Author name code: henriques ADS astronomy entries on 2022-09-14 =author:"Henriques, V.M.J." OR author:"Henriques, Vasco" ------------------------------------------------------------------------ Title: The Solar ALMA Science Archive (SALSA). First release, SALAT, and FITS header standard Authors: Henriques, Vasco M. J.; Jafarzadeh, Shahin; Guevara Gómez, Juan Camilo; Eklund, Henrik; Wedemeyer, Sven; Szydlarski, Mikołaj; Haugan, Stein Vidar H.; Mohan, Atul Bibcode: 2022A&A...659A..31H Altcode: 2021arXiv210902374H In December 2016, the Atacama Large Millimeter/submillimeter Array (ALMA) carried out the first regular observations of the Sun. These early observations and the reduction of the respective data posed a challenge due to the novelty and complexity of observing the Sun with ALMA. The difficulties with producing science-ready, time-resolved imaging products in a format familiar to and usable by solar physicists based on the measurement sets delivered by ALMA had limited the availability of such data to this point. With the development of the Solar ALMA Pipeline, it has now become possible to routinely reduce such data sets. As a result, a growing number of science-ready solar ALMA data sets are now offered in the form of the Solar ALMA Science Archive (SALSA). So far, SALSA contains primarily time series of single-pointing interferometric images at cadences of one or two seconds, accompanied by the respective single-dish full-disc solar images. The data arrays are provided in FITS format. We also present the first version of a standardised header format that accommodates future expansions and fits within the scope of other standards including the ALMA Science Archive itself and SOLARNET. The headers include information designed to aid the reproduction of the imaging products from the raw data. Links to co-observations, if available, with a focus on those of the Interface Region Imaging Spectrograph, are also provided. SALSA is accompanied by the Solar ALMA Library of Auxiliary Tools (SALAT), which contains Interactive Data Language and Python routines for convenient loading and a quick-look analysis of SALSA data.

Movies associated to Figs. 3 and 4 are available at https://www.aanda.org Title: Evidence of the multi-thermal nature of spicular downflows. Impact on solar atmospheric heating Authors: Bose, Souvik; Rouppe van der Voort, Luc; Joshi, Jayant; Henriques, Vasco M. J.; Nóbrega-Siverio, Daniel; Martínez-Sykora, Juan; De Pontieu, Bart Bibcode: 2021A&A...654A..51B Altcode: 2021arXiv210802153B 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.

Movies associated to Figs. 1-3, 8, and 10 are available at https://www.aanda.org Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio, Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart; Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa, Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler, Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun, Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres, Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.; Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini, Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena; Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor; Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael; Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli, Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys, Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.; Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis, Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson, Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.; Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.; Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava, Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas, Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST Instrument Scientists; DKIST Science Working Group; DKIST Critical Science Plan Community Bibcode: 2021SoPh..296...70R Altcode: 2020arXiv200808203R The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute. Title: Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae Authors: Rouppe van der Voort, Luc H. M.; Joshi, Jayant; Henriques, Vasco M. J.; Bose, Souvik Bibcode: 2021A&A...648A..54R Altcode: 2021arXiv210111321R 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 Hα and Hβ 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 Hβ 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.

Movies associated to Figs. 1 and 6 are available at https://www.aanda.org Title: Spicules and downflows in the solar chromosphere Authors: Bose, Souvik; Joshi, Jayant; Henriques, Vasco M. J.; Rouppe van der Voort, Luc Bibcode: 2021A&A...647A.147B Altcode: 2021arXiv210107829B 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 Hα 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 Hα 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 Hα 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.

Movies are available at https://www.aanda.org Title: An overall view of temperature oscillations in the solar chromosphere with ALMA Authors: Jafarzadeh, S.; Wedemeyer, S.; Fleck, B.; Stangalini, M.; Jess, D. B.; Morton, R. J.; Szydlarski, M.; Henriques, V. M. J.; Zhu, X.; Wiegelmann, T.; Guevara Gómez, J. C.; Grant, S. D. T.; Chen, B.; Reardon, K.; White, S. M. Bibcode: 2021RSPTA.37900174J Altcode: 2021RSTPA.379..174J; 2020arXiv201001918J By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb-Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3-5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of Hα line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Transverse motions in sunspot super-penumbral fibrils Authors: Morton, R. J.; Mooroogen, K.; Henriques, V. M. J. Bibcode: 2021RSPTA.37900183M Altcode: 2020arXiv201207394M Sunspots have played a key role in aiding our understanding of magnetohydrodynamic (MHD) wave phenomena in the Sun's atmosphere, and it is well known they demonstrate a number of wave phenomena associated with slow MHD modes. Recent studies have shown that transverse wave modes are present throughout the majority of the chromosphere. Using high-resolution Ca II 8542 Å observations from the Swedish Solar Telescope, we provide the first demonstration that the chromospheric super-penumbral fibrils, which span out from the sunspot, also show ubiquitous transverse motions. We interpret these motions as transverse waves, in particular the MHD kink mode. We compile the statistical properties of over 2000 transverse motions to find distributions for periods and amplitudes, finding they are broadly consistent with previous observations of chromospheric transverse waves in quiet Sun fibrils. The very presence of the waves in super-penumbral fibrils raises important questions about how they are generated, and could have implications for our understanding of how MHD wave energy is transferred through the atmosphere of a sunspot.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Downflowing umbral flashes as evidence of standing waves in sunspot umbrae Authors: Felipe, T.; Henriques, V. M. J.; de la Cruz Rodríguez, J.; Socas-Navarro, H. Bibcode: 2021A&A...645L..12F Altcode: 2021arXiv210104188F Context. Umbral flashes are sudden brightenings commonly visible in the core of some chromospheric lines. Theoretical and numerical modeling suggests that they are produced by the propagation of shock waves. According to these models and early observations, umbral flashes are associated with upflows. However, recent studies have reported umbral flashes in downflowing atmospheres.
Aims: We aim to understand the origin of downflowing umbral flashes. We explore how the existence of standing waves in the umbral chromosphere impacts the generation of flashed profiles.
Methods: We performed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. The Stokes profiles of the Ca II 8542 Å line were synthesized with the NICOLE code.
Results: For freely propagating waves, the chromospheric temperature enhancements of the oscillations are in phase with velocity upflows. In this case, the intensity core of the Ca II 8542 Å atmosphere is heated during the upflowing stage of the oscillation. However, a different scenario with a resonant cavity produced by the sharp temperature gradient of the transition region leads to chromospheric standing oscillations. In this situation, temperature fluctuations are shifted backward and temperature enhancements partially coincide with the downflowing stage of the oscillation. In umbral flash events produced by standing oscillations, the reversal of the emission feature is produced when the oscillation is downflowing. The chromospheric temperature keeps increasing while the atmosphere is changing from a downflow to an upflow. During the appearance of flashed Ca II 8542 Å cores, the atmosphere is upflowing most of the time, and only 38% of the flashed profiles are associated with downflows.
Conclusions: We find a scenario that remarkably explains the recent empirical findings of downflowing umbral flashes as a natural consequence of the presence of standing oscillations above sunspot umbrae. Title: Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra Authors: Henriques, Vasco M. J.; Nelson, Chris J.; Rouppe van der Voort, Luc H. M.; Mathioudakis, Mihalis Bibcode: 2020A&A...642A.215H Altcode: 2020arXiv200805482H 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. 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: Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines (Corrigendum) Authors: Bose, Souvik; Henriques, Vasco M. J.; Joshi, Jayant; Rouppe van der Voort, Luc Bibcode: 2020A&A...637C...1B Altcode: No abstract at ADS Title: Multi-scale observations of thermal non-equilibrium cycles in coronal loops Authors: Froment, C.; Antolin, P.; Henriques, V. M. J.; Kohutova, P.; Rouppe van der Voort, L. H. M. Bibcode: 2020A&A...633A..11F Altcode: 2019arXiv191109710F 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.

Movies associated to Figs. 3-5, and 8 are available at https://www.aanda.org Title: High-resolution spectropolarimetric observations of the temporal evolution of magnetic fields in photospheric bright points Authors: Keys, P. H.; Reid, A.; Mathioudakis, M.; Shelyag, S.; Henriques, V. M. J.; Hewitt, R. L.; Del Moro, D.; Jafarzadeh, S.; Jess, D. B.; Stangalini, M. Bibcode: 2020A&A...633A..60K Altcode: 2019arXiv191108436K Context. Magnetic bright points (MBPs) are dynamic, small-scale magnetic elements often found with field strengths of the order of a kilogauss within intergranular lanes in the photosphere.
Aims: Here we study the evolution of various physical properties inferred from inverting high-resolution full Stokes spectropolarimetry data obtained from ground-based observations of the quiet Sun at disc centre.
Methods: Using automated feature-tracking algorithms, we studied 300 MBPs and analysed their temporal evolution as they evolved to kilogauss field strengths. These properties were inferred using both the NICOLE and SIR Stokes inversion codes. We employ similar techniques to study radiative magnetohydrodynamical simulations for comparison with our observations.
Results: Evidence was found for fast (∼30-100 s) amplification of magnetic field strength (by a factor of 2 on average) in MBPs during their evolution in our observations. Similar evidence for the amplification of fields is seen in our simulated data.
Conclusions: Several reasons for the amplifications were established, namely, strong downflows preceding the amplification (convective collapse), compression due to granular expansion and mergers with neighbouring MBPs. Similar amplification of the fields and interpretations were found in our simulations, as well as amplification due to vorticity. Such a fast amplification will have implications for a wide array of topics related to small-scale fields in the lower atmosphere, particularly with regard to propagating wave phenomena in MBPs. Title: Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines Authors: Bose, Souvik; Henriques, Vasco M. J.; Joshi, Jayant; Rouppe van der Voort, Luc Bibcode: 2019A&A...631L...5B Altcode: 2019arXiv191005533B 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. Title: The magnetic properties of photospheric magnetic bright points with high-resolution spectropolarimetry Authors: Keys, Peter H.; Reid, Aaron; Mathioudakis, Mihalis; Shelyag, Sergiy; Henriques, Vasco M. J.; Hewitt, Rebecca L.; Del Moro, Dario; Jafarzadeh, Shahin; Jess, David B.; Stangalini, Marco Bibcode: 2019MNRAS.488L..53K Altcode: 2019MNRAS.tmpL..98K; 2019MNRAS.tmpL..95K; 2019arXiv190607687K 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. Title: Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes Authors: Bose, Souvik; Henriques, Vasco M. J.; Rouppe van der Voort, Luc; Pereira, Tiago M. D. Bibcode: 2019A&A...627A..46B Altcode: 2019arXiv190508264B 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.

Data of semi-empirical model atmospheres are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A46. Title: VizieR Online Data Catalog: Sunspot penumbra and umbral flashes models (Bose+, 2019) Authors: Bose, S.; Henriques, V. M. J.; Rouppe van der Voort, L.; Pereira, T. M. D. Bibcode: 2019yCat..36270046B Altcode: The tables included in this paper describes the stratification of atmospheric parameters for the cool and the hot penumbra, umbral flash and the quiet surrounding respectively as a function of geometric height and optical depth.

(4 data files). Title: Spectropolarimetric Inversions of the Ca II 8542 Å Line in an M-class Solar Flare Authors: Kuridze, D.; Henriques, V. M. J.; Mathioudakis, M.; Rouppe van der Voort, L.; de la Cruz Rodríguez, J.; Carlsson, M. Bibcode: 2018ApJ...860...10K Altcode: 2018arXiv180500487K 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. Title: Penumbral Waves Driving Solar Fan-shaped Chromospheric Jets Authors: Reid, A.; Henriques, V. M. J.; Mathioudakis, M.; Samanta, T. Bibcode: 2018ApJ...855L..19R Altcode: 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. Title: Penumbral Waves driving Solar chromospheric fan-shaped jets Authors: Reid, A.; Henriques, V. M. J.; Mathioudakis, M.; Samanta, T. Bibcode: 2018arXiv180207537R Altcode: We use H$\alpha$ imaging spectroscopy taken via the Swedish 1-m Solar Telescope (SST) 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\ks, 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 (SDO), indicating a very hot jet front, leaving behind optically thick cool plasma in its wake. Title: Dynamics of internetwork chromospheric fibrils: Basic properties and magnetohydrodynamic kink waves Authors: Mooroogen, K.; Morton, R. J.; Henriques, V. Bibcode: 2017A&A...607A..46M Altcode: 2017arXiv170803500M
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. Title: The formation of small-scale umbral brightenings in sunspot atmospheres Authors: Nelson, C. J.; Henriques, V. M. J.; Mathioudakis, M.; Keenan, F. P. Bibcode: 2017A&A...605A..14N Altcode: 2017arXiv170505617N 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. Title: Spectroscopic Inversions of the Ca II 8542 Å Line in a C-class Solar Flare Authors: Kuridze, D.; Henriques, V.; Mathioudakis, M.; Koza, J.; Zaqarashvili, T. V.; Rybák, J.; Hanslmeier, A.; Keenan, F. P. Bibcode: 2017ApJ...846....9K Altcode: 2017arXiv170800472K We study the C8.4-class solar flare SOL2016-05-14T11:34 UT using high-resolution spectral imaging in the Ca II 8542 Å line obtained with the CRISP imaging spectropolarimeter on the Swedish 1 m Solar Telescope. Spectroscopic inversions of the Ca II 8542 Å line using the non-LTE code NICOLE are used to investigate the evolution of the temperature and velocity structure in the flaring chromosphere. A comparison of the temperature stratification in flaring and non-flaring areas reveals strong footpoint heating during the flare peak in the lower atmosphere. The temperature of the flaring footpoints between {log} {τ }500 ≈ -2.5 {and} -3.5, where τ 500 is the continuum optical depth at 500 nm, is ∼ 5{--}6.5 {kK} close to the flare peak, reducing gradually to ∼ 5 {kK}. The temperature in the middle and upper chromosphere, between {log} {τ }500≈ -3.5 and -5.5, is estimated to be ∼6.5-20 kK, decreasing to preflare temperatures, ∼5-10 kK, after approximately 15 minutes. However, the temperature stratification of the non-flaring areas is unchanged. The inverted velocity fields show that the flaring chromosphere is dominated by weak downflowing condensations at the formation height of Ca II 8542 Å. Title: A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils Authors: Henriques, V. M. J.; Mathioudakis, M.; Socas-Navarro, H.; de la Cruz Rodríguez, J. Bibcode: 2017ApJ...845..102H Altcode: 2017arXiv170605311H 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. Title: Chromospheric Inversions of a Micro-flaring Region Authors: Reid, A.; Henriques, V.; Mathioudakis, M.; Doyle, J. G.; Ray, T. Bibcode: 2017ApJ...845..100R Altcode: 2017arXiv170700029R 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. Title: Kelvin-Helmholtz Instability in Solar Chromospheric Jets: Theory and Observation Authors: Kuridze, D.; Zaqarashvili, T. V.; Henriques, V.; Mathioudakis, M.; Keenan, F. P.; Hanslmeier, A. Bibcode: 2016ApJ...830..133K Altcode: 2016arXiv160801497K Using data obtained by the high-resolution CRisp Imaging SpectroPolarimeter instrument on the Swedish 1 m Solar Telescope, we investigate the dynamics and stability of quiet-Sun chromospheric jets observed at the disk center. Small-scale features, such as rapid redshifted and blueshifted excursions, appearing as high-speed jets in the wings of the Hα line, are characterized by short lifetimes and rapid fading without any descending behavior. To study the theoretical aspects of their stability without considering their formation mechanism, we model chromospheric jets as twisted magnetic flux tubes moving along their axis, and use the ideal linear incompressible magnetohydrodynamic approximation to derive the governing dispersion equation. Analytical solutions of the dispersion equation indicate that this type of jet is unstable to Kelvin-Helmholtz instability (KHI), with a very short (few seconds) instability growth time at high upflow speeds. The generated vortices and unresolved turbulent flows associated with the KHI could be observed as a broadening of chromospheric spectral lines. Analysis of the Hα line profiles shows that the detected structures have enhanced line widths with respect to the background. We also investigate the stability of a larger-scale Hα jet that was ejected along the line of sight. Vortex-like features, rapidly developing around the jet’s boundary, are considered as evidence of the KHI. The analysis of the energy equation in the partially ionized plasma shows that ion-neutral collisions may lead to fast heating of the KH vortices over timescales comparable to the lifetime of chromospheric jets. Title: The Effects of Transients on Photospheric and Chromospheric Power Distributions Authors: Samanta, T.; Henriques, V. M. J.; Banerjee, D.; Krishna Prasad, S.; Mathioudakis, M.; Jess, D.; Pant, V. Bibcode: 2016ApJ...828...23S Altcode: 2016arXiv160406289S We have observed a quiet-Sun region with the Swedish 1 m Solar Telescope equipped with the CRISP Imaging SpectroPolarimeter. High-resolution, high-cadence, Hα line scanning images were taken to observe different layers of the solar atmosphere from the photosphere to upper chromosphere. We study the distribution of power in different period bands at different heights. Power maps of the upper photosphere and the lower chromosphere show suppressed power surrounding the magnetic-network elements, known as “magnetic shadows.” These also show enhanced power close to the photosphere, traditionally referred to as “power halos.” The interaction between acoustic waves and inclined magnetic fields is generally believed to be responsible for these two effects. In this study we explore whether small-scale transients can influence the distribution of power at different heights. We show that the presence of transients, like mottles, Rapid Blueshifted Excursions (RBEs), and Rapid Redshifted Excursions (RREs), can strongly influence the power maps. The short and finite lifetime of these events strongly affects all power maps, potentially influencing the observed power distribution. We show that Doppler-shifted transients like RBEs and RREs that occur ubiquitously can have a dominant effect on the formation of the power halos in the quiet Sun. For magnetic shadows, transients like mottles do not seem to have a significant effect on the power suppression around 3 minutes, and wave interaction may play a key role here. Our high-cadence observations reveal that flows, waves, and shocks manifest in the presence of magnetic fields to form a nonlinear magnetohydrodynamic system. Title: Magnetic Flux Cancellation in Ellerman Bombs Authors: Reid, A.; Mathioudakis, M.; Doyle, J. G.; Scullion, E.; Nelson, C. J.; Henriques, V.; Ray, T. Bibcode: 2016ApJ...823..110R Altcode: 2016arXiv160307100R Ellerman Bombs (EBs) are often found to be co-spatial with bipolar photospheric magnetic fields. We use Hα imaging spectroscopy along with Fe I 6302.5 Å spectropolarimetry from the Swedish 1 m Solar Telescope (SST), combined with data from the Solar Dynamic Observatory, to study EBs and the evolution of the local magnetic fields at EB locations. EBs are found via an EB detection and tracking algorithm. Using NICOLE inversions of the spectropolarimetric data, we find that, on average, (3.43 ± 0.49) × 1024 erg of stored magnetic energy disappears from the bipolar region during EB burning. The inversions also show flux cancellation rates of 1014-1015 Mx s-1 and temperature enhancements of 200 K at the detection footpoints. We investigate the near-simultaneous flaring of EBs due to co-temporal flux emergence from a sunspot, which shows a decrease in transverse velocity when interacting with an existing, stationary area of opposite polarity magnetic flux, resulting in the formation of the EBs. We also show that these EBs can be fueled further by additional, faster moving, negative magnetic flux regions. Title: High-cadence observations of spicular-type events on the Sun Authors: Shetye, J.; Doyle, J. G.; Scullion, E.; Nelson, C. J.; Kuridze, D.; Henriques, V.; Woeger, F.; Ray, T. Bibcode: 2016A&A...589A...3S Altcode: 2016arXiv160108087S Context. Chromospheric observations taken at high-cadence and high-spatial resolution show a range of spicule-like features, including Type-I, Type-II (as well as rapid blue-shifted excursions (RBEs) and rapid red-shifted excursions (RREs) which are thought to be on-disk counterparts of Type-II spicules) and those which seem to appear within a few seconds, which if interpreted as flows would imply mass flow velocities in excess of 1000 km s-1.
Aims: This article seeks to quantify and study rapidly appearing spicular-type events. We also compare the multi-object multi-frame blind deconvolution (MOMFBD) and speckle reconstruction techniques to understand if these spicules are more favourably observed using a particular technique.
Methods: We use spectral imaging observations taken with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m Solar Telescope. Data was sampled at multiple positions within the Hα line profile for both an on-disk and limb location.
Results: The data is host to numerous rapidly appearing features which are observed at different locations within the Hα line profile. The feature's durations vary between 10-20 s and lengths around 3500 km. Sometimes, a time delay in their appearance between the blue and red wings of 3-5 s is evident, whereas, sometimes they are near simultaneous. In some instances, features are observed to fade and then re-emerge at the same location several tens of seconds later.
Conclusions: We provide the first statistical analysis of these spicules and suggest that these observations can be interpreted as the line-of-sight (LOS) movement of highly dynamic spicules moving in and out of the narrow 60 mÅ transmission filter that is used to observe in different parts of the Hα line profile. The LOS velocity component of the observed fast chromospheric features, manifested as Doppler shifts, are responsible for their appearance in the red and blue wings of Hα line. Additional work involving data at other wavelengths is required to investigate the nature of their possible wave-like activity. Title: Quiet-Sun Hα Transients and Corresponding Small-scale Transition Region and Coronal Heating Authors: Henriques, V. M. J.; Kuridze, D.; Mathioudakis, M.; Keenan, F. P. Bibcode: 2016ApJ...820..124H Altcode: 2016arXiv160204820H Rapid blue- and redshifted excursions (RBEs and RREs) are likely to be the on-disk counterparts of Type II spicules. Recently, heating signatures from RBEs/RREs have been detected in IRIS slit-jaw images dominated by transition region (TR) lines around network patches. Additionally, signatures of Type II spicules have been observed in Atmospheric Imaging Assembly (AIA) diagnostics. The full-disk, ever-present nature of the AIA diagnostics should provide us with sufficient statistics to directly determine how important RBEs and RREs are to the heating of the TR and corona. We find, with high statistical significance, that at least 11% of the low coronal brightenings detected in a quiet-Sun region in He II 304 Å can be attributed to either RBEs or RREs as observed in Hα, and a 6% match of Fe IX 171 Å detected events to RBEs or RREs with very similar statistics for both types of Hα features. We took a statistical approach that allows for noisy detections in the coronal channels and provides us with a lower, but statistical significant, bound. Further, we consider matches based on overlapping features in both time and space, and find strong visual indications of further correspondence between coronal events and co-evolving but non-overlapping, RBEs and RREs. Title: The Dynamics of Rapid Redshifted and Blueshifted Excursions in the Solar Hα Line Authors: Kuridze, D.; Henriques, V.; Mathioudakis, M.; Erdélyi, R.; Zaqarashvili, T. V.; Shelyag, S.; Keys, P. H.; Keenan, F. P. Bibcode: 2015ApJ...802...26K Altcode: 2015arXiv150106205K We analyze high temporal and spatial resolution time-series of spectral scans of the Hα line obtained with the CRisp Imaging SpectroPolarimeter instrument mounted on the Swedish Solar Telescope. The data reveal highly dynamic, dark, short-lived structures known as Rapid Redshifted and Blueshifted Excursions (RREs, RBEs) that are on-disk absorption features observed in the red and blue wings of spectral lines formed in the chromosphere. We study the dynamics of RREs and RBEs by tracking their evolution in space and time, measuring the speed of the apparent motion, line of sight (LOS) Doppler velocity, and transverse velocity of individual structures. A statistical study of their measured properties shows that RREs and RBEs have similar occurrence rates, lifetimes, lengths, and widths. They also display non-periodic, nonlinear transverse motions perpendicular to their axes at speeds of 4-31 km s-1. Furthermore, both types of structures either appear as high speed jets and blobs that are directed outwardly from a magnetic bright point with speeds of 50-150 km s-1, or emerge within a few seconds. A study of the different velocity components suggests that the transverse motions along the LOS of the chromospheric flux tubes are responsible for the formation and appearance of these redshifted/blueshifted structures. The short lifetime and fast disappearance of the RREs/RBEs suggests that, similar to type II spicules, they are rapidly heated to transition region or even coronal temperatures. We speculate that the Kelvin-Helmholtz instability triggered by observed transverse motions of these structures may be a viable mechanism for their heating. Title: Stable umbral chromospheric structures Authors: Henriques, V. M. J.; Scullion, E.; Mathioudakis, M.; Kiselman, D.; Gallagher, P. T.; Keenan, F. P. Bibcode: 2015A&A...574A.131H Altcode: 2014arXiv1412.6100H
Aims: We seek to understand the morphology of the chromosphere in sunspot umbra. We investigate if the horizontal structures observed in the spectral core of the Ca II H line are ephemeral visuals caused by the shock dynamics of more stable structures, and examine their relationship with observables in the H-alpha line.
Methods: Filtergrams in the core of the Ca II H and H-alpha lines as observed with the Swedish 1-m Solar Telescope are employed. We utilise a technique that creates composite images and tracks the flash propagation horizontally.
Results: We find 0.̋15 wide horizontal structures, in all of the three target sunspots, for every flash where the seeing is moderate to good. Discrete dark structures are identified that are stable for at least two umbral flashes, as well as systems of structures that live for up to 24 min. We find cases of extremely extended structures with similar stability, with one such structure showing an extent of 5''. Some of these structures have a correspondence in H-alpha, but we were unable to find a one-to-one correspondence for every occurrence. If the dark streaks are formed at the same heights as umbral flashes, there are systems of structures with strong departures from the vertical for all three analysed sunspots.
Conclusions: Long-lived Ca II H filamentary horizontal structures are a common and likely ever-present feature in the umbra of sunspots. If the magnetic field in the chromosphere of the umbra is indeed aligned with the structures, then the present theoretical understanding of the typical umbra needs to be revisited.

Movies associated to Figs. 3 and 4 are available in electronic form at http://www.aanda.org Title: Ca II H sunspot tomography from the photosphere to the chromosphere Authors: Henriques, V. M. J.; Kiselman, D. Bibcode: 2013A&A...557A...5H Altcode:
Aims: We aim at gaining insight into the thermal properties of different small-scale structures related to sunspots.
Methods: We use filtergrams in the Ca ii H filter at the Swedish 1-m Solar Telescope to study the relationship between fine structure at different heights in a sunspot.
Results: The methods for destretching and aligning the different image data work well. The magnetic spine structure in the outer parts of the sunspot penumbra is found to be associated with higher intensities in the Ca ii H wing passbands but with less steep vertical temperature gradients. Dark lanes in a light bridge behave very similarly to dark cores in penumbral filaments. Fibril structures are seen in the line-core images over the umbra and penumbra.
Conclusions: The observations add support to the idea that penumbral filaments, light bridges, and umbral dots are caused by similar processes of overturning convection. Observations in the Ca ii H & K wings are a promising observable, complementing others, for testing simulation results for sunspots at high spatial resolution. Title: Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra Authors: Scharmer, G. B.; de la Cruz Rodriguez, J.; Sütterlin, P.; Henriques, V. M. J. Bibcode: 2013A&A...553A..63S Altcode: 2012arXiv1211.5776S We discuss NICOLE inversions of Fe i 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15 arcsec. We report on narrow, radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km s-1. The locations of these downflows close to the spines agree with predictions from the convective gap model (the "gappy penumbra") proposed six years ago, and more recent three-dimensional magnetohydrodynamic simulations. We also confirm the existence of strong convective flows throughout the entire penumbra, showing the expected correlation between temperature and vertical velocity, and having vertical root mean square velocities of about 1.2 km s-1. Title: Three-dimensional mapping of fine structure in the solar atmosphere Authors: Henriques, Vasco M. J. Bibcode: 2013PhDT.........2H Altcode: The effects on image formation through a tilted interference filter in a converging beam are investigated and an adequate compensation procedure is established. A method that compensates for small-scale seeing distortions is also developed with the aim of co-aligning non-simultaneous solar images from different passbands. These techniques are applied to data acquired with a narrow tiltable filter at the Swedish 1-meter Solar Telescope. Tilting provides a way to scan the wing of the Ca II H line. The resulting images are used to map the temperature stratification and vertical temperature gradients in a solar active region containing a sunspot at a resolution approaching 0''10. The data are compared with hydro-dynamical quiet sun models and magneto-hydrodynamic models of plage. The comparison gives credence to the observational techniques, the analysis methods, and the simulations. Vertical temperature gradients are lower in magnetic structures than in non-magnetic.

Line-of-sight velocities and magnetic field properties in the penumbra of the same sunspot are estimated using the CRISP imaging spectropolarimeter and straylight compensation adequate for the data. These reveal a pattern of upflows and downflows throughout the entire penumbra including the interior penumbra. A correlation with intensity positively identifies these flows as convective in origin. The vertical convective signatures are observed everywhere, but the horizontal Evershed flow is observed to be confined to areas of nearly horizontal magnetic field.

The relation between temperature gradient and total circular polarization in magnetically sensitive lines is investigated in different structures of the penumbra. Penumbral dark cores are prominent in total circular polarization and temperature gradient maps. These become longer and more contiguous with increasing height. Dark fibril structures over bright regions are observed in the Ca II H line core, above both the umbra and penumbra. Title: Three-dimensional mapping of fine structure in the solar atmosphere Authors: Henriques, Vasco Manuel de Jorge Bibcode: 2013PhDT.......405H Altcode: No abstract at ADS Title: Three-dimensional temperature mapping of solar photospheric fine structure using Ca ii H filtergrams Authors: Henriques, V. M. J. Bibcode: 2012A&A...548A.114H Altcode: 2012arXiv1210.4168H Context. The wings of the Ca ii H and K lines provide excellent photospheric temperature diagnostics. At the Swedish 1-m Solar Telescope (SST), the blue wing of Ca ii H is scanned with a narrowband interference filter mounted on a rotation stage. This provides up to 0farcs10 spatial resolution filtergrams at high cadence that are concurrent with other diagnostics at longer wavelengths.
Aims: The aim is to develop observational techniques that provide photospheric temperature stratification at the highest spatial resolution possible and use them to compare simulations and observations at different heights.
Methods: We use filtergrams in the Ca ii H blue wing that were obtained with a tiltable interference filter at the SST. Synthetic observations are produced from three-dimensional (3D) hydro and magneto-hydrodynamic numerical simulations and degraded to match the observations. The temperature structure obtained from applying the method to the synthetic data is compared with the known structure in the simulated atmospheres and with observations of an active region. Cross-correlation techniques using restored non-simultaneous continuum images are used to reduce high-altitude, small-scale seeing signal introduced from the non-simultaneity of the frames when differentiating data.
Results: Temperature extraction using high-resolution filtergrams in the Ca ii H blue wing works reasonably well when tested with simulated 3D atmospheres. The cross-correlation technique successfully compensates for the problem of small-scale seeing differences and provides a measure of the spurious signal from this source in differentiated data. Synthesized data from the simulated atmospheres (including pores) match well the observations morphologically at different observed heights and in vertical temperature gradients. Title: SST/CRISP observations of convective flows in a sunspot penumbra Authors: Scharmer, G. B.; Henriques, V. M. J. Bibcode: 2012A&A...540A..19S Altcode: 2011arXiv1109.1301S Context. Recent discoveries of intensity correlated downflows in the interior of a sunspot penumbra provide direct evidence for overturning convection, adding to earlier strong indications of convection from filament dynamics observed far from solar disk center, and supporting recent simulations of sunspots.
Aims: Using spectropolarimetric observations obtained at a spatial resolution approaching 0.1 arcsec with the Swedish 1-m Solar Telescope (SST) and its spectropolarimeter CRISP, we investigate whether the convective downflows recently discovered in the C i line at 538.03 nm can also be detected in the wings of the Fe i line at 630.15 nm.
Methods: We make azimuthal fits of the measured LOS velocities in the core and wings of the 538 nm and 630 nm lines to disentangle the vertical and horizontal flows. To investigate how these depend on the continuum intensity, the azimuthal fits are made separately for each intensity bin. By using spatially high-pass filtered measurements of the LOS component of the magnetic field, the flow properties are determined separately for magnetic spines (relatively strong and vertical field) and inter-spines (weaker and more horizontal field).
Results: The dark convective downflows discovered recently in the 538.03 nm line are evident also in the 630.15 nm line, and have similar strength. This convective signature is the same in spines and inter-spines. However, the strong radial (Evershed) outflows are found only in the inter-spines.
Conclusions: At the spatial resolution of the present SST/CRISP data, the small-scale intensity pattern seen in continuum images is strongly related to a convective up/down flow pattern that exists everywhere in the penumbra. Earlier failures to detect the dark convective downflows in the interior penumbra can be explained by inadequate spatial resolution in the observed data. Title: A tilted interference filter in a converging beam Authors: Löfdahl, M. G.; Henriques, V. M. J.; Kiselman, D. Bibcode: 2011A&A...533A..82L Altcode: 2011arXiv1108.1234L Context. Narrow-band interference filters can be tuned toward shorter wavelengths by tilting them from the perpendicular to the optical axis. This can be used as a cheap alternative to real tunable filters, such as Fabry-Pérot interferometers and Lyot filters. At the Swedish 1-meter Solar Telescope, such a setup is used to scan through the blue wing of the Ca ii H line. Because the filter is mounted in a converging beam, the incident angle varies over the pupil, which causes a variation of the transmission over the pupil, different for each wavelength within the passband. This causes broadening of the filter transmission profile and degradation of the image quality.
Aims: We want to characterize the properties of our filter, at normal incidence as well as at different tilt angles. Knowing the broadened profile is important for the interpretation of the solar images. Compensating the images for the degrading effects will improve the resolution and remove one source of image contrast degradation. In particular, we need to solve the latter problem for images that are also compensated for blurring caused by atmospheric turbulence.
Methods: We simulate the process of image formation through a tilted interference filter in order to understand the effects. We test the hypothesis that they are separable from the effects of wavefront aberrations for the purpose of image deconvolution. We measure the filter transmission profile and the degrading PSF from calibration data.
Results: We find that the filter transmission profile differs significantly from the specifications. We demonstrate how to compensate for the image-degrading effects. Because the filter tilt effects indeed appear to be separable from wavefront aberrations in a useful way, this can be done in a final deconvolution, after standard image restoration with Multi-Frame Blind Deconvolution/Phase Diversity based methods. We illustrate the technique with real data. Title: Detection of Convective Downflows in a Sunspot Penumbra Authors: Scharmer, G. B.; Henriques, V. M. J.; Kiselman, D.; de la Cruz Rodríguez, J. Bibcode: 2011Sci...333..316S Altcode: The fine structure and dynamics of sunspots and the strong outflow in their outer filamentary part—the penumbra—have puzzled astronomers for more than a century. Recent theoretical models and three-dimensional numerical simulations explain the penumbral filaments and their radiative energy output as the result of overturning convection. Here, we describe the detection of ubiquitous, relatively dark downward flows of up to 1 kilometer per second (km/s) in the interior penumbra, using imaging spectropolarimetric data from the Swedish 1-meter Solar Telescope. The dark downflows are omnipresent in the interior penumbra, distinguishing them from flows in arched flux tubes, and are associated with strong (3 to 3.5 km/s) radial outflows. They are thus part of a penumbral convective flow pattern, with the Evershed flow representing the horizontal component of that convection. Title: Photospheric Temperatures from Ca IIH Authors: Henriques, V. M. J.; Kiselman, D.; van Noort, M. Bibcode: 2010ASSP...19..511H Altcode: 2010mcia.conf..511H The temperature stratification in the upper photosphere can be extracted from Ca II H&K spectrograms following Shine and Linsky (1974) by assuming LTE, the Eddington-Barbier approximation, hydrostatic equilibrium, and that Ca II is mostly in the ground state. Rouppe van der Voort (2002) confirmed that these assumptions were solid for a wide range in the Ca IIK wings and further developed the method including forward computation using MULTI (Carlsson 1986). Title: Temperature stratification in the Sun's photosphere in high horizontal resolution using Ca II H filtergrams. Authors: Henriques, V. M. J.; Kiselman, D. Bibcode: 2009MmSAI..80..639H Altcode: A method to extract the temperature stratification in the Sun's photosphere using filtergrams is presented along with some high resolution results. The data was acquired with the Swedish 1-m Solar Telescope (SST) using a tunable filter in the Ca II H blue wing. Each full scan is completed in the order of seconds thus allowing for the full resolution of the SST and reasonable depth sampling to be obtained simultaneously in a shorter time than that of the evolution time scale of the photosphere. We test the quality of the method by applying it to a set of synthetic images (obtained through radiative transfer on 3D HD and MHD simulation snapshots followed by degradation) and comparing the output with the known 3D simulated atmosphere. Fine structure around bright points becomes evident in both the temperature gradient maps computed from a set of test observations and synthetic images obtained from MHD simulations. Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.; Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz Rodríguez, J. Bibcode: 2008ESPM...12..2.5S Altcode: We present first observations of sunspots with the imaging spectropolarimeter CRISP, recently installed at the Swedish 1-m Solar Telescope (SST) on La Palma. This spectropolarimeter is based on a high-fidelity dual Fabry-Perot filter system.

Two liquid crystals and a polarizing beam splitter are used to reduce seeing induced I,Q,U,V crosstalk by simultaneously recording images with two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously records broadband images through the pre-filter of the FPI filter system, allowing image reconstruction and co-alignment of images of different polarization states and at different wavelengths in Zeeman sensitive spectral lines.

The first data, recorded in April 2008, demonstrate the capability of this system to record high cadence, high S/N polarimetric data with a spatial resolution at or close to the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss the analysis of first spectropolarimetric data for sunspots, based on Milne-Eddington inversion techniques. Title: Temperature structure from Ca II H wing inversions Authors: Henriques, V.; van Noort Kiselman, M. D. Bibcode: 2008ESPM...12.2.76H Altcode: A method in development to extract temperature at different depths from the Ca II H&K lines using filtergrams is presented along with preliminary inversion results. The inversions give information up to a height of 200-300 km.

We make use of the Swedish 1-m Solar Telescope (SST) blue filter setup which allows for simultaneous observations in four filter positions including a 1-Å tunable filter that scans through the Ca H blue wing.