Author name code: rouppevandervoort ADS astronomy entries on 2022-09-14 =author:"Rouppe van der Voort, Luc H.M." OR =author:"Rouppe van der Voort, Luc" OR =author:"Rouppe van der Voort, L.H.M." OR =author:"Rouppe van der Voort, L." OR author:"van der Voort, L.R." ------------------------------------------------------------------------ Title: Properties of ubiquitous magnetic reconnection events in the lower solar atmosphere Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M. Bibcode: 2022A&A...664A..72J Altcode: 2022arXiv220308172J Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high-quality Hβ observations indicate that Ellerman bombs are more common than previously thought, and it was estimated that at any time, about half a million Ellerman bombs are present in the quiet Sun.
Aims: We performed an extensive statistical characterization of the quiet-Sun Ellerman bombs (QSEBs) in these new Hβ observations.
Methods: We analyzed a 1 h dataset of the quiet Sun observed with the Swedish 1-m Solar Telescope that consists of spectral imaging in the Hβ and Hα lines as well as spectropolarimetric imaging in Fe I 6173 Å. We used the k-means clustering and the 3D connected component labeling techniques to automatically detect QSEBs.
Results: We detected a total of 2809 QSEBs. The lifetimes vary between 9 s and 20.5 min, with a median of 1.14 min. The maximum area ranges between 0.0016 and 0.2603 Mm2, with a median of 0.018 Mm2. The maximum brightness in the Hβ wing varies between 1.06 and 2.76 with respect to the average wing intensity. A subset (14%) of the QSEBs displays enhancement of the Hβ line core. On average, the line core brightening appears 0.88 min after the onset of brightening in the wings, and the distance between these brightenings is 243 km. This gives rise to an apparent propagation speed ranging between −14.3 and +23.5 km s−1, with an average that is upward propagating at +4.4 km s−1. The average orientation is nearly parallel to the limbward direction. QSEBs are nearly uniformly distributed over the field of view, but we find empty areas with the size of mesogranulation. QSEBs are located more frequently near the magnetic network, where they are often larger, live longer, and are brighter.
Conclusions: We conclude that QSEBs are ubiquitous in the quiet Sun and appear everywhere, except in areas of mesogranular size with the weakest magnetic fields (BLOS ≲ 50 G). Our observations support the interpretation of reconnection along vertically extended current sheets.

Movies associated to Figs. 1 and 3 are available at https://www.aanda.org Title: A textbook example of magnetic flux emergence leading to EBs, UV bursts, surges and EUV signatures Authors: Cabello, Iballa; Moreno-Insertis, . Fernando, , Prof; Rouppe van der Voort, Luc; Bose, Souvik; Nóbrega Siverio, Daniel Bibcode: 2022cosp...44.2531C Altcode: Small-scale eruptive phenomena (like Ellerman bombs (EBs), UV bursts, surges) constitute both a true challenge and an opportunity for progress in understanding the solar atmosphere since they involve very different layers from the photosphere to the low corona. In our work, we are aiming to characterize small-scale eruptive phenomena related to emerging flux regions. In particular, we use coordinated observations from the Swedish $1-$m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO, both HMI and AIA) to analyze an episode of magnetic flux emergence in an enhanced network that leads to an EB, a UV burst, a cool surge, and coronal signatures in the EUV. Through Milne-Eddington inversions of the {\ion{Fe}{I}} 6302 \AA\ line observed with SST/CRISP we obtain high-resolution (0.057"/pixel) magnetograms that allow us to reliably measure the magnetic field at the photosphere. A comparison with the corresponding SDO/HMI magnetograms reveals that this type of small-scale events are barely discernible in low-resolution (1") observations. During the emergence, a roundish dark bubble is visible in {\ion{Ca}{II} K} 3933 \AA\ at the location where the two opposite polarities of the emerging dipole are splitting apart. Several minutes later, indirect evidence of reconnection is found above the positive polarity of the dipole through the appearance of an EB in the wings of the {H$\alpha$} 6563 \AA\ and {\ion{Ca}{II} K} 3933 \AA\ lines from SST, and also in the SDO/AIA 1600 and 1700 \AA~data. Later, a surge shows up as an elongated structure visible in absorption in {H$\alpha$} and {\ion{Ca}{II} K}, extending over 12 Mm projected size on the disk. The shape of the surge is also apparent as an absorption feature in the SDO/AIA channels. Simultaneously with the surge (and at the location where the EB had appeared earlier on) a UV burst is clearly discernible as a strong and bright emission feature both in IRIS/SJI 1400 and 2796 \AA. Interestingly, this UV burst also has counterpart in SDO/AIA 94, 171, 193, 211, 304, and 335 \AA, meaning that we can find multi-thermal plasma up to a few MK in the reconnection site. This observation clearly shows the impact of the emergence of new magnetic field from the photosphere through the chromosphere and transition region and up into the corona. In addition, it provides an illustrative case to test new realistic simulations. Title: Quantifying Properties of Photospheric Magnetic Cancellations in the Quiet Sun Internetwork Authors: Ledvina, Vincent E.; Kazachenko, Maria D.; Criscuoli, Serena; Tilipman, Dennis; Ermolli, Ilaria; Falco, Mariachiara; Guglielmino, Salvatore; Jafarzadeh, Shahin; van der Voort, Luc Rouppe; Zuccarello, Francesca Bibcode: 2022ApJ...934...38L Altcode: 2022arXiv220604644L We analyzed spectropolarimetric data from the Swedish 1 m Solar Telescope to investigate the physical properties of small-scale magnetic cancellations in the quiet Sun photosphere. Specifically, we looked at the full Stokes polarization profiles along the Fe I 557.6 nm and of the Fe I 630.1 nm lines measured by the CRisp Imaging SpectroPolarimeter to study the temporal evolution of the line-of-sight magnetic field during 42.5 minutes of quiet Sun evolution. From this magnetogram sequence, we visually identified 38 cancellation events. We then used the Yet Another Feature Tracking Algorithm to characterize the physical properties of these magnetic cancellations. We found on average 1.6 × 1016 Mx of magnetic flux canceled in each event with an average cancellation rate of 3.8 × 1014 Mx s-1. The derived canceled flux is associated with strong downflows, with an average speed of V LOS ≍ 1.1 km s-1. Our results show that the average lifetime of each event is 9.2 minutes with an average of 44.8% of initial magnetic flux being canceled. Our estimates of magnetic fluxes provide a lower limit since studied magnetic cancellation events have magnetic field values that are very close to the instrument noise level. We observed no horizontal magnetic fields at the cancellation sites and therefore cannot conclude whether the events are associated with structures that could cause magnetic reconnection. Title: On the relationship between spicules and coronal bright points Authors: Bose, Souvik; De Pontieu, Bart; Rouppe van der Voort, Luc; Nóbrega Siverio, Daniel Bibcode: 2022cosp...44.2522B Altcode: Coronal bright points (CBPs) are a set of small-scale, lower coronal loop systems connecting opposite magnetic polarities and are primarily characterized by enhanced emission in the extreme ultraviolet (EUV) wavelengths and X-rays. Being ubiquitous they are thought to play a definite role in heating the solar corona. This study aims to explore the chromospheric components associated with a CBP by focusing on spicules and small-scaled flux emergence. We used high-resolution observations in H$\beta$ and Fe I 617.3 nm spectral lines obtained from the Swedish 1-m Solar Telescope (SST) in coordination with the images acquired from the Atmospheric Imaging Assembly (AIA) instrument on-board the Solar Dynamics Observatory (SDO). On-disk spicules were automatically detected by employing advanced image processing techniques on the Dopplergrams derived from H$\beta$, and Mile-Eddington inversions of the Fe I 617.3 nm line provided the photospheric vector magnetic field. The AIA co-observations were co-aligned to SST with the latter serving as a reference. We find abundant occurrences of chromospheric spicules close to the "footpoints" of the CBP. The orientation of the spicules is predominantly aligned along with CBP loops which further indicates that they form a fundamental part of the same magnetic structure. Several examples of the spatio-temporal evolution indicate that much of the chromospheric plasma is heated to coronal temperatures implying that spicules potentially supply mass and energy to the CBP loops. Furthermore, we study chromospheric and corresponding coronal responses to two magnetic flux emergence events and their impact on the dynamics of the CBP. This study presents unique and unambiguous evidence that connects chromospheric spicular dynamics and flux emergence with a CBP for the very first time using high-resolution observations. Title: Chromospheric emission from nanoflare heating in RADYN simulations Authors: Bakke, H.; Carlsson, M.; Rouppe van der Voort, L.; Gudiksen, B. V.; Polito, V.; Testa, P.; De Pontieu, B. Bibcode: 2022A&A...659A.186B Altcode: 2022arXiv220111961B Context. Heating signatures from small-scale magnetic reconnection events in the solar atmosphere have proven to be difficult to detect through observations. Numerical models that reproduce flaring conditions are essential in understanding how nanoflares may act as a heating mechanism of the corona.
Aims: We study the effects of non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN simulations of nanoflare heated loops to investigate the diagnostic potential of chromospheric emission from small-scale events.
Methods: The Mg II h and k, Ca II H and K, Ca II 854.2 nm, and Hα and Hβ chromospheric lines were synthesised from various RADYN models of coronal loops subject to electron beams of nanoflare energies. The contribution function to the line intensity was computed to better understand how the atmospheric response to the non-thermal electrons affects the formation of spectral lines and the detailed shape of their spectral profiles.
Results: The spectral line signatures arising from the electron beams highly depend on the density of the loop and the lower cutoff energy of the electrons. Low-energy (5 keV) electrons deposit their energy in the corona and transition region, producing strong plasma flows that cause both redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons deposit their energy in the lower transition region and chromosphere, resulting in increased emission from local heating. Our results indicate that effects from small-scale events can be observed with ground-based telescopes, expanding the list of possible diagnostics for the presence and properties of nanoflares. 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: Penumbral decay observed in active region NOAA 12585 Authors: Murabito, M.; Guglielmino, S. L.; Ermolli, I.; Romano, P.; Jafarzadeh, S.; Rouppe van der Voort, L. H. M. Bibcode: 2021A&A...653A..93M Altcode: 2021arXiv210604936M Context. The physical conditions leading the sunspot penumbra decay are poorly understood so far.
Aims: We investigate the photospheric magnetic and velocity properties of a sunspot penumbra during the decay phase to advance the current knowledge of the conditions leading to this process.
Methods: A penumbral decay was observed with the CRISP instrument at the Swedish 1 m Solar Telescope on 2016 September 4 and 5 in the active region NOAA 12585. During these days, full-Stokes spectropolarimetric scans along the Fe I 630 nm line pair were acquired over more than one hour. We inverted these observations with the VFISV code to obtain the evolution of the magnetic and velocity properties. We complement the study with data from instruments on board the Solar Dynamics Observatory and Hinode space missions.
Results: The studied penumbra disappears progressively in time and space. The magnetic flux evolution seems to be linked to the presence of moving magnetic features (MMFs). Decreasing Stokes V signals are observed. Evershed flows and horizontal fields were detected even after the disappearance of the penumbral sector.
Conclusions: The analyzed penumbral decay seems to result from the interaction between opposite polarity fields in type III MMFs and penumbra, while the presence of overlying canopies regulates the evolution in the different penumbral sectors.

Movies associated with Fig. 6 are available at https://www.aanda.org 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: High Resolution Observations of the Low Atmospheric Response to Small Heating Events in Active Regions Authors: Testa, P.; Rouppe van der Voort, L.; De Pontieu, B.; Bakke, H. Bibcode: 2020AGUFMSH004..03T Altcode: We investigate the low atmospheric response to small heating events (nano/micro-flares) by analyzing coordinated active region observations with IRIS and SST, and also using the simultaneous SDO/AIA observations to study the coronal emission. The events we observe as intense brightenings in the chromosphere and transition region are clearly associated with heating of the overlying loops to high temperatures (5-10MK), as is clear from the strong Fe XVIII emission observed in the 94A SDO/AIA passband. Some of the chromospheric brightenings have been observed with the SST with the CRISP instrument in Ca II 8542 and H-alpha, as well as with the new CHROMIS instrument in Ca II K, which provides unprecedented resolution. We will present the rich spectral diagnostics offered by IRIS and SST for these events, including insights we obtained by applying a k-means clustering analysis to the SST and IRIS spectra during the heating events. Our observations will help provide more stringent constraints on the properties of non-thermal particles in nanoflares and microflares. 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: Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M.; de la Cruz Rodríguez, Jaime Bibcode: 2020A&A...641L...5J Altcode: 2020arXiv200614975J Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the lower solar atmosphere. We analyze high spatial resolution observations (0.″1) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the field of view for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier Hα observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightenings found in the Hβ line wings also persist in the line core with a temporal delay and spatial offset toward the nearest solar limb. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the lower solar atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.

Movies associated to Figs. 1-3, B1, and B2 are available at https://www.aanda.org 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: A multi-diagnostic spectral analysis of penumbral microjets Authors: Drews, Ainar; Rouppe van der Voort, Luc Bibcode: 2020A&A...638A..63D Altcode: 2020arXiv200502608D Context. Penumbral microjets (PMJs) are short-lived, jet-like objects found in the penumbra of sunspots. They were first discovered in chromospheric lines and have later also been shown to exhibit signals in transition region (TR) lines. Their origin and manner of evolution is not yet settled.
Aims: We perform a comprehensive analysis of PMJs through the use of spectral diagnostics that span from photospheric to TR temperatures to constrain PMJ properties. Methods We employed high-spatial-resolution Swedish 1-m Solar Telescope observations in the Ca II 8542 Å and H α lines, IRIS slit-jaw images, and IRIS spectral observations in the Mg II h & k lines, the Mg II 2798.75 Å & 2798.82 Å triplet blend, the C II 1334 Å & 1335 Å lines, and the Si IV 1394 Å & 1403 Å lines. We derived a wide range of spectral diagnostics from these and investigated other secondary phenomena associated with PMJs.
Results: We find that PMJs exhibit varying degrees of signal in all of our studied spectral lines. We find low or negligible Doppler velocities and velocity gradients throughout our diagnostics and all layers of the solar atmosphere associated with these. Dark features in the inner wings of H α and Ca II 8542 Å imply that PMJs form along pre-existing fibril structures. We find evidence for upper photospheric heating in a subset of PMJs through emission in the wings of the Mg II triplet lines. There is little evidence for ubiquitous twisting motion in PMJs. There is no marked difference in onset-times for PMJ brightenings in different spectral lines.
Conclusions: PMJs most likely exhibit only very modest mass-motions, contrary to earlier suggestions. We posit that PMJs form at upper photospheric or chromospheric heights at pre-existing fibril structures.

Movies associated to Appendix B are available at https://www.aanda.org 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: Ellerman bombs and UV bursts: reconnection at different atmospheric layers Authors: Ortiz, Ada; Hansteen, Viggo H.; Nóbrega-Siverio, Daniel; Rouppe van der Voort, Luc Bibcode: 2020A&A...633A..58O Altcode: 2019arXiv191010736O The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst other dynamical phenomena, Ellerman bombs (EBs), which are observed in the wings of Hα and are due to magnetic reconnection in the photosphere below the chromospheric canopy. Signs of magnetic reconnection are also observed in other spectral lines, typical of the chromosphere or the transition region. An example are the ultraviolet (UV) bursts observed in the transition region lines of Si IV and the upper chromospheric lines of Mg II. In this work we analyze high-cadence, high-resolution coordinated observations between the Swedish 1m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα images from the SST provide us with the positions, timings, and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 133 (C II, transition region), 140 (Si IV, transition region), and 279.6 (Mg II k, core, upper chromosphere) nm as well as spectroscopy in the far- and near-ultraviolet from the fast spectrograph raster allow us to study the possible chromospheric and transition region counterparts of those EBs. Our main goal is to study the possible temporal and spatial relationship between several reconnection events at different layers in the atmosphere (namely EBs and UV bursts), the timing history between them, and the connection of these dynamical phenomena to the ejection of surges in the chromosphere. We also investigate the properties of an extended UV burst and their variations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. These results support and expand those obtained from recent numerical simulations of magnetic flux emergence.

The movies are available at https://www.aanda.org 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: Solar Hα features with hot onsets. IV. Network fibrils Authors: Rutten, Robert J.; Rouppe van der Voort, Luc H. M.; De Pontieu, Bart Bibcode: 2019A&A...632A..96R Altcode: 2019arXiv190809315R Even in quiet areas underneath coronal holes the solar chromosphere contains ubiquitous heating events. They tend to be small scale and short lived, hence difficult to identify. Here we do not address their much-debated contribution to outer-atmosphere heating, but their aftermaths. We performed a statistical analysis of high-resolution observations in the Balmer Hα line to suggest that many slender dark Hα fibrils spreading out from network represent cooling gas that outlines tracks of preceding rapid type II spicule events or smaller similar but as yet unresolved heating agents in which the main gas constituent, hydrogen, ionizes at least partially. Subsequent recombination then causes dark Hα fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the Hα chromosphere around network results from intermittent, frequent small-scale prior heating.

Movies associated to Fig. 3 and blinkers are available at https://www.aanda.org Title: Science Requirement Document (SRD) for the European Solar Telescope (EST) (2nd edition, December 2019) Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.; Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.; Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse, K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.; Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller, T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno, J.; Utz, D.; Zuccarello, F. Bibcode: 2019arXiv191208650S Altcode: The European Solar Telescope (EST) is a research infrastructure for solar physics. It is planned to be an on-axis solar telescope with an aperture of 4 m and equipped with an innovative suite of spectro-polarimetric and imaging post-focus instrumentation. The EST project was initiated and is driven by EAST, the European Association for Solar Telescopes. EAST was founded in 2006 as an association of 14 European countries. Today, as of December 2019, EAST consists of 26 European research institutes from 18 European countries. The Preliminary Design Phase of EST was accomplished between 2008 and 2011. During this phase, in 2010, the first version of the EST Science Requirement Document (SRD) was published. After EST became a project on the ESFRI roadmap 2016, the preparatory phase started. The goal of the preparatory phase is to accomplish a final design for the telescope and the legal governance structure of EST. A major milestone on this path is to revisit and update the Science Requirement Document (SRD). The EST Science Advisory Group (SAG) has been constituted by EAST and the Board of the PRE-EST EU project in November 2017 and has been charged with the task of providing with a final statement on the science requirements for EST. Based on the conceptual design, the SRD update takes into account recent technical and scientific developments, to ensure that EST provides significant advancement beyond the current state-of-the-art. The present update of the EST SRD has been developed and discussed during a series of EST SAG meetings. The SRD develops the top-level science objectives of EST into individual science cases. Identifying critical science requirements is one of its main goals. Those requirements will define the capabilities of EST and the post-focus instrument suite. The technical requirements for the final design of EST will be derived from the SRD. Title: Ellerman bombs and UV bursts: reconnection at different atmospheric layers Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Nobrega, D. E.; Rouppe van der Voort, L. Bibcode: 2019AGUFMSH13B..06H Altcode: The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst many other dynamical phenomena, the appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed in the wings of H(alpha) and are highly likely to be due to reconnection in the photosphere, below the chromospheric canopy. Signs of the reconnection process are also observed in several other spectral lines, typical of the chromosphere or the transition region. An example are the UV bursts observed in the transition region lines of Si IV and the upper chromospheric lines of Mg II. In this work we analyze high cadence, high resolution coordinated observations between the Swedish 1-m Solar Telescope (SST) and the Interface Region Ima ging Spectrograph (IRIS) spacecraft in order to study the possible relationship between reconnection events at different layers in the atmosphere and, in particular, the timing history between them. H(alpha) images from the SST provide us with the positions, timings and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 1330 (C II, transition region), 1400 (Si IV, transition region) and 2796 (Mg II k, core, upper chromosphere) Ã…, as well as spectroscopy in the far and near ultraviolet from the fast spectrograph raster, allow us to study the possible chromospheric/transition region counterparts of those photospheric EBs. Our main goal is to study whether there is a temporal and spatial relationship between the appearance of an EB and the appearance of a UV burst and the connection of these dynamical phenomena to the appearance of surges in the chromosphere. We also investigate in detail the properties of an extended UV burst and their v ariations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. This scenario is in agreement with the most recent 3D numerical experiments modeling flux emergence. 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: 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: 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: 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: 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: Penumbral microjets at high spatial and temporal resolution Authors: Rouppe van der Voort, Luc H. M.; Drews, Ainar Bibcode: 2019A&A...626A..62R Altcode: 2019arXiv190502981R Context. Sunspot observations in chromospheric spectral lines have revealed short-lived linear bright transients that are commonly referred to as penumbral microjets (PMJs). Details on the origin and physical nature of PMJs are to a large extend still unknown.
Aims: We aim to characterize the dynamical nature of PMJs to provide guidance for future modeling efforts.
Methods: We analyzed high spatial (0.̋1) and temporal resolution (1 s) Ca II H filtergram (0.1 nm bandwidth) observations of a sunspot that were obtained on two consecutive days with the Swedish 1 m Solar Telescope.
Results: We find that PMJs appear to be the rapid brightening of an already existing (faint) fibril. The rapid brightening is the fast increase (typically less than 10 s) in intensity over significant length (several hundreds of kilometers) of the existing fibril. For most PMJs, no clear root or source from where the brightening appears to originate can be identified. After the fast onset, about half of the PMJs have tops that move with an apparent velocity of between 5 and 14 km s-1, most of them upward. No significant motion of the top is observed in the other PMJs. About one-third of the PMJs split into two parallel and coevolving linear features during the later phases of their lifetimes.
Conclusions: We conclude that mass flows can play only a limited role in the onset phase of PMJs. It is more likely that we see the effect of a fast heating front.

Movies are available at https://www.aanda.org Title: Energetics of magnetic transients in a solar active region plage Authors: Chitta, L. P.; Sukarmadji, A. R. C.; Rouppe van der Voort, L.; Peter, H. Bibcode: 2019A&A...623A.176C Altcode: 2019arXiv190201650C Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface.
Aims: We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations.
Methods: We used photospheric Fe I 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code.
Results: The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m-2 through the photosphere.
Conclusions: We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops. Title: Observationally Based Models of Penumbral Microjets Authors: Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A.; Rouppe van der Voort, L.; Scharmer, G. B.; Carlsson, M. Bibcode: 2019ApJ...870...88E Altcode: 2018arXiv181107881E We study the polarization signals and physical parameters of penumbral microjets (PMJs) by using high spatial resolution data taken in the Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP and CHROMIS instruments at the Swedish 1 m Solar Telescope. We infer their physical parameters, such as physical observables in the photosphere and chromospheric velocity diagnostics, by different methods, including inversions of the observed Stokes profiles with the STiC code. PMJs harbor overall brighter Ca II K line profiles and conspicuous polarization signals in Ca II 854.2 nm, specifically in circular polarization that often shows multiple lobes mainly due to the shape of Stokes I. They usually overlap photospheric regions with a sheared magnetic field configuration, suggesting that magnetic reconnections could play an important role in the origin of PMJs. The discrepancy between their low LOS velocities and the high apparent speeds reported on earlier, as well as the existence of different vertical velocity gradients in the chromosphere, indicate that PMJs might not be entirely related to mass motions. Instead, PMJs could be due to perturbation fronts induced by magnetic reconnections occurring in the deep photosphere that propagate through the chromosphere. This reconnection may be associated with current heating that produces temperature enhancements from the temperature minimum region. Furthermore, enhanced collisions with electrons could also increase the coupling to the local conditions at higher layers during the PMJ phase, giving a possible explanation for the enhanced emission in the overall Ca II K profiles emerging from these transients. 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: 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: Chromospheric counterparts of solar transition region unresolved fine structure loops Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Hansteen, Viggo H.; De Pontieu, Bart Bibcode: 2018A&A...611L...6P Altcode: 2018arXiv180304415P Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models.

Two movies associated to Fig. 1 are available at https://www.aanda.org Title: Ellerman bombs and UV bursts: reconnection at different atmospheric layers? Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Rouppe van der Voort, L. Bibcode: 2017AGUFMSH43A2801H Altcode: The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst many other phenomena, the appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed in the wings of H(alpha) and are highly likely to be due to reconnection in the photosphere, below the chromospheric canopy. However, signs of the reconnection process are also observed in several other spectral lines, typical of the chromosphere or transition region. An example are the UV bursts observed in the transition region lines of Si IV. In this work we analyze high cadence coordinated observations between the 1-m Swedish Solar Telescope and the IRIS spacecraft in order to study the possible relationship between reconnection events at different layers in the atmosphere, and in particular, the timing history between them. High cadence, high resolution H-alpha images from the SST provide us with the positions, timings and trajectories of Ellerman bombs in an emerging flux region. Simultaneous co-aligned IRIS slit-jaw images at 1400 and 1330 A and detailed Si IV spectra from the fast spectrograph raster allow us to study the transition region counterparts of those photospheric Ellerman bombs. Our main goal is to study whether there is a temporal relationship between the appearance of an EB and the appearance of a UV burst. Eventually we would like to investigate whether reconnection happens at discrete heights, or as a reconnection sheet spanning several layers at the same time. Title: Surges and Si IV Bursts in the Solar Atmosphere: Understanding IRIS and SST Observations through RMHD Experiments Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis, F.; Rouppe van der Voort, L. Bibcode: 2017ApJ...850..153N Altcode: 2017arXiv171008928N Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as Hα 6563 \mathringA and Ca II 8542 \mathringA . However, whether there is a response to the surge appearance and evolution in the Si IV lines or, in fact, in many other transition region lines has not been studied. In this paper, we analyze a simultaneous episode of an Hα surge and a Si IV burst that occurred on 2016 September 03 in active region AR 12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. For the first time, we report emission of Si IV within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si IV patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si IV, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the nonequilibrium ionization of silicon. Through spectral synthesis, we explain several features of the observations. We show that the presence of Si IV emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes. 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: CRISPRED: CRISP imaging spectropolarimeter data reduction pipeline Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.; Hillberg, T.; Rouppe van der Voort, L. Bibcode: 2017ascl.soft08003D Altcode: CRISPRED reduces data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). It performs fitting routines, corrects optical aberrations from atmospheric turbulence as well as from the optics, and compensates for inter-camera misalignments, field-dependent and time-varying instrumental polarization, and spatial variation in the detector gain and in the zero level offset (bias). It has an object-oriented IDL structure with computationally demanding routines performed in C subprograms called as dynamically loadable modules (DLMs). Title: Beam electrons as a source of Hα flare ribbons Authors: Druett, Malcolm; Scullion, Eamon; Zharkova, Valentina; Matthews, Sarah; Zharkov, Sergei; Rouppe van der Voort, Luc Bibcode: 2017NatCo...815905D Altcode: The observations of solar flare onsets show rapid increase of hard and soft X-rays, ultra-violet emission with large Doppler blue shifts associated with plasma upflows, and Hα hydrogen emission with red shifts up to 1-4 Å. Modern radiative hydrodynamic models account well for blue-shifted emission, but struggle to reproduce closely the red-shifted Hα lines. Here we present a joint hydrodynamic and radiative model showing that during the first seconds of beam injection the effects caused by beam electrons can reproduce Hα line profiles with large red-shifts closely matching those observed in a C1.5 flare by the Swedish Solar Telescope. The model also accounts closely for timing and magnitude of upward motion to the corona observed 29 s after the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory. Title: On the generation of solar spicules and Alfvénic waves Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; Carlsson, M.; Pereira, T. M. D. Bibcode: 2017Sci...356.1269M Altcode: 2017arXiv171007559M In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves. Title: Microjets in the penumbra of a sunspot Authors: Drews, Ainar; Rouppe van der Voort, Luc Bibcode: 2017A&A...602A..80D Altcode: 2017arXiv170206078D Context. Penumbral microjets (PMJs) are short-lived jets found in the penumbra of sunspots, first observed in wide-band Ca II H line observations as localized brightenings, and are thought to be caused by magnetic reconnection. Earlier work on PMJs has focused on smaller samples of by-eye selected events and case studies.
Aims: It is our goal to present an automated study of a large sample of PMJs to place the basic statistics of PMJs on a sure footing and to study the PMJ Ca II 8542 Å spectral profile in detail.
Methods: High spatial resolution and spectrally well-sampled observations in the Ca II 8542 Å line obtained from the Swedish 1-m Solar Telescope (SST) were reduced by a principle component analysis and subsequently used in the automated detection of PMJs using the simple machine learning algorithm k-nearest neighbour. PMJ detections were verified with co-temporal Ca II H line observations.
Results: We find a total of 453 tracked PMJ events, 4253 PMJs detections tallied over all timeframes, and a detection rate of 21 events per timestep. From these, an average length, width and lifetime of 640 km, 210 km and 90 s are obtained. The average PMJ Ca II 8542 Å line profile is characterized by enhanced inner wings, often in the form of one or two distinct peaks, and a brighter line core as compared to the quiet-Sun average. Average blue and red peak positions are determined at - 10.4 km s-1 and + 10.2 km s-1 offsets from the Ca II 8542 Å line core. We find several clusters of PMJ hot-spots within the sunspot penumbra, in which PMJ events occur in the same general area repeatedly over time.
Conclusions: Our results indicate smaller average PMJs sizes and longer lifetimes compared to previously published values, but with statistics still in the same orders of magnitude. The investigation and analysis of the PMJ line profiles strengthens the proposed heating of PMJs to transition region temperatures. The presented statistics on PMJs form a solid basis for future investigations and numerical modelling of PMJs. Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson, M.; Rouppe van der Voort, L.; Leenaarts, J. Bibcode: 2017ApJ...839...22H Altcode: 2017arXiv170402872H A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma “bombs” (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere. Title: Solar Hα features with hot onsets. II. A contrail fibril Authors: Rutten, R. J.; Rouppe van der Voort, L. H. M. Bibcode: 2017A&A...597A.138R Altcode: 2016arXiv160907616R The solar chromosphere observed in Hα consists mostly of narrow fibrils. The longest typically originate in network or plage and arch far over adjacent internetwork. We use data from multiple telescopes to analyze one well-observed example in a quiet area. It resulted from the earlier passage of an accelerating disturbance in which the gas was heated to high temperature as in the spicule-II phenomenon. After this passage a dark Hα fibril appeared as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset and subsequent visibilities in various diagnostics and conclude that such Hα fibrils can indeed be contrail phenomena, not indicative of the thermodynamic and magnetic environment when they are observed but of more dynamic happenings before. They do not connect across internetwork cells but represent launch tracks of heating events and chart magnetic field during launch, not at present. 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: Kinematics and Magnetic Properties of a Light Bridge in a Decaying Sunspot Authors: Falco, M.; Borrero, J. M.; Guglielmino, S. L.; Romano, P.; Zuccarello, F.; Criscuoli, S.; Cristaldi, A.; Ermolli, I.; Jafarzadeh, S.; Rouppe van der Voort, L. Bibcode: 2016SoPh..291.1939F Altcode: 2016arXiv160607229F; 2016SoPh..tmp..107F We present the results obtained by analysing high spatial and spectral resolution data of the solar photosphere acquired by the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011 of a large sunspot with a light bridge (LB) observed in NOAA AR 11263. These data are complemented by simultaneous Hinode Spectropolarimeter (SP) observation in the Fe I 630.15 nm and 630.25 nm lines. The continuum intensity map shows a discontinuity in the radial distribution of the penumbral filaments in correspondence with the LB, which shows a dark lane (≈0.3″ wide and ≈8.0″ long) along its main axis. The available data were inverted with the Stokes Inversion based on Response functions (SIR) code and physical parameters maps were obtained. The line-of-sight (LOS) velocity of the plasma along the LB derived from the Doppler effect shows motions towards and away from the observer up to 0.6 kms−1 that are lower in value than the LOS velocities observed in the neighbouring penumbral filaments. The noteworthy result is that we find motions towards the observer of up to 0.6 kms−1 in the dark lane where the LB is located between two umbral cores, while the LOS velocity motion towards the observer is strongly reduced where the LB is located between an umbral core at one side and penumbral filaments on the other side. Statistically, the LOS velocities correspond to upflows or downflows, and comparing these results with Hinode/SP data, we conclude that the surrounding magnetic field configuration (whether more or less inclined) could have a role in maintaining the conditions for the process of plasma pile-up along the dark lane. The results obtained from our study support and confirm outcomes of recent magneto-hydrodynamic simulations showing upflows along the main axis of an LB. 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: Emergence of Granular-sized Magnetic Bubbles Through the Solar Atmosphere. III. The Path to the Transition Region Authors: Ortiz, Ada; Hansteen, Viggo H.; Bellot Rubio, Luis Ramón; de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Carlsson, Mats; Rouppe van der Voort, Luc Bibcode: 2016ApJ...825...93O Altcode: 2016arXiv160400302O We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Hα 656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0.″14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C II, transition region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far- and near-ultraviolet, of which we have used the Si IV 140.3 and the Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona. Title: The Appearance of Spicules in High Resolution Observations of Ca II H and Hα Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Carlsson, Mats Bibcode: 2016ApJ...824...65P Altcode: 2016arXiv160403116P Solar spicules are chromospheric fibrils that appear everywhere on the Sun, yet their origin is not understood. Using high resolution observations of spicules obtained with the Swedish 1 m Solar Telescope, we aim to understand how spicules appear in filtergrams and Dopplergrams, how they compare in Ca II H and Hα filtergrams, and what can make them appear and disappear. We find that spicules display a rich and detailed spatial structure, and show a distribution of transverse velocities that, when aligned with the line of sight, can make them appear at different Hα wing positions. They become more abundant at positions closer to the line core, reflecting a distribution of Doppler shifts and widths. In Hα width maps they stand out as bright features both on disk and off limb, reflecting their large Doppler motions and possibly higher temperatures than in the typical Hα formation region. Spicule lifetimes measured from narrowband images at only a few positions will be an underestimate because Doppler shifts can make them disappear prematurely from such images; for such cases, width maps are a more robust tool. In Hα and Ca II H filtergrams, off-limb spicules essentially have the same properties, appearance, and evolution. We find that the sudden appearance of spicules can be explained by Doppler shifts from their transverse motions, and does not require other convoluted explanations. Title: Fan-shaped jets above the light bridge of a sunspot driven by reconnection Authors: Robustini, Carolina; Leenaarts, Jorrit; de la Cruz Rodriguez, Jaime; Rouppe van der Voort, Luc Bibcode: 2016A&A...590A..57R Altcode: 2015arXiv150807927R We report on a fan-shaped set of high-speed jets above a strongly magnetized light bridge (LB) of a sunspot observed in the Hα line. We study the origin, dynamics, and thermal properties of the jets using high-resolution imaging spectroscopy in Hα from the Swedish 1m Solar Telescope and data from the Solar Dynamics Observatory and Hinode. The Hα jets have lengths of 7-38 Mm, are impulsively accelerated to a speed of ~100 km s-1 close to photospheric footpoints in the LB, and exhibit a constant deceleration consistent with solar effective gravity. They are predominantly launched from one edge of the light bridge, and their footpoints appear bright in the Hα wings. Atmospheric Imaging Assembly data indicates elongated brightenings that are nearly co-spatial with the Hα jets. We interpret them as jets of transition region temperatures. The magnetic field in the light bridge has a strength of 0.8-2 kG and it is nearly horizontal. All jet properties are consistent with magnetic reconnection as the driver.

Movies associated to Figs. 1 and 2 are available in electronic form at http://www.aanda.org Title: Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M. Bibcode: 2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Title: On the Active Region Bright Grains Observed in the Transition Region Imaging Channels of IRIS Authors: Skogsrud, H.; Rouppe van der Voort, L.; De Pontieu, B. Bibcode: 2016ApJ...817..124S Altcode: 2015arXiv151205263S The Interface Region Imaging Spectrograph (IRIS) provides spectroscopy and narrow band slit-jaw (SJI) imaging of the solar chromosphere and transition region at unprecedented spatial and temporal resolutions. Combined with high-resolution context spectral imaging of the photosphere and chromosphere as provided by the Swedish 1 m Solar Telescope (SST), we can now effectively trace dynamic phenomena through large parts of the solar atmosphere in both space and time. IRIS SJI 1400 images from active regions, which primarily sample the transition region with the Si IV 1394 and 1403 Å lines, reveal ubiquitous bright “grains” which are short-lived (two to five minute) bright roundish small patches of sizes 0.″5-1.″7 that generally move limbward with velocities up to about 30 km s-1. In this paper, we show that many bright grains are the result of chromospheric shocks impacting the transition region. These shocks are associated with dynamic fibrils (DFs), most commonly observed in Hα. We find that the grains show the strongest emission in the ascending phase of the DF, that the emission is strongest toward the top of the DF, and that the grains correspond to a blueshift and broadening of the Si IV lines. We note that the SJI 1400 grains can also be observed in the SJI 1330 channel which is dominated by C II lines. Our observations show that a significant part of the active region transition region dynamics is driven from the chromosphere below rather than from coronal activity above. We conclude that the shocks that drive DFs also play an important role in the heating of the upper chromosphere and lower transition region. Title: Beyond spicule dynamics: spicule and fibril spectroscopy at high spatial and temporal resolution Authors: Mendes Domingos Pereira, T.; Rouppe van der Voort, L. Bibcode: 2015AGUFMSH31B2407M Altcode: Solar spicules are chromospheric fibrils observed at the solar limb. They are observed everywhere in the Sun, but their origin is not yet understood. Much of our understanding of spicules has been obtained through filtergram observations and/or focused on the dynamics of spicules. Spectroscopic studies have been usually limited by spatial extent/resolution, temporal resolution, or variable seeing. In this work we make use of a unique time series of imaging spectroscopy at high spatial and temporal resolution, obtained with the Swedish Solar Telescope under excellent seeing and coordinated with the IRIS mission. With these data we characterize the evolution of spectra along quiet Sun fibrils and spicules, and discuss what makes them visible in filtergrams and sets them apart from other chromospheric fibrils. With combined H-alpha and Ca II H high-resolution observations we also discuss how spicules appear in these two lines, a long standing issue that has been interpreted in conflicting ways. Finally, using the wide range of IRIS diagnostics we put together the spectral evolution of spicules through the chromosphere and transition region. Title: Observing Cascades of Solar Bullets at High Resolution. II. Authors: Scullion, E.; Engvold, O.; Lin, Y.; Rouppe van der Voort, L. Bibcode: 2015ApJ...814..123S Altcode: High resolution observations from the Swedish 1-m Solar Telescope revealed bright, discrete, blob-like structures (which we refer to as solar bullets) in the Hα 656.28 nm line core that appear to propagate laterally across the solar atmosphere as clusters in active regions (ARs). These small-scale structures appear to be field aligned and many bullets become triggered simultaneously and traverse collectively as a cluster. Here, we conduct a follow-up study on these rapidly evolving structures with coincident observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly. With the co-aligned data sets, we reveal (a) an evolving multithermal structure in the bullet cluster ranging from chromospheric to at least transition region temperatures, (b) evidence for cascade-like behavior and corresponding bidirectional motions in bullets within the cluster, which indicate that there is a common source of the initial instability leading to bullet formation, and (c) a direct relationship between co-incident bullet velocities observed in Hα and He ii 30.4 nm and an inverse relationship with respect to bullet intensity in these channels. We find evidence supporting that bullets are typically composed of a cooler, higher density core detectable in Hα with a less dense, hotter, and fainter co-moving outer sheath. Bullets unequivocally demonstrate the finely structured nature of the AR corona. We have no clear evidence for bullets being associated with locally heated (or cooled), fast flowing plasma. Fast MHD pulses (such as solitons) could best describe the dynamic properties of bullets whereas the presence of a multithermal structure is new. Title: Hα Line Profile Asymmetries and the Chromospheric Flare Velocity Field Authors: Kuridze, D.; Mathioudakis, M.; Simões, P. J. A.; Rouppe van der Voort, L.; Carlsson, M.; Jafarzadeh, S.; Allred, J. C.; Kowalski, A. F.; Kennedy, M.; Fletcher, L.; Graham, D.; Keenan, F. P. Bibcode: 2015ApJ...813..125K Altcode: 2015arXiv151001877K The asymmetries observed in the line profiles of solar flares can provide important diagnostics of the properties and dynamics of the flaring atmosphere. In this paper the evolution of the Hα and Ca ii λ8542 lines are studied using high spatial, temporal, and spectral resolution ground-based observations of an M1.1 flare obtained with the Swedish 1 m Solar Telescope. The temporal evolution of the Hα line profiles from the flare kernel shows excess emission in the red wing (red asymmetry) before flare maximum and excess in the blue wing (blue asymmetry) after maximum. However, the Ca ii λ8542 line does not follow the same pattern, showing only a weak red asymmetry during the flare. RADYN simulations are used to synthesize spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. We show that the red asymmetry observed in Hα is not necessarily associated with plasma downflows, and the blue asymmetry may not be related to plasma upflows. Indeed, we conclude that the steep velocity gradients in the flaring chromosphere modify the wavelength of the central reversal in the Hα line profile. The shift in the wavelength of maximum opacity to shorter and longer wavelengths generates the red and blue asymmetries, respectively. 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: Magnetic Upflow Events in the Quiet-Sun Photosphere. I. Observations Authors: Jafarzadeh, S.; Rouppe van der Voort, L.; de la Cruz Rodríguez, J. Bibcode: 2015ApJ...810...54J Altcode: 2015arXiv150707355J Rapid magnetic upflows in the quiet-Sun photosphere were recently uncovered from both Sunrise/IMaX and Hinode/SOT observations. Here, we study magnetic upflow events (MUEs) from high-quality, high- (spatial, temporal, and spectral) resolution, and full Stokes observations in four photospheric magnetically sensitive Fe i lines centered at 5250.21, 6173.34, 6301.51, and 6302.50 Å acquired with the Swedish Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line Stokes V signals from those in the far blue wing whose signal-to-noise ratio (S/N) ≥slant 7. We find a larger number of MUEs at any given time (2.0× {10}-2 arcsec-2), larger by one to two orders of magnitude, than previously reported. The MUEs appear to fall into four classes presenting different shapes of Stokes V profiles with (I) asymmetric double lobes, (II) single lobes, (III) double-humped (two same-polarity lobes), and (IV) three lobes (an extra blueshifted bump in addition to double lobes), of which less than half are single-lobed. We also find that MUEs are almost equally distributed in network and internetwork areas and they appear in the interior or at the edge of granules in both regions. Distributions of physical properties, except for horizontal velocity, of the MUEs (namely, Stokes V signal, size, line-of-sight velocity, and lifetime) are almost identical for the different spectral lines in our data. A bisector analysis of our spectrally resolved observations shows that these events host modest upflows and do not show a direct indication of the presence of supersonic upflows reported earlier. Our findings reveal that the numbers, types (classes), and properties determined for MUEs can strongly depend on the detection techniques used and the properties of the employed data, namely, S/Ns, resolutions, and wavelengths. 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: On the Temporal Evolution of Spicules Observed with IRIS, SDO, and Hinode Authors: Skogsrud, H.; Rouppe van der Voort, L.; De Pontieu, B.; Pereira, T. M. D. Bibcode: 2015ApJ...806..170S Altcode: 2015arXiv150502525S Spicules are ubiquitous, fast moving jets observed off-limb in chromospheric spectral lines. Combining the recently launched Interface Region Imaging Spectrograph with the Solar Dynamics Observatory and Hinode, we have a unique opportunity to study spicules simultaneously in multiple passbands and from a seeing free environment. This makes it possible to study their thermal evolution over a large range of temperatures. A recent study showed that spicules appear in several chromospheric and transition region spectral lines, suggesting that spicules continue their evolution in hotter passbands after they fade from Ca ii H. In this follow-up paper, we answer some of the questions that were raised in the introductory study. In addition, we study spicules off-limb in C ii 1330 Å for the first time. We find that Ca ii H spicules are more similar to Mg ii 2976 Å spicules than initially reported. For a sample of 54 spicules, we find that 44% of Si iv 1400 Å spicules are brighter toward the top; 56% of the spicules show an increase in Si iv emission when the Ca ii H component fades. We find several examples of spicules that fade from passbands other than Ca ii H, and we observe that if a spicule fades from a passband, it also generally fades from the passbands with lower formation temperatures. We discuss what these new, multi-spectral results mean for the classification of type I and II spicules. 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: On Fibrils and Field Lines: the Nature of Hα Fibrils in the Solar Chromosphere Authors: Leenaarts, Jorrit; Carlsson, Mats; Rouppe van der Voort, Luc Bibcode: 2015ApJ...802..136L Altcode: 2015arXiv150200295L Observations of the solar chromosphere in the line core of the Hα line show dark elongated structures called fibrils that show swaying motion. We performed a three-dimensional radiation-MHD simulation of a network region and computed synthetic Hα images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes, and phase speeds of the simulated fibrils are consistent with observations. We find that some fibrils trace out the same field line along the fibril’s length, while other fibrils sample different field lines at different locations along their length. Fibrils sample the same field lines on a timescale of ∼200 s. This is shorter than their own lifetime. Fibril-threading field lines carry slow-mode waves, as well as transverse waves propagating with the Alfvén speed. Transverse waves propagating in opposite directions cause an interference pattern with complex apparent phase speeds. The relationship between fibrils and field lines is governed by constant migration and swaying of the field lines, their mass loading and draining, and their visibility in Hα. Field lines are visible where they lie close to the optical depth unity surface. The location of the latter is at a height at which the column mass reaches a certain fixed value. The visibility of the field line is thus determined by its own mass density and by the mass density of the material above it. Using the swaying motion of fibrils as a tracer of chromospheric transverse oscillations must be done with caution. Title: Internetwork Chromospheric Bright Grains Observed With IRIS and SST Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson, Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt, Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan; Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli, Sarah; Kankelborg, Charles Bibcode: 2015ApJ...803...44M Altcode: 2015arXiv150203490M The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet Sun. These bright grains are seen with the IRIS 1330, 1400, and 2796 Å slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1 m Solar Telescope (SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H 3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å, Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.} 33), temporal, and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images (SJIs) with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 Å slit-jaw filter comes from both the Mg ii k core and wings. The signal in the C ii and Si iv lines is too weak to explain the presence of grains in the 1300 and 1400 Å SJIs and we conclude that the grain signal in these passbands comes mostly from the continuum. Although weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C ii spectra. For some grains, a spectral signature can be found in IRIS Si iv. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region. Title: CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.; Hillberg, T.; Rouppe van der Voort, L. Bibcode: 2015A&A...573A..40D Altcode: 2014arXiv1406.0202D The production of science-ready data from major solar telescopes requires expertise beyond that of the typical observer. This is a consequence of the increasing complexity of instruments and observing sequences, which require calibrations and corrections for instrumental and seeing effects that are not only difficult to measure, but are also coupled in ways that require careful analysis in the design of the correction procedures. Modern space-based telescopes have data-processing pipelines capable of routinely producing well-characterized data products. High resolution imaging spectropolarimeters at ground-based telescopes need similar data pipelines.We present new methods for flat-fielding spectropolarimetric data acquired with telecentric Fabry-Perot instruments and a new approach for accurate camera co-alignment for image restoration. We document a procedure that forms the basis of current state-of-the-art processing of data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). By collecting, implementing, and testing a suite of computer programs, we have defined a data reduction pipeline for this instrument. This pipeline, CRISPRED, streamlines the process of making science-ready data.It is implemented and operated in IDL, with time-consuming steps delegated to C.CRISPRED will also be the basis for the data pipeline of the forthcoming CHROMIS instrument. Title: Heating Signatures in the Disk Counterparts of Solar Spicules in Interface Region Imaging Spectrograph Observations Authors: Rouppe van der Voort, L.; De Pontieu, B.; Pereira, T. M. D.; Carlsson, M.; Hansteen, V. Bibcode: 2015ApJ...799L...3R Altcode: 2014arXiv1412.4531R We use coordinated observations with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify the disk counterpart of type II spicules in upper-chromospheric and transition region (TR) diagnostics. These disk counterparts were earlier identified through short-lived asymmetries in chromospheric spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We find clear signatures of RBEs and RREs in Mg II h & k, often with excursions of the central h3 and k3 absorption features in concert with asymmetries in co-temporal and co-spatial Hα spectral profiles. We find spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si IV 1394 and 1403 Å spectral lines and interpret this as a sign that type II spicules are heated to at least TR temperatures, supporting other recent work. These C II and Si IV spectral signals are weaker for a smaller network region than for more extended network regions in our data. A number of bright features around extended network regions observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified as network jets, can be clearly connected to Hα RBEs and/or RREs in our coordinated data. We speculate that at least part of the diffuse halo around network regions in the IRIS SJI 1330 and 1400 images can be attributed to type II spicules with insufficient opacity in the C II and Si IV lines to stand out as single features in these passbands. Title: Unresolved Fine-scale Structure in Solar Coronal Loop-tops Authors: Scullion, E.; Rouppe van der Voort, L.; Wedemeyer, S.; Antolin, P. Bibcode: 2014ApJ...797...36S Altcode: 2014arXiv1409.1920S New and advanced space-based observing facilities continue to lower the resolution limit and detect solar coronal loops in greater detail. We continue to discover even finer substructures within coronal loop cross-sections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop substructures, through taking advantage of the resolving power of the Swedish 1 m Solar Telescope/CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory/Atmospheric Image Assembly (AIA). High-resolution imaging of the chromospheric Hα 656.28 nm spectral line core and wings can, under certain circumstances, allow one to deduce the topology of the local magnetic environment of the solar atmosphere where its observed. Here, we study post-flare coronal loops, which become filled with evaporated chromosphere that rapidly condenses into chromospheric clumps of plasma (detectable in Hα) known as a coronal rain, to investigate their fine-scale structure. We identify, through analysis of three data sets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded substructures. Many cool strands even extend fully intact from loop-top to footpoint. We discover that coronal loop fine-scale strands can appear bunched with as many as eight parallel strands within an AIA coronal loop cross-section. The strand number density versus cross-sectional width distribution, as detected by CRISP within AIA-defined coronal loops, most likely peaks at well below 100 km, and currently, 69% of the substructure strands are statistically unresolved in AIA coronal loops. Title: On the Multi-threaded Nature of Solar Spicules Authors: Skogsrud, H.; Rouppe van der Voort, L.; De Pontieu, B. Bibcode: 2014ApJ...795L..23S Altcode: 2014arXiv1410.1334S A dominant constituent in the dynamic chromosphere is spicules. Spicules at the limb appear as relatively small and dynamic jets that are observed to stick out everywhere. Many papers emphasize the important role spicules might play in the energy and mass balance of the chromosphere and corona. However, many aspects of spicules remain a mystery. In this Letter, we shed more light on the multi-threaded nature of spicules and their torsional component. We use high spatial, spectral, and temporal resolution observations from the Swedish 1 m Solar Telescope in the Hα spectral line. The data target the limb, and we extract spectra from spicules far out from the limb to reduce the line-of-sight superposition effect. We discover that many spicules display very asymmetric spectra with some even showing multiple peaks. To quantify this asymmetry, we use a double-Gaussian fitting procedure and find an average velocity difference between the single-Gaussian components to be between 20 and 30 km s-1 for a sample of 57 spicules. We observe that spicules show significant substructure where one spicule consists of many "threads." We interpret the asymmetric spectra as a line-of-sight superposition of threads in one spicule and therefore have a measure for a perpendicular flow inside spicules that will be important for future numerical models to reproduce. In addition, we show examples of λ - x slices perpendicular across spicules and find spectral tilts in individual threads, providing further evidence for the complex dynamical nature of spicules. Title: On the prevalence of small-scale twist in the solar chromosphere and transition region Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.; Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.; Martinez-Sykora, J. Bibcode: 2014Sci...346D.315D Altcode: 2014arXiv1410.6862D The solar chromosphere and transition region (TR) form an interface between the Sun’s surface and its hot outer atmosphere. There, most of the nonthermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33-arc second) observations with NASA’s Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub-arc second scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-meter Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere. Title: Dynamic Properties along the Neutral Line of a Delta Spot Inferred from High-resolution Observations Authors: Cristaldi, A.; Guglielmino, S. L.; Zuccarello, F.; Romano, P.; Falco, M.; Rouppe van der Voort, L.; de la Cruz Rodríguez, J.; Ermolli, I.; Criscuoli, S. Bibcode: 2014ApJ...789..162C Altcode: Delta (δ) spots are complex magnetic configurations of sunspots characterized by umbrae of opposite polarity sharing a common penumbra. In order to investigate the fine structure of the region separating the two magnetic polarities of a δ spot, we studied the morphology, the magnetic configuration, and the velocity field in such a region using observations of active region (AR) NOAA 11267 obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope on 2011 August 6. The analysis of CRISP data shows upflows and downflows of ~ ± 3 km s-1 in proximity of the δ spot polarity inversion line (PIL), and horizontal motions along the PIL of the order of ~1 km s-1. The results obtained from the SIR inversion of CRISP data also indicate that the transverse magnetic field in the brighter region separating the two opposite magnetic polarities of the δ spot is tilted about ~45° with respect to the PIL. Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations confirm the presence of motions of ~ ± 3 km s-1 in proximity of the PIL, which were observed to last 15 hr. From the data analyzed, we conclude that the steady, persistent, and subsonic motions observed along the δ spot PIL can be interpreted as being due to Evershed flows occurring in the penumbral filaments that show a curved, wrapped configuration. The fluting of the penumbral filaments and their bending, continuously increased by the approaching motion of the negative umbra toward the positive one, give rise to the complex line-of-sight velocity maps that we observed. Title: IRIS Observations of Twist in the Low Solar Atmosphere Authors: De Pontieu, Bart; Rouppe van der Voort, Luc; Pereira, Tiago M. D.; Skogsrud, Haakon; McIntosh, Scott W.; Carlsson, Mats; Hansteen, Viggo Bibcode: 2014AAS...22431302D Altcode: The Interface Region Imaging Spectrograph (IRIS) small explorer was launched in June 2013. IRIS’s high-resolution (0.33 arcsec), high-cadence (2s) images and spectra reveal a solar chromosphere and transition region that is riddled with twist. This is evidenced by the presence of ubiquitous torsional motions on very small (subarcsec) spatial scales. These motions occur in active regions, quiet Sun and coronal holes on a variety of structures such as spicules at the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as low-lying loops. We use IRIS data and observations from the Swedish Solar Telescope (SST) in La Palma, Spain to describe these motions quantitatively, study their propagation, and illustrate how such strong twisting motions are often associated with significant and rapid heating to at least transition region temperatures. Title: Emergence of Granular-sized Magnetic Bubbles through the Solar Atmosphere. I. Spectropolarimetric Observations and Simulations Authors: Ortiz, Ada; Bellot Rubio, Luis R.; Hansteen, Viggo H.; de la Cruz Rodríguez, Jaime; Rouppe van der Voort, Luc Bibcode: 2014ApJ...781..126O Altcode: 2013arXiv1312.5735O We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.''14. Simultaneous full Stokes observations of the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca II 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca II 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s-1 and expands at a horizontal speed of 4 km s-1. In about 3.5 minutes it travels some 1100 km to reach the mid chromosphere, implying an average ascent speed of 5.2 km s-1. The maximum separation attained by the magnetic legs is 6.''6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90° in the magnetic bubble interior, along with temperature deficits of up to 250 K at log τ = -2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes. Title: IRIS observations of twist in the low solar atmosphere Authors: De Pontieu, Bart; Pereira, Tiago; Rouppe van der Voort, Luc; Skogsrud, Hakon Bibcode: 2014cosp...40E.654D Altcode: The Interface Region Imaging Spectrograph (IRIS) small explorer was launched in June 2013. IRIS's high-resolution (0.33 arcsec), high-cadence (2s) images and spectra reveal a solar chromosphere and transition region that is riddled with twist. This is evidenced by the presence of ubiquitous torsional motions on very small (subarcsec) spatial scales. These motions occur in active regions, quiet Sun and coronal holes on a variety of structures such as spicules at the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as low-lying loops. We use IRIS data and observations from the Swedish Solar Telescope (SST) in La Palma, Spain to describe these motions quantitatively and illustrate how such strong twisting motions are often associated with significant and rapid heating to at least transition region temperatures. Title: Effects of flux emergence in the outer solar atmosphere. Observational advances Authors: Ortiz Carbonell, Ada; De Pontieu, Bart; Bellot Rubio, L. R.; Hansteen, Viggo; Rouppe van der Voort, Luc; Carlsson, Mats Bibcode: 2014cosp...40E2387O Altcode: We study granular sized magnetic flux emergence events that occur in a flux emergence region in NOAA 11850 on September 25, 2013. During that time, the first co-observing campaign between the Swedish 1 m Solar Telescope and the IRIS spacecraft was carried out. Simultaneous observations of the Halpha 656.28 nm and Ca II 854.2 nm chromospheric lines, and the Fe I 630.25 nm photospheric line, were made with the CRISP/SST spectropolarimeter reaching a spatial resolution of 0."14. At the same time, IRIS was performing a four-step dense raster of the said emerging flux region, taking slit-jaw images at 133 (C II transiti on region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere), obtaining thus the highest resolution images ever taken of the upper chromosphere and transition region. The photospheric and chromospheric properties of the emerging magnetic flux bubbles have been described in detail in Ortiz et al. (2014). However, in the current work we are able to follow such lower atmosphere observations of flux emergence up to the transition region with unprecedented spatial and temporal resolution. We describe the properties (size, time delays, lifetime, velocities, temperature) of the observed signatures of flux emergence in the transition region. We believe this may be an important mechanism of transporting energy and magnetic flux to the upper layers of the solar atmosphere, namely the transition region and corona, at least in cases when active regions are formed by flux emerging through the photosphere. * Ortiz et al. (2014) ApJ 781, 126 Title: Short Dynamic Fibrils in Sunspot Chromospheres Authors: Rouppe van der Voort, L.; de la Cruz Rodríguez, J. Bibcode: 2013ApJ...776...56R Altcode: 2013arXiv1308.3893R Sunspot chromospheres display vigorous oscillatory signatures when observed using chromospheric diagnostics such as the strong Ca II lines and Hα. New high-resolution sunspot observations from the Swedish 1 m Solar Telescope show the ubiquitous presence of small-scale, periodic, jet-like features that move up and down. This phenomenon has not been described before. The typical width of these features is about 0.''3 and they display clear parabolic trajectories in space-time diagrams. The maximum extension of the top of the jets is lowest in the umbra, a few 100 km, and progressively longer further away from the umbra in the penumbra, with the longest extending more than 1000 km. These jets resemble the dynamic fibrils found in plage regions but at smaller extensions. Local thermodynamic equilibrium inversion of spectropolarimetric Ca II 8542 observations enabled a comparison of the magnetic field inclination and properties of these short jets. We find that the most extended of these jets also have longer periods and tend to be located in regions with more horizontal magnetic fields. These results are direct observational confirmation of the mechanism of long-period waves propagating along inclined magnetic fields into the solar chromosphere. This mechanism was identified earlier as the driver of dynamic fibrils in plage, part of the mottles in the quiet Sun, and the type I spicules at the limb. The sunspot dynamic fibrils that we report here represent a new class of manifestation of this mechanism, distinct from the transient penumbral and umbral micro-jets reported earlier. 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: Are Giant Tornadoes the Legs of Solar Prominences? Authors: Wedemeyer, Sven; Scullion, Eamon; Rouppe van der Voort, Luc; Bosnjak, Antonija; Antolin, Patrick Bibcode: 2013ApJ...774..123W Altcode: 2013arXiv1306.2661W Observations in the 171 Å channel of the Atmospheric Imaging Assembly of the space-borne Solar Dynamics Observatory show tornado-like features in the atmosphere of the Sun. These giant tornadoes appear as dark, elongated, and apparently rotating structures in front of a brighter background. This phenomenon is thought to be produced by rotating magnetic field structures that extend throughout the atmosphere. We characterize giant tornadoes through a statistical analysis of properties such as spatial distribution, lifetimes, and sizes. A total number of 201 giant tornadoes are detected in a period of 25 days, suggesting that, on average, about 30 events are present across the whole Sun at a time close to solar maximum. Most tornadoes appear in groups and seem to form the legs of prominences, thus serving as plasma sources/sinks. Additional Hα observations with the Swedish 1 m Solar Telescope imply that giant tornadoes rotate as a structure, although they clearly exhibit a thread-like structure. We observe tornado groups that grow prior to the eruption of the connected prominence. The rotation of the tornadoes may progressively twist the magnetic structure of the prominence until it becomes unstable and erupts. Finally, we investigate the potential relation of giant tornadoes to other phenomena, which may also be produced by rotating magnetic field structures. A comparison to cyclones, magnetic tornadoes, and spicules implies that such events are more abundant and short-lived the smaller they are. This comparison might help to construct a power law for the effective atmospheric heating contribution as a function of spatial scale. Title: Physical properties of a sunspot chromosphere with umbral flashes Authors: de la Cruz Rodríguez, J.; Rouppe van der Voort, L.; Socas-Navarro, H.; van Noort, M. Bibcode: 2013A&A...556A.115D Altcode: 2013arXiv1304.0752D We present new high-resolution spectro-polarimetric Ca IIλ8542 observations of umbral flashes in sunspots. At nearly 0.18 arcsec, and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). From a fast Fourier transform analysis, we find a trend of decreasing power at high temporal frequencies at those locations with more horizontal magnetic fields, for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.

Movie and Appendices A and B are available in electronic form at http://www.aanda.org Title: A Detailed Comparison between the Observed and Synthesized Properties of a Simulated Type II Spicule Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit; Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc Bibcode: 2013ApJ...771...66M Altcode: 2013arXiv1305.2397M We have performed a three-dimensional radiative MHD simulation of the solar atmosphere. This simulation shows a jet-like feature that shows similarities to the type II spicules observed for the first time with Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the solar disk are associated with these spicules. Observational results suggest they may contribute significantly in supplying the corona with hot plasma. We perform a detailed comparison of the properties of the simulated jet with those of type II spicules (observed with Hinode) and RBEs (with ground-based instruments). We analyze a wide variety of synthetic emission and absorption lines from the simulations including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition region and coronal temperatures (10,000 K to several million K). We compare their synthetic intensities, line profiles, Doppler shifts, line widths, and asymmetries with observations from Hinode/SOT and EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many properties of the synthetic observables resemble the observations, and we describe in detail the physical processes that lead to these observables. Detailed analysis of the synthetic observables provides insight into how observations should be analyzed to derive information about physical variables in such a dynamic event. For example, we find that line-of-sight superposition in the optically thin atmosphere requires the combination of Doppler shifts and spectral line asymmetry to determine the velocity in the jet. In our simulated type II spicule, the lifetime of the asymmetry of the transition region lines is shorter than that of the coronal lines. Other properties differ from the observations, especially in the chromospheric lines. The mass density of the part of the spicule with a chromospheric temperature is too low to produce significant opacity in chromospheric lines. The synthetic Ca II 8542 Å and Hα profiles therefore do not show signal resembling RBEs. These and other discrepancies are described in detail, and we discuss which mechanisms and physical processes may need to be included in the MHD simulations to mimic the thermodynamic processes of the chromosphere and corona, in particular to reproduce type II spicules. Title: Magnetic tornadoes and chromospheric swirls - Definition and classification Authors: Wedemeyer, Sven; Scullion, Eamon; Steiner, Oskar; de la Cruz Rodriguez, Jaime; Rouppe van der Voort, L. H. M. Bibcode: 2013JPhCS.440a2005W Altcode: 2013arXiv1303.0179W Chromospheric swirls are the observational signatures of rotating magnetic field structures in the solar atmosphere, also known as magnetic tornadoes. Swirls appear as dark rotating features in the core of the spectral line of singly ionized calcium at a wavelength of 854.2 nm. This signature can be very subtle and difficult to detect given the dynamic changes in the solar chromosphere. Important steps towards a systematic and objective detection method are the compilation and characterization of a statistically significant sample of observed and simulated chromospheric swirls. Here, we provide a more exact definition of the chromospheric swirl phenomenon and also present a first morphological classification of swirls with three types: (I) Ring, (II) Split, (III) Spiral. We also discuss the nature of the magnetic field structures connected to tornadoes and the influence of limited spatial resolution on the appearance of their photospheric footpoints. 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: Interplay of Three Kinds of Motion in the Disk Counterpart of Type II Spicules: Upflow, Transversal, and Torsional Motions Authors: Sekse, D. H.; Rouppe van der Voort, L.; De Pontieu, B.; Scullion, E. Bibcode: 2013ApJ...769...44S Altcode: 2013arXiv1304.2304S Recently, it was shown that the complex dynamical behavior of spicules has to be interpreted as the result of simultaneous action of three kinds of motion: (1) field aligned flows, (2) swaying motions, and (3) torsional motions. We use high-quality observations from the CRisp Imaging SpectroPolarimeter at the Swedish 1-m Solar Telescope to investigate signs of these different kinetic modes in spicules on the disk. Earlier, rapid blue-shifted excursions (RBEs), short-lived absorption features in the blue wing of chromospheric spectral lines, were identified as the disk counterpart of type II spicules. Here we report the existence of similar absorption features in the red wing of the Ca II 8542 and Hα lines: rapid redshifted excursions (RREs). RREs are found over the whole solar disk and are located in the same regions as RBEs: in the vicinity of magnetic field concentrations. RREs have similar characteristics as RBEs: they have similar lengths, widths, lifetimes, and average Doppler velocity. The striking similarity of RREs to RBEs implies that RREs are a manifestation of the same physical phenomenon and that spicules harbor motions that can result in a net redshift when observed on-disk. We find that RREs are less abundant than RBEs: the RRE/RBE detection count ratio is about 0.52 at disk center and 0.74 near the limb. We interpret the higher number of RBEs and the decreased imbalance toward the limb as an indication that field-aligned upflows have a significant contribution to the net Dopplershift of the structure. Most RREs and RBEs are observed in isolation, but we find many examples of parallel and touching RRE/RBE pairs which appear to be part of the same spicule. We interpret the existence of these RRE/RBE pairs and the observation that many RREs and RBEs have varying Dopplershift along their width as signs that torsional motion is an important characteristic of spicules. The fact that most RBEs and RREs are observed in isolation agrees with the idea that transversal swaying motion is another important kinetic mode. We find examples of transitions from RRE to RBE and vice versa. These transitions sometimes appear to propagate along the structure with speeds between 18 and 108 km s-1 and can be interpreted as the sign of a transverse (Alfvénic) wave. Title: Heating of the Magnetic Chromosphere: Observational Constraints from Ca II λ8542 Spectra Authors: de la Cruz Rodríguez, J.; De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M. Bibcode: 2013ApJ...764L..11D Altcode: 2013arXiv1301.3141D The heating of the Sun's chromosphere remains poorly understood. While progress has been made on understanding what drives the quiet-Sun internetwork chromosphere, chromospheric heating in strong magnetic field regions continues to present a difficult challenge, mostly because of a lack of observational constraints. We use high-resolution spectropolarimetric data from the Swedish 1 m Solar Telescope to identify the location and spatio-temporal properties of heating in the magnetic chromosphere. In particular, we report the existence of raised-core spectral line profiles in the Ca II λ8542 line. These profiles are characterized by the absence of an absorption line core, showing a quasi-flat profile between λ ≈ ±0.5 Å, and are abundant close to magnetic bright points and plage. Comparison with three-dimensional MHD simulations indicates that such profiles occur when the line of sight goes through an "elevated temperature canopy" associated with the expansion with height of the magnetic field of flux concentrations. This temperature canopy in the simulations is caused by ohmic dissipation where there are strong magnetic field gradients. The raised-core profiles are thus indicators of locations of increased chromospheric heating. We characterize the location and temporal and spatial properties of such profiles in our observations, thus providing much stricter constraints on theoretical models of chromospheric heating mechanisms than before. Title: On the Temporal Evolution of the Disk Counterpart of Type II Spicules in the Quiet Sun Authors: Sekse, D. H.; Rouppe van der Voort, L.; De Pontieu, B. Bibcode: 2013ApJ...764..164S Altcode: 2012arXiv1212.4988S The newly established type II spicule has been speculated to provide enough hot plasma to play an important role in the mass loading and heating of the solar corona. With the identification of rapid blueshifted excursions (RBEs) as the on-disk counterpart of type II spicules we have analyzed three different high-quality timeseries with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope on La Palma and subjected to an automated detection routine to detect a large number of RBEs for statistical purposes. Our observations are of a quiet-Sun region at disk center and we find lower Doppler velocities, 15-40 km s-1, and Doppler widths, 2-15 km s-1, of RBEs than in earlier coronal hole studies, 30-50 km s-1 and 7-23 km s-1, respectively. In addition, we examine the spatial dependence of Doppler velocities and widths along the RBE axis and conclude that there is no clear trend to this over the field of view or in individual RBEs in the quiet Sun at disk center. These differences with previous coronal hole studies are attributed to the more varying magnetic field configuration in quiet-Sun conditions. Using an extremely high-cadence data set has allowed us to improve greatly on the determination of lifetimes of RBEs, which we find to range from 5 to 60 s with an average lifetime of 30 s, as well as the transverse motions in RBEs, with transverse velocities up to 55 km s-1 and averaging 12 km s-1. Furthermore, our measurements of the recurrence rates of RBEs provide important new constraints on coronal heating by spicules. We also see many examples of a sinusoidal wave pattern in the transverse motion of RBEs with periods averaging 54 s and amplitudes from 21.5 to 129 km which agrees well with previous studies of wave motion in spicules at the limb. We interpret the appearance of RBEs over their full length within a few seconds as the result of a combination of three kinds of motions as is earlier reported for spicules. Finally, we look at the temporal connection between Hα and Ca II 8542 RBEs and find that Ca II 8542 RBEs in addition to being located closer to the footpoint also appear before the Hα RBEs. This connection between Ca II 8542 and Hα supports the idea that heating is occurring in spicules and contributes more weight to the prominence of spicules as a source for heating and mass loading of the corona. Title: Comparing Simultaneous Measurements of two High-Resolution Imaging Spectropolarimeters: The `Göttingen' FPI@VTT and CRISP@SST Authors: Bello González, N.; Bellot Rubio, L. R.; Ortiz, A.; Rezaei, R.; Rouppe van der Voort, L.; Schlichenmaier, R. Bibcode: 2012ASPC..463..251B Altcode: 2012arXiv1204.1023B In July 2009, the leading spot of the active region NOAA11024 was observed simultaneously and independently with the ‘Göttingen’ FPI at VTT and CRISP at SST, i.e., at two different sites, telescopes, instruments and using different spectral lines. The data processing and data analysis have been carried out independently with different techniques. Maps of physical parameters retrieved from 2D spectro-polarimetric data observed with ‘Göttingen’ FPI and CRISP show an impressive agreement. In addition, the ‘Göttingen’ FPI maps also exhibit a notable resemblance with simultaneous TIP (spectrographic) observations. The consistency in the results demonstrates the excellent capabilities of these observing facilities. Besides, it confirms the solar origin of the detected signals and the reliability of FPI-based spectro-polarimeters. 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: Temporal Evolution of Velocity and Magnetic Field in and around Umbral Dots Authors: Watanabe, Hiroko; Bellot Rubio, Luis R.; de la Cruz Rodríguez, Jaime; Rouppe van der Voort, Luc Bibcode: 2012ApJ...757...49W Altcode: 2012arXiv1207.6006W We study the temporal evolution of umbral dots (UDs) using measurements from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope. Scans of the magnetically sensitive 630 nm iron lines were performed under stable atmospheric conditions for 71 minutes with a cadence of 63 s. These observations allow us to investigate the magnetic field and velocity in and around UDs at a resolution approaching 0farcs13. From the analysis of 339 UDs, we draw the following conclusions: (1) UDs show clear hints of upflows, as predicted by magnetohydrodynamic simulations. By contrast, we could not find systematic downflow signals. Only in very deep layers, we detect localized downflows around UDs, but they do not persist in time. (2) We confirm that UDs exhibit weaker and more inclined fields than their surroundings, as reported previously. However, UDs that have strong fields above 2000 G or are in the decay phase show enhanced and more vertical fields. (3) There are enhanced fields at the migration front of UDs detached from penumbral grains, as if their motion were impeded by the ambient field. (4) Long-lived UDs travel longer distances with slower proper motions. Our results appear to confirm some aspects of recent numerical simulations of magnetoconvection in the umbra (e.g., the existence of upflows in UDs), but not others (e.g., the systematic weakening of the magnetic field at the position of UDs). 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: Velocities in Magnetoconvective Structures inside Sunspot Umbrae Authors: Ortiz, A.; Rubio, L. B.; Rouppe van der Voort, L. Bibcode: 2012ASPC..454..217O Altcode: We study the velocity field of umbral dots at a resolution of 0.″14. Our analysis is based on full Stokes spectropolarimetric measurements of a pore taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. We find that most umbral dots are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges with velocities of up to 1000 m s-1 and enhanced net circular polarization signals. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields. Title: Statistical Properties of the Disk Counterparts of Type II Spicules from Simultaneous Observations of Rapid Blueshifted Excursions in Ca II 8542 and Hα Authors: Sekse, D. H.; Rouppe van der Voort, L.; De Pontieu, B. Bibcode: 2012ApJ...752..108S Altcode: 2012arXiv1204.2943S Spicules were recently found to exist as two different types when a new class of so-called type II spicules was discovered at the solar limb with the Solar Optical Telescope on board the Hinode spacecraft. These type II spicules have been linked with on-disk observations of rapid blueshifted excursions (RBEs) in the Hα and Ca II 8542 lines. Here we analyze observations optimized for the detection of RBEs in both Hα and Ca II 8542 lines simultaneously at a high temporal cadence taken with the Crisp Imaging Spectropolarimeter at the Swedish Solar Telescope on La Palma. In this study, we used a high-quality time sequence for RBEs at different blueshifts and employed an automated detection routine to detect a large number of RBEs in order to expand on the statistics of RBEs. We find that the number of detected RBEs is strongly dependent on the associated Doppler velocity of the images on which the search is performed. Automatic detection of RBEs at lower velocities increases the estimated number of RBEs to the same order of magnitude expected from limb spicules. This shows that RBEs and type II spicules are indeed exponents of the same phenomenon. Furthermore, we provide solid evidence that Ca II 8542 RBEs are connected to Hα RBEs and are located closer to the network regions with the Hα RBEs being a continuation of the Ca II 8542 RBEs. Our results show that RBEs have an average lifetime of 83.9 s when observed in both spectral lines and that the Doppler velocities of RBEs range from 10 to 25 km s-1 in Ca II 8542 and 30 to 50 km s-1 in Hα. In addition, we automatically determine the transverse motion of a much larger sample of RBEs than previous studies, and find that, just like type II spicules, RBEs undergo significant transverse motions of the order of 5-10 km s-1. Finally, we find that the intergranular jets discovered at Big Bear Solar Observatory are a subset of RBEs. Title: Prominence seismology using ground- and space-based observations Authors: Ballester, J. L.; Arregui, I.; Oliver, R.; Terradas, J.; Soler, R.; Lin, Y.; Engvold, O.; Langagen, O.; Rouppe van der Voort, L. H. M. Bibcode: 2012EAS....55..169B Altcode: Ground- and space-based observations have confirmed the presence of oscillatory motions in prominences and they have been interpreted in terms of magnetohydrodynamic (MHD) waves. This interpretation opens the door to perform prominence seismology, whose main aim is to determine physical parameters in magnetic and plasma structures (prominences) that are difficult to measure by direct means. Here, two prominence seismology applications are presented. Title: Ubiquitous Torsional Motions in Type II Spicules Authors: De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M.; Rutten, R. J.; Hansteen, V. H.; Watanabe, H. Bibcode: 2012ApJ...752L..12D Altcode: 2012arXiv1205.5006D Spicules are long, thin, highly dynamic features that jut out ubiquitously from the solar limb. They dominate the interface between the chromosphere and corona and may provide significant mass and energy to the corona. We use high-quality observations with the Swedish 1 m Solar Telescope to establish that so-called type II spicules are characterized by the simultaneous action of three different types of motion: (1) field-aligned flows of order 50-100 km s-1, (2) swaying motions of order 15-20 km s-1, and (3) torsional motions of order 25-30 km s-1. The first two modes have been studied in detail before, but not the torsional motions. Our analysis of many near-limb and off-limb spectra and narrowband images using multiple spectral lines yields strong evidence that most, if not all, type II spicules undergo large torsional modulation and that these motions, like spicule swaying, represent Alfvénic waves propagating outward at several hundred km s-1. The combined action of the different motions explains the similar morphology of spicule bushes in the outer red and blue wings of chromospheric lines, and needs to be taken into account when interpreting Doppler motions to derive estimates for field-aligned flows in spicules and determining the Alfvénic wave energy in the solar atmosphere. Our results also suggest that large torsional motion is an ingredient in the production of type II spicules and that spicules play an important role in the transport of helicity through the solar atmosphere. Title: Magnetic tornadoes as energy channels into the solar corona Authors: Wedemeyer-Böhm, Sven; Scullion, Eamon; Steiner, Oskar; Rouppe van der Voort, Luc; de La Cruz Rodriguez, Jaime; Fedun, Viktor; Erdélyi, Robert Bibcode: 2012Natur.486..505W Altcode: Heating the outer layers of the magnetically quiet solar atmosphere to more than one million kelvin and accelerating the solar wind requires an energy flux of approximately 100 to 300 watts per square metre, but how this energy is transferred and dissipated there is a puzzle and several alternative solutions have been proposed. Braiding and twisting of magnetic field structures, which is caused by the convective flows at the solar surface, was suggested as an efficient mechanism for atmospheric heating. Convectively driven vortex flows that harbour magnetic fields are observed to be abundant in the photosphere (the visible surface of the Sun). Recently, corresponding swirling motions have been discovered in the chromosphere, the atmospheric layer sandwiched between the photosphere and the corona. Here we report the imprints of these chromospheric swirls in the transition region and low corona, and identify them as observational signatures of rapidly rotating magnetic structures. These ubiquitous structures, which resemble super-tornadoes under solar conditions, reach from the convection zone into the upper solar atmosphere and provide an alternative mechanism for channelling energy from the lower into the upper solar atmosphere. Title: Dynamics of the Solar Magnetic Bright Points Derived from Their Horizontal Motions Authors: Chitta, L. P.; van Ballegooijen, A. A.; Rouppe van der Voort, L.; DeLuca, E. E.; Kariyappa, R. Bibcode: 2012ApJ...752...48C Altcode: 2012arXiv1204.4362C The subarcsecond bright points (BPs) associated with the small-scale magnetic fields in the lower solar atmosphere are advected by the evolution of the photospheric granules. We measure various quantities related to the horizontal motions of the BPs observed in two wavelengths, including the velocity autocorrelation function. A 1 hr time sequence of wideband Hα observations conducted at the Swedish 1 m Solar Telescope (SST) and a 4 hr Hinode G-band time sequence observed with the Solar Optical Telescope are used in this work. We follow 97 SST and 212 Hinode BPs with 3800 and 1950 individual velocity measurements, respectively. For its high cadence of 5 s as compared to 30 s for Hinode data, we emphasize more the results from SST data. The BP positional uncertainty achieved by SST is as low as 3 km. The position errors contribute 0.75 km2 s-2 to the variance of the observed velocities. The raw and corrected velocity measurements in both directions, i.e., (vx , vy ), have Gaussian distributions with standard deviations of (1.32, 1.22) and (1.00, 0.86) km s-1, respectively. The BP motions have correlation times of about 22-30 s. We construct the power spectrum of the horizontal motions as a function of frequency, a quantity that is useful and relevant to the studies of generation of Alfvén waves. Photospheric turbulent diffusion at timescales less than 200 s is found to satisfy a power law with an index of 1.59. 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: Dynamics of the Photospheric Bright Points Observed With SST and Hinode Authors: Chitta, Lakshmi Pradeep; van Ballegooijen, A.; Rouppe van der Voort, L.; DeLuca, E.; Kariyappa, R. Bibcode: 2012AAS...22020614C Altcode: 2012AAS...22020614P The horizontal motions of the solar magnetic bright points (BPs) observed in two wavelengths (SST Halpha and Hinode/SOT G-band) is studied in detail. With emphasis on SST results: the velocity distribution of horizontal motions is found to be a Gaussian. The auto-correlations of observed velocities is also obtained. An empirical fit to the observed auto-correlation gives us a positional uncertainty of 3 km and the error in the velocity measurements to be 0.87 km s$^{-1}$. Due to the non-Lorentzian, cusp-like nature of the auto-correlation, the power spectrum of the BP motions shows enhanced power at frequencies exceeding 0.02 Hz. The diffusion of magnetic field due to granular evolution at short timescales is found to satisfy a power law with a slope of 1.59. Title: The Formation of the Hα Line in the Solar Chromosphere Authors: Leenaarts, J.; Carlsson, M.; Rouppe van der Voort, L. Bibcode: 2012ApJ...749..136L Altcode: 2012arXiv1202.1926L We use state-of-the-art radiation-MHD simulations and three-dimensional (3D) non-LTE radiative transfer computations to investigate Hα line formation in the solar chromosphere and apply the results of this investigation to develop the potential of Hα as a diagnostic of the chromosphere. We show that one can accurately model Hα line formation assuming statistical equilibrium and complete frequency redistribution provided the computation of the model atmosphere included non-equilibrium ionization of hydrogen and the Lyα and Lyβ line profiles are described by Doppler profiles. We find that 3D radiative transfer is essential in modeling hydrogen lines due to the low photon destruction probability in Hα. The Hα opacity in the upper chromosphere is mainly sensitive to the mass density and only weakly sensitive to the temperature. We find that the Hα line-core intensity is correlated with the average formation height: The larger the average formation height is, the lower the intensity will be. The line-core width is a measure of the gas temperature in the line-forming region. The fibril-like dark structures seen in Hα line-core images computed from our model atmosphere are tracing magnetic field lines. These structures are caused by field-aligned ridges of enhanced chromospheric mass density that raise their average formation height, and therefore make them appear dark against their deeper-formed surroundings. We compare with observations, and find that the simulated line-core widths are very similar to the observed ones, without the need for additional microturbulence. Title: High-resolution observations of type II spicules Authors: Rouppe van der Voort, Luc Bibcode: 2012decs.confE..41R Altcode: Type II spicules are a class of spicules that is connected with mass-loading and heating of the solar corona. Type II spicules are characterized by short lifetimes and high apparent velocities. Combined with their narrow spatial widths, type II spicules are challenging to observe. We use the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope on La Palma to observe type II spicules at the limb and their counterparts on the solar disk, the so-called "Rapid Blue-shifted Excursions" (RBEs). The combination of adaptive optics and image post-processing allows CRISP to attain high resolution simultaneously in the spatial, temporal and spectral domains. Here we present results from the analysis of several high-quality data sets which allow to constrain the physical properties of type II spicules. Title: Small-scale rotating magnetic flux structures as alternative energy channels into the low corona Authors: Wedemeyer-Böhm; , Sven; Scullion; , Eamon; Steiner; , Oskar; Rouppe van der Voort, Luc; de la Cruz Rodriguez, Jaime; Erdelyi, Robertus; Fedun, Viktor Bibcode: 2012decs.confE..67W Altcode: Vortex flows are frequently observed in the downflow areas in the lanes between granules. The magnetic field is advected and trapped by these flows in the low photosphere. Consequently, the rotation of a vortex flow is transferred to the atmospheric layers above by means of the magnetic flux structure. This effect results in so-called swirls, which are observed in the chromosphere. New simultaneous observations with the Swedish Solar Telescope and the Solar Dynamics Observatory reveal that chromospheric swirls can have a coronal counterpart. This finding implies that the rotating flux structure couples the layers of the solar atmosphere from the photosphere to the (low) corona. Three-dimensional numerical simulations confirm this picture and reproduce the swirl signature. A combined analysis of the simulations and observations implies that such small-scale rotating flux structures could provide an alternative mechanism for channeling substantial energy from the photosphere into the upper solar atmosphere. Title: Type-II spicules: Heating and magnetic field properties from aligned CRISP/SST and SDO observations Authors: Scullion, E.; Rouppe van der Voort, L.; de la Cruz Rodriguez, J. Bibcode: 2012decs.confE..44S Altcode: Over the past decade there has been a resurgence in the study of small-scale chromospheric jets known, classically, as spicules. Recent observations have lead us to conclude that there are two distinct varieties of spicule, namely, slower type-I (i.e. mottles, dynamic fibrils, H-alpha spicules etc.) and faster type-II (RBEs: Rapid Blue-shift Excursions on-disk). Such events dominate the dynamics of the chromosphere. Joint SDO (Solar Dynamics Observatory) and Hinode observations have revealed that fast spicules are the source of hot plasma channelling into the corona. Here we report on the properties of this widespread heating with observations from the high resolution CRISP (CRisp Imaging SpectroPolarimeter) instrument at the SST (1-m Swedish Solar Telescope, La Palma) and co-aligned SDO data. Furthermore, we reveal new insight into the formation of type-II spicules through considering the distribution of RBEs with respect to the photospheric magnetic field (via CRISP). Title: Small-scale, Dynamic Bright Blobs in Solar Filaments and Active Regions Authors: Lin, Y.; Engvold, O.; Rouppe van der Voort, L. H. M. Bibcode: 2012ApJ...747..129L Altcode: 2012arXiv1201.0406L High-cadence high spatial resolution observations in Hα with the Swedish 1 m Solar Telescope on La Palma have revealed the existence of small-scale highly dynamic bright blobs. A fast wavelength tuning spectropolarimeter provides spectral information of these structures. The blobs slide along thin magnetic threads at speeds in the range from 45 km s-1 to 111 km s-1. The blobs have a slightly elongated shape and their lengths increase by a factor of three from close to half an arcsecond when they first appear until they disappear one to two minutes later. The brightest blobs show the highest speed. The widths of the Hα line emission of the blobs correspond to non-thermal velocities in the plasma less than 10 km s-1, which implies that they are not the result of shock-driven heating. The dynamic character of the bright blobs is similar to what can be expected from an MHD fast-mode pulse. Title: Observing the Fine Structure of Loops through High-resolution Spectroscopic Observations of Coronal Rain with the CRISP Instrument at the Swedish Solar Telescope Authors: Antolin, P.; Rouppe van der Voort, L. Bibcode: 2012ApJ...745..152A Altcode: 2011arXiv1112.0656A Observed in cool chromospheric lines, such as Hα or Ca II H, coronal rain corresponds to cool and dense plasma falling from coronal heights. Considered as a peculiar sporadic phenomenon of active regions, it has not received much attention since its discovery more than 40 years ago. Yet, it has been shown recently that a close relationship exists between this phenomenon and the coronal heating mechanism. Indeed, numerical simulations have shown that this phenomenon is most likely due to a loss of thermal equilibrium ensuing from a heating mechanism acting mostly toward the footpoints of loops. We present here one of the first high-resolution spectroscopic observations of coronal rain, performed with the CRisp Imaging Spectro Polarimeter (CRISP) instrument at the Swedish Solar Telescope. This work constitutes the first attempt to assess the importance of coronal rain in the understanding of the coronal magnetic field in active regions. With the present resolution, coronal rain is observed to literally invade the entire field of view. A large statistical set is obtained in which dynamics (total velocities and accelerations), shapes (lengths and widths), trajectories (angles of fall of the blobs), and thermodynamic properties (temperatures) of the condensations are derived. Specifically, we find that coronal rain is composed of small and dense chromospheric cores with average widths and lengths of ~310 km and ~710 km, respectively, average temperatures below 7000 K, displaying a broad distribution of falling speeds with an average of ~70 km s-1, and accelerations largely below the effective gravity along loops. Through estimates of the ion-neutral coupling in the blobs we show that coronal rain acts as a tracer of the coronal magnetic field, thus supporting the multi-strand loop scenario, and acts as a probe of the local thermodynamic conditions in loops. We further elucidate its potential in coronal heating. We find that the cooling in neighboring strands occurs simultaneously in general suggesting a similar thermodynamic evolution among strands, which can be explained by a common footpoint heating process. Constraints for coronal heating models of loops are thus provided. Estimates of the fraction of coronal volume with coronal rain give values between 7% and 30%. Estimates of the occurrence time of the phenomenon in loops set times between 5 and 20 hr, implying that coronal rain may be a common phenomenon, in agreement with the frequent observations of cool downflows in extreme-ultraviolet lines. The coronal mass drain rate in the form of coronal rain is estimated to be on the order of 5 × 109 g s-1, a significant quantity compared to the estimate of mass flux into the corona from spicules. Title: Generation and propagation of Alfvenic waves in spicules Authors: De Pontieu, B.; Okamoto, T. J.; Rouppe van der Voort, L.; Hansteen, V. H.; Carlsson, M. Bibcode: 2011AGUFMSH13B1956D Altcode: Both spicules and Alfven waves have recently been implicated in playing a role in the heating of the outer atmosphere. Yet we do not know how spicules or Alfven waves are generated. Here we focus on the properties of Alfvenic waves in spicules and their role in forming spicules. We use high-resolution observations taken with the Solar Optical Telescope onboard Hinode, and with the CRISP Fabry-Perot Interferometer at the Swedish Solar Telescope (SST) in La Palma to study the generation and propagation of Alfvenic waves in spicules and their disk counterparts. Using automated detection algorithms to identify propagating waves in limb spicules, we find evidence for both up- and downward propagating as well as standing waves. Our data suggests significant reflection of waves in and around spicules and provides constraints for theoretical models of spicules and wave propagation through the chromosphere. We also show observational evidence (using SST data) of the generation of Alfven waves and the role they play in forming spicules. 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: Quiet-Sun imaging asymmetries in Na I D1 compared with other strong Fraunhofer lines Authors: Rutten, R. J.; Leenaarts, J.; Rouppe van der Voort, L. H. M.; de Wijn, A. G.; Carlsson, M.; Hansteen, V. Bibcode: 2011A&A...531A..17R Altcode: 2011arXiv1104.4307R Imaging spectroscopy of the solar atmosphere using the Na I D1 line yields marked asymmetry between the blue and red line wings: sampling a quiet-Sun area in the blue wing displays reversed granulation, whereas sampling in the red wing displays normal granulation. The Mg I b2 line of comparable strength does not show this asymmetry, nor does the stronger Ca II 8542 Å line. We demonstrate the phenomenon with near-simultaneous spectral images in Na I D1, Mg I b2, and Ca II 8542 Å from the Swedish 1-m Solar Telescope. We then explain it with line-formation insights from classical 1D modeling and with a 3D magnetohydrodynamical simulation combined with NLTE spectral line synthesis that permits detailed comparison with the observations in a common format. The cause of the imaging asymmetry is the combination of correlations between intensity and Dopplershift modulation in granular overshoot and the sensitivity to these of the steep profile flanks of the Na I D1 line. The Mg I b2 line has similar core formation but much wider wings due to larger opacity buildup and damping in the photosphere. Both lines obtain marked core asymmetry from photospheric shocks in or near strong magnetic concentrations, less from higher-up internetwork shocks that produce similar asymmetry in the spatially averaged Ca II 8542 Å profile. Title: Upflows in the Central Dark Lane of Sunspot Light Bridges Authors: Rouppe van der Voort, L.; Bellot Rubio, L. R.; Ortiz, A. Bibcode: 2010ApJ...718L..78R Altcode: 2010arXiv1006.4578R We use high spatial and spectral resolution observations obtained with the CRisp Imaging SpectroPolarimeter at the Swedish 1 m Solar Telescope to analyze the velocity profile of granular light bridges (LBs) in a sunspot. We find upflows associated with the central dark lanes of the LBs. From bisectors in the Fe I 630.15 nm line we find that the magnitude of the upflows varies with height, with the strongest upflows being deeper in the atmosphere. Typical upflow velocities measured from the 70% bisector are around 500 m s-1 with peaks above 1 km s-1. The upflows in the central dark lane are surrounded by downflows of weaker magnitude, sometimes concentrated in patches with enhanced velocities reaching up to 1.1 km s-1. A small spatial offset between the upflows and the continuum dark lane is interpreted as a line-of-sight effect due to the elevated nature of the dark lane and the LB above the umbral surroundings. Our observations show that the central dark lane in granular LBs is not equivalent to the intergranular lanes of normal photospheric granulation that host convective downflows. These results support recent MHD simulations of magneto-convection in sunspot atmospheres. Title: Downflows in Sunspot Umbral Dots Authors: Ortiz, A.; Bellot Rubio, L. R.; Rouppe van der Voort, L. Bibcode: 2010ApJ...713.1282O Altcode: 2010arXiv1003.1897O We study the velocity field of umbral dots (UDs) at a resolution of 0farcs14. Our analysis is based on full Stokes measurements of a pore taken with the Crisp Imaging Spectro-Polarimeter at the Swedish 1 m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. In addition, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most UDs are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges, with sizes of about 0farcs25, velocities of up to 1000 m s-1, and enhanced net circular polarization signals. The downflows evolve rapidly and have lifetimes of only a few minutes. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields. Title: Observation and analysis of chromospheric magnetic fields . Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; van Noort, M.; Rouppe van der Voort, L. Bibcode: 2010MmSAI..81..716D Altcode: 2010arXiv1004.0698D The solar chromosphere is a vigorously dynamic region of the sun, where waves and magnetic fields play an important role. To improve chromospheric diagnostics, we present new observations in Ca II 8542 carried out with the SST/CRISP on La Palma, working in full-Stokes mode. We measured Stokes line profiles in active regions. The line profiles observed close to the solar limb show signals in all four Stokes parameters, while profiles observed close to disk center only show signals above the noise level in Stokes I and V. We used the NLTE inversion code 'NICOLE' to derive atmospheric parameters in umbral flashes present in a small round sunspot without penumbra. Title: Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope Authors: Ortiz, A.; Rouppe van der Voort, L. H. M. Bibcode: 2010ASSP...19..150O Altcode: 2009arXiv0902.4150O; 2010mcia.conf..150O CRISP (Crisp Imaging Spectro-polarimeter), the new spectropolarimeter at the Swedish 1-m Solar Telescope, opens a new perspective in solar polarimetry. With better spatial resolution (0.13″) than Hinode in the Fe I 6302 Å lines and similar polarimetric sensitivity reached through postprocessing, CRISP complements the SP spectropolarimeter onboard Hinode. We present some of the data that we obtained in our June 2008 campaign and preliminary results from LTE inversions of a pore containing umbral dots. 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. Title: Small-scale swirl events in the quiet Sun chromosphere Authors: Wedemeyer-Böhm, S.; Rouppe van der Voort, L. Bibcode: 2009A&A...507L...9W Altcode: 2009arXiv0910.2226W Context: Recent progress in instrumentation enables solar observations with high resolution simultaneously in the spatial, temporal, and spectral domains.
Aims: We use such high-resolution observations to study small-scale structures and dynamics in the chromosphere of the quiet Sun.
Methods: We analyse time series of spectral scans through the Ca ii 854.2 nm spectral line obtained with the CRISP instrument at the Swedish 1-m Solar Telescope. The targets are quiet Sun regions inside coronal holes close to disc-centre.
Results: The line core maps exhibit relatively few fibrils compared to what is normally observed in quiet Sun regions outside coronal holes. The time series show a chaotic and dynamic scene that includes spatially confined “swirl” events. These events feature dark and bright rotating patches, which can consist of arcs, spiral arms, rings or ring fragments. The width of the fragments typically appears to be of the order of only 0.2 arcsec, which is close to the effective spatial resolution. They exhibit Doppler shifts of -2 to -4 km s-1 but sometimes up to -7 km s-1, indicating fast upflows. The diameter of a swirl is usually of the order of 2´´. At the location of these swirls, the line wing and wide-band maps show close groups of photospheric bright points that move with respect to each other.
Conclusions: A likely explanation is that the relative motion of the bright points twists the associated magnetic field in the chromosphere above. Plasma or propagating waves may then spiral upwards guided by the magnetic flux structure, thereby producing the observed intensity signature of Doppler-shifted ring fragments.

The movie is only available in electronic form at http://www.aanda.org

Marie Curie Intra-European Fellow of the European Commission. Title: Swaying Threads of a Solar Filament Authors: Lin, Y.; Soler, R.; Engvold, O.; Ballester, J. L.; Langangen, Ø.; Oliver, R.; Rouppe van der Voort, L. H. M. Bibcode: 2009ApJ...704..870L Altcode: 2009arXiv0909.2792L From recent high-resolution observations obtained with the Swedish 1 m Solar Telescope in La Palma, we detect swaying motions of individual filament threads in the plane of the sky. The oscillatory characters of these motions are comparable with oscillatory Doppler signals obtained from corresponding filament threads. Simultaneous recordings of motions in the line of sight and in the plane of the sky give information about the orientation of the oscillatory plane. These oscillations are interpreted in the context of the magnetohydrodynamic (MHD) theory. Kink MHD waves supported by the thread body are proposed as an explanation of the observed thread oscillations. On the basis of this interpretation and by means of seismological arguments, we give an estimation of the thread Alfvén speed and magnetic field strength by means of seismological arguments. Title: On the continuum intensity distribution of the solar photosphere Authors: Wedemeyer-Böhm, S.; Rouppe van der Voort, L. Bibcode: 2009A&A...503..225W Altcode: 2009arXiv0905.0705W Context: For many years, there seemed to be significant differences between the continuum intensity distributions derived from observations and simulations of the solar photosphere.
Aims: In order to settle the discussion on these apparent discrepancies, we present a detailed comparison between simulations and seeing-free observations that takes into account the crucial influence of instrumental image degradation.
Methods: We use a set of images of quiet Sun granulation taken in the blue, green and red continuum bands of the Broadband Filter Imager of the Solar Optical Telescope (SOT) onboard Hinode. The images are deconvolved with point spread functions (PSF) that account for non-ideal contributions due to instrumental stray-light and imperfections. In addition, synthetic intensity images are degraded with the corresponding PSFs. The results are compared with respect to spatial power spectra, intensity histograms, and the centre-to-limb variation of the intensity contrast.
Results: The intensity distribution of SOT granulation images is broadest for the blue continuum at disc-centre and narrows towards the limb and for longer wavelengths. The distributions are relatively symmetric close to the limb but exhibit a growing asymmetry towards disc-centre. The intensity contrast, which is connected to the width of the distribution, is found to be (12.8 ± 0.5)%, (8.3 ± 0.4)%, and (6.2 ± 0.2)% at disc-centre for blue, green, and red continuum, respectively. Removing the influence of the PSF unveils much broader intensity distributions with a secondary component that is otherwise only visible as an asymmetry between the darker and brighter than average part of the distribution. The contrast values increase to (26.7 ± 1.3)%, (19.4 ± 1.4)%, and (16.6 ± 0.7)% for blue, green, and red continuum, respectively. The power spectral density of the images exhibits a pronounced peak at spatial scales characteristic for the granulation pattern and a steep decrease towards smaller scales. The observational findings like the absolute values and centre-to-limb variation of the intensity contrast, intensity histograms, and power spectral density are well matched with corresponding synthetic observables from three-dimensional radiation (magneto-)hydrodynamic simulations.
Conclusions: We conclude that the intensity contrast of the solar continuum intensity is higher than usually derived from ground-based observations and is well reproduced by modern radiation (magneto-)hydrodynamic models. Properly accounting for image degradation effects is of crucial importance for comparisons between observations and numerical models. Title: Spectropolarimetric Diagnostics at the Solar Photosphere near the Limb Authors: Yelles Chaouche, L.; Solanki, S. K.; Rouppe van der Voort, L.; van Noort, M. Bibcode: 2009ASPC..405..189Y Altcode: 2009arXiv0910.2885Y In the present work, we investigate the formation of Stokes profiles and spectro-polarimetric diagnostics in an active region plage near the limb. We use 3-D radiation-MHD simulations with unipolar fields of an average strength of 400 G, which is largely concentrated in flux tubes in which the field reaches typical kilo-Gauss values. We generate synthetic Stokes spectra by radiative transfer calculations, then we degrade the simulated Stokes signal to account for observational conditions. The synthetic data treated in this manner are compared with and found to roughly reproduce spectro-polarimetric high-resolution observations at μ=0.39 obtained by the SOUP instrument with the Swedish 1-m Solar Telescope at the beginning of 2006. Title: Three-Dimensional Non-LTE Radiative Transfer Computation of the CA 8542 Infrared Line From a Radiation-MHD Simulation Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rouppe van der Voort, L. Bibcode: 2009ApJ...694L.128L Altcode: 2009arXiv0903.0791L The interpretation of imagery of the solar chromosphere in the widely used Ca II 854.2 nm infrared line is hampered by its complex, three-dimensional, and non-LTE formation. Forward modeling is required to aid understanding. We use a three-dimensional non-LTE radiative transfer code to compute synthetic Ca II 854.2 nm images from a radiation-MHD simulation of the solar atmosphere spanning from the convection zone to the corona. We compare the simulation with observations obtained with the CRISP filter at the Swedish 1 m Solar Telescope. We find that the simulation reproduces dark patches in the blue line wing caused by Doppler shifts, brightenings in the line core caused by upward-propagating shocks, and thin dark elongated structures in the line core that form the interface between upward and downward gas motion in the chromosphere. The synthetic line core is narrower than the observed one, indicating that the Sun exhibits both more vigorous large-scale dynamics as well as small scale motions that are not resolved within the simulation, presumably owing to a lack of spatial resolution. Title: The solar continuum intensity distribution. Settling the conflict between observations and simulations Authors: Wedemeyer-Böhm, S.; Rouppe van der Voort, L. Bibcode: 2009MmSAI..80..635W Altcode: 2009arXiv0908.3758W For many years, there seemed to be significant differences between the continuum intensity distributions derived from observations and simulations of the solar photosphere. In order to settle the discussion on these apparent discrepancies, we present a detailed comparison between simulations and seeing-free observations that takes into account the crucial influence of instrumental image degradation. We use a set of images of quiet Sun granulation taken in the blue, green and red continuum bands of the Broadband Filter Imager of the Solar Optical Telescope (SOT) onboard Hinode. The images are deconvolved with Point Spread Functions (PSF) that account for non-ideal contributions due to instrumental stray-light and imperfections. In addition, synthetic intensity images are degraded with the corresponding PSFs. The results are compared with respect to spatial power spectra, intensity histograms, and the centre-to-limb variation of the intensity contrast. The observational findings are well matched with corresponding synthetic observables from three-dimensional radiation (magneto-)hydrodynamic simulations. We conclude that the intensity contrast of the solar continuum intensity is higher than usually derived from ground-based observations and is well reproduced by modern numerical simulations. Properly accounting for image degradation effects is of crucial importance for comparisons between observations and numerical models. It finally settles the traditionally perceived conflict between observations and simulations. Title: Stokes imaging polarimetry using image restoration at the Swedish 1-m solar telescope Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M. Bibcode: 2008A&A...489..429V Altcode: 2008arXiv0805.4296V Aims: We aim to achieve both high spatial resolution and high polarimetric sensitivity, using an earth-based 1m-class solar telescope, for the study of magnetic fine structure on the surface of the Sun.
Methods: We use a setup with 3 high-speed, low-noise cameras to construct datasets with interleaved polarimetric states, particularly suitable for Multi-Object Multi-Frame Blind Deconvolution image restorations. We discuss the polarimetric calibration routine and various potential sources of error in the results.
Results: We obtained near diffraction limited images, which have a noise level of ≈ 10-3 I_cont. We confirm that dark cores have a weaker magnetic field and a lower inclination angle with respect to the solar surface than the edges of the penumbral filament. We demonstrate that the magnetic field strength in faculae-striations is significantly lower than in other nearby parts of the faculae. Title: On small active region filaments, fibrils and surges Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort, L. H. M.; van Noort, M. Bibcode: 2008AdSpR..42..803L Altcode: High resolution Hα images and magnetograms (0.2 arc s) of an active region were obtained in alternating time series at 42 s cadences using the Swedish 1-m Solar Telescope on 2004 August 21. The Hα filtergrams reveal an active region filament and surges consisting of thread-like structures which have widths similar to the widths of chromospheric fibrils, both recorded down to the resolution limit in the best images. All observed structures in the active region appear highly dynamic. Fibrils show counterstreaming strongly resembling the counterstreaming threads in filaments. Streaming, along the threads of surges extending more than 10 arc s, is higher in speed (∼20 km s-1) than in the filament and fibrils and appears to flow independently over and above the chromospheric fibrils. Blue shifts seen in the Hα Dopplergrams confirm the outward mass motion of the surges. However, in at least one case, we also see simultaneous downflows from the same site but in the opposite direction and downward toward the chromosphere. We suggest that the site between these two outward and downward flows identifies the place where magnetic reconnection could occur and thereby cause of the surge. This appears to imply that the reconnection site is in the high chromosphere or low corona. Title: Moat Flow in the Vicinity of Sunspots for Various Penumbral Configurations Authors: Vargas Domínguez, S.; Rouppe van der Voort, L.; Bonet, J. A.; Martínez Pillet, V.; Van Noort, M.; Katsukawa, Y. Bibcode: 2008ApJ...679..900V Altcode: 2008arXiv0802.1457V High-resolution time series of sunspots have been obtained with the Swedish 1 m Solar Telescope between 2003 and 2006 at different locations on the solar disk. Proper motions in seven different active regions have been studied. The analysis was performed by applying local correlation tracking to every series of sunspots, each of them more than 40 minutes long. The sunspots' shapes include a different variety of penumbral configurations. We report on the systematic behavior of the large-scale outflows surrounding the sunspots, commonly known as moat flows, that are essentially present only when preceded by a penumbra not tangential but perpendicular to the sunspot border. We present one case for which this rule appears not to be confirmed. We speculate that the magnetic neutral line, which is located in the vicinity of the anomalous region, might be responsible for blocking the outflow. These new results confirm the systematic and strong relation between the moat flows and the existence of penumbrae. A comparative statistical study between moats and standard granulation is also performed. Title: Measurements of Plasma Motions in Dynamic Fibrils Authors: Langangen, Øystein; Rouppe van der Voort, Luc; Lin, Yong Bibcode: 2008ApJ...673.1201L Altcode: 2007arXiv0710.1011L We present a 40 minute time series of filtergrams from the red and blue wings of the Hα line in an active region near the solar disk center. From these filtergrams we construct both Dopplergrams and summed "line center" images. Several dynamic fibrils (DFs) are identified in the summed images. The data are used to simultaneously measure the proper motion and the Doppler signals in DFs. For calibration of the Doppler signals, we use spatially resolved spectrograms of a similar active region. Significant variations in the calibration constant for different solar features are observed, and only regions containing DFs have been used in order to reduce calibration errors. We find a coherent behavior of the Doppler velocity and the proper motion that clearly demonstrates that the evolution of DFs involves plasma motion. The Doppler velocities are found to be a factor of 2-3 smaller than velocities derived from proper motions in the image plane. The difference can be explained by the radiative processes involved, as the Doppler velocity is a result of the local atmospheric velocity weighted with the response function. As a result, the Doppler velocity originates from a wide range of heights in the atmosphere. This is contrasted with the proper-motion velocity, which is measured from the sharply defined bright tops of the DFs and is therefore a very local velocity measure. The Doppler signal originates from well below the top of the DF. Finally, we discuss how this difference, together with the lower spatial resolution of older observations, has contributed to some of the confusion about the identity of DFs, spicules, and mottles. Title: Spectroscopic Measurements of Dynamic Fibrils in the Ca II λ8662 Line Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo; De Pontieu, Bart Bibcode: 2008ApJ...673.1194L Altcode: 2007arXiv0710.0247L We present high spatial resolution spectroscopic measurements of dynamic fibrils (DFs) in the Ca II λ8662 line. These data show clear Doppler shifts in the identified DFs, which demonstrates that at least a subset of DFs are actual mass motions in the chromosphere. A statistical analysis of 26 DFs reveals a strong and statistically significant correlation between the maximal velocity and the deceleration. The range of the velocities and the decelerations are substantially lower, about a factor 2, in our spectroscopic observations compared to the earlier results based on proper motion in narrowband images. There are fundamental differences in the different observational methods; when DFs are observed spectroscopically, the measured Doppler shifts are a result of the atmospheric velocity, weighted with the response function to velocity over an extended height. When the proper motion of DFs is observed in narrowband images, the movement of the top of the DF is observed. This point is sharply defined because of the high contrast between the DF and the surroundings. The observational differences between the two methods are examined by several numerical experiments using both numerical simulations and a time series of narrowband Hα images. With basis in the simulations we conclude that the lower maximal velocity is explained by the low formation height of the Ca IR line. We conclude that the present observations support the earlier result that DFs are driven by magnetoacoustic shocks excited by convective flows and p-modes. Title: Evidence of Traveling Waves in Filament Threads Authors: Lin, Y.; Engvold, O.; Rouppe van der Voort, L. H. M.; van Noort, M. Bibcode: 2007SoPh..246...65L Altcode: High-resolution Hα filtergrams (0.2″) obtained with the Swedish 1-m Solar Telescope resolve numerous very thin, thread-like structures in solar filaments. The threads are believed to represent thin magnetic flux tubes that must be longer than the observable threads. We report on evidence for small-amplitude (1 - 2 km s−1) waves propagating along a number of threads with an average phase velocity of 12 km s−1 and a wavelength of 4″. The oscillatory period of individual threads vary from 3 to 9 minutes. Temporal variation of the Doppler velocities averaged over a small area containing a number of individual threads shows a short-period (3.6 minutes) wave pattern. These short-period oscillations could possibly represent fast modes in accordance with numerical fibril models proposed by Díaz et al. (Astron. Astrophys.379, 1083, 2001). In some cases, it is clear that the propagating waves are moving in the same direction as the mass flows. Title: What are 'Faculae'? Authors: Berger, T. E.; Title, A. M.; Tarbell, T.; Rouppe van der Voort, L.; Löfdahl, M. G.; Scharmer, G. B. Bibcode: 2007ASPC..369..103B Altcode: We present very high resolution filtergram and magnetogram observations of solar faculae taken at the Swedish 1-meter Solar Telescope (SST) on La Palma. Three datasets with average line-of-sight angles of 16, 34, and 53 degrees are analyzed. The average radial extent of faculae is at least 400~km. In addition we find that contrast versus magnetic flux density is nearly constant for faculae at a given disk position. These facts and the high resolution images and movies reveal that faculae are not the interiors of small flux tubes - they are granules seen through the transparency caused by groups of magnetic elements or micropores ``in front of'' the granules. Previous results which show a strong dependency of facular contrast on magnetic flux density were caused by bin-averaging of lower resolution data leading to a mixture of the signal from bright facular walls and the associated intergranular lanes and micropores. The findings are relevant to studies of total solar irradiance (TSI) that use facular contrast as a function of disk position and magnetic field in order to model the increase in TSI with increasing sunspot activity. Title: Relationships between magnetic foot points and G-band bright structures Authors: Ishikawa, R.; Tsuneta, S.; Kitakoshi, Y.; Katsukawa, Y.; Bonet, J. A.; Vargas Domínguez, S.; Rouppe van der Voort, L. H. M.; Sakamoto, Y.; Ebisuzaki, T. Bibcode: 2007A&A...472..911I Altcode: 2008arXiv0802.1765I Aims:Magnetic elements are thought to be described by flux tube models, and are well reproduced by MHD simulations. However, these simulations are only partially constrained by observations. We observationally investigate the relationship between G-band bright points and magnetic structures to clarify conditions, which make magnetic structures bright in G-band.
Methods: The G-band filtergrams together with magnetograms and dopplergrams were taken for a plage region covered by abnormal granules as well as ubiquitous G-band bright points, using the Swedish 1-m Solar Telescope (SST) under very good seeing conditions.
Results: High magnetic flux density regions are not necessarily associated with G-band bright points. We refer to the observed extended areas with high magnetic flux density as magnetic islands to separate them from magnetic elements. We discover that G-band bright points tend to be located near the boundary of such magnetic islands. The concentration of G-band bright points decreases with inward distance from the boundary of the magnetic islands. Moreover, G-band bright points are preferentially located where magnetic flux density is higher, given the same distance from the boundary. There are some bright points located far inside the magnetic islands. Such bright points have higher minimum magnetic flux density at the larger inward distance from the boundary. Convective velocity is apparently reduced for such high magnetic flux density regions regardless of whether they are populated by G-band bright points or not. The magnetic islands are surrounded by downflows.
Conclusions: These results suggest that high magnetic flux density, as well as efficient heat transport from the sides or beneath, are required to make magnetic elements bright in G-band. Title: Contrast Analysis of Solar Faculae and Magnetic Bright Points Authors: Berger, T. E.; Rouppe van der Voort, L.; Löfdahl, M. Bibcode: 2007ApJ...661.1272B Altcode: The morphology and contrast of small-scale solar magnetic elements at four disk positions is analyzed. The data were obtained at the Swedish 1 m Solar Telescope (SST) over 3 yr (2003-2005). Two of the data sets have disk positions near disk center (average μ=cosθ>0.8) and show numerous ``magnetic bright points'' (MBPs), and two are sufficiently limbward to show prominent ``faculae'' (average μ<=0.6). The filtergrams are obtained in the 430.5 nm G band and 436.4 nm ``continuum'' bandpasses; the magnetograms are Fe I 630.25 nm Stokes V images taken with the Solar Optical Universal Polarimeter (SOUP) tunable filter. In all images we achieve nearly diffraction-limited resolution (~100 km in the G band). Analysis shows that MBPs and faculae are distinct radiative signatures of the magnetic field: MBPs have a constant or slightly decreasing contrast with increasing magnetogram signal, while facular contrast increases linearly with magnetogram signal. Faculae are much larger than MBPs, with an average radial width of 400 km. The observations support recent modeling showing that faculae are granules seen through the opacity reduction provided by magnetic elements (or groups thereof), while MBPs are caused by lateral radiation leakage scattering from deeper layers of the magnetic element. Title: High-Resolution Observations and Numerical Simulations of Chromospheric Fibrils and Mottles Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2007ASPC..368...65D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in Hα) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations.

In addition, we present some preliminary observations of quiet Sun jets or mottles. We find that the mechanism that produces fibrils in active regions is most likely also at work in quiet Sun regions, although it is modified by the weaker magnetic field and the presence of more mixed-polarity. A comparison with numerical simulations suggests that the weaker magnetic field in quiet Sun allows for significantly stronger (than in active regions) transverse motions that are superposed on the field-aligned, shock-driven motions. This leads to a more dynamic, and much more complex environment than in active region plage. In addition, our observations of the mixed polarity environment in quiet Sun regions suggest that other mechanisms, such as reconnection, may well play a significant role in the formation of some quiet Sun jets. Simultaneous high-resolution magnetograms (such as those provided by Hinode), as well as numerical simulations that take into account a whole variety of different magnetic configurations, will be necessary to determine the relative importance in quiet Sun of, respectively, the fibril-mechanism and reconnection. Title: Chromospheric Spectrometry at High Spatial Resolution Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L.; Hansteen, V. H.; de Pontieu, B. Bibcode: 2007ASPC..368..145L Altcode: In this summary we present spectrograms and images of the chromosphere obtained in a co-observation campaign with the SST and the DOT. The data are used to identify and measure the Doppler shifts of dynamic fibrils. Quantitative comparison with the results of

\citep{ol-2006ApJ...647L..73H} requires compensation for several observational issues. Title: On the Moat-Penumbra Relation Authors: Vargas Domínguez, S.; Bonet, J. A.; Martínez Pillet, V.; Katsukawa, Y.; Kitakoshi, Y.; Rouppe van der Voort, L. Bibcode: 2007ApJ...660L.165V Altcode: 2007astro.ph..2713V Proper motions in a sunspot group with a δ-configuration and close to the solar disk center have been studied by employing local correlation tracking techniques. The analysis is based on a more than 1 hr time series of G-band images. Radial outflows with a mean speed of 0.67 km s-1 have been detected around the spots, the well-known sunspots moats. However, these outflows are not found in those umbral core sides without penumbra. Moreover, moat flows are only found in those sides of penumbrae located in the direction marked by the penumbral filaments. Penumbral sides perpendicular to them show no moat flow. These results strongly suggest a relation between the moat flow and the well-known, filament-aligned Evershed flow. The standard picture of a moat flow originating from a blocking of the upward propagation of heat is discussed in some detail. Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen, V. H.; Carlsson, M.; van Noort, M. Bibcode: 2007ApJ...660L.169R Altcode: 2007astro.ph..3535R We present high spatial and high temporal resolution observations of the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on La Palma. We observe that many mottles, jetlike features in the quiet Sun, display clear up- and downward motions along their main axis. In addition, many mottles show vigorous transverse displacements. Unique identification of the mottles throughout their lifetime is much harder than for their active region counterpart, dynamic fibrils. This is because many seem to lack a sharply defined edge at their top, and significant fading often occurs throughout their lifetime. For those mottles that can be reliably tracked, we find that the mottle tops often undergo parabolic paths. We find a linear correlation between the deceleration these mottles undergo and the maximum velocity they reach, similar to what was found earlier for dynamic fibrils. Combined with an analysis of oscillatory properties, we conclude that at least part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed-polarity environment and vigorous dynamics suggest that reconnection may play a significant role in the formation of some quiet-Sun jets. Title: Observations and Simulations of Fibrils and Mottles Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort, Luc; van Noort, Michiel; Carlsson, Mats Bibcode: 2007astro.ph..2081D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in H-alpha) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. In addition, we present some preliminary observations of quiet Sun jets or mottles. We find that the mechanism that produces fibrils in active regions is most likely also at work in quiet Sun regions, although it is modified by the weaker magnetic field and the presence of more mixed-polarity. A comparison with numerical simulations suggests that the weaker magnetic field in quiet Sun allows for significantly stronger (than in active regions) transverse motions that are superposed on the field-aligned, shock-driven motions. This leads to a more dynamic, and much more complex environment than in active region plage. In addition, our observations of the mixed polarity environment in quiet Sun regions suggest that other mechanisms, such as reconnection, may well play a significant role in the formation of some quiet Sun jets. Title: Velocities Measured in Small-Scale Solar Magnetic Elements Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort, Luc; Stein, R. F. Bibcode: 2007ApJ...655..615L Altcode: 2006astro.ph.11741L We have obtained high-resolution spectrograms of small-scale magnetic structures with the Swedish 1-m Solar Telescope. We present Doppler measurements at 0.2" spatial resolution of bright points, ribbons, and flowers, and their immediate surroundings, in the C I λ5380.3 line (formed in the deep photosphere) and the two Fe I lines at 5379.6 and 5386.3 Å. The velocity inside the flowers and ribbons are measured to be almost zero, while we observe downflows at the edges. These downflows are increasing with decreasing height. We also analyze realistic magnetoconvective simulations to obtain a better understanding of the interpretation of the observed signal. We calculate how the Doppler signal depends on the velocity field in various structures. Both the smearing effect of the nonnegligible width of this velocity response function along the line of sight and of the smearing from the telescope and atmospheric point-spread function are discussed. These studies lead us to the conclusion that the velocity inside the magnetic elements is really upflow of order 1-2 km s-1, while the downflows at the edges really are much stronger than observed, of order 1.5-3.3 km s-1. Title: High-Resolution Observations and Modeling of Dynamic Fibrils Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2007ApJ...655..624D Altcode: 2007astro.ph..1786D We present unprecedented high-resolution Hα observations, obtained with the Swedish 1 m Solar Telescope, that, for the first time, spatially and temporally resolve dynamic fibrils in active regions on the Sun. These jetlike features are similar to mottles or spicules in quiet Sun. We find that most of these fibrils follow almost perfect parabolic paths in their ascent and descent. We measure the properties of the parabolic paths taken by 257 fibrils and present an overview of the deceleration, maximum velocity, maximum length, and duration, as well as their widths and the thickness of a bright ring that often occurs above dynamic fibrils. We find that the observed deceleration of the projected path is typically only a fraction of solar gravity and incompatible with a ballistic path at solar gravity. We report on significant differences of fibril properties between those occurring above a dense plage region and those above a less dense plage region where the magnetic field seems more inclined from the vertical. We compare these findings to advanced numerical two-dimensional radiative MHD simulations and find that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. Detailed comparison of observed and simulated fibril properties shows striking similarities of the values for deceleration, maximum velocity, maximum length, and duration. We compare our results with observations of mottles and find that a similar mechanism is most likely at work in the quiet Sun. Title: High Spatial Resolution Observations of Solar Magnetic Structures Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.; Hansteen, V. Bibcode: 2006ASPC..354...37R Altcode: We present observations of the dynamic evolution of photospheric magnetic structures in the G-band, continuum, magnetograms and Dopplergrams. The observations were obtained with the Swedish one-m Solar Telescope on La Palma. Using adaptive optics and the Multi-Object Multi-Frame Blind Deconvolution image restoration technique, we obtained several datasets at close to the diffraction limit of the telescope (0.1 arcsec) over long periods of time. We show examples of the dynamical evolution of different magnetic structures: the advection of individual bright points by the granular flow, the formation and fragmentation of flux sheets, and the continuous transition between micro-pores, elongated ribbons and more circular ``flowers''. Narrow sheets with downdrafts are found right at the edges of magnetic field concentrations. Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2006AGUFMSH23B0359D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. We will present results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in Hα) above and about active region plage. These jets are similar to mottles and spicules in quiet Sun. Our analysis is based on a time series of extremely high-resolution (120 km) images taken in Hα linecenter at 1 second cadence, obtained by the Oslo group at the SST in October 2005. The 78 min long time series for the first time, spatially and temporally resolves dynamic fibrils in active regions. Our analysis shows that most of the fibrils follow almost perfect parabolic paths in their ascent and descent. We measure the properties of the parabolic paths taken by 257 different dynamic fibrils, and find that the observed deceleration of the projected path is typically only a fraction of solar gravity, and incompatible with a ballistic path at solar gravity. We report on significant differences of measured fibril properties between those occurring in association with a dense plage region, and those above a less dense plage region where the magnetic field seems more inclined away from the vertical. We compare these observational findings to advanced numerical 2D radiative MHD simulations, and find that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. Detailed comparison of the properties of fibrils found in our observations and those in our numerical simulations shows striking similarities of the values for deceleration, maximum velocity, maximum length and duration. The numerical simulations also reproduce the correlations we observe between various fibrils properties, as well as the regional differences, taking into account the different magnetic configuration for the various regions. We compare our results with observations of mottles and find that a similar mechanism is most likely at work in the quiet Sun. Title: Solar Image Restoration by use of Multi-Object Multi-Frame Blind Deconvolution Authors: van Noort, M.; Rouppe van der Voort, L.; Löfdahl, M. Bibcode: 2006ASPC..354...55V Altcode: We present examples of the application of the image restoration method of Multi-Object Multi-Frame Blind Deconvolution to observations obtained with the Swedish one-meter Solar Telescope on La Palma. This restoration method is an extension of Joint Phase Diverse Speckle image restoration. Multiple realizations of multiple objects can now be restored jointly, facilitating near-perfect alignment between different objects. This greatly reduces false signals in the determination of derived quantities, such as magnetograms, Dopplergrams and G-band-continuum difference images. Title: High-Resolution Observations of Fast Events in the Solar Chromosphere Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M. Bibcode: 2006ApJ...648L..67V Altcode: We present new, high spatial and high temporal resolution observations of the Sun in Hα obtained with the Swedish 1 m Solar Telescope on La Palma. The combined use of adaptive optics and image restoration techniques yielded a near-diffraction-limited time series with a cadence of 3 frames s-1 of two different active regions. The unique combination of high temporal and spatial resolution reveals to us the existence of highly dynamic structures, moving at velocities of up to 240 km s-1, and high-velocity waves in the chromosphere. The rapid motions appear to be common, as they are observed in two data sets recorded in succession at different locations on the solar disk. The dynamic events are probably associated with reconfigurations of the magnetic field. Title: Rapid Temporal Variability of Faculae: High-Resolution Observations and Modeling Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort, L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G. Bibcode: 2006ApJ...646.1405D Altcode: We present high-resolution G-band observations (obtained with the Swedish 1 m Solar Telescope) of the rapid temporal variability of faculae, which occurs on granular timescales. By combining these observations with magnetoconvection simulations of a plage region, we show that much of this variability is not intrinsic to the magnetic field concentrations that are associated with faculae, but rather a phenomenon associated with the normal evolution and splitting of granules. We also show examples of facular variability caused by changes in the magnetic field, with movies of dynamic behavior of the striations that dominate much of the facular appearance at 0.1" resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying. Title: High resolution observations of solar magnetic fields Authors: Rouppe van der Voort, L. Bibcode: 2006IAUJD...3E..77R Altcode: With the development of adaptive optics systems and image restoration techniques, high spatial resolution observations of small scale magnetic fields have become more frequent. I will discuss some of the recent studies that reveal the intricate structure and dynamics of solar magnetic fields with increasing clarity. Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2006ApJ...647L..73H Altcode: 2006astro.ph..7332H The formation of jets such as dynamic fibrils, mottles, and spicules in the solar chromosphere is one of the most important, but also most poorly understood, phenomena of the Sun's magnetized outer atmosphere. We use extremely high resolution observations from the Swedish 1 m Solar Telescope combined with advanced numerical modeling to show that in active regions these jets are a natural consequence of upwardly propagating slow-mode magnetoacoustic shocks. These shocks form when waves generated by convective flows and global p-mode oscillations in the lower lying photosphere leak upward into the magnetized chromosphere. We find excellent agreement between observed and simulated jet velocities, decelerations, lifetimes, and lengths. Our findings suggest that previous observations of quiet-Sun spicules and mottles may also be interpreted in light of a shock-driven mechanism. Title: High Resolution Spectropolarimetry of Penumbral Formation with IBIS Authors: Reardon, Kevin; Casini, R.; Cavallini, F.; Tomczyk, S.; Rouppe van der Voort, L.; Van Noort, M.; Woeger, F.; Socas Navarro, H.; IBIS Team Bibcode: 2006SPD....37.3503R Altcode: 2006BAAS...38..260R We present the results of first spectropolarimetric observations made with the Interferometric Bidimensional Spectrometer (IBIS) at the NSO/Dunn Solar Telescope. The use of narrowband imaging and post-facto reconstruction techniques allows for observations close to the diffraction limit of the vector magnetic field. We will show observations of the the formation of an individual penumbral filament around a small pore. We measure the magnetic field and velocity field of the forming penumbral filament. The spectropolarimetric mode of IBIS will be available to the community in the fall of 2006. Title: Dynamics of an active region filament, fibrils and surges in high resolution Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort, L. H. M.; van Noort, M. Bibcode: 2006cosp...36.3193L Altcode: 2006cosp.meet.3193L High resolution H alpha and magnetograms 0 2 arc sec of an active region were obtained in alternating time series at 42 sec cadences using the Swedish 1-m Solar Telescope on 2004 August 21 The 79 times 52 arcsec field of view was centered at N11 W5 The H alpha filtergrams reveal an active region filament and surges consisting of thread-like structures which have widths similar to the widths of chromospheric fibrils both recorded down to the resolution limit in the best images All observed structures in the active region are highly dynamic The flow speeds in some active filament threads 25 km s -1 are higher than typical speeds in quiescent filament threads Fibrils show counterstreaming strongly resembling the counterstreaming threads in filaments The transverse speeds of mass within fibrils are comparable to the typical speeds of mass flows in quiescent filament threads sim 10 km s -1 Streaming along the threads of surges extending more than 10 arc sec is higher in speed sim 20 km s -1 than in the filament and fibrils and appears to flow over the chromospheric fibrils Small surges near one end of the filament appear to flow into the filament thereby mass seems to be added to the filament Blue shifts seen in the H alpha Dopplergrams confirm the outward mass motion of the surges However in at least one case we also see simultaneous red shifts from the same site in the opposite direction toward the chromosphere We suggest that the site between these two opposite motions identifies the place where magnetic Title: The Diagnostic Potential of the MG i 4571.1 Å Line Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L. Bibcode: 2005ESASP.600E..65L Altcode: 2005ESPM...11...65L; 2005dysu.confE..65L No abstract at ADS Title: The wings of Ca II H and K as solar fluxtube diagnostics Authors: Sheminova, V. A.; Rutten, R. J.; Rouppe van der Voort, L. H. M. Bibcode: 2005A&A...437.1069S Altcode: We combine high-resolution Ca II H and K spectrograms from the Swedish Vacuum Solar Telescope with standard fluxtube modeling to derive photospheric temperature and velocity stratifications within individual magnetic elements in plage near a sunspot. We find that 1D on-axis modeling gives better consistency than spatial averaging over flaring-fluxtube geometry. Our best-fit temperature stratifications suggest that magnetic elements are close to radiative equilibrium throughout their photospheres. Their brightness excess throughout the H and K wings compared with the quiet photosphere is primarily due to low density, not to mechanical heating. We conclude that the extended H and K wings provide excellent fine-structure diagnostics for both high-resolution observations and simulations of the solar photosphere. Title: The temperature gradient in and around solar magnetic fluxtubes Authors: Sheminova, V. A.; Rutten, R. J.; Rouppe van der Voort, L. H. M. Bibcode: 2005KFNTS...5..110S Altcode: We use spectra covering the outer part of the extended wing of the solar Ca II K line observed at high angular resolution with the Swedish Vacuum Solar Telescope to test standard solar fluxtube models. The wings of the Ca II resonance lines are formed in LTE both with regard to excitation (source function) and to ionization (opacity) and, therefore, sample temperature stratifications in relatively straightforward fashion. We obtain best fits by combining steeper temperature gradients than those in the standard models for both the tube inside and the tube environment. Similarly steep gradients are also determined from a numerical magnetoconvection simulation by the late A. S. Gadun. It is found that the energy balance in the individual magnetic elements appears to be close to radiative equilibrium throughout the photosphere. Title: High resolution magnetogram measurements of solar faculae Authors: Berger, T. E.; Rouppe van der Voort, L.; Lofdahl, M. G. Bibcode: 2005AGUSMSP31A..02B Altcode: We present new images of magnetic elements near the solar limb ("faculae") along with magnetogram measurements and contrast profiles. Imaging magnetogram observations were made of AR 10377 at μ = cosθ = 0.6 on 06-June-2003 using the Swedish 1-m Solar Telescope; the data are the highest spatial resolution magnetogram measurements of faculae to date. Contrary to previous lower resolution measurements, we find no correlation between facular contrast and magnetic flux density at a given disk position. Increasing magnetic flux density in a region implies an increasing prevalence of micropores. Previous observations which lacked sufficient spatial resolution to discern dark micropore "floors" from bright facular walls find a strong non-linear dependence of facular contrast on magnetic flux density, with decreasing contrast beyond a certain flux density. We show instead that the observed contrast of bright facular walls is independent of magnetic flux density when properly segmented from dark micropores. The observations are useful for examining the detailed structure of faculae including the dark lanes found on the disk-center side of many faculae (explained by several recent 3D MHD numerical simulations). The average radial profile for 678 faculae segmented from the dataset is very nearly gaussian with a FWHM radial extent of 265 km and an extended tail on the limbward side, as predicted by current MHD simulations. Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical evolution Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson, M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E. Bibcode: 2005A&A...435..327R Altcode: Small magnetic structures can be seen in G-band filtergrams as isolated bright points, strings of bright points and dark micro-pores. At a resolution of 0.1 arcsec, new forms of magnetic structures are found in strong field areas: elongated “ribbons” and more circular “flowers”. In this paper we study the temporal evolution of these small scale magnetic structures. In strong-field regions the time-evolution is more that of a magnetic fluid than that of collections of flux-tubes that keep their identity. We find that the granular flow concentrates the magnetic field into flux sheets that are visible as thin bright features in the filtergrams. Weak upflows are found in the flux sheets and downflows in the immediate surroundings. The flux sheets often become unstable to a fluting instability and the edges buckle. The sheets tend to break up into strings of bright points, still with weak upflows in the magnetic elements and zero velocity or downflows between them. Where there are larger flux concentrations we find ribbons, flowers and micro-pores. There is a continuous transition between these forms and they evolve from one form to another. The appearance is mostly determined by the horizontal size - larger structures are dark (micro-pores), narrower structures are ribbon shaped and the flowers are the smallest in extent. All these structures have darker inner parts and a bright edge. The plasma is found to be at rest in the ribbons, with small concentrations of weak upflow sites. Narrow sheets with downdrafts are found right at the edges of the magnetic field concentrations. Title: Solar Filaments and Photospheric Network Authors: Lin, Yong; Wiik, Jun Elin; Engvold, Oddbjørn; Van Der Voort, Luc Rouppe; Frank, Zoe A. Bibcode: 2005SoPh..227..283L Altcode: The locations of barbs of quiescent solar filaments are compared with the photospheric/chromospheric network, which thereby serves as a proxy of regions with enhanced concentrations of magnetic flux. The study covers quiet regions, where also the photospheric network as represented by flow converging regions, i.e., supergranular cell boundaries, contain largely weak magnetic fields. It is shown that close to 65% of the observed end points of barbs falls within the network boundaries. The remaining fraction points into the inner areas of the network cells. This confirms earlier findings (Lin et al., Solar Physics, 2004) that quiescent filaments are basically connected with weaker magnetic fields in the photosphere below. Title: Solar magnetic elements at 0.1 arcsec resolution. General appearance and magnetic structure Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.; Title, A.; Scharmer, G. Bibcode: 2004A&A...428..613B Altcode: New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were obtained at the recently commissioned Swedish 1-m Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.1 arcsec) structures and find new forms of magnetic structures in this particular region. A cotemporal Ca II H-line image is used to examine the low-chromosphere of network elements. A cotemporal Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx cm-2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s-1. We find that magnetic flux in this region as seen in both the magnetogram and the G-band image is typically structured into larger, amorphous, ``ribbons'' which are not resolved into individual flux tubes. The measured magnetic flux density in the ribbon structures ranges from 300 to 1500 Mx cm-2, the higher values occurring at localized concentrations embedded within the ribbons. The Dopplergram indicates relative downflows associated with all magnetic elements with some indication that higher downflows occur adjacent to the peak magnetic flux location. The mean absolute magnetic flux density of the remnant plage network is about 130 Mx cm-2; in the lowest flux regions of the field-of-view, the mean absolute flux density is approximately 60 Mx cm-2. Within these quiet regions we do not find evidence of pervasive kilo-gauss strength magnetic elements as seen in recent high resolution internetwork studies. In general, the observations confirm recent 3-dimensional numerical simulations which show that the magnetic field in high-density regions such as plage is concentrated in complex structures that are not generally composed of discrete magnetic flux tubes.

Appendices are only available in electronic form at http://www.edpsciences.org Title: Observations of solar magnetic elements with 0.1" resolution Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl, M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.; Title, A. M.; Scharmer, G. Bibcode: 2004AAS...204.2005B Altcode: 2004BAAS...36..686B New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were taken with the Swedish 1-meter Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.''1) structures and find new forms of magnetic structures in this particular region. A simultaneous Ca II H-line image is used to examine the low-chromosphere of network elements. A simultaneous Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.''18) FWHM with a flux sensitivity of approximately 130 Mx cm-2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s-1. Magnetic flux in this region as seen in both the magnetogram and the G-band image is typically structured into larger, amorphous, ``ribbons'' with a wide range of flux density values, rather than isolated kilogauss flux tubes. We also present filtergrams and magnetograms of magnetic elements at the solar limb showing that solar faculae are resolved into bright granular walls that appear to project 350 to 500 km above the photosphere. Title: Penumbral structure at 0.1 arcsec resolution. I. General appearance and power spectra Authors: Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Kiselman, D.; Scharmer, G. B. Bibcode: 2004A&A...414..717R Altcode: We analyse sunspot filtergrams of unprecedented quality obtained by \citet{scharmer02dark} with the Swedish 1-m Solar Telescope on La Palma. The observations comprise images in three different wavelength bands: 488, 436, and 430 nm (G-band). We find that there are still unresolved penumbral filaments which must have widths smaller than 80 km. The fine structuring along the filaments is limited. Penumbral grains have internal structure and look like they are split or crossed by narrow dark structures. We calculate intensity power spectra of the penumbra from images that are corrected for seeing using the Phase Diversity technique. The effects of high order aberrations that are not corrected for are estimated to be too low to be consistent with a flat power spectrum. The penumbral power spectra do not show any signs of bumps or peaks that could correspond to a preferred scale at 0\farcs35 for the width of penumbral filaments. We argue that the power spectrum is not a very reliable source of information concerning preferred scales. Title: Observations at 0{. ^{primeprime}}1 Resolution of the Dynamic Evolution of Magnetic Elements Authors: Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo Bibcode: 2004IAUS..223..207C Altcode: 2005IAUS..223..207C We present observations of the dynamic evolution of photospheric magnetic elements in the G-band, magnetograms and Dopplergrams. The observations were obtained with the Swedish 1m Solar Telescope on La Palma at close to the diffraction limit of 0{. ^{primeprime}}1. In the most quiet regions we observe individual bright points in the G-band with corresponding magnetic signal in the magnetograms. Where the filling factor of the magnetic field is larger, the bright points interact when advected by the granular and super-granular flow-fields, flux sheets form and fragment. The plage region of the decaying active region is filled with more complex topologies like ribbon structures with darker interior and bright, knotted edges. These change into flower-like shape when small in extent and into micro-pores when the flux region is larger in extent. The magnetic elements in the plage region are associated with upflows with strong downflows in the immediate vicinity in the low-field region. Title: Observations of Umbral Flashes Authors: Rouppe van der Voort, L. H. M.; Krijger, J. M. Bibcode: 2003csss...12..607R Altcode: We present observations of oscillations in the chromosphere of the umbra of sunspots. The observations were obtained with the Swedish Vacuum Solar Telescope (SVST) and the Dutch Open Telescope (DOT) on La Palma, comprising spectrograms and filtergrams in the Ca II H line. The sawtooth pattern in the spectroscopic time evolution of the Ca II H core is shown as well as evidence for a connection between umbral flashes and running penumbral waves from image sequences. Running waves, coherent over a large fraction of the penumbra, seem to be excited by flashes that occur close to the umbra-penumbral boundary. Comparing the intensity oscillations in the Ca II H line with TRACE observations in the 1600 Å passband, we find a phase difference of approximately 25 ° with 1600 Å leading the Ca II H intensity oscillation which we attribute to complex dynamical behaviour. Title: La Palma observations of umbral flashes Authors: Rouppe van der Voort, L. H. M.; Rutten, R. J.; Sütterlin, P.; Sloover, P. J.; Krijger, J. M. Bibcode: 2003A&A...403..277R Altcode: We present high-quality Ca II H & K data showing chromospheric flashes in sunspot umbrae collected with the Swedish Vacuum Solar Telescope, the Dutch Open Telescope, and the Swedish 1-m Solar Telescope at the Roque de los Muchachos Observatory on La Palma. Differential movies, time slices, spectrograms, and Fourier power maps demonstrate that umbral flashes and running penumbral waves are closely related oscillatory phenomena, combining upward shock propagation with coherent wave spreading over the entire spot. We attribute the flash brightening to large redshift by post-shock material higher up. We find no obvious relation between umbral dots and umbral flashes. Title: On the time variability of the Evershed effect Authors: Rouppe van der Voort, L. H. M. Bibcode: 2003A&A...397..757R Altcode: The time variability of the Evershed effect in the penumbra of a sunspot is studied from a time series of high-spatial resolution spectrograms covering the wings of the Ca Ii K line accompanied by Ca Ii K slit-jaw images obtained with the Swedish Vacuum Solar Telescope. Simultaneous G-band observations from the Dutch Open Telescope are used to study the penumbral scene around the spectrograph slit on the photospheric level. Four spectral lines are selected to probe the Evershed effect at different heights in the penumbral atmosphere. The Evershed flow is found to show variations on a time scale of 8-14 min which is a confirmation of earlier observations from tunable filter instruments. The largest amplitudes in the variation are found where the Evershed flow is strongest. No systematic height dependence is found for the propagation of the velocity variations. On several occasions, an increase in the Evershed signal is associated with an intensity increase. In the weak Evershed signal detected in bright filaments, no significant variability is found. As an alternative to the interpretation of temporal enhancements in the flow speed, it is speculated that temporal height variations of a flow channel hosting a steady Evershed flow can explain the observed variations in the Evershed effect. Title: Sunspot structure and dynamics Authors: Rouppe van der Voort, Luc Bibcode: 2003PhDT.......159R Altcode: Fundamental physical processes occur on small spatial scales in the solar atmosphere and high-spatial-resolution observations are necessary to test theoretical models. This thesis presents high-spatial-resolution observations of sunspots obtained with the Swedish solar telescopes on La Palma: the Swedish Vacuum Solar Telescope (SVST) and its successor the Swedish 1-m Solar Telescope (SST). Spectroscopic observations are used to derive semi-empirical models of the temperature stratification of fine-structure elements in the sunspot penumbra by inversion of the radiation temperatures in the damping wings of the Ca II K line. These models are used to investigate the line formation of spectral line blends in the Ca II K wings. Line-core Dopplershift measurements of a selection of these spectral line blends are used to investigate the height dependence of bulk velocities in the penumbral atmosphere, including the Evershed flow. Examples are found of concentrations of the Evershed flow in the form of channels that tend to coincide with dark filaments. A weak correlation is found between reduced intensity and enhanced velocity but there exist examples of strong flows in bright structures. Numerical tests of flow channels in the penumbral atmosphere suggest that the bulk of the Evershed flow is concentrated in the lower atmosphere and has a velocity slower than 6 km s1. No indication is found of the Evershed flow being elevated above the continuum. A time series of spectrograms with the spectrograph slit positioned at a quasi-fixed position in the penumbra shows that the Evershed flow is variable on a time scale of 8-14 minutes. The time variability does not display any significant height dependence. SST observations of sunspots at unprecedented resolution (better than 0.12 arcseconds) reveal bright penumbral filaments with dark cores. The dark cores have apparent widths of 90 km but are not resolved. Other examples of very narrow dark features in the magnetic-field dominated regions outside sunspots are also found. Observations from the SVST, SST and the Dutch Open Telescope are used to study chromospheric oscillations in the sunspot umbra and penumbra. These observations comprise time series of spectrograms and narrow-band filtergrams covering the line-cores of the Ca II H and K lines. Umbral flashes and running penumbral waves are found to be closely related oscillatory phenomena, combining upward shock propagation with coherent wave spreading over the entire sunspot. Flash brightening is attributed to large redshift by post-shock material higher in the atmosphere. Title: First Results from the Swedish 1-m Solar Telescope Authors: Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Rouppe van der Voort, L. H. M. Bibcode: 2003ASPC..307....3S Altcode: No abstract at ADS Title: Dark cores in sunspot penumbral filaments Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan; Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M. Bibcode: 2002Natur.420..151S Altcode: Sunspot umbrae-the dark central regions of the spots-are surrounded by brighter filamentary penumbrae, the existence of which remains largely inexplicable. The penumbral filaments contain magnetic fields with varying inclinations and are associated with flowing gas, but discriminating between theoretical models has been difficult because the structure of the filaments has not hitherto been resolved. Here we report observations of penumbral filaments that reveal dark cores inside them. We cannot determine the nature of these dark cores, but their very existence provides a crucial test for any model of penumbrae. Our images also reveal other very small structures, in line with the view that many of the fundamental physical processes in the solar photosphere occur on scales smaller than 100km. Title: Penumbral structure and kinematics from high-spatial-resolution observations of Ca II K Authors: Rouppe van der Voort, L. H. M. Bibcode: 2002A&A...389.1020R Altcode: High-spatial-resolution spectra of the Ca Ii K line obtained with the Swedish Vacuum Solar Telescope (SVST) on La Palma are used to study the penumbra of a sunspot. The observed radiation temperatures in the Ca Ii K wing are used to derive the temperature stratification of fine-structure elements in the penumbra. It is found that in general, over the observed atmospheric depth range, penumbral structures keep their relative brightness identity with respect to their local surroundings, i.e., bright (dark) structures in the lower photosphere remain bright (dark) in the upper photosphere. Hot structures have a larger temperature difference between the bottom and the top of the photosphere than cool structures. Three semi-empirical atmosphere models, a cool, hot and mean model, are presented as being representative for the temperature stratification of penumbral fine structure. The mean temperature distribution of the centre-side penumbra is found to be up to 50 K hotter in the higher photosphere as compared to the limb-side penumbra. Hot structures being more numerous in the centre-side penumbra can account for this difference. These are primarily found near the outer penumbral boundary. It is suggested that the asymmetry can be explained by a differential line-of-sight effect that is caused by isotherms in bright structures having in the higher photosphere a tilt angle of approximately 7° with the horizontal, pointing downward towards the outer boundary. Line blends in the extended Ca Ii K wing are selected to study the Evershed effect and its height dependence. At a number of locations, the Evershed effect is found to be concentrated in channels which have a tendency to coincide with dark filaments. A weak correlation between brightness and velocity signal is found but also a number of bright structures with a significant Evershed signal. Simple numerical tests of flow channels in the penumbral atmosphere are performed to confront existing theoretical models with the observations. From these experiments it is found that the bulk of the flow must be concentrated in the lower atmospheric layers, i.e., below 200 km, and must have a velocity not higher than 6 km s-1. A channel width of 200 km is found to give the best reproduction of the observed velocities, so that the flow is either concentrated in a single channel or in a bundle of narrower channels. No direct indication is found of the Evershed channels being elevated above the continuum, and it is estimated that the flow channels reach down to at least 50 km above the continuum. Based on observations made with the Swedish Vacuum Solar Telescope which was operated on the island of La Palma by the Royal Swedish Academy of Sciences in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Title: Study of the Structure and Dynamics of the Penumbra of Sunspots (CD-ROM Directory: contribs/rouppe) Authors: Rouppe van der Voort, L. Bibcode: 2001ASPC..223..744R Altcode: 2001csss...11..744R No abstract at ADS Title: Proxy Magnetometry of the Photosphere: Why are G-Band Bright Points so Bright? Authors: Rutten, R. J.; Kiselman, D.; Rouppe van der Voort, L.; Plez, B. Bibcode: 2001ASPC..236..445R Altcode: 2001aspt.conf..445R No abstract at ADS