Author name code: wedemeyer ADS astronomy entries on 2022-09-14 author:"Wedemeyer-Boehm, Sven" OR author:"Wedemeyer, Sven" ------------------------------------------------------------------------ Title: Propagation of transverse waves in the solar chromosphere probed at different heights with ALMA sub-bands Authors: Guevara Gómez, Juan Camilo; Jafarzadeh, Shahin; Wedemeyer, Sven; Szydlarski, Mikolaj Bibcode: 2022arXiv220812070G Altcode: The Atacama Large Millimeter/sub-millimeter Array (ALMA) has provided us with an excellent diagnostic tool for studies of the dynamics of the Solar chromosphere, albeit through a single receiver band at one time presently. Each ALMA band consists of four sub-bands that are comprised of several spectral channels. To date, however, the spectral domain has been neglected in favour of ensuring optimal imaging, so that time-series observations have been mostly limited to full-band data products, thereby limiting studies to a single chromospheric layer. Here, we report the first observations of a dynamical event (i.e. wave propagation) for which the ALMA Band 3 data (centred at 3\,mm; 100\,GHz) is split into a lower and an upper sideband. In principle, this approach is aimed at mapping slightly different layers in the Solar atmosphere. The side-band data were reduced together with the Solar ALMA Pipeline (SoAP), resulting in time series of brightness-temperature maps for each side-band. Through a phase analysis of a magnetically quiet region, where purely acoustic waves are expected to dominate, the average height difference between the two side-bands is estimated as $73\pm16$~km. Furthermore, we examined the propagation of transverse waves in small-scale bright structures by means of wavelet phase analysis between oscillations at the two atmospheric heights. We find 6\% of the waves to be standing, while 54\% and 46\% of the remaining waves are propagating upwards and downwards, respectively, with absolute propagating speeds on the order of $\approx96$~km/s, resulting in a mean energy flux of $3800$\,W/m$^2$. Title: EMISSA (Exploring millimetre indicators of solar-stellar activity). II. Towards a robust indicator of stellar activity Authors: Mohan, A.; Wedemeyer, S.; Hauschildt, P. H.; Pandit, S.; Saberi, M. Bibcode: 2022A&A...664L...9M Altcode: 2022arXiv220804217M Context. An activity indicator, which can provide a robust quantitative mapping between the stellar activity and the physical properties of its atmosphere, is important in exploring the evolution of the observed active phenomena across main-sequence stars of different spectral types. Common activity indicators do provide qualitative correlations with physical properties such as Teff and the rotation period, among others. However, due to the large variability in their values, even for a single star, defining robust quantitative mappings between activity and physical properties is difficult. Millimetre (mm) wavelengths probe the different atmospheric layers within the stellar chromosphere, providing a tomographic view of the atmospheric dynamics.
Aims: The project aims to define a robust activity indicator by characterising mm brightness temperature spectra (TB(ν)) of the cool main-sequence stars (Teff ∼ 5000-7000 K) compiled by Paper I in this series. The sample contains 13 stars, including the Sun.
Methods: We derived the mm TB(ν) spectral indices (αmm) for cool stars, including the Sun, based on observations in the 30-1000 GHz range. The derived values for αmm are explored as a function of various physical parameters and empirical power-law functions were derived. We also compared αmm estimates with other activity indicators.
Results: Despite the estimation errors, αmm values could distinguish the cool stars well, unlike common activity indicators. The low estimation errors on the derived trends of αmm vs. physical parameters suggest that αmm could be a robust activity indicator.
Conclusions: We note that αmm, which is linked to chromospheric thermal stratification and activity in cool stars, can well distinguish and physically characterise the stars more robustly than common activity indicators. We emphasise the need for multi-frequency data across the mm band for stars, with a range of physical parameters and gathered at multiple epochs during their activity cycles. This will help to explore αmm in a statistically robust manner and to study the emergence of chromospheric heating on the main sequence. Title: Characterisation of bright chromospheric and oscillating small-scale features observed with ALMA Authors: Guevara Gómez, Juan Camilo; Jafarzadeh, Shahin; Wedemeyer, Sven Bibcode: 2022cosp...44.2549G Altcode: Small-scale chromospheric bright features exhibiting oscillations in brightness temperature, size and horizontal velocity were identified and traced in both solar ALMA observations in Bands 3 ($\sim$3 mm) and 6 ($\sim$1.2 mm) and in a Bifrost simulation resembling the ALMA observation. In total, 492 and 175 features were present in the observations and simulation respectively. Particularly, an anti-correlation between brightness temperature and size is present in the oscillations which might imply that these features are associated with fast-sausage MHD modes. We have performed a wavelet analysis to quantify the periods of oscillation for these three quantities as well as the phase angles between temperature and size. The outcome of a statistical analysis shows that the found periods correspond to high frequency oscillations in photosphere and chromosphere by diagnostics at other wavelengths. We have also estimated the energy carried by these waves and discussed the results from a statistical point of view. Specifically, we compared the outcomes between the the two ALMA frequency bands as they are considered to be formed at distinct heights and used the simulation to discuss the context of the observational results. Title: Formation of activity indicators in a 3D model atmosphere Authors: Pandit, Sneha; Wedemeyer, Sven Bibcode: 2022cosp...44.2555P Altcode: The Sun, being the nearest star, can be used as a reference case for solar-like stars due to the availability of many spatiotemporally resolved solar spectra. Amongst several spectral lines, some of the strongest chromospheric diagnostics are the Ca II H & K lines which can be used to gauge the temperature stratification of the atmosphere as the line core and wings are formed in different regions of the solar atmosphere. Furthermore, the H$\alpha$ line is a tracer for the magnetic structures and its line core gives an estimate of the mass density. These two diagnostics together can provide insights into the stellar structure. The 1.5D radiation transfer codes RH and Multi3D are used to obtain synthetic spectra for the Ca II lines and the H$\alpha$ line from an enhanced network atmosphere model simulated with the state-of-the-art Bifrost code. The activity indices generated from these lines could further be used to compare the spectra of sun-like stars with the solar spectrum. These indices can shed light on the physical properties like temperature stratification, magnetic structures, mass density distribution in the stellar atmospheres. Meanwhile, brightness temperatures from ALMA observations provide a new complementary view on the activity and the thermal structure of stellar atmospheres. The synthetic Ca II and H$\alpha$ spectra are therefore compared to corresponding millimetre continuum maps. The overall aim of the presented study is to establish more robust solar/stellar activity indicators using ALMA observations in comparison with classical diagnostics. Title: Solar/Stellar atmospheric tomography with mm - cm data: Initial catalogue of main sequence stars and results Authors: Mohan, Atul; Wedemeyer, Sven; Pandit, Sneha; Saberi, Maryam; Hauschildt, Peter Bibcode: 2022cosp...44.2495M Altcode: Millimeter - Centimeter (10 - 1000 GHz) bands are particularly sensitive to emission from various outer atmospheric layers of main sequence stars, with shorter wavelengths probing deeper layers. This makes the study of mm - cm spectra (S$_{obs}$) a unique tool to study the vertical atmospheric stratification of these stars as a function of various physical parameters. A major challenge in the field of mm astronomy had been the lack of a sensitive interferometer which can not only detect the faint $\sim$ 10 - 100 $\mu$Jy level atmospheric fluxes robustly from these stars, but also provide enough spatial resolution needed to distinguish any emission from companion stars and stellar disks. With the advent of the Atacama Large Millimeter/Sub-millimeter array, the aforementioned hurdles have been overcome to a great extent for at least the stars within a distance of 10\,pc.\\ In this work, we gathered the archival data of main sequence stars robustly detected with ALMA, and additionally reported 10 - 80 GHz fluxes if any, from other modern interferometric arrays like ATCA, JVLA etc. The data for the Sun were also gathered and the disk-averaged fluxes were recorded to get the sun-as-a-star spectrum. The resulting sample comprises of 12 main sequence stars plus the sun-as-a-star covering an effective temperature (T$_{eff}$) range of 3000 - 10000 K. We compared their observed fluxes against respective purely photospheric model fluxes obtained using the PHOENIX code (S$_{model}$) and obtained the upper atmospheric excess energy spectrum ($\Delta$S/S$_{model}$ = S$_{obs}$/S$_{model}$ - 1 ). We find evidence for a stratified atmosphere which gets progressively hotter with height in cool stars (T$_{eff}$ < 7000 K). The spectral index of the observed mm-cm brightness spectrum, was characterised for every star in the sample with sufficient data. The steepness was found to decrease with T$_{eff}$, possibly hinting at steeper thermal gradients in cooler stars. This study demonstrates the potential of mm - cm band diagnostics to perform atmospheric tomography in cool stars and that long duration light curves for these stars can be powerful tools for studying the dynamics across different layers. This calls for the need to have dedicated long duration surveys of nearby cool stars in mm - cm bands assisted by state of the art modelling to better understand the nature of atmospheric stratification and dynamics in cool stars as a function of stellar type. Title: First detection of AlF line emission towards M-type AGB stars Authors: Saberi, M.; Khouri, T.; Velilla-Prieto, L.; Fonfría, J. P.; Vlemmings, W. H. T.; Wedemeyer, S. Bibcode: 2022A&A...663A..54S Altcode: 2022arXiv220403284S The nucleosynthesis production of fluorine (F) is still a matter of debate. Asymptotic giant branch (AGB) stars are one of the main candidates for F production. However, their contribution to the total F budget is not fully known due to the lack of observations. In this paper, we report the detection of aluminium monofluoride (AlF) line emission, one of the two main carriers of F in the gas-phase in the outflow of evolved stars, towards five nearby oxygen-rich (M-type) AGB stars. We studied the Atacama large millimetre/sub-millimetre array (ALMA) observations of AlF (v = 0, J = 4—3, 9-8, 10-9, and 15-14) and (v = 1, J = 7-6) line emission towards o Ceti, and (v = 0, J = 7-6 and 15-14) lines towards R Leo. We also report a tentative detection of AlF (v = 0, J = 7-6) line in IK Tau, (v = 0, J = 15-14) line towards R Dor, and (v = 0, J = 7-6 and J = 15-14) lines in W Hya. From spatially resolved observations, we estimated the AlF emitting region with a radius ~11R for o Ceti and ~9R for R Leo. From population diagram analysis, we report the AlF column densities of ~5.8 × 1015 cm−2 and ~3 × 1015 cm−2 for o Ceti and R Leo, respectively, within these regions. For o Ceti, we used the C18O (v = 0, J = 3-2) observations to estimate the H2 column density of the emitting region. We found a fractional abundance of fAIF/H2 ~ (2.5 ± 1.7) × 10−8. This gives a lower limit on the F budget in o Ceti and is compatible with the solar F budget fF/H2 = (5 ± 2) × 10−8. For R Leo, a fractional abundance fAIF/H2 = (1.2 ± 0.5) × 10−8 is estimated. For other sources, we cannot precisely determine the emitting region based on the available data. Assuming an emitting region with a radius of ~11R and the rotational temperatures derived for o Ceti and R Leo, we crudely approximated the AlF column density to be ~(1.2−1.5) × 1015 cm−2 in W Hya, ~(2.5−3.0) × 1014 cm−2 in R Dor, and ~(0.6−1.0) × 1016 cm−2 in IK Tau. These result in fractional abundances within a range of fAIF/H2 ~ (0.1 − 4) × 10−8 in W Hya, R Dor, and IK Tau. Title: The Atacama Large Aperture Submillimeter Telescope: Key science drivers Authors: Ramasawmy, Joanna; Klaassen, Pamela D.; Cicone, Claudia; Mroczkowski, Tony K.; Chen, Chian-Chou; Cornish, Thomas; Lima da Cunha, Elisabete; Hatziminaoglou, Evanthia; Johnstone, Doug; Liu, Daizhong; Perrott, Yvette; Schimek, Alice; Stanke, Thomas; Wedemeyer, Sven Bibcode: 2022arXiv220703914R Altcode: The Atacama Large Aperture Submillimeter Telescope (AtLAST) is a concept for a 50m class single-dish telescope that will provide high sensitivity, fast mapping of the (sub-)millimeter sky. Expected to be powered by renewable energy sources, and to be constructed in the Atacama desert in the 2030s, AtLAST's suite of up to six state-of-the-art instruments will take advantage of its large field of view and high throughput to deliver efficient continuum and spectroscopic observations of the faint, large-scale emission that eludes current facilities. Here we present the key science drivers for the telescope characteristics, and discuss constraints that the transformational science goals place on future instrumentation. Title: A Genetic Algorithm to Model Solar Radio Active Regions From 3D Magnetic Field Extrapolations Authors: de Oliveira e Silva, Alexandre José; Selhorst, Caius Lucius; Costa, Joaquim E. R.; Simões, Paulo J. A.; Giménez de Castro, Carlos Guillermo; Wedemeyer, Sven; White, Stephen M.; Brajša, Roman; Valio, Adriana Bibcode: 2022FrASS...9.1118D Altcode: 2022arXiv220503385S; 2022arXiv220503385J In recent decades our understanding of solar active regions (ARs) has improved substantially due to observations made with better angular resolution and wider spectral coverage. While prior AR observations have shown that these structures were always brighter than the quiet Sun at centimeter wavelengths, recent observations at millimeter and submillimeter wavelengths have shown ARs with well defined dark umbrae. Given this new information, it is now necessary to update our understanding and models of the solar atmosphere in active regions. In this work, we present a data-constrained model of the AR solar atmosphere, in which we use brightness temperature measurements of NOAA 12470 at three radio frequencies: 17, 100 and 230 GHz. The observations at 17 GHz were made by the Nobeyama Radioheliograph (NoRH), while the observations at 100 and 230 GHz were obtained by the Atacama Large Millimeter/submillimeter Array (ALMA). Based on our model, which assumes that the radio emission originates from thermal free-free and gyroresonance processes, we calculate radio brightness temperature maps that can be compared with the observations. The magnetic field at distinct atmospheric heights was determined in our modelling process by force-free field extrapolation using photospheric magnetograms taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). In order to determine the best plasma temperature and density height profiles necessary to match the observations, the model uses a genetic algorithm that modifies a standard quiet Sun atmospheric model. Our results show that the height of the transition region (TR) of the modelled atmosphere varies with the type of region being modelled: for umbrae the TR is located at 1080 ± 20 km above the solar surface; for penumbrae, the TR is located at 1800 ± 50 km; and for bright regions outside sunspots, the TR is located at 2000 ± 100 km. With these results, we find good agreement with the observed AR brightness temperature maps. Our modelled AR can be used to estimate the emission at frequencies without observational coverage. Title: Power distribution of oscillations in the atmosphere of a plage region. Joint observations with ALMA, IRIS, and SDO Authors: Narang, Nancy; Chandrashekhar, Kalugodu; Jafarzadeh, Shahin; Fleck, Bernhard; Szydlarski, Mikołaj; Wedemeyer, Sven Bibcode: 2022A&A...661A..95N Altcode: 2022arXiv220211547N Context. Joint observations of the Atacama Large Millimeter/Submillimeter Array (ALMA) with other solar observatories can provide a wealth of opportunities for understanding the coupling between different layers of the solar atmosphere.
Aims: We present a statistical analysis of the power distribution of oscillations in a plage region in active region NOAA AR12651, which was observed jointly with ALMA, the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO).
Methods: We employ coordinated ALMA Band 6 (1.25 mm) brightness temperature maps, IRIS slit-jaw images in the 2796 Å passband, and observations in six passbands (1600 Å, 304 Å, 131 Å, 171 Å, 193 Å, and 211 Å) from the Atmospheric Imaging Assembly (AIA) on board SDO. We perform Lomb-Scargle transforms to study the distribution of oscillation power by means of dominant period maps and power maps. We study the spatial association of oscillations through the atmosphere, with a focus on the correlation of the power distribution of ALMA oscillations with others.
Results: We do not observe any significant association of ALMA oscillations with IRIS and AIA oscillations. While the global behavior of the dominant ALMA oscillations shows a similarity with that of the transition region and coronal passbands of AIA, the ALMA dominant period maps and power maps do not show any correlation with those from the other passbands. The spatial distribution of dominant periods and power in different period intervals of ALMA oscillations is uncorrelated with those of any other passbands.
Conclusions: We speculate that the non-association of ALMA oscillations with those of IRIS and AIA is due to significant variations in the height of formation of the millimeter continuum observed by ALMA. Additionally, the fact that ALMA directly maps the brightness temperature, in contrast to the intensity observations by IRIS and AIA, can result in the very different intrinsic nature of the ALMA oscillations compared to the IRIS and AIA oscillations. Title: The Solar ALMA Science Archive (SALSA). First release, SALAT, and FITS header standard Authors: Henriques, Vasco M. J.; Jafarzadeh, Shahin; Guevara Gómez, Juan Camilo; Eklund, Henrik; Wedemeyer, Sven; Szydlarski, Mikołaj; Haugan, Stein Vidar H.; Mohan, Atul Bibcode: 2022A&A...659A..31H Altcode: 2021arXiv210902374H In December 2016, the Atacama Large Millimeter/submillimeter Array (ALMA) carried out the first regular observations of the Sun. These early observations and the reduction of the respective data posed a challenge due to the novelty and complexity of observing the Sun with ALMA. The difficulties with producing science-ready, time-resolved imaging products in a format familiar to and usable by solar physicists based on the measurement sets delivered by ALMA had limited the availability of such data to this point. With the development of the Solar ALMA Pipeline, it has now become possible to routinely reduce such data sets. As a result, a growing number of science-ready solar ALMA data sets are now offered in the form of the Solar ALMA Science Archive (SALSA). So far, SALSA contains primarily time series of single-pointing interferometric images at cadences of one or two seconds, accompanied by the respective single-dish full-disc solar images. The data arrays are provided in FITS format. We also present the first version of a standardised header format that accommodates future expansions and fits within the scope of other standards including the ALMA Science Archive itself and SOLARNET. The headers include information designed to aid the reproduction of the imaging products from the raw data. Links to co-observations, if available, with a focus on those of the Interface Region Imaging Spectrograph, are also provided. SALSA is accompanied by the Solar ALMA Library of Auxiliary Tools (SALAT), which contains Interactive Data Language and Python routines for convenient loading and a quick-look analysis of SALSA data.

Movies associated to Figs. 3 and 4 are available at https://www.aanda.org Title: Final Report for SAG 21: The Effect of Stellar Contamination on Space-based Transmission Spectroscopy Authors: Rackham, Benjamin V.; Espinoza, Néstor; Berdyugina, Svetlana V.; Korhonen, Heidi; MacDonald, Ryan J.; Montet, Benjamin T.; Morris, Brett M.; Oshagh, Mahmoudreza; Shapiro, Alexander I.; Unruh, Yvonne C.; Quintana, Elisa V.; Zellem, Robert T.; Apai, Dániel; Barclay, Thomas; Barstow, Joanna K.; Bruno, Giovanni; Carone, Ludmila; Casewell, Sarah L.; Cegla, Heather M.; Criscuoli, Serena; Fischer, Catherine; Fournier, Damien; Giampapa, Mark S.; Giles, Helen; Iyer, Aishwarya; Kopp, Greg; Kostogryz, Nadiia M.; Krivova, Natalie; Mallonn, Matthias; McGruder, Chima; Molaverdikhani, Karan; Newton, Elisabeth R.; Panja, Mayukh; Peacock, Sarah; Reardon, Kevin; Roettenbacher, Rachael M.; Scandariato, Gaetano; Solanki, Sami; Stassun, Keivan G.; Steiner, Oskar; Stevenson, Kevin B.; Tregloan-Reed, Jeremy; Valio, Adriana; Wedemeyer, Sven; Welbanks, Luis; Yu, Jie; Alam, Munazza K.; Davenport, James R. A.; Deming, Drake; Dong, Chuanfei; Ducrot, Elsa; Fisher, Chloe; Gilbert, Emily; Kostov, Veselin; López-Morales, Mercedes; Line, Mike; Močnik, Teo; Mullally, Susan; Paudel, Rishi R.; Ribas, Ignasi; Valenti, Jeff A. Bibcode: 2022arXiv220109905R Altcode: Study Analysis Group 21 (SAG21) of the Exoplanet Exploration Program Analysis Group (ExoPAG) was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, and plages. This SAG has brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current needs that can be addressed in this context to make the most of transit studies from current NASA facilities like HST and JWST. The analysis produced 14 findings, which fall into three Science Themes encompassing (1) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities ("The Sun as the Stellar Benchmark"), (2) how stars other than the Sun extend our knowledge of heterogeneities ("Surface Heterogeneities of Other Stars") and (3) how to incorporate information gathered for the Sun and other stars into transit studies ("Mapping Stellar Knowledge to Transit Studies"). Title: The Sun at millimeter wavelengths. III. Impact of the spatial resolution on solar ALMA observations Authors: Eklund, Henrik; Wedemeyer, Sven; Szydlarski, Mikołaj; Jafarzadeh, Shahin Bibcode: 2021A&A...656A..68E Altcode: 2021arXiv210913826E Context. Interferometric observations of the Sun with the Atacama Large Millimeter/sub-millimeter Array (ALMA) provide valuable diagnostic tools for studying the small-scale dynamics of the solar atmosphere.
Aims: The aims are to perform estimations of the observability of the small-scale dynamics as a function of spatial resolution for regions with different characteristic magnetic field topology facilitate a more robust analysis of ALMA observations of the Sun.
Methods: A three-dimensional model of the solar atmosphere from the radiation-magnetohydrodynamic code Bifrost was used to produce high-cadence observables at millimeter and submillimeter wavelengths. The synthetic observables for receiver bands 3-10 were degraded to the angular resolution corresponding to ALMA observations with different configurations of the interferometric array from the most compact, C1, to the more extended, C7. The observability of the small-scale dynamics was analyzed in each case. The analysis was thus also performed for receiver bands and resolutions that are not commissioned so far for solar observations as a means for predicting the potential of future capabilities.
Results: The minimum resolution required to study the typical small spatial scales in the solar chromosphere depends on the characteristic properties of the target region. Here, a range from quiet Sun to enhanced network loops is considered. Limited spatial resolution affects the observable signatures of dynamic small-scale brightening events in the form of reduced brightness temperature amplitudes, potentially leaving them undetectable, and even shifts in the times at which the peaks occur of up to tens of seconds. Conversion factors between the observable brightness amplitude and the original amplitude in the fully resolved simulation are provided that can be applied to observational data in principle, but are subject to wavelength-dependent uncertainties. Predictions of the typical appearance at the different combinations of receiver band, array configuration, and properties of the target region are conducted.
Conclusions: The simulation results demonstrate the high scientific potential that ALMA already has with the currently offered capabilities for solar observations. For the study of small-scale dynamic events, however, the spatial resolution is still crucial, and wide array configurations are preferable. In any case, it is essential to take the effects due to limited spatial resolution into account in the analysis of observational data. Finally, the further development of observing capabilities including wider array configurations and advanced imaging procedures yields a high potential for future ALMA observations of the Sun. Title: EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity). I. The initial millimeter-centimeter main-sequence star sample Authors: Mohan, A.; Wedemeyer, S.; Pandit, S.; Saberi, M.; Hauschildt, P. H. Bibcode: 2021A&A...655A.113M Altcode: 2021arXiv211013339M Context. Due to their wide wavelength coverage across the millimeter to centimeter (mm-cm) range and their increased sensitivity, modern interferometric arrays facilitate observations of the thermal and non-thermal radiation that is emitted from different layers in the outer atmospheres of stars.
Aims: We study the spectral energy distribution (Sobs(ν)) of main-sequence stars based on archival observations in the mm-cm range with the aim to study their atmospheric stratification as a function of stellar type.
Methods: The main-sequence stars with significant detection in mm bands were identified in the ALMA Science Archive. These data were then complemented with spectral flux data in the extreme ultraviolet to cm range as compiled from various catalogues and observatory archives. We compared the resultant Sobs(ν) of each star with a photospheric emission model (Smod(ν)) calculated with the PHOENIX code. The departures of Sobs(ν) from Smod(ν) were quantified in terms of a spectral flux excess parameter (ΔS∕Smod) and studied as a function of stellar type.
Results: The initial sample consists of 12 main-sequence stars across a broad range of spectral types from A1 to M3.5 and the Sun-as-a-star as reference. The stars with Teff = 3000-7000 K (F-M type) showed a systematically higher Sobs(ν) than Smod(ν) in the mm-cm range. Their ΔS∕Smod exhibits a monotonic rise with decreasing frequency. The steepness of this rise is higher for cooler stars in the Teff = 3000-7000 K range, although the single fully convective star (Teff ~ 3000 K) in the sample deviates from this trend. Meanwhile, Sobs(ν) of the A-type stars agrees with Smod(ν) within errors.
Conclusions: The systematically high ΔS∕Smod in F-M stars indicates hotter upper atmospheric layers, that is, a chromosphere and corona in these stars, like for the Sun. The mm-cm ΔS∕Smod spectrum offers a way to estimate the efficiency of the heating mechanisms across various outer atmospheric layers in main-sequence stars, and thereby to understand their structure and activity. We emphasise the need for dedicated surveys of main-sequence stars in the mm-cm range. Title: Dynamics of small-scale dark features observed by the Atacama Large Millimeter/Submillimeter Array (ALMA) Authors: Guevara Gómez, Juan Camilo; Wedemeyer, Sven Bibcode: 2021csss.confE.227G Altcode: Observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA) now allow for more quantitative determination of temperatures in the chromosphere. ALMA observations of Quiet Sun regions in receiver Band 3 (3 mm; 100 GHz) exhibit small-scale features that resemble a mesh-like chromospheric pattern, similar to what was earlier detected with other chromospheric diagnostics. Tracking the features in time results in lifetimes, velocities and sizes, which are analysed within the context given by co-aligned images of the Solar Dynamics Observatory (SDO). Here we present a statistical analysis and compare the results with a 3D MHD simulation with Bifrost of an enhanced network which has been degraded to the ALMA spatial resolution. The match between observations and simulations suggests that the dark features are post-shock regions. The thermodynamics and kinetic properties derived from the ALMA observations will therefore enhance our understanding of the small-scale dynamics and heating of the solar chromosphere and its potential implications for other solar-like stars. Title: An overall view of temperature oscillations in the solar chromosphere with ALMA Authors: Jafarzadeh, S.; Wedemeyer, S.; Fleck, B.; Stangalini, M.; Jess, D. B.; Morton, R. J.; Szydlarski, M.; Henriques, V. M. J.; Zhu, X.; Wiegelmann, T.; Guevara Gómez, J. C.; Grant, S. D. T.; Chen, B.; Reardon, K.; White, S. M. Bibcode: 2021RSPTA.37900174J Altcode: 2021RSTPA.379..174J; 2020arXiv201001918J By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb-Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3-5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of Hα line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: High-frequency oscillations in small chromospheric bright features observed with Atacama Large Millimetre/Submillimetre Array Authors: Guevara Gómez, J. C.; Jafarzadeh, S.; Wedemeyer, S.; Szydlarski, M.; Stangalini, M.; Fleck, B.; Keys, P. H. Bibcode: 2021RSPTA.37900184G Altcode: 2020arXiv200804179G We report detection of oscillations in brightness temperature, size and horizontal velocity of three small bright features in the chromosphere of a plage/enhanced-network region. The observations, which were taken with high temporal resolution (i.e. 2 s cadence) with the Atacama large millimetre/ submillimetre array (ALMA) in Band 3 (centred at 3 mm; 100 GHz), exhibit three small-scale features with oscillatory behaviour with different, but overlapping, distributions of period on the order of, on average, 90 ± 22 s, 110 ± 12 s and 66 ± 23 s, respectively. We find anti-correlations between perturbations in brightness, temperature and size of the three features, which suggest the presence of fast sausage-mode waves in these small structures. In addition, the detection of transverse oscillations (although with a larger uncertainty) may also suggest the presence of Alfvénic oscillations which are likely representative of kink waves. This work demonstrates the diagnostic potential of high-cadence observations with ALMA for detecting high-frequency magnetohydrodynamic waves in the solar chromosphere. Such waves can potentially channel a vast amount of energy into the outer atmosphere of the Sun.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Characterization of shock wave signatures at millimetre wavelengths from Bifrost simulations Authors: Eklund, Henrik; Wedemeyer, Sven; Snow, Ben; Jess, David B.; Jafarzadeh, Shahin; Grant, Samuel D. T.; Carlsson, Mats; Szydlarski, Mikołaj Bibcode: 2021RSPTA.37900185E Altcode: 2020arXiv200805324E Observations at millimetre wavelengths provide a valuable tool to study the small-scale dynamics in the solar chromosphere. We evaluate the physical conditions of the atmosphere in the presence of a propagating shock wave and link that to the observable signatures in mm-wavelength radiation, providing valuable insights into the underlying physics of mm-wavelength observations. A realistic numerical simulation from the three-dimensional radiative magnetohydrodynamic code Bifrost is used to interpret changes in the atmosphere caused by shock wave propagation. High-cadence (1 s) time series of brightness temperature (Tb) maps are calculated with the Advanced Radiative Transfer code at the wavelengths 1.309 mm and 1.204 mm, which represents opposite sides of spectral band 6 of the Atacama Large Millimeter/submillimeter Array (ALMA). An example of shock wave propagation is presented. The brightness temperatures show a strong shock wave signature with large variation in formation height between approximately 0.7 and 1.4 Mm. The results demonstrate that millimetre brightness temperatures efficiently track upwardly propagating shock waves in the middle chromosphere. In addition, we show that the gradient of the brightness temperature between wavelengths within ALMA band 6 can potentially be used as a diagnostics tool in understanding the small-scale dynamics at the sampled layers.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Dynamics of small-scale dark features observed by the Atacama Large Millimeter/submillimeter Array (ALMA) Authors: Guevara Gómez, Juan Camilo; Wedemeyer, Sven Bibcode: 2021cosp...43E.974G Altcode: Solar ALMA observations exhibit small-scale features resembling the mesh-like chromospheric pattern that was earlier detected with other chromospheric diagnostics. In this work, we present a first statistical study of the dynamics of dark features observed using ALMA Band 3 receivers (~3 mm; 100 GHz). By tracing the features in time, we estimate their lifetimes, velocities and sizes, which are analysed within the context given by co-aligned images of the Solar Dynamics Observatory (SDO). Furthermore, the results are compared with those obtained from applying the same method to a 3D MHD simulation with Bifrost of an enhanced network which has been degraded to the ALMA spatial resolution. Title: ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.; Sainz Dalda, Alberto Bibcode: 2021ApJ...906...83C Altcode: 2020arXiv201205970C We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate a plage from its surrounding regions than other methods commonly used. We caution that isolating a plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that λ = 1.25 mm free-free emission in plage regions observed with the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition-region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph (IRIS)/Si IV at 1393 Å. We also show that IRIS/Mg II h and k are not as well correlated with ALMA/Band6 as was previously thought, and we discuss discrepancies with previous works. Lastly, we report indications of chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations. Title: ALMA and IRIS Observations of the Solar Chromosphere. I. An On-disk Type II Spicule Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.; Sainz Dalda, Alberto Bibcode: 2021ApJ...906...82C Altcode: 2020arXiv200512717C We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph. The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers nonequilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature. Title: ALMA and IRIS Observations Highlighting the Dynamics and Structure of Chromospheric Plage Authors: Chintzoglou, G.; De Pontieu, B.; Martinez-Sykora, J.; Hansteen, V. H.; de la Cruz Rodriguez, J.; Szydlarski, M.; Jafarzadeh, S.; Wedemeyer, S.; Bastian, T.; Sainz Dalda, A. Bibcode: 2020AGUFMSH0010009C Altcode: We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers non-equilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature. In addition, we discuss the strikingly high degree of similarity between chromospheric plage features observed in ALMA/Band6 and IRIS/\ion{Si}{4} (also reproduced in our model) suggesting that ALMA/Band6 does not observe in the low chromosphere as previously thought but rather observes the upper chromospheric parts of structures such as spicules and other bright structures above plage at geometric heights near transition region temperatures. We also show that IRIS/\ion{Mg}{2} is not as well correlated with ALMA/Band6 as was previously thought. For these comparisons, we propose and employ a novel empirical method for the determination of plage regions, which seems to better isolate plage from its surrounding regions as compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care to mitigate statistical bias in quantitative comparisons between different chromospheric observables. Lastly, we report indications for chromospheric heating due to traveling shocks supported by the ALMA/Band6 observations. Title: The Sun at millimeter wavelengths. II. Small-scale dynamic events in ALMA Band 3 Authors: Eklund, Henrik; Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Guevara Gómez, Juan Camilo Bibcode: 2020A&A...644A.152E Altcode: 2020arXiv201006400E Context. Solar observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) facilitate studies of the atmosphere of the Sun at chromospheric heights at high spatial and temporal resolution at millimeter wavelengths.
Aims: ALMA intensity data at millimeter(mm)-wavelengths are used for a first detailed systematic assessment of the occurrence and properties of small-scale dynamical features in the quiet Sun.
Methods: We analyzed ALMA Band 3 data (∼3 mm/100 GHz) with a spatial resolution of ∼1.4-2.1 arcsec and a duration of ∼40 min together with SDO/HMI magnetograms. The temporal evolution of the mm maps is studied to detect pronounced dynamical features, which then are connected to dynamical events via a k-means clustering algorithm. We studied the physical properties of the resulting events and explored whether or not they show properties consistent with propagating shock waves. For this purpose, we calculated observable shock wave signatures at mm wavelengths from one- and three-dimensional model atmospheres.
Results: We detect 552 dynamical events with an excess in brightness temperature (ΔTb) of at least ≥400 K. The events show a large variety in size up to ∼9″, amplitude ΔTb up to ∼1200 K with typical values in the range ∼450-750 K, and lifetime at full width at half maximum of ΔTb of between ∼43 and 360 s, with typical values between ∼55 and 125 s. Furthermore, many of the events show signature properties suggesting that they are likely produced by propagating shock waves.
Conclusions: There are a lot of small-scale dynamic structures detected in the Band 3 data, even though the spatial resolution sets limitations on the size of events that can be detected. The number of dynamic signatures in the ALMA mm data is very low in areas with photospheric footpoints with stronger magnetic fields, which is consistent with the expectation for propagating shock waves. Title: The Sun at millimeter wavelengths. I. Introduction to ALMA Band 3 observations Authors: Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Eklund, Henrik; Guevara Gomez, Juan Camilo; Bastian, Tim; Fleck, Bernhard; de la Cruz Rodriguez, Jaime; Rodger, Andrew; Carlsson, Mats Bibcode: 2020A&A...635A..71W Altcode: 2020arXiv200102185W Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started regular observations of the Sun in 2016, first offering receiver Band 3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around 1.25 mm (239 GHz).
Aims: Here we present an initial study of one of the first ALMA Band 3 observations of the Sun. Our aim is to characterise the diagnostic potential of brightness temperatures measured with ALMA on the Sun.
Methods: The observation covers a duration of 48 min at a cadence of 2 s targeting a quiet Sun region at disc-centre. Corresponding time series of brightness temperature maps are constructed with the first version of the Solar ALMA Pipeline and compared to simultaneous observations with the Solar Dynamics Observatory (SDO).
Results: The angular resolution of the observations is set by the synthesised beam, an elliptical Gaussian that is approximately 1.4″ × 2.1″ in size. The ALMA maps exhibit network patches, internetwork regions, and elongated thin features that are connected to large-scale magnetic loops, as confirmed by a comparison with SDO maps. The ALMA Band 3 maps correlate best with the SDO/AIA 171 Å, 131 Å, and 304 Å channels in that they exhibit network features and, although very weak in the ALMA maps, imprints of large-scale loops. A group of compact magnetic loops is very clearly visible in ALMA Band 3. The brightness temperatures in the loop tops reach values of about 8000-9000 K and in extreme moments up to 10 000 K.
Conclusions: ALMA Band 3 interferometric observations from early observing cycles already reveal temperature differences in the solar chromosphere. The weak imprint of magnetic loops and the correlation with the 171, 131, and 304 SDO channels suggests, however, that the radiation mapped in ALMA Band 3 might have contributions from a wider range of atmospheric heights than previously assumed, but the exact formation height of Band 3 needs to be investigated in more detail. The absolute brightness temperature scale as set by total power measurements remains less certain and must be improved in the future. Despite these complications and the limited angular resolution, ALMA Band 3 observations have a large potential for quantitative studies of the small-scale structure and dynamics of the solar chromosphere.

Movies are available at https://www.aanda.org Title: The multi-thermal chromosphere. Inversions of ALMA and IRIS data Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; Leenaarts, J.; Chintzoglou, G.; De Pontieu, B.; Wedemeyer, S.; Szydlarski, M. Bibcode: 2020A&A...634A..56D Altcode: 2019arXiv191209886D Context. Numerical simulations of the solar chromosphere predict a diverse thermal structure with both hot and cool regions. Observations of plage regions in particular typically feature broader and brighter chromospheric lines, which suggests that they are formed in hotter and denser conditions than in the quiet Sun, but also implies a nonthermal component whose source is unclear.
Aims: We revisit the problem of the stratification of temperature and microturbulence in plage and the quiet Sun, now adding millimeter (mm) continuum observations provided by the Atacama Large Millimiter Array (ALMA) to inversions of near-ultraviolet Interface Region Imaging Spectrograph (IRIS) spectra as a powerful new diagnostic to disentangle the two parameters. We fit cool chromospheric holes and track the fast evolution of compact mm brightenings in the plage region.
Methods: We use the STiC nonlocal thermodynamic equilibrium (NLTE) inversion code to simultaneously fit real ultraviolet and mm spectra in order to infer the thermodynamic parameters of the plasma.
Results: We confirm the anticipated constraining potential of ALMA in NLTE inversions of the solar chromosphere. We find significant differences between the inversion results of IRIS data alone compared to the results of a combination with the mm data: the IRIS+ALMA inversions have increased contrast and temperature range, and tend to favor lower values of microturbulence (∼3-6 km s-1 in plage compared to ∼4-7 km s-1 from IRIS alone) in the chromosphere. The average brightness temperature of the plage region at 1.25 mm is 8500 K, but the ALMA maps also show much cooler (∼3000 K) and hotter (∼11 000 K) evolving features partially seen in other diagnostics. To explain the former, the inversions require the existence of localized low-temperature regions in the chromosphere where molecules such as CO could form. The hot features could sustain such high temperatures due to non-equilibrium hydrogen ionization effects in a shocked chromosphere - a scenario that is supported by low-frequency shock wave patterns found in the Mg II lines probed by IRIS. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array - from continuum to magnetic fields Authors: Wedemeyer, Sven; Szydlarski, Mikolaj; Rodriguez, Jaime de la Cruz; Jafarzadeh, Shahin Bibcode: 2020IAUS..354...24W Altcode: The Atacama Large Millimeter/submillimeter Array offers regular observations of our Sun since 2016. After an extended period of further developing and optimizing the post-processing procedures, first scientific results are now produced. While the first observing cycles mostly provided mosaics and time series of continuum brightness temperature maps with a cadence of 1-2s, additional receiver bands and polarization capabilities will be offered in the future. Currently, polarization capabilities are offered for selected receiver bands but not yet for solar observing. An overview of the recent development, first scientific results and potential of solar magnetic field measurements with ALMA will be presented. Title: Solar Astronomy with ALMA Authors: Wedemeyer, Sven Bibcode: 2019asrc.confE..12W Altcode: Solar observing with ALMA is offered as a non-standard mode since Cycle 4. The requirements for such observations are different from many other observations with ALMA in the sense that the mapped atmospheric layers of the Sun evolve on very short timescales and the primary beam being filled with complex emission. High-cadence (snapshot) imaging is needed for such a dynamic target but is very challenging. The effort and time that went into developing the observing mode seems well justified given that ALMA provides a new complementary view at a part of the solar atmosphere that is still elusive in many aspects. The solar observing campaigns with ALMA are co-ordinated with a number of space-borne and ground-based telescopes covering the UV to IR range. Co-ordinating such strictly simultaneous multi-telescope observations adds another layer of complexity but results in rich data sets covering all layers of the solar atmosphere while probing different properties of the atmospheric gas. Since Cycle 4, the imaging procedures for solar ALMA observations have been significantly improved and science-ready data are being produced. I will give a brief overview over ALMA's diagnostic potential for the Sun and challenges with carrying out solar observations and post-processing the data. First examples for Band 3 and Band 6 data are presented and illustrate the dynamic nature of the solar atmosphere, featuring, among other things, the imprint of magnetic fields and propagating shock waves. Title: Extreme Precision Radial Velocity Working Group Authors: Gaudi, Scott; Blackwood, Gary; Howard, Andrew; Latham, David; Fischer, Debra; Ford, Eric; Cegla, Heather; Plavchan, Peter; Quirrenbach, Andreas; Burt, Jennifer; Mamajek, Eric; Beichman, Chas; Bender, Chad; Crass, Jonathan; Diddams, Scott; Dumusque, Xavier; Eastman, Jason; Fulton, BJ; Halverson, Sam; Haywood, Raphaelle; Hearty, Fred; Leifer, Stephanie; Loehner-Boettcher, Johannes; Mortier, Annelies; Reiners, Ansgar; Robertson, Paul; Roy, Arpita; Schwab, Christian; Seifahrt, Andreas; Szentgyorgyi, Andrew; Terrien, Ryan; Teske, Johanna; Thompson, Samantha; Vasisht, Gautam; Aigrain, Suzanne; Bedell, Megan; Bernstein, Rebecca; Blackman, Ryan; Blake, Cullen; Buchhave, Lars; Callas, John; Ciardi, David; Chaplain, William; Cisewski-Kehe, Jessi; Collier-Cameron, Andrew; Cornachione, Matthew; Meunier, Nadege; Ninan, Joe; O'Meara, John; Ong, Joel; Wang, Sharon; Wedemeyer-Boehm, Sven; Zhao, Lily; Boss, Alan; Oppenheimer, Rebecca; Pitman, Joe; Poyneer, Lisa; Ridgeway, Stephen Bibcode: 2019BAAS...51g.232G Altcode: 2019astro2020U.232G NASA and NSF are jointly commissioning a community-based “Extreme Precision Radial Velocity (EPRV) working Group” to develop a blueprint for a strategy for an EPRV initiative. The purpose of this white paper is to describe the (EPRV) Working Group, whose primary responsibility is to lay out the blueprint mentioned above. Title: VizieR Online Data Catalog: HAT-P-26 differential transit photometry (von Essen+, 2019) Authors: von Essen, C.; Wedemeyer, S.; Sosa, M. S.; Hjorth, M.; Parkash, V.; Freudenthal, J.; Mallonn, M.; Miculan, R. G.; Zibecchi, L.; Cellone, S.; Torres, A. F. Bibcode: 2019yCat..36280116V Altcode: 11 differential photometry time series of primary transit events of HAT-P-26b in the R band.

(1 data file). Title: Multiwavelength High-resolution Observations of Chromospheric Swirls in the Quiet Sun Authors: Shetye, Juie; Verwichte, Erwin; Stangalini, Marco; Judge, Philip G.; Doyle, J. G.; Arber, Tony; Scullion, Eamon; Wedemeyer, Sven Bibcode: 2019ApJ...881...83S Altcode: We report observations of small-scale swirls seen in the solar chromosphere. They are typically 2 Mm in diameter and last around 10 minutes. Using spectropolarimetric observations obtained by the CRisp Imaging Spectro-Polarimeter at the Swedish 1 m Solar Telescope, we identify and study a set of swirls in chromospheric Ca II 8542 Å and Hα lines as well as in the photospheric Fe I line. We have three main areas of focus. First, we compare the appearance, morphology, dynamics, and associated plasma parameters between the Ca II and Hα channels. Rotation and expansion of the chromospheric swirl pattern are explored using polar plots. Second, we explore the connection to underlying photospheric magnetic concentration (MC) dynamics. MCs are tracked using the SWAMIS tracking code. The swirl center and MC remain cospatial and share similar periods of rotation. Third, we elucidate the role swirls play in modifying chromospheric acoustic oscillations and found a temporary reduction in wave period during swirls. We use cross-correlation wavelets to examine the change in period and phase relations between different wavelengths. The physical picture that emerges is that a swirl is a flux tube that extends above an MC in a downdraft region in an intergranular lane. The rotational motion of the MC matches the chromospheric signatures. We could not determine whether a swirl is a gradual response to the photospheric motion or an actual propagating Alfvénic wave. Title: Indications for transit-timing variations in the exo-Neptune HAT-P-26b Authors: von Essen, C.; Wedemeyer, S.; Sosa, M. S.; Hjorth, M.; Parkash, V.; Freudenthal, J.; Mallonn, M.; Miculán, R. G.; Zibecchi, L.; Cellone, S.; Torres, A. F. Bibcode: 2019A&A...628A.116V Altcode: 2019arXiv190406360V Upon its discovery, the low-density transiting Neptune HAT-P-26b showed a 2.1σ detection drift in its spectroscopic data, while photometric data showed a weak curvature in the timing residuals, the confirmation of which required further follow-up observations. To investigate this suspected variability, we observed 11 primary transits of HAT-P-26b between March, 2015, and July, 2018. For this, we used the 2.15 m Jorge Sahade Telescope placed in San Juan, Argentina, and the 1.2 m STELLA and the 2.5 m Nordic Optical Telescope, both located in the Canary Islands, Spain. To add to valuable information on the transmission spectrum of HAT-P-26b, we focused our observations in the R-band only. To contrast the observed timing variability with possible stellar activity, we carried out a photometric follow-up of the host star over three years. We carried out a global fit to the data and determined the individual mid-transit times focusing specifically on the light curves that showed complete transit coverage. Using bibliographic data corresponding to both ground and space-based facilities, plus our new characterized mid-transit times derived from parts-per-thousand precise photometry, we observed indications of transit timing variations in the system, with an amplitude of 4 min and a periodicity of 270 epochs. The photometric and spectroscopic follow-up observations of this system will be continued in order to rule out any aliasing effects caused by poor sampling and the long-term periodicity.

The transit photometry (time, flux, error) and the long term monitoring in three bands 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/628/A116 Title: Kepler Object of Interest Network. III. Kepler-82f: a new non-transiting 21 M planet from photodynamical modelling Authors: Freudenthal, J.; von Essen, C.; Ofir, A.; Dreizler, S.; Agol, E.; Wedemeyer, S.; Morris, B. M.; Becker, A. C.; Deeg, H. J.; Hoyer, S.; Mallonn, M.; Poppenhaeger, K.; Herrero, E.; Ribas, I.; Boumis, P.; Liakos, A. Bibcode: 2019A&A...628A.108F Altcode: 2019arXiv190706534F Context. The Kepler Object of Interest Network (KOINet) is a multi-site network of telescopes around the globe organised for follow-up observations of transiting planet candidate Kepler objects of interest with large transit timing variations (TTVs). The main goal of KOINet is the completion of their TTV curves as the Kepler telescope stopped observing the original Kepler field in 2013.
Aims: We ensure a comprehensive characterisation of the investigated systems by analysing Kepler data combined with new ground-based transit data using a photodynamical model. This method is applied to the Kepler-82 system leading to its first dynamic analysis.
Methods: In order to provide a coherent description of all observations simultaneously, we combine the numerical integration of the gravitational dynamics of a system over the time span of observations with a transit light curve model. To explore the model parameter space, this photodynamical model is coupled with a Markov chain Monte Carlo algorithm.
Results: The Kepler-82b/c system shows sinusoidal TTVs due to their near 2:1 resonance dynamical interaction. An additional chopping effect in the TTVs of Kepler-82c hints to a further planet near the 3:2 or 3:1 resonance. We photodynamically analysed Kepler long- and short-cadence data and three new transit observations obtained by KOINet between 2014 and 2018. Our result reveals a non-transiting outer planet with a mass of mf = 20.9 ± 1.0 M near the 3:2 resonance to the outermost known planet, Kepler-82c. Furthermore, we determined the densities of planets b and c to the significantly more precise values ρb = 0.98-0.14+0.10 g cm-3 and ρc = 0.494-0.077+0.066 g cm-3.

Ground-based photometry is 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/628/A108 Title: VizieR Online Data Catalog: Photometry of Kepler-82b and c transits (Freudenthal+, 2019) Authors: Freudenthal, J.; von Essen, C.; Ofir, A.; Dreizler, S.; Agol, E.; Wedemeyer, S.; Morris, B. M.; Becker, A. C.; Deeg, H. J.; Hoyer, S.; Mallonn, M.; Poppenhaeger, K.; Herrero, E.; Ribas, I.; Boumis, P.; Liakos, A. Bibcode: 2019yCat..36280108F Altcode: All our observations were carried out using R-band filter. We provide the differential photometry with varying reference stars for each observation together with the used detrending components. We measured one Kepler-82b and two Kepler-82c transits in three observations between 2014 and 2018.

(4 data files). Title: High-cadence imaging of the Sun Authors: Wedemeyer, Sven Bibcode: 2019adw..confE..47W Altcode: A challenge of observing the Sun with ALMA is that the primary beam covers only a small region on the Sun and is therefore filled with a complex radiation pattern, which evolves on extremely short time scales of only seconds and below. Fully exploiting ALMA's possibilities therefore requires imaging at very high cadence. Accordingly, there is much potential for improving the solar observing modes and post-processing of data. The development study "High-cadence imaging of the Sun", which started in 2018, aims at developing a high-cadence imaging capability by employing realistic test cases based on state-of-the-art numerical simulations, the Solar ALMA Simulator tool, and a solar simulation pipeline. Comparisons between the input and output of the simulation pipeline will reveal how well imaging at high cadence performs and how to optimize it. We present an overview of the study and its current state. Title: First Spectral Analysis of a Solar Plasma Eruption Using ALMA Authors: Rodger, Andrew S.; Labrosse, Nicolas; Wedemeyer, Sven; Szydlarski, Mikolaj; Simões, Paulo J. A.; Fletcher, Lyndsay Bibcode: 2019ApJ...875..163R Altcode: 2019arXiv190201319R The aim of this study is to demonstrate how the logarithmic millimeter continuum gradient observed using the Atacama Large Millimeter/submillimeter Array (ALMA) may be used to estimate optical thickness in the solar atmosphere. We discuss how using multiwavelength millimeter measurements can refine plasma analysis through knowledge of the absorption mechanisms. Here we use subband observations from the publicly available science verification (SV) data, while our methodology will also be applicable to regular ALMA data. The spectral resolving capacity of ALMA SV data is tested using the enhancement coincident with an X-ray bright point and from a plasmoid ejection event near active region NOAA12470 observed in Band 3 (84-116 GHz) on 2015 December 17. We compute the interferometric brightness temperature light curve for both features at each of the four constituent subbands to find the logarithmic millimeter spectrum. We compared the observed logarithmic spectral gradient with the derived relationship with optical thickness for an isothermal plasma to estimate the structures’ optical thicknesses. We conclude, within 90% confidence, that the stationary enhancement has an optical thickness between 0.02 ≤ τ ≤ 2.78, and that the moving enhancement has 0.11 ≤ τ ≤ 2.78, thus both lie near to the transition between optically thin and thick plasma at 100 GHz. From these estimates, isothermal plasmas with typical Band 3 background brightness temperatures would be expected to have electron temperatures of ∼7370-15300 K for the stationary enhancement and between ∼7440 and 9560 K for the moving enhancement, thus demonstrating the benefit of subband ALMA spectral analysis. Title: The solar chromosphere at millimetre and ultraviolet wavelengths. I. Radiation temperatures and a detailed comparison Authors: Jafarzadeh, S.; Wedemeyer, S.; Szydlarski, M.; De Pontieu, B.; Rezaei, R.; Carlsson, M. Bibcode: 2019A&A...622A.150J Altcode: 2019arXiv190105763J Solar observations with the Atacama Large Millimeter/submillimeter Array (ALMA) provide us with direct measurements of the brightness temperature in the solar chromosphere. We study the temperature distributions obtained with ALMA Band 6 (in four sub-bands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of, a sunspot, comprising quasi-quiet and active regions with different amounts of underlying magnetic fields. We compare these temperatures with those obtained at near- and far-ultraviolet (UV) wavelengths (and with the line-core intensities of the optically-thin far-UV spectra), co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer. These include the emission peaks and cores of the Mg II k 279.6 nm and Mg II h 280.4 nm lines as well as the line cores of C II 133.4 nm, O I 135.6 nm, and Si IV 139.4 nm, sampling the mid-to-high chromosphere and the low transition region. Splitting the ALMA sub-bands resulted in an slight increase of spatial resolution in individual temperature maps, thus, resolving smaller-scale structures compared to those produced with the standard averaging routines. We find that the radiation temperatures have different, though somewhat overlapping, distributions in different wavelengths and in the various magnetic regions. Comparison of the ALMA temperatures with those of the UV diagnostics should, however, be interpreted with great caution, the former is formed under the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE. The mean radiation temperature of the ALMA Band 6 is similar to that extracted from the IRIS C II line in all areas with exception of the sunspot and pores where the C II poses higher radiation temperatures. In all magnetic regions, the Mg II lines associate with the lowest mean radiation temperatures in our sample. These will provide constraints for future numerical models. Title: First high-resolution look at the quiet Sun with ALMA at 3mm Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos, S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu, T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V. Bibcode: 2018A&A...619L...6N Altcode: 2018arXiv181005223N We present an overview of high-resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun regions were observed at a resolution of up to 2.5″ by 4.5″. We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full-disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detect, for the first time, spicular structures, rising up to 15″ above the limb with a width down to the image resolution and brightness temperature of ∼1800 K above the local background. No trace of spicules, either in emission or absorption, is found on the disk. Our results highlight the potential of ALMA for the study of the quiet chromosphere. Title: Kepler Object of Interest Network. II. Photodynamical modelling of Kepler-9 over 8 years of transit observations Authors: Freudenthal, J.; von Essen, C.; Dreizler, S.; Wedemeyer, S.; Agol, E.; Morris, B. M.; Becker, A. C.; Mallonn, M.; Hoyer, S.; Ofir, A.; Tal-Or, L.; Deeg, H. J.; Herrero, E.; Ribas, I.; Khalafinejad, S.; Hernández, J.; Rodríguez S., M. M. Bibcode: 2018A&A...618A..41F Altcode: 2018arXiv180700007F Context. The Kepler Object of Interest Network (KOINet) is a multi-site network of telescopes around the globe organised to follow up transiting planet-candidate Kepler objects of interest (KOIs) with large transit timing variations (TTVs). Its main goal is to complete their TTV curves, as the Kepler telescope no longer observes the original Kepler field.
Aims: Combining Kepler and new ground-based transit data we improve the modelling of these systems. To this end, we have developed a photodynamical model, and we demonstrate its performance using the Kepler-9 system as an example.
Methods: Our comprehensive analysis combines the numerical integration of the system's dynamics over the time span of the observations along with the transit light curve model. This provides a coherent description of all observations simultaneously. This model is coupled with a Markov chain Monte Carlo algorithm, allowing for the exploration of the model parameter space.
Results: Applied to the Kepler-9 long cadence data, short cadence data, and 13 new transit observations collected by KOINet between the years 2014 and 2017, our modelling provides well constrained predictions for the next transits and the system's parameters. We have determined the densities of the planets Kepler-9b and 9c to the very precise values of ρb = 0.439 ± 0.023 g cm-3 and ρc = 0.322 ± 0.017 g cm-3. Our analysis reveals that Kepler-9c will stop transiting in about 30 yr due to strong dynamical interactions between Kepler-9b and 9c, near 2:1 resonance, leading to a periodic change in inclination.
Conclusions: Over the next 30 years, the inclination of Kepler-9c (-9b) will decrease (increase) slowly. This should be measurable by a substantial decrease (increase) in the transit duration, in as soon as a few years' time. Observations that contradict this prediction might indicate the presence of additional objects in this system. If this prediction turns out to be accurate, this behaviour opens up a unique chance to scan the different latitudes of a star: high latitudes with planet c and low latitudes with planet b.

Ground-based photometry is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/618/A41 Title: VizieR Online Data Catalog: KOINet. Study of exoplanet systems via TTVs (von Essen+, 2018) Authors: von Essen, C.; Ofir, A.; Dreizler, S.; Agol, E.; Freudenthal, J.; Hernandez, J.; Wedemeyer, S.; Parkash, V.; Deeg, H. J.; Hoyer, S.; Morris, B. M.; Becker, A. C.; Sun, L.; Gu, S. H.; Herrero, E.; Tal-Or, L.; Poppenhaeger, K.; Mallonn, M.; Albrecht, S.; Khalafinejad, S.; Boumis, P.; Delgado-Correal, C.; Fabrycky, D. C.; Janulis, R.; Lalitha, S.; Liakos, A.; Mikolaitis, S.; Moyano D'Angelo, M. L.; Sokov, E.; Pakstiene, E.; Popov, A.; Krushinsky, V.; Ribas, I.; Rodriguez, M. M. S.; Rusov, S.; Sokova, I.; Tautvaisiene, G.; Wang, X. Bibcode: 2018yCat..36150079V Altcode: All our observations were carried out using R-band filter. We provide the differential photometry with varying reference stars for each observation together with the used detrending components. We provide four transits of KOI-0410.01, and one transit of KOI-0902.01, KOI-0525.01, and KOI-0760.01, respectively. The telescope nomenclature, in agreement with the names of the files, is the following:

IAC0.8: IAC80 telescope (0.8m), Instituto de Astrofisica de Canarias, Spain. ARC3.5: Apache Point Observatory (3.5 m), USA. NOT2.5: Nordic Optical Telescope (2.5 m), Spain. YO2.4: Yunnan Observatories (2.4 m), PR China.

(8 data files). Title: VizieR Online Data Catalog: Photometry of Kepler-9b and c transits (Freudenthal+, 2018) Authors: Freudenthal, J.; von Essen, C.; Dreizler, S.; Wedemeyer, S.; Agol, E.; Morris, B. M.; Becker, A. C.; Mallonn, M.; Hoyer, S.; Ofir, A.; Tal-Or, L.; Deeg, H. J.; Herrero, E.; Ribas, I.; Khalafinejad, S.; Hernandez, J.; Rodriguez, S. M. M. Bibcode: 2018yCat..36180041F Altcode: All our observations were carried out using R-band filter. We provide the differential photometry with varying reference stars for each observation together with the used detrending components. We measured five Kepler-9b and four Kepler-9c transits in thirteen observations between 2014 and 2017.

(14 data files). Title: Kepler Object of Interest Network. I. First results combining ground- and space-based observations of Kepler systems with transit timing variations Authors: von Essen, C.; Ofir, A.; Dreizler, S.; Agol, E.; Freudenthal, J.; Hernández, J.; Wedemeyer, S.; Parkash, V.; Deeg, H. J.; Hoyer, S.; Morris, B. M.; Becker, A. C.; Sun, L.; Gu, S. H.; Herrero, E.; Tal-Or, L.; Poppenhaeger, K.; Mallonn, M.; Albrecht, S.; Khalafinejad, S.; Boumis, P.; Delgado-Correal, C.; Fabrycky, D. C.; Janulis, R.; Lalitha, S.; Liakos, A.; Mikolaitis, Š.; Moyano D'Angelo, M. L.; Sokov, E.; Pakštienė, E.; Popov, A.; Krushinsky, V.; Ribas, I.; Rodríguez S., M. M.; Rusov, S.; Sokova, I.; Tautvaišienė, G.; Wang, X. Bibcode: 2018A&A...615A..79V Altcode: 2018arXiv180106191V During its four years of photometric observations, the Kepler space telescope detected thousands of exoplanets and exoplanet candidates. One of Kepler's greatest heritages has been the confirmation and characterization of hundreds of multi-planet systems via transit timing variations (TTVs). However, there are many interesting candidate systems displaying TTVs on such long timescales that the existing Kepler observations are of insufficient length to confirm and characterize them by means of this technique. To continue with Kepler's unique work, we have organized the "Kepler Object of Interest Network" (KOINet), a multi-site network formed of several telescopes located throughout America, Europe, and Asia. The goals of KOINet are to complete the TTV curves of systems where Kepler did not cover the interaction timescales well, to dynamically prove that some candidates are true planets (or not), to dynamically measure the masses and bulk densities of some planets, to find evidence for non-transiting planets in some of the systems, to extend Kepler's baseline adding new data with the main purpose of improving current models of TTVs, and to build a platform that can observe almost anywhere on the northern hemisphere, at almost any time. KOINet has been operational since March 2014. Here we show some promising first results obtained from analyzing seven primary transits of KOI-0410.01, KOI-0525.01, KOI-0760.01, and KOI-0902.01, in addition to the Kepler data acquired during the first and second observing seasons of KOINet. While carefully choosing the targets we set demanding constraints on timing precision (at least 1 min) and photometric precision (as good as one part per thousand) that were achieved by means of our observing strategies and data analysis techniques. For KOI-0410.01, new transit data revealed a turnover of its TTVs. We carried out an in-depth study of the system, which is identified in the NASA Data Validation Report as a false positive. Among others, we investigated a gravitationally bound hierarchical triple star system and a planet-star system. While the simultaneous transit fitting of ground- andspace-based data allowed for a planet solution, we could not fully reject the three-star scenario. New data, already scheduled in the upcoming 2018 observing season, will set tighter constraints on the nature of the system.

Ground-based photometry is 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/615/A79 Title: Observations of the solar chromosphere with ALMA and comparison with theoretical models Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold O.; Kuhar, Matej; Kobelski, Adam; Wedemeyer, Sven; White, Stephen M.; Ludwig, Hans-G.; Temmer, Manuela; Saar, Steven H.; Selhorst, Caius L. Bibcode: 2018csss.confE..37B Altcode: 2018arXiv181207293B In this work we use solar observations with the ALMA radio telescope at the wavelength of 1.21 mm. The aim of the analysisis to improve understanding of the solar chromosphere, a dynamic layer in the solar atmosphere between the photosphere andcorona. The study has an observational and a modeling part. In the observational part full-disc solar images are analyzed.Based on a modied FAL atmospheric model, radiation models for various observed solar structures are developed. Finally, theobservational and modeling results are compared and discussed. Title: First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun Authors: Brajša, R.; Sudar, D.; Benz, A. O.; Skokić, I.; Bárta, M.; De Pontieu, B.; Kim, S.; Kobelski, A.; Kuhar, M.; Shimojo, M.; Wedemeyer, S.; White, S.; Yagoubov, P.; Yan, Y. Bibcode: 2018A&A...613A..17B Altcode: 2017arXiv171106130B Context. Various solar features can be seen in emission or absorption on maps of the Sun in the millimetre and submillimetre wavelength range. The recently installed Atacama Large Millimetre/submillimetre Array (ALMA) is capable of observing the Sun in that wavelength range with an unprecedented spatial, temporal and spectral resolution. To interpret solar observations with ALMA, the first important step is to compare solar ALMA maps with simultaneous images of the Sun recorded in other spectral ranges.
Aims: The first aim of the present work is to identify different structures in the solar atmosphere seen in the optical, infrared, and EUV parts of the spectrum (quiet Sun, active regions, prominences on the disc, magnetic inversion lines, coronal holes and coronal bright points) in a full-disc solar ALMA image. The second aim is to measure the intensities (brightness temperatures) of those structures and to compare them with the corresponding quiet Sun level.
Methods: A full-disc solar image at 1.21 mm obtained on December 18, 2015, during a CSV-EOC campaign with ALMA is calibrated and compared with full-disc solar images from the same day in Hα line, in He I 1083 nm line core, and with various SDO images (AIA at 170 nm, 30.4 nm, 21.1 nm, 19.3 nm, and 17.1 nm and HMI magnetogram). The brightness temperatures of various structures are determined by averaging over corresponding regions of interest in the calibrated ALMA image.
Results: Positions of the quiet Sun, active regions, prominences on the disc, magnetic inversion lines, coronal holes and coronal bright points are identified in the ALMA image. At the wavelength of 1.21 mm, active regions appear as bright areas (but sunspots are dark), while prominences on the disc and coronal holes are not discernible from the quiet Sun background, despite having slightly less intensity than surrounding quiet Sun regions. Magnetic inversion lines appear as large, elongated dark structures and coronal bright points correspond to ALMA bright points.
Conclusions: These observational results are in general agreement with sparse earlier measurements at similar wavelengths. The identification of coronal bright points represents the most important new result. By comparing ALMA and other maps, it was found that the ALMA image was oriented properly and that the procedure of overlaying the ALMA image with other images is accurate at the 5 arcsec level. The potential of ALMA for physics of the solar chromosphere is emphasised. Title: Solar Observations with ALMA Authors: Wedemeyer, Sven Bibcode: 2018iss..confE..38W Altcode: The continuum intensity at millimeter wavelengths can serve as an essentially linear thermometer of the plasma in a thin layer in the atmosphere of the Sun, whereas the polarisation of the received radiation is a measure for the longitudinal magnetic field component in the same layer. The enormous leap in terms of spatial resolution with the Atacama Large Millimeter/submillimeter Array (ALMA) now makes it possible to observe the intricate fine-structure of the solar atmosphere at sufficiently high spatial, temporal, and spectral resolution, thus enabling studies of a wide range of scientific topics in solar physics that had been inaccessible at millimeter wavelengths before. The radiation observed by ALMA originates mostly from the chromosphere - a complex and dynamic layer between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer solar atmosphere. ALMA observations of the solar chromosphere, which are offered as a regular capability since 2016, therefore have the potential to make important contributions towards the solution of fundamental questions in solar physics with implications for our understanding of stars in general. In this presentation, I will give a short description of ALMA's solar observing mode, it challenges and opportunities, and selected science cases in combination with numerical simulations and coordinated observations at other wavelengths. ALMA's scientific potential for studying the dynamic small-scale pattern of the solar chromosphere is illustrated with first results from Cycle 4. Title: Exploring the Sun with ALMA Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu, B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson, H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S. M.; Yan, Y. Bibcode: 2018Msngr.171...25B Altcode: The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory opens a new window onto the Universe. The ability to perform continuum imaging and spectroscopy of astrophysical phenomena at millimetre and submillimetre wavelengths with unprecedented sensitivity opens up new avenues for the study of cosmology and the evolution of galaxies, the formation of stars and planets, and astrochemistry. ALMA also allows fundamentally new observations to be made of objects much closer to home, including the Sun. The Sun has long served as a touchstone for our understanding of astrophysical processes, from the nature of stellar interiors, to magnetic dynamos, non-radiative heating, stellar mass loss, and energetic phenomena such as solar flares. ALMA offers new insights into all of these processes. Title: Impact of magnetic fields on the structure of convective atmospheres of red giant stars Authors: Klevas, J.; Kučinskas, A.; Wedemeyer, S.; Ludwig, H. -G. Bibcode: 2018CoSka..48..280K Altcode: We use 3D magnetohydrodynamic CO5BOLD model atmospheres to study the interplay between magnetic fields and convection in the atmospheres of red giant stars. We find that vortex-like structures occur prominently in stars with stronger magnetic fields and lead to alterations of their thermal structures. Title: A comparison of solar ALMA observations and model based predictions of the brightness temperature Authors: Brajša, R.; Kuhar, M.; Benz, A. O.; Skokić, I.; Sudar, D.; Wedemeyer, S.; Báarta, M.; De Pontieu, B.; Kim, S.; Kobelski, A.; Shimojo, M.; White, S.; Yagoubov, P.; Yan, Y.; Ludwig, H. G.; Temmer, M.; Saar, S. H.; Selhorst, C. L.; Beuc, R. Bibcode: 2018CEAB...42....1B Altcode: The new facility Atacama Large Millimeter/submillimeter Array (ALMA) is capable of observing the Sun in the wavelength range from 0.3 mm to 10 mm with an unprecedented spatial, temporal and spectral resolution. The first aim of the present work is to identify different structures in the solar atmosphere (quiet Sun, active regions, filaments on the disc, and coronal holes) in a full disc solar ALMA image at 1.21 mm obtained on December 18, 2015 during a CSV-EOC campaign. It is compared with full disc solar images from the same day in the Hα line (Cerro Tololo Observatory, NISP), and at three EUV wavelengths (30.4 nm, 21.1 nm, 17.1 nm; a composite SDO image). Positions of the quiet Sun areas, active regions, filaments on the disc, and coronal holes are identified in the ALMA image. To interpret solar observations with ALMA it is important to compare the measured and calculated intensities of various solar structures. So, the second aim of this work is to calculate the intensity (brightness temperature) for those structures (quiet Sun, active regions, filaments on the disc, and coronal holes) for a broad wavelength range (from 0.3 mm to 10 mm), closely related to that of the ALMA, and to compare the results with available ALMA observations. Thermal bremsstrahlung is the dominant radiation mechanism for explanation of the observed phenomena. A procedure for calculating the brightness temperature for a given wavelength and model atmosphere, which integrates the radiative transfer equation for thermal bremsstrahlung, is used. At the wavelength of 1.21 mm active regions appear as bright areas, while filaments on the disc and coronal holes are not discernible from the quiet Sun background. The models generally agree with the observed results: Active regions are bright primarily due to higher densities, filaments can appear bright, dark or not at all and coronal holes cannot be easily identified. Title: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. VI. First chromosphere model of a late-type giant Authors: Wedemeyer, Sven; Kučinskas, Arūnas; Klevas, Jonas; Ludwig, Hans-Günter Bibcode: 2017A&A...606A..26W Altcode: 2017arXiv170509641W
Aims: Although observational data unequivocally point to the presence of chromospheres in red giant stars, no attempts have been made so far to model them using 3D hydrodynamical model atmospheres. We therefore compute an exploratory 3D hydrodynamical model atmosphere for a cool red giant in order to study the dynamical and thermodynamic properties of its chromosphere, as well as the influence of the chromosphere on its observable properties.
Methods: Three-dimensional radiation hydrodynamics simulations are carried out with the CO5BOLD model atmosphere code for a star with the atmospheric parameters (Teff ≈ 4010 K, log g = 1.5, [ M / H ] = 0.0), which are similar to those of the K-type giant star Aldebaran (α Tau). The computational domain extends from the upper convection zone into the chromosphere (7.4 ≥ log τRoss ≥ - 12.8) and covers several granules in each horizontal direction. Using this model atmosphere, we compute the emergent continuum intensity maps at different wavelengths, spectral line profiles of Ca II K, the Ca II infrared triplet line at 854.2 nm, and Hα, as well as the spectral energy distribution (SED) of the emergent radiative flux.
Results: The initial model quickly develops a dynamical chromosphere that is characterised by propagating and interacting shock waves. The peak temperatures in the chromospheric shock fronts reach values of up to 5000 K, although the shock fronts remain quite narrow. Similar to the Sun, the gas temperature distribution in the upper layers of red giant stars is composed of a cool component due to adiabatic cooling in the expanding post-shock regions and a hot component due to shock waves. For this red giant model, the hot component is a rather flat high-temperature tail, which nevertheless affects the resulting average temperatures significantly.
Conclusions: The simulations show that the atmospheres of red giant stars are dynamic and intermittent. Consequently, many observable properties cannot be reproduced with static 1D models, but require advanced 3D hydrodynamical modelling. Furthermore, including a chromosphere in the models might produce significant contributions to the emergent UV flux. Title: Solar Commissioning Observations of the Sun with ALMA Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.; Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim, Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven Bibcode: 2017SPD....4820402W Altcode: PI-led science observations have commenced with the Atacama Large Millimeter-submillimeter Array (ALMA) following an extensive commissioning effort. This talk will summarize that effort and discuss some of the scientific results derived from the commissioning data. As the solar cycle declines, ALMA observations will mainly address chromospheric science topics. Examples of data obtained during commissioning, both from the interferometer and from single-dish observations, will be presented. The temperatures of the layers that ALMA is most sensitive to have been determined for the two frequency bands currently used for solar observations. Curious behavior in a sunspot umbra and an observations of a small chromospheric ejection will be discussed. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric Imaging Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.; Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.; Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.; Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder, S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.; Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y. Bibcode: 2017SoPh..292...87S Altcode: 2017arXiv170403236S Observations of the Sun at millimeter and submillimeter wavelengths offer a unique probe into the structure, dynamics, and heating of the chromosphere; the structure of sunspots; the formation and eruption of prominences and filaments; and energetic phenomena such as jets and flares. High-resolution observations of the Sun at millimeter and submillimeter wavelengths are challenging due to the intense, extended, low-contrast, and dynamic nature of emission from the quiet Sun, and the extremely intense and variable nature of emissions associated with energetic phenomena. The Atacama Large Millimeter/submillimeter Array (ALMA) was designed with solar observations in mind. The requirements for solar observations are significantly different from observations of sidereal sources and special measures are necessary to successfully carry out this type of observations. We describe the commissioning efforts that enable the use of two frequency bands, the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum interferometric-imaging observations of the Sun with ALMA. Examples of high-resolution synthesized images obtained using the newly commissioned modes during the solar-commissioning campaign held in December 2015 are presented. Although only 30 of the eventual 66 ALMA antennas were used for the campaign, the solar images synthesized from the ALMA commissioning data reveal new features of the solar atmosphere that demonstrate the potential power of ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning efforts will continue to enable new and unique solar observing capabilities. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota, A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales, A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki, W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.; Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.; Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann, G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y. Bibcode: 2017SoPh..292...88W Altcode: 2017arXiv170504766W The Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that uses the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions, we derive quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at λ =1.3 mm. These values have statistical uncertainties of about 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of about 25, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plages are among the hotter features, while a large sunspot umbra shows up as a depression, and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images. Title: Vortex flows in the solar chromosphere. I. Automatic detection method Authors: Kato, Y.; Wedemeyer, S. Bibcode: 2017A&A...601A.135K Altcode: 2017arXiv170206032K Solar "magnetic tornadoes" are produced by rotating magnetic field structures that extend from the upper convection zone and the photosphere to the corona of the Sun. Recent studies show that these kinds of rotating features are an integral part of atmospheric dynamics and occur on a large range of spatial scales. A systematic statistical study of magnetic tornadoes is a necessary next step towards understanding their formation and their role in mass and energy transport in the solar atmosphere. For this purpose, we develop a new automatic detection method for chromospheric swirls, meaning the observable signature of solar tornadoes or, more generally, chromospheric vortex flows and rotating motions. Unlike existing studies that rely on visual inspections, our new method combines a line integral convolution (LIC) imaging technique and a scalar quantity that represents a vortex flow on a two-dimensional plane. We have tested two detection algorithms, based on the enhanced vorticity and vorticity strength quantities, by applying them to three-dimensional numerical simulations of the solar atmosphere with CO5BOLD. We conclude that the vorticity strength method is superior compared to the enhanced vorticity method in all aspects. Applying the method to a numerical simulation of the solar atmosphere reveals very abundant small-scale, short-lived chromospheric vortex flows that have not been found previously by visual inspection. Title: 3D hydrodynamical COBOLD simulations of a chromosphere of a red giant Authors: Klevas, J.; Wedemeyer, S.; Kučinskas, A.; Ludwig, H. -G. Bibcode: 2017MmSAI..88..100K Altcode: We present the results of a 3D hydrodynamical simulation of a cool red giant star with a chromosphere. The simulation was performed using a 3D hydrodynamic COBOLD model atmosphere (Teff=4000 K, {log g}=1.5 cgs, [M/H]=0.0 [dex]), which was extended outwards to include chromospheric layers. We synthesized the spectral energy distribution of a model atmosphere including chromosphere and compare it to the spectral energy distribution of a model atmosphere without the chromosphere. We find that adding a model chromosphere leads to a significant increase of the radiative flux at wavelengths smaller than 300 nm. The increase in the UV flux is attributed to the presence of shock waves, which provide additional heating in the chromosphere. 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: Lower solar atmosphere and magnetism at ultra-high spatial resolution Authors: Collet, Remo; Criscuoli, Serena; Ermolli, Ilaria; Fabbian, Damian; Guerreiro, Nuno; Haberreiter, Margit; Peck, Courtney; Pereira, Tiago M. D.; Rempel, Matthias; Solanki, Sami K.; Wedemeyer-Boehm, Sven Bibcode: 2016arXiv161202348C Altcode: We present the scientific case for a future space-based telescope aimed at very high spatial and temporal resolution imaging of the solar photosphere and chromosphere. Previous missions (e.g., HINODE, SUNRISE) have demonstrated the power of observing the solar photosphere and chromosphere at high spatial resolution without contamination from Earth's atmosphere. We argue here that increased spatial resolution (from currently 70 km to 25 km in the future) and high temporal cadence of the observations will vastly improve our understanding of the physical processes controlling solar magnetism and its characteristic scales. This is particularly important as the Sun's magnetic field drives solar activity and can significantly influence the Sun-Earth system. At the same time a better knowledge of solar magnetism can greatly improve our understanding of other astrophysical objects. Title: Phoenix Meets CO5BOLD: 3D NLTE Radiative Transfer Calculations For M-Dwarf Chromospheres Authors: De Gennaro Aquino, Ivan; Hauschildt, Peter H.; Wedemeyer, Sven Bibcode: 2016csss.confE.149D Altcode: M-dwarf atmospheres are phenomenologically so rich that is currently impossible to include all the physical processes in one astrophysical simulation code. 1D models have greatly improved our understanding of the radiative properties of M-dwarf photospheres and important achievements have been obtained in 1D and 3D magneto-hydrodynamic simulations. Using a snapshot from a CO5BOLD M-dwarf simulation as input model, we use the 3D atmosphere code PHOENIX/3D to compute the radiative properties of a M-dwarf photosphere-chromosphere atmosphere with NLTE treatment for several atomic species and background atomic and molecular opacities. Title: Chromospheric and Coronal Wave Generation in a Magnetic Flux Sheath Authors: Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen, Boris; Wedemeyer, Sven; Carlsson, Mats Bibcode: 2016ApJ...827....7K Altcode: 2016arXiv160608826K Using radiation magnetohydrodynamic simulations of the solar atmospheric layers from the upper convection zone to the lower corona, we investigate the self-consistent excitation of slow magneto-acoustic body waves (slow modes) in a magnetic flux concentration. We find that the convective downdrafts in the close surroundings of a two-dimensional flux slab “pump” the plasma inside it in the downward direction. This action produces a downflow inside the flux slab, which encompasses ever higher layers, causing an upwardly propagating rarefaction wave. The slow mode, excited by the adiabatic compression of the downflow near the optical surface, travels along the magnetic field in the upward direction at the tube speed. It develops into a shock wave at chromospheric heights, where it dissipates, lifts the transition region, and produces an offspring in the form of a compressive wave that propagates further into the corona. In the wake of downflows and propagating shock waves, the atmosphere inside the flux slab in the chromosphere and higher tends to oscillate with a period of ν ≈ 4 mHz. We conclude that this process of “magnetic pumping” is a most plausible mechanism for the direct generation of longitudinal chromospheric and coronal compressive waves within magnetic flux concentrations, and it may provide an important heat source in the chromosphere. It may also be responsible for certain types of dynamic fibrils. Title: Ssalmon - The Solar Simulations For The Atacama Large Millimeter Observatory Network Authors: Wedemeyer, Sven; Ssalmon Group Bibcode: 2016csss.confE..84W Altcode: The Atacama Large Millimeter/submillimeter Array (ALMA) provides a new powerful tool for observing the solar chromosphere at high spatial, temporal, and spectral resolution, which will allow for addressing a wide range of scientific topics in solar physics. Numerical simulations of the solar atmosphere and modeling of instrumental effects are valuable tools for constraining, preparing and optimizing future observations with ALMA and for interpreting the results. In order to co-ordinate related activities, the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated on September 1st, 2014, in connection with the NA- and EU-led solar ALMA development studies. As of April, 2015, SSALMON has grown to 83 members from 18 countries (plus ESO and ESA). Another important goal of SSALMON is to promote the scientific potential of solar science with ALMA, which has resulted in two major publications so far. During 2015, the SSALMON Expert Teams produced a White Paper with potential science cases for Cycle 4, which will be the first time regular solar observations will be carried out. Registration and more information at http://www.ssalmon.uio.no. Title: Testing Wave Propagation Properties in the Solar Chromosphere with ALMA and IRIS Authors: Fleck, Bernard; Straus, Thomas; Wedemeyer, Sven Bibcode: 2016SPD....47.0102F Altcode: Waves and oscillations are interesting not only from the point of view that they can propagate energy into the chromosphere and dissipate that energy to produce non-radiative heating, they also carry information about the structure of the atmosphere in which they propagate. Since the late 80s there is substantial evidence that the chromospheric wave field is dominated by a non-propagating component, presumably resulting from wave reflection at the transition region. Observations of Doppler oscillations measured in the Ca II infrared tripet lines, Ca II K, and He 10830 all show vanishing phase lags (i.e. vanishing travel time differences) between the various lines, in particular also for frequencies above the cut-off frequency. Why is the apparent phase speed of high frequency acoustic waves in the chromosphere so high? Are these results misleading because of complex radiation transfer effects in these optically thick lines? ALMA, which acts as a linear thermometer of the solar chromosphere, will provide measurements of the local plasma conditions that should be, at least in principle, much easier to interpret. Multi-wavelength time series of ALMA observations of the temperature fluctuations of inter-network oscillations should allow travel time measurements between different heights as these disturbances propagate through the chromosphere and thus should finally settle the long-standing question about the propagation characteristics of high frequency acoustic waves in the chromosphere. We plan to combine ALMA mm-observations with high resolution IRIS observations in the Mg II h and k lines, and until ALMA observations are available, will study the expected signals using time series of mm-maps from 3D radiation hydrodynamics simulations that are being prepared within the framework of the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON). Title: Solar Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.; Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.; Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA Solar Development Team Bibcode: 2016ASPC..504..327K Altcode: The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint North American, European, and East Asian project that opens the mm-sub mm wavelength part of the electromagnetic spectrum for general astrophysical exploration, providing high-resolution imaging in frequency bands currently ranging from 84 GHz to 950 GHz (300 microns to 3 mm). It is located in the Atacama desert in northern Chile at an elevation of 5000 m. Despite being a general purpose instrument, provisions have been made to enable solar observations with ALMA. Radiation emitted at ALMA wavelengths originates mostly from the chromosphere, which plays an important role in the transport of matter and energy, and the in heating the outer layers of the solar atmosphere. Despite decades of research, the solar chromosphere remains a significant challenge: both to observe, owing to the complicated formation mechanisms of currently available diagnostics; and to understand, as a result of the complex nature of the structure and dynamics of the chromosphere. ALMA has the potential to change the scene substantially as it serves as a nearly linear thermometer at high spatial and temporal resolution, enabling us to study the complex interaction of magnetic fields and shock waves and yet-to-be-discovered dynamical processes. Moreover, ALMA will play an important role in the study of energetic emissions associated with solar flares at sub-THz frequencies. 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: New Eyes on the Sun — Solar Science with ALMA Authors: Wedemeyer, S. Bibcode: 2016Msngr.163...15W Altcode: In Cycle 4, which starts in October 2016, the Atacama Large Millimeter/submillimeter Array (ALMA) will be open for regular observations of the Sun for the first time. ALMA’s impressive capabilities have the potential to revolutionise our understanding of our host star, with far-reaching implications for our knowledge about stars in general. The radiation emitted at ALMA wavelengths originates mostly from the chromosphere — a complex and dynamic layer between the photosphere and the corona that is prominent during solar eclipses. Despite decades of intensive research, the chromosphere is still elusive due to its complex nature and the resulting challenges to its observation. ALMA will change the scene substantially by opening up a new window on the Sun, promising answers to long-standing questions. Title: Synthetic activity indicators for M-type dwarf stars Authors: Wedemeyer, Sven; Ludwig, Hans-Günter Bibcode: 2016IAUS..320..303W Altcode: 2015arXiv151106153W Here, we present a set of time-dependent 3D RMHD simulations of a M-dwarf star representative of AD Leo, which extend from the upper convection zone into the chromosphere. The 3D model atmospheres are characterized by a very dynamic and intermittent structure on small spatial and temporal scales and a wealth of physical processes, which by nature cannot be described by means of 1D static model atmospheres. Artificial observations of these models imply that a combination of complementary diagnostics such as Ca II lines and the continuum intensity from UV to millimeter wavelengths, probe various properties of the dynamics, thermal and magnetic structure of the photosphere and the chromosphere and thus provide measures of stellar activity, which can be compared to observations. The complicated magnetic field structure and its imprint in synthetic diagnostics may have important implications for the understanding and characterization of stellar activity and with it possibly for the evaluation of planetary habitability around active M-dwarf stars. Title: ALMA Observations of the Sun in Cycle 4 and Beyond Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.; Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.; Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson, M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.; Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov, V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões, P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig, A.; White, S.; Yagoubov, P.; Zaqarashvili, T. Bibcode: 2016arXiv160100587W Altcode: This document was created by the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) in preparation of the first regular observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), which are anticipated to start in ALMA Cycle 4 in October 2016. The science cases presented here demonstrate that a large number of scientifically highly interesting observations could be made already with the still limited solar observing modes foreseen for Cycle 4 and that ALMA has the potential to make important contributions to answering long-standing scientific questions in solar physics. With the proposal deadline for ALMA Cycle 4 in April 2016 and the Commissioning and Science Verification campaign in December 2015 in sight, several of the SSALMON Expert Teams composed strategic documents in which they outlined potential solar observations that could be feasible given the anticipated technical capabilities in Cycle 4. These documents have been combined and supplemented with an analysis, resulting in recommendations for solar observing with ALMA in Cycle 4. In addition, the detailed science cases also demonstrate the scientific priorities of the solar physics community and which capabilities are wanted for the next observing cycles. The work on this White Paper effort was coordinated in close cooperation with the two international solar ALMA development studies led by T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be further updated until the beginning of Cycle 4 in October 2016. In particular, we plan to adjust the technical capabilities of the solar observing modes once finally decided and to further demonstrate the feasibility and scientific potential of the included science cases by means of numerical simulations of the solar atmosphere and corresponding simulated ALMA observations. Title: Solar Simulations for the Atacama Large Millimeter Observatory Network Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Shimojo, M. Bibcode: 2015ASPC..499..341W Altcode: 2015arXiv150206379W The Atacama Large Millimeter/submillimeter Array (ALMA) will be a valuable tool for observing the chromosphere of our Sun at (sub-)millimeter wavelengths at high spatial, temporal and spectral resolution and as such has great potential to address long-standing scientific questions in solar physics. In order to make the best use of this scientific opportunity, the Solar Simulations for the Atacama Large Millimeter Observatory Network has been initiated. A key goal of this international collaboration is to support the preparation and interpretation of future observations of the Sun with ALMA. Title: ALMA's High-Cadence Imaging Capabilities for Solar Observations Authors: Wedemeyer, S.; Parmer, A. Bibcode: 2015ASPC..499..343W Altcode: 2015arXiv150203580W The Atacama Large Millimeter/submillimeter Array offers an unprecedented view of our Sun at sub-/millimeter wavelengths. The high spatial, temporal, and spectral resolution facilitates the measurement of gas temperatures and magnetic fields in the solar chromosphere with high precision. The anticipated results will revolutionize our understanding of the solar atmosphere and may in particular result in major steps towards solving the coronal heating problem. Based on state-of-the-art 3D radiation magnetohydrodynamic simulations, we calculate the emergent continuum intensity (and thus brightness temperature maps) in the wavelength range accessed by ALMA and simulate instrumental effects for different array configurations. First results show that the local gas temperature can be closely mapped with ALMA and that much of the complex small-scale chromospheric pattern can be resolved. Title: SSALMON - The Solar Simulations for the Atacama Large Millimeter Observatory Network Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu, B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.; Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.; Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier, A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M. Bibcode: 2015AdSpR..56.2679W Altcode: 2015arXiv150205601W The Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated in 2014 in connection with two ALMA development studies. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new tool, which can also observe the Sun at high spatial, temporal, and spectral resolution. The international SSALMONetwork aims at co-ordinating the further development of solar observing modes for ALMA and at promoting scientific opportunities for solar physics with particular focus on numerical simulations, which can provide important constraints for the observing modes and can aid the interpretation of future observations. The radiation detected by ALMA originates mostly in the solar chromosphere - a complex and dynamic layer between the photosphere and corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Potential targets include active regions, prominences, quiet Sun regions, flares. Here, we give a brief overview over the network and potential science cases for future solar observations with ALMA. Title: Fast Single-Dish Scans of the Sun Using ALMA Authors: Phillips, N.; Hills, R.; Bastian, T.; Hudson, H.; Marson, R.; Wedemeyer, S. Bibcode: 2015ASPC..499..347P Altcode: 2015arXiv150206122P We have implemented control and data-taking software that makes it possible to scan the beams of individual ALMA antennas to perform quite complex patterns while recording the signals at high rates. We conducted test observations of the Sun in September and December, 2014. The data returned have excellent quality; in particular they allow us to characterize the noise and signal fluctuations present in this kind of observation. The fast-scan experiments included both Lissajous patterns covering rectangular areas, and “double-circle” patterns of the whole disk of the Sun and smaller repeated maps of specific disk-shaped targets. With the latter we find that we can achieve roughly Nyquist sampling of the Band 6 (230 GHz) beam in 60 s over a region 300” in diameter. These maps show a peak-to-peak brightness-temperature range of up to 1000 K, while the time-series variability at any given point appears to be of order 0.5% RMS over times of a few minutes. We thus expect to be able to separate the noise contributions due to transparency fluctuations from variations in the Sun itself. Such timeseries have many advantages, in spite of the non-interferometric observations. In particular such data should make it possible to observe microflares in active regions and nanoflares in any part of the solar disk and low corona. Title: Solar ALMA Observations - A New View of Our Host Star Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Shimojo, M.; Hales, A.; Yagoubov, P.; Hudson, H. Bibcode: 2015ASPC..499..345W Altcode: 2015arXiv150206397W ALMA provides the necessary spatial, temporal and spectral resolution to explore central questions in contemporary solar physics with potentially far-reaching implications for stellar atmospheres and plasma physics. It can uniquely constraint the thermal and magnetic field structure in the solar chromosphere with measurements that are highly complementary to simultaneous observations with other ground-based and space-borne instruments. Here, we highlight selected science cases. Title: On the Evolution of Magnetic White Dwarfs Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.; Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P. Bibcode: 2015ApJ...812...19T Altcode: 2015arXiv150905398T We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (Teff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with Teff ≲ 10,000 K cool significantly slower than non-magnetic degenerates. Title: The statistical properties of vortex flows in the solar atmosphere Authors: Wedemeyer, Sven; Kato, Yoshiaki; Steiner, Oskar Bibcode: 2015IAUGA..2256852W Altcode: Rotating magnetic field structures associated with vortex flows on the Sun, also known as “magnetic tornadoes”, may serve as waveguides for MHD waves and transport mass and energy upwards through the atmosphere. Magnetic tornadoes may therefore potentially contribute to the heating of the upper atmospheric layers in quiet Sun regions.Magnetic tornadoes are observed over a large range of spatial and temporal scales in different layers in quiet Sun regions. However, their statistical properties such as size, lifetime, and rotation speed are not well understood yet because observations of these small-scale events are technically challenging and limited by the spatial and temporal resolution of current instruments. Better statistics based on a combination of high-resolution observations and state-of-the-art numerical simulations is the key to a reliable estimate of the energy input in the lower layers and of the energy deposition in the upper layers. For this purpose, we have developed a fast and reliable tool for the determination and visualization of the flow field in (observed) image sequences. This technique, which combines local correlation tracking (LCT) and line integral convolution (LIC), facilitates the detection and study of dynamic events on small scales, such as propagating waves. Here, we present statistical properties of vortex flows in different layers of the solar atmosphere and try to give realistic estimates of the energy flux which is potentially available for heating of the upper solar atmosphere Title: The Atacama Large Millimeter/Submillimeter Array: a New Asset for Solar and Heliospheric Physics Authors: Bastian, Timothy S.; Barta, Miroslav; Brajsa, Roman; Chen, Bin; De Pontieu, Bart; Fleishman, Gregory; Gary, Dale; Hales, Antonio; Hills, Richard; Hudson, Hugh; Iwai, Kazamasu; Shimojo, Masumi; White, Stephen; Wedemeyer, Sven; Yan, Yihua Bibcode: 2015IAUGA..2257295B Altcode: The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint North American, European, and East Asian interferometric array that opens the mm-submm wavelength part of the electromagnetic spectrum for general astrophysical exploration, providing high-resolution imaging in frequency bands ranging from 86 to 950 GHz. Despite being a general purpose instrument, provisions have been made to enable solar observations with ALMA. Radiation emitted at ALMA wavelengths originates mostly from the chromosphere, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. In this paper we describe recent efforts to ensure that ALMA can be usefully exploited by the scientific community to address outstanding questions in solar physics. We summarize activities under North American and European ALMA development studies, including instrument testing, calibration and imaging strategies, a science simulations. With the support of solar observations, ALMA joins next-generation groundbased instruments that can be used alone or in combination with other ground-based and space-based instruments to address outstanding questions in solar and heliospheric physics. Opportunities for the wider community to contribute to these efforts will be highlighted. Title: Synthetic activity indicators for M-type dwarf stars Authors: Wedemeyer, Sven; Ludwig, Hans-Günter; Hauschildt, Peter; De Gennaro Aquino, Ivan Bibcode: 2015IAUGA..2255174W Altcode: Our understanding of the Sun has been substantially progressed owing to the advances in high-resolution observations during the last decades. These observations guided the development of numerical simulation codes for stellar atmospheres towards unprecedented levels of realism and complexity. Such 3D radiation magnetohydrodynamic (RMHD) codes can be applied and adapted to cooler stars. Here, we present a set of time-dependent 3D RMHD simulations for dwarf stars of spectral type M (representative of AD Leo). "M-dwarfs" are the most abundant stars in our galaxy and known to exhibit mega-flares. Comparisons of M-dwarf models with the Sun as fundamental reference case reveal differences and similarities, which lead to important insights into the structure and dynamics of quiescent "background" atmospheres. The models, which extend from the upper convection zone into the chromosphere, have different initial magnetic field strengths (up to 500G) and topologies, representing regions with different activity levels. The 3D model atmospheres are characterized by a very dynamic and intermittent structure on small spatial and temporal scales, final field strengths reaching a few kG and a wealth of physical processes, which by nature cannot be described by means of 1D static model atmospheres.Synthetic observables, i.e. spectra and intensity images, are calculated by using these models as input for detailed radiative transfer calculations and can be combined into synthetic full stellar disks, thus simulating spatially unresolved observations of M-dwarfs. The considered diagnostics, like, e.g., Halpha, Ca II lines, or the continuum intensity from UV to millimeter wavelengths, sample various properties of the dynamics, thermal and magnetic structure of the photosphere and the chromosphere and thus provide measures of stellar activity, which can be compared to observations. The complicated magnetic field structure and its imprint in synthetic diagnostics may have important implications for the understanding and characterization of stellar activity and with it possibly for the evaluation of planetary habitability around active M-dwarf stars. Title: SSALMON - The Solar Simulations for the Atacama Large Millimeter Observatory Network Authors: Wedemeyer, Sven; Bastian, Timothy S.; Brajsa, Roman; Barta, Miroslav Bibcode: 2015IAUGA..2257466W Altcode: The Atacama Large Millimeter/submillimeter Array (ALMA) provides a new powerful tool for observing the solar chromosphere at high spatial, temporal, and spectral resolution, which will allow for addressing fundamental scientific questions. Based on first solar test observations, observing strategies for regular solar campaigns are currently under development. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help in this respect, constraining and optimizing future observing modes for ALMA. On September 1st, 2014, the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) has been initiated with the aim to co-ordinate related activities and to promote the scientific potential of ALMA observations of the Sun. The network is connected to two currently ongoing ALMA development studies. As of March 18th, 57 scientists from 15 countries have joined the international SSALMONetwork. Among the affiliations are NRAO, ESO, NAOJ, the Czech ALMA ARC node at Ondrejov, ESA and many more. Since March 2015, we are building up expert teams, which work on specific tasks in preparation of future regular ALMA observations (expected to start in late 2016) and their interpretation. Registration and more information at http://www.ssalmon.uio.no. Title: Solar ALMA observations - A revolutionizing new view at our host star Authors: Wedemeyer, Sven; Brajsa, Roman; Bastian, Timothy S.; Barta, Miroslav; Hales, Antonio; Yagoubov, Pavel; Hudson, Hugh; Loukitcheva, Maria; Fleishman, Gregory Bibcode: 2015IAUGA..2256732W Altcode: Observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA) have a large potential for revolutionizing our understanding of our host star with far reaching implications for stars in general. The radiation emitted at ALMA wavelengths originates mostly from the chromosphere - a complex and dynamic layer between the photosphere and the corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere.Despite decades of intensive research, the chromosphere is still elusive and challenging to observe owing to the complicated formation mechanisms of currently available diagnostics. ALMA will change the scene substantially as it serves as a nearly linear thermometer at high spatial, temporal, and spectral resolution, enabling us to study the complex interaction of magnetic fields and shock waves and yet-to-be-discovered dynamical processes. Furthermore, radio recombination and molecular lines may have great diagnostic potential but need to be investigated first. These unprecedented capabilities promise important new findings for a large range of topics in solar physics including the structure, dynamics and energy balance of quiet Sun regions, active regions and sunspots, flares and prominences. As a part of ongoing development studies, an international network has been initiated, which aims at defining and preparing key solar science with ALMA through simulation studies: SSALMON -- Solar Simulations for the Atacama Large Millimeter Observatory Network (http://ssalmon.uio.no). Here, we give an overview of potential science cases. Title: Solar Observations with the Atacama Large Millimeter/submillimeter Array Authors: Wedemeyer, Sven Bibcode: 2015IAUGA..2252221W Altcode: The interferometric Atacama Large Millimeter/submillimeter Array (ALMA) has already demonstrated its impressive capabilities by observing a large variety of targets ranging from protoplanetary disks to galactic nuclei. ALMA is also capable of observing the Sun and has been used for five solar test campaigns so far. The technically challenging solar observing modes are currently under development and regular observations are expected to begin in late 2016.ALMA consists of 66 antennas located in the Chilean Andes at an altitude of 5000 m and is a true leap forward in terms of spatial resolution at millimeter wavelengths. The resolution of reconstructed interferometric images of the Sun is anticipated to be close to what current optical solar telescopes can achieve. In combination with the high temporal and spectral resolution, these new capabilities open up new parameter spaces for solar millimeter observations.The solar radiation at wavelengths observed by ALMA originates from the chromosphere, where the height of the sampled layer increases with selected wavelength. The continuum intensity is linearly correlated to the local gas temperature in the probed layer, which makes ALMA essentially a linear thermometer. During flares, ALMA can detect additional non-thermal emission contributions. Measurements of the polarization state facilitate the valuable determination of the chromospheric magnetic field. In addition, spectrally resolved observations of radio recombination and molecular lines may yield great diagnostic potential, which has yet to be investigated and developed.Many different scientific applications for a large range of targets from quiet Sun to active regions and prominences are possible, ranging from ultra-high cadence wave studies to flare observations. ALMA, in particular in combination with other ground-based and space-borne instruments, will certainly lead to fascinating new findings, which will advance our understanding of the atmosphere of our Sun. Here we give an overview of ALMA's capabilities and potential science cases. Title: Observing the Sun with ALMA: A New Window into Solar Physics Authors: Bastian, Timothy S.; Shimojo, Masumi; Wedemeyer-Bohm, Sven; ALMA North American Solar Development Team Bibcode: 2015AAS...22541301B Altcode: The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint North American, European, and East Asian interferometric array that opens the mm-submm wavelength part of the electromagnetic spectrum for general astrophysical exploration, providing high resolution imaging in frequency bands. Despite being a general purpose instrument, provisions have been made to enable solar observations with ALMA, thereby offering a new window into solar physics. Radiation emitted at ALMA wavelengths originates mostly from the chromosphere, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Despite decades of intensive research, an understanding of the chromosphere is still elusive, and challenging to observe owing to the complicated formation mechanisms of currently available diagnostics. ALMA will change the scene substantially as it serves as a nearly linear thermometer at high spatial, temporal, and spectral resolution, enabling us to study the complex interaction of magnetic fields and shock waves and yet-to-be-discovered dynamical processes.Moreover, ALMA will play an important role in the study of energetic emissions associated with solar flares at sub-THz frequencies.This presentations introduces ALMA to the solar physcis community and motivates the science that can be addressed by ALMA using a number of examples based on 3D MHD simulations. In addition, the means by which ALMA is used to acquire and calibrate solar observations will be discussed. Finally, we encourage potential users to join us in further defining and articulating the exciting science to be explored with this fundamentally new instrument. 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 plasma flow inside magnetic tornadoes on the Sun Authors: Wedemeyer, Sven; Steiner, Oskar Bibcode: 2014PASJ...66S..10W Altcode: 2014PASJ..tmp...98W; 2014arXiv1406.7270W High-resolution observations with the Swedish 1-m Solar Telescope (SST) and the Solar Dynamics Observatory (SDO) reveal rotating magnetic field structures that extend from the solar surface into the chromosphere and the corona. These so-called magnetic tornadoes are primarily detected as rings or spirals of rotating plasma in the Ca II 854.2 nm line core (also known as chromospheric swirls). Detailed numerical simulations show that the observed chromospheric plasma motion is caused by the rotation of magnetic field structures, which again are driven by photospheric vortex flows at their footpoints. Under the right conditions, two vortex flow systems are stacked on top of each other. We refer to the lower vortex, which extends from the low photosphere into the convection zone, as intergranular vortex flow (IVF). Once a magnetic field structure is co-located with an IVF, the rotation is mediated into the upper atmospheric layers and an atmospheric vortex flow (AVF, or magnetic tornado) is generated. In contrast to the recent work by Shelyag et al. (2013, ApJ, 776, L4), we demonstrate that particle trajectories in a simulated magnetic tornado indeed follow spirals and argue that the properties of the trajectories decisively depend on the location in the atmosphere and the strength of the magnetic field. Title: The Detection of Upwardly Propagating Waves Channeling Energy from the Chromosphere to the Low Corona Authors: Freij, N.; Scullion, E. M.; Nelson, C. J.; Mumford, S.; Wedemeyer, S.; Erdélyi, R. Bibcode: 2014ApJ...791...61F Altcode: 2014arXiv1408.4621F There have been ubiquitous observations of wave-like motions in the solar atmosphere for decades. Recent improvements to space- and ground-based observatories have allowed the focus to shift to smaller magnetic structures on the solar surface. In this paper, high-resolution ground-based data taken using the Swedish 1 m Solar Telescope is combined with co-spatial and co-temporal data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) satellite to analyze running penumbral waves (RPWs). RPWs have always been thought to be radial wave propagation that occurs within sunspots. Recent research has suggested that they are in fact upwardly propagating field-aligned waves (UPWs). Here, RPWs within a solar pore are observed for the first time and are interpreted as UPWs due to the lack of a penumbra that is required to support RPWs. These UPWs are also observed co-spatially and co-temporally within several SDO/AIA elemental lines that sample the transition region and low corona. The observed UPWs are traveling at a horizontal velocity of around 17 ± 0.5 km s-1 and a minimum vertical velocity of 42 ± 21 km s-1. The estimated energy of the waves is around 150 W m-2, which is on the lower bound required to heat the quiet-Sun corona. This is a new, yet unconsidered source of wave energy within the solar chromosphere and low corona. 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: 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: Is the Sun Lighter than the Earth? Isotopic CO in the Photosphere, Viewed through the Lens of Three-dimensional Spectrum Synthesis Authors: Ayres, Thomas R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.; Wedemeyer-Böhm, S. Bibcode: 2013ApJ...765...46A Altcode: 2013arXiv1301.5281A We consider the formation of solar infrared (2-6 μm) rovibrational bands of carbon monoxide (CO) in CO5BOLD 3D convection models, with the aim of refining abundances of the heavy isotopes of carbon (13C) and oxygen (18O, 17O), to compare with direct capture measurements of solar wind light ions by the Genesis Discovery Mission. We find that previous, mainly 1D, analyses were systematically biased toward lower isotopic ratios (e.g., R 2312C/13C), suggesting an isotopically "heavy" Sun contrary to accepted fractionation processes that were thought to have operated in the primitive solar nebula. The new 3D ratios for 13C and 18O are R 23 = 91.4 ± 1.3 (R = 89.2) and R 68 = 511 ± 10 (R = 499), where the uncertainties are 1σ and "optimistic." We also obtained R 67 = 2738 ± 118 (R = 2632), but we caution that the observed 12C17O features are extremely weak. The new solar ratios for the oxygen isotopes fall between the terrestrial values and those reported by Genesis (R 68 = 530, R 67 = 2798), although including both within 2σ error flags, and go in the direction favoring recent theories for the oxygen isotope composition of Ca-Al inclusions in primitive meteorites. While not a major focus of this work, we derive an oxygen abundance, epsilonO ~ 603 ± 9 ppm (relative to hydrogen; log epsilon ~ 8.78 on the H = 12 scale). The fact that the Sun is likely lighter than the Earth, isotopically speaking, removes the necessity of invoking exotic fractionation processes during the early construction of the inner solar system. Title: Isotopic CO in the Solar Photosphere, Viewed Through the Lens of 3D Spectrum Synthesis Authors: Ayres, T. R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.; Wedemeyer-Bohm, S. Bibcode: 2013LPI....44.3038A Altcode: 2013LPICo1719.3038A New analyses of CO isotopologue abundances in the solar photosphere are now consistent with Genesis solar wind results, although ^17O error bars are still large. Title: Three-dimensional magnetohydrodynamic simulations of M-dwarf chromospheres Authors: Wedemeyer, S.; Ludwig, H. -G.; Steiner, O. Bibcode: 2013AN....334..137W Altcode: 2013csss...17..137W; 2012arXiv1207.2342W We present first results from three-dimensional radiation magnetohydrodynamic simulations of M-type dwarf stars with CO5BOLD. The local models include the top of the convection zone, the photosphere, and the chromosphere. The results are illustrated for models with an effective temperature of 3240 K and a gravitational acceleration of {log g = 4.5}, which represent analogues of AD Leo. The models have different initial magnetic field strengths and field topologies. This first generation of models demonstrates that the atmospheres of M dwarfs are highly dynamic and intermittent. Magnetic fields and propagating shock waves produce a complicated fine-structure, which is clearly visible in synthetic intensity maps in the core of the Ca II K spectral line and also at millimeter wavelengths. The dynamic small-scale pattern cannot be described by means of one-dimensional models, which has important implications for the construction of semi-empirical model atmospheres and thus for the interpretation of observations in general. Detailed three-dimensional numerical simulations are valuable in this respect. Furthermore, such models facilitate the analysis of small-scale processes, which cannot be observed on stars but nevertheless might be essential for understanding M-dwarf atmospheres and their activity. An example are so-called ``magnetic tornadoes'', which have recently been found on the Sun and are presented here in M-dwarf models for the first time. Title: The CO5BOLD analysis tool. Authors: Wedemeyer, S. Bibcode: 2013MSAIS..24...96W Altcode: 2013arXiv1301.1497W The interactive IDL-based CO5BOLD Analysis Tool (CAT) was developed to facilitate an easy and quick analysis of numerical simulation data produced with the 2D/3D radiation magnetohydrodynamics code CO5BOLD. The basic mode of operation is the display and analysis of cross-sections through a model either as 2D slices or 1D graphs. A wide range of physical quantities can be selected. Further features include the export of models into VAPOR format or the output of images and animations. A short overview including scientific analysis examples is given. Title: Solar carbon monoxide: poster child for 3D effects . Authors: Ayres, T. R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.; Wedemeyer-Böhm, S. Bibcode: 2013MSAIS..24...85A Altcode: Photospheric infrared (2-6 mu m) rovibrational bands of carbon monoxide (CO) provide a tough test for 3D convection models such as those calculated using CO5BOLD. The molecular formation is highly temperature-sensitive, and thus responds in an exaggerated way to thermal fluctuations in the dynamic atmosphere. CO, itself, is an important tracer of the oxygen abundance, a still controversial issue in solar physics; as well as the heavy isotopes of carbon (13C) and oxygen (18O, 17O), which, relative to terrestrial values, are fingerprints of fractionation processes that operated in the primitive solar nebula. We show how 3D models impact the CO line formation, and add in a second constraint involving the near-UV Ca RIPTSIZE II line wings, which also are highly temperature sensitive, but in the opposite sense to the molecules. We find that our reference CO5BOLD snapshots appear to be slightly too cool on average in the outer layers of the photosphere where the CO absorptions and Ca RIPTSIZE II wing emissions arise. We show, further, that previous 1D modeling was systematically biased toward higher oxygen abundances and lower isotopic ratios (e.g., R23equiv 12C/13C), suggesting an isotopically ``heavy'' Sun contrary to direct capture measurements of solar wind light ions by the Genesis Discovery Mission. New 3D ratios for the oxygen isotopes are much closer to those reported by Genesis, and the associated oxygen abundance from CO now is consistent with the recent Caffau et al. study of atomic oxygen. Some lingering discrepancies perhaps can be explained by magnetic bright points. Solar CO demonstrates graphically the wide gulf that can occur between a 3D analysis and 1D. 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: 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: Simulations of stellar convection with CO5BOLD Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O. Bibcode: 2012JCoPh.231..919F Altcode: 2011arXiv1110.6844F High-resolution images of the solar surface show a granulation pattern of hot rising and cooler downward-sinking material - the top of the deep-reaching solar convection zone. Convection plays a role for the thermal structure of the solar interior and the dynamo acting there, for the stratification of the photosphere, where most of the visible light is emitted, as well as for the energy budget of the spectacular processes in the chromosphere and corona. Convective stellar atmospheres can be modeled by numerically solving the coupled equations of (magneto)hydrodynamics and non-local radiation transport in the presence of a gravity field. The CO5BOLD code described in this article is designed for so-called "realistic" simulations that take into account the detailed microphysics under the conditions in solar or stellar surface layers (equation-of-state and optical properties of the matter). These simulations indeed deserve the label "realistic" because they reproduce the various observables very well - with only minor differences between different implementations. The agreement with observations has improved over time and the simulations are now well-established and have been performed for a number of stars. Still, severe challenges are encountered when it comes to extending these simulations to include ideally the entire star or substellar object: the strong stratification leads to completely different conditions in the interior, the photosphere, and the corona. Simulations have to cover spatial scales from the sub-granular level to the stellar diameter and time scales from photospheric wave travel times to stellar rotation or dynamo cycle periods. Various non-equilibrium processes have to be taken into account. Last but not least, realistic simulations are based on detailed microphysics and depend on the quality of the input data, which can be the actual accuracy limiter. This article provides an overview of the physical problem and the numerical solution and the capabilities of CO5BOLD, illustrated with a number of applications. Title: Non-equilibrium calcium ionisation in the solar atmosphere Authors: Wedemeyer-Böhm, S.; Carlsson, M. Bibcode: 2011A&A...528A...1W Altcode: 2011arXiv1101.2211W Context. The chromosphere of the Sun is a temporally and spatially very varying medium for which the assumption of ionisation equilibrium is questionable.
Aims: Our aim is to determine the dominant processes and timescales for the ionisation equilibrium of calcium under solar chromospheric conditions.
Methods: The study is based on numerical simulations with the RADYN code, which combines hydrodynamics with a detailed solution of the radiative transfer equation. The calculations include a detailed non-equilibrium treatment of hydrogen, calcium, and helium. Next to an hour long simulation sequence, additional simulations are produced, for which the stratification is slightly perturbed so that a ionisation relaxation timescale can be determined. The simulations are characterised by upwards propagating shock waves, which cause strong temperature fluctuations and variations of the (non-equilibrium) ionisation degree of calcium.
Results: The passage of a hot shock front leads to a strong net ionisation of Ca II, rapidly followed by net recombination. The relaxation timescale of the calcium ionisation state is found to be of the order of a few seconds at the top of the photosphere and 10 to 30 s in the upper chromosphere. At heights around 1 Mm, we find typical values around 60 s and in extreme cases up to ~150 s. Generally, the timescales are significantly reduced in the wakes of ubiquitous hot shock fronts. The timescales can be reliably determined from a simple analysis of the eigenvalues of the transition rate matrix. The timescales are dominated by the radiative recombination from Ca III into the metastable Ca II energy levels of the 4d 2D term. These transitions depend strongly on the density of free electrons and therefore on the (non-equilibrium) ionisation degree of hydrogen, which is the main electron donor.
Conclusions: The ionisation/recombination timescales derived here are too long for the assumption of an instantaneous ionisation equilibrium to be valid and, on the other hand, are not long enough to warrant an assumption of a constant ionisation fraction. Fortunately, the ionisation degree of Ca ii remains small in the height range, where the cores of the H, K, and the infrared triplet lines are formed. We conclude that the difference due to a detailed treatment of Ca ionisation has only negligible impact on the modelling of spectral lines of Ca ii and the plasma properties under the conditions in the quiet solar chromosphere. Title: CO5BOLD: COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with l=2,3 Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven; Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner; Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga; Steiner, Oskar Bibcode: 2010ascl.soft11014F Altcode: CO5BOLD - nickname COBOLD - is the short form of "COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with l=2,3".

It is used to model solar and stellar surface convection. For solar-type stars only a small fraction of the stellar surface layers are included in the computational domain. In the case of red supergiants the computational box contains the entire star. Recently, the model range has been extended to sub-stellar objects (brown dwarfs).

CO5BOLD solves the coupled non-linear equations of compressible hydrodynamics in an external gravity field together with non-local frequency-dependent radiation transport. Operator splitting is applied to solve the equations of hydrodynamics (including gravity), the radiative energy transfer (with a long-characteristics or a short-characteristics ray scheme), and possibly additional 3D (turbulent) diffusion in individual sub steps. The 3D hydrodynamics step is further simplified with directional splitting (usually). The 1D sub steps are performed with a Roe solver, accounting for an external gravity field and an arbitrary equation of state from a table.

The radiation transport is computed with either one of three modules:

MSrad module: It uses long characteristics. The lateral boundaries have to be periodic. Top and bottom can be closed or open ("solar module").

LHDrad module: It uses long characteristics and is restricted to an equidistant grid and open boundaries at all surfaces (old "supergiant module").

SHORTrad module: It uses short characteristics and is restricted to an equidistant grid and open boundaries at all surfaces (new "supergiant module").

The code was supplemented with an (optional) MHD version [Schaffenberger et al. (2005)] that can treat magnetic fields. There are also modules for the formation and advection of dust available. The current version now contains the treatment of chemical reaction networks, mostly used for the formation of molecules [Wedemeyer-Böhm et al. (2005)], and hydrogen ionization [Leenaarts & Wedemeyer-Böhm (2005)], too.

CO5BOLD is written in Fortran90. The parallelization is done with OpenMP directives. Title: Are there variations in Earth's global mean temperature related to the solar activity? Authors: Kjeldseth-Moe, Olav; Wedemeyer-Böhm, Sven Bibcode: 2010IAUS..264..320K Altcode: We have analyzed the record of Earth's global temperature variations between 1850 and 2007 looking for signals of periodic variations and compared our results with solar activity variations in the same time period. Significant periods are found at 9.4, 10.6 and 20.9 years. These periodic variations may be caused by solar activity. However, and amazingly enough, we also find at least 17 other significant periodic variations in addition to expected variations with periods of 1 year and of half a year. The result is considered in terms of solar related forcing mechanisms. These may be variable solar heating associated with the small changes in solar irradiance over the solar cycle, or direct effects of interactions between variable magnetic fields carried by the solar wind and particles and fields in interplanetary space or in the Earth's ionosphere. Title: Small-scale structure and dynamics of the chromospheric magnetic field Authors: Wedemeyer-Böhm, S. Bibcode: 2010MmSAI..81..693W Altcode: 2009arXiv0911.5639W Recent advances in observational performance and numerical simulations have revolutionised our understanding of the solar chromosphere. This concerns in particular the structure and dynamics on small spatial and temporal scales. As a result, the picture of the solar chromosphere changed from an idealised static and plane-parallel stratification to a complex compound of intermittent domains, which are dynamically coupled to the layers below and above. In this picture, the chromosphere in a stricter sense is associated with the typical fibrilar structure shaped by magnetic fields like it is known from images taken in the Halpha line core. In internetwork regions below this layer, there exists a domain with propagating shock waves and weak magnetic fields, which both probably interact with the overlying large scale field. The existence of such a sub-canopy domain certainly depends on the properties of the overlying field. Details of the structure of the lower atmosphere can therefore be expected to vary significantly from location to location. Here, high-resolution observations, which were obtained with the CRISP filter at the Swedish Solar Telescope, are used to derive qualitative constraints for the atmospheric structure of quiet-Sun regions. Title: NLTE spectral synthesis based on 3D MHD convection simulations -understanding the role of the magnetic field in intensity variations Authors: Haberreiter, Margit; Wedemeyer-Boehm, Sven; Rast, Mark Bibcode: 2010cosp...38..132H Altcode: 2010cosp.meet..132H While the magnetic field is considered to be the main driver for Solar Spectral Irradiance (SSI) variations, the detailed physical mechanisms that explain this relation are not yet fully understood. In this paper we analyze the effect of small scale magnetic field on the intensity in Ca II 393.4 nm and various continuum wavelengths calculated with the NLTE radiative tranfer code SolMod3D. The code calculates NLTE level populations and line spectra based on 3D MHD simulations carried out with CO5BOLD. This enables us to study in great detail the effect of the varying small scale magnetic field on intensity variations. The results are important for a better understanding of the role of small-scale magnetic field in irradiance variations. Title: Toward the analysis of waves in the solar atmosphere based on NLTE spectral synthesis from 3D MHD simulations. Authors: Haberreiter, M.; Finsterle, W.; McIntosh, S.; Wedemeyer-Böhm, S. Bibcode: 2010MmSAI..81..782H Altcode: 2010arXiv1001.5086H From the analysis of Dopplergrams in the K I 7699 Å and Na I 5890 Å spectral lines observed with the Magneto-Optical filter at Two Heights (MOTH) experiment during the austral summer in 2002-03 we find upward traveling waves in magnetic regions. Our analysis shows that the dispersion relation of these waves strongly depends on whether the wave is detected in the low-beta or high-beta regime. Moreover, the observed dispersion relation does not show the expected decrease of the acoustic cut-off frequency for the field guided slow magnetic wave. Instead, we detected an increase of the travel times below the acoustic cut-off frequency and at the same time a decrease of the travel time above it. To study the formation height of the spectral lines employed by MOTH in greater detail we are currently in the process of employing 3D MHD simulations carried out with CO5BOLD to perform NLTE spectral synthesis. Title: Recovering the line-of-sight magnetic field in the chromosphere from Ca II IR spectra Authors: Wöger, F.; Wedemeyer-Böhm, S.; Uitenbroek, H.; Rimmele, T. Bibcode: 2010MmSAI..81..598W Altcode: 2009arXiv0912.3467W We propose a method to derive the line-of-sight magnetic flux density from measurements in the chromospheric Ca II IR line at 854.2 nm. The method combines two well-understood techniques, the center-of-gravity and bisector method, in a single hybrid technique. The technique is tested with magneto-static simulations of a flux tube. We apply the method to observations with the Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn Solar Telescope of the NSO/SP to investigate the morphology of the lower chromosphere, with focus on the chromospheric counterparts to the underlying photospheric magnetic flux elements. Title: Morphology and Dynamics of Photospheric and Chromospheric Magnetic Fields Authors: Wöger, F.; Wedemeyer-Böhm, S.; Rimmele, T. Bibcode: 2009ASPC..415..319W Altcode: 2009arXiv0912.3285W We use joint observations obtained with the Hinode space observatory and the Interferometric Bidimensional Spectrometer (IBIS) installed at the DST of the NSO/SP to investigate the morphology and dynamics of (a) non-magnetic and (b) magnetic regions in the fluctosphere. In inter-network regions with no significant magnetic flux contributions above the detection limit of IBIS, we find intensity structures with similar characteristics as those seen in numerical simulations by Wedemeyer-Böhm et al. (2008) The magnetic flux elements in the network are stable and seem to resemble the spatially extended counterparts to the underlying photospheric magnetic elements. We will explain some of the difficulties in deriving the magnetic field vector from observations of the fluctosphere. Title: The Horizontal Magnetic Field of the Quiet Sun: Numerical Simulations in Comparison to Observations with Hinode Authors: Steiner, O.; Rezaei, R.; Schlichenmaier, R.; Schaffenberger, W.; Wedemeyer-Böhm, S. Bibcode: 2009ASPC..415...67S Altcode: 2009arXiv0904.2030S Three-dimensional magnetohydrodynamic simulations of the surface layers of the Sun intrinsically produce a predominantly horizontal magnetic field in the photosphere. This is a robust result in the sense that it arises from simulations with largely different initial and boundary conditions for the magnetic field. While the disk-center synthetic circular and linear polarization signals agree with measurements from Hinode, their center-to-limb variation sensitively depends on the height variation of the horizontal and the vertical field component and they seem to be at variance with the observed behavior. Title: Morphology and Dynamics of the Low Solar Chromosphere Authors: Wöger, F.; Wedemeyer-Böhm, S.; Uitenbroek, H.; Rimmele, T. R. Bibcode: 2009ApJ...706..148W Altcode: 2009arXiv0910.1381W The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn Solar Telescope of the NSO/SP is used to investigate the morphology and dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere below. The study addresses in particular the structure of magnetic elements that extend into these layers. We choose different quiet-Sun regions inside and outside the coronal holes. In inter-network regions with no significant magnetic flux contributions above the detection limit of IBIS, we find intensity structures with the characteristics of a shock wave pattern. The magnetic flux elements in the network are long lived and seem to resemble the spatially extended counterparts to the underlying photospheric magnetic elements. We suggest a modification to common methods to derive the line-of-sight magnetic field strength and explain some of the difficulties in deriving the magnetic field vector from observations of the fluctosphere. 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: 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: Coupling from the Photosphere to the Chromosphere and the Corona Authors: Wedemeyer-Böhm, S.; Lagg, A.; Nordlund, Å. Bibcode: 2009SSRv..144..317W Altcode: 2008SSRv..tmp..171W; 2008arXiv0809.0987W The atmosphere of the Sun is characterized by a complex interplay of competing physical processes: convection, radiation, conduction, and magnetic fields. The most obvious imprint of the solar convection and its overshooting in the low atmosphere is the granulation pattern. Beside this dominating scale there is a more or less smooth distribution of spatial scales, both towards smaller and larger scales, making the Sun essentially a multi-scale object. Convection and overshooting give the photosphere its face but also act as drivers for the layers above, namely the chromosphere and corona. The magnetic field configuration effectively couples the atmospheric layers on a multitude of spatial scales, for instance in the form of loops that are anchored in the convection zone and continue through the atmosphere up into the chromosphere and corona. The magnetic field is also an important structuring agent for the small, granulation-size scales, although (hydrodynamic) shock waves also play an important role—especially in the internetwork atmosphere where mostly weak fields prevail. Based on recent results from observations and numerical simulations, we attempt to present a comprehensive picture of the atmosphere of the quiet Sun as a highly intermittent and dynamic system. 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: Coupling from the Photosphere to the Chromosphere and the Corona Authors: Wedemeyer-Böhm, S.; Lagg, A.; Nordlund, Å. Bibcode: 2009odsm.book..317W Altcode: The atmosphere of the Sun is characterized by a complex interplay of competing physical processes: convection, radiation, conduction, and magnetic fields. The most obvious imprint of the solar convection and its overshooting in the low atmosphere is the granulation pattern. Beside this dominating scale there is a more or less smooth distribution of spatial scales, both towards smaller and larger scales, making the Sun essentially a multi-scale object. Convection and overshooting give the photosphere its face but also act as drivers for the layers above, namely the chromosphere and corona. The magnetic field configuration effectively couples the atmospheric layers on a multitude of spatial scales, for instance in the form of loops that are anchored in the convection zone and continue through the atmosphere up into the chromosphere and corona. The magnetic field is also an important structuring agent for the small, granulation-size scales, although (hydrodynamic) shock waves also play an important role—especially in the internetwork atmosphere where mostly weak fields prevail. Based on recent results from observations and numerical simulations, we attempt to present a comprehensive picture of the atmosphere of the quiet Sun as a highly intermittent and dynamic system. Title: Observational Evidence for Shocks in the Solar Photosphere - New TESOS/VTT Results Authors: Rybak, J.; Kucera, A.; Hanslmeier, A.; Woehl, H.; Wedemeyer-Boehm, S.; Steiner, O. Bibcode: 2008ESPM...12.2.36R Altcode: High-resolution spectroscopic observations recently acquired with the TESOS spectrometer at the Vacuum Tower Telescope (VTT, Observatorio del Teide, Tenerife) are used to test predictions regarding strongly dynamic events in the photosphere as obtained from three-dimensional numerical simulations with the CO5BOLD-code.

Time series of two-dimensional maps of the Fe I 543.4 nm spectral line profile at different centre-to-limb positions are investigated in a statistical sense by comparing the distributions of individual spectral parameters derived from observations with the corresponding distributions from synthesized spectra calculated with the LINFOR3D code from the simulations. Appropriate degradation of the synthesized spectra was applied in order to take the limited spatial resolution of the telescope, seeing effects, and the scattered instrumental light into account.

At the actual spatial resolution of 0.5 arc sec, the statistics show that signatures of the photospheric dynamics, including the most dynamical events like occasional supersonic flows of plasma in the nearly horizontal direction, are very similar in both observations and simulations.

Discrepancies are found only for those spectral parameters (residual line intensity, Doppler line core shifts), which are affected by non-LTE effects, since non-LTE effects are not taken into account in the synthesis of the Fe I 543.4nm spectral line. Title: The Horizontal Internetwork Magnetic Field: Numerical Simulations in Comparison to Observations with Hinode Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm, S. Bibcode: 2008ESPM...12.3.22S Altcode: Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe I 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas it is negative in the convectively unstable layer below it. Title: Radiation hydrodynamics simulations of stellar surface convection Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Guenter; Wedemeyer-Boehm, Sven Bibcode: 2008asd..soft...36F Altcode: The code is used to perform radiation hydrodynamics simulations of the convective surface layers and the photosphere of cool stars. Title: Point spread functions for the Solar optical telescope onboard Hinode Authors: Wedemeyer-Böhm, S. Bibcode: 2008A&A...487..399W Altcode: 2008arXiv0804.4536W Aims: We investigate the combined point spread function (PSF) of the Broadband Filter Imager (BFI) and the Solar Optical Telescope (SOT) onboard the Hinode spacecraft.
Methods: Observations of the Mercury transit from November 2006 and the solar eclipse(s) from 2007 are used to determine the PSFs of SOT for the blue, green, and red continuum channels of the BFI. For each channel, we calculate large grids of theoretical point spread functions by convolution of the ideal diffraction-limited PSF and Voigt profiles. These PSFs are applied to artificial images of an eclipse and a Mercury transit. The comparison of the resulting artificial intensity profiles across the terminator and the corresponding observed profiles yields a quality measure for each case. The optimum PSF for each observed image is indicated by the best fit.
Results: The observed images of the Mercury transit and the eclipses exhibit a clear proportional relation between the residual intensity and the overall light level in the telescope. In addition, there is an anisotropic stray-light contribution. These two factors make it very difficult to pin down a single unique PSF that can account for all observational conditions. Nevertheless, the range of possible PSF models can be limited by using additional constraints like the pre-flight measurements of the Strehl ratio.
Conclusions: The BFI/SOT operate close to the diffraction limit and have only a rather small stray-light contribution. The FWHM of the PSF is broadened by only ~1% with respect to the diffraction-limited case, while the overall Strehl ratio is ~0.8. In view of the large variations - best seen in the residual intensities of eclipse images - and the dependence on the overall light level and position in the FOV, a range of PSFs should be considered instead of a single PSF per wavelength. The individual PSFs of that range allow then the determination of error margins for the quantity under investigation. Nevertheless, the stray-light contributions are found to be best matched with Voigt functions with the parameters σ = 0.008 arcsec and γ = 0.004 arcsec, 0.005 arcsec, and 0.006 arcsec for the blue, green, and red continuum channels, respectively. Title: The Horizontal Internetwork Magnetic Field: Numerical Simulations in Comparison to Observations with Hinode Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm, S. Bibcode: 2008ApJ...680L..85S Altcode: 2008arXiv0801.4915S Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe I 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas the Poynting flux is negative in the convectively unstable layer below it. Title: Small-scale structure and dynamics of the lower solar atmosphere Authors: Wedemeyer-Böhm, Sven; Wöger, Friedrich Bibcode: 2008IAUS..247...66W Altcode: 2007IAUS..247...66W; 2007arXiv0710.4776W The chromosphere of the quiet Sun is a highly intermittent and dynamic phenomenon. Three-dimensional radiation (magneto-)hydrodynamic simulations exhibit a mesh-like pattern of hot shock fronts and cool expanding post-shock regions in the sub-canopy part of the inter-network. This domain might be called “fluctosphere”. The pattern is produced by propagating shock waves, which are excited at the top of the convection zone and in the photospheric overshoot layer. New high-resolution observations reveal a ubiquitous small-scale pattern of bright structures and dark regions in-between. Although it qualitatively resembles the picture seen in models, more observations e.g. with the future ALMA are needed for thorough comparisons with present and future models. Quantitative comparisons demand for synthetic intensity maps and spectra for the three-dimensional (magneto-)hydrodynamic simulations. The necessary radiative transfer calculations, which have to take into account deviations from local thermodynamic equilibrium, are computationally very involved so that no reliable results have been produced so far. Until this task becomes feasible, we have to rely on careful qualitative comparisons of simulations and observations. Here we discuss what effects have to be considered for such a comparison. Nevertheless we are now on the verge of assembling a comprehensive picture of the solar chromosphere in inter-network regions as dynamic interplay of shock waves and structuring and guiding magnetic fields. Title: Hinode observations reveal boundary layers of magnetic elements in the solar photosphere Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier, R.; Schmidt, W.; Lites, B. W. Bibcode: 2007A&A...476L..33R Altcode: 2007arXiv0711.0408R Aims:We study the structure of the magnetic elements in network-cell interiors.
Methods: A quiet Sun area close to the disc centre was observed with the spectro-polarimeter of the Solar Optical Telescope on board the Hinode space mission, which yielded the best spatial resolution ever achieved in polarimetric data of the Fe I 630 nm line pair. For comparison and interpretation, we synthesize a similar data set from a three-dimensional magneto-hydrodynamic simulation.
Results: We find several examples of magnetic elements, either roundish (tube) or elongated (sheet), which show a central area of negative Stokes-V area asymmetry framed or surrounded by a peripheral area with larger positive asymmetry. This pattern was predicted some eight years ago on the basis of numerical simulations. Here, we observationally confirm its existence for the first time.
Conclusions: We gather convincing evidence that this pattern of Stokes-V area asymmetry is caused by the funnel-shaped boundary of magnetic elements that separates the flux concentration from the weak-field environment. On this basis, we conclude that electric current sheets induced by such magnetic boundary layers are common in the photosphere. Title: Variation of the Stokes-V area asymmetry across magnetic elements Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier, R.; Lites, B. W. Bibcode: 2007AN....328..706R Altcode: No abstract at ADS Title: Inter-network regions of the Sun at millimetre wavelengths Authors: Wedemeyer-Böhm, S.; Ludwig, H. G.; Steffen, M.; Leenaarts, J.; Freytag, B. Bibcode: 2007A&A...471..977W Altcode: 2007arXiv0705.2304W Aims:The continuum intensity at wavelengths around 1 mm provides an excellent way to probe the solar chromosphere and thus valuable input for the ongoing controversy on the thermal structure and the dynamics of this layer. The synthetic continuum intensity maps for near-millimetre wavelengths presented here demonstrate the potential of future observations of the small-scale structure and dynamics of internetwork regions on the Sun.
Methods: The synthetic intensity/brightness temperature maps are calculated on basis of three-dimensional radiation (magneto-)hydrodynamic (MHD) simulations. The assumption of local thermodynamic equilibrium (LTE) is valid for the source function. The electron densities are also treated in LTE for most maps but also in non-LTE for a representative model snapshot. Quantities like intensity contrast, intensity contribution functions, spatial and temporal scales are analysed in dependence on wavelength and heliocentric angle.
Results: While the millimetre continuum at 0.3 mm originates mainly from the upper photosphere, the longer wavelengths considered here map the low and middle chromosphere. The effective formation height increases generally with wavelength and also from disk-centre towards the solar limb. The average intensity contribution functions are usually rather broad and in some cases they are even double-peaked as there are contributions from hot shock waves and cool post-shock regions in the model chromosphere. The resulting shock-induced thermal structure translates to filamentary brightenings and fainter regions in between. Taking into account the deviations from ionisation equilibrium for hydrogen gives a less strong variation of the electron density and with it of the optical depth. The result is a narrower formation height range although the intensity maps still are characterised by a highly complex pattern. The average brightness temperature increases with wavelength and towards the limb although the wavelength-dependence is reversed for the MHD model and the NLTE brightness temperature maps. The relative contrast depends on wavelength in the same way as the average intensity but decreases towards the limb. The dependence of the brightness temperature distribution on wavelength and disk-position can be explained with the differences in formation height and the variation of temperature fluctuations with height in the model atmospheres. The related spatial and temporal scales of the chromospheric pattern should be accessible by future instruments.
Conclusions: Future high-resolution millimetre arrays, such as the Atacama Large Millimeter Array (ALMA), will be capable of directly mapping the thermal structure of the solar chromosphere. Simultaneous observations at different wavelengths could be exploited for a tomography of the chromosphere, mapping its three-dimensional structure, and also for tracking shock waves. The new generation of millimetre arrays will be thus of great value for understanding the dynamics and structure of the solar atmosphere. Title: Non-equilibrium Hydrogen Ionization in the Solar Atmosphere Authors: Leenaarts, J.; Wedemeyer-Böhm, S.; Carlsson, M.; Hansteen, V. H. Bibcode: 2007ASPC..368..103L Altcode: The assumption of statistical equilibrium for atomic level populations of hydrogen does not hold under the conditions of the chromosphere due to the low density and the short dynamic timescale. In order to calculate the hydrogen ionization balance and the electron density one has to solve the time-dependent rate equations. We present results from 2D and 3D radiation-magneto-hydrodynamics simulations of the solar atmosphere incorporating the time-dependent rate equations for hydrogen. Both the hydrogen ionization degree and the electron density in our models are much more constant than LTE and statistical equilibrium theory predict. These simulations provide multi-dimensional model atmospheres with realistic electron densities and hydrogen level populations that can be used in detailed radiative transfer modeling. Title: Dynamic models of the sun from the convection zone to the chromosphere Authors: Wedemeyer-Böhm, Sven Bibcode: 2007IAUS..239...52W Altcode: 2006astro.ph.10327W The chromosphere in internetwork regions of the quiet Sun was regarded as a static and homogeneous layer for a long time. Thanks to advances in observations and numerical modelling, the wave nature of these atmospheric regions received increasing attention during the last decade. Recent three-dimensional radiation magnetohydrodynamic simulations with CO5BOLD feature the chromosphere of internetwork regions as a dynamic and intermittent phenomenon. It is a direct product of interacting waves that form a mesh-like pattern of hot shock fronts and cool post-shock regions. The waves are excited self-consistently at the top of the convection zone. In the middle chromosphere above an average height of 1000 km, plasma beta gets larger than one and magnetic fields become more important. The model chromosphere exhibits a magnetic field that is much more homogeneous than in the layers below and evolves much faster. That includes fast propagating (MHD) waves. Further improvements of the simulations like time-dependent hydrogen ionisation are currently in progress. This class of models is capable of explaining apparently contradicting diagnostics such as carbon monoxide and UV emission at the same time. Title: What is Heating the Quiet-Sun Chromosphere? Authors: Wedemeyer-Böhm, S.; Steiner, O.; Bruls, J.; Rammacher, W. Bibcode: 2007ASPC..368...93W Altcode: 2006astro.ph.12627W It is widely believed that the heating of the chromosphere in quiet-Sun internetwork regions is provided by dissipation of acoustic waves that are excited by the convective motions close to the top of the convection zone and in the photospheric overshoot layer. This view lately became challenged by observations suggesting that the acoustic energy flux into the chromosphere is too low, by a factor of at least ten. Based on a comparison of TRACE data with synthetic image sequences for a three-dimensional simulation extending from the top layers of the convection zone to the middle chromosphere, we come to the contradicting conclusion that the acoustic flux in the model provides sufficient energy for heating the solar chromosphere of internetwork regions. The role of a weak magnetic field and associated electric current sheets is also discussed. Title: First local helioseismic experiments with CO5BOLD Authors: Steiner, O.; Vigeesh, G.; Krieger, L.; Wedemeyer-Böhm, S.; Schaffenberger, W.; Freytag, B. Bibcode: 2007AN....328..323S Altcode: 2007astro.ph..1029S With numerical experiments we explore the feasibility of using high frequency waves for probing the magnetic fields in the photosphere and the chromosphere of the Sun. We track a plane-parallel, monochromatic wave that propagates through a non-stationary, realistic atmosphere, from the convection-zone through the photosphere into the magnetically dominated chromosphere, where it gets refracted and reflected. We compare the wave travel time between two fixed geometrical height levels in the atmosphere (representing the formation height of two spectral lines) with the topography of the surface of equal magnetic and thermal energy density (the magnetic canopy or β=1 contour) and find good correspondence between the two. We conclude that high frequency waves indeed bear information on the topography of the `magnetic canopy'. Title: Carbon monoxide in the solar atmosphere. II. Radiative cooling by CO lines Authors: Wedemeyer-Böhm, S.; Steffen, M. Bibcode: 2007A&A...462L..31W Altcode: 2006astro.ph.12197W Aims: The role of carbon monoxide as a cooling agent for the thermal structure of the mid-photospheric to low-chromospheric layers of the solar atmosphere in internetwork regions is investigated.
Methods: The treatment of radiative cooling via spectral lines of carbon monoxide (CO) has been added to the radiation chemo-hydrodynamics code CO5BOLD. The radiation transport has now been solved in a continuum band with Rosseland mean opacity and an additional band with CO opacity. The latter is calculated as a Planck mean over the CO band between 4.4 and 6.2 μm. The time-dependent CO number density is derived from the solution of a chemical reaction network.
Results: The CO opacity indeed causes additional cooling at the fronts of propagating shock waves in the chromosphere. There, the time-dependent approach results in a higher CO number density compared to the equilibrium case and hence in a larger net radiative cooling rate. The average gas temperature stratification of the model atmosphere, however, is only reduced by roughly 100 K. Also the temperature fluctuations and the CO number density are only affected to small extent. A numerical experiment without dynamics shows that the CO cooling process works in principle and drives the atmosphere to a cool radiative equilibrium state. At chromospheric heights, the radiative relaxation of the atmosphere to a cool state takes several 1000 s. The CO cooling process thus would seem to be too slow compared to atmospheric dynamics to be responsible for the very cool temperature regions observed in the solar atmosphere.
Conclusions: . The hydrodynamical timescales in our solar atmosphere model are much too short to allow for the radiative relaxation to a cool state, thus suppressing the potential thermal instability due to carbon monoxide as a cooling agent. Apparently, the thermal structure and dynamics of the outer model atmosphere are instead determined primarily by shock waves. Title: On the fine structure of the quiet solar Ca II K atmosphere Authors: Tritschler, A.; Schmidt, W.; Uitenbroek, H.; Wedemeyer-Böhm, S. Bibcode: 2007A&A...462..303T Altcode: 2006astro.ph.11402T Aims:We investigate the morphological, dynamical, and evolutionary properties of the internetwork and network fine structure of the quiet sun at disk centre.
Methods: The analysis is based on a ~6 h time sequence of narrow-band filtergrams centred on the inner-wing Ca II K2v reversal at 393.3 nm. To examine the temporal evolution of network and internetwork areas separately we employ a double-Gaussian decomposition of the mean intensity distribution. An autocorrelation analysis is performed to determine the respective characteristic time scales. In order to analyse statistical properties of the fine structure we apply image segmentation techniques.
Results: The results for the internetwork are related to predictions derived from numerical simulations of the quiet sun. The average evolutionary time scale of the internetwork in our observations is 52 s. Internetwork grains show a tendency to appear on a mesh-like pattern with a mean cell size of ~4-5 arcsec. Based on this size and the spatial organisation of the mesh we speculate that this pattern is related to the existence of photospheric downdrafts as predicted by convection simulations. The image segmentation shows that typical sizes of both network and internetwork grains are in the order of 1.6 arcsec. Title: Dynamic Hydrogen Ionization in Simulations of the Solar Chromosphere Authors: Leenaarts, J.; Wedemeyer-Böhm, S. Bibcode: 2006ASPC..354..306L Altcode: Since the assumption of statistical equilibrium does not hold under the conditions of the dynamical solar chromosphere, the time dependence of the rate equations has to be taken into account when calculating ionization stages of elements. We present a method based on the work by Sollum (1999) to calculate the dynamic hydrogen ionization degree and electron density in the 3-D radiation-hydrodynamics code CO^5BOLD. In our model chromosphere, both quantities are more constant over time and horizontal position than LTE theory predicts. We compare synthetic brightness temperature images at λ=1~mm calculated with LTE and time-dependent NLTE electron densities. Both formation height and average brightness temperature change significantly compared to LTE when using time-dependent electron densities. Title: A First Three-Dimensional Model for the Carbon Monoxide Concentration in the Solar Atmosphere Authors: Wedemeyer-Böhm, S.; Kamp, I.; Freytag, B.; Bruls, J.; Steffen, M. Bibcode: 2006ASPC..354..301W Altcode: The time-dependent and self-consistent treatment of carbon monoxide (CO) has been added to the radiation chemo-hydrodynamics code CO5BOLD. It includes the solution of a chemical reaction network and the advection of the resulting particle densities with the hydrodynamic flow field. Here we present a first 3D simulation of the non-magnetic solar photosphere and low chromosphere, calculated with the upgraded code. In the resulting model, the highest amount of CO is located in the cool regions of the reversed granulation pattern in the middle photosphere. A large fraction of carbon is bound by CO throughout the chromosphere with exception of hot shock waves where the CO concentration is strongly reduced. The distribution of carbon monoxide is very inhomogeneous due to co-existing regions of hot and cool gas caused by the hydrodynamic flow. High-resolution observations of CO could thus provide important constraints for the thermal structure of the solar photosphere and chromosphere. Title: Time-dependent hydrogen ionisation in 3D simulations of the solar chromosphere. Methods and first results Authors: Leenaarts, J.; Wedemeyer-Böhm, S. Bibcode: 2006A&A...460..301L Altcode: 2006astro.ph..8620L Context: .The hydrogen ionisation degree deviates substantially from statistical equilibrium under the conditions of the solar chromosphere. A realistic description of this atmospheric layer thus must account for time-dependent non-equilibrium effects.
Aims: .Advancing the realism of numerical simulations of the solar chromosphere by improved numerical treatment of the relevant physics will provide more realistic models that are essential for interpretation of existing and future observations.
Methods: .An approximate method for solving the rate equations for the hydrogen populations was extended and implemented in the three-dimensional radiation (magneto-)hydrodynamics code CO^5BOLD. The method is based on a model atom with six energy levels and fixed radiative rates. It has been tested extensively in one-dimensional simulations. The extended method has been used to create a three-dimensional model that extends from the upper convection zone to the chromosphere.
Results: .The ionisation degree of hydrogen in our time-dependent simulation is comparable to the corresponding equilibrium value up to 500 km above optical depth unity. Above this height, the non-equilibrium ionisation degree is fairly constant over time and space, and tends to be at a value set by hot propagating shock waves. The hydrogen level populations and electron density are much more constant than the corresponding values for statistical equilibrium, too. In contrast, the equilibrium ionisation degree varies by more than 20 orders of magnitude between hot, shocked regions and cool, non-shocked regions.
Conclusions: .The simulation shows for the first time in 3D that the chromospheric hydrogen ionisation degree and electron density cannot be calculated in equilibrium. Our simulation can provide realistic values of those quantities for detailed radiative transfer computations. Title: A New Method for Comparing Numerical Simulations with Spectroscopic Observations of the Solar Photosphere Authors: Rybák, J.; Kučera, A.; Wöhl, H.; Wedemeyer-Böhm, S.; Steiner, O. Bibcode: 2006ASPC..354...77R Altcode: A method for comparing high-resolution spectroscopic observations of the solar photosphere with numerical simulations of convection in the solar photosphere is presented.

It is based on the comparison of the granular continuum contrast obtained from both the observations and the synthetic spectra, when the latter are calculated from numerical simulations using a particular type of data degradation. This method can be used post facto when a minimum of auxiliary information on characteristics of the telescope/spectrograph and on seeing conditions is available.

Here, the method is applied to results of numerical simulations computed with the CO5BOLD code and high-resolution spectroscopic observations obtained with the VTT on Tenerife. Title: Holistic MHD-Simulation from the Convection Zone to the Chromosphere Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.; Freytag, B. Bibcode: 2006ASPC..354..345S Altcode: A three-dimensional magnetohydrodynamic simulation of the integral layers from the convection zone to the chromosphere has been carried out. The simulation represents magnetoconvection in a quiet network-cell interior. The following preliminary new results are obtained: The chromospheric magnetic field is very dynamic with a continuous rearrangement of magnetic flux on a time scale of less than one~minute. Rapidly moving magnetic filaments (rarely exceeding 40~G) form in the compression zone downstream and along propagating shock fronts that are present throughout the chromosphere. The magnetic filaments rapidly move, form, and dissolve with the shock waves. Flux concentrations strongly expand through the photosphere into a more homogeneous, space filling chromospheric field. ``Canopy fields'' form on a granular scale above largely field-free granule centers leading to a mesh-work of current sheets in a height range between approximately 400 and 900~km. Title: High Resolution Time Series of Narrowband Ca IIK Images in the Chromosphere Authors: Wöger, F.; Wedemeyer-Böhm, S.; Schmidt, W.; von der Lühe, O. Bibcode: 2006ASPC..354..284W Altcode: We have observed a region of quiet Sun near disk center with the Vacuum Tower Telescope (VTT) of the Kiepenheuer-Institut für Sonnenphysik at the Observatorio del Teide, Tenerife, Spain in April 2005 in several wavelengths. Observations were made at the Ca II K line at 393.3 nm, using a Lyot filter with a bandwidth of 30 ± FWHM, centered at the K_{2v} emission peak; at the Hα line at 656.3 nm, using a Lyot filter (25 ± FWHM) centered at line core, and in the G-band (430.5 nm), using an interference filter (1 nm FWHM). We acquired a two-hour long sequence of images at a cadence of ten seconds and a spatial resolution of about 0.3 arcsec. We present our Ca observations of excellent spatial resolution which show morphological structures in internetwork regions similar in form, size and lifetime to those present in recent numerical models of the solar chromosphere. Title: Observation of a short-lived pattern in the solar chromosphere Authors: Wöger, F.; Wedemeyer-Böhm, S.; Schmidt, W.; von der Lühe, O. Bibcode: 2006A&A...459L...9W Altcode: 2006astro.ph..9382W Aims.In this work we investigate the dynamic behavior of inter-network regions of the solar chromosphere.
Methods: .We observed the chromosphere of the quiet Sun using a narrow-band Lyot filter centered at the Ca II K2v emission peak with a bandpass of 0.3 Å. We achieved a spatial resolution of on average 0.7 arcsec at a cadence of 10 s.
Results: .In the inter-network we find a mesh-like pattern that features bright grains at the vertices. The pattern has a typical spatial scale of 1.95 arcsec and a mean evolution time scale of 53 s with a standard deviation of 10 s. A comparison of our results with a recent three-dimensional radiation hydrodynamical model implies that the observed pattern is of chromospheric origin. The measured time scales are not compatible with those of reversed granulation in the photosphere although the appearance is similar. A direct comparison between network and inter-network structure shows that their typical time scales differ by at least a factor of two.
Conclusions: .The existence of a rapidly evolving small-scale pattern in the inter-network regions supports the picture of the lower chromosphere as a highly dynamical and intermittent phenomenon. Title: Simulations of Magnetohydrodynamics and CO Formation from the Convection Zone to the Chromosphere Authors: Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O.; Steffen, M.; Freytag, B.; Kamp, I. Bibcode: 2005ESASP.596E..16W Altcode: 2005ccmf.confE..16W No abstract at ADS Title: Magnetohydrodynamic Simulation from the Convection Zone to the Chromosphere Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.; Freytag, B. Bibcode: 2005ESASP.596E..65S Altcode: 2005ccmf.confE..65S No abstract at ADS Title: Carbon monoxide in the solar atmosphere. I. Numerical method and two-dimensional models Authors: Wedemeyer-Böhm, S.; Kamp, I.; Bruls, J.; Freytag, B. Bibcode: 2005A&A...438.1043W Altcode: 2005astro.ph..3496W The radiation hydrodynamic code CO5BOLD has been supplemented with the time-dependent treatment of chemical reaction networks. Advection of particle densities due to the hydrodynamic flow field is also included. The radiative transfer is treated frequency-independently, i.e. grey, so far. The upgraded code has been applied to two-dimensional simulations of carbon monoxide (CO) in the non-magnetic solar photosphere and low chromosphere. For this purpose a reaction network has been constructed, taking into account the reactions that are most important for the formation and dissociation of CO under the physical conditions of the solar atmosphere. The network has been strongly reduced to 27 reactions, involving the chemical species H, H2, C, O, CO, CH, OH and a representative metal. The resulting CO number density is highest in the cool regions of the reversed granulation pattern at mid-photospheric heights and decreases strongly above. There, the CO abundance stays close to a value of 8.3 on the usual logarithmic abundance scale with [H] = 12 but is reduced in hot shock waves which are a ubiquitous phenomenon of the model atmosphere. For comparison, the corresponding equilibrium densities have been calculated, based on the reaction network but also under the assumption of instantaneous chemical equilibrium by applying the Rybicki & Hummer (RH) code. Owing to the short chemical timescales, the assumption holds for a large fraction of the atmosphere, in particular the photosphere. In contrast, the CO number density deviates strongly from the corresponding equilibrium value in the vicinity of chromospheric shock waves. Simulations with altered reaction networks clearly show that the formation channel via hydroxide (OH) is the most important one under the conditions of the solar atmosphere. Title: The shock-patterned solar chromosphere in the light of ALMA Authors: Wedemeyer-Böhm, S.; Ludwig, H. -G.; Steffen, M.; Freytag, B.; Holweger, H. Bibcode: 2005ESASP.560.1035W Altcode: 2005csss...13.1035W; 2005astro.ph..9747W Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer et al. (2004) suggest that the solar chromosphere is highly structured in space and time on scales of only 1000 km and 20-25 sec, resp.. The resulting pattern consists of a network of hot gas and enclosed cool regions which are due to the propagation and interaction of shock fronts. In contrast to many other diagnostics, the radio continuum at millimeter wavelengths is formed in LTE, and provides a rather direct measure of the thermal structure. It thus facilitates the comparison between numerical model and observation. While the involved time and length scales are not accessible with todays equipment for that wavelength range, the next generation of instruments, such as the Atacama Large Millimeter Array (ALMA), will provide a big step towards the required resolution. Here we present results of radiative transfer calculations at mm and sub-mm wavelengths with emphasis on spatial and temporal resolution which are crucial for the ongoing discussion about the chromospheric temperature structure. Title: DOT tomography of the solar atmosphere. III. Observations and simulations of reversed granulation Authors: Leenaarts, J.; Wedemeyer-Böhm, S. Bibcode: 2005A&A...431..687L Altcode: We compare high-quality image sequences from the Dutch Open Telescope (DOT) with synthetic image sequences obtained from 3D radiation-hydrodynamics simulations of the solar granulation. In particular, we study the subsonic brightness pattern observed in the wings of Ca II HK. The simulations reproduce the observed intensity contrast, time scales, and Fourier behaviour rather well. Most differences can be attributed to the resolution difference between the observations and the simulation and the small geometrical extent of the simulation. We conclude that magnetic fields play no major role in the formation of reversed granulation. Title: Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.; Holweger, H. Bibcode: 2004A&A...414.1121W Altcode: 2003astro.ph.11273W Three-dimensional numerical simulations with CO5, a new radiation hydrodynamics code, result in a dynamic, thermally bifurcated model of the non-magnetic chromosphere of the quiet Sun. The 3D model includes the middle and low chromosphere, the photosphere, and the top of the convection zone, where acoustic waves are excited by convective motions. While the waves propagate upwards, they steepen into shocks, dissipate, and deposit their mechanienergy as heat in the chromosphere. Our numerical simulations show for the first time a complex 3D structure of the chromospheric layers, formed by the interaction of shock waves. Horizontal temperature cross-sections of the model chromosphere exhibit a network of hot filaments and enclosed cool regions. The horizontal pattern evolves on short time-scales of the order of typically 20-25 s, and has spatial scales comparable to those of the underlying granulation. The resulting thermal bifurcation, i.e., the co-existence of cold and hot regions, provides temperatures high enough to produce the observed chromospheric UV emission and - at the same time - temperatures cold enough to allow the formation of molecules (e.g., carbon monoxide). Our 3D model corroborates the finding by \citet{carlsson94} that the chromospheric temperature rise of semi-empirical models does not necessarily imply an increase in the average gas temperature but can be explained by the presence of substantial spatial and temporal temperature inhomogeneities. Title: Modelling the Chromospheric Background Pattern of the Non-magnetic Sun Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Günter; Holweger, Hartmut Bibcode: 2003ANS...324R..66W Altcode: 2003ANS...324..I07W No abstract at ADS Title: Multi-dimensional radiation hydrodynamic simulations of the non-magnetic solar atmosphere Authors: Wedemeyer, Sven Bibcode: 2003PhDT.......190W Altcode: No abstract at ADS Title: Acoustic Waves in the Solar Chromosphere - Numerical Simulations with COBOLD Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.; Holweger, H. Bibcode: 2003IAUS..210P..C1W Altcode: No abstract at ADS Title: 3-D hydrodynamic simulations of the solar chromosphere Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.; Holweger, H. Bibcode: 2003AN....324..410W Altcode: We present first results of three-dimensional numerical simulations of the non-magnetic solar chromosphere, computed with the radiation hydrodynamics code CO5BOLD. Acoustic waves which are excited at the top of the convection zone propagate upwards into the chromosphere where the waves steepen into shocks. The interaction of the waves leads to the formation of complex structures which evolve on short time scales. Consequently, the model chromosphere is highly dynamical, inhomogeneous, and thermally bifurcated. Title: Statistical equilibrium and photospheric abundance of silicon in the Sun and in Vega Authors: Wedemeyer, S. Bibcode: 2001A&A...373..998W Altcode: Based on detailed non-LTE calculations, an updated determination of the abundance of silicon in the Sun and Vega is presented. The model atom includes neutral and singly ionized stages of silicon with 115 energy levels and 84 line transitions. Non-LTE effects are found to be quite small in the Sun. The mean non-LTE abundance correction is -0.010 dex with respect to standard LTE calculations, leading to a solar abundance of log ɛNLTE = 7.550 ± 0.056. For the prototype A0 V star Vega the non-LTE effects are small, too. With a non-LTE abundance correction of Δ log ɛ = -0.054, a silicon abundance of -0.599 dex with respect to the Sun. This confirms the classification of Vega as a mild λ Boo star Title: Acoustic Energy Generated by Convection: 3-D Numerical Simulations for the Sun Authors: Wedemeyer, Sven; Freytag, Bernd; Holweger, Hartmut; Ludwig, Hans-Günter; Steffen, Matthias Bibcode: 2001AGM....18..P01W Altcode: Dissipation of acoustic waves may be an efficient heating mechanism for the lower and middle chromosphere of the quiet Sun. The basic idea is that turbulent motions at the top of the solar convection zone generate acoustic waves which propagate upwards and dissipate in the lower and middle chromosphere, transporting energy into the higher layers. But still the question remains if this amount of energy is sufficient to explain the observed temperature increase without invoking magnetic fields. With a new version of the COBOLD radiation hydrodynamics code we are able to compute 3-D models extending all the way from the upper convection zone to the middle chromosphere. First 3-D simulations reveal a complex, inhomogenous and highly dynamical structure of the lower and middle chromosphere which evolves on rather short timescales. On small spatial dimensions very cool regions are present next to a "network" of hotter matter. The code is being developed further to provide a more detailed analysis and comparison with observations. Title: Radiation Hydrodynamics Simulations of the Solar Chromosphere Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Holweger, Hartmut Bibcode: 2000AGM....17..P01W Altcode: While heating of the solar corona is commonly attributed to reconnection of magnetic field lines, the mechanism responsible for heating the chromosphere of the quiet Sun, away from active regions, is still under debate1,2. The basic question which we will address in this contribution i s: Can generation of acoustic waves by turbulent convection in photospheric and subphotospheric layers explain the chromospheric emission of the quiet Sun? With a new 3D radiation hydrodynamics code3 we are able to compute models ex tending from the upper convection zone to the middle chromosphere. The code can handle shocks with a minimum of numerical dissipation. Therefore generation and propagation of acoustic waves can be investigated, permitting the evaluation of wave dissipation in the chromosphere in a physically consistent manner. We present first results and discuss the principal problems and future prospects. Title: Silicon as a cosmic reference element: a reassessment of the solar SI abundance Authors: Wedemeyer, Sven; Holweger, Hartmut; Steffen, Matthias Bibcode: 1999AGAb...15..113V Altcode: 1999AGM....15..P53V; 1999AGM....15..P54W Silicon is an important reference elements for comparing various types of cosmic matter with the Sun. The most widely used sources of solar (photospheric) abundances, the compilation by Anders & Grevesse (1989) and its updates (e.g. Grevesse & Sauval 1998), are based on standard abundance analyses employing 1D solar models and, in most cases, assuming LTE. We report NLTE calculations for Si and a first attempt to determine the effect of horizontal temperature inhomogeneities associated with convection on the photospheric abundance of Si. We combine the result with that obtained previously for O and Fe (Aellig et al. 1999; Schnabel et al. 1999) and compare the photospheric Si/Fe, Si/O and Si/H abundance ratios with literature data for meteorites, the corona and solar wind, energetic particles and galactic B stars and H ii regions. References: Aellig M.R., Holweger H., Bochsler P., et al., 1999, Solar Wind Nine, AIP Conf. Proc. Vol. 471, 255 Anders E., Grevesse N., 1989, Geochim. Comochim. Acta 53, 197 Grevesse N., Sauval A.J., 1998, Space Sci. Rev. 85, 161 Schnabel R., Kock M., Holweger H., 1999, A&A 342, 610