6361/202141034. Full text in Research Archive Show summary
Context. The physical conditions leading the sunspot penumbra decay are poorly understood so far. Aims. We investigate the photospheric magnetic and velocity properties of a sunspot penumbra during the decay phase to advance the current knowledge of the conditions leading to this process. Methods. A penumbral decay was observed with the CRISP instrument at the Swedish 1 m Solar Telescope on 2016 September 4 and 5 in the active region NOAA 12585. During these days, full-Stokes spectropolarimetric scans along the Fe  I 630 nm line pair were acquired over more than one hour. We inverted these observations with the VFISV code to obtain the evolution of the magnetic and velocity properties. We complement the study with data from instruments on board the Solar Dynamics Observatory and Hinode space missions. Results. The studied penumbra disappears progressively in time and space. The magnetic flux evolution seems to be linked to the presence of moving magnetic features (MMFs). Decreasing Stokes V signals are observed. Evershed flows and horizontal fields were detected even after the disappearance of the penumbral sector. Conclusions. The analyzed penumbral decay seems to result from the interaction between opposite polarity fields in type III MMFs and penumbra, while the presence of overlying canopies regulates the evolution in the different penumbral sectors.
Libbrecht, Tine; Bjørgen, Johan Pires; Leenaarts, Jorrit; Rodríguez, Jaime de la Cruz; Hansteen, Viggo Haraldson & Joshi, Jayant (2021). Line formation of He I D3 and He I 10 830 Å in a small-scale reconnection event. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 652. doi: 10.1051/0004-6361/202039788. Full text in Research Archive Show summary
Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection events that occur in the region of the upper photosphere to the chromosphere. It has recently been discovered that these events can have emission signatures in the He  I D 3 and He  I 10 830 Å lines, suggesting that their temperatures are higher than previously expected. Aims. We aim to explain the line formation of He  I D 3 and He  I 10 830 Å in small-scale reconnection events. Methods. We used a simulated EB in a Bifrost-generated radiative magnetohydrodynamics snapshot. The resulting He  I D 3 and He  I 10 830 Å line intensities were synthesized in 3D using the non-local thermal equilibrium (non-LTE) Multi3D code. The presence of coronal extreme UV (EUV) radiation was included self-consistently. We compared the synthetic helium spectra with observed raster scans of EBs in He  I 10 830 Å and He  I D 3 obtained at the Swedish Solar Telescope with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. Results. Emission in He  I D 3 and He  I 10 830 Å is formed in a thin shell around the EB at a height of ∼0.8 Mm, while the He  I D 3 absorption is formed above the EB at ∼4 Mm. The height at which the emission is formed corresponds to the lower boundary of the EB, where the temperature increases rapidly from 6 × 10 3 K to 10 6 K. The synthetic line profiles at a heliocentric angle of μ = 0.27 are qualitatively similar to the observed profiles at the same μ-angle in dynamics, broadening, and line shape: emission in the wing and absorption in the line core. The opacity in He  I D 3 and He  I 10 830 Å is generated through photoionization-recombination driven by EUV radiation that is locally generated in the EB at temperatures in the range of 2 × 10 4 − 2 × 10 6 K and electron densities between 10 11 and 10 13 cm −3 . The synthetic emission signals are a result of coupling to local conditions in a thin shell around the EB, with temperatures between 7 × 10 3 and 10 4 K and electron densities ranging from ∼10 12 to 10 13 cm −3 . This shows that both strong non-LTE and thermal processes play a role in the formation of He  I D 3 and He  I 10 830 Å in the synthetic EB/UV burst that we studied. Conclusions. In conclusion, the synthetic He  I D 3 and He  I 10 830 Å emission signatures are an indicator of temperatures of at least 2 × 10 4 K; in this case, as high as ∼10 6 K.
Santos, Luiz F.G. Dos; Bose, Souvik; Salvatelli, Valentina; Neuberg, Brad; Cheung, Mark C. M. & Janvier, Miho [Show all 10 contributors for this article] (2021). Multichannel autocalibration for the Atmospheric Imaging Assembly using machine learning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 648. doi: 10.1051/0004-6361/202040051. Full text in Research Archive Show summary
Context. Solar activity plays a quintessential role in affecting the interplanetary medium and space weather around Earth. Remote-sensing instruments on board heliophysics space missions provide a pool of information about solar activity by measuring the solar magnetic field and the emission of light from the multilayered, multithermal, and dynamic solar atmosphere. Extreme-UV (EUV) wavelength observations from space help in understanding the subtleties of the outer layers of the Sun, that is, the chromosphere and the corona. Unfortunately, instruments such as the Atmospheric Imaging Assembly (AIA) on board the NASA Solar Dynamics Observatory (SDO), suffer from time-dependent degradation that reduces their sensitivity. The current best calibration techniques rely on flights of sounding rockets to maintain absolute calibration. These flights are infrequent, complex, and limited to a single vantage point, however. Aims. We aim to develop a novel method based on machine learning (ML) that exploits spatial patterns on the solar surface across multiwavelength observations to autocalibrate the instrument degradation. Methods. We established two convolutional neural network (CNN) architectures that take either single-channel or multichannel input and trained the models using the SDOML dataset. The dataset was further augmented by randomly degrading images at each epoch, with the training dataset spanning nonoverlapping months with the test dataset. We also developed a non-ML baseline model to assess the gain of the CNN models. With the best trained models, we reconstructed the AIA multichannel degradation curves of 2010–2020 and compared them with the degradation curves based on sounding-rocket data. Results. Our results indicate that the CNN-based models significantly outperform the non-ML baseline model in calibrating instrument degradation. Moreover, multichannel CNN outperforms the single-channel CNN, which suggests that cross-channel relations between different EUV channels are important to recover the degradation profiles. The CNN-based models reproduce the degradation corrections derived from the sounding-rocket cross-calibration measurements within the experimental measurement uncertainty, indicating that it performs equally well as current techniques. Conclusions. Our approach establishes the framework for a novel technique based on CNNs to calibrate EUV instruments. We envision that this technique can be adapted to other imaging or spectral instruments operating at other wavelengths.
Bose, Souvik; Joshi, Jayant; Henriques, Vasco Manuel de Jorge & Voort, Luc Rouppe van der (2021). Spicules and downflows in the solar chromosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 647. doi: 10.1051/0004-6361/202040014. Full text in Research Archive Show summary
Context. High-speed downflows have been observed in the solar transition region (TR) and lower corona for many decades. Despite their abundance, it has been hard to find signatures of such downflows in the solar chromosphere. Aims. In this work, we target an enhanced network region which shows ample occurrences of rapid spicular downflows in the H α spectral line, which could potentially be linked to high-speed TR downflowing counterparts. Methods. We used the k -means algorithm to classify the spectral profiles of on-disk spicules in H α and Ca  II K data observed from the Swedish 1 m Solar Telescope and employed an automated detection method based on advanced morphological image processing operations to detect such downflowing features, in conjunction with rapid blue-shifted and red-shifted excursions (RBEs and RREs). Results. We report the existence of a new category of RREs (termed as downflowing RRE) for the first time that, contrary to earlier interpretation, are associated with chromospheric field aligned downflows moving toward the strong magnetic field regions. Statistical analysis performed on nearly 20 000 RBEs and 15 000 RREs (including the downflowing counterparts), which were detected in our 97 min long dataset, shows that the downflowing RREs are very similar to RBEs and RREs except for their oppositely directed plane-of-sky motion. Furthermore, we also find that RBEs, RREs, and downflowing RREs can be represented by a wide range of spectral profiles with varying Doppler offsets, and H α line core widths, both along and perpendicular to the spicule axis, that causes them to be associated with multiple substructures which evolve together. Conclusions. We speculate that these rapid plasma downflows could well be the chromospheric counterparts of the commonly observed TR downflows.
Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge & Bose, Souvik (2021). Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 648. doi: 10.1051/0004-6361/202040171. Full text in Research Archive Show summary
Context. Ellerman bombs are regions with enhanced Balmer line wing emission and mark magnetic reconnection in the deep solar atmosphere in active regions and the quiet Sun. They are often found in regions where opposite magnetic polarities are in close proximity. Recent high-resolution observations suggest that Ellerman bombs are more prevalent than previously thought. Aims. We aim to determine the occurrence of Ellerman bombs in the penumbra of sunspots. Methods. We analyzed high spatial resolution observations of sunspots in the Balmer H α and H β lines as well as auxiliary continuum channels obtained with the Swedish 1-m Solar Telescope and applied the k -means clustering technique to systematically detect and characterize Ellerman Bombs. Results. Features with all the defining characteristics of Ellerman bombs are found in large numbers over the entire penumbra. The true prevalence of these events is only fully appreciated in the H β line due to the highest spatial resolution and lower chromospheric opacity. We find that the penumbra hosts some of the highest Ellerman bomb densities, surpassed only by the moat in the immediate surroundings of the sunspot. Some penumbral Ellerman bombs show flame morphology and rapid dynamical evolution. Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km s −1 and sometimes more than 10 km s −1 . Many penumbral Ellerman bombs migrate from the inner to the outer penumbra over hundreds of km, and some continue moving beyond the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are found in the vicinity of regions with opposite magnetic polarity. Conclusions. We conclude that reconnection is a near continuous process in the low atmosphere of the penumbra of sunspots that manifest in the form of penumbral Ellerman bombs. These are so prevalent that they may be a major sink of sunspot magnetic energy.
Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge; Siverio, Daniel Elias Nóbrega & Martinez-Sykora, Juan [Show all 7 contributors for this article] (2021). Evidence of the multi-thermal nature of spicular downflows. Impact on solar atmospheric heating. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 654(51). doi: 10.1051/0004-6361/202141404. Full text in Research Archive Show summary
Context. Spectroscopic observations of the emission lines formed in the solar transition region commonly show persistent downflows on the order of 10−15 km s −1 . The cause of such downflows, however, is still not fully clear and has remained a matter of debate. Aims. We aim to understand the cause of such downflows by studying the coronal and transition region responses to the recently reported chromospheric downflowing rapid redshifted excursions (RREs) and their impact on the heating of the solar atmosphere. Methods. We have used two sets of coordinated data from the Swedish 1 m Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory for analyzing the response of the downflowing RREs in the transition region and corona. To provide theoretical support, we use an already existing 2.5D magnetohydrodynamic simulation of spicules performed with the Bifrost code. Results. We find ample occurrences of downflowing RREs and show several examples of their spatio-temporal evolution, sampling multiple wavelength channels ranging from the cooler chromospheric to the hotter coronal channels. These downflowing features are thought to be likely associated with the returning components of the previously heated spicular plasma. Furthermore, the transition region Doppler shifts associated with them are close to the average redshifts observed in this region, which further implies that these flows could (partly) be responsible for the persistent downflows observed in the transition region. We also propose two mechanisms – (i) a typical upflow followed by a downflow and (ii) downflows along a loop –from the perspective of a numerical simulation that could explain the ubiquitous occurrence of such downflows. A detailed comparison between the synthetic and observed spectral characteristics reveals a distinctive match and further suggests an impact on the heating of the solar atmosphere. Conclusions. We present evidence that suggests that at least some of the downflowing RREs are the chromospheric counterparts of the transition region and lower coronal downflows.
Ortiz, Ada; Hansteen, Viggo; Siverio, Daniel Elias Nóbrega & Voort, Luc Rouppe van der (2020). Ellerman bombs and UV bursts: reconnection at different atmospheric layers. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 633(A58). doi: 10.1051/0004-6361/201936574. Full text in Research Archive Show summary
The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst other dynamical phenomena, Ellerman bombs (EBs), which are observed in the wings of Hα and are due to magnetic reconnection in the photosphere below the chromospheric canopy. Signs of magnetic reconnection are also observed in other spectral lines, typical of the chromosphere or the transition region. An example are the ultraviolet (UV) bursts observed in the transition region lines of Si IV and the upper chromospheric lines of Mg II. In this work we analyze high-cadence, high-resolution coordinated observations between the Swedish 1m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα images from the SST provide us with the positions, timings, and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 133 (C II, transition region), 140 (Si IV, transition region), and 279.6 (Mg II k, core, upper chromosphere) nm as well as spectroscopy in the far- and near-ultraviolet from the fast spectrograph raster allow us to study the possible chromospheric and transition region counterparts of those EBs. Our main goal is to study the possible temporal and spatial relationship between several reconnection events at different layers in the atmosphere (namely EBs and UV bursts), the timing history between them, and the connection of these dynamical phenomena to the ejection of surges in the chromosphere. We also investigate the properties of an extended UV burst and their variations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. These results support and expand those obtained from recent numerical simulations of magnetic flux emergence.
Drews, Ainar & Voort, Luc Rouppe van der (2020). A multi-diagnostic spectral analysis of penumbral microjets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 638. doi: 10.1051/0004-6361/202037911. Full text in Research Archive Show summary
Context. Penumbral microjets (PMJs) are short-lived, jet-like objects found in the penumbra of sunspots. They were first discovered in chromospheric lines and have later also been shown to exhibit signals in transition region (TR) lines. Their origin and manner of evolution is not yet settled. Aims. We perform a comprehensive analysis of PMJs through the use of spectral diagnostics that span from photospheric to TR temperatures to constrain PMJ properties. Methods We employed high-spatial-resolution Swedish 1-m Solar Telescope observations in the Ca II 8542 Å and H α lines, IRIS slit-jaw images, and IRIS spectral observations in the Mg II h & k lines, the Mg II 2798.75 Å & 2798.82 Å triplet blend, the C II 1334 Å & 1335 Å lines, and the Si IV 1394 Å & 1403 Å lines. We derived a wide range of spectral diagnostics from these and investigated other secondary phenomena associated with PMJs. Results. We find that PMJs exhibit varying degrees of signal in all of our studied spectral lines. We find low or negligible Doppler velocities and velocity gradients throughout our diagnostics and all layers of the solar atmosphere associated with these. Dark features in the inner wings of H α and Ca II 8542 Å imply that PMJs form along pre-existing fibril structures. We find evidence for upper photospheric heating in a subset of PMJs through emission in the wings of the Mg II triplet lines. There is little evidence for ubiquitous twisting motion in PMJs. There is no marked difference in onset-times for PMJ brightenings in different spectral lines. Conclusions. PMJs most likely exhibit only very modest mass-motions, contrary to earlier suggestions. We posit that PMJs form at upper photospheric or chromospheric heights at pre-existing fibril structures.
Henriques, Vasco Manuel de Jorge; Nelson, Chris J.; Voort, Luc Rouppe van der & Mathioudakis, Mihalis (2020). Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 642. doi: 10.1051/0004-6361/202038538. Full text in Research Archive Show summary
Context. The chromosphere of the umbra of sunspots features an assortment of dynamic fine structures that are poorly understood and often studied separately. Small-scale umbral brightenings (SSUBs), umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral dark fibrils are found in any observation of the chromosphere with sufficient spatial resolution performed at the correct umbral flash stage and passband. Understanding these features means understanding the dynamics of the umbral chromosphere. Aims. We aim to fully understand the dynamics of umbral chromosphere through analysis of the relationships between distinct observed fine features and to produce complete models that explain both spectral profiles and the temporal evolution of the features. We seek to relate such understanding to umbral flashes. Methods. We studied the spatial and spectral co-evolution of SDFs, SSUBs, and umbral flashes in Ca  II 8542 Å spectral profiles. We produced models that generate the spectral profiles for all classes of features using non-local thermodynamic equilibrium radiative transfer with a recent version of the NICOLE inversion code. Results. We find that both bright SSUBs and dark SDF structures are described with a continuous feature in the parameter space that is distinct from the surroundings even in pixel-by-pixel inversions. We find a phase difference between such features and umbral flashes in both inverted line-of-sight velocities and timing of the brightenings. For umbral flashes themselves we resolve, for the first time in inversion-based semi-empirical modelling, the pre-flash downflows, post-flash upflows, and the counter-flows present during the umbral flash phase. We further present a simple time-dependent cartoon model that explains the dynamics and spectral profiles of both fine structure, dark and bright, and umbral flashes in umbral chromospheres. Conclusions. The similarity of the profiles between the brightenings and umbral flashes, the pattern of velocities obtained from the inversions, and the phase relationships between the structures all lead us to put forward that all dynamic umbral chromospheric structures observed to this date are a locally delayed or locally early portion of the oscillatory flow pattern that generates flashes, secondary to the steepening large-scale acoustic waves at its source. Essentially, SSUBs are part of the same shock or merely compression front responsible for the spatially larger umbral flash phenomenon, but out of phase with the broader oscillation.
Frogner, Lars; Gudiksen, Boris Vilhelm & Bakke, Helle (2020). Accelerated particle beams in a 3D simulation of the quiet Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 643. doi: 10.1051/0004-6361/202038529. Full text in Research Archive Show summary
Context. Observational and theoretical evidence suggest that beams of accelerated particles are produced in flaring events of all sizes in the solar atmosphere, from X-class flares to nanoflares. Current models of these types of particles in flaring loops assume an isolated 1D atmosphere. Aims. A more realistic environment for modelling accelerated particles can be provided by 3D radiative magnetohydrodynamics codes. Here, we present a simple model for particle acceleration and propagation in the context of a 3D simulation of the quiet solar atmosphere, spanning from the convection zone to the corona. We then examine the additional transport of energy introduced by the particle beams. Methods. The locations of particle acceleration associated with magnetic reconnection were identified by detecting changes in magnetic topology. At each location, the parameters of the accelerated particle distribution were estimated from local conditions. The particle distributions were then propagated along the magnetic field, and the energy deposition due to Coulomb collisions with the ambient plasma was computed. Results. We find that particle beams originate in extended acceleration regions that are distributed across the corona. Upon reaching the transition region, they converge and produce strands of intense heating that penetrate the chromosphere. Within these strands, beam heating consistently dominates conductive heating below the bottom of the transition region. This indicates that particle beams qualitatively alter the energy transport even outside of active regions.
Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Show all 33 contributors for this article] (2020). High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038732. Full text in Research Archive Show summary
NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe  I lines and spectrally resolved imaging in the chromospheric Ca  II 8542 Å, H α , and Ca  II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.
Joshi, Jayant; Voort, Luc Rouppe van der & Rodriguez, Jaime de la Cruz (2020). Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038769. Full text in Research Archive Show summary
Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the lower solar atmosphere. We analyze high spatial resolution observations (0 . 1) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the H β line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the field of view for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier H α observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightenings found in the H β line wings also persist in the line core with a temporal delay and spatial offset toward the nearest solar limb. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the lower solar atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.
Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2019). Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 631. doi: 10.1051/0004-6361/201936617. Full text in Research Archive Show summary
We characterize, for the first time, type-II spicules in Ca II K 3934 Å using the CHROMIS instrument at the Swedish 1 m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K3 minimum best representing the velocity of the spicules. The K2 features are either suppressed by the Doppler-shifted K3 or enhanced via increased contribution from the lower layers, leading to strongly enhanced but unshifted K2 peaks, with widening towards the line core as consistent with upper-layer opacity removal via Doppler-shift. We identify spicule spectra in concurrent IRIS Mg II k 2796Å observations with very similar properties. Using our interpretation of spicule chromospheric line formation, we produce synthetic profiles that match observations.
Vissers, Gregal Joan Maria; Rodriguez, Jaime de la Cruz; Libbrecht, Tine; Voort, Luc Rouppe van der; Scharmer, Göran B. & Carlsson, Mats (2019). Dissecting bombs and bursts: Non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 627. doi: 10.1051/0004-6361/201833560. Full text in Research Archive Show summary
Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. As they are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide useful insight into the overall evolution of active regions. Here we present results from inversions of SST/CRISP and CHROMIS, as well as IRIS data of such transient events. Combining information from the Mg II h & k, Si IV, and Ca II 8542 Å and Ca II H & K lines, we aim to characterise their temperature and velocity stratification, as well as their magnetic field configuration. We find average temperature enhancements of a few thousand kelvin, close to the classical temperature minimum and similar to previous studies, but localised peak temperatures of up to 10 000–15 000 K from Ca II inversions. Including Mg II appears to generally dampen these temperature enhancements to below 8000 K, while Si IV requires temperatures in excess of 10 000 K at low heights, but may also be reproduced with secondary temperature enhancements of 35 000–60 000 K higher up. However, reproducing Si IV comes at the expense of overestimating the Mg II emission. The line-of-sight velocity maps show clear bi-directional jet signatures for some events and strong correlation with substructure in the intensity images in general. Absolute line-of-sight velocities range between 5 and 20 km s−1 on average, with slightly larger velocities towards, rather than away from, the observer. The inverted magnetic field parameters show an enhancement of the horizontal field co-located with the brightenings at heights similar to that of the temperature increase. We are thus able to largely reproduce the observational properties of Ellerman bombs with the UV burst signature (e.g. intensities, profile asymmetries, morphology, and bi-directional jet signatures), with temperature stratifications peaking close to the classical temperature minimum. Correctly modelling the Si IV emission in agreement with all other diagnostics is however an outstanding issue and remains paramount in explaining its apparent coincidence with Hα emission. Fine-tuning the approach (accounting for resolution differences, fitting localised temperature enhancements, and/or performing spatially coupled inversions) is likely necessary in order to obtain better agreement between all considered diagnostics. © ESO 2019
Keys, Peter H.; Reid, Aaron; Mathioudakis, Mihalis; Shelyag, Sergiy; Henriques, Vasco Manuel de Jorge & Hewitt, Rebecca L. [Show all 10 contributors for this article] (2019). The magnetic properties of photospheric magnetic bright points with high-resolution spectropolarimetry. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 488(1), p. L53–L58. doi: 10.1093/mnrasl/slz097. Full text in Research Archive Show summary
Magnetic bright points (MBPs) are small-scale magnetic elements ubiquitous across the solar disc, with the prevailing theory suggesting that they form due to the process of convective collapse. Employing a unique full Stokes spectropolarimetric data set of a quiet Sun region close to disc centre obtained with the Swedish Solar Telescope, we look at general trends in the properties of magnetic bright points. In total we track 300 MBPs in the data set and we employ NICOLE inversions to ascertain various parameters for the bright points such as line-of-sight magnetic field strength and line-of-sight velocity, for comparison. We observe a bimodal distribution in terms of maximum magnetic field strength in the bright points with peaks at ∼480 G and ∼1700 G, although we cannot attribute the kilogauss fields in this distribution solely to the process of convective collapse. Analysis of muram simulations does not return the same bimodal distribution. However, the simulations provide strong evidence that the emergence of new flux and diffusion of this new flux play a significant role in generating the weak bright point distribution seen in our observations.
Pozuelo, Sara Esteban; Rodriguez, Jaime de la Cruz; Drews, Ainar; Voort, Luc Rouppe van der; Scharmer, Göran B. & Carlsson, Mats (2019). Observationally Based Models of Penumbral Microjets. The Astrophysical Journal (ApJ). ISSN 0004-637X. 870(2). doi: 10.3847/1538-4357/aaf28a. Full text in Research Archive
Froment, Clara; Antolin, Patrick; Henriques, Vasco Manuel de Jorge; Kohutova, Petra & Voort, Luc Rouppe van der (2019). Multi-scale observations of thermal non-equilibrium cycles in coronal loops. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 633. doi: 10.1051/0004-6361/201936717. Full text in Research Archive Show summary
Context. Thermal non-equilibrium (TNE) is a phenomenon that can occur in solar coronal loops when the heating is quasi-constant and highly-stratified. Under such heating conditions, coronal loops undergo cycles of evaporation and condensation. The recent observations of ubiquitous long-period intensity pulsations in coronal loops and their relationship with coronal rain have demonstrated that understanding the characteristics of TNE cycles is an essential step in constraining the circulation of mass and energy in the corona. Aims. We report unique observations with the Solar Dynamics Observatory (SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured thermal properties across the extreme spatiotemporal scales covered by TNE processes. Methods. Within the same coronal loop bundle, we captured 6 h period coronal intensity pulsations in SDO/AIA and coronal rain observed off-limb in the chromospheric Hα and Ca II K spectral lines with SST/CRISP and SST/CHROMIS. We combined a multi-thermal analysis of the cycles with AIA and an extensive spectral characterisation of the rain clumps with the SST. Results. We find clear evidence of evaporation-condensation cycles in the corona which are linked with periodic coronal rain showers. The high-resolution spectroscopic instruments at the SST reveal the fine-structured rain strands and allow us to probe the cooling phase of one of the cycles down to chromospheric temperatures. Conclusions. These observations reinforce the link between long-period intensity pulsations and coronal rain. They also demonstrate the capability of TNE to shape the dynamics of active regions on the large scales as well as on the smallest scales currently resolvable.
Rutten, Robert; Voort, Luc Rouppe van der & Pontieu, Bart Walter De (2019). Solar Hα features with hot onsets: IV. Network fibrils. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 632. doi: 10.1051/0004-6361/201936113. Full text in Research Archive Show summary
Even in quiet areas underneath coronal holes the solar chromosphere contains ubiquitous heating events. They tend to be small scale and short lived, hence difficult to identify. Here we do not address their much-debated contribution to outer-atmosphere heating, but their aftermaths. We performed a statistical analysis of high-resolution observations in the Balmer Hα line to suggest that many slender dark Hα fibrils spreading out from network represent cooling gas that outlines tracks of preceding rapid type II spicule events or smaller similar but as yet unresolved heating agents in which the main gas constituent, hydrogen, ionizes at least partially. Subsequent recombination then causes dark Hα fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the Hα chromosphere around network results from intermittent, frequent small-scale prior heating.
Voort, Luc Rouppe van der & Drews, Ainar (2019). Penumbral microjets at high spatial and temporal resolution. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 626. doi: 10.1051/0004-6361/201935343. Full text in Research Archive Show summary
Context. Sunspot observations in chromospheric spectral lines have revealed short-lived linear bright transients that are commonly referred to as penumbral microjets (PMJs). Details on the origin and physical nature of PMJs are to a large extend still unknown. Aims. We aim to characterize the dynamical nature of PMJs to provide guidance for future modeling efforts. Methods. We analyzed high spatial (0.̋1) and temporal resolution (1 s) Ca II H filtergram (0.1 nm bandwidth) observations of a sunspot that were obtained on two consecutive days with the Swedish 1 m Solar Telescope. Results. We find that PMJs appear to be the rapid brightening of an already existing (faint) fibril. The rapid brightening is the fast increase (typically less than 10 s) in intensity over significant length (several hundreds of kilometers) of the existing fibril. For most PMJs, no clear root or source from where the brightening appears to originate can be identified. After the fast onset, about half of the PMJs have tops that move with an apparent velocity of between 5 and 14 km s−1, most of them upward. No significant motion of the top is observed in the other PMJs. About one-third of the PMJs split into two parallel and coevolving linear features during the later phases of their lifetimes. Conclusions. We conclude that mass flows can play only a limited role in the onset phase of PMJs. It is more likely that we see the effect of a fast heating front. ESO 2019
Bose, Souvik; Henriques, Vasco Manuel de Jorge; Voort, Luc Rouppe van der & Pereira, Tiago Mendes Domingos (2019). Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 627. doi: 10.1051/0004-6361/201935289. Full text in Research Archive Show summary
Context: The solar chromosphere and the lower transition region are believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution (CRD) in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims: We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods: We used spectropolarimetric observations of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar Telescope and used NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These were used to synthesize Mg II h&k spectra using the RH1.5D code, which we compared with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results: Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot down-flows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions: We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot.
Vissers, Gregal Joan Maria; Voort, Luc Rouppe van der & Rutten, Robert (2019). Automating Ellerman bomb detection in ultraviolet continua. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 626. doi: 10.1051/0004-6361/201834811. Full text in Research Archive Show summary
Ellerman bombs are transient brightenings in the wings of Hα 6563 Å that pinpoint photospheric sites of magnetic reconnection in solar active regions. Their partial visibility in the 1600 Å and 1700 Å continua registered routinely by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) offers a unique opportunity to inventory such magnetic-field disruptions throughout the AIA database if a reliable recipe for their detection can be formulated. This is done here. We have improved and applied an Hα Ellerman bomb detection code to ten data sets spanning viewing angles from solar disc centre to the limb. They combine high-quality Hα imaging spectroscopy from the Swedish 1 m Solar Telescope with simultaneous AIA imaging around 1600 Å and 1700 Å. A trial grid of brightness, lifetime and area constraints is imposed on the AIA images to define optimal recovery of the 1735 Ellerman bombs detected in Hα. The best results when optimising simultaneously for recovery fraction and reliability are obtained from 1700 Å images by requiring 5σ brightening above the average 1700 Å nearby quiet-Sun intensity, lifetime above one minute, area of 1–18 AIA pixels. With this recipe 27% of the AIA detections are Hα-detected Ellerman bombs while it recovers 19% of these (of which many are smaller than the AIA resolution). Better yet, among the top 10% AIA 1700 Å detections selected with combined brightness, lifetime and area thresholds as many as 80% are Hα Ellerman bombs. Automated selection of the best 1700 Å candidates therefore opens the entire AIA database for detecting most of the more significant photospheric reconnection events. This proxy is applicable as a flux-dynamics tell-tale in studying any Earth-side solar active region since early 2010 up to the present.
Chitta, L. P.; Sukarmadji, A. R. C.; Voort, Luc Rouppe van der & Peter, H. (2019). Energetics of magnetic transients in a solar active region plage. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 623. doi: 10.1051/0004-6361/201834548. Full text in Research Archive Show summary
Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface. Aims. We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations. Methods. We used photospheric Fe I 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code. Results. The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m−2 through the photosphere. Conclusions. We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops.
Reid, A.; Henriques, Vasco Manuel de Jorge; Mathioudakis, M. & Samanta, T. (2018). Penumbral Waves Driving Solar Fan-shaped Chromospheric Jets. Astrophysical Journal Letters. ISSN 2041-8205. 855(2), p. 1–6. doi: 10.3847/2041-8213/aab155. Full text in Research Archive Show summary
We use Hα imaging spectroscopy taken via the Swedish 1 m Solar Telescope to investigate the occurrence of fan-shaped jets at the solar limb. We show evidence for near-simultaneous photospheric reconnection at a sunspot edge leading to the jets appearance, with upward velocities of 30 km s−1, and extensions up to 8 Mm. The brightening at the base of the jets appears recurrent, with a periodicity matching that of the nearby sunspot penumbra, implying running penumbral waves could be the driver of the jets. The jets' constant extension velocity implies that a driver counteracting solar gravity exists, possibly as a result of the recurrent reconnection erupting material into the chromosphere. These jets also show signatures in higher temperature lines captured from the Solar Dynamics Observatory, indicating a very hot jet front, leaving behind optically thick cool plasma in its wake. © 2018 IOP Publishing
Kuridze, D.; Henriques, Vasco Manuel de Jorge; Mathioudakis, M.; Voort, Luc Rouppe van der; Rodriguez, Jaime de la Cruz & Carlsson, Mats (2018). Spectropolarimetric Inversions of the Ca II 8542Å Line in an M-class Solar Flare. The Astrophysical Journal (ApJ). ISSN 0004-637X. 860(1). doi: 10.3847/1538-4357/aac26d. Full text in Research Archive Show summary
We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using high-resolution full Stokes imaging spectropolarimetry of the Ca ii 8542 Å line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semiempirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and nonflaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post-flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and nonflaring atmospheres shows that during the flare, the Ca ii 8542 Å line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation, which led to results similar to those obtained with the NICOLE inversions.
Pereira, Tiago Mendes Domingos; Voort, Luc Rouppe van der; Hansteen, Viggo & Pontieu, Bart Walter De (2018). Chromospheric counterparts of solar transition region unresolved fine structure loops. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 611. doi: 10.1051/0004-6361/201832762. Full text in Research Archive Show summary
Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models. © 2018 ESO
Bakke, Helle; Frogner, Lars & Gudiksen, Boris Vilhelm (2018). Non-thermal electrons from solar nanoflares In a 3D radiative MHD simulation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 620. doi: 10.1051/0004-6361/201834129. Full text in Research Archive Show summary
Context: We introduce a model for including accelerated particles in pure magnetohydrodynamics (MHD) simulations of the solar atmosphere. Aims: We show that the method is viable and produces results that enhance the realism of MHD simulations of the solar atmosphere. Methods: The acceleration of high-energy electrons in solar flares is an accepted fact, but is not included in the most advanced 3D simulations of the solar atmosphere. The effect of the acceleration is not known, and here we introduce a simple method to account for the ability of the accelerated electrons to move energy from the reconnection sites and into the dense transition zone and chromosphere. Results: The method was only run for a short time and with low reconnection energies, but this showed that the reconnection process itself changes, and that there is a clear effect on the observables at the impact sites of the accelerated electrons. Further work will investigate the effect on the reconnection sites and the impact sites in detail.
Bose, Souvik & Nagaraju, K (2018). On the Variability of the Solar Mean Magnetic Field: Contributions from Various Magnetic Features on the Surface of the Sun. The Astrophysical Journal (ApJ). ISSN 0004-637X. 862(1). doi: 10.3847/1538-4357/aaccf1. Full text in Research Archive Show summary
The solar mean magnetic field (SMMF) is referred to as the disk-averaged line-of-sight (LOS) magnetic field that also reflects the polarity imbalance of the magnetic field on the Sun. The origin of the SMMF has been debated over the past few decades, with one school of thought suggesting that the contribution to the SMMF is mostly due to the large-scale magnetic field structure, also called the background magnetic field, whereas other and more recent studies have indicated that active regions have a major contribution to the observed SMMF. In this paper, we re-investigate the issue of the origin of the SMMF by decomposing the solar disk into plages, networks, sunspots, and background regions, thereby calculating the variation in the observed SMMF due to each of these features. We have used full-disk images from Solar Dynamics Observatory (SDO)/AIA recorded at 1600 Å for earmarking plages, networks, and background regions and 4500 Å images for separating the sunspots. The LOS fields corresponding to each of these regions are estimated from the co-temporal SDO/Helioseismic and Magnetic Imager full-disk magnetograms. The temporal variation of the SMMF shows a near one-to-one correspondence with that of the background field regions, suggesting that they constitute the major component of the observed SMMF. A linear regression analysis based on the coefficient of determination shows that the background field dominates and accounts for 89% of the variation in the SMMF, whereas the magnetic field from the other features accounts for the rest 11%.
Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert; Nelson, Chris J. & Huang, Zhenghua [Show all 19 contributors for this article] (2018). Solar Ultraviolet Bursts. Space Science Reviews. ISSN 0038-6308. 214:120(8). doi: 10.1007/s11214-018-0551-0. Full text in Research Archive Show summary
The term “ultraviolet (UV) burst” is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016–2017.
Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Scharmer, G. B.; Rodriguez, Jaime de la Cruz; Martinez-Sykora, Juan & Nobrega-Siverio, Daniel [Show all 12 contributors for this article] (2017). Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere. Astrophysical Journal Letters. ISSN 2041-8205. 851(1). doi: 10.3847/2041-8213/aa99dd. Full text in Research Archive Show summary
Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry–Pérot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si iv 1403 Å UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca ii K 3934 Å line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0farcs2 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere. © 2017. The American Astronomical Society. All rights reserved.
Mooroogen, K.; Morton, R. J. & Henriques, Vasco Manuel de Jorge (2017). Dynamics of internetwork chromospheric fibrils: Basic properties and magnetohydrodynamic kink waves. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 607. doi: 10.1051/0004-6361/201730926. Full text in Research Archive Show summary
Aims. Current observational instruments are now providing data with the necessary temporal and spatial cadences required to examine highly dynamic, fine-scale magnetic structures in the solar atmosphere. Using the spectroscopic imaging capabilities of the Swedish Solar Telescope, we aim to provide the first investigation on the nature and dynamics of elongated absorption features (fibrils) observed in Hα in the internetwork. Methods. We observe and identify a number of internetwork fibrils, which form away from the kilogauss, network magnetic flux, and we provide a synoptic view on their behaviour. The internetwork fibrils are found to support wave-like behaviour, which we interpret as magnetohydrodynamic (MHD) kink waves. The properties of these waves, that is, amplitude, period, and propagation speed, are measured from time-distance diagrams and we attempt to exploit them via magneto-seismology in order to probe the variation of plasma properties along the wave-guides. Results. We found that the Internetwork (IN) fibrils appear, disappear, and re-appear on timescales of tens of minutes, suggesting that they are subject to repeated heating. No clear photospheric footpoints for the fibrils are found in photospheric magnetograms or Hα wing images. However, we suggest that they are magnetised features as the majority of them show evidence of supporting propagating MHD kink waves, with a modal period of 120 s. Additionally, one IN fibril is seen to support a flow directed along its elongated axis, suggesting a guiding field. The wave motions are found to propagate at speeds significantly greater than estimates for typical chromospheric sound speeds. Through their interpretation as kink waves, the measured speeds provide an estimate for local average Alfvén speeds. Furthermore, the amplitudes of the waves are also found to vary as a function of distance along the fibrils, which can be interpreted as evidence of stratification of the plasma in the neighbourhood of the IN fibril. © ESO, 2017
Rutten, Robert & Rouppe, Van Der Voort Luc (2017). Solar Hα features with hot onsets: II. A contrail fibril. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 597. doi: 10.1051/0004-6361/201527560. Full text in Research Archive Show summary
The solar chromosphere observed in Hα consists mostly of narrow fibrils. The longest typically originate in network or plage and arch far over adjacent internetwork. We use data from multiple telescopes to analyze one well-observed example in a quiet area. It resulted from the earlier passage of an accelerating disturbance in which the gas was heated to high temperature as in the spicule-II phenomenon. After this passage a dark Hα fibril appeared as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset and subsequent visibilities in various diagnostics and conclude that such Hα fibrils can indeed be contrail phenomena, not indicative of the thermodynamic and magnetic environment when they are observed but of more dynamic happenings before. They do not connect across internetwork cells but represent launch tracks of heating events and chart magnetic field during launch, not at present. © ESO, 2017
Nobrega-Siverio, Daniel; Martinez-Sykora, Juan; Moreno-Insertis, F. & Voort, Luc Rouppe van der (2017). Surges and Si IV Bursts in the Solar Atmosphere: Understanding IRIS and SST Observations through RMHD Experiments. The Astrophysical Journal (ApJ). ISSN 0004-637X. 850(2). doi: 10.3847/1538-4357/aa956c. Full text in Research Archive Show summary
© 2017. The American Astronomical Society. All rights reserved.
Nelson, C. J.; Henriques, Vasco Manuel de Jorge; Mathioudakis, M. & Keenan, F. P. (2017). The formation of small-scale umbral brightenings in sunspot atmospheres. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 605. doi: 10.1051/0004-6361/201730467. Full text in Research Archive Show summary
Context. Sunspot atmospheres have been shown to be highly inhomogeneous hosting both quasi-stable and transient features, such as small-scale umbral brightenings (previously named “umbral micro-jets”) and dark fibril-like events. Aims. We seek to understand the morphological properties and formation mechanisms of small-scale umbral brightenings (analogous to umbral micro-jets). In addition, we aim to understand whether links between these events and short dynamic fibrils, umbral flashes, and umbral dots can be established. Methods. A Swedish 1 m Solar Telescope (SST) filtergram time-series sampling the Ca ii H line and a CRisp Imaging Spectro-Polarimeter (SST/CRISP) full-Stokes 15-point Ca ii 8542 Å line scan dataset were used. The spatial resolutions of these datasets are close to 0.1′′ and 0.18′′ with cadences of 1.4 s and 29 s, respectively. These data allowed us to construct light-curves, plot line profiles, and to perform a weak-field approximation in order to infer the magnetic field strength. Results. The average lifetime and lengths of the 54 small-scale brightenings identified in the sunspot umbra are found to be 44.2 s (σ = 20 s) and 0.56′′ (σ = 0.14′′), respectively. The spatial positioning and morphological evolution of these events in Ca ii H filtergrams was investigated finding no evidence of parabolic or ballistic profiles nor a preference for co-spatial formation with umbral flashes. Line scans in Ca ii 8542 Å and the presence of Stokes V profile reversals provided evidence that these events could form in a similar manner to umbral flashes in the chromosphere (i.e. through the formation of shocks either due to the steepening of localised wavefronts or due to the impact of returning material from short dynamic fibrils, a scenario we find evidence for). The application of the weak-field approximation indicated that changes in the line-of-sight magnetic field were not responsible for the modifications to the line profile and suggested that thermodynamic effects are, in fact, the actual cause of the increased emission. Finally, a sub-set of small-scale brightenings were observed to form at the foot-points of short dynamic fibrils. Conclusions. The small-scale umbral brightenings studied here do not appear to be jet-like in nature. Instead they appear to be evidence of shock formation in the lower solar atmosphere. We found no correlation between the spatial locations where these events were observed and the occurrence of umbral dots and umbral flashes. These events have lifetimes and spectral signatures comparable to umbral flashes and are located at the footpoints of short dynamic fibrils, during or at the end of the red-shifted stage. It is possible that these features form due to the shocking of fibrilar material in the lower atmosphere upon its return under gravity. © ESO, 2017
Kuridze, D.; Henriques, Vasco Manuel de Jorge; Mathioudakis, M.; Koza, J.; Zaqarashvili, T. V. & Rybak, J [Show all 8 contributors for this article] (2017). Spectroscopic Inversions of the Ca II 8542 Å Line in a C-class Solar Flare. The Astrophysical Journal (ApJ). ISSN 0004-637X. 846(1). doi: 10.3847/1538-4357/aa83b9. Full text in Research Archive Show summary
© 2017. The American Astronomical Society. All rights reserved.
Henriques, Vasco Manuel de Jorge; Mathioudakis, M.; Socas-Navarro, H. & Rodriguez, Jaime de la Cruz (2017). A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils. The Astrophysical Journal (ApJ). ISSN 0004-637X. 845(2). doi: 10.3847/1538-4357/aa7ca4. Full text in Research Archive Show summary
We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca ii λ8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier "sunspot plumes," which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers. © 2017. The American Astronomical Society. All rights reserved.
Drews, Ainar & Voort, Luc Rouppe van der (2017). Microjets in the penumbra of a sunspot. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 602. doi: 10.1051/0004-6361/201630312. Full text in Research Archive Show summary
Context. Penumbral microjets (PMJs) are short-lived jets found in the penumbra of sunspots, first observed in wide-band Ca ii H line observations as localized brightenings, and are thought to be caused by magnetic reconnection. Earlier work on PMJs has focused on smaller samples of by-eye selected events and case studies. Aims. It is our goal to present an automated study of a large sample of PMJs to place the basic statistics of PMJs on a sure footing and to study the PMJ Ca ii 8542 Å spectral profile in detail. Methods. High spatial resolution and spectrally well-sampled observations in the Ca ii 8542 Å line obtained from the Swedish 1-m Solar Telescope (SST) were reduced by a principle component analysis and subsequently used in the automated detection of PMJs using the simple machine learning algorithm k-nearest neighbour. PMJ detections were verified with co-temporal Ca ii H line observations. Results. We find a total of 453 tracked PMJ events, 4253 PMJs detections tallied over all timeframes, and a detection rate of 21 events per timestep. From these, an average length, width and lifetime of 640 km, 210 km and 90 s are obtained. The average PMJ Ca ii 8542 Å line profile is characterized by enhanced inner wings, often in the form of one or two distinct peaks, and a brighter line core as compared to the quiet-Sun average. Average blue and red peak positions are determined at − 10.4 km s-1 and + 10.2 km s-1 offsets from the Ca ii 8542 Å line core. We find several clusters of PMJ hot-spots within the sunspot penumbra, in which PMJ events occur in the same general area repeatedly over time. Conclusions. Our results indicate smaller average PMJs sizes and longer lifetimes compared to previously published values, but with statistics still in the same orders of magnitude. The investigation and analysis of the PMJ line profiles strengthens the proposed heating of PMJs to transition region temperatures. The presented statistics on PMJs form a solid basis for future investigations and numerical modelling of PMJs. © ESO, 2017
Reid, A.; Henriques, Vasco Manuel de Jorge; Mathioudakis, M.; Doyle, J. G. & Ray, T. (2017). Chromospheric Inversions of a Micro-flaring Region. The Astrophysical Journal (ApJ). ISSN 0004-637X. 845(2). doi: 10.3847/1538-4357/aa7de0. Full text in Research Archive Show summary
We use spectropolarimetric observations of the Ca ii 8542 Å line, taken from the Swedish 1 m Solar Telescope, in an attempt to recover dynamic activity in a micro-flaring region near a sunspot via inversions. These inversions show localized mean temperature enhancements of ~1000 K in the chromosphere and upper photosphere, along with co-spatial bi-directional Doppler shifting of 5–10 km s−1. This heating also extends along a nearby chromospheric fibril, which is co-spatial to 10–15 km s−1 downflows. Strong magnetic flux cancellation is also apparent in one of the footpoints, and is concentrated in the chromosphere. This event more closely resembles that of an Ellerman Bomb, though placed slightly higher in the atmosphere than what is typically observed. © 2017. The American Astronomical Society. All rights reserved.
Martinez-Sykora, Juan; Pontieu, Bart Walter De; Hansteen, Viggo; Voort, Luc Rouppe van der; Carlsson, Mats & Pereira, Tiago Mendes Domingos (2017). On the generation of solar spicules and Alfvenic waves. Science. ISSN 0036-8075. 356(6344), p. 1269–1272. doi: 10.1126/science.aah5412. Full text in Research Archive Show summary
In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves. This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science on 23 Jun 2017, DOI: 10.1126/science.aah5412
Druett, Malcolm; Scullion, Eamon; Zharkova, Valentina; Matthews, Sarah; Zharkov, Sergei & Voort, Luc Rouppe van der (2017). Beam electrons as a source of Hα flare ribbons. Nature Communications. 8. doi: 10.1038/ncomms15905. Full text in Research Archive Show summary
The observations of solar flare onsets show rapid increase of hard and soft X-rays, ultra-violet emission with large Doppler blue shifts associated with plasma upflows, and Hα hydrogen emission with red shifts up to 1–4 Å. Modern radiative hydrodynamic models account well for blue-shifted emission, but struggle to reproduce closely the red-shifted Hα lines. Here we present a joint hydrodynamic and radiative model showing that during the first seconds of beam injection the effects caused by beam electrons can reproduce Hα line profiles with large red-shifts closely matching those observed in a C1.5 flare by the Swedish Solar Telescope. The model also accounts closely for timing and magnitude of upward motion to the corona observed 29 s after the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory.
Hansteen, Viggo; Archontis, Vasilis; Pereira, Tiago Mendes Domingos; Carlsson, Mats; Voort, Luc Rouppe van der & Leenaarts, Jorrit (2017). Bombs and Flares at the Surface and Lower Atmosphere of the Sun. The Astrophysical Journal (ApJ). ISSN 0004-637X. 839(1). doi: 10.3847/1538-4357/aa6844. Full text in Research Archive Show summary
A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma "bombs" (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere. © 2017. The American Astronomical Society. All rights reserved.
Rouppe, Van Der Voort Luc; Rutten, Robert & Vissers, Gregal Joan Maria (2016). Reconnection brightenings in the quiet solar photosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 592. doi: 10.1051/0004-6361/201628889. Full text in Research Archive Show summary
We describe a new quiet-Sun phenomenon which we call quiet-Sun Ellerman-like brightenings (QSEB). QSEBs are similar to Ellerman bombs (EB) in some respects but differ significantly in others. EBs are transient brightenings of the wings of the Balmer Hα line that mark strong-field photospheric reconnection in complex active regions. QSEBs are similar but smaller and less intense Balmer-wing brightenings that occur in quiet areas away from active regions. In the Hα wing, we measure typical lengths of less than 0.5 arcsec, widths of 0.23 arcsec, and lifetimes of less than a minute. We discovered them using high-quality Hα imaging spectrometry from the Swedish 1-m Solar Telescope (SST) and show that, in lesser-quality data, they cannot be distinguished from more ubiquitous facular brightenings, nor in the UV diagnostics currently available from space platforms. We add evidence from concurrent SST spectropolarimetry that QSEBs also mark photospheric reconnection events, but in quiet regions on the solar surface. Reproduced with permission from Astronomy & Astrophysics, © ESO
Ortiz, Ada; Hansteen, Viggo; Rubio, L. R. Bellot; Rodriguez, Jaime de la Cruz; Pontieu, Bart Walter De & Carlsson, Mats [Show all 7 contributors for this article] (2016). Emergence of Granular-Sized Magnetic Bubbles through the Solar Atmosphere. III. The Path to the Transition Region. The Astrophysical Journal (ApJ). ISSN 0004-637X. 825(2). doi: 10.3847/0004-637X/825/2/93. Full text in Research Archive Show summary
We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Hα 656.28 nm and Ca ii 854.2 nm lines, plus the photospheric Fe i 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0farcs14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C ii, transition region), 140 (Si iv, transition region), 279.6 (Mg ii k, core, upper chromosphere), and 283.2 nm (Mg ii k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far- and near-ultraviolet, of which we have used the Si iv 140.3 and the Mg ii k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona. Reproduced with permission from the Astrophysical Journal. © IOP Publishing
Wedemeyer, Sven; Bastian, T.; Brajsa, R.; Hudson, H.; Fleishman, G. & Loukitcheva, M. [Show all 38 contributors for this article] (2016). Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun. Space Science Reviews. ISSN 0038-6308. 200(1), p. 1–73. doi: 10.1007/s11214-015-0229-9. Full text in Research Archive
Robustini, Carolina; Leenaarts, Jorrit; Rodriguez, Jaime de la Cruz & Rouppe, Van Der Voort Luc (2016). Fan-shaped jets above the light bridge of a sunspot driven by reconnection. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 590. doi: 10.1051/0004-6361/201528022. Full text in Research Archive Show summary
We report on a fan-shaped set of high-speed jets above a strongly magnetized light bridge (LB) of a sunspot observed in the Hα line. We study the origin, dynamics, and thermal properties of the jets using high-resolution imaging spectroscopy in Hα from the Swedish 1m Solar Telescope and data from the Solar Dynamics Observatory and Hinode. The Hα jets have lengths of 7–38 Mm, are impulsively accelerated to a speed of ~100 km s-1 close to photospheric footpoints in the LB, and exhibit a constant deceleration consistent with solar effective gravity. They are predominantly launched from one edge of the light bridge, and their footpoints appear bright in the Hα wings. Atmospheric Imaging Assembly data indicates elongated brightenings that are nearly co-spatial with the Hα jets. We interpret them as jets of transition region temperatures. The magnetic field in the light bridge has a strength of 0.8−2 kG and it is nearly horizontal. All jet properties are consistent with magnetic reconnection as the driver. Reproduced with permission from Astronomy & Astrophysics, © ESO
Pereira, Tiago Mendes Domingos; Rouppe, Van Der Voort Luc & Carlsson, Mats (2016). The Appearance of Spicules in High Resolution Observations of Ca II H and Hα. The Astrophysical Journal (ApJ). ISSN 0004-637X. 824(2). doi: 10.3847/0004-637X/824/2/65. Full text in Research Archive Show summary
Solar spicules are chromospheric fibrils that appear everywhere on the Sun, yet their origin is not understood. Using high resolution observations of spicules obtained with the Swedish 1 m Solar Telescope, we aim to understand how spicules appear in filtergrams and Dopplergrams, how they compare in Ca ii H and Hα filtergrams, and what can make them appear and disappear. We find that spicules display a rich and detailed spatial structure, and show a distribution of transverse velocities that, when aligned with the line of sight, can make them appear at different Hα wing positions. They become more abundant at positions closer to the line core, reflecting a distribution of Doppler shifts and widths. In Hα width maps they stand out as bright features both on disk and off limb, reflecting their large Doppler motions and possibly higher temperatures than in the typical Hα formation region. Spicule lifetimes measured from narrowband images at only a few positions will be an underestimate because Doppler shifts can make them disappear prematurely from such images; for such cases, width maps are a more robust tool. In Hα and Ca ii H filtergrams, off-limb spicules essentially have the same properties, appearance, and evolution. We find that the sudden appearance of spicules can be explained by Doppler shifts from their transverse motions, and does not require other convoluted explanations. Reproduced with permission from the Astrophysical Journal. © IOP Publishing
Skogsrud, Håkon; Rouppe, Van Der Voort Luc & Pontieu, Bart Walter De (2016). On the active region bright grains observed in the transition region imaging channels of IRIS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 817:124(2). doi: 10.3847/0004-637X/817/2/124. Full text in Research Archive Show summary
The Interface Region Imaging Spectrograph (IRIS) provides spectroscopy and narrow band slit-jaw (SJI) imaging of the solar chromosphere and transition region at unprecedented spatial and temporal resolutions. Combined with high-resolution context spectral imaging of the photosphere and chromosphere as provided by the Swedish 1 m Solar Telescope (SST), we can now effectively trace dynamic phenomena through large parts of the solar atmosphere in both space and time. IRIS SJI 1400 images from active regions, which primarily sample the transition region with the Si iv 1394 and 1403 Å lines, reveal ubiquitous bright "grains" which are short-lived (two to five minute) bright roundish small patches of sizes 0farcs5–1farcs7 that generally move limbward with velocities up to about 30 km s−1. In this paper, we show that many bright grains are the result of chromospheric shocks impacting the transition region. These shocks are associated with dynamic fibrils (DFs), most commonly observed in Hα. We find that the grains show the strongest emission in the ascending phase of the DF, that the emission is strongest toward the top of the DF, and that the grains correspond to a blueshift and broadening of the Si iv lines. We note that the SJI 1400 grains can also be observed in the SJI 1330 channel which is dominated by C ii lines. Our observations show that a significant part of the active region transition region dynamics is driven from the chromosphere below rather than from coronal activity above. We conclude that the shocks that drive DFs also play an important role in the heating of the upper chromosphere and lower transition region. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

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Voort, Luc Rouppe van der (2021). Sola på nært hold. Astronomi. ISSN 0802-7587. p. 32–35. Full text in Research Archive
Voort, Luc Rouppe van der (2021). Vulkanutbrudd på La Palma. Astronomi. ISSN 0802-7587. p. 55–55. Full text in Research Archive
Voort, Luc Rouppe van der (2021). Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae. Full text in Research Archive
Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2019). Characterization and formation of Ca II K and Mg II k spectra of spicules. Full text in Research Archive
Froment, Clara; Antolin, Patrick; Henriques, V. M. J. & Voort, Luc Rouppe van der (2018). Multi-scale observations of thermal nonequilibrium cycles in coronal loops. Full text in Research Archive
Froment, Clara; Antolin, Patrick; Henriques, V. M. J. & Voort, Luc Rouppe van der (2018). Multi-scale observations of thermal nonequilibrium cycles in coronal loops. Full text in Research Archive
Voort, Luc Rouppe van der (2018). Spicules with SST. Full text in Research Archive
Voort, Luc Rouppe van der (2018). Alignment between IRIS and ground-based data. Full text in Research Archive
Bose, Souvik & Voort, Luc Rouppe van der (2021). On the dynamics of spicules and mass flows in the solar atmosphere. Universitetet i Oslo. Full text in Research Archive Show summary
The atmosphere of the Sun is envisioned as composed of inherently complex, non-homogeneous, and dynamic layers. A detailed understanding of the physical processes involved in these layers is still lacking. For example, it is largely unknown why the outermost layer of the Sun’s atmosphere (corona) is so much hotter than the photosphere by millions of degrees. Astrophysicists think that the layer sandwiched between the photosphere and the corona, known as the interface region, may hold the key to a better understanding of the nature of this controversy. With the help of coordinated high-resolution ground and space-based observations from the Swedish 1-m Solar Telescope (SST) on La Palma, Spain, and NASA’s Interface Region Imaging Spectrograph (IRIS), I aim to unlock some of the mysteries surrounding the dynamics of the interface region with a focus on small-scale jets, known as “spicules”. Spicules are found almost everywhere on the Sun’s surface and at any given moment there can be as many as 10 million of them rapidly shooting outwards. Because of their “omnipresence”, it is suggested that they play a major role in energizing the outer atmospheric layers of the Sun. This thesis focuses on their physical characteristics, dynamics, and their role in the mass-balance and heating of the solar atmosphere.
Drews, Ainar; Voort, Luc Rouppe van der & Carlsson, Mats (2021). On microjets in sunspot penumbrae. Universitetet i Oslo. Full text in Research Archive Show summary
Penumbral microjets (PMJs) are intriguing flashes of light found in the twilight region of sunspots, the penumbra, and they have typical lengths of hundreds of kilometres. Quite some finesse is required to tease out the origins, precise properties, and how PMJs actually move through the Sun’s plasma. PMJs are likely caused by the process of magnetic reconnection, in which “tangled” magnetic field lines suddenly “snap” and realign, releasing energy. This process is typically associated with much larger solar flares. Accordingly, the study of PMJs offers insight into this process at a much smaller scale. The thesis focuses on highly detailed observations from the Swedish 1-m Solar Telescope and NASA’s Interface Region Imaging Spectrograph. A wide range of advanced methods were used, including an automated detection scheme for PMJs involving the machine-learning algorithm k-Nearest Neighbour. Multiple sets of observations were analysed using detailed spectral diagnostics, ranging across many spectral lines. Ultimately, the work accomplished great strides forward in the characterization of PMJs, especially in how they move through the atmosphere and how they evolve in time.

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Published May 23, 2019 2:21 PM - Last modified Aug. 28, 2024 11:11 AM

Participants

Luc Rouppe van der Voort University of Oslo
Vasco Manuel de Jorge Henriques University of Oslo
Jayant Joshi University of Oslo
Souvik Bose University of Oslo
Helle Bakke University of Oslo

Detailed list of participants