Author name code: vissers ADS astronomy entries on 2022-09-14 author:"Vissers, Gregal J.M." ------------------------------------------------------------------------ Title: Active region chromospheric magnetic fields. Observational inference versus magnetohydrostatic modelling Authors: Vissers, G. J. M.; Danilovic, S.; Zhu, X.; Leenaarts, J.; Díaz Baso, C. J.; da Silva Santos, J. M.; de la Cruz Rodríguez, J.; Wiegelmann, T. Bibcode: 2022A&A...662A..88V Altcode: 2021arXiv210902943V Context. A proper estimate of the chromospheric magnetic fields is thought to improve modelling of both active region and coronal mass ejection evolution. However, because the chromospheric field is not regularly obtained for sufficiently large fields of view, estimates thereof are commonly obtained through data-driven models or field extrapolations, based on photospheric boundary conditions alone and involving pre-processing that may reduce details and dynamic range in the magnetograms.
Aims: We investigate the similarity between the chromospheric magnetic field that is directly inferred from observations and the field obtained from a magnetohydrostatic (MHS) extrapolation based on a high-resolution photospheric magnetogram.
Methods: Based on Swedish 1-m Solar Telescope Fe I 6173 Å and Ca II 8542 Å observations of NOAA active region 12723, we employed the spatially regularised weak-field approximation (WFA) to derive the vector magnetic field in the chromosphere from Ca II, as well as non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca II to infer a model atmosphere for selected regions. Milne-Eddington inversions of Fe I serve as photospheric boundary conditions for the MHS model that delivers the three-dimensional field, gas pressure, and density self-consistently.
Results: For the line-of-sight component, the MHS chromospheric field generally agrees with the non-LTE inversions and WFA, but tends to be weaker by 16% on average than these when larger in magnitude than 300 G. The observationally inferred transverse component is systematically stronger, up to an order of magnitude in magnetically weaker regions, but the qualitative distribution with height is similar to the MHS results. For either field component, the MHS chromospheric field lacks the fine structure derived from the inversions. Furthermore, the MHS model does not recover the magnetic imprint from a set of high fibrils connecting the main polarities.
Conclusions: The MHS extrapolation and WFA provide a qualitatively similar chromospheric field, where the azimuth of the former is better aligned with Ca II 8542 Å fibrils than that of the WFA, especially outside strong-field concentrations. The amount of structure as well as the transverse field strengths are, however, underestimated by the MHS extrapolation. This underscores the importance of considering a chromospheric magnetic field constraint in data-driven modelling of active regions, particularly in the context of space weather predictions. Title: Heating of the solar chromosphere through current dissipation Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.; Rempel, M. Bibcode: 2022A&A...661A..59D Altcode: 2022arXiv220203955D Context. The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger.
Aims: We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST).
Methods: We used Milne-Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional (3D) stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions.
Results: We find enhanced heating rates in the upper chromosphere of up to ∼5 kW m−2, where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution (∼1.2″). Band 3 brightness temperatures reach about ∼104 K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of ∼500 G in the chromosphere.
Conclusions: We are able to quantitatively reproduce many of the observed features including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.

The movie is available at https://www.aanda.org Title: COCOPLOT: COlor COllapsed PLOTting software Using colour to view 3D data as a 2D image Authors: Druett, Malcolm K.; Pietrow, Alexander G. M.; Vissers, Gregal J. M.; Robustini, Carolina; Calvo, Flavio Bibcode: 2022RASTI...1...29D Altcode: 2021arXiv211110786D Most modern solar observatories deliver data products formatted as 3D spatio-temporal data cubes, that contain additional, higher dimensions with spectral and/or polarimetric information. This multi-dimensional complexity presents a major challenge when browsing for features of interest in several dimensions simultaneously. We developed the COlor COllapsed PLOTting (COCOPLOT) software as a quick-look and context image software, to convey spectral profile or time evolution from all the spatial pixels ($x,y$) in a 3D [$n_x,n_y,n_\lambda$] or [$n_x,n_y,n_t$] data cube as a single image, using color. This can avoid the need to scan through many wavelengths, creating difference and composite images when searching for signals satisfying multiple criteria. Filters are generated for the red, green, and blue channels by selecting values of interest to highlight in each channel, and their weightings. These filters are combined with the data cube over the third dimension axis to produce an $n_x \times n_y \times 3$ cube displayed as one true color image. Some use cases are presented for data from the Swedish 1-m Solar Telescope (SST) and IRIS, including H$\alpha$ solar flare data, a comparison with $k$-means clustering for identifying asymmetries in the Ca II K line and off-limb coronal rain in IRIS C II slit-jaw images. These illustrate identification by color alone using COCOPLOT of locations including line wing or central enhancement, broadening, wing absorption, and sites with intermittent flows or time-persistent features. COCOPLOT is publicly available in both IDL and Python. Title: COCOPLOT: COlor COllapsed PLOTting software Authors: Druett, Malcolm K.; Pietrow, Alexander G. M.; Vissers, Gregal J. M.; Robustini, Carolina Bibcode: 2021ascl.soft11008D Altcode: The COCOPLOT (COlor COllapsed PLOTting) quick-look and context image code conveys spectral profile information from all of the spatial pixels in a 3D datacube as a single image using color. It can also identify and expose temporal behavior and display and highlight solar features. COCOPLOT thus aids in identifying regions of interest quickly. The software is available in Python and IDL, and can be used as a standalone package or integrated into other software. Title: ISP-SST/ISPy: ISPy release v0.2.0 Authors: Díaz Baso, Carlos José; Vissers, Gregal; Calvo, Flavio; Pietrow, Alexander; Yadav, Rahul; De La Cruz Rodríguez, Jaime; Zivadinovic, Lazar Bibcode: 2021zndo...5608441D Altcode: 2021zndo...5608441J This new version includes all previous features (basic functionality for SST data I/O and calibration, image alignment and inversion result visualisations) and the following new ones: neuralDenoising, potentialExtrapolation, field-dependent crosstalk, mapping to CEA coordinates, neuralEstimation, cube visualisation and routines for creating SST spectral line programs. Title: SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP Authors: Löfdahl, Mats G.; Hillberg, Tomas; de la Cruz Rodríguez, Jaime; Vissers, Gregal; Andriienko, Oleksii; Scharmer, Göran B.; Haugan, Stein V. H.; Fredvik, Terje Bibcode: 2021A&A...653A..68L Altcode: 2018arXiv180403030L Context. Data from ground-based, high-resolution solar telescopes can only be used for science with calibrations and processing, which requires detailed knowledge about the instrumentation. Space-based solar telescopes provide science-ready data, which are easier to work with for researchers whose expertise is in the interpretation of data. Recently, data-processing pipelines for ground-based instruments have been constructed.
Aims: We aim to provide observers with a user-friendly data pipeline for data from the Swedish 1-meter Solar Telescope (SST) that delivers science-ready data together with the metadata needed for proper interpretation and archiving.
Methods: We briefly describe the CHROMospheric Imaging Spectrometer (CHROMIS) instrument, including its (pre)filters, as well as recent upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and polarization optics. We summarize the processing steps from raw data to science-ready data cubes in FITS files. We report calibrations and compensations for data imperfections in detail. Misalignment of Ca II data due to wavelength-dependent dispersion is identified, characterized, and compensated for. We describe intensity calibrations that remove or reduce the effects of filter transmission profiles as well as solar elevation changes. We present REDUX, a new version of the MOMFBD image restoration code, with multiple enhancements and new features. It uses projective transforms for the registration of multiple detectors. We describe how image restoration is used with CRISP and CHROMIS data. The science-ready output is delivered in FITS files, with metadata compliant with the SOLARNET recommendations. Data cube coordinates are specified within the World Coordinate System (WCS). Cavity errors are specified as distortions of the WCS wavelength coordinate with an extension of existing WCS notation. We establish notation for specifying the reference system for Stokes vectors with reference to WCS coordinate directions. The CRIsp SPectral EXplorer (CRISPEX) data-cube browser has been extended to accept SSTRED output and to take advantage of the SOLARNET metadata.
Results: SSTRED is a mature data-processing pipeline for imaging instruments, developed and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP spectropolarimeter. SSTRED delivers well-characterized, science-ready, archival-quality FITS files with well-defined metadata. The SSTRED code, as well as REDUX and CRISPEX, is freely available through git repositories. Title: Non-LTE inversions of a confined X2.2 flare. I. The vector magnetic field in the photosphere and chromosphere Authors: Vissers, G. J. M.; Danilovic, S.; de la Cruz Rodríguez, J.; Leenaarts, J.; Morosin, R.; Díaz Baso, C. J.; Reid, A.; Pomoell, J.; Price, D. J.; Inoue, S. Bibcode: 2021A&A...645A...1V Altcode: 2020arXiv200901537V Context. Obtaining an accurate measurement of magnetic field vector in the solar atmosphere is essential for studying changes in field topology during flares and reliably modelling space weather.
Aims: We tackle this problem by applying various inversion methods to a confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017 and comparing the photospheric and chromospheric magnetic field vector with the results of two numerical models of this event.
Methods: We obtained the photospheric magnetic field from Milne-Eddington and (non-)local thermal equilibrium (non-LTE) inversions of Hinode SOT/SP Fe I 6301.5 Å and 6302.5 Å. The chromospheric field was obtained from a spatially regularised weak-field approximation (WFA) and non-LTE inversions of Ca II 8542 Å observed with CRISP at the Swedish 1 m Solar Telescope. We investigated the field strengths and photosphere-to-chromosphere shear in the field vector.
Results: The LTE- and non-LTE-inferred photospheric magnetic field components are strongly correlated across several optical depths in the atmosphere, with a tendency towards a stronger field and higher temperatures in the non-LTE inversions. For the chromospheric field, the non-LTE inversions correlate well with the spatially regularised WFA, especially in terms of the line-of-sight field strength and field vector orientation. The photosphere exhibits coherent strong-field patches of over 4.5 kG, co-located with similar concentrations exceeding 3 kG in the chromosphere. The obtained field strengths are up to two to three times higher than in the numerical models, while the photosphere-to-chromosphere shear close to the polarity inversion line is more concentrated and structured.
Conclusions: In the photosphere, the assumption of LTE for Fe I line formation does not yield significantly different magnetic field results in comparison to the non-LTE case, while Milne-Eddington inversions fail to reproduce the magnetic field vector orientation where Fe I is in emission. In the chromosphere, the non-LTE-inferred field is excellently approximated by the spatially regularised WFA. Our inversions confirm the locations of flux rope footpoints that have been predicted by numerical models. However, pre-processing and lower spatial resolution lead to weaker and smoother field in the models than what our data indicate. This highlights the need for higher spatial resolution in the models to better constrain pre-eruptive flux ropes. Title: Probing chromospheric heating with millimeter interferometry Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White, S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S. Bibcode: 2020AGUFMSH0010001D Altcode: Observations at visible and ultraviolet wavelengths have shown that solar active regions host different kinds of small-scale, transient, bright structures that are believed to be heating events resulting from the release of magnetic energy in the low atmosphere of the Sun, especially at the early stages of flux emergence. It is of great scientific interest to be able to accurately infer temperatures and formation heights of the most localized events, which are still matter of debate, in the hope of learning about the evolution of active regions where occasionally more energetic phenomena lead to much larger outbursts that propagate across the Solar System. The millimeter (mm) continuum is a new complementary diagnostic for chromospheric heating that is now available thanks to the Atacama Large Millimeter/submillimeter Array (ALMA).

We report on the first ALMA 3 mm observations of small-scale heating events in a solar active region. In contrast with the low-amplitude brightness temperature variations in the quiet-Sun, the interferometric maps show that the active region consists of long, warm, fibril-like structures that connect magnetic concentrations of opposite polarity and often flare up along with compact, flickering mm-bursts -- reminiscent of ultraviolet bursts -- with brightness temperatures of up to 14000 K at 1.2" scales. These events also show simultaneous EUV emission observed by the Solar Dynamics Observatory (SDO). We find a weak correlation between the photospheric bright patches and the 3 mm continuum brightness and, in particular, we do not detect any mm counterpart of Ellerman bombs which confirms that they are photospheric phenomena.

Our observations and modelling highlight the diagnostic capabilities of ALMA for local heating in solar active regions and emphasize the need for coordinated observations with IRIS and DKIST in the future. Title: ALMA observations of transient heating in a solar active region Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White, S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H. Bibcode: 2020A&A...643A..41D Altcode: 2020arXiv200614564D
Aims: We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA).
Methods: We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis.
Results: We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts - reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 - 340 km s-1) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

Movie associated to Fig. 5 is available at https://www.aanda.org Title: Stratification of canopy magnetic fields in a plage region. Constraints from a spatially-regularized weak-field approximation method Authors: Morosin, Roberta; de la Cruz Rodríguez, Jaime; Vissers, Gregal J. M.; Yadav, Rahul Bibcode: 2020A&A...642A.210M Altcode: 2020arXiv200614487M Context. The role of magnetic fields in the chromospheric heating problem remains greatly unconstrained. Most theoretical predictions from numerical models rely on a magnetic configuration, field strength, and connectivity; the details of which have not been well established with observational studies for many chromospheric scenarios. High-resolution studies of chromospheric magnetic fields in plage are very scarce or non existent in general.
Aims: Our aim is to study the stratification of the magnetic field vector in plage regions. Previous studies predict the presence of a magnetic canopy in the chromosphere that has not yet been studied with full-Stokes observations. We use high-spatial resolution full-Stokes observations acquired with the CRisp Imaging Spectro-Polarimeter (CRISP) at the Swedish 1-m Solar Telescope in the Mg I 5173 Å, Na I 5896 Å and Ca II 8542 Å lines.
Methods: We have developed a spatially-regularized weak-field approximation (WFA) method, based on the idea of spatial regularization. This method allows for a fast computation of magnetic field maps for an extended field of view. The fidelity of this new technique has been assessed using a snapshot from a realistic 3D magnetohydrodynamics simulation.
Results: We have derived the depth-stratification of the line-of-sight component of the magnetic field from the photosphere to the chromosphere in a plage region. The magnetic fields are concentrated in the intergranular lanes in the photosphere and expand horizontally toward the chromosphere, filling all the space and forming a canopy. Our results suggest that the lower boundary of this canopy must be located around 400 - 600 km from the photosphere. The mean canopy total magnetic field strength in the lower chromosphere (z ≈ 760 km) is 658 G. At z = 1160 km, we estimate ⟨B∥⟩ ≈ 417 G.
Conclusions: In this study we propose a modification to the WFA that improves its applicability to data with a worse signal-to-noise ratio. We have used this technique to study the magnetic properties of the hot chromospheric canopy that is observed in plage regions. The methods described in this paper provide a quick and reliable way of studying multi layer magnetic field observations without the many difficulties inherent to other inversion methods. Title: High-resolution observations of the solar photosphere, chromosphere, and transition region. A database of coordinated IRIS and SST observations Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson, M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.; Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques, V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.; Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.; Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.; Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski, M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P. Bibcode: 2020A&A...641A.146R Altcode: 2020arXiv200514175R 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, Hα, 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. Title: Dissecting bombs and bursts: non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht, T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M. Bibcode: 2019A&A...627A.101V Altcode: 2019arXiv190502035V 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. Title: Automating Ellerman bomb detection in ultraviolet continua Authors: Vissers, Gregal J. M.; Rouppe van der Voort, Luc H. M.; Rutten, Robert J. Bibcode: 2019A&A...626A...4V Altcode: 2019arXiv190107975V 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. Title: Solar Ultraviolet Bursts Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.; Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr Bibcode: 2018SSRv..214..120Y Altcode: 2018arXiv180505850Y 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. Title: Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.; de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.; Carlsson, M.; Vissers, G. Bibcode: 2017ApJ...851L...6R Altcode: 2017arXiv171104581R 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 0.″2 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. Title: Observing the Formation of Flare-driven Coronal Rain Authors: Scullion, E.; Rouppe van der Voort, L.; Antolin, P.; Wedemeyer, S.; Vissers, G.; Kontar, E. P.; Gallagher, P. T. Bibcode: 2016ApJ...833..184S Altcode: 2016arXiv161009255S Flare-driven coronal rain can manifest from rapidly cooled plasma condensations near coronal loop tops in thermally unstable postflare arcades. We detect five phases that characterize the postflare decay: heating, evaporation, conductive cooling dominance for ∼120 s, radiative/enthalpy cooling dominance for ∼4700 s, and finally catastrophic cooling occurring within 35-124 s, leading to rain strands with a periodicity of 55-70 s. We find an excellent agreement between the observations and model predictions of the dominant cooling timescales and the onset of catastrophic cooling. At the rain-formation site, we detect comoving, multithermal rain clumps that undergo catastrophic cooling from ∼1 MK to ∼22,000 K. During catastrophic cooling, the plasma cools at a maximum rate of 22,700 K s-1 in multiple loop-top sources. We calculated the density of the extreme-ultraviolet (EUV) plasma from the differential emission measure of the multithermal source employing regularized inversion. Assuming a pressure balance, we estimate the density of the chromospheric component of rain to be 9.21 × 1011 ± 1.76 × 1011 cm-3, which is comparable with quiescent coronal rain densities. With up to eight parallel strands in the EUV loop cross section, we calculate the mass loss rate from the postflare arcade to be as much as 1.98 × 1012 ± 4.95 × 1011 g s-1. Finally, we reveal a close proximity between the model predictions of {10}5.8 K and the observed properties between {10}5.9 and {10}6.2 K, which defines the temperature onset of catastrophic cooling. The close correspondence between the observations and numerical models suggests that indeed acoustic waves (with a sound travel time of 68 s) could play an important role in redistributing energy and sustaining the enthalpy-based radiative cooling. Title: Reconnection brightenings in the quiet solar photosphere Authors: Rouppe van der Voort, Luc H. M.; Rutten, Robert J.; Vissers, Gregal J. M. Bibcode: 2016A&A...592A.100R Altcode: 2016arXiv160603675R 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.

The movies are available in electronic form at http://www.aanda.org Title: Evidence for a Transition Region Response to Penumbral Microjets in Sunspots Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Carlsson, M. Bibcode: 2015ApJ...811L..33V Altcode: 2015arXiv150901402V Penumbral microjets (PMJs) are short-lived, fine-structured, and bright jets that are generally observed in chromospheric imaging of the penumbra of sunspots. Here we investigate their potential transition region signature by combining observations with the Swedish 1-m Solar Telescope in the Ca ii H and Ca ii 8542 Å lines with ultraviolet imaging and spectroscopy obtained with the Interface Region Imaging Spectrograph (IRIS), which includes the C ii 1334/1335 Å, Si iv 1394/1403 Å, and Mg ii h & k 2803/2796 Å lines. We find a clear corresponding signal in the IRIS Mg ii k, C ii, and Si iv slit-jaw images, typically offset spatially from the Ca ii signature in the direction along the jets: from base to top, the PMJs are predominantly visible in Ca ii, Mg ii k, and C ii/Si iv, suggesting progressive heating to transition region temperatures along the jet extent. Hence, these results support the suggestion from earlier studies that PMJs may heat to transition region temperatures. Title: Ellerman Bombs at High Resolution. III. Simultaneous Observations with IRIS and SST Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Rutten, R. J.; Carlsson, M.; De Pontieu, B. Bibcode: 2015ApJ...812...11V Altcode: 2015arXiv150700435V Ellerman bombs (EBs) are transient brightenings of the extended wings of the solar Balmer lines in emerging active regions. We describe their properties in the ultraviolet lines sampled by the Interface Region Imaging Spectrograph (IRIS), using simultaneous imaging spectroscopy in Hα with the Swedish 1-m Solar Telescope (SST) and ultraviolet images from the Solar Dynamics Observatory for Ellerman bomb detection and identification. We select multiple co-observed EBs for detailed analysis. The IRIS spectra strengthen the view that EBs mark reconnection between bipolar kilogauss fluxtubes with the reconnection and the resulting bi-directional jet located within the solar photosphere and shielded by overlying chromospheric fibrils in the cores of strong lines. The spectra suggest that the reconnecting photospheric gas underneath is heated sufficiently to momentarily reach stages of ionization normally assigned to the transition region and the corona. We also analyze similar outburst phenomena that we classify as small flaring arch filaments and ascribe to reconnection at a higher location. They have different morphologies and produce hot arches in million-Kelvin diagnostics. Title: Ellerman Bombs at High Resolution. IV. Visibility in Na I and Mg I Authors: Rutten, R. J.; Rouppe van der Voort, L. H. M.; Vissers, G. J. M. Bibcode: 2015ApJ...808..133R Altcode: 2015arXiv150604426R Ellerman bombs are transient brightenings of the wings of the solar Balmer lines that mark reconnection in the photosphere. Ellerman noted in 1917 that he did not observe such brightenings in the Na i D and Mg i b lines. This non-visibility should constrain EB interpretation, but has not been addressed in published bomb modeling. We therefore test Ellerman’s observation and confirm it using high-quality imaging spectrometry with the Swedish 1-m Solar Telescope. However, we find a diffuse brightness in these lines that seems to result from prior EBs. We tentatively suggest this is a post-bomb hot-cloud phenomenon also found in recent EB spectroscopy in the ultraviolet. Title: The Multithermal and Multi-stranded Nature of Coronal Rain Authors: Antolin, P.; Vissers, G.; Pereira, T. M. D.; Rouppe van der Voort, L.; Scullion, E. Bibcode: 2015ApJ...806...81A Altcode: 2015arXiv150404418A We analyze coordinated observations of coronal rain in loops, spanning chromospheric, transition region (TR), and coronal temperatures with sub-arcsecond spatial resolution. Coronal rain is found to be a highly multithermal phenomenon with a high degree of co-spatiality in the multi-wavelength emission. EUV darkening and quasi-periodic intensity variations are found to be strongly correlated with coronal rain showers. Progressive cooling of coronal rain is observed, leading to a height dependence of the emission. A fast-slow two-step catastrophic cooling progression is found, which may reflect the transition to optically thick plasma states. The intermittent and clumpy appearance of coronal rain at coronal heights becomes more continuous and persistent at chromospheric heights just before impact, mainly due to a funnel effect from the observed expansion of the magnetic field. Strong density inhomogeneities of 0\buildrel{\prime\prime}\over{.} 2-0\buildrel{\prime\prime}\over{.} 5 are found, in which a transition from temperatures of 105 to 104 K occurs. The 0\buildrel{\prime\prime}\over{.} 2-0\buildrel{\prime\prime}\over{.} 8 width of the distribution of coronal rain is found to be independent of temperature. The sharp increase in the number of clumps at the coolest temperatures, especially at higher resolution, suggests that the bulk distribution of the rain remains undetected. Rain clumps appear organized in strands in both chromospheric and TR temperatures. We further find structure reminiscent of the magnetohydrodynamic (MHD) thermal mode (also known as entropy mode), thereby suggesting an important role of thermal instability in shaping the basic loop substructure. Rain core densities are estimated to vary between 2 × 1010 and 2.5× {{10}11} cm-3, leading to significant downward mass fluxes per loop of 1-5 × 109 g s-1, thus suggesting a major role in the chromosphere-corona mass cycle. Title: Ellerman bombs: Advances driven by high-resolution observations Authors: Vissers, Gregal Bibcode: 2014cosp...40E3533V Altcode: Ellerman bombs, transient brightenings that have traditionally been observed in the wings of the Balmer Halpha line, are a ubiquitous phenomenon in the lower atmosphere of active regions with considerable flux emergence. These explosive events display sub-arcsecond fine structure, fast dynamical evolution and their energies tend to fall in the nanoflare ballpark. Over the past decade and a half, several high-resolution ground-based and space-based telescopes have contributed greatly to further characterising Ellerman bombs, offering a view in spectral diagnostics ranging from the UV to the infrared. I will highlight some of the recent advances that have been made - both observationally and from a theoretical point of view - in determining their properties (quantitative morphology, energies, flows and proper motion, driving mechanism, etc.), and discuss the potential of observations from relatively new space-based telescopes, such as SDO and IRIS, to add to our understanding of this phenomenon. Title: Ellerman Bombs at High Resolution. II. Triggering, Visibility, and Effect on Upper Atmosphere Authors: Vissers, Gregal J. M.; Rouppe van der Voort, Luc H. M.; Rutten, Robert J. Bibcode: 2013ApJ...774...32V Altcode: 2013arXiv1307.1547V We use high-resolution imaging spectroscopy with the Swedish 1-m Solar Telescope (SST) to study the transient brightenings of the wings of the Balmer Hα line in emerging active regions that are called Ellerman bombs. Simultaneous sampling of Ca II 8542 Å with the SST confirms that most Ellerman bombs also occur in the wings of this line, but with markedly different morphology. Simultaneous images from the Solar Dynamics Observatory (SDO) show that Ellerman bombs are also detectable in the photospheric 1700 Å continuum, again with differing morphology. They are also observable in 1600 Å SDO images, but with much contamination from C IV emission in transition-region features. Simultaneous SST spectropolarimetry in Fe I 6301 Å shows that Ellerman bombs occur at sites of strong-field magnetic flux cancellation between small bipolar strong-field patches that rapidly move together over the solar surface. Simultaneous SDO images in He II 304 Å, Fe IX 171 Å, and Fe XIV 211 Å show no clear effect of the Ellerman bombs on the overlying transition region and corona. These results strengthen our earlier suggestion, based on Hα morphology alone, that the Ellerman bomb phenomenon is a purely photospheric reconnection phenomenon. Title: Ellerman bombs: fallacies, fads, usage Authors: Rutten, Robert J.; Vissers, Gregal J. M.; Rouppe van der Voort, Luc H. M.; Sütterlin, Peter; Vitas, Nikola Bibcode: 2013JPhCS.440a2007R Altcode: 2013arXiv1304.1364R Ellerman bombs are short-lived brightenings of the outer wings of Hα that occur in active regions with much flux emergence. We point out fads and fallacies in the extensive Ellerman bomb literature, discuss their appearance in various spectral diagnostics, and advocate their use as indicators of field reconfiguration in active-region topography using AIA 1700 Å images. Title: On-Disk Coronal Rain Authors: Antolin, Patrick; Vissers, Gregal; Rouppe van der Voort, Luc Bibcode: 2012SoPh..280..457A Altcode: 2012SoPh..tmp...78A; 2012arXiv1203.2077A Small and elongated, cool and dense blob-like structures are being reported with high resolution telescopes in physically different regions throughout the solar atmosphere. Their detection and the understanding of their formation, morphology, and thermodynamical characteristics can provide important information on their hosting environment, especially concerning the magnetic field, whose understanding constitutes a major problem in solar physics. An example of such blobs is coronal rain, a phenomenon of thermal non-equilibrium observed in active region loops, which consists of cool and dense chromospheric blobs falling along loop-like paths from coronal heights. So far, only off-limb coronal rain has been observed, and few reports on the phenomenon exist. In the present work, several data sets of on-disk Hα observations with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) are analyzed. A special family of on-disk blobs is selected for each data set, and a statistical analysis is carried out on their dynamics, morphology, and temperature. All characteristics present distributions which are very similar to reported coronal rain statistics. We discuss possible interpretations considering other similar blob-like structures reported so far and show that a coronal rain interpretation is the most likely one. The chromospheric nature of the blobs and the projection effects (which eliminate all direct possibilities of height estimation) on one side, and their small sizes, fast dynamics, and especially their faint character (offering low contrast with the background intensity) on the other side, are found as the main causes for the absence until now of the detection of this on-disk coronal rain counterpart. Title: Implications for Coronal Heating from Coronal Rain Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort, L.; Vissers, G.; Hansteen, V. Bibcode: 2012ASPC..454..171A Altcode: Coronal rain is a phenomenon above active regions in which cool plasma condensations fall down from coronal heights. Numerical simulations of loops have shown that such condensations can naturally form in the case of footpoint concentrated heating through the “catastrophic cooling” mechanism. In this work we analize high resolution limb observations in Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of SST and derive statistical properties. We further investigate the link between coronal rain and the coronal heating mechanisms by performing 1.5-D MHD simulations of a loop subject to footpoint heating and to Alfvén waves generated in the photosphere. It is found that if a loop is heated predominantly from Alfvén waves coronal rain is inhibited due to the characteristic uniform heating they produce. Hence coronal rain can point both to the spatial distribution of the heating and to the agent of the heating itself, thus acting as a marker for coronal heating mechanisms. Title: A Sharp Look at Coronal Rain with Hinode/SOT and SST/CRISP Authors: Antolin, P.; Carlsson, M.; Rouppe van der Voort, L.; Verwichte, E.; Vissers, G. Bibcode: 2012ASPC..455..253A Altcode: 2012arXiv1202.0787A The tropical wisdom that when it is hot and dense we can expect rain might also apply to the Sun. Indeed, observations and numerical simulations have showed that strong heating at footpoints of loops, as is the case for active regions, puts their coronae out of thermal equilibrium, which can lead to a phenomenon known as catastrophic cooling. Following local pressure loss in the corona, hot plasma locally condenses in these loops and dramatically cools down to chromospheric temperatures. These blobs become bright in Hα and Ca ii H in time scales of minutes, and their dynamics seem to be subject more to internal pressure changes in the loop rather than to gravity. They thus become trackers of the magnetic field, which results in the spectacular coronal rain that is observed falling down coronal loops. In this work we report on high resolution observations of coronal rain with the Solar Optical Telescope (SOT) on Hinode and CRISP at the Swedish Solar Telescope (SST). A statistical study is performed in which properties such as velocities and accelerations of coronal rain are derived. We show how this phenomenon can constitute a diagnostic tool for the internal physical conditions inside loops. Furthermore, we analyze transverse oscillations of strand-like condensations composing coronal rain falling in a loop, and discuss the possible nature of the wave. This points to the important role that coronal rain can play in the fields of coronal heating and coronal seismology. Title: Flocculent Flows in the Chromospheric Canopy of a Sunspot Authors: Vissers, Gregal; Rouppe van der Voort, Luc Bibcode: 2012ApJ...750...22V Altcode: 2012arXiv1202.5453V High-quality imaging spectroscopy in the Hα line, obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m solar Telescope (SST) at La Palma and covering a small sunspot and its surroundings, is studied. They exhibit ubiquitous flows both along fibrils making up the chromospheric canopy away from the spot and in the superpenumbra. We term these flows "flocculent" to describe their intermittent character, that is, morphologically reminiscent of coronal rain. The flocculent flows are investigated further in order to determine their dynamic and morphological properties. For the measurement of their characteristic velocities, accelerations, and sizes, we employ a new versatile analysis tool, the CRisp SPectral EXplorer (CRISPEX), which we describe in detail. Absolute velocities on the order of 7.2-82.4 km s-1 are found, with an average value of 36.5 ± 5.9 km s-1 and slightly higher typical velocities for features moving toward the sunspot than away. These velocities are much higher than those determined from the shift of the line core, which shows patches around the sunspot with velocity enhancements of up to 10-15 km s-1 (both red- and blueshifted). Accelerations are determined for a subsample of features that show clear accelerating or decelerating behavior, yielding an average of 270 ± 63 m s-2 and 149 ± 63 m s-2 for the accelerating and decelerating features, respectively. Typical flocculent features measure 627 ± 44 km in length and 304 ± 30 km in width. On average, 68 features are detected per minute, with an average lifetime of 67.7 ± 8.8 s. The dynamics and phenomenology of the flocculent flows suggest they may be driven by a siphon flow, where the flocculence could arise from a density perturbation close to one of the footpoints or along the loop structure. Title: Ellerman Bombs at High Resolution. I. Morphological Evidence for Photospheric Reconnection Authors: Watanabe, Hiroko; Vissers, Gregal; Kitai, Reizaburo; Rouppe van der Voort, Luc; Rutten, Robert J. Bibcode: 2011ApJ...736...71W Altcode: 2011arXiv1105.4008W High-resolution imaging-spectroscopy movies of solar active region NOAA 10998 obtained with the Crisp Imaging Spectropolarimeter at the Swedish 1-m Solar Telescope show very bright, rapidly flickering, flame-like features that appear intermittently in the wings of the Balmer Hα line in a region with moat flows and likely some flux emergence. They show up at regular Hα blue-wing bright points that outline the magnetic network, but flare upward with much larger brightness and distinct "jet" morphology seen from aside in the limbward view of these movies. We classify these features as Ellerman bombs and present a morphological study of their appearance at the unprecedented spatial, temporal, and spectral resolution of these observations. The bombs appear along the magnetic network with footpoint extents up to 900 km. They show apparent travel away from the spot along the pre-existing network at speeds of about 1 km s-1. The bombs flare repetitively with much rapid variation at timescales of seconds only, in the form of upward jet-shaped brightness features. These reach heights of 600-1200 km and tend to show blueshifts; some show bi-directional Doppler signature and some seem accompanied with an Hα surge. They are not seen in the core of Hα due to shielding by overlying chromospheric fibrils. The network where they originate has normal properties. The morphology of these jets strongly supports deep-seated photospheric reconnection of emergent or moat-driven magnetic flux with pre-existing strong vertical network fields as the mechanism underlying the Ellerman bomb phenomenon. Title: Dynamics of fine structure in the atmosphere of solar active regions Authors: Vissers, Gregal Joan María Bibcode: 2011PhDT.......106V Altcode: No abstract at ADS Title: Coronal Rain as a Marker for Coronal Heating Mechanisms Authors: Antolin, P.; Shibata, K.; Vissers, G. Bibcode: 2010ApJ...716..154A Altcode: 2009arXiv0910.2383A Reported observations in Hα, Ca II H, and K or other chromospheric lines of coronal rain trace back to the days of the Skylab mission. Corresponding to cool and dense plasma, coronal rain is often observed falling down along coronal loops in active regions. A physical explanation for this spectacular phenomenon has been put forward thanks to numerical simulations of loops with footpoint-concentrated heating, a heating scenario in which cool condensations naturally form in the corona. This effect has been termed "catastrophic cooling" and is the predominant explanation for coronal rain. In this work, we further investigate the link between this phenomenon and the heating mechanisms acting in the corona. We start by analyzing observations of coronal rain at the limb in the Ca II H line performed by the Hinode satellite, and derive interesting statistical properties concerning the dynamics. We then compare the observations with 1.5-dimensional MHD simulations of loops being heated by small-scale discrete events concentrated toward the footpoints (that could come, for instance, from magnetic reconnection events), and by Alfvén waves generated at the photospheric level. Both our observation and simulation results suggest that coronal rain is a far more common phenomenon than previously thought. Also, we show that the structure and dynamics of condensations are far more sensitive to the internal pressure changes in loops than to gravity. Furthermore, it is found that if a loop is predominantly heated from Alfvén waves, coronal rain is inhibited due to the characteristic uniform heating they produce. Hence, coronal rain may not only point to the spatial distribution of the heating in coronal loops but also to the agent of the heating itself. We thus propose coronal rain as a marker for coronal heating mechanisms. Title: On-disk Counterparts of Type II Spicules in the Ca II 854.2 nm and Hα Lines Authors: Rouppe van der Voort, L.; Leenaarts, J.; de Pontieu, B.; Carlsson, M.; Vissers, G. Bibcode: 2009ApJ...705..272R Altcode: 2009arXiv0909.2115R Recently, a second type of spicules was discovered at the solar limb with the Solar Optical Telescope onboard the Japanese Hinode spacecraft. These previously unrecognized type II spicules are thin chromospheric jets that are shorter lived (10-60 s) and that show much higher apparent upward velocities (of order 50-100 km s-1) than the classical spicules. Since they have been implicated in providing hot plasma to coronal loops, their formation, evolution, and properties are important ingredients for a better understanding of the mass and energy balance of the low solar atmosphere. Here, we report on the discovery of the disk counterparts of type II spicules using spectral imaging data in the Ca II 854.2 nm and Hα lines with the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope in La Palma. We find rapid blueward excursions in the line profiles of both chromospheric lines that correspond to thin, jet-like features that show apparent velocities of order 50 km s-1. These blueward excursions seem to form a separate absorbing component with Doppler shifts of order 20 and 50 km s-1 for the Ca II 854.2 nm and Hα line, respectively. We show that the appearance, lifetimes, longitudinal and transverse velocities, and occurrence rate of these rapid blue excursions on the disk are very similar to those of the type II spicules at the limb. A detailed study of the spectral line profiles in these events suggests that plasma is accelerated along the jet, and plasma is being heated throughout the short lifetime of the event.