Author name code: carlsson ADS astronomy entries on 2022-09-14 author:"Carlsson, Mats" ------------------------------------------------------------------------ Title: Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h and k Lines Authors: Rachmeler, L. A.; Bueno, J. Trujillo; McKenzie, D. E.; Ishikawa, R.; Auchère, F.; Kobayashi, K.; Kano, R.; Okamoto, T. J.; Bethge, C. W.; Song, D.; Ballester, E. Alsina; Belluzzi, L.; Pino Alemán, T. del; Ramos, A. Asensio; Yoshida, M.; Shimizu, T.; Winebarger, A.; Kobelski, A. R.; Vigil, G. D.; Pontieu, B. De; Narukage, N.; Kubo, M.; Sakao, T.; Hara, H.; Suematsu, Y.; Štěpán, J.; Carlsson, M.; Leenaarts, J. Bibcode: 2022ApJ...936...67R Altcode: 2022arXiv220701788R The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h and k spectral region around 2800 Å along a 200″ slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near-ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg II h and k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semiempirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry. Title: Formation and heating of chromospheric fibrils in a radiation-MHD simulation Authors: Druett, M. K.; Leenaarts, J.; Carlsson, M.; Szydlarski, M. Bibcode: 2022A&A...665A...6D Altcode: 2021arXiv211208245D
Aims: We examine the movements of mass elements within dense fibrils using passive tracer particles (corks) in order to understand the creation and destruction processes of fibrils.
Methods: Simulated fibrils were selected at times when they were visible in a Hα image proxy. The corks were selected within fibril Hα formation regions. From this set, we selected a cork and constructed the field line passing through it. Other fibrilar corks close to this field line were also selected and pathlines were constructed, revealing the locations of the mass elements forwards and backwards in time. Finally, we analysed the forces acting on these mass elements.
Results: The main process of fibrilar loading in the simulation is different to the mass loading scenario in which waves steepen into shocks and push material upwards along the field lines from locations near their footpoints. The twisted, low-lying field lines were destabilised and then they untwisted, lifting the material trapped above their apexes via the Lorentz force. Subsequently, the majority of the mass drained down the field lines towards one or both footpoints under the influence of gravity. Material with large horizontal velocities could also be elevated in rising field lines, creating somewhat parabolic motions, but the material was not generally moving upward along a stationary magnetic field line during loading.
Conclusions: The processes observed in the simulation are additional scenarios that are plausible. The criteria for observing such events are described in this work. We note that it is desirable for our simulations to also be able to form more densely packed fibrils from material fed from the base of field footpoints. The experimental parameters required to achieve this are also discussed in this paper.

Movies associated to Figs. 1, 4, 9, 14 are available at https://www.aanda.org Title: Coronal oscillations in the self-consistent 3D MHD simulations of the solar atmosphere Authors: Kohutova, Petra; Antolin, Patrick; Carlsson, Mats; Popovas, Andrius Bibcode: 2022cosp...44.2494K Altcode: Solar coronal loops are commonly subject to oscillations. Coronal oscillations are typically studied using highly idealised models of magnetic flux-tubes. In order to improve our understanding of coronal oscillations, it is necessary to consider the effect of realistic magnetic field topology and evolution. To do this, we study excitation, evolution and damping of coronal oscillations in three-dimensional self-consistent simulations of solar atmosphere spanning from convection zone to solar corona using the radiation-MHD code Bifrost. We use forward-modelled EUV emission and three-dimensional tracing of magnetic field to analyse oscillatory behaviour of individual magnetic loops. We show that coronal loop oscillations are abundant in such models and the oscillation modes and characteristics match those detected in solar observations. Finally, we discuss the dynamics and variability of the oscillating loops and the implications for coronal seismology. Title: Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel, Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan Bibcode: 2022cosp...44.2503F Altcode: There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. Title: Stirring the Base of the Solar Wind: On Heat Transfer and Vortex Formation Authors: Finley, Adam J.; Brun, Sacha A.; Carlsson, Mats; Szydlarski, Mikolaj; Hansteen, Viggo; Shoda, Munehito Bibcode: 2022arXiv220702878F Altcode: Current models of the solar wind must approximate (or ignore) the small-scale dynamics within the solar atmosphere, however these are likely important in shaping the emerging wave-turbulence spectrum and ultimately heating/accelerating the coronal plasma. The Bifrost code produces realistic simulations of the solar atmosphere that facilitate the analysis of spatial and temporal scales which are currently at, or beyond, the limit of modern solar telescopes. For this study, the Bifrost simulation is configured to represent the solar atmosphere in a coronal hole region, from which the fast solar wind emerges. The simulation extends from the upper-convection zone (2.5 Mm below the photosphere) to the low-corona (14.5 Mm above the photosphere), with a horizontal extent of 24 Mm x 24 Mm. The twisting of the coronal magnetic field by photospheric flows, efficiently injects energy into the low-corona. Poynting fluxes of up to $2-4$ kWm$^{-2}$ are commonly observed inside twisted magnetic structures with diameters in the low-corona of 1 - 5 Mm. Torsional Alfvén waves are favourably transmitted along these structures, and will subsequently escape into the solar wind. However, reflections of these waves from the upper boundary condition make it difficult to unambiguously quantify the emerging Alfvén wave-energy flux. This study represents a first step in quantifying the conditions at the base of the solar wind using Bifrost simulations. It is shown that the coronal magnetic field is readily braided and twisted by photospheric flows. Temperature and density contrasts form between regions with active stirring motions and those without. Stronger whirlpool-like flows in the convection, concurrent with magnetic concentrations, launch torsional Alfvén waves up through the magnetic funnel network, which are expected to enhance the turbulent generation of magnetic switchbacks in the solar wind. Title: Abundance diagnostics in active regions with Solar Orbiter/SPICE Authors: Giunta, Alessandra; Peter, Hardi; Parenti, Susanna; Buchlin, Eric; Thompson, William; Auchere, Frederic; Kucera, Therese; Carlsson, Mats; Janvier, Miho; Fludra, Andrzej; Hassler, Donald M.; Grundy, Timothy; Sidher, Sunil; Guest, Steve; Leeks, Sarah; Fredvik, Terje; Young, Peter Bibcode: 2022cosp...44.2583G Altcode: With the launch of Solar Orbiter in February 2020, we are now able to fully explore the link between the solar activity on the Sun and the inner heliosphere. Elemental abundance measurements provide a key tracer to probe the source regions of the solar wind and to track it from the solar surface and corona to the heliosphere. Abundances of elements with low first ionisation potential (FIP) are enhanced in the corona relative to high-FIP elements, with respect to the photosphere. This is known as the FIP effect, which is measured as abundance bias (FIP bias) of low and high FIP elements. This effect is vital for understanding the flow of mass and energy through the solar atmosphere. The comparison between in-situ and remote sensing composition data, coupled with modelling, will allow us to trace back the source of heliospheric plasma. Solar Orbiter has a unique combination of in-situ and remote sensing instruments that will help to make such a comparison. In particular, the SPICE (Spectral Imaging of the Coronal Environment) EUV spectrometer records spectra in two wavelength bands, 70.4-79.0 nm and 97.3-104.9 nm. SPICE is designed to provide spectroheliograms using a core set of emission lines arising from ions of both low-FIP and high-FIP elements such as C, N, O, Ne, Mg, S and Fe. These lines are formed over a wide range of temperatures from 20,000 K to over 1 million K, enabling the analysis of the different layers of the solar atmosphere. SPICE spectroheliograms can be processed to produce FIP bias maps, which can be compared to in-situ measurements of the solar wind composition of the same elements. During the Solar Orbiter Cruise Phase, SPICE observed several active regions. We will present some of these observations and discuss the SPICE diagnostic potential to derive relative abundances (e.g., Mg/Ne) and the FIP bias in those regions. Title: The SPICE spectrograph on Solar Orbiter: an introduction and results from the first Orbits Authors: Auchère, Frédéric; Peter, Hardi; Parenti, Susanna; Buchlin, Eric; Thompson, William; Auchere, Frederic; Teriaca, Luca; Kucera, Therese; Carlsson, Mats; Janvier, Miho; Fludra, Andrzej; Giunta, Alessandra; Schuehle, Udo; Hassler, Donald M.; Grundy, Timothy; Sidher, Sunil; Fredvik, Terje; Plowman, Joseph; Aznar Cuadrado, Regina Bibcode: 2022cosp...44.1338A Altcode: The Spectral Imaging of the Coronal Environment (SPICE) instrument is the EUV imaging spectrometer on board the Solar Orbiter mission. With its ability to derive physical properties of the coronal plasma, SPICE is a key component of the payload to establish the connection between the source regions and the in-situ measurements of the solar wind. The spacecraft was successfully launched in February 2020 and completed its cruise phase in December 2021. During this period, the remote sensing instruments were mostly operated during limited periods of time for 'checkout' engineering activities and synoptic observations. Nonetheless, several of these periods provided enough opportunities already to obtain new insights on coronal physics. During the march 2022 perihelion - close to 0.3 AU - SPICE will provide its highest spatial resolution data so far. Coordinated observations between the remote sensing and in-situ instruments will provide the first opportunity to use the full potential of the Solar Orbiter mission. We will review the instrument characteristics and present initial results from the cruise phase and first close encounter. Title: Effects of spatial resolution on inferences of atmospheric quantities from simulations Authors: Moe, Thore E.; Pereira, Tiago M. D.; Carlsson, Mats Bibcode: 2022A&A...662A..80M Altcode: 2022arXiv220408849M Context. Small-scale processes are thought to be important for the dynamics of the solar atmosphere. While numerical resolution fundamentally limits their inclusion in magnetohydronamic (MHD) simulations, real observations at the same nominal resolution should still contain imprints of subresolution effects. This means that the synthetic observables from a simulation of a given resolution might not be directly comparable to real observables at the same resolution. It is thus of interest to investigate how inferences based on synthetic spectra from simulations with different numerical resolutions compare, and whether these differences persist after the spectra have been spatially degraded to a common resolution
Aims: We aim to compare synthetic spectra obtained from realistic 3D radiative magnetohydrodynamic (rMHD) simulations run at different numerical resolutions from the same initial atmosphere, using very simple methods for inferring line-of-sight velocities and magnetic fields. Additionally we examine how the differing spatial resolution impacts the results retrieved from the STiC inversion code.
Methods: We used the RH 1.5D code to synthesize the photospheric Fe I 617.33 line in local thermodynamic equilibrium (LTE), and the chromospheric Ca II 854.209 line in non-LTE from three MHD simulation snapshots of differing spatial resolution. The simulations were produced by the Bifrost code, using horizontal grid spacing of 6 km, 12 km, and 23 km, respectively. They were started from the exact same atmosphere, and the snapshots were taken after the same exact elapsed time. The spectra obtained from the high-resolution snapshots were spatially degraded to match the lowest resolution. Simple methods, such as the center-of-gravity approach and the weak field approximation, were then used to estimate line-of-sight velocities and magnetic fields for the three cases after degradation. Finally, the spectra were input into the STiC inversion code and the retrieved line-of-sight velocities and magnetic field strengths, as well as the temperatures, from the inversions were compared.
Results: We find that while the simple inferences for all three simulations reveal the same large-scale tendencies, the higher resolutions yield more fine-grained structures and more extreme line-of-sight velocities and magnetic fields in concentrated spots even after spatial smearing. We also see indications that the imprints of subresolution effects on the degraded spectra result in systematic errors in the inversions, and that these errors increase with the amount of subresolution effects included. Fortunately, however, we find that successively including more subresolution yields smaller additional effects; that is to say, there is a clear trend of diminishing importance for progressively finer subresolution effects. Title: An approximate recipe of chromospheric radiative losses for solar flares Authors: Hong, J.; Carlsson, M.; Ding, M. D. Bibcode: 2022A&A...661A..77H Altcode: 2022arXiv220307630H Context. Radiative losses in the chromosphere are very important for the energy balance of the Sun. There have been efforts to make simple lookup tables for chromospheric radiative losses in the quiet Sun. During solar flares, the atmospheric conditions are quite different, and the currently available recipe is constructed from semi-empirical models. How these recipes work in flare conditions remains to be evaluated.
Aims: We aim to construct an approximate recipe of chromospheric radiative losses for solar flares.
Methods: We tabulate the optically thin radiative loss, escape probability, and ionization fraction using a grid of flare models from radiative hydrodynamic simulations as our dataset.
Results: We provide new lookup tables to calculate chromospheric radiative losses for flares. Compared with previous recipes, our recipe provides a better approximation of the detailed radiative losses for flares. Title: The Atmospheric Response to High Nonthermal Electron-beam Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope Authors: Kowalski, Adam F.; Allred, Joel C.; Carlsson, Mats; Kerr, Graham S.; Tremblay, Pier-Emmanuel; Namekata, Kosuke; Kuridze, David; Uitenbroek, Han Bibcode: 2022ApJ...928..190K Altcode: 2022arXiv220113349K Redshifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of the Interface Region Imaging Spectrograph. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam-generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of the hydrogen Balmer series should be readily detected in observations. To accurately interpret the upcoming spectral data of flares with the DKIST, we incorporate nonideal, nonadiabatic line-broadening profiles of hydrogen into the RADYN code. These improvements allow time-dependent predictions for the extreme Balmer line wing enhancements in solar flares. We study two chromospheric condensation models, which cover a range of electron-beam fluxes (1 - 5 × 1011 erg s-1 cm-2) and ambient electron densities (1 - 60 × 1013 cm-3) in the flare chromosphere. Both models produce broadening and redshift variations within 10 s of the onset of beam heating. In the chromospheric condensations, there is enhanced spectral broadening due to large optical depths at Hα, Hβ, and Hγ, while the much lower optical depth of the Balmer series H12-H16 provides a translucent window into the smaller electron densities in the beam-heated layers below the condensation. The wavelength ranges of typical DKIST/ViSP spectra of solar flares will be sufficient to test the predictions of extreme hydrogen wing broadening and accurately constrain large densities in chromospheric condensations. Title: Chromospheric emission from nanoflare heating in RADYN simulations Authors: Bakke, H.; Carlsson, M.; Rouppe van der Voort, L.; Gudiksen, B. V.; Polito, V.; Testa, P.; De Pontieu, B. Bibcode: 2022A&A...659A.186B Altcode: 2022arXiv220111961B Context. Heating signatures from small-scale magnetic reconnection events in the solar atmosphere have proven to be difficult to detect through observations. Numerical models that reproduce flaring conditions are essential in understanding how nanoflares may act as a heating mechanism of the corona.
Aims: We study the effects of non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN simulations of nanoflare heated loops to investigate the diagnostic potential of chromospheric emission from small-scale events.
Methods: The Mg II h and k, Ca II H and K, Ca II 854.2 nm, and Hα and Hβ chromospheric lines were synthesised from various RADYN models of coronal loops subject to electron beams of nanoflare energies. The contribution function to the line intensity was computed to better understand how the atmospheric response to the non-thermal electrons affects the formation of spectral lines and the detailed shape of their spectral profiles.
Results: The spectral line signatures arising from the electron beams highly depend on the density of the loop and the lower cutoff energy of the electrons. Low-energy (5 keV) electrons deposit their energy in the corona and transition region, producing strong plasma flows that cause both redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons deposit their energy in the lower transition region and chromosphere, resulting in increased emission from local heating. Our results indicate that effects from small-scale events can be observed with ground-based telescopes, expanding the list of possible diagnostics for the presence and properties of nanoflares. Title: Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). I. Coronal Heating Authors: De Pontieu, Bart; Testa, Paola; Martínez-Sykora, Juan; Antolin, Patrick; Karampelas, Konstantinos; Hansteen, Viggo; Rempel, Matthias; Cheung, Mark C. M.; Reale, Fabio; Danilovic, Sanja; Pagano, Paolo; Polito, Vanessa; De Moortel, Ineke; Nóbrega-Siverio, Daniel; Van Doorsselaere, Tom; Petralia, Antonino; Asgari-Targhi, Mahboubeh; Boerner, Paul; Carlsson, Mats; Chintzoglou, Georgios; Daw, Adrian; DeLuca, Edward; Golub, Leon; Matsumoto, Takuma; Ugarte-Urra, Ignacio; McIntosh, Scott W.; the MUSE Team Bibcode: 2022ApJ...926...52D Altcode: 2021arXiv210615584D The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of a multislit extreme ultraviolet (EUV) spectrograph (in three spectral bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in two passbands around 195 Å and 304 Å). MUSE will provide unprecedented spectral and imaging diagnostics of the solar corona at high spatial (≤0.″5) and temporal resolution (down to ~0.5 s for sit-and-stare observations), thanks to its innovative multislit design. By obtaining spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX-Fe XXI 108 Å) covering a wide range of transition regions and coronal temperatures along 37 slits simultaneously, MUSE will, for the first time, "freeze" (at a cadence as short as 10 s) with a spectroscopic raster the evolution of the dynamic coronal plasma over a wide range of scales: from the spatial scales on which energy is released (≤0.″5) to the large-scale (~170″ × 170″) atmospheric response. We use numerical modeling to showcase how MUSE will constrain the properties of the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and the large field of view on which state-of-the-art models of the physical processes that drive coronal heating, flares, and coronal mass ejections (CMEs) make distinguishing and testable predictions. We describe the synergy between MUSE, the single-slit, high-resolution Solar-C EUVST spectrograph, and ground-based observatories (DKIST and others), and the critical role MUSE plays because of the multiscale nature of the physical processes involved. In this first paper, we focus on coronal heating mechanisms. An accompanying paper focuses on flares and CMEs. Title: Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). II. Flares and Eruptions Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola; De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito, Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin, Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul; Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub, Leon; The Bibcode: 2022ApJ...926...53C Altcode: 2021arXiv210615591C Current state-of-the-art spectrographs cannot resolve the fundamental spatial (subarcseconds) and temporal (less than a few tens of seconds) scales of the coronal dynamics of solar flares and eruptive phenomena. The highest-resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by the Interface Region Imaging Spectrograph for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), subarcsecond-resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al., which focuses on investigating coronal heating with MUSE. Title: Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE): II. Flares and Eruptions Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola; De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito, Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin, Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah; Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon Bibcode: 2021AGUFMSH51A..08C Altcode: Current state-of-the-art spectrographs cannot resolve the fundamental spatial (sub-arcseconds) and temporal scales (less than a few tens of seconds) of the coronal dynamics of solar flares and eruptive phenomena. The highest resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by IRIS for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), sub-arcsecond resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics, and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al. (2021, also submitted to SH-17), which focuses on investigating coronal heating with MUSE. Title: First observations from the SPICE EUV spectrometer on Solar Orbiter Authors: Fludra, A.; Caldwell, M.; Giunta, A.; Grundy, T.; Guest, S.; Leeks, S.; Sidher, S.; Auchère, F.; Carlsson, M.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.; Buchlin, É.; Caminade, S.; DeForest, C.; Fredvik, T.; Haberreiter, M.; Harra, L.; Janvier, M.; Kucera, T.; Müller, D.; Parenti, S.; Schmutz, W.; Schühle, U.; Solanki, S. K.; Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R.; Chitta, L. P. Bibcode: 2021A&A...656A..38F Altcode: 2021arXiv211011252F
Aims: We present first science observations taken during the commissioning activities of the Spectral Imaging of the Coronal Environment (SPICE) instrument on the ESA/NASA Solar Orbiter mission. SPICE is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths. In this paper we illustrate the possible types of observations to give prospective users a better understanding of the science capabilities of SPICE.
Methods: We have reviewed the data obtained by SPICE between April and June 2020 and selected representative results obtained with different slits and a range of exposure times between 5 s and 180 s. Standard instrumental corrections have been applied to the raw data.
Results: The paper discusses the first observations of the Sun on different targets and presents an example of the full spectra from the quiet Sun, identifying over 40 spectral lines from neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur, magnesium, and iron. These lines cover the temperature range between 20 000 K and 1 million K (10 MK in flares), providing slices of the Sun's atmosphere in narrow temperature intervals. We provide a list of count rates for the 23 brightest spectral lines. We show examples of raster images of the quiet Sun in several strong transition region lines, where we have found unusually bright, compact structures in the quiet Sun network, with extreme intensities up to 25 times greater than the average intensity across the image. The lifetimes of these structures can exceed 2.5 hours. We identify them as a transition region signature of coronal bright points and compare their areas and intensity enhancements. We also show the first above-limb measurements with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and far off limb measurements in the equatorial plane in Mg IX, Ne VIII, and O VI lines. We discuss the potential to use abundance diagnostics methods to study the variability of the elemental composition that can be compared with in situ measurements to help confirm the magnetic connection between the spacecraft location and the Sun's surface, and locate the sources of the solar wind.
Conclusions: The SPICE instrument successfully performs measurements of EUV spectra and raster images that will make vital contributions to the scientific success of the Solar Orbiter mission. Title: Solar oxygen abundance Authors: Bergemann, Maria; Hoppe, Richard; Semenova, Ekaterina; Carlsson, Mats; Yakovleva, Svetlana A.; Voronov, Yaroslav V.; Bautista, Manuel; Nemer, Ahmad; Belyaev, Andrey K.; Leenaarts, Jorrit; Mashonkina, Lyudmila; Reiners, Ansgar; Ellwarth, Monika Bibcode: 2021MNRAS.508.2236B Altcode: 2021arXiv210901143B; 2021MNRAS.tmp.1964B Motivated by the controversy over the surface metallicity of the Sun, we present a re-analysis of the solar photospheric oxygen (O) abundance. New atomic models of O and Ni are used to perform non-local thermodynamic equilibrium (NLTE) calculations with 1D hydrostatic (MARCS) and 3D hydrodynamical (Stagger and Bifrost) models. The Bifrost 3D MHD simulations are used to quantify the influence of the chromosphere. We compare the 3D NLTE line profiles with new high-resolution, R$\approx 700\, 000$, spatially resolved spectra of the Sun obtained using the IAG FTS instrument. We find that the O I lines at 777 nm yield the abundance of log A(O) = 8.74 ± 0.03 dex, which depends on the choice of the H-impact collisional data and oscillator strengths. The forbidden [O I] line at 630 nm is less model dependent, as it forms nearly in LTE and is only weakly sensitive to convection. However, the oscillator strength for this transition is more uncertain than for the 777 nm lines. Modelled in 3D NLTE with the Ni I blend, the 630 nm line yields an abundance of log A(O) = 8.77 ± 0.05 dex. We compare our results with previous estimates in the literature and draw a conclusion on the most likely value of the solar photospheric O abundance, which we estimate at log A(O) = 8.75 ± 0.03 dex. Title: Diagnostic capabilities of spectropolarimetric observations for understanding solar phenomena. I. Zeeman-sensitive photospheric lines Authors: Quintero Noda, C.; Barklem, P. S.; Gafeira, R.; Ruiz Cobo, B.; Collados, M.; Carlsson, M.; Martínez Pillet, V.; Orozco Suárez, D.; Uitenbroek, H.; Katsukawa, Y. Bibcode: 2021A&A...652A.161Q Altcode: 2021arXiv210605084Q Future ground-based telescopes will expand our capabilities for simultaneous multi-line polarimetric observations in a wide range of wavelengths, from the near-ultraviolet to the near-infrared. This creates a strong demand to compare candidate spectral lines to establish a guideline of the lines that are most appropriate for each observation target. We focused in this first work on Zeeman-sensitive photospheric lines in the visible and infrared. We first examined their polarisation signals and response functions using a 1D semi-empirical atmosphere. Then we studied the spatial distribution of the line core intensity and linear and circular polarisation signals using a realistic 3D numerical simulation. We ran inversions of synthetic profiles, and we compared the heights at which we obtain a high correlation between the input and the inferred atmosphere. We also used this opportunity to revisit the atomic information we have on these lines and computed the broadening cross-sections due to collisions with neutral hydrogen atoms for all the studied spectral lines. The results reveal that four spectral lines stand out from the rest for quiet-Sun and network conditions: Fe I 5250.2, 6302, 8468, and 15 648 Å. The first three form higher in the atmosphere, and the last line is mainly sensitive to the atmospheric parameters at the bottom of the photosphere. However, as they reach different heights, we strongly recommend using at least one of the first three candidates together with the Fe I 15 648 Å line to optimise our capabilities for inferring the thermal and magnetic properties of the lower atmosphere. Title: Mapping of Solar Magnetic Fields from the Photosphere to the Top of the Chromosphere with CLASP2 Authors: McKenzie, D.; Ishikawa, R.; Trujillo Bueno, J.; Auchere, F.; del Pino Aleman, T.; Okamoto, T.; Kano, R.; Song, D.; Yoshida, M.; Rachmeler, L.; Kobayashi, K.; Narukage, N.; Kubo, M.; Ishikawa, S.; Hara, H.; Suematsu, Y.; Sakao, T.; Bethge, C.; De Pontieu, B.; Vigil, G.; Winebarger, A.; Alsina Ballester, E.; Belluzzi, L.; Stepan, J.; Asensio Ramos, A.; Carlsson, M.; Leenaarts, J. Bibcode: 2021AAS...23810603M Altcode: Coronal heating, chromospheric heating, and the heating & acceleration of the solar wind, are well-known problems in solar physics. Additionally, knowledge of the magnetic energy that powers solar flares and coronal mass ejections, important drivers of space weather, is handicapped by imperfect determination of the magnetic field in the sun's atmosphere. Extrapolation of photospheric magnetic measurements into the corona is fraught with difficulties and uncertainties, partly due to the vastly different plasma beta between the photosphere and the corona. Better results in understanding the coronal magnetic field should be derived from measurements of the magnetic field in the chromosphere. To that end, we are pursuing quantitative determination of the magnetic field in the chromosphere, where plasma beta transitions from greater than unity to less than unity, via ultraviolet spectropolarimetry. The CLASP2 mission, flown on a sounding rocket in April 2019, succeeded in measuring all four Stokes polarization parameters in UV spectral lines formed by singly ionized Magnesium and neutral Manganese. Because these ions produce spectral lines under different conditions, CLASP2 thus was able to quantify the magnetic field properties at multiple heights in the chromosphere simultaneously, as shown in the recent paper by Ishikawa et al. In this presentation we will report the findings of CLASP2, demonstrating the variation of magnetic fields along a track on the solar surface and as a function of height in the chromosphere; and we will illustrate what is next for the CLASP missions and the demonstration of UV spectropolarimetry in the solar chromosphere. Title: Chromospheric Heating by Magnetohydrodynamic Waves and Instabilities Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson, M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.; Murawski, K.; Zaqarashvili, T. V. Bibcode: 2021JGRA..12629097S Altcode: 2021arXiv210402010S The importance of the chromosphere in the mass and energy transport within the solar atmosphere is now widely recognized. This review discusses the physics of magnetohydrodynamic waves and instabilities in large-scale chromospheric structures as well as in magnetic flux tubes. We highlight a number of key observational aspects that have helped our understanding of the role of the solar chromosphere in various dynamic processes and wave phenomena, and the heating scenario of the solar chromosphere is also discussed. The review focuses on the physics of waves and invokes the basics of plasma instabilities in the context of this important layer of the solar atmosphere. Potential implications, future trends and outstanding questions are also delineated. Title: A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS) Authors: De Pontieu, Bart; Polito, Vanessa; Hansteen, Viggo; Testa, Paola; Reeves, Katharine K.; Antolin, Patrick; Nóbrega-Siverio, Daniel Elias; Kowalski, Adam F.; Martinez-Sykora, Juan; Carlsson, Mats; McIntosh, Scott W.; Liu, Wei; Daw, Adrian; Kankelborg, Charles C. Bibcode: 2021SoPh..296...84D Altcode: 2021arXiv210316109D The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. IRIS is the highest resolution observatory to provide seamless coverage of spectra and images from the photosphere into the low corona. The unique combination of near- and far-ultraviolet spectra and images at sub-arcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion-neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of waves, the acceleration of non-thermal particles, and various small-scale instabilities. IRIS has provided insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nano-flares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvénic waves, energy release and jet-like dynamics associated with braiding of magnetic-field lines, the role of turbulence and the tearing-mode instability in reconnection, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and various other mechanisms in triggering and driving CMEs. IRIS observations have also been used to elucidate the physical mechanisms driving the solar irradiance that impacts Earth's upper atmosphere, and the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine-learning techniques have played a key role. With the advent of exciting new instrumentation both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST) and the Atacama Large Millimeter/submillimeter Array (ALMA), and space-based, e.g. the Parker Solar Probe and the Solar Orbiter, we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges. Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio, Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart; Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa, Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler, Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun, Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres, Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.; Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini, Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena; Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor; Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael; Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli, Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys, Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.; Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis, Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson, Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.; Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.; Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava, Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas, Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST Instrument Scientists; DKIST Science Working Group; DKIST Critical Science Plan Community Bibcode: 2021SoPh..296...70R Altcode: 2020arXiv200808203R The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute. Title: Mapping solar magnetic fields from the photosphere to the base of the corona Authors: Ishikawa, Ryohko; Bueno, Javier Trujillo; del Pino Alemán, Tanausú; Okamoto, Takenori J.; McKenzie, David E.; Auchère, Frédéric; Kano, Ryouhei; Song, Donguk; Yoshida, Masaki; Rachmeler, Laurel A.; Kobayashi, Ken; Hara, Hirohisa; Kubo, Masahito; Narukage, Noriyuki; Sakao, Taro; Shimizu, Toshifumi; Suematsu, Yoshinori; Bethge, Christian; De Pontieu, Bart; Dalda, Alberto Sainz; Vigil, Genevieve D.; Winebarger, Amy; Ballester, Ernest Alsina; Belluzzi, Luca; Štěpán, Jiří; Ramos, Andrés Asensio; Carlsson, Mats; Leenaarts, Jorrit Bibcode: 2021SciA....7.8406I Altcode: 2021arXiv210301583I Routine ultraviolet imaging of the Sun's upper atmosphere shows the spectacular manifestation of solar activity; yet we remain blind to its main driver, the magnetic field. Here we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg II $h$ & $k$ and Mn I) and visible (Fe I) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere the field strengths reach more than 300 gauss, strongly correlated with the Mg II $k$ line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere. Title: Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner, O.; Vigeesh, G. Bibcode: 2021RSPTA.37900170F Altcode: 2020arXiv200705847F There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. The height dependence of wave power, in particular of high-frequency waves, varies by up to two orders of magnitude between the models, and the phase difference spectra of several models show unexpected features, including ±180° phase jumps.

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: Relative coronal abundance diagnostics with Solar Orbiter/SPICE Authors: Zambrana Prado, N.; Buchlin, E.; Peter, H.; Young, P. R.; Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Aznar Cuadrado, R.; Caminade, S.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.; Parenti, S.; Schmutz, W. K.; Schühle, U.; Sidher, S.; Teriaca, L.; Thompson, W. T.; Williams, D. Bibcode: 2020AGUFMSH038..09Z Altcode: Linking solar activity on the surface and in the corona to the inner heliosphere is one of Solar Orbiter's main goals. Its UV spectrometer SPICE (SPectral Imaging of the Coronal Environment) will provide relative abundance measurements which will be key in this quest as different structures on the Sun have different abundances as a consequence of the FIP (First Ionization Potential) effect. Solar Orbiter's unique combination of remote sensing and in-situ instruments coupled with observation from other missions such as Parker Solar Probe will allow us to compare in-situ and remote sensing composition data. With the addition of modeling, these new results will allow us to trace back the source of heliospheric plasma. As high telemetry will not always be available with SPICE, we have developed a method for measuring relative abundances that is both telemetry efficient and reliable. Unlike methods based on Differential Emission Measure (DEM) inversion, the Linear Combination Ratio (LCR) method does not require a large number of spectral lines. This new method is based on linear combinations of UV spectral lines. The coefficients of the combinations are optimized such that the ratio of two linear combinations of radiances would yield the relative abundance of two elements. We present some abundance diagnostics tested on different combinations of spectral lines observable by SPICE. Title: Dynamics and thermal structure in the quiet Sun seen by SPICE Authors: Peter, H.; Aznar Cuadrado, R.; Schühle, U.; Teriaca, L.; Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Buchlin, E.; Caminade, S.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.; Parenti, S.; Schmutz, W. K.; Sidher, S.; Thompson, W. T.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH038..03P Altcode: We will present some of the early data of the Spectral Imaging of the Coronal Environment (SPICE) instrument on Solar Orbiter. One of the unique features of SPICE is its capability to record a wide range of wavelengths in the extreme UV with the possibility to record spectral lines giving access to a continuous plasma temperature range from 10.000 K to well above 1 MK. The data taken so far were for commissioning purposes and they can be used for a preliminary evaluation of the science performance of the instrument. Here we will concentrate on sample spectra covering the whole wavelength region and on the early raster maps acquired in bright lines in the quiet Sun close to disk center. Looking at different quiet Sun features we investigate the thermal structure of the atmosphere and flow structures. For this we apply fits to the spectral profiles and check the performance in terms of Doppler shifts and line widths to retrieve the structure of the network in terms of dynamics. While the amount of data available so far is limited, we will have a first look on how quiet Sun plasma responds to heating events. For this, we will compare spectral lines forming at different temperatures recorded at strictly the same time. Title: A Journey of Exploration to the Polar Regions of a Star: Probing the Solar Poles and the Heliosphere from High Helio-Latitude Authors: Finsterle, W.; Harra, L.; Andretta, V.; Appourchaux, T.; Baudin, F.; Bellot Rubio, L.; Birch, A.; Boumier, P.; Cameron, R. H.; Carlsson, M.; Corbard, T.; Davies, J. A.; Fazakerley, A. N.; Fineschi, S.; Gizon, L. C.; Harrison, R. A.; Hassler, D.; Leibacher, J. W.; Liewer, P. C.; Macdonald, M.; Maksimovic, M.; Murphy, N.; Naletto, G.; Nigro, G.; Owen, C. J.; Martinez-Pillet, V.; Rochus, P. L.; Romoli, M.; Sekii, T.; Spadaro, D.; Veronig, A. Bibcode: 2020AGUFMSH0110005F Altcode: A mission to view the solar poles from high helio-latitudes (above 60°) will build on the experience of Solar Orbiter as well as a long heritage of successful solar missions and instrumentation (e.g. SOHO, STEREO, Hinode, SDO), but will focus for the first time on the solar poles, enabling scientific investigations that cannot be done by any other mission. One of the major mysteries of the Sun is the solar cycle. The activity cycle of the Sun drives the structure and behaviour of the heliosphere and is, of course, the driver of space weather. In addition, solar activity and variability provides fluctuating input into the Earth climate models, and these same physical processes are applicable to stellar systems hosting exoplanets. One of the main obstructions to understanding the solar cycle, and hence all solar activity, is our current lack of understanding of the polar regions. We describe a mission concept that aims to address this fundamental issue. In parallel, we recognise that viewing the Sun from above the polar regions enables further scientific advantages, beyond those related to the solar cycle, such as unique and powerful studies of coronal mass ejection processes, from a global perspective, and studies of coronal structure and activity in polar regions. Not only will these provide important scientific advances for fundamental stellar physics research, they will feed into our understanding of impacts on the Earth and other planets' space environment. Title: First Results From SPICE EUV Spectrometer on Solar Orbiter Authors: Fludra, A.; Caldwell, M.; Giunta, A. S.; Grundy, T.; Guest, S.; Sidher, S.; Auchere, F.; Carlsson, M.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.; Buchlin, E.; Caminade, S.; DeForest, C.; Fredvik, T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Leeks, S.; Mueller, D.; Parenti, S.; Schmutz, W. K.; Schühle, U.; Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH038..02F Altcode: SPICE (Spectral Imaging of Coronal Environment) is one of the remote sensing instruments onboard Solar Orbiter. It is an EUV imaging spectrometer observing the Sun in two wavelength bands: 69.6-79.4 nm and 96.6-105.1 nm. SPICE is capable of recording full spectra in these bands with exposures as short as 1s. SPICE is the only Solar Orbiter instrument that can measure EUV spectra from the disk and low corona of the Sun and record all spectral lines simultaneously. SPICE uses one of three narrow slits, 2"x11', 4''x11', 6''x11', or a wide slit 30''x14'. The primary mirror can be scanned in a direction perpendicular to the slit, allowing raster images of up to 16' in size.

We present an overview of the first SPICE data taken on several days during the instrument commissioning carried out by the RAL Space team between 2020 April 21 and 2020 June 14. We also include results from SPICE observations at the first Solar Orbiter perihelion at 0.52AU, taken between June 16-21st. We give examples of full spectra from the quiet Sun near disk centre and provide a list of key spectral lines emitted in a range of temperatures between 10,000 K and over 1 million K, from neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur and magnesium. We show examples of first raster images in several strong lines, obtained with different slits and a range of exposure times between 5s and 180s. We describe the temperature coverage and density diagnostics, determination of plasma flows, and discuss possible applications to studies of the elemental abundances in the corona. We also show the first off-limb measurements with SPICE, as obtained when the spacecraft pointed at the limb. Title: Calibrating optical distortions in the Solar Orbiter SPICE spectrograph Authors: Thompson, W. T.; Schühle, U.; Young, P. R.; Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.; Buchlin, E.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.; Parenti, S.; Caminade, S.; Schmutz, W. K.; Teriaca, L.; Williams, D.; Sidher, S. Bibcode: 2020AGUFMSH0360029T Altcode: The Spectral Imaging of the Coronal Environment (SPICE) instrument on Solar Orbiter is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths from 70.4-79.0 nm and 97.3-104.9 nm. A single-mirror off-axis paraboloid focuses the solar image onto the entrance slit of the spectrometer section. A Toroidal Variable Line Space (TVLS) grating images the entrance slit onto a pair of MCP-intensified APS detectors. Ray-tracing analysis prior to launch showed that the instrument was subject to a number of small image distortions which need to be corrected in the final data product. We compare the ray tracing results with measurements made in flight. Co-alignment with other telescopes on Solar Orbiter will also be examined. Title: First results from the EUI and SPICE observations of Alpha Leo near Solar Orbiter first perihelion Authors: Buchlin, E.; Teriaca, L.; Giunta, A. S.; Grundy, T.; Andretta, V.; Auchere, F.; Peter, H.; Berghmans, D.; Carlsson, M.; Fludra, A.; Harra, L.; Hassler, D.; Long, D.; Rochus, P. L.; Schühle, U.; Aznar Cuadrado, R.; Caldwell, M.; Caminade, S.; DeForest, C.; Fredvik, T.; Gissot, S.; Heerlein, K.; Janvier, M.; Kraaikamp, E.; Kucera, T. A.; Müller, D.; Parenti, S.; Schmutz, W. K.; Sidher, S.; Smith, P.; Stegen, K.; Thompson, W. T.; Verbeeck, C.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH0360024B Altcode: On June 16th 2020 Solar Orbiter made a dedicated observing campaign where the spacecraft pointed to the solar limb to allow some of the high resolution instruments to observe the ingress (at the east limb) and later the egress (west limb) of the occultation of the star Alpha Leonis by the solar disk. The star was chosen because its luminosity and early spectral type ensure high and stable flux at wavelengths between 100 and 122 nanometers, a range observed by the High Resolution EUI Lyman alpha telescope (HRI-LYA) and by the long wavelength channel of the SPICE spectrograph. Star observations, when feasible, allow to gather a great deal of information on the instrument performances, such as the radiometric performance and the instrument optical point spread function (PSF).

We report here the first results from the above campaign for the two instruments. Title: Solar Orbiter: connecting remote sensing and in situ measurements Authors: Horbury, T. S.; Auchere, F.; Antonucci, E.; Berghmans, D.; Bruno, R.; Carlsson, M.; del Toro Iniesta, J. C.; Fludra, A.; Harra, L.; Hassler, D.; Heinzel, P.; Howard, R. A.; Krucker, S.; Livi, S. A.; Long, D.; Louarn, P.; Maksimovic, M.; Mueller, D.; Owen, C. J.; Peter, H.; Rochus, P. L.; Rodriguez-Pacheco, J.; Romoli, M.; Schühle, U.; Solanki, S. K.; Teriaca, L.; Wimmer-Schweingruber, R. F.; Zouganelis, Y.; Laker, R. Bibcode: 2020AGUFMSH038..10H Altcode: A key science goal of the Solar Orbiter mission is to make connections between phenomena on the Sun and their manifestations in interplanetary space. To that end, the spacecraft carries a carefully tailored payload of six remote sensing instruments and four making in situ measurements. During June 2020, while the spacecraft was around 0.5 AU from the Sun, the remote sensing instruments operated for several days. While this was primarily an engineering activity, the resulting observations provided outstanding measurements and represent the ideal first opportunity to investigate the potential for making connections between the remote sensing and in situ payloads on Solar Orbiter.

We present a preliminary analysis of the available remote sensing and in situ observations, showing how connections can be made, and discuss the potential for further, more precise mapping to be performed as the mission progresses. Title: First results from combined EUI and SPICE observations of Lyman lines of Hydrogen and He II Authors: Teriaca, L.; Aznar Cuadrado, R.; Giunta, A. S.; Grundy, T.; Parenti, S.; Auchere, F.; Vial, J. C.; Fludra, A.; Berghmans, D.; Carlsson, M.; Harra, L.; Hassler, D.; Long, D.; Peter, H.; Rochus, P. L.; Schühle, U.; Buchlin, E.; Caldwell, M.; Caminade, S.; DeForest, C.; Fredvik, T.; Gissot, S.; Heerlein, K.; Janvier, M.; Kraaikamp, E.; Kucera, T. A.; Mueller, D.; Schmutz, W. K.; Sidher, S.; Smith, P.; Stegen, K.; Thompson, W. T.; Verbeeck, C.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH0360003T Altcode: The Solar Orbiter spacecraft carries a powerful set of remote sensing instruments that allow studying the solar atmosphere with unprecedented diagnostic capabilities. Many such diagnostics require the simultaneous usage of more than one instrument. One example of that is the capability, for the first time, to obtain (near) simultaneous spatially resolved observations of the emission from the first three lines of the Lyman series of hydrogen and of He II Lyman alpha. In fact, the SPectral Imaging of the Coronal Environment (SPICE) spectrometer can observe the Lyman beta and gamma lines in its long wavelength (SPICE-LW) channel, the High Resolution Lyman Alpha (HRI-LYA) telescope of the Extreme Ultraviolet Imager (EUI) acquires narrow band images in the Lyman alpha line while the Full Disk Imager (FSI) of EUI can take images dominated by the Lyman alpha line of ionized Helium at 30.4 nm (FSI-304). Being hydrogen and helium the main components of our star, these very bright transitions play an important role in the energy budget of the outer atmosphere via radiative losses and the measurement of their profiles and radiance ratios is a fundamental constraint to any comprehensive modelization effort of the upper solar chromosphere and transition region. Additionally, monitoring their average ratios can serve as a check out for the relative radiometric performance of the two instruments throughout the mission. Although the engineering data acquired so far are far from ideal in terms of time simultaneity (often only within about 1 h) and line coverage (often only Lyman beta was acquired by SPICE and not always near simultaneous images from all three telescopes are available) the analysis we present here still offers a great opportunity to have a first look at the potential of this diagnostic from the two instruments. In fact, we have identified a series of datasets obtained at disk center and at various positions at the solar limb that allow studying the Lyman alpha to beta radiance ratio and their relation to He II 30.4 as a function of the position on the Sun (disk center versus limb and quiet Sun versus coronal holes). Title: Coordination within the remote sensing payload on the Solar Orbiter mission Authors: Auchère, F.; Andretta, V.; Antonucci, E.; Bach, N.; Battaglia, M.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Caminade, S.; Carlsson, M.; Carlyle, J.; Cerullo, J. J.; Chamberlin, P. C.; Colaninno, R. C.; Davila, J. M.; De Groof, A.; Etesi, L.; Fahmy, S.; Fineschi, S.; Fludra, A.; Gilbert, H. R.; Giunta, A.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Howard, R. A.; Hurford, G.; Kleint, L.; Kolleck, M.; Krucker, S.; Lagg, A.; Landini, F.; Long, D. M.; Lefort, J.; Lodiot, S.; Mampaey, B.; Maloney, S.; Marliani, F.; Martinez-Pillet, V.; McMullin, D. R.; Müller, D.; Nicolini, G.; Orozco Suarez, D.; Pacros, A.; Pancrazzi, M.; Parenti, S.; Peter, H.; Philippon, A.; Plunkett, S.; Rich, N.; Rochus, P.; Rouillard, A.; Romoli, M.; Sanchez, L.; Schühle, U.; Sidher, S.; Solanki, S. K.; Spadaro, D.; St Cyr, O. C.; Straus, T.; Tanco, I.; Teriaca, L.; Thompson, W. T.; del Toro Iniesta, J. C.; Verbeeck, C.; Vourlidas, A.; Watson, C.; Wiegelmann, T.; Williams, D.; Woch, J.; Zhukov, A. N.; Zouganelis, I. Bibcode: 2020A&A...642A...6A Altcode: Context. To meet the scientific objectives of the mission, the Solar Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing (RS) instruments designed for joint operations with inter-instrument communication capabilities. Indeed, previous missions have shown that the Sun (imaged by the RS instruments) and the heliosphere (mainly sampled by the IS instruments) should be considered as an integrated system rather than separate entities. Many of the advances expected from Solar Orbiter rely on this synergistic approach between IS and RS measurements.
Aims: Many aspects of hardware development, integration, testing, and operations are common to two or more RS instruments. In this paper, we describe the coordination effort initiated from the early mission phases by the Remote Sensing Working Group. We review the scientific goals and challenges, and give an overview of the technical solutions devised to successfully operate these instruments together.
Methods: A major constraint for the RS instruments is the limited telemetry (TM) bandwidth of the Solar Orbiter deep-space mission compared to missions in Earth orbit. Hence, many of the strategies developed to maximise the scientific return from these instruments revolve around the optimisation of TM usage, relying for example on onboard autonomy for data processing, compression, and selection for downlink. The planning process itself has been optimised to alleviate the dynamic nature of the targets, and an inter-instrument communication scheme has been implemented which can be used to autonomously alter the observing modes. We also outline the plans for in-flight cross-calibration, which will be essential to the joint data reduction and analysis.
Results: The RS instrument package on Solar Orbiter will carry out comprehensive measurements from the solar interior to the inner heliosphere. Thanks to the close coordination between the instrument teams and the European Space Agency, several challenges specific to the RS suite were identified and addressed in a timely manner. Title: The Solar Orbiter SPICE instrument. An extreme UV imaging spectrometer Authors: SPICE Consortium; Anderson, M.; Appourchaux, T.; Auchère, F.; Aznar Cuadrado, R.; Barbay, J.; Baudin, F.; Beardsley, S.; Bocchialini, K.; Borgo, B.; Bruzzi, D.; Buchlin, E.; Burton, G.; Büchel, V.; Caldwell, M.; Caminade, S.; Carlsson, M.; Curdt, W.; Davenne, J.; Davila, J.; Deforest, C. E.; Del Zanna, G.; Drummond, D.; Dubau, J.; Dumesnil, C.; Dunn, G.; Eccleston, P.; Fludra, A.; Fredvik, T.; Gabriel, A.; Giunta, A.; Gottwald, A.; Griffin, D.; Grundy, T.; Guest, S.; Gyo, M.; Haberreiter, M.; Hansteen, V.; Harrison, R.; Hassler, D. M.; Haugan, S. V. H.; Howe, C.; Janvier, M.; Klein, R.; Koller, S.; Kucera, T. A.; Kouliche, D.; Marsch, E.; Marshall, A.; Marshall, G.; Matthews, S. A.; McQuirk, C.; Meining, S.; Mercier, C.; Morris, N.; Morse, T.; Munro, G.; Parenti, S.; Pastor-Santos, C.; Peter, H.; Pfiffner, D.; Phelan, P.; Philippon, A.; Richards, A.; Rogers, K.; Sawyer, C.; Schlatter, P.; Schmutz, W.; Schühle, U.; Shaughnessy, B.; Sidher, S.; Solanki, S. K.; Speight, R.; Spescha, M.; Szwec, N.; Tamiatto, C.; Teriaca, L.; Thompson, W.; Tosh, I.; Tustain, S.; Vial, J. -C.; Walls, B.; Waltham, N.; Wimmer-Schweingruber, R.; Woodward, S.; Young, P.; de Groof, A.; Pacros, A.; Williams, D.; Müller, D. Bibcode: 2020A&A...642A..14S Altcode: 2019arXiv190901183A; 2019arXiv190901183S
Aims: The Spectral Imaging of the Coronal Environment (SPICE) instrument is a high-resolution imaging spectrometer operating at extreme ultraviolet wavelengths. In this paper, we present the concept, design, and pre-launch performance of this facility instrument on the ESA/NASA Solar Orbiter mission.
Methods: The goal of this paper is to give prospective users a better understanding of the possible types of observations, the data acquisition, and the sources that contribute to the instrument's signal.
Results: The paper discusses the science objectives, with a focus on the SPICE-specific aspects, before presenting the instrument's design, including optical, mechanical, thermal, and electronics aspects. This is followed by a characterisation and calibration of the instrument's performance. The paper concludes with descriptions of the operations concept and data processing.
Conclusions: The performance measurements of the various instrument parameters meet the requirements derived from the mission's science objectives. The SPICE instrument is ready to perform measurements that will provide vital contributions to the scientific success of the Solar Orbiter mission. Title: The Solar Orbiter mission. Science overview Authors: Müller, D.; St. Cyr, O. C.; Zouganelis, I.; Gilbert, H. R.; Marsden, R.; Nieves-Chinchilla, T.; Antonucci, E.; Auchère, F.; Berghmans, D.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rochus, P.; Rodriguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Bruno, R.; Carlsson, M.; Fludra, A.; Harra, L.; Hassler, D. M.; Livi, S.; Louarn, P.; Peter, H.; Schühle, U.; Teriaca, L.; del Toro Iniesta, J. C.; Wimmer-Schweingruber, R. F.; Marsch, E.; Velli, M.; De Groof, A.; Walsh, A.; Williams, D. Bibcode: 2020A&A...642A...1M Altcode: 2020arXiv200900861M
Aims: Solar Orbiter, the first mission of ESA's Cosmic Vision 2015-2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.
Methods: The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.
Results: Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission's science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.

ARRAY(0x207ce98) Title: On the Formation of Lyman β and the O I 1027 and 1028 Å Spectral Lines Authors: Hasegawa, Takahiro; Noda, Carlos Quintero; Shimizu, Toshifumi; Carlsson, Mats Bibcode: 2020ApJ...900...34H Altcode: 2020arXiv200812556H We study the potential of Lyman β and the O I 1027 and 1028 Å spectral lines to help in understanding the properties of the chromosphere and transition region (TR). The oxygen transitions are located in the wing of Lyman β, which is a candidate spectral line for the solar missions Solar Orbiter/Spectral Imaging of the Coronal Environment and Solar-C (EUVST). We examine the general spectroscopic properties of the three transitions in the quiet Sun by synthesizing them assuming nonlocal thermal equilibrium and taking into account partial redistribution effects. We estimate the heights where the spectral lines are sensitive to the physical parameters, computing the response functions to temperature and velocity using a 1D semiempirical atmospheric model. We also synthesize the intensity spectrum using the 3D enhanced network simulation computed with the BIFROST code. The results indicate that Lyman β is sensitive to the temperature from the middle chromosphere to the TR, while it is mainly sensitive to the line-of-sight (LOS) velocity at the lower atmospheric layers, around 2000 km above the optical surface. The O I lines form lower in the middle chromosphere, being sensitive to the LOS velocities at heights lower than those covered by Lyman β. The spatial distribution of the intensity signals computed with the BIFROST atmosphere, as well as the inferred velocities from the line core Doppler shift, confirms the previous results. Therefore, these results indicate that the spectral window at 1025 Å contains several spectral lines that complement each other to seamlessly trace the thermal structure and gas dynamics from the middle chromosphere to the lower TR. Title: High-resolution observations of the solar photosphere, chromosphere, and transition region. A database of coordinated IRIS and SST observations Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson, M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.; Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques, V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.; Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.; Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.; Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski, M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P. Bibcode: 2020A&A...641A.146R Altcode: 2020arXiv200514175R NASA's Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe I lines and spectrally resolved imaging in the chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes. Title: Ambipolar diffusion in the Bifrost code Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis, F.; Carlsson, M. Bibcode: 2020A&A...638A..79N Altcode: 2020arXiv200411927N Context. Ambipolar diffusion is a physical mechanism related to the drift between charged and neutral particles in a partially ionized plasma that is key to many different astrophysical systems. However, understanding its effects is challenging due to basic uncertainties concerning relevant microphysical aspects and the strong constraints it imposes on the numerical modeling.
Aims: Our aim is to introduce a numerical tool that allows us to address complex problems involving ambipolar diffusion in which, additionally, departures from ionization equilibrium are important or high resolution is needed. The primary application of this tool is for solar atmosphere calculations, but the methods and results presented here may also have a potential impact on other astrophysical systems.
Methods: We have developed a new module for the stellar atmosphere Bifrost code that improves its computational capabilities of the ambipolar diffusion term in the generalized Ohm's law. This module includes, among other things, collision terms adequate to processes in the coolest regions in the solar chromosphere. As the main feature of the module, we have implemented the super time stepping (STS) technique, which allows an important acceleration of the calculations. We have also introduced hyperdiffusion terms to guarantee the stability of the code.
Results: We show that to have an accurate value for the ambipolar diffusion coefficient in the solar atmosphere it is necessary to include as atomic elements in the equation of state not only hydrogen and helium, but also the main electron donors like sodium, silicon, and potassium. In addition, we establish a range of criteria to set up an automatic selection of the free parameters of the STS method that guarantees the best performance, optimizing the stability and speed for the ambipolar diffusion calculations. We validate the STS implementation by comparison with a self-similar analytical solution. Title: Chromospheric Bubbles in Solar Flares Authors: Reid, A.; Zhigulin, B.; Carlsson, M.; Mathioudakis, M. Bibcode: 2020ApJ...894L..21R Altcode: 2020arXiv200510586R We analyze a grid of radiative hydrodynamic simulations of solar flares to study the energy balance and response of the atmosphere to nonthermal electron beam heating. The appearance of chromospheric bubbles is one of the most notable features that we find in the simulations. These pockets of chromospheric plasma get trapped between the transition region and the lower atmosphere as it is superheated by the particle beam. The chromospheric bubbles are seen in the synthetic spectra, appearing as an additional component to Balmer line profiles with high Doppler velocities as high as 200 km s-1. Their signatures are also visible in the wings of Ca II 8542 Å line profiles. These bubbles of chromospheric plasma are driven upward by a wave front that is induced by the shock of energy deposition, and require a specific heating rate and atmospheric location to manifest. 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: Observational constraints on the origin of the elements. II. 3D non-LTE formation of Ba II lines in the solar atmosphere Authors: Gallagher, A. J.; Bergemann, M.; Collet, R.; Plez, B.; Leenaarts, J.; Carlsson, M.; Yakovleva, S. A.; Belyaev, A. K. Bibcode: 2020A&A...634A..55G Altcode: 2019arXiv191003898G Context. The pursuit of more realistic spectroscopic modelling and consistent abundances has led us to begin a new series of papers designed to improve current solar and stellar abundances of various atomic species. To achieve this, we have begun updating the three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer code, MULTI3D, and the equivalent one-dimensional (1D) non-LTE radiative transfer code, MULTI 2.3.
Aims: We examine our improvements to these codes by redetermining the solar barium abundance. Barium was chosen for this test as it is an important diagnostic element of the s-process in the context of galactic chemical evolution. New Ba II + H collisional data for excitation and charge exchange reactions computed from first principles had recently become available and were included in the model atom. The atom also includes the effects of isotopic line shifts and hyperfine splitting.
Methods: A grid of 1D LTE barium lines were constructed with MULTI 2.3 and fit to the four Ba II lines available to us in the optical region of the solar spectrum. Abundance corrections were then determined in 1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium abundance was computed from these corrections.
Results: We present for the first time the full 3D non-LTE barium abundance of A(Ba) = 2.27 ± 0.02 ± 0.01, which was derived from four individual fully consistent barium lines. Errors here represent the systematic and random errors, respectively. Title: Ion-neutral Interactions and Nonequilibrium Ionization in the Solar Chromosphere Authors: Martínez-Sykora, Juan; Leenaarts, Jorrit; De Pontieu, Bart; Nóbrega-Siverio, Daniel; Hansteen, Viggo H.; Carlsson, Mats; Szydlarski, Mikolaj Bibcode: 2020ApJ...889...95M Altcode: 2019arXiv191206682M The thermal structure of the chromosphere is regulated through a complex interaction of various heating processes, radiative cooling, and the ionization degree of the plasma. Here, we study the impact on the thermal properties of the chromosphere when including the combined action of nonequilibrium ionization (NEI) of hydrogen and helium and ion-neutral interaction effects. We have performed a 2.5D radiative magnetohydrodynamic simulation using the Bifrost code. This model includes ion-neutral interaction effects by solving the generalized Ohm' s law (GOL) as well as NEI for hydrogen and helium. The GOL equation includes ambipolar diffusion and the Hall term. We compare this simulation with another simulation that computes the ionization in local thermodynamic equilibrium (LTE) including ion-neutral interaction effects. Our numerical models reveal substantial thermal differences in magneto-acoustic shocks, the wake behind the shocks, spicules, low-lying magnetic loops, and the transition region. In particular, we find that heating through ambipolar diffusion in shock wakes is substantially less efficient, while in the shock fronts themselves it is more efficient, under NEI conditions than when assuming LTE. Title: Cosmic ray interactions in the solar atmosphere Authors: Hudson, Hugh S.; MacKinnon, Alec; Szydlarski, Mikolaj; Carlsson, Mats Bibcode: 2020MNRAS.491.4852H Altcode: 2019arXiv191001186H; 2019MNRAS.tmp.3116H High-energy particles enter the solar atmosphere from Galactic or solar coronal sources, and produce 'albedo' emission from the quiet Sun that is now observable across a wide range of photon energies. The interaction of high-energy particles in a stellar atmosphere depends essentially upon the joint variation of the magnetic field and plasma density, which heretofore has been characterized parametrically as P ∝ Bα with P the gas pressure and B the magnitude of the magnetic field. We re-examine that parametrization by using a self-consistent 3D MHD model (Bifrost) and show that this relationship tends to P ∝ B3.5 ± 0.1 based on the visible portions of the sample of open-field flux tubes in such a model, but with large variations from point to point. This scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere, which will have a strong effect on the prediction of the emission anisotropy (limb brightening). The simulations show that much of the open flux in coronal holes originates in weak-field regions within the granular pattern of the convective motions seen in the simulations. Title: Capabilities of bisector analysis of the Si I 10 827 Å line for estimating line-of-sight velocities in the quiet Sun Authors: González Manrique, S. J.; Quintero Noda, C.; Kuckein, C.; Ruiz Cobo, B.; Carlsson, M. Bibcode: 2020A&A...634A..19G Altcode: 2020arXiv200100508G We examine the capabilities of a fast and simple method to infer line-of-sight (LOS) velocities from observations of the photospheric Si I 10 827 Å line. This spectral line is routinely observed together with the chromospheric He I 10 830 Å triplet as it helps to constrain the atmospheric parameters. We study the accuracy of bisector analysis and a line core fit of Si I 10 827 Å. We employ synthetic profiles starting from the Bifrost enhanced network simulation. The profiles are computed solving the radiative transfer equation, including non-local thermodynamic equilibrium effects on the determination of the atomic level populations of Si I. We found a good correlation between the inferred velocities from bisectors taken at different line profile intensities and the original simulation velocity at given optical depths. This good correlation means that we can associate bisectors taken at different line-profile percentages with atmospheric layers that linearly increase as we scan lower spectral line intensities. We also determined that a fit to the line-core intensity is robust and reliable, providing information about atmospheric layers that are above those accessible through bisectors. Therefore, by combining both methods on the Si I 10 827 Å line, we can seamlessly trace the quiet-Sun LOS velocity stratification from the deep photosphere to higher layers until around logτ = -3.5 in a fast and straightforward way. This method is ideal for generating quick-look reference images for future missions like the Daniel K. Inoue Solar Telescope and the European Solar Telescope, for example. Title: Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora, J.; Carlsson, M.; Szydlarski, M. Bibcode: 2020A&A...633A..66N Altcode: 2019arXiv191201015N Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized.
Aims: We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process.
Methods: Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar diffusion term of the generalized Ohm's law.
Results: Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere.

Movies associated to Figs. 2-5, 8, 9, and A.1 are available at https://www.aanda.org Title: The SPICE (Spectral Imaging of the Coronal Environment) Ultraviolet Imaging Spectrograph Investigation Authors: Hassler, D.; Auchere, F.; Carlsson, M.; Fludra, A.; Giunta, A. S.; Mueller, D.; Peter, H.; Parenti, S.; Teriaca, L.; Fredvik, T. Bibcode: 2019AGUFMSH24A..02H Altcode: One of the primary objectives of the Solar Orbiter mission is to link remote sensing observations of the solar surface structures with in-situ observations of solar wind streams. The SPICE (Spectral Imaging of the Coronal Environment) instrument will characterize the plasma properties of regions near the Sun to directly compare with in-situ measurements from both Solar Orbiter & Parker Solar Probe. Specifically, SPICE will map outflow velocities of surface features to solar wind structures with similar composition (FIP, M/q) measured in-situ by the SWA/HIS instrument on Solar Orbiter. These observations will help discriminate models of solar wind origin by matching composition signatures in solar wind streams to surface feature composition, and discriminate physical processes that inject material from closed structures into solar wind streams.

This presentation will provide an overview of the SPICE investigation, including science & measurement objective, instrument design, capabilities and performance as measured during calibration prior to delivery to the Solar Orbiter spacecraft. The presentation will also provide a description of the operations concept and data processing during the mission. Title: Science Requirement Document (SRD) for the European Solar Telescope (EST) (2nd edition, December 2019) Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.; Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.; Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse, K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.; Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller, T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno, J.; Utz, D.; Zuccarello, F. Bibcode: 2019arXiv191208650S Altcode: The European Solar Telescope (EST) is a research infrastructure for solar physics. It is planned to be an on-axis solar telescope with an aperture of 4 m and equipped with an innovative suite of spectro-polarimetric and imaging post-focus instrumentation. The EST project was initiated and is driven by EAST, the European Association for Solar Telescopes. EAST was founded in 2006 as an association of 14 European countries. Today, as of December 2019, EAST consists of 26 European research institutes from 18 European countries. The Preliminary Design Phase of EST was accomplished between 2008 and 2011. During this phase, in 2010, the first version of the EST Science Requirement Document (SRD) was published. After EST became a project on the ESFRI roadmap 2016, the preparatory phase started. The goal of the preparatory phase is to accomplish a final design for the telescope and the legal governance structure of EST. A major milestone on this path is to revisit and update the Science Requirement Document (SRD). The EST Science Advisory Group (SAG) has been constituted by EAST and the Board of the PRE-EST EU project in November 2017 and has been charged with the task of providing with a final statement on the science requirements for EST. Based on the conceptual design, the SRD update takes into account recent technical and scientific developments, to ensure that EST provides significant advancement beyond the current state-of-the-art. The present update of the EST SRD has been developed and discussed during a series of EST SAG meetings. The SRD develops the top-level science objectives of EST into individual science cases. Identifying critical science requirements is one of its main goals. Those requirements will define the capabilities of EST and the post-focus instrument suite. The technical requirements for the final design of EST will be derived from the SRD. Title: Cosmic Rays Across the Rainbow Bridge: Particle Interactions in a Magnetized Plasma Atmosphere Authors: Hudson, H. S.; MacKinnon, A.; Szydlarski, M.; Carlsson, M. Bibcode: 2019AGUFMSH21B..02H Altcode: High-energy particles enter the solar atmosphere from Galactic or solar sources, producing gamma-ray "albedo'' radiations. These emissions come from the quiet Sun and from the large-scale corona, as well as from the vicinity of flares, and have been observed across a wide range of photon energies (MeV to GeV). The interaction of high-energy particles in the solar atmosphere depends essentially upon the joint variation of the magnetic field and the gas, and this has heretofore generally been characterized parametrically as P ~ Balpha (Zweibel & Haber 1983), with P the gas pressure and B the magnitude of the magnetic field. We have checked this relationship with a Bifrost 3D MHD model, approximating the particle transport as a guiding-center motion along "open'' (large-scale) fieldlines. We find alpha ~ 2.2 in the strongest (kG) fields in the simulation, but individual field structures have widely disparate relationships. The scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere and to their incessant dynamics. We discuss this and other aspects of the interactions of high-energy particles with the Sun (the "solar Størmer problem''). Title: Co-spatial velocity and magnetic swirls in the simulated solar photosphere Authors: Liu, Jiajia; Carlsson, Mats; Nelson, Chris J.; Erdélyi, Robert Bibcode: 2019A&A...632A..97L Altcode: 2019arXiv191110923L Context. Velocity or intensity swirls have now been shown to be widely present throughout the photosphere and chromosphere. It has been suggested that these events could contribute to the heating of the upper solar atmosphere, via exciting Alfvén pulses, which could carry significant amounts of energy. However, the conjectured necessary physical conditions for their excitation, that the magnetic field rotates co-spatially and co-temporally with the velocity field, has not been verified.
Aims: We aim to understand whether photospheric velocity swirls exist co-spatially and co-temporally with photospheric magnetic swirls, in order to demonstrate the link between swirls and pulses.
Methods: The automated swirl detection algorithm (ASDA) is applied to the photospheric horizontal velocity and vertical magnetic fields obtained from a series of realistic numerical simulations using the radiative magnetohydrodynamics (RMHD) code Bifrost. The spatial relationship between the detected velocity and magnetic swirls is further investigated via a well-defined correlation index (CI) study.
Results: On average, there are ∼63 short-lived photospheric velocity swirls (with lifetimes mostly less than 20 s, and average radius of ∼37 km and rotating speeds of ∼2.5 km s-1) detected in a field of view (FOV) of 6 × 6 Mm-2, implying a total population of velocity swirls of ∼1.06 × 107 in the solar photosphere. More than 80% of the detected velocity swirls are found to be accompanied by local magnetic concentrations in intergranular lanes. On average, ∼71% of the detected velocity swirls have been found to co-exist with photospheric magnetic swirls with the same rotating direction.
Conclusions: The co-temporal and co-spatial rotation in the photospheric velocity and magnetic fields provide evidence that the conjectured condition for the excitation of Alfvén pulses by photospheric swirls is fulfilled. Title: Modeling Mg II during Solar Flares. II. Nonequilibrium Effects Authors: Kerr, Graham S.; Carlsson, Mats; Allred, Joel C. Bibcode: 2019ApJ...885..119K Altcode: 2019arXiv190913300K To extract the information that the Mg II NUV spectra (observed by the Interface Region Imaging Spectrograph) carry about the chromosphere during solar flares, and to validate models of energy transport via model-data comparison, forward modeling is required. The assumption of statistical equilibrium (SE) is typically used to obtain the atomic level populations from snapshots of flare atmospheres, due to computational necessity. However, it is possible that relying on SE could lead to spurious results. We compare solving the atomic level populations via SE versus a nonequilibrium (NEQ) time-dependent approach. This was achieved using flare simulations from RADYN alongside the minority species version MS_RADYN from which the time-dependent Mg II atomic level populations and radiation transfer were computed in complete frequency redistribution. The impacts on the emergent profiles, lightcurves, line ratios, and formation heights are discussed. In summary we note that NEQ effects during flares are typically important only in the initial stages and for a short period following the cessation of the energy injection. An analysis of the timescales of ionization equilibrium reveals that for most of the duration of the flare, when the temperatures and densities are sufficiently enhanced, the relaxation timescales are short (τ relax < 0.1 s), so that the equilibrium solution is an adequate approximation. These effects vary with the size of the flare, however. In weaker flares, effects can be more pronounced. We recommend that NEQ effects be considered when possible but that SE is sufficient at most stages of the flare. Title: Observational constraints on the origin of the elements. I. 3D NLTE formation of Mn lines in late-type stars Authors: Bergemann, Maria; Gallagher, Andrew J.; Eitner, Philipp; Bautista, Manuel; Collet, Remo; Yakovleva, Svetlana A.; Mayriedl, Anja; Plez, Bertrand; Carlsson, Mats; Leenaarts, Jorrit; Belyaev, Andrey K.; Hansen, Camilla Bibcode: 2019A&A...631A..80B Altcode: 2019arXiv190505200B Manganese (Mn) is a key Fe-group element, commonly employed in stellar population and nucleosynthesis studies to explore the role of SN Ia. We have developed a new non-local thermodynamic equilibrium (NLTE) model of Mn, including new photo-ionisation cross-sections and new transition rates caused by collisions with H and H- atoms. We applied the model in combination with one-dimensional (1D) LTE model atmospheres and 3D hydrodynamical simulations of stellar convection to quantify the impact of NLTE and convection on the line formation. We show that the effects of NLTE are present in Mn I and, to a lesser degree, in Mn II lines, and these increase with metallicity and with the effective temperature of a model. Employing 3D NLTE radiative transfer, we derive a new abundance of Mn in the Sun, A(Mn) = 5.52 ± 0.03 dex, consistent with the element abundance in C I meteorites. We also applied our methods to the analysis of three metal-poor benchmark stars. We find that 3D NLTE abundances are significantly higher than 1D LTE. For dwarfs, the differences between 1D NLTE and 3D NLTE abundances are typically within 0.15 dex, however, the effects are much larger in the atmospheres of giants owing to their more vigorous convection. We show that 3D NLTE successfully solves the ionisation and excitation balance for the RGB star HD 122563 that cannot be achieved by 1D LTE or 1D NLTE modelling. For HD 84937 and HD 140283, the ionisation balance is satisfied, however, the resonance Mn I triplet lines still show somewhat lower abundances compared to the high-excitation lines. Our results for the benchmark stars confirm that 1D LTE modelling leads to significant systematic biases in Mn abundances across the full wavelength range from the blue to the IR. We also produce a list of Mn lines that are not significantly biased by 3D and can be reliably, within the 0.1 dex uncertainty, modelled in 1D NLTE.

The new cross-sections and rate coefficients 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/cat/J/A+A/631/A80The atomic model is available at https://keeper.mpdl.mpg.de/f/1ce2a838074b49fc9424/?dl=1 Title: A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flare Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.; Testa, P.; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.; Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen, B.; McIntosh, S. W. Bibcode: 2019NatAs...3..160C Altcode: 2018NatAs...3..160C Solar and stellar flares are the most intense emitters of X-rays and extreme ultraviolet radiation in planetary systems1,2. On the Sun, strong flares are usually found in newly emerging sunspot regions3. The emergence of these magnetic sunspot groups leads to the accumulation of magnetic energy in the corona. When the magnetic field undergoes abrupt relaxation, the energy released powers coronal mass ejections as well as heating plasma to temperatures beyond tens of millions of kelvins. While recent work has shed light on how magnetic energy and twist accumulate in the corona4 and on how three-dimensional magnetic reconnection allows for rapid energy release5,6, a self-consistent model capturing how such magnetic changes translate into observable diagnostics has remained elusive. Here, we present a comprehensive radiative magnetohydrodynamics simulation of a solar flare capturing the process from emergence to eruption. The simulation has sufficient realism for the synthesis of remote sensing measurements to compare with observations at visible, ultraviolet and X-ray wavelengths. This unifying model allows us to explain a number of well-known features of solar flares7, including the time profile of the X-ray flux during flares, origin and temporal evolution of chromospheric evaporation and condensation, and sweeping of flare ribbons in the lower atmosphere. Furthermore, the model reproduces the apparent non-thermal shape of coronal X-ray spectra, which is the result of the superposition of multi-component super-hot plasmas8 up to and beyond 100 million K. Title: VizieR Online Data Catalog: Mn lines 3D NLTE formation in late-type stars (Bergemann+, 2019) Authors: Bergemann, M.; Gallagher, A. G.; Eitner, P.; Bautista, M.; Collet, R.; Yakovleva, S.; Mayriedl, A.; Plez, B.; Carlsson, M.; Leenaarts, J.; Belyaev, A. K.; Hansen, C. Bibcode: 2019yCat..36310080B Altcode: Model rate coefficients in cm3/s for neutralization and de-exci processes in collisions of MnI+H and MnII+H-, as well as MnII+H and MnIII+H- for temperatures from T=1000K to T=10000K.

Total photoionization cross section for MnI levels.

(24 data files). Title: Modeling Mg II During Solar Flares. I. Partial Frequency Redistribution, Opacity, and Coronal Irradiation Authors: Kerr, Graham S.; Allred, Joel C.; Carlsson, Mats Bibcode: 2019ApJ...883...57K Altcode: 2019arXiv190805329K The Interface Region Imaging Spectrograph has routinely observed the flaring Mg II near-ultraviolet (NUV) spectrum, offering excellent diagnostic potential and a window into the location of energy deposition. A number of studies have forward-modeled both the general properties of these lines and specific flare observations. Generally these have forward-modeled radiation via post-processing of snapshots from hydrodynamic flare simulations through radiation transfer codes. There has, however, not been a study of how the physics included in these radiation transport codes affects the solution. A baseline setup for forward-modeling Mg II in flares is presented and contrasted with approaches that add or remove complexity. It is shown for Mg II that (1) partial frequency distribution (PRD) is still required during flare simulations despite the increased densities; (2) using full angle-dependent PRD affects the solution but takes significantly longer to process a snapshot; (3) including Mg I in non-LTE (NLTE) results in negligible differences to the Mg II lines but does affect the NUV quasi-continuum; (4) only hydrogen and Mg II need to be included in NLTE; (5) ideally the nonequilibrium hydrogen populations, with nonthermal collisional rates, should be used rather than the statistical equilibrium populations; (6) an atom consisting of only the ground state, h and k upper levels, and continuum level is insufficient to model the resonance lines; and (7) irradiation from a hot, dense flaring transition region can affect the formation of Mg II. We discuss modifications to the RH code allowing straightforward inclusion of the transition region and coronal irradiation in flares. Title: New View of the Solar Chromosphere Authors: Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo H. Bibcode: 2019ARA&A..57..189C Altcode: The solar chromosphere forms a crucial, yet complex and until recently poorly understood, interface between the solar photosphere and the heliosphere. Advances in high-resolution instrumentation, adaptive optics, image reconstruction techniques, and space-based observatories allow unprecedented high-resolution views of the finely structured and highly dynamic chromosphere. Dramatic progress in numerical computations allows 3D radiative magnetohydrodynamic forward models to take the place of the previous generation of 1D semiempirical atmosphere models. These new models provide deep insight into complex nonlocal thermodynamic equilibrium chromospheric diagnostics and enable physics-based interpretations of observations. This combination of modeling and observations has led to new insights into the role of shock waves, transverse magnetic waves, magnetic reconnection and flux emergence in the chromospheric energy balance, the formation of spicules, the impact of ion-neutral interactions, and the connectivity between chromosphere and transition region. During the next few years, the advent of new instrumentation (integral-field-unit spectropolarimetry) and observatories (ALMA, DKIST), coupled with novel inversion codes and expansion of existing numerical models to deal with ever more complex physical processes (including multifluid approaches), is expected to lead to major new insights into the dominant heating processes in the chromosphere and beyond. Title: Modelling of Flare Processes: A Comparison of the Two RHD Codes FLARIX and RADYN Authors: Kašparová, Jana; Carlsson, Mats; Heinzel, Petr; Varady, Michal Bibcode: 2019ASPC..519..141K Altcode: We present a comparison of two autonomous, methodologically different radiation hydrodynamic codes, FLARIX and RADYN, and their use to model the solar flare processes. Both codes can model the time evolution of a 1D atmosphere heated by a specified process, e.g. by electron beams propagating from the injection site in the corona down to the lower atmosphere. In such a scenario time scales can be rather short and lead to fast heating on even sub-second time scales. Our aim is to compare the FLARIX and RADYN codes using exactly the same setup and model conditions. Although such a comparison has never been done successfully before for this type of codes, we will present a close agreement between the time evolution of the modelled atmospheric structure for a test case of electron beam heating. Title: Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons Authors: Zhu, Yingjie; Kowalski, Adam F.; Tian, Hui; Uitenbroek, Han; Carlsson, Mats; Allred, Joel C. Bibcode: 2019ApJ...879...19Z Altcode: 2019arXiv190412285Z Observations from the Interface Region Imaging Spectrograph often reveal significantly broadened and non-reversed profiles of the Mg II h, k and triplet lines at flare ribbons. To understand the formation of these optically thick Mg II lines, we perform plane-parallel radiative hydrodynamics modeling with the RADYN code, and then recalculate the Mg II line profiles from RADYN atmosphere snapshots using the radiative transfer code RH. We find that the current RH code significantly underestimates the Mg II h and k Stark widths. By implementing semiclassical perturbation approximation results of quadratic Stark broadening from the STARK-B database in the RH code, the Stark broadenings are found to be one order of magnitude larger than those calculated from the current RH code. However, the improved Stark widths are still too small, and another factor of 30 has to be multiplied to reproduce the significantly broadened lines and adjacent continuum seen in observations. Nonthermal electrons, magnetic fields, three-dimensional effects, or electron density effects may account for this factor. Without modifying the RADYN atmosphere, we have also reproduced non-reversed Mg II h and k profiles, which appear when the electron beam energy flux is decreasing. These profiles are formed at an electron density of ∼8 × 1014 cm-3 and a temperature of ∼1.4 × 104 K, where the source function slightly deviates from the Planck function. Our investigation also demonstrates that at flare ribbons the triplet lines are formed in the upper chromosphere, close to the formation heights of the h and k lines. Title: The Response of the Lyα Line in Different Flare Heating Models Authors: Hong, Jie; Li, Ying; Ding, M. D.; Carlsson, Mats Bibcode: 2019ApJ...879..128H Altcode: 2019arXiv190513356H The solar Lyα line is the strongest line in the ultraviolet waveband, and is greatly enhanced during solar flares. Here we present radiative hydrodynamic simulations of solar flares under different heating models, and calculate the response of this line taking into account nonequilibrium ionization of hydrogen and partial frequency redistribution. We find that in nonthermal heating models, the Lyα line can show a red or blue asymmetry corresponding to the chromospheric evaporation or condensation, respectively. The asymmetry may change from red to blue if the electron beam flux is large enough to produce a significant chromospheric condensation region. In the Lyα intensity light curve, a dip appears when the change of asymmetry occurs. In thermal models, the Lyα line intensity peaks quickly and then falls, and the profile has an overall red asymmetry, which is similar to the profiles from heating by a soft electron beam. The Lyα profile shows a single red peak at the end of thermal heating, and the whole line is formed in a very small height range. Title: Chromospheric polarimetry through multiline observations of the 850 nm spectral region III: Chromospheric jets driven by twisted magnetic fields Authors: Quintero Noda, C.; Iijima, H.; Katsukawa, Y.; Shimizu, T.; Carlsson, M.; de la Cruz Rodríguez, J.; Ruiz Cobo, B.; Orozco Suárez, D.; Oba, T.; Anan, T.; Kubo, M.; Kawabata, Y.; Ichimoto, K.; Suematsu, Y. Bibcode: 2019MNRAS.486.4203Q Altcode: 2019MNRAS.tmp.1081N; 2019arXiv190409151Q We investigate the diagnostic potential of the spectral lines at 850 nm for understanding the magnetism of the lower atmosphere. For that purpose, we use a newly developed 3D simulation of a chromospheric jet to check the sensitivity of the spectral lines to this phenomenon as well as our ability to infer the atmospheric information through spectropolarimetric inversions of noisy synthetic data. We start comparing the benefits of inverting the entire spectrum at 850 nm versus only the Ca II 8542 Å spectral line. We found a better match of the input atmosphere for the former case, mainly at lower heights. However, the results at higher layers were not accurate. After several tests, we determined that we need to weight more the chromospheric lines than the photospheric ones in the computation of the goodness of the fit. The new inversion configuration allows us to obtain better fits and consequently more accurate physical parameters. Therefore, to extract the most from multiline inversions, a proper set of weights needs to be estimated. Besides that, we conclude again that the lines at 850 nm, or a similar arrangement with Ca II 8542 Å plus Zeeman-sensitive photospheric lines, pose the best-observing configuration for examining the thermal and magnetic properties of the lower solar atmosphere. Title: Dissecting bombs and bursts: non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht, T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M. Bibcode: 2019A&A...627A.101V Altcode: 2019arXiv190502035V Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. As they are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide useful insight into the overall evolution of active regions. Here we present results from inversions of SST/CRISP and CHROMIS, as well as IRIS data of such transient events. Combining information from the Mg II h & k, Si IV, and Ca II 8542 Å and Ca II H & K lines, we aim to characterise their temperature and velocity stratification, as well as their magnetic field configuration. We find average temperature enhancements of a few thousand kelvin, close to the classical temperature minimum and similar to previous studies, but localised peak temperatures of up to 10 000-15 000 K from Ca II inversions. Including Mg II appears to generally dampen these temperature enhancements to below 8000 K, while Si IV requires temperatures in excess of 10 000 K at low heights, but may also be reproduced with secondary temperature enhancements of 35 000-60 000 K higher up. However, reproducing Si IV comes at the expense of overestimating the Mg II emission. The line-of-sight velocity maps show clear bi-directional jet signatures for some events and strong correlation with substructure in the intensity images in general. Absolute line-of-sight velocities range between 5 and 20 km s-1 on average, with slightly larger velocities towards, rather than away from, the observer. The inverted magnetic field parameters show an enhancement of the horizontal field co-located with the brightenings at heights similar to that of the temperature increase. We are thus able to largely reproduce the observational properties of Ellerman bombs with the UV burst signature (e.g. intensities, profile asymmetries, morphology, and bi-directional jet signatures), with temperature stratifications peaking close to the classical temperature minimum. Correctly modelling the Si IV emission in agreement with all other diagnostics is however an outstanding issue and remains paramount in explaining its apparent coincidence with Hα emission. Fine-tuning the approach (accounting for resolution differences, fitting localised temperature enhancements, and/or performing spatially coupled inversions) is likely necessary in order to obtain better agreement between all considered diagnostics. Title: Radiative MHD Simulation of a Solar Flare Authors: Cheung, Mark; Rempel, Matthias D.; Chintzoglou, Georgios; Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto; DeRosa, Marc L.; Malanushenko, Anna; Hansteen, Viggo; Carlsson, Mats; De Pontieu, Bart; Gudiksen, Boris; McIntosh, Scott W. Bibcode: 2019AAS...23431005C Altcode: We present a radiative MHD simulation of a solar flare. The computational domain captures the near-surface layers of the convection zone and overlying atmosphere. Inspired by the observed evolution of NOAA Active Region (AR) 12017, a parasitic bipolar region is imposed to emerge in the vicinity of a pre-existing sunspot. The emergence of twisted magnetic flux generates shear flows that create a pre-existing flux rope underneath the canopy field of the sunspot. Following erosion of the overlying bootstrapping field, the flux rope erupts. Rapid release of magnetic energy results in multi-wavelength synthetic observables (including X-ray spectra, narrowband EUV images, Doppler shifts of EUV lines) that are consistent with flare observations. This works suggests the super-position of multi-thermal, superhot (up to 100 MK) plasma may be partially responsible for the apparent non-thermal shape of coronal X-ray sources in flares. Implications for remote sensing observations of other astrophysical objects is also discussed. This work is an important stepping stone toward high-fidelity data-driven MHD models. Title: Ellerman bombs and UV bursts: transient events in chromospheric current sheets Authors: Hansteen, V.; Ortiz, A.; Archontis, V.; Carlsson, M.; Pereira, T. M. D.; Bjørgen, J. P. Bibcode: 2019A&A...626A..33H Altcode: 2019arXiv190411524H Context. Ellerman bombs (EBs), observed in the photospheric wings of the Hα line, and UV bursts, observed in the transition region Si IV line, are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere.
Aims: We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5 Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship.
Methods: The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the 1.5D RH code and the fully 3D MULTI3D code.
Results: The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic spectra in the Hα and Si IV 139.376 nm lines both show characteristics that are typical of the observations. These synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.

The movie associated to Fig. 3 is available at https://www.aanda.org Title: On the Origin of the Magnetic Energy in the Quiet Solar Chromosphere Authors: Martínez-Sykora, Juan; Hansteen, Viggo H.; Gudiksen, Boris; Carlsson, Mats; De Pontieu, Bart; Gošić, Milan Bibcode: 2019ApJ...878...40M Altcode: 2019arXiv190404464M The presence of magnetic field is crucial in the transport of energy through the solar atmosphere. Recent ground-based and space-borne observations of the quiet Sun have revealed that magnetic field accumulates at photospheric heights, via a local dynamo or from small-scale flux emergence events. However, most of this small-scale magnetic field may not expand into the chromosphere due to the entropy drop with height at the photosphere. Here we present a study that uses a high-resolution 3D radiative MHD simulation of the solar atmosphere with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field to reveal that (1) the net magnetic flux from the simulated quiet photosphere is not sufficient to maintain a chromospheric magnetic field (on average), (2) processes in the lower chromosphere, in the region dominated by magnetoacoustic shocks, are able to convert kinetic energy into magnetic energy, (3) the magnetic energy in the chromosphere increases linearly in time until the rms of the magnetic field strength saturates at roughly 4-30 G (horizontal average) due to conversion from kinetic energy, (4) and that the magnetic features formed in the chromosphere are localized to this region. Title: Mg II NUV Spectra in Solar Flares: Modelling approaches and Velocity Diagnostics Authors: Kerr, Graham S.; Allred, Joel C.; Carlsson, Mats Bibcode: 2019AAS...23420413K Altcode: The Interface Region Imaging Spectrograph (IRIS) has observed the Mg II NUV spectrum (the h & k resonance and the subordinate triplet), in hundreds of solar flares. These lines form throughout the chromosphere, offering excellent diagnostic potential and a window into the location of flare energy deposition. A number of studies have attempted to forward model both the general properties of these lines in flares, and specific flare observations. Generally, these investigations have post-processed snapshots of solar atmospheres from hydrodynamic or radiation hydrodynamic flare simulations through advanced radiation transport codes. There has, however, not been a survey of how the physics included in these radiation transport codes affects the solution. We present here a detailed study showing what physics must be included when forward modelling these lines, including the use of partial redistribution (PRD), the specific treatment of (PRD), the need for a sufficiently large model atom, the inclusion of Mg I in NLTE, the inclusion of other species in NLTE, and the impact of using non-equilibrium hydrogen populations. Futher, we contrast Mg II computed in statistical equilibrium with a computation that includes non-equilibrium effects. Finally, we apply commonly used observational metrics for inferring Doppler speeds to the synthetic spectra, demonstrating that optical depth and radiation transfer effects can confuse the interpretation of those measurements. Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter Authors: McKenzie, D. E.; Ishikawa, R.; Trujillo Bueno, J.; Auchére, F.; Rachmeler, L. A.; Kubo, M.; Kobayashi, K.; Winebarger, A. R.; Bethge, C. W.; Narukage, N.; Kano, R.; Ishikawa, S.; de Pontieu, B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Štěpán, J.; del Pino Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A. Bibcode: 2019ASPC..526..361M Altcode: The hydrogen Lyman-α line at 121.6 nm and the Mg k line at 279.5 nm are especially relevant for deciphering the magnetic structure of the chromosphere since their line-center signals are formed in the chromosphere and transition region, with unique sensitivities to magnetic fields. We propose the Chromospheric LAyer Spectro-Polarimeter (CLASP2), to build upon the success of the first CLASP flight, which measured the linear polarization in H I Lyman-α. The existing CLASP instrument will be refitted to measure all four Stokes parameters in the 280 nm range, including variations due to the anisotropic radiation pumping, the Hanle effect, and the Zeeman effect. Title: Comparison of Scattering Polarization Signals Observed by CLASP: Possible Indication of the Hanle Effect Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.; Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa, Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; De Pomtieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R. Bibcode: 2019ASPC..526..305I Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP; Kano et al. 2012; Kobayashi et al. 2012; Kubo et al. 2014) observed, for the first time, the linear polarization produced by scattering processes in the hydrogen Lyman-α (121.57 nm) and Si III (120.56 nm) lines of the solar disk radiation. The complexity of the observed scattering polarization (i.e., conspicuous spatial variations in Q/I and U/I at spatial scales of 10″-20″ and the absence of center-to- limb variation at the Lyman-α center; see Kano et al. 2017) motivated us to search for possible hints of the operation of the Hanle effect by comparing: (a) the Lyman-α line center signal, for which the critical field strength (BH) for the onset of the Hanle effect is 53 G, (b) the Lyman-α wing, which is insensitive to the Hanle effect, and (c) the Si III line, whose BH = 290 G. We focus on four regions with different total unsigned photospheric magnetic fluxes (estimated from SDO/HMI observations), and compare the corresponding U/I spatial variations in the Lyman-α wing, Lyman-α center, and Si III line. The U/I signal in the Lyman-α wing shows an antisymmetric spatial distribution, which is caused by the presence of a bright structure in all the selected regions, regardless of the total unsigned photospheric magnetic flux. In an internetwork region, the Lyman-α center shows an antisymmetric spatial variation across the selected bright structure, but it does not show it in other more magnetized regions. In the Si III line, the spatial variation of U/I deviates from the above-mentioned antisymmetric shape as the total unsigned photospheric magnetic flux increases. We argue that a plausible explanation of this differential behavior is the operation of the Hanle effect.

This work, presented in an oral contribution at this Workshop, has been published on The Astrophysical Journal (Ishikawa et al. 2017). 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: SI IV Resonance Line Emission during Solar Flares: Non-LTE, Nonequilibrium, Radiation Transfer Simulations Authors: Kerr, Graham S.; Carlsson, Mats; Allred, Joel C.; Young, Peter R.; Daw, Adrian N. Bibcode: 2019ApJ...871...23K Altcode: 2018arXiv181111075K The Interface Region Imaging Spectrograph routinely observes the Si IV resonance lines. When analyzing quiescent observations of these lines, it has typically been assumed that they form under optically thin conditions. This is likely valid for the quiescent Sun, but this assumption has also been applied to the more extreme flaring scenario. We used 36 electron-beam-driven radiation hydrodynamic solar flare simulations, computed using the RADYN code, to probe the validity of this assumption. Using these simulated atmospheres, we solved the radiation transfer equations to obtain the non-LTE, nonequilibrium populations, line profiles, and opacities for a model silicon atom, including charge exchange processes. This was achieved using the “minority species” version of RADYN. The inclusion of charge exchange resulted in a substantial fraction of Si IV at cooler temperatures than those predicted by ionization equilibrium. All simulations with an injected energy flux F> 5× {10}10 erg cm-2 s-1 resulted in optical depth effects on the Si IV emission, with differences in both intensity and line shape compared to the optically thin calculation. Weaker flares (down to F ≈ 5 × 109 erg cm-2 s-1) also resulted in Si IV emission forming under optically thick conditions, depending on the other beam parameters. When opacity was significant, the atmospheres generally had column masses in excess of 5 × 10-6 g cm-2 over the temperature range 40-100 kK, and the Si IV formation temperatures were between 30 and 60 kK. We urge caution when analyzing Si IV flare observations, or when computing synthetic emission without performing a full radiation transfer calculation. Title: Observationally Based Models of Penumbral Microjets Authors: Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A.; Rouppe van der Voort, L.; Scharmer, G. B.; Carlsson, M. Bibcode: 2019ApJ...870...88E Altcode: 2018arXiv181107881E We study the polarization signals and physical parameters of penumbral microjets (PMJs) by using high spatial resolution data taken in the Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP and CHROMIS instruments at the Swedish 1 m Solar Telescope. We infer their physical parameters, such as physical observables in the photosphere and chromospheric velocity diagnostics, by different methods, including inversions of the observed Stokes profiles with the STiC code. PMJs harbor overall brighter Ca II K line profiles and conspicuous polarization signals in Ca II 854.2 nm, specifically in circular polarization that often shows multiple lobes mainly due to the shape of Stokes I. They usually overlap photospheric regions with a sheared magnetic field configuration, suggesting that magnetic reconnections could play an important role in the origin of PMJs. The discrepancy between their low LOS velocities and the high apparent speeds reported on earlier, as well as the existence of different vertical velocity gradients in the chromosphere, indicate that PMJs might not be entirely related to mass motions. Instead, PMJs could be due to perturbation fronts induced by magnetic reconnections occurring in the deep photosphere that propagate through the chromosphere. This reconnection may be associated with current heating that produces temperature enhancements from the temperature minimum region. Furthermore, enhanced collisions with electrons could also increase the coupling to the local conditions at higher layers during the PMJ phase, giving a possible explanation for the enhanced emission in the overall Ca II K profiles emerging from these transients. Title: Study of the polarization produced by the Zeeman effect in the solar Mg I b lines Authors: Quintero Noda, C.; Uitenbroek, H.; Carlsson, M.; Orozco Suárez, D.; Katsukawa, Y.; Shimizu, T.; Ruiz Cobo, B.; Kubo, M.; Oba, T.; Kawabata, Y.; Hasegawa, T.; Ichimoto, K.; Anan, T.; Suematsu, Y. Bibcode: 2018MNRAS.481.5675Q Altcode: 2018arXiv181001067Q; 2018MNRAS.tmp.2566Q The next generation of solar observatories aim to understand the magnetism of the solar chromosphere. Therefore, it is crucial to understand the polarimetric signatures of chromospheric spectral lines. For this purpose, we here examine the suitability of the three Fraunhofer Mg I b1, b2, and b4 lines at 5183.6, 5172.7, and 5167.3 Å, respectively. We start by describing a simplified atomic model of only six levels and three line transitions for computing the atomic populations of the 3p-4s (multiplet number 2) levels involved in the Mg I b line transitions assuming non-local thermodynamic conditions and considering only the Zeeman effect using the field-free approximation. We test this simplified atom against more complex ones finding that, although there are differences in the computed profiles, they are small compared with the advantages provided by the simple atom in terms of speed and robustness. After comparing the three Mg I lines, we conclude that the most capable one is the b2 line as b1 forms at similar heights and always shows weaker polarization signals, while b4 is severely blended with photospheric lines. We also compare Mg I b2 with the K I D1 and Ca II 8542 Å lines finding that the former is sensitive to the atmospheric parameters at heights that are in between those covered by the latter two lines. This makes Mg I b2 an excellent candidate for future multiline observations that aim to seamlessly infer the thermal and magnetic properties of different features in the lower solar atmosphere. Title: CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region Authors: Trujillo Bueno, J.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Ishikawa, R.; Kano, R.; Winebarger, A.; Auchère, F.; Narukage, N.; Kobayashi, K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M. Bibcode: 2018ApJ...866L..15T Altcode: 2018arXiv180908865T The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the Q/I line-center signals. Using an analytical model, we first show that the geometric complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the Q/I and U/I line-center signals. Second, we introduce a statistical description of the solar atmosphere based on a 3D model derived from a state-of-the-art radiation magnetohydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect. Title: A Statistical Inference Method for Interpreting the CLASP Observations Authors: Štěpán, J.; Trujillo Bueno, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Kano, R.; Winebarger, A.; Auchère, F.; Ishikawa, R.; Narukage, N.; Kobayashi, K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M. Bibcode: 2018ApJ...865...48S Altcode: 2018arXiv180802725S On 2015 September 3, the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) successfully measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation, revealing conspicuous spatial variations in the Q/I and U/I signals. Via the Hanle effect, the line-center Q/I and U/I amplitudes encode information on the magnetic field of the chromosphere-corona transition region, but they are also sensitive to the three-dimensional structure of this corrugated interface region. With the help of a simple line-formation model, here we propose a statistical inference method for interpreting the Lyα line-center polarization observed by CLASP. Title: Current State of UV Spectro-Polarimetry and its Future Direction Authors: Ishikawa, Ryohko; Sakao, Taro; Katsukawa, Yukio; Hara, Hirohisa; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Kubo, Masahito; Auchere, Frederic; De Pontieu, Bart; Winebarger, Amy; Kobayashi, . Ken; Kano, Ryouhei; Narukage, Noriyuki; Trujillo Bueno, Javier; Song, Dong-uk; Manso Sainz, Rafael; Asensio Ramos, Andres; Leenaarts, Jorritt; Carlsson, Mats; Bando, Takamasa; Ishikawa, Shin-nosuke; Tsuneta, Saku; Belluzzi, Luca; Suematsu, Yoshinori; Giono, Gabriel; Yoshida, Masaki; Goto, Motoshi; Del Pino Aleman, Tanausu; Stepan, Jiri; Okamoto, Joten; Tsuzuki, Toshihiro; Uraguchi, Fumihiro; Champey, Patrick; Alsina Ballester, Ernest; Casini, Roberto; McKenzie, David; Rachmeler, Laurel; Bethge, Christian Bibcode: 2018cosp...42E1564I Altcode: To obtain quantitative information on the magnetic field in low beta regions (i.e., upper chromosphere and above) has been increasingly important to understand the energetic phenomena of the outer solar atmosphere such as flare, coronal heating, and the solar wind acceleration. In the UV range, there are abundant spectral lines that originate in the upper chromosphere and transition region. However, the Zeeman effect in these spectral lines does not give rise to easily measurable polarization signals because of the weak magnetic field strength and the larger Doppler broadening compared with the Zeeman effect. Instead, the Hanle effect in UV lines is expected to be a suitable diagnostic tool of the magnetic field in the upper atmospheric layers. To investigate the validity of UV spectro-polarimetry and the Hanle effect, the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP), which is a NASA sounding- rocket experiment, was launched at White Sands in US on September 3, 2015. During its 5 minutes ballistic flight, it successfully performed spectro-polarimetric observations of the hydrogen Lyman-alpha line (121.57 nm) with an unprecedentedly high polarization sensitivity of 0.1% in this wavelength range. CLASP observed the linear polarization produced by scattering process in VUV lines for the first time and detected the polarization signals which indicate the operation of the Hanle effect. Following the success of CLASP, we are confident that UV spectro-polarimetry is the way to proceed, and we are planning the second flight of CLASP (CLASP2: Chromospheric LAyer SpectroPolarimeter 2). For this second flight we will carry out spectro-polarimetry in the Mg II h and k lines around 280 nm, with minimum modifications of the CLASP1 instrument. The linear polarization in the Mg II k line is induced by scattering processes and the Hanle effect, being sensitive to magnetic field strengths of 5 to 50 G. In addition, the circular polarizations in the Mg II h and k lines induced by the Zeeman effect can be measurable in at least plage and active regions. The combination of the Hanle and Zeeman effects could help us to more reliably infer the magnetic fields of the upper solar chromosphere. CLASP2 was selected for flight and is being developed for launch in the spring of 2019.Based on these sounding rocket experiments (CLASP1 and 2), we aim at establishing the strategy and refining the instrument concept for future space missions to explore the enigmatic atmospheric layers via UV spectro-polarimetry. Title: Impact of Type II Spicules in the Corona: Simulations and Synthetic Observables Authors: Martínez-Sykora, Juan; De Pontieu, Bart; De Moortel, Ineke; Hansteen, Viggo H.; Carlsson, Mats Bibcode: 2018ApJ...860..116M Altcode: 2018arXiv180506475M The role of type II spicules in the corona has been a much debated topic in recent years. This paper aims to shed light on the impact of type II spicules in the corona using novel 2.5D radiative MHD simulations, including ion-neutral interaction effects with the Bifrost code. We find that the formation of simulated type II spicules, driven by the release of magnetic tension, impacts the corona in various manners. Associated with the formation of spicules, the corona exhibits (1) magneto-acoustic shocks and flows, which supply mass to coronal loops, and (2) transversal magnetic waves and electric currents that propagate at Alfvén speeds. The transversal waves and electric currents, generated by the spicule’s driver and lasting for many minutes, are dissipated and heat the associated loop. These complex interactions in the corona can be connected with blueshifted secondary components in coronal spectral lines (red-blue asymmetries) observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart of type II spicules and propagating coronal disturbances observed with the 171 Å and 193 Å SDO/AIA channels. Title: Disentangling flows in the solar transition region Authors: Zacharias, P.; Hansteen, V. H.; Leenaarts, J.; Carlsson, M.; Gudiksen, B. V. Bibcode: 2018A&A...614A.110Z Altcode: 2018arXiv180407513Z Context. The measured average velocities in solar and stellar spectral lines formed at transition region temperatures have been difficult to interpret. The dominant redshifts observed in the lower transition region naturally leads to the question of how the upper layers of the solar (and stellar) atmosphere can be maintained. Likewise, no ready explanation has been made for the average blueshifts often found in upper transition region lines. However, realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere are able to reproduce the observed dominant line shifts and may thus hold the key to resolve these issues.
Aims: These new 3D rMHD simulations aim to shed light on how mass flows between the chromosphere and corona and on how the coronal mass is maintained. These simulations give new insights into the coupling of various atmospheric layers and the origin of Doppler shifts in the solar transition region and corona.
Methods: The passive tracer particles, so-called corks, allow the tracking of parcels of plasma over time and thus the study of changes in plasma temperature and velocity not only locally, but also in a co-moving frame. By following the trajectories of the corks, we can investigate mass and energy flows and understand the composition of the observed velocities.
Results: Our findings show that most of the transition region mass is cooling. The preponderance of transition region redshifts in the model can be explained by the higher percentage of downflowing mass in the lower and middle transition region. The average upflows in the upper transition region can be explained by a combination of both stronger upflows than downflows and a higher percentage of upflowing mass. The most common combination at lower and middle transition region temperatures are corks that are cooling and traveling downward. For these corks, a strong correlation between the pressure gradient along the magnetic field line and the velocity along the magnetic field line has been observed, indicating a formation mechanism that is related to downward propagating pressure disturbances. Corks at upper transition region temperatures are subject to a rather slow and highly variable but continuous heating process.
Conclusions: Corks are shown to be an essential tool in 3D rMHD models in order to study mass and energy flows. We have shown that most transition region plasma is cooling after having been heated slowly to upper transition region temperatures several minutes before. Downward propagating pressure disturbances are identified as one of the main mechanisms responsible for the observed redshifts at transition region temperatures.

The movie associated to Fig. 3 is available at http://www.aanda.org Title: Spectropolarimetric Inversions of the Ca II 8542 Å Line in an M-class Solar Flare Authors: Kuridze, D.; Henriques, V. M. J.; Mathioudakis, M.; Rouppe van der Voort, L.; de la Cruz Rodríguez, J.; Carlsson, M. Bibcode: 2018ApJ...860...10K Altcode: 2018arXiv180500487K We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using high-resolution full Stokes imaging spectropolarimetry of the Ca II 8542 Å line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semiempirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and nonflaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post-flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and nonflaring atmospheres shows that during the flare, the Ca II 8542 Å line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation, which led to results similar to those obtained with the NICOLE inversions. Title: Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields Authors: Gošić, M.; de la Cruz Rodríguez, J.; De Pontieu, B.; Bellot Rubio, L. R.; Carlsson, M.; Esteban Pozuelo, S.; Ortiz, A.; Polito, V. Bibcode: 2018ApJ...857...48G Altcode: 2018arXiv180207392G The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg II h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating. Title: Chromospheric heating during flux emergence in the solar atmosphere Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Danilovic, Sanja; Scharmer, Göran; Carlsson, Mats Bibcode: 2018A&A...612A..28L Altcode: 2017arXiv171200474L Context. The radiative losses in the solar chromosphere vary from 4 kW m-2 in the quiet Sun, to 20 kW m-2 in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aim. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region.
Methods: We have used observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα, and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line profiles and in addition perform multi-line, multi-species, non-local thermodynamic equilibrium (non-LTE) inversions to estimate the spatial and temporal variation of the chromospheric structure.
Results: We investigate which spectral features of Ca II K contribute to the frequency-integrated Ca II K brightness, which we use as a tracer of chromospheric radiative losses. The majority of the radiative losses are not associated with localised high-Ca II K-brightness events, but instead with a more gentle, spatially extended, and persistent heating. The frequency-integrated Ca II K brightness correlates strongly with the total linear polarization in the Ca II 854.2 nm, while the Ca II K profile shapes indicate that the bulk of the radiative losses occur in the lower chromosphere. Non-LTE inversions indicate a transition from heating concentrated around photospheric magnetic elements below log τ500 = -3 to a more space-filling and time-persistent heating above log τ500 = -4. The inferred gas temperature at log τ500 = -3.8 correlates strongly with the total linear polarization in the Ca II 854.2 nm line, suggesting that that the heating rate correlates with the strength of the horizontal magnetic field in the low chromosphere.

Movies attached to Figs. 1 and 4 are available at https://www.aanda.org/ Title: Non-LTE Calculations of the Fe I 6173 Å Line in a Flaring Atmosphere Authors: Hong, Jie; Ding, M. D.; Li, Ying; Carlsson, Mats Bibcode: 2018ApJ...857L...2H Altcode: 2018arXiv180309912H The Fe I 6173 Å line is widely used in the measurements of vector magnetic fields by instruments including the Helioseismic and Magnetic Imager (HMI). We perform non-local thermodynamic equilibrium calculations of this line based on radiative hydrodynamic simulations in a flaring atmosphere. We employ both a quiet-Sun atmosphere and a penumbral atmosphere as the initial one in our simulations. We find that, in the quiet-Sun atmosphere, the line center is obviously enhanced during an intermediate flare. The enhanced emission is contributed from both radiative backwarming in the photosphere and particle beam heating in the lower chromosphere. A blue asymmetry of the line profile also appears due to an upward mass motion in the lower chromosphere. If we take a penumbral atmosphere as the initial atmosphere, the line has a more significant response to the flare heating, showing a central emission and an obvious asymmetry. The low spectral resolution of HMI would indicate some loss of information, but the enhancement and line asymmetry are still kept. By calculating polarized line profiles, we find that the Stokes I and V profiles can be altered as a result of flare heating. Thus the distortion of this line has a crucial influence on the magnetic field measured from this line, and one should be cautious in interpreting the magnetic transients observed frequently in solar flares. Title: Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN Simulations Authors: Polito, V.; Testa, P.; Allred, J.; De Pontieu, B.; Carlsson, M.; Pereira, T. M. D.; Gošić, Milan; Reale, Fabio Bibcode: 2018ApJ...856..178P Altcode: 2018arXiv180405970P We present the results of 1D hydrodynamic simulations of coronal loops that are subject to nanoflares, caused by either in situ thermal heating or nonthermal electron (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTEs with high enough low-energy cutoff ({E}{{C}}) deposit energy in the lower TR and chromosphere, causing blueshifts (up to ∼20 km s-1) in the IRIS Si IV lines, which thermal conduction cannot reproduce. The {E}{{C}} threshold value for the blueshifts depends on the total energy of the events (≈5 keV for 1024 erg, up to 15 keV for 1025 erg). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and {E}{{C}}. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (<109 cm-3). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94 Å) for single strands is below current detectability and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg II chromospheric lines can help further constrain the properties of the heating mechanisms. Title: Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere Authors: Bjørgen, Johan P.; Sukhorukov, Andrii V.; Leenaarts, Jorrit; Carlsson, Mats; de la Cruz Rodríguez, Jaime; Scharmer, Göran B.; Hansteen, Viggo H. Bibcode: 2018A&A...611A..62B Altcode: 2017arXiv171201045B Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST), can observe the chromosphere in the H and K lines of Ca II at high spatial and spectral resolution. Accurate modeling as well as an understanding of the formation of these lines are needed to interpret the SST/CHROMIS observations. Such modeling is computationally challenging because these lines are influenced by strong departures from local thermodynamic equilibrium, three-dimensional radiative transfer, and partially coherent resonance scattering of photons. Aim. We aim to model the Ca II H and K lines in 3D model atmospheres to understand their formation and to investigate their diagnostic potential for probing the chromosphere.
Methods: We model the synthetic spectrum of Ca II using the radiative transfer code Multi3D in three different radiation-magnetohydrodynamic model atmospheres computed with the Bifrost code. We classify synthetic intensity profiles according to their shapes and study how their features are related to the physical properties in the model atmospheres. We investigate whether the synthetic data reproduce the observed spatially-averaged line shapes, center-to-limb variation and compare this data with SST/CHROMIS images.
Results: The spatially-averaged synthetic line profiles show too low central emission peaks, and too small separation between the peaks. The trends of the observed center-to-limb variation of the profiles properties are reproduced by the models. The Ca II H and K line profiles provide a temperature diagnostic of the temperature minimum and the temperature at the formation height of the emission peaks. The Doppler shift of the central depression is an excellent probe of the velocity in the upper chromosphere. Title: Hα and Hβ emission in a C3.3 solar flare: comparison between observations and simulations Authors: Zuccarello, F.; Simoes, P. J. D. A.; Capparelli, V.; Fletcher, L.; Romano, P.; Mathioudakis, M.; Cauzzi, G.; Carlsson, M.; Kuridze, D.; Keys, P. Bibcode: 2017AGUFMSH41A2742Z Altcode: This work is based on the analysis of an extremely rare set of simultaneous observations of a C3.3 solar flare in the Hα and Hβ lines at high spatial and temporal resolution, which were acquired at the Dunn Solar Telescope. Images of the C3.3 flare (SOL2014-04-22T15:22) made at various wavelengths along the Hα line profile by the Interferometric Bidimensional Spectrometer (IBIS) and in the Hβ with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband imager are analyzed to obtain the intensity evolution. The analysis shows that Hα and Hβ intensity excesses in three identified flare footpoints are well correlated in time. In the stronger footpoints, the typical value of the the Hα/Hβ intensity ratio observed is ∼ 0.4 - 0.5, in broad agreement with values obtained from a RADYN non-LTE simulation driven by an electron beam with parameters constrained by observations. The weaker footpoint has a larger Hα/Hβ ratio, again consistent with a RADYN simulation but with a smaller energy flux. Title: An IRIS Optically Thin View of the Dynamics of the Solar Chromosphere Authors: Carlsson, M. Bibcode: 2017AGUFMSH41C..04C Altcode: We analyze the formation of the O I 1356 and Cl I 1351 lines and show that they are formed in the mid-chromosphere and are optically thin. Their non-thermal line-widths are thus a direct measure of the velocity field along the line of sight. We use this insight to analyze a large set of observations from the Interface Region Imaging Spectrograph (IRIS) to study the dynamics of the Solar Chromosphere. Title: Constraints on active region coronal heating properties from observations and modeling of chromospheric, transition region, and coronal emission Authors: Testa, P.; Polito, V.; De Pontieu, B.; Carlsson, M.; Reale, F.; Allred, J. C.; Hansteen, V. H. Bibcode: 2017AGUFMSH43A2804T Altcode: We investigate coronal heating properties in active region cores in non-flaring conditions, using high spatial, spectral, and temporal resolution chromospheric/transition region/coronal observations coupled with detailed modeling. We will focus, in particular, on observations with the Interface Region Imaging Spectrograph (IRIS), joint with observations with Hinode (XRT and EIS) and SDO/AIA. We will discuss how these observations and models (1D HD and 3D MHD, with the RADYN and Bifrost codes) provide useful diagnostics of the coronal heating processes and mechanisms of energy transport. Title: Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.; de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.; Carlsson, M.; Vissers, G. Bibcode: 2017ApJ...851L...6R Altcode: 2017arXiv171104581R Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV 1403 Å UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca II K 3934 Å line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.″2 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere. Title: Chromospheric polarimetry through multiline observations of the 850-nm spectral region - II. A magnetic flux tube scenario Authors: Quintero Noda, C.; Kato, Y.; Katsukawa, Y.; Oba, T.; de la Cruz Rodríguez, J.; Carlsson, M.; Shimizu, T.; Orozco Suárez, D.; Ruiz Cobo, B.; Kubo, M.; Anan, T.; Ichimoto, K.; Suematsu, Y. Bibcode: 2017MNRAS.472..727Q Altcode: 2017arXiv170801333Q In this publication, we continue the work started in Quintero Noda et al., examining this time a numerical simulation of a magnetic flux tube concentration. Our goal is to study if the physical phenomena that take place in it, in particular, the magnetic pumping, leaves a specific imprint on the examined spectral lines. We find that the profiles from the interior of the flux tube are periodically doppler shifted following an oscillation pattern that is also reflected in the amplitude of the circular polarization signals. In addition, we analyse the properties of the Stokes profiles at the edges of the flux tube discovering the presence of linear polarization signals for the Ca II lines, although they are weak with an amplitude around 0.5 per cent of the continuum intensity. Finally, we compute the response functions to perturbations in the longitudinal field, and we estimate the field strength using the weak-field approximation. Our results indicate that the height of formation of the spectral lines changes during the magnetic pumping process, which makes the interpretation of the inferred magnetic field strength and its evolution more difficult. These results complement those from previous works, demonstrating the capabilities and limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry in a very dynamic and complex atmosphere. Title: Hα and Hβ Emission in a C3.3 Solar Flare: Comparison between Observations and Simulations Authors: Capparelli, Vincenzo; Zuccarello, Francesca; Romano, Paolo; Simões, Paulo J. A.; Fletcher, Lyndsay; Kuridze, David; Mathioudakis, Mihalis; Keys, Peter H.; Cauzzi, Gianna; Carlsson, Mats Bibcode: 2017ApJ...850...36C Altcode: 2017arXiv171004067C The hydrogen Balmer series is a basic radiative loss channel from the flaring solar chromosphere. We report here on the analysis of an extremely rare set of simultaneous observations of a solar flare in the {{H}}α and {{H}}β lines, at high spatial and temporal resolutions, that were acquired at the Dunn Solar Telescope. Images of the C3.3 flare (SOL2014-04-22T15:22) made at various wavelengths along the {{H}}α line profile by the Interferometric Bidimensional Spectrometer (IBIS) and in the {{H}}β with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband imager are analyzed to obtain the intensity evolution. The {{H}}α and {{H}}β intensity excesses in three identified flare footpoints are well-correlated in time. We examine the ratio of {{H}}α to {{H}}β flare excess, which was proposed by previous authors as a possible diagnostic of the level of electron-beam energy input. In the stronger footpoints, the typical value of the the {{H}}α /H β intensity ratio observed is ∼0.4-0.5, in broad agreement with values obtained from a RADYN non-LTE simulation driven by an electron beam with parameters constrained (as far as possible) by observation. The weaker footpoint has a larger {{H}}α /H β ratio, again consistent with a RADYN simulation, but with a smaller energy flux. The {{H}}α line profiles observed have a less prominent central reversal than is predicted by the RADYN results, but can be brought into agreement if the {{H}}α -emitting material has a filling factor of around 0.2-0.3. Title: The Formation of IRIS Diagnostics. IX. The Formation of the C I 135.58 NM Line in the Solar Atmosphere Authors: Lin, Hsiao-Hsuan; Carlsson, Mats; Leenaarts, Jorrit Bibcode: 2017ApJ...846...40L Altcode: 2017arXiv170809426L The C I 135.58 nm line is located in the wavelength range of NASA’s Interface Region Imaging Spectrograph (IRIS) small explorer mission. We study the formation and diagnostic potential of this line by means of non local-thermodynamic-equilibrium modeling, employing both 1D and 3D radiation-magnetohydrodynamic models. The C I/C II ionization balance is strongly influenced by photoionization by Lyα emission. The emission in the C I 135.58 nm line is dominated by a recombination cascade and the line forming region is optically thick. The Doppler shift of the line correlates strongly with the vertical velocity in its line forming region, which is typically located at 1.5 Mm height. With IRIS, the C I 135.58 nm line is usually observed together with the O I 135.56 nm line, and from the Doppler shift of both lines, we obtain the velocity difference between the line forming regions of the two lines. From the ratio of the C I/O I line core intensity, we can determine the distance between the C I and the O I forming layers. Combined with the velocity difference, the velocity gradient at mid-chromospheric heights can be derived. The C I/O I total intensity line ratio is correlated with the inverse of the electron density in the mid-chromosphere. We conclude that the C I 135.58 nm line is an excellent probe of the middle chromosphere by itself, and together with the O I 135.56 nm line the two lines provide even more information, which complements other powerful chromospheric diagnostics of IRIS such as the Mg II h and k lines and the C II lines around 133.5 nm. Title: Solar polarimetry through the K I lines at 770 nm Authors: Quintero Noda, C.; Uitenbroek, H.; Katsukawa, Y.; Shimizu, T.; Oba, T.; Carlsson, M.; Orozco Suárez, D.; Ruiz Cobo, B.; Kubo, M.; Anan, T.; Ichimoto, K.; Suematsu, Y. Bibcode: 2017MNRAS.470.1453Q Altcode: 2017arXiv170510002Q We characterize the K I D1 & D2 lines in order to determine whether they could complement the 850 nm window, containing the Ca II infrared triplet lines and several Zeeman sensitive photospheric lines, that was studied previously. We investigate the effect of partial redistribution on the intensity profiles, their sensitivity to changes in different atmospheric parameters, and the spatial distribution of Zeeman polarization signals employing a realistic magnetohydrodynamic simulation. The results show that these lines form in the upper photosphere at around 500 km, and that they are sensitive to the line-of-sight velocity and magnetic field strength at heights where neither the photospheric lines nor the Ca II infrared lines are. However, at the same time, we found that their sensitivity to the temperature essentially comes from the photosphere. Then, we conclude that the K I lines provide a complement to the lines in the 850 nm window for the determination of atmospheric parameters in the upper photosphere, especially for the line-of-sight velocity and the magnetic field. Title: Two-dimensional Radiative Magnetohydrodynamic Simulations of Partial Ionization in the Chromosphere. II. Dynamics and Energetics of the Low Solar Atmosphere Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats; Hansteen, Viggo H.; Nóbrega-Siverio, Daniel; Gudiksen, Boris V. Bibcode: 2017ApJ...847...36M Altcode: 2017arXiv170806781M We investigate the effects of interactions between ions and neutrals on the chromosphere and overlying corona using 2.5D radiative MHD simulations with the Bifrost code. We have extended the code capabilities implementing ion-neutral interaction effects using the generalized Ohm’s law, I.e., we include the Hall term and the ambipolar diffusion (Pedersen dissipation) in the induction equation. Our models span from the upper convection zone to the corona, with the photosphere, chromosphere, and transition region partially ionized. Our simulations reveal that the interactions between ionized particles and neutral particles have important consequences for the magnetothermodynamics of these modeled layers: (1) ambipolar diffusion increases the temperature in the chromosphere; (2) sporadically the horizontal magnetic field in the photosphere is diffused into the chromosphere, due to the large ambipolar diffusion; (3) ambipolar diffusion concentrates electrical currents, leading to more violent jets and reconnection processes, resulting in (3a) the formation of longer and faster spicules, (3b) heating of plasma during the spicule evolution, and (3c) decoupling of the plasma and magnetic field in spicules. Our results indicate that ambipolar diffusion is a critical ingredient for understanding the magnetothermodynamic properties in the chromosphere and transition region. The numerical simulations have been made publicly available, similar to previous Bifrost simulations. This will allow the community to study realistic numerical simulations with a wider range of magnetic field configurations and physics modules than previously possible. Title: Impact of Type II Spicules into the Corona Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats; Hansteen, Viggo H.; Pereira, Tiago M. D. Bibcode: 2017SPD....4810403M Altcode: In the lower solar atmosphere, the chromosphere is permeated by jets, in which plasma is propelled at speeds of 50-150 km/s into the Sun’s atmosphere or corona. Although these spicules may play a role in heating the million-degree corona and are associated with Alfvén waves that help drive the solar wind, their generation remains mysterious. We implemented in the radiative MHD Bifrost code the effects of partial ionization using the generalized Ohm’s law. This code also solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. The ion-neutral collision frequency is computed using recent studies that improved the estimation of the cross sections under chromospheric conditions (Vranjes & Krstic 2013). Self-consistently driven jets (spicules type II) in magnetohydrodynamic simulations occur ubiquitously when magnetic tension is confined and transported upwards through interactions between ions and neutrals, and impulsively released to drive flows, heat plasma, generate Alfvén waves, and may play an important role in maintaining the substructure of loop fans. This mechanism explains how spicular plasma can be heated to millions of degrees and how Alfvén waves are generated in the chromosphere. Title: Realistic radiative MHD simulation of a solar flare Authors: Rempel, Matthias D.; Cheung, Mark; Chintzoglou, Georgios; Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto; DeRosa, Marc L.; Viktorovna Malanushenko, Anna; Hansteen, Viggo H.; De Pontieu, Bart; Carlsson, Mats; Gudiksen, Boris; McIntosh, Scott W. Bibcode: 2017SPD....4840001R Altcode: We present a recently developed version of the MURaM radiative MHD code that includes coronal physics in terms of optically thin radiative loss and field aligned heat conduction. The code employs the "Boris correction" (semi-relativistic MHD with a reduced speed of light) and a hyperbolic treatment of heat conduction, which allow for efficient simulations of the photosphere/corona system by avoiding the severe time-step constraints arising from Alfven wave propagation and heat conduction. We demonstrate that this approach can be used even in dynamic phases such as a flare. We consider a setup in which a flare is triggered by flux emergence into a pre-existing bipolar active region. After the coronal energy release, efficient transport of energy along field lines leads to the formation of flare ribbons within seconds. In the flare ribbons we find downflows for temperatures lower than ~5 MK and upflows at higher temperatures. The resulting soft X-ray emission shows a fast rise and slow decay, reaching a peak corresponding to a mid C-class flare. The post reconnection energy release in the corona leads to average particle energies reaching 50 keV (500 MK under the assumption of a thermal plasma). We show that hard X-ray emission from the corona computed under the assumption of thermal bremsstrahlung can produce a power-law spectrum due to the multi-thermal nature of the plasma. The electron energy flux into the flare ribbons (classic heat conduction with free streaming limit) is highly inhomogeneous and reaches peak values of about 3x1011 erg/cm2/s in a small fraction of the ribbons, indicating regions that could potentially produce hard X-ray footpoint sources. We demonstrate that these findings are robust by comparing simulations computed with different values of the saturation heat flux as well as the "reduced speed of light". Title: RADYN Simulations of Non-thermal and Thermal Models of Ellerman Bombs Authors: Hong, Jie; Carlsson, Mats; Ding, M. D. Bibcode: 2017ApJ...845..144H Altcode: 2017arXiv170705514H Ellerman bombs (EBs) are brightenings in the Hα line wings that are believed to be caused by magnetic reconnection in the lower atmosphere. To study the response and evolution of the chromospheric line profiles, we perform radiative hydrodynamic simulations of EBs using both non-thermal and thermal models. Overall, these models can generate line profiles that are similar to observations. However, in non-thermal models we find dimming in the Hα line wings and continuum when the heating begins, while for the thermal models dimming occurs only in the Hα line core, and with a longer lifetime. This difference in line profiles can be used to determine whether an EB is dominated by non-thermal heating or thermal heating. In our simulations, if a higher heating rate is applied, then the Hα line will be unrealistically strong and there are still no clear UV burst signatures. Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter Authors: Rachmeler, Laurel; E McKenzie, David; Ishikawa, Ryohko; Trujillo Bueno, Javier; Auchère, Frédéric; Kobayashi, Ken; Winebarger, Amy; Bethge, Christian; Kano, Ryouhei; Kubo, Masahito; Song, Donguk; Narukage, Noriyuki; Ishikawa, Shin-nosuke; De Pontieu, Bart; Carlsson, Mats; Yoshida, Masaki; Belluzzi, Luca; Stepan, Jiri; del Pino Alemná, Tanausú; Ballester, Ernest Alsina; Asensio Ramos, Andres Bibcode: 2017SPD....4811010R Altcode: We present the instrument, science case, and timeline of the CLASP2 sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter) sounding rocket flight in 2015 resulted in the first-ever linear polarization measurements of solar hydrogen Lyman-alpha line, which is sensitive to the Hanle effect and can be used to constrain the magnetic field and geometric complexity of the upper chromosphere. Ly-alpha is one of several upper chromospheric lines that contain magnetic information. In the spring of 2019, we will re-fly the modified CLASP telescope to measure the full Stokes profile of Mg II h & k near 280 nm. This set of lines is sensitive to the upper chromospheric magnetic field via both the Hanle and the Zeeman effects. Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter Authors: Rachmeler, Laurel A.; McKenzie, D. E.; Ishikawa, R.; Trujillo-Bueno, J.; Auchere, F.; Kobayashi, K.; Winebarger, A.; Bethge, C.; Kano, R.; Kubo, M.; Song, D.; Narukage, N.; Ishikawa, S.; De Pontieu, B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Stepan, J.; del Pino Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A. Bibcode: 2017shin.confE..79R Altcode: We present the instrument, science case, and timeline of the CLASP2 sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter) sounding rocket flight in 2015 resulted in the first-ever linear polarization measurements of solar hydrogen Lyman-alpha line, which is sensitive to the Hanle effect and can be used to constrain the magnetic field and geometric complexity of the upper chromosphere. Ly-alpha is one of several upper chromospheric lines that contain magnetic information. In the spring of 2019, we will re-fly the modified CLASP telescope to measure the full Stokes profile of Mg II h & k near 280 nm. This set of lines is sensitive to the upper chromospheric magnetic field via both the Hanle and the Zeeman effects. Title: On the generation of solar spicules and Alfvénic waves Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; Carlsson, M.; Pereira, T. M. D. Bibcode: 2017Sci...356.1269M Altcode: 2017arXiv171007559M In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves. Title: Millimeter radiation from a 3D model of the solar atmosphere. II. Chromospheric magnetic field Authors: Loukitcheva, M.; White, S. M.; Solanki, S. K.; Fleishman, G. D.; Carlsson, M. Bibcode: 2017A&A...601A..43L Altcode: 2017arXiv170206018L
Aims: We use state-of-the-art, three-dimensional non-local thermodynamic equilibrium (non-LTE) radiative magnetohydrodynamic simulations of the quiet solar atmosphere to carry out detailed tests of chromospheric magnetic field diagnostics from free-free radiation at millimeter and submillimeter wavelengths (mm/submm).
Methods: The vertical component of the magnetic field was deduced from the mm/submm brightness spectra and the degree of circular polarization synthesized at millimeter frequencies. We used the frequency bands observed by the Atacama Large Millimeter/Submillimeter Array (ALMA) as a convenient reference. The magnetic field maps obtained describe the longitudinal magnetic field at the effective formation heights of the relevant wavelengths in the solar chromosphere.
Results: The comparison of the deduced and model chromospheric magnetic fields at the spatial resolution of both the model and current observations demonstrates a good correlation, but has a tendency to underestimate the model field. The systematic discrepancy of about 10% is probably due to averaging of the restored field over the heights contributing to the radiation, weighted by the strength of the contribution. On the whole, the method of probing the longitudinal component of the magnetic field with free-free emission at mm/submm wavelengths is found to be applicable to measurements of the weak quiet-Sun magnetic fields. However, successful exploitation of this technique requires very accurate measurements of the polarization properties (primary beam and receiver polarization response) of the antennas, which will be the principal factor that determines the level to which chromospheric magnetic fields can be measured.
Conclusions: Consequently, high-resolution and high-precision observations of circularly polarized radiation at millimeter wavelengths can be a powerful tool for producing chromospheric longitudinal magnetograms. Title: Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.; Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa, Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R. Bibcode: 2017ApJ...841...31I Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter is a sounding rocket experiment that has provided the first successful measurement of the linear polarization produced by scattering processes in the hydrogen Lyα line (121.57 nm) radiation of the solar disk. In this paper, we report that the Si III line at 120.65 nm also shows scattering polarization and we compare the scattering polarization signals observed in the Lyα and Si III lines in order to search for observational signatures of the Hanle effect. We focus on four selected bright structures and investigate how the U/I spatial variations vary between the Lyα wing, the Lyα core, and the Si III line as a function of the total unsigned photospheric magnetic flux estimated from Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations. In an internetwork region, the Lyα core shows an antisymmetric spatial variation across the selected bright structure, but it does not show it in other more magnetized regions. In the Si III line, the spatial variation of U/I deviates from the above-mentioned antisymmetric shape as the total unsigned photospheric magnetic flux increases. A plausible explanation of this difference is the operation of the Hanle effect. We argue that diagnostic techniques based on the scattering polarization observed simultaneously in two spectral lines with very different sensitivities to the Hanle effect, like Lyα and Si III, are of great potential interest for exploring the magnetism of the upper solar chromosphere and transition region. Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson, M.; Rouppe van der Voort, L.; Leenaarts, J. Bibcode: 2017ApJ...839...22H Altcode: 2017arXiv170402872H A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma “bombs” (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere. Title: Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration Authors: Giono, G.; Ishikawa, R.; Narukage, N.; Kano, R.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Bando, T.; Hara, H.; Suematsu, Y.; Winebarger, A.; Kobayashi, K.; Auchère, F.; Trujillo Bueno, J.; Tsuneta, S.; Shimizu, T.; Sakao, T.; Cirtain, J.; Champey, P.; Asensio Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M. Bibcode: 2017SoPh..292...57G Altcode: The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding rocket instrument designed to measure for the first time the linear polarization of the hydrogen Lyman-α line (121.6 nm). The instrument was successfully launched on 3 September 2015 and observations were conducted at the solar disc center and close to the limb during the five-minutes flight. In this article, the disc center observations are used to provide an in-flight calibration of the instrument spurious polarization. The derived in-flight spurious polarization is consistent with the spurious polarization levels determined during the pre-flight calibration and a statistical analysis of the polarization fluctuations from solar origin is conducted to ensure a 0.014% precision on the spurious polarization. The combination of the pre-flight and the in-flight polarization calibrations provides a complete picture of the instrument response matrix, and a proper error transfer method is used to confirm the achieved polarization accuracy. As a result, the unprecedented 0.1% polarization accuracy of the instrument in the vacuum ultraviolet is ensured by the polarization calibration. Title: Discovery of Scattering Polarization in the Hydrogen Lyα Line of the Solar Disk Radiation Authors: Kano, R.; Trujillo Bueno, J.; Winebarger, A.; Auchère, F.; Narukage, N.; Ishikawa, R.; Kobayashi, K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Goto, M.; Belluzzi, L.; Štěpán, J.; Asensio Ramos, A.; Manso Sainz, R.; Champey, P.; Cirtain, J.; De Pontieu, B.; Casini, R.; Carlsson, M. Bibcode: 2017ApJ...839L..10K Altcode: 2017arXiv170403228K There is a thin transition region (TR) in the solar atmosphere where the temperature rises from 10,000 K in the chromosphere to millions of degrees in the corona. Little is known about the mechanisms that dominate this enigmatic region other than the magnetic field plays a key role. The magnetism of the TR can only be detected by polarimetric measurements of a few ultraviolet (UV) spectral lines, the Lyα line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV spectrum) being of particular interest given its sensitivity to the Hanle effect (the magnetic-field-induced modification of the scattering line polarization). We report the discovery of linear polarization produced by scattering processes in the Lyα line, obtained with the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket experiment. The Stokes profiles observed by CLASP in quiet regions of the solar disk show that the Q/I and U/I linear polarization signals are of the order of 0.1% in the line core and up to a few percent in the nearby wings, and that both have conspicuous spatial variations with scales of ∼10 arcsec. These observations help constrain theoretical models of the chromosphere-corona TR and extrapolations of the magnetic field from photospheric magnetograms. In fact, the observed spatial variation from disk to limb of polarization at the line core and wings already challenge the predictions from three-dimensional magnetohydrodynamical models of the upper solar chromosphere. Title: Hydrogen Balmer Line Broadening in Solar and Stellar Flares Authors: Kowalski, Adam F.; Allred, Joel C.; Uitenbroek, Han; Tremblay, Pier-Emmanuel; Brown, Stephen; Carlsson, Mats; Osten, Rachel A.; Wisniewski, John P.; Hawley, Suzanne L. Bibcode: 2017ApJ...837..125K Altcode: 2017arXiv170203321K The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTE radiative-transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are overbroadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a “multithread” model improves the agreement with the observations. We revisit the three-component phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a “hot spot” atmosphere heated by an ultrarelativistic electron beam with reasonable filling factors: ∼0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission. Title: Chromospheric polarimetry through multiline observations of the 850-nm spectral region Authors: Quintero Noda, C.; Shimizu, T.; Katsukawa, Y.; de la Cruz Rodríguez, J.; Carlsson, M.; Anan, T.; Oba, T.; Ichimoto, K.; Suematsu, Y. Bibcode: 2017MNRAS.464.4534Q Altcode: 2016arXiv161006651Q Future solar missions and ground-based telescopes aim to understand the magnetism of the solar chromosphere. We performed a supporting study in Quintero Noda et al. focused on the infrared Ca II 8542 Å line and we concluded that it is one of the best candidates because it is sensitive to a large range of atmospheric heights, from the photosphere to the middle chromosphere. However, we believe that it is worth trying to improve the results produced by this line observing additional spectral lines. In that regard, we examined the neighbourhood solar spectrum looking for spectral lines which could increase the sensitivity to the atmospheric parameters. Interestingly, we discovered several photospheric lines which greatly improve the photospheric sensitivity to the magnetic field vector. Moreover, they are located close to a second chromospheric line which also belongs to the Ca II infrared triplet, I.e. the Ca II 8498 Å line, and enhances the sensitivity to the atmospheric parameters at chromospheric layers. We conclude that the lines in the vicinity of the Ca II 8542 Å line not only increase its sensitivity to the atmospheric parameters at all layers, but also they constitute an excellent spectral window for chromospheric polarimetry. Title: The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29 Authors: Kowalski, Adam F.; Allred, Joel C.; Daw, Adrian; Cauzzi, Gianna; Carlsson, Mats Bibcode: 2017ApJ...836...12K Altcode: 2016arXiv160907390K The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright continuum emission in the far- and near-ultraviolet (NUV) and highly asymmetric chromospheric emission lines, providing long-sought constraints on the heating mechanisms of the lower atmosphere in solar flares. We analyze the continuum and emission line data from the Interface Region Imaging Spectrograph (IRIS) of the brightest flaring magnetic footpoints in this flare. We compare the NUV spectra of the brightest pixels to new radiative-hydrodynamic predictions calculated with the RADYN code using constraints on a nonthermal electron beam inferred from the collisional thick-target modeling of hard X-ray data from Reuven Ramaty High Energy Solar Spectroscopic Imager. We show that the atmospheric response to a high beam flux density satisfactorily achieves the observed continuum brightness in the NUV. The NUV continuum emission in this flare is consistent with hydrogen (Balmer) recombination radiation that originates from low optical depth in a dense chromospheric condensation and from the stationary beam-heated layers just below the condensation. A model producing two flaring regions (a condensation and stationary layers) in the lower atmosphere is also consistent with the asymmetric Fe II chromospheric emission line profiles observed in the impulsive phase. Title: On the generation of solar spicules and Alfvén waves Authors: Carlsson, Mats Bibcode: 2017psio.confE..46C Altcode: No abstract at ADS Title: Formation of the helium extreme-UV resonance lines Authors: Golding, T. P.; Leenaarts, J.; Carlsson, M. Bibcode: 2017A&A...597A.102G Altcode: 2016arXiv161000352G Context. While classical models successfully reproduce intensities of many transition region lines, they predict helium extreme-UV (EUV) line intensities roughly an order of magnitude lower than the observed value.
Aims: Our aim is to determine the relevant formation mechanism(s) of the helium EUV resonance lines capable of explaining the high intensities under quiet Sun conditions.
Methods: We synthesised and studied the emergent spectra from a 3D radiation-magnetohydrodynamics simulation model. The effects of coronal illumination and non-equilibrium ionisation of hydrogen and helium are included self-consistently in the numerical simulation.
Results: Radiative transfer calculations result in helium EUV line intensities that are an order of magnitude larger than the intensities calculated under the classical assumptions. The enhanced intensity of He Iλ584 is primarily caused by He II recombination cascades. The enhanced intensity of He IIλ304 and He IIλ256 is caused primarily by non-equilibrium helium ionisation.
Conclusions: The analysis shows that the long standing problem of the high helium EUV line intensities disappears when taking into account optically thick radiative transfer and non-equilibrium ionisation effects. Title: A Chromospheric Flare Model Consisting of Two Dynamical Layers: Critical Tests from IRIS Data of Solar Flares Authors: Kowalski, Adam; Allred, Joel C.; Daw, Adrian N.; Cauzzi, Gianna; Carlsson, Mats; Inglis, Andrew; O'Neill, Aaron; Mathioudakis, Mihalis; Uitenbroek, Han Bibcode: 2017AAS...22933902K Altcode: Recent 1D radiative-hydrodynamic simulations of flares have shown that a heated, chromospheric compression layer and a stationary layer, just below the compression, are produced in response to high flux electron beam heating. The hot blackbody-like continuum and redshifted intensity in singly ionized chromospheric lines in these model predictions are generally consistent with broad wavelength coverage spectra of M dwarf flares and with high spectral resolution observations of solar flares, respectively. We critically test this two-component chromospheric flare model against the Fe II profiles and NUV continuum brightness for several X-class solar flares observed with the Interface Region Imaging Spectrograph (IRIS). We present several new predictions for the Daniel K. Inoue Solar Telescope (DKIST). Title: SOLARNET WP 100: Access to Science Data Centres. Space missions Authors: Carlsson, Mats Bibcode: 2017psio.confE.101C Altcode: No abstract at ADS Title: Discovery of Ubiquitous Fast-Propagating Intensity Disturbances by the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) Authors: Kubo, M.; Katsukawa, Y.; Suematsu, Y.; Kano, R.; Bando, T.; Narukage, N.; Ishikawa, R.; Hara, H.; Giono, G.; Tsuneta, S.; Ishikawa, S.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auchère, F.; Trujillo Bueno, J.; Asensio Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M. Bibcode: 2016ApJ...832..141K Altcode: High-cadence observations by the slit-jaw (SJ) optics system of the sounding rocket experiment known as the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances that recurrently propagate in either the chromosphere or the transition region or both at a speed much higher than the speed of sound. The CLASP/SJ instrument provides a time series of two-dimensional images taken with broadband filters centered on the Lyα line at a 0.6 s cadence. The multiple fast-propagating intensity disturbances appear in the quiet Sun and in an active region, and they are clearly detected in at least 20 areas in a field of view of 527″ × 527″ during the 5 minute observing time. The apparent speeds of the intensity disturbances range from 150 to 350 km s-1, and they are comparable to the local Alfvén speed in the transition region. The intensity disturbances tend to propagate along bright elongated structures away from areas with strong photospheric magnetic fields. This suggests that the observed fast-propagating intensity disturbances are related to the magnetic canopy structures. The maximum distance traveled by the intensity disturbances is about 10″, and the widths are a few arcseconds, which are almost determined by a pixel size of 1.″03. The timescale of each intensity pulse is shorter than 30 s. One possible explanation for the fast-propagating intensity disturbances observed by CLASP is magnetohydrodynamic fast-mode waves. Title: On the Misalignment between Chromospheric Features and the Magnetic Field on the Sun Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats; Hansteen, Viggo Bibcode: 2016ApJ...831L...1M Altcode: 2016arXiv160702551M Observations of the upper chromosphere show an enormous amount of intricate fine structure. Much of this comes in the form of linear features, which are most often assumed to be well aligned with the direction of the magnetic field in the low plasma β regime that is thought to dominate the upper chromosphere. We use advanced radiative magnetohydrodynamic simulations, including the effects of ion-neutral interactions (using the generalized Ohm’s law) in the partially ionized chromosphere, to show that the magnetic field is often not well aligned with chromospheric features. This occurs where the ambipolar diffusion is large, I.e., ions and neutral populations decouple as the ion-neutral collision frequency drops, allowing the field to slip through the neutral population; where currents perpendicular to the field are strong; and where thermodynamic timescales are longer than or similar to those of ambipolar diffusion. We find this often happens in dynamic spicule or fibril-like features at the top of the chromosphere. This has important consequences for field extrapolation methods, which increasingly use such upper chromospheric features to help constrain the chromospheric magnetic field: our results invalidate the underlying assumption that these features are aligned with the field. In addition, our results cast doubt on results from 1D hydrodynamic models, which assume that plasma remains on the same field lines. Finally, our simulations show that ambipolar diffusion significantly alters the amount of free energy available in the coronal part of our simulated volume, which is likely to have consequences for studies of flare initiation. Title: The cause of spatial structure in solar He I 1083 nm multiplet images Authors: Leenaarts, Jorrit; Golding, Thomas; Carlsson, Mats; Libbrecht, Tine; Joshi, Jayant Bibcode: 2016A&A...594A.104L Altcode: 2016arXiv160800838L Context. The He I 1083 nm is a powerful diagnostic for inferring properties of the upper solar chromosphere, in particular for the magnetic field. The basic formation of the line in one-dimensional models is well understood, but the influence of the complex three-dimensional structure of the chromosphere and corona has however never been investigated. This structure must play an essential role because images taken in He I 1083 nm show structures with widths down to 100 km.
Aims: We aim to understand the effect of the three-dimensional temperature and density structure in the solar atmosphere on the formation of the He I 1083 nm line.
Methods: We solved the non-LTE radiative transfer problem assuming statistical equilibrium for a simple nine-level helium atom that nevertheless captures all essential physics. As a model atmosphere we used a snapshot from a 3D radiation-MHD simulation computed with the Bifrost code. Ionising radiation from the corona was self-consistently taken into account.
Results: The emergent intensity in the He I 1083 nm is set by the source function and the opacity in the upper chromosphere. The former is dominated by scattering of photospheric radiation and does not vary much with spatial location. The latter is determined by the photonionisation rate in the He I ground state continuum, as well as the electron density in the chromosphere. The spatial variation of the flux of ionising radiation is caused by the spatially-structured emissivity of the ionising photons from material at T ≈ 100 kK in the transition region. The hotter coronal material produces more ionising photons, but the resulting radiation field is smooth and does not lead to small-scale variation of the UV flux. The corrugation of the transition region further increases the spatial variation of the amount of UV radiation in the chromosphere. Finally we find that variations in the chromospheric electron density also cause strong variation in He I 1083 nm opacity. We compare our findings to observations using SST, IRIS and SDO/AIA data.

A movie associated to Fig. 4 is available at http://www.aanda.org 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: The SPICE Spectral Imager on Solar Orbiter: Linking the Sun to the Heliosphere Authors: Fludra, Andrzej; Haberreiter, Margit; Peter, Hardi; Vial, Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina; Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson, William; Gabriel, Alan; Morris, Nigel; Caldwell, Martin; Auchere, Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.; DeForest, Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne; Janvier, Miho; Wimmer-Schweingruber, Robert; Griffin, Douglas; Davila, Joseph; Giunta, Alessandra; Waltham, Nick; Eccleston, Paul; Gottwald, Alexander; Klein, Roman; Hanley, John; Walls, Buddy; Howe, Chris; Schuehle, Udo Bibcode: 2016cosp...41E.607F Altcode: The SPICE (Spectral Imaging of the Coronal Environment) instrument is one of the key remote sensing instruments onboard the upcoming Solar Orbiter Mission. SPICE has been designed to contribute to the science goals of the mission by investigating the source regions of outflows and ejection processes which link the solar surface and corona to the heliosphere. In particular, SPICE will provide quantitative information on the physical state and composition of the solar atmosphere plasma. For example, SPICE will access relative abundances of ions to study the origin and the spatial/temporal variations of the 'First Ionization Potential effect', which are key signatures to trace the solar wind and plasma ejections paths within the heliosphere. Here we will present the instrument and its performance capability to attain the scientific requirements. We will also discuss how different observation modes can be chosen to obtain the best science results during the different orbits of the mission. To maximize the scientific return of the instrument, the SPICE team is working to optimize the instrument operations, and to facilitate the data access and their exploitation. Title: Solar abundances with the SPICE spectral imager on Solar Orbiter Authors: Giunta, Alessandra; Haberreiter, Margit; Peter, Hardi; Vial, Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina; Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson, William; Bocchialini, Karine; Gabriel, Alan; Morris, Nigel; Caldwell, Martin; Auchere, Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.; DeForest, Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne; Janvier, Miho; Wimmer-Schweingruber, Robert; Griffin, Douglas; Baudin, Frederic; Davila, Joseph; Fludra, Andrzej; Waltham, Nick; Eccleston, Paul; Gottwald, Alexander; Klein, Roman; Hanley, John; Walls, Buddy; Howe, Chris; Schuehle, Udo; Gyo, Manfred; Pfiffner, Dany Bibcode: 2016cosp...41E.681G Altcode: Elemental composition of the solar atmosphere and in particular abundance bias of low and high First Ionization Potential (FIP) elements are a key tracer of the source regions of the solar wind. These abundances and their spatio-temporal variations, as well as the other plasma parameters , will be derived by the SPICE (Spectral Imaging of the Coronal Environment) EUV spectral imager on the upcoming Solar Orbiter mission. SPICE is designed to provide spectroheliograms (spectral images) using a core set of emission lines arising from ions of both low-FIP and high-FIP elements. These lines are formed over a wide range of temperatures, enabling the analysis of the different layers of the solar atmosphere. SPICE will use these spectroheliograms to produce dynamic composition maps of the solar atmosphere to be compared to in-situ measurements of the solar wind composition of the same elements (i.e. O, Ne, Mg, Fe). This will provide a tool to study the connectivity between the spacecraft (the Heliosphere) and the Sun. We will discuss the SPICE capabilities for such composition measurements. Title: Emergence of Granular-sized Magnetic Bubbles Through the Solar Atmosphere. III. The Path to the Transition Region Authors: Ortiz, Ada; Hansteen, Viggo H.; Bellot Rubio, Luis Ramón; de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Carlsson, Mats; Rouppe van der Voort, Luc Bibcode: 2016ApJ...825...93O Altcode: 2016arXiv160400302O We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Hα 656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0.″14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C II, transition region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far- and near-ultraviolet, of which we have used the Si IV 140.3 and the Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona. Title: The Appearance of Spicules in High Resolution Observations of Ca II H and Hα Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Carlsson, Mats Bibcode: 2016ApJ...824...65P Altcode: 2016arXiv160403116P Solar spicules are chromospheric fibrils that appear everywhere on the Sun, yet their origin is not understood. Using high resolution observations of spicules obtained with the Swedish 1 m Solar Telescope, we aim to understand how spicules appear in filtergrams and Dopplergrams, how they compare in Ca II H and Hα filtergrams, and what can make them appear and disappear. We find that spicules display a rich and detailed spatial structure, and show a distribution of transverse velocities that, when aligned with the line of sight, can make them appear at different Hα wing positions. They become more abundant at positions closer to the line core, reflecting a distribution of Doppler shifts and widths. In Hα width maps they stand out as bright features both on disk and off limb, reflecting their large Doppler motions and possibly higher temperatures than in the typical Hα formation region. Spicule lifetimes measured from narrowband images at only a few positions will be an underestimate because Doppler shifts can make them disappear prematurely from such images; for such cases, width maps are a more robust tool. In Hα and Ca II H filtergrams, off-limb spicules essentially have the same properties, appearance, and evolution. We find that the sudden appearance of spicules can be explained by Doppler shifts from their transverse motions, and does not require other convoluted explanations. Title: Physics & Diagnostics of the Drivers of Solar Eruptions Authors: Cheung, Mark; Rempel, Matthias D.; Martinez-Sykora, Juan; Testa, Paola; Hansteen, Viggo H.; Viktorovna Malanushenko, Anna; Sainz Dalda, Alberto; DeRosa, Marc L.; De Pontieu, Bart; Carlsson, Mats; Chen, Feng; McIntosh, Scott W.; Gudiksen, Boris Bibcode: 2016SPD....47.0607C Altcode: We provide an update on our NASA Heliophysics Grand Challenges Research (HGCR) project on the ‘Physics & Diagnostics of the Drivers of Solar Eruptions’. This presentation will focus on results from a data-inspired, 3D radiative MHD model of a solar flare. The model flare results from the interaction of newly emerging flux with a pre-existing active region. Synthetic observables from the model reproduce observational features compatible with actual flares. These include signatures of coronal magnetic reconnection, chromospheric evaporation, EUV flare arcades, sweeping motion of flare ribbons and sunquakes. Title: Spectro-polarimetric observation in UV with CLASP to probe the chromosphere and transition region Authors: Kano, Ryouhei; Ishikawa, Ryohko; Winebarger, Amy R.; Auchère, Frédéric; Trujillo Bueno, Javier; Narukage, Noriyuki; Kobayashi, Ken; Bando, Takamasa; Katsukawa, Yukio; Kubo, Masahito; Ishikawa, Shin-Nosuke; Giono, Gabriel; Hara, Hirohisa; Suematsu, Yoshinori; Shimizu, Toshifumi; Sakao, Taro; Tsuneta, Saku; Ichimoto, Kiyoshi; Goto, Motoshi; Cirtain, Jonathan W.; De Pontieu, Bart; Casini, Roberto; Manso Sainz, Rafael; Asensio Ramos, Andres; Stepan, Jiri; Belluzzi, Luca; Carlsson, Mats Bibcode: 2016SPD....4710107K Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a NASA sounding-rocket experiment that was performed in White Sands in the US on September 3, 2015. During its 5-minute ballistic flight, CLASP successfully made the first spectro-polarimetric observation in the Lyman-alpha line (121.57 nm) originating in the chromosphere and transition region. Since the Lyman-alpha polarization is sensitive to magnetic field of 10-100 G by the Hanle effect, we aim to infer the magnetic field information in such upper solar atmosphere with this experiment.The obtained CLASP data showed that the Lyman-alpha scattering polarization is about a few percent in the wings and the order of 0.1% in the core near the solar limb, as it had been theoretically predicted, and that both polarization signals have a conspicuous spatio-temporal variability. CLASP also observed another upper-chromospheric line, Si III (120.65 nm), whose critical field strength for the Hanle effect is 290 G, and showed a measurable scattering polarization of a few % in this line. The polarization properties of the Si III line could facilitate the interpretation of the scattering polarization observed in the Lyman-alpha line.In this presentation, we would like to show how the upper chromosphere and transition region are seen in the polarization of these UV lines and discuss the possible source of these complicated polarization signals. Title: Division G Commission 36: Theory of Stellar Atmospheres Authors: Puls, Joachim; Hubeny, Ivan; Asplund, Martin; Allard, France; Allende Prieto, Carlos; Ayres, Thomas R.; Carlsson, Mats; Gustafsson, Bengt; Kudritzki, Rolf-Peter; Ryabchikova, Tatiana A. Bibcode: 2016IAUTA..29..453P Altcode: Different from previous triennial reports, this report covers the activities of IAU Commission 36 `Theory of Stellar Atmospheres' over the past six years†, and will be the last report from the `old' Commission 36. After the General Assembly in Honolulu (August 2015), a new Commission `Stellar and Planetary Atmospheres' (C.G5, under Division G, `Stars and Stellar Physics') has come into life, and will continue our work devoted to the outer envelopes of stars, as well as extend it to the atmospheres of planets (see Sect. 4). Title: Modeling Repeatedly Flaring δ Sunspots Authors: Chatterjee, Piyali; Hansteen, Viggo; Carlsson, Mats Bibcode: 2016PhRvL.116j1101C Altcode: 2016arXiv160100749C Active regions (ARs) appearing on the surface of the Sun are classified into α , β , γ , and δ by the rules of the Mount Wilson Observatory, California on the basis of their topological complexity. Amongst these, the δ sunspots are known to be superactive and produce the most x-ray flares. Here, we present results from a simulation of the Sun by mimicking the upper layers and the corona, but starting at a more primitive stage than any earlier treatment. We find that this initial state consisting of only a thin subphotospheric magnetic sheet breaks into multiple flux tubes which evolve into a colliding-merging system of spots of opposite polarity upon surface emergence, similar to those often seen on the Sun. The simulation goes on to produce many exotic δ sunspot associated phenomena: repeated flaring in the range of typical solar flare energy release and ejective helical flux ropes with embedded cool-dense plasma filaments resembling solar coronal mass ejections. Title: Non-equilibrium Helium Ionization in an MHD Simulation of the Solar Atmosphere Authors: Golding, Thomas Peter; Leenaarts, Jorrit; Carlsson, Mats Bibcode: 2016ApJ...817..125G Altcode: 2015arXiv151204738G The ionization state of the gas in the dynamic solar chromosphere can depart strongly from the instantaneous statistical equilibrium commonly assumed in numerical modeling. We improve on earlier simulations of the solar atmosphere that only included non-equilibrium hydrogen ionization by performing a 2D radiation-magnetohydrodynamics simulation featuring non-equilibrium ionization of both hydrogen and helium. The simulation includes the effect of hydrogen Lyα and the EUV radiation from the corona on the ionization and heating of the atmosphere. Details on code implementation are given. We obtain helium ion fractions that are far from their equilibrium values. Comparison with models with local thermodynamic equilibrium (LTE) ionization shows that non-equilibrium helium ionization leads to higher temperatures in wavefronts and lower temperatures in the gas between shocks. Assuming LTE ionization results in a thermostat-like behavior with matter accumulating around the temperatures where the LTE ionization fractions change rapidly. Comparison of DEM curves computed from our models shows that non-equilibrium ionization leads to more radiating material in the temperature range 11-18 kK, compared to models with LTE helium ionization. We conclude that non-equilibrium helium ionization is important for the dynamics and thermal structure of the upper chromosphere and transition region. It might also help resolve the problem that intensities of chromospheric lines computed from current models are smaller than those observed. Title: A publicly available simulation of an enhanced network region of the Sun Authors: Carlsson, Mats; Hansteen, Viggo H.; Gudiksen, Boris V.; Leenaarts, Jorrit; De Pontieu, Bart Bibcode: 2016A&A...585A...4C Altcode: 2015arXiv151007581C Context. The solar chromosphere is the interface between the solar surface and the solar corona. Modelling of this region is difficult because it represents the transition from optically thick to thin radiation escape, from gas-pressure domination to magnetic-pressure domination, from a neutral to an ionised state, from MHD to plasma physics, and from near-equilibrium (LTE) to non-equilibrium conditions.
Aims: Our aim is to provide the community with realistic simulations of the magnetic solar outer atmosphere. This will enable detailed comparison of existing and upcoming observations with synthetic observables from the simulations, thereby elucidating the complex interactions of magnetic fields and plasma that are crucial for our understanding of the dynamic outer atmosphere.
Methods: We used the radiation magnetohydrodynamics code Bifrost to perform simulations of a computational volume with a magnetic field topology similar to an enhanced network area on the Sun.
Results: The full simulation cubes are made available from the Hinode Science Data Centre Europe. The general properties of the simulation are discussed, and limitations are discussed.

The Hinode Science Data Centre Europe (http://www.sdc.uio.no/search/simulations). 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: Probing the Sun with ALMA: Observations and Simulations Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.; Carlsson, M. Bibcode: 2015ASPC..499..349L Altcode: 2015arXiv150805686L ALMA will open a new chapter in the study of the Sun by providing a leap in spatial resolution and sensitivity compared to currently available mm wavelength observations. In preparation of ALMA, we have carried out a large number of observational tests and state-of-the-art radiation MHD simulations. Here we review the best available observations of the Sun at millimeter wavelengths.Using state of the art radiation MHD simulations of the solar atmosphere we demonstrate the huge potential of ALMA observations for uncovering the nature of the solar chromosphere. We show that ALMA will not only provide a reliable probe of the thermal structure and dynamics of the chromosphere, it will also open up a powerful new diagnostic of magnetic field at chromospheric heights, a fundamentally important, but so far poorly known parameter. Title: New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation Authors: Kowalski, Adam F.; Hawley, S. L.; Carlsson, M.; Allred, J. C.; Uitenbroek, H.; Osten, R. A.; Holman, G. Bibcode: 2015SoPh..290.3487K Altcode: 2015SoPh..tmp...61K; 2015arXiv150307057K The heating mechanism at high densities during M-dwarf flares is poorly understood. Spectra of M-dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T ≈104K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at λ ≤3 646 Å, and 3) an apparent pseudo-continuum of blended high-order Balmer lines between λ =3 646 Å and λ ≈3 900 Å. These properties are not reproduced by models that employ a typical "solar-type" flare heating level of ≤1011ergcm−2s−1 in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological three-component interpretation. We present a new 1D radiative-hydrodynamic model of an M-dwarf flare from precipitating nonthermal electrons with a high energy flux of 1013ergcm−2s−1. The simulation produces bright near-ultraviolet and optical continuum emission from a dense (n >1015cm−3), hot (T ≈12 000 -13 500 K) chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T ≈104K blackbody-like continuum component and a low Balmer jump ratio result from optically thick Balmer (∞ →n =2 ) and Paschen recombination (∞ →n =3 ) radiation, and thus the properties of the flux spectrum are caused by blue (λ ≈4 300 Å) light escaping over a larger physical depth range than by red (λ ≈6 700 Å) and near-ultraviolet (λ ≈3 500 Å) light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high-order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares. Title: Heating of the Solar Chromosphere and Connections to the Transition Region and Corona Authors: Carlsson, M. Bibcode: 2015AGUFMSH23D..01C Altcode: We present observations of the chromosphere and transition region from IRIS and the SST. These observations are interpreted using 1D semi-empirical numerical experiments, 1D radiaton-MHD simulations and 3D "realistic" radiation-MHD simulations spanning the solar atmosphere from the convection zone to the corona. A picture is emerging of the energy balance in the solar chromosphere and connections to the transition region and corona. Title: Impact of the Ion-Neutral Interaction Effects in the Solar Chromosphere Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.; Carlsson, M. Bibcode: 2015AGUFMSH31B2411M Altcode: The complexity of the chromosphere is due to various regime changes that take place across it. Consequently, the interpretation of chromospheric observations is a challenging task. It is thus crucial to combine these observations with advanced radiative-MHD numerical modeling. Because the photosphere, chromosphere and transition region are partially ionized, the interaction between ionized and neutral particles has important consequences on the magneto-thermodynamics of these regions. We implemented the effects of partial ionization using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011) which solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. We perform 2.5D simulations which combines large and small scales structures. This leads to a highly dynamic chromosphere with large variety of physical processes which have not been reproduced with smaller simulations. The implementation of partial ionization effects impact our modeled radiative-MHD atmosphere, such as producing chromospheric heating and diffusion of photospheric magnetic field into the upper-chromosphere. We will also focus on which observables of these processes can be revealed with chromospheric observations. Title: Hα Line Profile Asymmetries and the Chromospheric Flare Velocity Field Authors: Kuridze, D.; Mathioudakis, M.; Simões, P. J. A.; Rouppe van der Voort, L.; Carlsson, M.; Jafarzadeh, S.; Allred, J. C.; Kowalski, A. F.; Kennedy, M.; Fletcher, L.; Graham, D.; Keenan, F. P. Bibcode: 2015ApJ...813..125K Altcode: 2015arXiv151001877K The asymmetries observed in the line profiles of solar flares can provide important diagnostics of the properties and dynamics of the flaring atmosphere. In this paper the evolution of the Hα and Ca ii λ8542 lines are studied using high spatial, temporal, and spectral resolution ground-based observations of an M1.1 flare obtained with the Swedish 1 m Solar Telescope. The temporal evolution of the Hα line profiles from the flare kernel shows excess emission in the red wing (red asymmetry) before flare maximum and excess in the blue wing (blue asymmetry) after maximum. However, the Ca ii λ8542 line does not follow the same pattern, showing only a weak red asymmetry during the flare. RADYN simulations are used to synthesize spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. We show that the red asymmetry observed in Hα is not necessarily associated with plasma downflows, and the blue asymmetry may not be related to plasma upflows. Indeed, we conclude that the steep velocity gradients in the flaring chromosphere modify the wavelength of the central reversal in the Hα line profile. The shift in the wavelength of maximum opacity to shorter and longer wavelengths generates the red and blue asymmetries, respectively. Title: The Formation of Iris Diagnostics. VIII. Iris Observations in the C II 133.5 nm Multiplet. Authors: Rathore, Bhavna; Pereira, Tiago M. D.; Carlsson, Mats; De Pontieu, Bart Bibcode: 2015ApJ...814...70R Altcode: 2015arXiv151004845R The C ii 133.5 nm multiplet has been observed by NASA’s Interface Region Imaging Spectrograph (IRIS) in unprecedented spatial resolution. The aims of this work are to characterize these new observations of the C ii lines, place them in context with previous work, and to identify any additional value the C ii lines bring when compared with other spectral lines. We make use of wide, long exposure IRIS rasters covering the quiet Sun and an active region. Line properties such as velocity shift and width are extracted from individual spectra and analyzed. The lines have a variety of shapes (mostly single-peak or double-peak), are strongest in active regions and weaker in the quiet Sun. The ratio between the 133.4 and 133.5 nm components is always less than 1.8, indicating that their radiation is optically thick in all locations. Maps of the C ii line widths are a powerful new diagnostic of chromospheric structures, and their line shifts are a robust velocity diagnostic. Compared with earlier quiet Sun observations, we find similar absolute intensities and mean line widths, but smaller redshifts; this difference can perhaps be attributed to differences in spectral resolution and spatial coverage. The C ii intensity maps are somewhat similar to those of transition region lines, but also share some features with chromospheric maps such as those from the Mg ii k line, indicating that they are formed between the upper chromosphere and transition region. C ii intensity, width, and velocity maps can therefore be used to gather additional information about the upper chromosphere. Title: Combined Modeling of Acceleration, Transport, and Hydrodynamic Response in Solar Flares. II. Inclusion of Radiative Transfer with RADYN Authors: Rubio da Costa, Fatima; Liu, Wei; Petrosian, Vahé; Carlsson, Mats Bibcode: 2015ApJ...813..133R Altcode: 2015arXiv150501549R Solar flares involve complex processes that are coupled and span a wide range of temporal, spatial, and energy scales. Modeling such processes self-consistently has been a challenge in the past. Here we present results from simulations that couple particle kinetics with hydrodynamics (HD) of the atmospheric plasma. We combine the Stanford unified Fokker-Planck code that models particle acceleration and transport with the RADYN HD code that models the atmospheric response to collisional heating by accelerated electrons through detailed radiative transfer calculations. We perform simulations using two different electron spectra, one an ad hoc power law and the other predicted by the model of stochastic acceleration by turbulence or plasma waves. Surprisingly, the later model, even with energy flux \ll {10}10 {erg} {{{s}}}-1 {{cm}}-2, can cause “explosive” chromospheric evaporation and drive stronger up- and downflows (and HD shocks). This is partly because our acceleration model, like many others, produces a spectrum consisting of a quasi-thermal component plus a power-law tail. We synthesize emission-line profiles covering different heights in the lower atmosphere, including Hα 6563 Å, He ii 304 Å, Ca ii K 3934 Å, and Si iv 1393 Å. One interesting result is the unusual high temperature (up to a few times 105 K) of the formation site of He ii 304 Å, which is expected owing to photoionization-recombination under flare conditions, compared to those in the quiet Sun dominated by collisional excitation. When compared with observations, our results can constrain the properties of nonthermal electrons and thus the poorly understood particle acceleration mechanism. Title: The Formation of IRIS Diagnostics. VII. The Formation of the OI 135.56 NM Line in the Solar Atmosphere Authors: Lin, Hsiao-Hsuan; Carlsson, Mats Bibcode: 2015ApJ...813...34L Altcode: 2015arXiv150906605L The O i 135.56 nm line is covered by NASA's Interface Region Imaging Spectrograph (IRIS) small explorer mission which studies how the solar atmosphere is energized. We study here the formation and diagnostic potential of this line by means of non-local thermodynamic equilibrium modeling employing both 1D semi-empirical and 3D radiation magnetohydrodynamic models. We study the basic formation mechanisms and derive a quintessential model atom that incorporates essential atomic physics for the formation of the O i 135.56 nm line. This atomic model has 16 levels and describes recombination cascades through highly excited levels by effective recombination rates. The ionization balance O i/O ii is set by the hydrogen ionization balance through charge exchange reactions. The emission in the O i 135.56 nm line is dominated by a recombination cascade and the line is optically thin. The Doppler shift of the maximum emission correlates strongly with the vertical velocity in its line forming region, which is typically located at 1.0-1.5 Mm height. The total intensity of the line emission is correlated with the square of the electron density. Since the O i 135.56 nm line is optically thin, the width of the emission line is a very good diagnostic of non-thermal velocities. We conclude that the O i 135.56 nm line is an excellent probe of the middle chromosphere, and compliments other powerful chromospheric diagnostics of IRIS such as the Mg ii h & k lines and the C ii lines around 133.5 nm. Title: Evidence for a Transition Region Response to Penumbral Microjets in Sunspots Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Carlsson, M. Bibcode: 2015ApJ...811L..33V Altcode: 2015arXiv150901402V Penumbral microjets (PMJs) are short-lived, fine-structured, and bright jets that are generally observed in chromospheric imaging of the penumbra of sunspots. Here we investigate their potential transition region signature by combining observations with the Swedish 1-m Solar Telescope in the Ca ii H and Ca ii 8542 Å lines with ultraviolet imaging and spectroscopy obtained with the Interface Region Imaging Spectrograph (IRIS), which includes the C ii 1334/1335 Å, Si iv 1394/1403 Å, and Mg ii h & k 2803/2796 Å lines. We find a clear corresponding signal in the IRIS Mg ii k, C ii, and Si iv slit-jaw images, typically offset spatially from the Ca ii signature in the direction along the jets: from base to top, the PMJs are predominantly visible in Ca ii, Mg ii k, and C ii/Si iv, suggesting progressive heating to transition region temperatures along the jet extent. Hence, these results support the suggestion from earlier studies that PMJs may heat to transition region temperatures. Title: The Formation of IRIS Diagnostics. VI. The Diagnostic Potential of the C II Lines at 133.5 nm in the Solar Atmosphere Authors: Rathore, Bhavna; Carlsson, Mats; Leenaarts, Jorrit; De Pontieu, Bart Bibcode: 2015ApJ...811...81R Altcode: 2015arXiv150804423R We use 3D radiation magnetohydrodynamic models to investigate how the thermodynamic quantities in the simulation are encoded in observable quantities, thus exploring the diagnostic potential of the C ii 133.5 nm lines. We find that the line core intensity is correlated with the temperature at the formation height but the correlation is rather weak, especially when the lines are strong. The line core Doppler shift is a good measure of the line-of-sight velocity at the formation height. The line width is both dependent on the width of the absorption profile (thermal and non-thermal width) and an opacity broadening factor of 1.2-4 due to the optically thick line formation with a larger broadening for double peak profiles. The C ii 133.5 nm lines can be formed both higher and lower than the core of the Mg ii k line depending on the amount of plasma in the 14-50 kK temperature range. More plasma in this temperature range gives a higher C ii 133.5 nm formation height relative to the Mg ii k line core. The synthetic line profiles have been compared with Interface Region Imaging Spectrograph observations. The derived parameters from the simulated line profiles cover the parameter range seen in observations but, on average, the synthetic profiles are too narrow. We interpret this discrepancy as a combination of a lack of plasma at chromospheric temperatures in the simulation box and too small non-thermal velocities. The large differences in the distribution of properties between the synthetic profiles and the observed ones show that the C ii 133.5 nm lines are powerful diagnostics of the upper chromosphere and lower transition region. Title: Numerical Simulations of Coronal Heating through Footpoint Braiding Authors: Hansteen, V.; Guerreiro, N.; De Pontieu, B.; Carlsson, M. Bibcode: 2015ApJ...811..106H Altcode: 2015arXiv150807234H Advanced three-dimensional (3D) radiative MHD simulations now reproduce many properties of the outer solar atmosphere. When including a domain from the convection zone into the corona, a hot chromosphere and corona are self-consistently maintained. Here we study two realistic models, with different simulated areas, magnetic field strength and topology, and numerical resolution. These are compared in order to characterize the heating in the 3D-MHD simulations which self-consistently maintains the structure of the atmosphere. We analyze the heating at both large and small scales and find that heating is episodic and highly structured in space, but occurs along loop-shaped structures, and moves along with the magnetic field. On large scales we find that the heating per particle is maximal near the transition region and that widely distributed opposite-polarity field in the photosphere leads to a greater heating scale height in the corona. On smaller scales, heating is concentrated in current sheets, the thicknesses of which are set by the numerical resolution. Some current sheets fragment in time, this process occurring more readily in the higher-resolution model leading to spatially highly intermittent heating. The large-scale heating structures are found to fade in less than about five minutes, while the smaller, local, heating shows timescales of the order of two minutes in one model and one minutes in the other, higher-resolution, model. Title: The Formation of IRIS Diagnostics. V. A Quintessential Model Atom of C II and General Formation Properties of the C II Lines at 133.5 nm Authors: Rathore, Bhavna; Carlsson, Mats Bibcode: 2015ApJ...811...80R Altcode: 2015arXiv150804365R The C ii 133.5 {nm} lines are important observables for the NASA/SMEX mission Interface Region Imaging Spectrograph. To make three-dimensional (3D) non-LTE radiative transfer computationally feasible, it is crucial to have a model atom with as few levels as possible while retaining the main physical processes. We here develop such a model atom and we study the general formation properties of the C ii lines. We find that a nine-level model atom of C i-C iii with the transitions treated assuming complete frequency redistribution (CRD) suffices to describe the C ii 133.5 {nm} lines. 3D scattering effects are important for the intensity in the core of the line. The lines are formed in the optically thick regime. The core intensity is formed in layers where the temperature is about 10 kK at the base of the transition region. The lines are 1.2-4 times wider than the atomic absorption profile due to the formation in the optically thick regime. The smaller opacity broadening happens for single peak intensity profiles where the chromospheric temperature is low with a steep source function increase into the transition region, the larger broadening happens when there is a temperature increase from the photosphere to the low chromosphere leading to a local source function maximum and a double peak intensity profile with a central reversal. Assuming optically thin formation with the standard coronal approximation leads to several errors: neglecting photoionization severly underestimates the amount of C ii at temperatures below 16 kK, erroneously shifts the formation from 10 kK to 25 kK, and leads to too low intensities. Title: Ellerman Bombs at High Resolution. III. Simultaneous Observations with IRIS and SST Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Rutten, R. J.; Carlsson, M.; De Pontieu, B. Bibcode: 2015ApJ...812...11V Altcode: 2015arXiv150700435V Ellerman bombs (EBs) are transient brightenings of the extended wings of the solar Balmer lines in emerging active regions. We describe their properties in the ultraviolet lines sampled by the Interface Region Imaging Spectrograph (IRIS), using simultaneous imaging spectroscopy in Hα with the Swedish 1-m Solar Telescope (SST) and ultraviolet images from the Solar Dynamics Observatory for Ellerman bomb detection and identification. We select multiple co-observed EBs for detailed analysis. The IRIS spectra strengthen the view that EBs mark reconnection between bipolar kilogauss fluxtubes with the reconnection and the resulting bi-directional jet located within the solar photosphere and shielded by overlying chromospheric fibrils in the cores of strong lines. The spectra suggest that the reconnecting photospheric gas underneath is heated sufficiently to momentarily reach stages of ionization normally assigned to the transition region and the corona. We also analyze similar outburst phenomena that we classify as small flaring arch filaments and ascribe to reconnection at a higher location. They have different morphologies and produce hot arches in million-Kelvin diagnostics. Title: Observed Variability of the Solar Mg II h Spectral Line Authors: Schmit, D.; Bryans, P.; De Pontieu, B.; McIntosh, S.; Leenaarts, J.; Carlsson, M. Bibcode: 2015ApJ...811..127S Altcode: 2015arXiv150804714S The Mg ii h&k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double-Gaussian model to fit the Mg ii h profile for a full-Sun mosaic data set taken on 2014 August 24. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well as h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles that do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared to predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core that is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg ii profiles observed by IRIS. Title: Numerical Modeling of the Solar Chromosphere and Corona: What Has Been Done? What Should Be Done? Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B. Bibcode: 2015ASPC..498..141H Altcode: A number of increasingly sophisticated numerical simulations spanning the solar atmosphere from below the photosphere in the convection zone to far above in the corona have shed considerable insight into the role of the magnetic field in the structure and energetics of the Sun's outer layers. This development is strengthened by the wealth of observational data now coming on-line from both ground and space based observatories. In this talk we will concentrate on the successes and failures of the modeling effort thus far and discuss the inclusion of various effects not traditionally considered in the MHD description such as time dependent ionization, non-LTE radiative transfer, and generalized Ohm's law. Title: What Do IRIS Observations of Mg II k Tell Us about the Solar Plage Chromosphere? Authors: Carlsson, Mats; Leenaarts, Jorrit; De Pontieu, Bart Bibcode: 2015ApJ...809L..30C Altcode: 2015arXiv150804888C We analyze observations from the Interface Region Imaging Spectrograph of the Mg ii k line, the Mg ii UV subordinate lines, and the O i 135.6 {nm} line to better understand the solar plage chromosphere. We also make comparisons with observations from the Swedish 1-m Solar Telescope of the Hα line, the Ca ii 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly observations of the coronal 19.3 {nm} line. To understand the observed Mg ii profiles, we compare these observations to the results of numerical experiments. The single-peaked or flat-topped Mg ii k profiles found in plage imply a transition region at a high column mass and a hot and dense chromosphere of about 6500 K. This scenario is supported by the observed large-scale correlation between moss brightness and filled-in profiles with very little or absent self-reversal. The large wing width found in plage also implies a hot and dense chromosphere with a steep chromospheric temperature rise. The absence of emission in the Mg ii subordinate lines constrain the chromospheric temperature and the height of the temperature rise while the width of the O i 135.6 {nm} line sets a limit to the non-thermal velocities to around 7 km s-1. Title: A Unified Computational Model for Solar and Stellar Flares Authors: Allred, Joel C.; Kowalski, Adam F.; Carlsson, Mats Bibcode: 2015ApJ...809..104A Altcode: 2015arXiv150704375A We present a unified computational framework that can be used to describe impulsive flares on the Sun and on dMe stars. The models assume that the flare impulsive phase is caused by a beam of charged particles that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how flare-accelerated charged particles propagate down one-dimensional flux tubes and heat the stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere. These effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index. Title: The Formation of IRIS Diagnostics. IV. The Mg II Triplet Lines as a New Diagnostic for Lower Chromospheric Heating Authors: Pereira, Tiago M. D.; Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo Bibcode: 2015ApJ...806...14P Altcode: 2015arXiv150401733P A triplet of subordinate lines of Mg ii exists in the region around the h&k lines. In solar spectra these lines are seen mostly in absorption, but in some cases can become emission lines. The aim of this work is to study the formation of this triplet, and investigate any diagnostic value they can bring. Using 3D radiative magnetohydrodynamic simulations of quiet Sun and flaring flux emergence, we synthesize spectra and investigate how spectral features respond to the underlying atmosphere. We find that emission in the lines is rare and is typically caused by a steep temperature increase in the lower chromosphere (above 1500 K, with electron densities above 1017 m-3). In both simulations the lines are sensitive to temperature increases taking place at column masses ≳5 · 10-4 g cm-2. Additional information can also be inferred from the peak-to-wing ratio and shape of the line profiles. Using observations from NASA's Interface Region Imaging Spectrograph we find both absorption and emission line profiles with similar shapes to the synthetic spectra, which suggests that these lines represent a useful diagnostic that complements the Mg ii h&k lines. Title: IRIS observations and 3D `realistic' MHD models of the solar chromosphere Authors: Hansteen, V.; Carlsson, M.; Gudiksen, B. Bibcode: 2015hsa8.conf...19H Altcode: The Interface Region Imaging Spectrograph (IRIS) is a NASA ``Small Explorer'' mission. It was launched in late June 2013 and since then it has obtained spectra and images from the outer solar atmosphere at unprecedented spatial and temporal resolution. Its primary goal is to probe the photosphere-corona interface: the source region of outer atmosphere heating and dynamics and a region that has an extremely complicated interplay between plasma, radiation and magnetic field. The scientific justification for IRIS hinges on the capabilities of 3D magnetohydrodynamic models to allow the confident interpretation of observed data. The interplay between observations and modeling is discussed, illustrated with examples from recent IRIS observations. Title: First High-resolution Spectroscopic Observations of an Erupting Prominence Within a Coronal Mass Ejection by the Interface Region Imaging Spectrograph (IRIS) Authors: Liu, Wei; De Pontieu, Bart; Vial, Jean-Claude; Title, Alan M.; Carlsson, Mats; Uitenbroek, Han; Okamoto, Takenori J.; Berger, Thomas E.; Antolin, Patrick Bibcode: 2015ApJ...803...85L Altcode: 2015arXiv150204738L Spectroscopic observations of prominence eruptions associated with coronal mass ejections (CMEs), although relatively rare, can provide valuable plasma and three-dimensional geometry diagnostics. We report the first observations by the Interface Region Imaging Spectrograph mission of a spectacular fast CME/prominence eruption associated with an equivalent X1.6 flare on 2014 May 9. The maximum plane-of-sky and Doppler velocities of the eruption are 1200 and 460 km s-1, respectively. There are two eruption components separated by ∼200 km s-1 in Doppler velocity: a primary, bright component and a secondary, faint component, suggesting a hollow, rather than solid, cone-shaped distribution of material. The eruption involves a left-handed helical structure undergoing counterclockwise (viewed top-down) unwinding motion. There is a temporal evolution from upward eruption to downward fallback with less-than-free-fall speeds and decreasing nonthermal line widths. We find a wide range of Mg ii k/h line intensity ratios (less than ∼2 expected for optically-thin thermal emission): the lowest ever reported median value of 1.17 found in the fallback material, a comparably high value of 1.63 in nearby coronal rain, and intermediate values of 1.53 and 1.41 in the two eruption components. The fallback material exhibits a strong (\gt 5σ ) linear correlation between the k/h ratio and the Doppler velocity as well as the line intensity. We demonstrate that Doppler dimming of scattered chromospheric emission by the erupted material can potentially explain such characteristics. Title: Three-dimensional Radiative Transfer Simulations of the Scattering Polarization of the Hydrogen Lyα Line in a Magnetohydrodynamic Model of the Chromosphere-Corona Transition Region Authors: Štěpán, J.; Trujillo Bueno, J.; Leenaarts, J.; Carlsson, M. Bibcode: 2015ApJ...803...65S Altcode: 2015arXiv150106382S Probing the magnetism of the upper solar chromosphere requires measuring and modeling the scattering polarization produced by anisotropic radiation pumping in UV spectral lines. Here we apply PORTA (a novel radiative transfer code) to investigate the hydrogen Lyα line in a three-dimensional model of the solar atmosphere resulting from a state of the art magnetohydrodynamic (MHD) simulation. At full spatial resolution the linear polarization signals are very significant all over the solar disk, with a large fraction of the field of view (FOV) showing line-center amplitudes well above the 1% level. Via the Hanle effect the line-center polarization signals are sensitive to the magnetic field of the model's transition region, even when its mean field strength is only 15 G. The breaking of the axial symmetry of the radiation field produces significant forward-scattering polarization in Lyα, without the need of an inclined magnetic field. Interestingly, the Hanle effect tends to decrease such forward-scattering polarization signals in most of the points of the FOV. When the spatial resolution is degraded, the line-center polarization of Lyα drops below the 1% level, reaching values similar to those previously found in one-dimensional (1D) semi-empirical models (i.e., up to about 0.5 %). The center to limb variation (CLV) of the spatially averaged polarization signals is qualitatively similar to that found in 1D models, with the largest line-center amplitudes at μ =cos θ ≈ 0.4 (θ being the heliocentric angle). These results are important, both for designing the needed space-based instrumentation and for a reliable interpretation of future observations of the Lyα polarization. Title: The role of partial ionization effects in the chromosphere Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo; Carlsson, Mats Bibcode: 2015RSPTA.37340268M Altcode: 2015arXiv150302723M The energy for the coronal heating must be provided from the convection zone. However, the amount and the method by which this energy is transferred into the corona depend on the properties of the lower atmosphere and the corona itself. We review: (i) how the energy could be built in the lower solar atmosphere, (ii) how this energy is transferred through the solar atmosphere, and (iii) how the energy is finally dissipated in the chromosphere and/or corona. Any mechanism of energy transport has to deal with the various physical processes in the lower atmosphere. We will focus on a physical process that seems to be highly important in the chromosphere and not deeply studied until recently: the ion-neutral interaction effects in the chromosphere. We review the relevance and the role of the partial ionization in the chromosphere and show that this process actually impacts considerably the outer solar atmosphere. We include analysis of our 2.5D radiative magnetohydrodynamic simulations with the Bifrost code (Gudiksen et al. 2011 Astron. Astrophys. 531, A154 (doi:10.1051/0004-6361/201116520)) including the partial ionization effects on the chromosphere and corona and thermal conduction along magnetic field lines. The photosphere, chromosphere and transition region are partially ionized and the interaction between ionized particles and neutral particles has important consequences on the magneto-thermodynamics of these layers. The partial ionization effects are treated using generalized Ohm's law, i.e. we consider the Hall term and the ambipolar diffusion (Pedersen dissipation) in the induction equation. The interaction between the different species affects the modelled atmosphere as follows: (i) the ambipolar diffusion dissipates magnetic energy and increases the minimum temperature in the chromosphere and (ii) the upper chromosphere may get heated and expanded over a greater range of heights. These processes reveal appreciable differences between the modelled atmospheres of simulations with and without ion-neutral interaction effects. Title: On Fibrils and Field Lines: the Nature of Hα Fibrils in the Solar Chromosphere Authors: Leenaarts, Jorrit; Carlsson, Mats; Rouppe van der Voort, Luc Bibcode: 2015ApJ...802..136L Altcode: 2015arXiv150200295L Observations of the solar chromosphere in the line core of the Hα line show dark elongated structures called fibrils that show swaying motion. We performed a three-dimensional radiation-MHD simulation of a network region and computed synthetic Hα images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes, and phase speeds of the simulated fibrils are consistent with observations. We find that some fibrils trace out the same field line along the fibril’s length, while other fibrils sample different field lines at different locations along their length. Fibrils sample the same field lines on a timescale of ∼200 s. This is shorter than their own lifetime. Fibril-threading field lines carry slow-mode waves, as well as transverse waves propagating with the Alfvén speed. Transverse waves propagating in opposite directions cause an interference pattern with complex apparent phase speeds. The relationship between fibrils and field lines is governed by constant migration and swaying of the field lines, their mass loading and draining, and their visibility in Hα. Field lines are visible where they lie close to the optical depth unity surface. The location of the latter is at a height at which the column mass reaches a certain fixed value. The visibility of the field line is thus determined by its own mass density and by the mass density of the material above it. Using the swaying motion of fibrils as a tracer of chromospheric transverse oscillations must be done with caution. Title: Internetwork Chromospheric Bright Grains Observed With IRIS and SST Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson, Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt, Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan; Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli, Sarah; Kankelborg, Charles Bibcode: 2015ApJ...803...44M Altcode: 2015arXiv150203490M The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet Sun. These bright grains are seen with the IRIS 1330, 1400, and 2796 Å slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1 m Solar Telescope (SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H 3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å, Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.} 33), temporal, and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images (SJIs) with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 Å slit-jaw filter comes from both the Mg ii k core and wings. The signal in the C ii and Si iv lines is too weak to explain the presence of grains in the 1300 and 1400 Å SJIs and we conclude that the grain signal in these passbands comes mostly from the continuum. Although weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C ii spectra. For some grains, a spectral signature can be found in IRIS Si iv. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region. Title: A Unified Computational Model for Solar and Stellar Flares Authors: Allred, Joel; Kowalski, Adam; Carlsson, Mats Bibcode: 2015TESS....130207A Altcode: We describe a unified computational framework which can be used to model impulsive flares on the Sun and on dMe stars. The models are constructed assuming that the flare impulsive phase is caused by a beam of charged particles (primarily electrons and protons) that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and emission to dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how these flare-accelerated particles propagate down one dimensional flux tubes and heat the stellar atmosphere using Fokker-Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere, and these effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index. Title: Millimeter radiation from a 3D model of the solar atmosphere. I. Diagnosing chromospheric thermal structure Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; White, S. M. Bibcode: 2015A&A...575A..15L Altcode: 2015arXiv150102898L
Aims: We use advanced 3D non-local thermodynamic equilibrium radiative magnetohydrodynamic simulations of the solar atmosphere to carry out detailed tests of chromospheric diagnostics at millimeter and submillimeter wavelengths.
Methods: We focused on the diagnostics of the thermal structure of the chromosphere in the wavelength bands from 0.4 mm up to 9.6 mm that can be accessed with the Atacama Large Millimeter/Submillimeter Array (ALMA) and investigated how these diagnostics are affected by the instrumental resolution.
Results: We find that the formation height range of the millimeter radiation depends on the location in the simulation domain and is related to the underlying magnetic structure. Nonetheless, the brightness temperature is a reasonable measure of the gas temperature at the effective formation height at a given location on the solar surface. There is considerable scatter in this relationship, but this is significantly reduced when very weak magnetic fields are avoided. Our results indicate that although instrumental smearing reduces the correlation between brightness and temperature, millimeter brightness can still be used to reliably diagnose electron temperature up to a resolution of 1''. If the resolution is more degraded, then the value of the diagnostic diminishes rapidly.
Conclusions: We conclude that millimeter brightness can image the chromospheric thermal structure at the height at which the radiation is formed. Thus multiwavelength observations with ALMA with a narrow step in wavelength should provide sufficient information for a tomographic imaging of the chromosphere. Title: Homologous Helical Jets: Observations By IRIS, SDO, and Hinode and Magnetic Modeling With Data-Driven Simulations Authors: Cheung, Mark C. M.; De Pontieu, B.; Tarbell, T. D.; Fu, Y.; Tian, H.; Testa, P.; Reeves, K. K.; Martínez-Sykora, J.; Boerner, P.; Wülser, J. P.; Lemen, J.; Title, A. M.; Hurlburt, N.; Kleint, L.; Kankelborg, C.; Jaeggli, S.; Golub, L.; McKillop, S.; Saar, S.; Carlsson, M.; Hansteen, V. Bibcode: 2015ApJ...801...83C Altcode: 2015arXiv150101593C We report on observations of recurrent jets by instruments on board the Interface Region Imaging Spectrograph, Solar Dynamics Observatory (SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21, recurrent coronal jets were observed to emanate from NOAA Active Region 11793. Far-ultraviolet spectra probing plasma at transition region temperatures show evidence of oppositely directed flows with components reaching Doppler velocities of ±100 km s-1. Raster Doppler maps using a Si iv transition region line show all four jets to have helical motion of the same sense. Simultaneous observations of the region by SDO and Hinode show that the jets emanate from a source region comprising a pore embedded in the interior of a supergranule. The parasitic pore has opposite polarity flux compared to the surrounding network field. This leads to a spine-fan magnetic topology in the coronal field that is amenable to jet formation. Time-dependent data-driven simulations are used to investigate the underlying drivers for the jets. These numerical experiments show that the emergence of current-carrying magnetic field in the vicinity of the pore supplies the magnetic twist needed for recurrent helical jet formation. Title: Heating Signatures in the Disk Counterparts of Solar Spicules in Interface Region Imaging Spectrograph Observations Authors: Rouppe van der Voort, L.; De Pontieu, B.; Pereira, T. M. D.; Carlsson, M.; Hansteen, V. Bibcode: 2015ApJ...799L...3R Altcode: 2014arXiv1412.4531R We use coordinated observations with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify the disk counterpart of type II spicules in upper-chromospheric and transition region (TR) diagnostics. These disk counterparts were earlier identified through short-lived asymmetries in chromospheric spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We find clear signatures of RBEs and RREs in Mg II h & k, often with excursions of the central h3 and k3 absorption features in concert with asymmetries in co-temporal and co-spatial Hα spectral profiles. We find spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si IV 1394 and 1403 Å spectral lines and interpret this as a sign that type II spicules are heated to at least TR temperatures, supporting other recent work. These C II and Si IV spectral signals are weaker for a smaller network region than for more extended network regions in our data. A number of bright features around extended network regions observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified as network jets, can be clearly connected to Hα RBEs and/or RREs in our coordinated data. We speculate that at least part of the diffuse halo around network regions in the IRIS SJI 1330 and 1400 images can be attributed to type II spicules with insufficient opacity in the C II and Si IV lines to stand out as single features in these passbands. Title: IRIS diagnostics of non-thermal particles in coronal loops heated by nanoflares Authors: Testa, P.; De Pontieu, B.; Allred, J. C.; Carlsson, M.; Reale, F.; Daw, A. N. Bibcode: 2014AGUFMSH53D..08T Altcode: The variability of emission of the "moss", i.e., the upper transition region (TR) layer of high pressure loops in active regions, provides stringent constraints on the characteristics of heating events. We will discuss the new coronal heating diagnostics provided by the Interface Region Imaging Spectrograph (IRIS) together with SDO/AIA. IRIS provides imaging and spectral observations of the solar chromosphere and transition region, at high spatial (0.166 arcsec/pix) and temporal (down to ~1s) resolution at FUV and NUV wavelengths. We discuss how simultaneous IRIS and AIA observations, together with loop modeling (with the RADYN code) including chromosphere, transition region and corona, allow us to study impulsive heating events (nanoflares) and the energy transport mechanism between the corona and the lower atmospheric layers (thermal conduction vs. beams of non-thermal particles). We will show how the modeling of rapid moss brightenings provides diagnostics for the presence and properties of non-thermal particles in nanoflares, which are below the detectability threshold of hard X-ray observations. Title: Observables of Ion-Neutral Interaction Effects in the Solar Chromosphere Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.; Pereira, T. M. D.; Leenaarts, J.; Carlsson, M. Bibcode: 2014AGUFMSH51C4176M Altcode: The chromosphere and transition region constitute the interface between the solar surface and the corona and modulate the flow of mass and energy into the upper atmosphere. IRIS was launched in 2013 to study the chromosphere and transition region. The complexity of the chromosphere is due to various regime changes that take place across it, like: Hydrogen goes from predominantly neutral to predominantly ionized; the plasma behavior changes from collisional to collision-less; it goes from gas-pressure dominated to magnetically driven, etc. Consequently, the interpretation of chromospheric observations in general and those from IRIS, in particular, is a challenging task. It is thus crucial to combine IRIS observations with advanced radiative-MHD numerical modeling. Because the photosphere, chromosphere and transition region are partially ionized, the interaction between ionized and neutral particles has important consequences on the magneto-thermodynamics of these regions. We implemented the effects of partial ionization using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011) which solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. The implementation of partial ionization effects impact our modeled radiative-MHD atmosphere, such as producing chromospheric heating and diffusion of photospheric magnetic field into the upper-chromosphere. We will focus on which observables of these processes can be revealed with IRIS. Title: Probing the solar interface region Authors: De Pontieu, Bart; Title, Alan; Carlsson, Mats Bibcode: 2014Sci...346..315D Altcode: No abstract at ADS Title: Hot explosions in the cool atmosphere of the Sun Authors: Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D.; De Pontieu, B.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Martínez-Sykora, Juan; Kleint, L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V. Bibcode: 2014Sci...346C.315P Altcode: 2014arXiv1410.5842P The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by the Interface Region Imaging Spectrograph (IRIS) reveal that it is difficult to determine what is up and down, even in the cool 6000-kelvin photosphere just above the solar surface: This region hosts pockets of hot plasma transiently heated to almost 100,000 kelvin. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection. Title: A Sounding Rocket Experiment for the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) Authors: Kubo, M.; Kano, R.; Kobayashi, K.; Bando, T.; Narukage, N.; Ishikawa, R.; Tsuneta, S.; Katsukawa, Y.; Ishikawa, S.; Suematsu, Y.; Hara, H.; Shimizu, T.; Sakao, T.; Ichimoto, K.; Goto, M.; Holloway, T.; Winebarger, A.; Cirtain, J.; De Pontieu, B.; Casini, R.; Auchère, F.; Trujillo Bueno, J.; Manso Sainz, R.; Belluzzi, L.; Asensio Ramos, A.; Štěpán, J.; Carlsson, M. Bibcode: 2014ASPC..489..307K Altcode: A sounding-rocket experiment called the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is presently under development to measure the linear polarization profiles in the hydrogen Lyman-alpha (Lyα) line at 121.567 nm. CLASP is a vacuum-UV (VUV) spectropolarimeter to aim for first detection of the linear polarizations caused by scattering processes and the Hanle effect in the Lyα line with high accuracy (0.1%). This is a fist step for exploration of magnetic fields in the upper chromosphere and transition region of the Sun. Accurate measurements of the linear polarization signals caused by scattering processes and the Hanle effect in strong UV lines like Lyα are essential to explore with future solar telescopes the strength and structures of the magnetic field in the upper chromosphere and transition region of the Sun. The CLASP proposal has been accepted by NASA in 2012, and the flight is planned in 2015. Title: The unresolved fine structure resolved: IRIS observations of the solar transition region Authors: Hansteen, V.; De Pontieu, B.; Carlsson, M.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Pereira, T. M. D.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.; Martínez-Sykora, J. Bibcode: 2014Sci...346E.315H Altcode: 2014arXiv1412.3611H The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a long-standing problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, low-lying loops or loop segments at transition-region temperatures that vary rapidly, on the time scales of minutes. We argue that the existence of these loops solves a long-standing observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle. Title: Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares Authors: Testa, P.; De Pontieu, B.; Allred, J.; Carlsson, M.; Reale, F.; Daw, A.; Hansteen, V.; Martinez-Sykora, J.; Liu, W.; DeLuca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Tian, H.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli, S. Bibcode: 2014Sci...346B.315T Altcode: 2014arXiv1410.6130T The physical processes causing energy exchange between the Sun’s hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High-resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to 60 seconds) of intensity and velocity on small spatial scales (≲500 kilometers) at the footpoints of hot and dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of nonthermal electrons, which are generated in small impulsive (≲30 seconds) heating events called “coronal nanoflares.” The accelerated electrons deposit a sizable fraction of their energy (≲1025 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona. Title: Prevalence of small-scale jets from the networks of the solar transition region and chromosphere Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.; Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves, K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber, M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W. Bibcode: 2014Sci...346A.315T Altcode: 2014arXiv1410.6143T As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind. Title: On the prevalence of small-scale twist in the solar chromosphere and transition region Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.; Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.; Martinez-Sykora, J. Bibcode: 2014Sci...346D.315D Altcode: 2014arXiv1410.6862D The solar chromosphere and transition region (TR) form an interface between the Sun’s surface and its hot outer atmosphere. There, most of the nonthermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33-arc second) observations with NASA’s Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub-arc second scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-meter Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere. Title: An Interface Region Imaging Spectrograph First View on Solar Spicules Authors: Pereira, T. M. D.; De Pontieu, B.; Carlsson, M.; Hansteen, V.; Tarbell, T. D.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Wülser, J. P.; Martínez-Sykora, J.; Kleint, L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Tian, H.; Jaeggli, S.; Kankelborg, C. Bibcode: 2014ApJ...792L..15P Altcode: 2014arXiv1407.6360P Solar spicules have eluded modelers and observers for decades. Since the discovery of the more energetic type II, spicules have become a heated topic but their contribution to the energy balance of the low solar atmosphere remains unknown. Here we give a first glimpse of what quiet-Sun spicules look like when observed with NASA's recently launched Interface Region Imaging Spectrograph (IRIS). Using IRIS spectra and filtergrams that sample the chromosphere and transition region, we compare the properties and evolution of spicules as observed in a coordinated campaign with Hinode and the Atmospheric Imaging Assembly. Our IRIS observations allow us to follow the thermal evolution of type II spicules and finally confirm that the fading of Ca II H spicules appears to be caused by rapid heating to higher temperatures. The IRIS spicules do not fade but continue evolving, reaching higher and falling back down after 500-800 s. Ca II H type II spicules are thus the initial stages of violent and hotter events that mostly remain invisible in Ca II H filtergrams. These events have very different properties from type I spicules, which show lower velocities and no fading from chromospheric passbands. The IRIS spectra of spicules show the same signature as their proposed disk counterparts, reinforcing earlier work. Spectroheliograms from spectral rasters also confirm that quiet-Sun spicules originate in bushes from the magnetic network. Our results suggest that type II spicules are indeed the site of vigorous heating (to at least transition region temperatures) along extensive parts of the upward moving spicular plasma. Title: The Interface Region Imaging Spectrograph (IRIS) Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.; Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou, C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman, C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish, D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.; Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons, R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.; Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.; Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.; Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski, W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.; Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.; Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.; Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson, M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu, K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora, J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.; Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N. Bibcode: 2014SoPh..289.2733D Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33 - 0.4 arcsec spatial resolution, two-second temporal resolution, and 1 km s−1 velocity resolution over a field-of-view of up to 175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å, 1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw images in four different passbands (C II 1330, Si IV 1400, Mg II k 2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral rasters that sample regions up to 130 arcsec × 175 arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative-MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation. Title: Detection of Supersonic Downflows and Associated Heating Events in the Transition Region above Sunspots Authors: Kleint, L.; Antolin, P.; Tian, H.; Judge, P.; Testa, P.; De Pontieu, B.; Martínez-Sykora, J.; Reeves, K. K.; Wuelser, J. P.; McKillop, S.; Saar, S.; Carlsson, M.; Boerner, P.; Hurlburt, N.; Lemen, J.; Tarbell, T. D.; Title, A.; Golub, L.; Hansteen, V.; Jaeggli, S.; Kankelborg, C. Bibcode: 2014ApJ...789L..42K Altcode: 2014arXiv1406.6816K Interface Region Imaging Spectrograph data allow us to study the solar transition region (TR) with an unprecedented spatial resolution of 0.''33. On 2013 August 30, we observed bursts of high Doppler shifts suggesting strong supersonic downflows of up to 200 km s-1 and weaker, slightly slower upflows in the spectral lines Mg II h and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated with brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout the 2 hr observation, with average burst durations of about 20 s. The locations of these short-lived events appear to be the umbral and penumbral footpoints of EUV loops. Fast apparent downflows are observed along these loops in the SJIs and in the Atmospheric Imaging Assembly, suggesting that the loops are thermally unstable. We interpret the observations as cool material falling from coronal heights, and especially coronal rain produced along the thermally unstable loops, which leads to an increase of intensity at the loop footpoints, probably indicating an increase of density and temperature in the TR. The rain speeds are on the higher end of previously reported speeds for this phenomenon, and possibly higher than the free-fall velocity along the loops. On other observing days, similar bright dots are sometimes aligned into ribbons, resembling small flare ribbons. These observations provide a first insight into small-scale heating events in sunspots in the TR. Title: F-CHROMA.Flare Chromospheres: Observations, Models and Archives Authors: Cauzzi, Gianna; Fletcher, Lyndsay; Mathioudakis, Mihalis; Carlsson, Mats; Heinzel, Petr; Berlicki, Arek; Zuccarello, Francesca Bibcode: 2014AAS...22412339C Altcode: F-CHROMA is a collaborative project newly funded under the EU-Framework Programme 7 "FP7-SPACE-2013-1", involving seven different European research Institutes and Universities. The goal of F-CHROMA is to substantially advance our understanding of the physics of energy dissipation and radiation in the flaring solar atmosphere, with a particular focus on the flares' chromosphere. A major outcome of the F-CHROMA project will be the creation of an archive of chromospheric flare observations and models to be made available to the community for further research.In this poster we describe the structure and milestones of the project, the different activities planned, as well as early results. Emphasis will be given to the dissemination efforts of the project to make results of these activities available to and usable by the community. Title: IRIS Observations of Twist in the Low Solar Atmosphere Authors: De Pontieu, Bart; Rouppe van der Voort, Luc; Pereira, Tiago M. D.; Skogsrud, Haakon; McIntosh, Scott W.; Carlsson, Mats; Hansteen, Viggo Bibcode: 2014AAS...22431302D Altcode: The Interface Region Imaging Spectrograph (IRIS) small explorer was launched in June 2013. IRIS’s high-resolution (0.33 arcsec), high-cadence (2s) images and spectra reveal a solar chromosphere and transition region that is riddled with twist. This is evidenced by the presence of ubiquitous torsional motions on very small (subarcsec) spatial scales. These motions occur in active regions, quiet Sun and coronal holes on a variety of structures such as spicules at the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as low-lying loops. We use IRIS data and observations from the Swedish Solar Telescope (SST) in La Palma, Spain to describe these motions quantitatively, study their propagation, and illustrate how such strong twisting motions are often associated with significant and rapid heating to at least transition region temperatures. Title: Diagnostics of coronal heating and mechanisms of energy transport from IRIS and AIA observations of active region moss Authors: Testa, Paola; De Pontieu, Bart; Allred, Joel C.; Carlsson, Mats; Reale, Fabio; Daw, Adrian N.; Hansteen, Viggo Bibcode: 2014AAS...22431305T Altcode: The variability of emission of the "moss", i.e., the upper transition region (TR) layer of high pressure loops in active regions provides stringent constraints on the characteristics of heating events. The Interface Region Imaging Spectrograph (IRIS), launched in June 2013, provides imaging and spectral observations at high spatial (0.166 arcsec/pix), and temporal (down to ~1s) resolution at FUV and NUV wavelengths, and together with the high spatial and temporal resolution observations of SDO/AIA, can provide important insights into the coronal heating mechanisms. We present here an analysis of the temporal variability properties of moss regions at the footpoints of hot active region core loops undergoing heating, as observed by IRIS and AIA, covering emission from the corona to the transition region and the chromosphere. We model the observations using dynamic loop models (the Palermo-Harvard code, and RADYN, which also includes the effects of non-thermal particles) and discuss the implications on energy transport mechanisms (thermal conduction vs beams of non-thermal particles). Title: IRIS observations of the transition region above sunspots: oscillations and moving penumbral dots Authors: Tian, Hui; DeLuca, Ed; Weber, Mark A.; McKillop, Sean; Reeves, Kathy; Kleint, Lucia; Martinez-Sykora, Juan; De Pontieu, Bart; Carlsson, Mats Bibcode: 2014AAS...22431306T Altcode: NASA's IRIS mission is providing high-cadence and high-resolution observations of the solar transition region and chromosphere. We present results from IRIS observation of the transition region above sunspots. The major findings can be summarized as following: (1) The C II and Mg II line profiles are almost Gaussian in the sunspot umbra and clearly exhibit a deep reversal at the line center in the plage region, suggesting a greatly reduced opacity in the sunspot atmosphere. (2) Strongly nonlinear sunspot oscillations can be clearly identified in not only the slit jaw images of 2796Å, 1400Å and 1330Å, but also in spectra of the bright Mg II, C II and Si IV lines. The Si iv oscillation lags those of C ii and Mg ii by 3 and 12 seconds, respectively. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blue shift in Si iv , whereas the intensity enhancement slightly precedes the maximum blue shift in Mg ii . We find a positive correlation between the maximum velocity and deceleration. These results are consistent with numerical simulations of upward propagating magneto-acoustic shock waves. We also demonstrate that the strongly nonlinear line width oscillation, reported both previously and here, is spurious. (3) Many small-scale bright dots are present in the penumbral filaments and light bridges in SJI 1330Å and 1400Å images obtained in high-cadence observations. They are usually smaller than 1" and often just a couple of pixels wide. Some bright dots show apparent movement with a speed of 20-60 km/s(either outward or inward). The lifetime of these penumbral dots is mostly less than 1 min. The most obvious feature of the Si IV profiles in the bright dots is the enhanced line width. Besides that, the profile looks normal and no obvious fast flows are detected. The bright dots in the light bridges even show oscillation patterns. It's not clear whether these oscillations are triggered by the umbral oscillations or not. Title: Hybrid Kinetic and Radiative Hydrodynamic Simulations of Solar Flares and Comparison With Multiwavelength Observations Authors: Rubio Da Costa, Fatima; Petrosian, Vahe; Liu, Wei; Carlsson, Mats; Kleint, Lucia Bibcode: 2014AAS...22440906R Altcode: We present a unified simulation which combines two physical processes: how the particles are accelerated and the energy is transported along a coronal loop, and how the atmosphere responds. The “flare” code from Stanford University (Petrosian et al, 2001) models the stochastic acceleration and transport of particles and radiation of solar flares. It includes pitch angle diffusion and energy loss, and computes collisional heating to the background plasma and bremsstrahlung emission along the loop. The radiative hydrodynamic RADYN Code (Carlsson et al, 1992, 1996; Allred et al, 2005) computes the energy transport by the injected non-thermal electrons at the top of a 1D coronal loop. Recently, we have combined the two codes by updating the non-thermal heating in the RADYN code from the "flare" code, allowing us to develop a self-consistent simulation. In addition, we can now model more realistically the multi-wavelength emission of solar flares and compare it with observations, e.g., at optical wavelengths from IBIS at the Dunn Solar Telescope and in X-rays from RHESSI. The high resolution UV observations from the recently launched IRIS imaging spectrograph will be particularly useful in this regard. These will allow us to compare numerically modeled and observed emissions of solar flares in several lines using more robust simulations than possible before. Title: High-resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph Authors: Tian, H.; DeLuca, E.; Reeves, K. K.; McKillop, S.; De Pontieu, B.; Martínez-Sykora, J.; Carlsson, M.; Hansteen, V.; Kleint, L.; Cheung, M.; Golub, L.; Saar, S.; Testa, P.; Weber, M.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Kankelborg, C.; Jaeggli, S.; McIntosh, S. W. Bibcode: 2014ApJ...786..137T Altcode: 2014arXiv1404.6291T We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slit-jaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the sunspot. The intensity change is ~30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of ~10 km s-1 in Si IV. The Si IV oscillation lags those of C II and Mg II by ~3 and ~12 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si IV, whereas the intensity enhancement slightly precedes the maximum blueshift in Mg II. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves. Title: Detailed and Simplified Nonequilibrium Helium Ionization in the Solar Atmosphere Authors: Golding, Thomas Peter; Carlsson, Mats; Leenaarts, Jorrit Bibcode: 2014ApJ...784...30G Altcode: 2014arXiv1401.7562G Helium ionization plays an important role in the energy balance of the upper chromosphere and transition region. Helium spectral lines are also often used as diagnostics of these regions. We carry out one-dimensional radiation-hydrodynamics simulations of the solar atmosphere and find that the helium ionization is set mostly by photoionization and direct collisional ionization, counteracted by radiative recombination cascades. By introducing an additional recombination rate mimicking the recombination cascades, we construct a simplified three-level helium model atom consisting of only the ground states. This model atom is suitable for modeling nonequilibrium helium ionization in three-dimensional numerical models. We perform a brief investigation of the formation of the He I 10830 and He II 304 spectral lines. Both lines show nonequilibrium features that are not recovered with statistical equilibrium models, and caution should therefore be exercised when such models are used as a basis for interpretating observations. Title: The Effect of Isotopic Splitting on the Bisector and Inversions of the Solar Ca II 854.2 nm Line Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Kochukhov, Oleg; Carlsson, Mats Bibcode: 2014ApJ...784L..17L Altcode: 2014arXiv1401.5019L The Ca II 854.2 nm spectral line is a common diagnostic of the solar chromosphere. The average line profile shows an asymmetric core, and its bisector shows a characteristic inverse-C shape. The line actually consists of six components with slightly different wavelengths depending on the isotope of calcium. This isotopic splitting of the line has been taken into account in studies of non-solar stars, but never for the Sun. We performed non-LTE radiative transfer computations from three models of the solar atmosphere and show that the line-core asymmetry and inverse C-shape of the bisector of the 854.2 nm line can be explained by isotopic splitting. We confirm this finding by analyzing observations and showing that the line asymmetry is present irrespective of conditions in the solar atmosphere. Finally, we show that inversions based on the Ca II 854.2 nm line should take the isotopic splitting into account, otherwise the inferred atmospheres will contain erroneous velocity gradients and temperatures. Title: Diagnostics of coronal heating and mechanisms of energy transport from IRIS and AIA observations of active region moss Authors: Testa, Paola; Reale, Fabio; De Pontieu, Bart; Hansteen, Viggo; Carlsson, Mats; Allred, Joel; Daw, Adrian Bibcode: 2014cosp...40E3323T Altcode: The variability of emission of the "moss", i.e., the upper transition region (TR) layer of high pressure loops in active regions provides stringent constraints on the characteristics of heating events. The Interface Region Imaging Spectrograph (IRIS), launched in June 2013, provides imaging and spectral observations at high spatial (0.166 arcsec/pix), and temporal (down to ~1s) resolution at FUV and NUV wavelengths, and together with the high spatial and temporal resolution observations of SDO/AIA, can provide important insights into the coronal heating mechanisms. We present here an analysis of the temporal variability properties of moss regions at the footpoints of hot active region core loops undergoing heating, as observed by IRIS and AIA, covering emission from the corona to the transition region and the chromosphere. We model the observations using dynamic loop models (the Palermo-Harvard code, and RADYN, which also includes the effects of non-thermal particles) and discuss the implications on energy transport mechanisms (thermal conduction vs beams of non-thermal particles). Title: Measuring energy flux of magneto-acoustic wave in the magnetic elements by using IRIS Authors: Kato, Yoshiaki; De Pontieu, Bart; Martinez-Sykora, Juan; Hansteen, Viggo; Pereira, Tiago; Leenaarts, Jorritt; Carlsson, Mats Bibcode: 2014cosp...40E1423K Altcode: NASA's Interface Region Imaging Spectrograph (IRIS) has opened a new window to explore the chromospheric/coronal waves that potentially energize the solar atmosphere. By using an imaging spectrograph covering the Si IV and Mg II h&k lines as well as a slit-jaw imager centered at Si IV and Mg II k onboard IRIS, we can determine the nature of propagating magneto-acoustic waves just below and in the transition region. In this study, we compute the vertically emergent intensity of the Si IV and Mg II h&k lines from a time series of snapshots of a magnetic element in a two-dimensional Radiative MHD simulation from the Bifrost code. We investigate the synthetic line profiles to detect the slow magneto-acoustic body wave (slow mode) which becomes a slow shock at the lower chromosphere in the magnetic element. We find that the Doppler shift of the line core gives the velocity amplitude of the longitudinal magneto-acoustic body wave. The contribution function of the line core indicates that the formation of Mg II h&k lines is associated with the propagating shocks and therefore the time evolution of the line core intensity represents the propagating shocks projected on the optical surface. We will report on measurement of the energy flux of slow modes in the magnetic elements by using IRIS observations. Title: The Atmospheric Response to High Fluxes of Nonthermal Electrons during M Dwarf Flares Authors: Kowalski, Adam; Allred, J. C.; Carlsson, M.; Hawley, S. L.; Holman, G. D.; Mathioudakis, M.; Osten, R. A.; Uitenbroek, H. Bibcode: 2014AAS...22315117K Altcode: Flares are thought to be the result of magnetic fields in the stellar corona that undergo reconnection and accelerate charged particles into the lower atmosphere. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes can be used to constrain the heating mechanism of the lower stellar atmosphere. These observations show several ubiquitous properties of the continuum emission, which is not reproduced by models that use typical “solar-type” heating functions. We present results from a grid of new flare models using the RADYN code, which simultaneously calculates the radiative transfer and hydrodynamics on short timescales. We explore the atmospheric response to a short ~2 second burst of a very high heating rate from nonthermal electrons using a solar-type heating function, and we propose a new “M dwarf-type” heating variation that explains a range of observed spectral properties, such as ~10,000 K blackbody emission and a smooth continuum across the Balmer jump wavelength (3646A). Title: Comparison between IRIS Data and Numerical Models Authors: Carlsson, Mats; De Pontieu, Bart; Hansteen, Viggo; Pereira, Tiago; Leenaarts, Jorritt Bibcode: 2014cosp...40E.458C Altcode: The enigmatic chromosphere is the transition between the solar surface and the eruptive outer solar atmosphere. The chromosphere harbours and constrains the mass and energy loading processes that define the heating of the corona, the acceleration and the composition of the solar wind, and the energetics and triggering of solar outbursts (filament eruptions, flares, coronal mass ejections). The chromosphere is arguably the most difficult and least understood domain of solar physics. All at once it represents the transition from optically thick to thin radiation escape, from gas-pressure domination to magnetic-pressure domination, from neutral to ionised state, from MHD to plasma physics, and from near-equilibrium ("LTE") to non-equilibrium conditions. IRIS provides a leap in observational capability of the chromospheric plasma with an unprecedented combination of high spatial, temporal and spectral resolution in lines with diagnostic information all the way from the photosphere to the upper transition region. To fully extract this information it is necessary to combine the observations with numerical simulations that include a realistic description of the complicated physics of the chromosphere. In this talk, we will present such realistic simulations, spanning the solar atmosphere from the convection zone to the corona, and synthetic observations calculated from the simulations. These synthetic observations are compared with observations from IRIS. Title: Impact of the Partial Ionization in the solar atmosphere using 2.5D Radiative MHD Simulations Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo; Carlsson, Mats Bibcode: 2014cosp...40E2019M Altcode: The chromosphere/transition region constitute the interface between the solar surface and the corona and modulate the flow of mass and energy into the upper atmosphere. IRIS was launched in 2013 to study the chromosphere and transition region. The complexity of the chromosphere is due to various regime changes that take place across it, like: Hydrogen goes from predominantly neutral to predominantly ionized; the plasma behavior changes from collisional to collision-less; it goes from gas-pressure dominated to magnetically driven, etc. Consequently, the interpretation of chromospheric observations in general and those from IRIS, in particular, is a challenging task. It is thus crucial to combine IRIS observations with advanced radiative-MHD numerical modeling. Because the photosphere, chromosphere and transition region are partially ionized, the interaction between ionized and neutral particles has important consequences on the magneto-thermodynamics of these regions. We implemented the effects of partial ionization using generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011) which includes full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. I will describe the importance and impact of taking into account partial ionization effects in the modeled radiative-MHD atmosphere, such as chromospheric heating, photospheric magnetic field diffused into the upper-chromosphere which expands into the upper atmosphere filling the corona with mass, magnetic flux, energy and current, etc. Title: Effects of flux emergence in the outer solar atmosphere. Observational advances Authors: Ortiz Carbonell, Ada; De Pontieu, Bart; Bellot Rubio, L. R.; Hansteen, Viggo; Rouppe van der Voort, Luc; Carlsson, Mats Bibcode: 2014cosp...40E2387O Altcode: We study granular sized magnetic flux emergence events that occur in a flux emergence region in NOAA 11850 on September 25, 2013. During that time, the first co-observing campaign between the Swedish 1 m Solar Telescope and the IRIS spacecraft was carried out. Simultaneous observations of the Halpha 656.28 nm and Ca II 854.2 nm chromospheric lines, and the Fe I 630.25 nm photospheric line, were made with the CRISP/SST spectropolarimeter reaching a spatial resolution of 0."14. At the same time, IRIS was performing a four-step dense raster of the said emerging flux region, taking slit-jaw images at 133 (C II transiti on region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere), obtaining thus the highest resolution images ever taken of the upper chromosphere and transition region. The photospheric and chromospheric properties of the emerging magnetic flux bubbles have been described in detail in Ortiz et al. (2014). However, in the current work we are able to follow such lower atmosphere observations of flux emergence up to the transition region with unprecedented spatial and temporal resolution. We describe the properties (size, time delays, lifetime, velocities, temperature) of the observed signatures of flux emergence in the transition region. We believe this may be an important mechanism of transporting energy and magnetic flux to the upper layers of the solar atmosphere, namely the transition region and corona, at least in cases when active regions are formed by flux emerging through the photosphere. * Ortiz et al. (2014) ApJ 781, 126 Title: The Formation of IRIS Diagnostics. III. Near-ultraviolet Spectra and Images Authors: Pereira, T. M. D.; Leenaarts, J.; De Pontieu, B.; Carlsson, M.; Uitenbroek, H. Bibcode: 2013ApJ...778..143P Altcode: 2013arXiv1310.1926P The Mg II h&k lines are the prime chromospheric diagnostics of NASA's Interface Region Imaging Spectrograph (IRIS). In the previous papers of this series, we used a realistic three-dimensional radiative magnetohydrodynamics model to calculate the h&k lines in detail and investigated how their spectral features relate to the underlying atmosphere. In this work, we employ the same approach to investigate how the h&k diagnostics fare when taking into account the finite resolution of IRIS and different noise levels. In addition, we investigate the diagnostic potential of several other photospheric lines and near-continuum regions present in the near-ultraviolet (NUV) window of IRIS and study the formation of the NUV slit-jaw images. We find that the instrumental resolution of IRIS has a small effect on the quality of the h&k diagnostics; the relations between the spectral features and atmospheric properties are mostly unchanged. The peak separation is the most affected diagnostic, but mainly due to limitations of the simulation. The effects of noise start to be noticeable at a signal-to-noise ratio (S/N) of 20, but we show that with noise filtering one can obtain reliable diagnostics at least down to a S/N of 5. The many photospheric lines present in the NUV window provide velocity information for at least eight distinct photospheric heights. Using line-free regions in the h&k far wings, we derive good estimates of photospheric temperature for at least three heights. Both of these diagnostics, in particular the latter, can be obtained even at S/Ns as low as 5. Title: The Formation of IRIS Diagnostics. II. The Formation of the Mg II h&k Lines in the Solar Atmosphere Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek, H.; De Pontieu, B. Bibcode: 2013ApJ...772...90L Altcode: 2013arXiv1306.0671L NASA's Interface Region Imaging Spectrograph (IRIS) small explorer mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations requires forward modeling of Mg II h&k line formation from three-dimensional (3D) radiation-magnetohydrodynamic (RMHD) models. This paper is the second in a series where we undertake this modeling. We compute the vertically emergent h&k intensity from a snapshot of a dynamic 3D RMHD model of the solar atmosphere, and investigate which diagnostic information about the atmosphere is contained in the synthetic line profiles. We find that the Doppler shift of the central line depression correlates strongly with the vertical velocity at optical depth unity, which is typically located less than 200 km below the transition region (TR). By combining the Doppler shifts of the h and k lines we can retrieve the sign of the velocity gradient just below the TR. The intensity in the central line depression is anti-correlated with the formation height, especially in subfields of a few square Mm. This intensity could thus be used to measure the spatial variation of the height of the TR. The intensity in the line-core emission peaks correlates with the temperature at its formation height, especially for strong emission peaks. The peaks can thus be exploited as a temperature diagnostic. The wavelength difference between the blue and red peaks provides a diagnostic of the velocity gradients in the upper chromosphere. The intensity ratio of the blue and red peaks correlates strongly with the average velocity in the upper chromosphere. We conclude that the Mg II h&k lines are excellent probes of the very upper chromosphere just below the TR, a height regime that is impossible to probe with other spectral lines. They also provide decent temperature and velocity diagnostics of the middle chromosphere. Title: The Formation of IRIS Diagnostics. I. A Quintessential Model Atom of Mg II and General Formation Properties of the Mg II h&k Lines Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek, H.; De Pontieu, B. Bibcode: 2013ApJ...772...89L Altcode: 2013arXiv1306.0668L NASA's Interface Region Imaging Spectrograph (IRIS) space mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations will require forward modeling of Mg II h&k line formation from three-dimensional (3D) radiation-MHD models. This paper is the first in a series where we undertake this forward modeling. We discuss the atomic physics pertinent to h&k line formation, present a quintessential model atom that can be used in radiative transfer computations, and discuss the effect of partial redistribution (PRD) and 3D radiative transfer on the emergent line profiles. We conclude that Mg II h&k can be modeled accurately with a four-level plus continuum Mg II model atom. Ideally radiative transfer computations should be done in 3D including PRD effects. In practice this is currently not possible. A reasonable compromise is to use one-dimensional PRD computations to model the line profile up to and including the central emission peaks, and use 3D transfer assuming complete redistribution to model the central depression. Title: Chromospheric Lyman Alpha SpectroPolarimeter: CLASP Authors: Kobayashi, Ken; Kano, R.; Trujillo Bueno, J.; Winebarger, A. R.; Cirtain, J. W.; Bando, T.; De Pontieu, B.; Ishikawa, R.; Katsukawa, Y.; Kubo, M.; Narukage, N.; Sakao, T.; Tsuneta, S.; Auchère, F.; Asensio Ramos, A.; Belluzzi, L.; Carlsson, M.; Casini, R.; Hara, H.; Ichimoto, K.; Manso Sainz, R.; Shimizu, T.; Stepan, J.; Suematsu, Y.; Holloway, T. Bibcode: 2013SPD....44..142K Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a VUV spectropolarimeter optimized for measuring the linear polarization of the Lyman-alpha line (121.6 nm). The Lyman-alpha line is predicted to show linear polarization caused by atomic scattering in the chromosphere and modified by the magnetic field through the Hanle effect. The Hanle effect is sensitive to weaker magnetic fields than Zeeman effect, and is not canceled by opposing fields, making it sensitive to tangled or unresolved magnetic field structures. These factors make the Hanle effect a valuable tool for probing the magnetic field in the chromosphere above the quiet sun. To meet this goal, CLASP is designed to measure linear polarization with 0.1% polarization sensitivity at 0.01 nm spectral resolution and 10" spatial resolution. CLASP is scheduled to be launched in 2015. Title: A Detailed Comparison between the Observed and Synthesized Properties of a Simulated Type II Spicule Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit; Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc Bibcode: 2013ApJ...771...66M Altcode: 2013arXiv1305.2397M We have performed a three-dimensional radiative MHD simulation of the solar atmosphere. This simulation shows a jet-like feature that shows similarities to the type II spicules observed for the first time with Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the solar disk are associated with these spicules. Observational results suggest they may contribute significantly in supplying the corona with hot plasma. We perform a detailed comparison of the properties of the simulated jet with those of type II spicules (observed with Hinode) and RBEs (with ground-based instruments). We analyze a wide variety of synthetic emission and absorption lines from the simulations including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition region and coronal temperatures (10,000 K to several million K). We compare their synthetic intensities, line profiles, Doppler shifts, line widths, and asymmetries with observations from Hinode/SOT and EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many properties of the synthetic observables resemble the observations, and we describe in detail the physical processes that lead to these observables. Detailed analysis of the synthetic observables provides insight into how observations should be analyzed to derive information about physical variables in such a dynamic event. For example, we find that line-of-sight superposition in the optically thin atmosphere requires the combination of Doppler shifts and spectral line asymmetry to determine the velocity in the jet. In our simulated type II spicule, the lifetime of the asymmetry of the transition region lines is shorter than that of the coronal lines. Other properties differ from the observations, especially in the chromospheric lines. The mass density of the part of the spicule with a chromospheric temperature is too low to produce significant opacity in chromospheric lines. The synthetic Ca II 8542 Å and Hα profiles therefore do not show signal resembling RBEs. These and other discrepancies are described in detail, and we discuss which mechanisms and physical processes may need to be included in the MHD simulations to mimic the thermodynamic processes of the chromosphere and corona, in particular to reproduce type II spicules. Title: The Interface Region Imaging Spectrograph (IRIS) Authors: De Pontieu, Bart; Title, A. M.; Lemen, J.; Wuelser, J.; Tarbell, T. D.; Schrijver, C. J.; Golub, L.; Kankelborg, C.; Carlsson, M.; Hansteen, V. H.; Worden, S.; IRIS Team Bibcode: 2013SPD....44...03D Altcode: The solar chromosphere and transition region (TR) form a highly structured and dynamic interface region between the photosphere and the corona. This region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona. Nevertheless, the chromosphere remains poorly understood, because of the complexity of the required observational and analytical tools: the interface region is highly complex with transitions from optically thick to optically thin radiation, from pressure to magnetic field domination, and large density and temperature contrasts on small spatial scales. The Interface Region Imaging Spectrograph (IRIS) was selected for a NASA SMEX mission in 2009 and is scheduled to launch on 26-June-2013 (with first light scheduled for mid July). IRIS addresses critical questions: (1) Which types of non-thermal energy dominate in the chromosphere and beyond? (2) How does the chromosphere regulate mass and energy supply to the corona and heliosphere? (3) How do magnetic flux and matter rise through the lower atmosphere, and what role does flux emergence play in flares and mass ejections? These questions are addressed with a high-resolution near and far UV imaging spectrometer sensitive to emission from plasma at temperatures between 5,000 K and 10 MK. IRIS has a field-of-view of 120 arcsec, a spatial resolution of 0.4 arcsec, and velocity resolution of 0.5 km/s. The IRIS investigation includes a strong numerical modeling component based on advanced radiative MHD codes to facilitate interpretation of observations. We describe the IRIS instrumentation and numerical modeling, and present the plans for observations, calibration and data distribution. We will highlight some of the issues that IRIS observations can help resolve. More information can be found at http://iris.lmsal.com Title: A new approach to model particle acceleration and energy transfer in solar flares Authors: Rubio Da Costa, Fatima; Zuccarello, F.; Fletcher, L.; Labrosse, N.; Kasparova, J.; Prosecký, T.; Carlsson, M.; Petrosian, V.; Liu, W. Bibcode: 2013SPD....4440401R Altcode: Motivated by available observations of two different flares in Lyα and Hα, we model the conditions of the solar atmosphere using a radiation hydrodynamics code (RADYN, Carlsson & Stein, 1992) and analyze the energy transport carried by a beam of non-thermal electrons injected at the top of a 1D coronal loop. The numerical Lyα and Hα intensities match with the observations. The electron energy distribution is assumed to follow a power law of the form (E/Ec )-δ for energies greater than a cutoff value of Ec. Abbett & Hawley (1999) and Allred et al. (2005) assumed that the non-thermal electrons flux injected at the top of a flaring loop, the cut-off energy and the power law index are constant over time. An improvement was achieved by Allred & Hawley (2006), who modified the RADYN code in such a way that the input parameters were time dependent. Their inputs were based on observations of a flare obtained with RHESSI. By combining RADYN with the “flare” code from Stanford University which models the acceleration and transport of particles and radiation of solar flares in non-LTE regime, we can calculate the non-thermal electrons flux, the cut-off energy and the power law index at every simulated time step. The atmospheric parameters calculated by RADYN could in turn be used as updated inputs for "flare", providing several advantages over the results from Liu et al. (2009), who combined the particle acceleration code with a 1-D hydrodynamic code, improving the atmospheric conditions. Title: Numerical Simulations of Spicule Acceleration Authors: Guerreiro, N.; Carlsson, M.; Hansteen, V. Bibcode: 2013ApJ...766..128G Altcode: Observations in the Hα line of hydrogen and the H and K lines of singly ionized calcium on the solar limb reveal the existence of structures with jet-like behavior, usually designated as spicules. The driving mechanism for such structures remains poorly understood. Sterling et al. shed some light on the problem mimicking reconnection events in the chromosphere with a one-dimensional code by injecting energy with different spatial and temporal distributions and tracing the thermodynamic evolution of the upper chromospheric plasma. They found three different classes of jets resulting from these injections. We follow their approach but improve the physical description by including non-LTE cooling in strong spectral lines and non-equilibrium hydrogen ionization. Increased cooling and conversion of injected energy into hydrogen ionization energy instead of thermal energy both lead to weaker jets and smaller final extent of the spicules compared with Sterling et al. In our simulations we find different behavior depending on the timescale for hydrogen ionization/recombination. Radiation-driven ionization fronts also form. Title: Heating of the Magnetic Chromosphere: Observational Constraints from Ca II λ8542 Spectra Authors: de la Cruz Rodríguez, J.; De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M. Bibcode: 2013ApJ...764L..11D Altcode: 2013arXiv1301.3141D The heating of the Sun's chromosphere remains poorly understood. While progress has been made on understanding what drives the quiet-Sun internetwork chromosphere, chromospheric heating in strong magnetic field regions continues to present a difficult challenge, mostly because of a lack of observational constraints. We use high-resolution spectropolarimetric data from the Swedish 1 m Solar Telescope to identify the location and spatio-temporal properties of heating in the magnetic chromosphere. In particular, we report the existence of raised-core spectral line profiles in the Ca II λ8542 line. These profiles are characterized by the absence of an absorption line core, showing a quasi-flat profile between λ ≈ ±0.5 Å, and are abundant close to magnetic bright points and plage. Comparison with three-dimensional MHD simulations indicates that such profiles occur when the line of sight goes through an "elevated temperature canopy" associated with the expansion with height of the magnetic field of flux concentrations. This temperature canopy in the simulations is caused by ohmic dissipation where there are strong magnetic field gradients. The raised-core profiles are thus indicators of locations of increased chromospheric heating. We characterize the location and temporal and spatial properties of such profiles in our observations, thus providing much stricter constraints on theoretical models of chromospheric heating mechanisms than before. Title: The Effects of Spatio-temporal Resolution on Deduced Spicule Properties Authors: Pereira, Tiago M. D.; De Pontieu, Bart; Carlsson, Mats Bibcode: 2013ApJ...764...69P Altcode: 2012arXiv1212.2969P Spicules have been observed on the Sun for more than a century, typically in chromospheric lines such as Hα and Ca II H. Recent work has shown that so-called "type II" spicules may have a role in providing mass to the corona and the solar wind. In chromospheric filtergrams these spicules are not seen to fall back down, and they are shorter lived and more dynamic than the spicules that have been classically reported in ground-based observations. Observations of type II spicules with Hinode show fundamentally different properties from what was previously measured. In earlier work we showed that these dynamic type II spicules are the most common type, a view that was not properly identified by early observations. The aim of this work is to investigate the effects of spatio-temporal resolution in the classical spicule measurements. Making use of Hinode data degraded to match the observing conditions of older ground-based studies, we measure the properties of spicules with a semi-automated algorithm. These results are then compared to measurements using the original Hinode data. We find that degrading the data has a significant effect on the measured properties of spicules. Most importantly, the results from the degraded data agree well with older studies (e.g., mean spicule duration more than 5 minutes, and upward apparent velocities of about 25 km s-1). These results illustrate how the combination of spicule superposition, low spatial resolution and cadence affect the measured properties of spicules, and that previous measurements can be misleading. Title: The Interface Region Imaging Spectrograph (IRIS) Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Wuelser, J.; Tarbell, T. D.; Schrijver, C.; Golub, L.; Kankelborg, C. C.; Hansteen, V. H.; Carlsson, M. Bibcode: 2012AGUFMSH33D2256D Altcode: The solar chromosphere and transition region (TR) form a highly structured and dynamic interface region between the photosphere and the corona. This region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona. Nevertheless, the chromosphere remains poorly understood, because of the complexity of the required observational and analytical tools: the interface region is highly complex with transitions from optically thick to optically thin radiation, from pressure to magnetic field domination, and large density and temperature contrasts on small spatial scales. The Interface Region Imaging Spectrograph (IRIS) was selected for a NASA SMEX mission in 2009 and is scheduled to launch in early 2013. IRIS addresses critical questions: (1) Which types of non-thermal energy dominate in the chromosphere and beyond? (2) How does the chromosphere regulate mass and energy supply to the corona and heliosphere? (3) How do magnetic flux and matter rise through the lower atmosphere, and what role does flux emergence play in flares and mass ejections? These questions are addressed with a high-resolution near and far UV imaging spectrometer sensitive to emission from plasma at temperatures between 5,000 K and 10 MK. IRIS has a field-of-view of 120 arcsec, a spatial resolution of 0.4 arcsec, and velocity resolution of 0.5 km/s. The IRIS investigation includes a strong numerical modeling component based on advanced radiative MHD codes to facilitate interpretation of observations. We will describe the IRIS instrumentation and numerical modeling, and present the status of the IRIS observatory development. We will highlight some of the issues that IRIS observations can help resolve. Title: Chromospheric Magnetic Fields: Observations, Simulations and their Interpretation Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.; Leenaarts, J. Bibcode: 2012ASPC..463...15D Altcode: 2012arXiv1203.4577D The magnetic field of the quiet-Sun chromosphere remains a mystery for solar physicists. The reduced number of chromospheric lines are intrinsically hard to model and only a few of them are magnetically sensitive. In this work, we use a 3D numerical simulation of the outer layers of the solar atmosphere, to asses the reliability of non-LTE inversions, in this case applied to the Ca II λ8542 Å line. We show that NLTE inversions provide realistic estimates of physical quantities from synthetic observations. Title: 2nd ATST-EAST Workshop in Solar Physics: Magnetic Fields from the Photosphere to the Corona Authors: Rimmele, T. R.; Tritschler, A.; Wöger, F.; Collados Vera, M.; Socas-Navarro, H.; Schlichenmaier, R.; Carlsson, M.; Berger, T.; Cadavid, A.; Gilbert, P. R.; Goode, P. R.; Knölker, M. Bibcode: 2012ASPC..463.....R Altcode: No abstract at ADS Title: Quantifying Spicules Authors: Pereira, Tiago M. D.; De Pontieu, Bart; Carlsson, Mats Bibcode: 2012ApJ...759...18P Altcode: 2012arXiv1208.4404P Understanding the dynamic solar chromosphere is fundamental in solar physics. Spicules are an important feature of the chromosphere, connecting the photosphere to the corona, potentially mediating the transfer of energy and mass. The aim of this work is to study the properties of spicules over different regions of the Sun. Our goal is to investigate if there is more than one type of spicule, and how spicules behave in the quiet Sun, coronal holes, and active regions. We make use of high cadence and high spatial resolution Ca II H observations taken by Hinode/Solar Optical Telescope. Making use of a semi-automated detection algorithm, we self-consistently track and measure the properties of 519 spicules over different regions. We find clear evidence of two types of spicules. Type I spicules show a rise and fall and have typical lifetimes of 150-400 s and maximum ascending velocities of 15-40 km s-1, while type II spicules have shorter lifetimes of 50-150 s, faster velocities of 30-110 km s-1, and are not seen to fall down, but rather fade at around their maximum length. Type II spicules are the most common, seen in the quiet Sun and coronal holes. Type I spicules are seen mostly in active regions. There are regional differences between quiet-Sun and coronal hole spicules, likely attributable to the different field configurations. The properties of type II spicules are consistent with published results of rapid blueshifted events (RBEs), supporting the hypothesis that RBEs are their disk counterparts. For type I spicules we find the relations between their properties to be consistent with a magnetoacoustic shock wave driver, and with dynamic fibrils as their disk counterpart. The driver of type II spicules remains unclear from limb observations. Title: Investigating the Reliability of Coronal Emission Measure Distribution Diagnostics using Three-dimensional Radiative Magnetohydrodynamic Simulations Authors: Testa, Paola; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats Bibcode: 2012ApJ...758...54T Altcode: 2012arXiv1208.4286T Determining the temperature distribution of coronal plasmas can provide stringent constraints on coronal heating. Current observations with the Extreme ultraviolet Imaging Spectrograph (EIS) on board Hinode and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory provide diagnostics of the emission measure distribution (EMD) of the coronal plasma. Here we test the reliability of temperature diagnostics using three-dimensional radiative MHD simulations. We produce synthetic observables from the models and apply the Monte Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with the "true" distributions from the model, we assess the limitations of the diagnostics as a function of the plasma parameters and the signal-to-noise ratio of the data. We find that EMDs derived from EIS synthetic data reproduce some general characteristics of the true distributions, but usually show differences from the true EMDs that are much larger than the estimated uncertainties suggest, especially when structures with significantly different density overlap along the line of sight. When using AIA synthetic data the derived EMDs reproduce the true EMDs much less accurately, especially for broad EMDs. The differences between the two instruments are due to the: (1) smaller number of constraints provided by AIA data and (2) broad temperature response function of the AIA channels which provide looser constraints to the temperature distribution. Our results suggest that EMDs derived from current observatories may often show significant discrepancies from the true EMDs, rendering their interpretation fraught with uncertainty. These inherent limitations to the method should be carefully considered when using these distributions to constrain coronal heating. Title: The Hanle Effect of Lyα in a Magnetohydrodynamic Model of the Solar Transition Region Authors: Štěpán, J.; Trujillo Bueno, J.; Carlsson, M.; Leenaarts, J. Bibcode: 2012ApJ...758L..43S Altcode: 2012arXiv1208.4929S In order to understand the heating of the solar corona it is crucial to obtain empirical information on the magnetic field in its lower boundary (the transition region). To this end, we need to measure and model the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Lyα line. The interpretation of the observed Stokes profiles will require taking into account that the outer solar atmosphere is highly structured and dynamic, and that the height of the transition region may well vary from one place in the atmosphere to another. Here, we report on the Lyα scattering polarization signals we have calculated in a realistic model of an enhanced network region, resulting from a state-of-the-art radiation magnetohydrodynamic simulation. This model is characterized by spatially complex variations of the physical quantities at transition region heights. The results of our investigation lead us to emphasize that scattering processes in the upper solar chromosphere should indeed produce measurable linear polarization in Lyα. More importantly, we show that via the Hanle effect the model's magnetic field produces significant changes in the emergent Q/I and U/I profiles. Therefore, we argue that by measuring the polarization signals produced by scattering processes and the Hanle effect in Lyα and contrasting them with those computed in increasingly realistic atmospheric models, we should be able to decipher the magnetic, thermal, and dynamic structure of the upper chromosphere and transition region of the Sun. Title: Chromospheric Lyman-alpha spectro-polarimeter (CLASP) Authors: Kano, Ryouhei; Bando, Takamasa; Narukage, Noriyuki; Ishikawa, Ryoko; Tsuneta, Saku; Katsukawa, Yukio; Kubo, Masahito; Ishikawa, Shin-nosuke; Hara, Hirohisa; Shimizu, Toshifumi; Suematsu, Yoshinori; Ichimoto, Kiyoshi; Sakao, Taro; Goto, Motoshi; Kato, Yoshiaki; Imada, Shinsuke; Kobayashi, Ken; Holloway, Todd; Winebarger, Amy; Cirtain, Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier; Štepán, Jiří; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Auchère, Frédéric; Carlsson, Mats Bibcode: 2012SPIE.8443E..4FK Altcode: One of the biggest challenges in heliophysics is to decipher the magnetic structure of the solar chromosphere. The importance of measuring the chromospheric magnetic field is due to both the key role the chromosphere plays in energizing and structuring the outer solar atmosphere and the inability of extrapolation of photospheric fields to adequately describe this key boundary region. Over the last few years, significant progress has been made in the spectral line formation of UV lines as well as the MHD modeling of the solar atmosphere. It is found that the Hanle effect in the Lyman-alpha line (121.567 nm) is a most promising diagnostic tool for weaker magnetic fields in the chromosphere and transition region. Based on this groundbreaking research, we propose the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) to NASA as a sounding rocket experiment, for making the first measurement of the linear polarization produced by scattering processes and the Hanle effect in the Lyman-alpha line (121.567 nm), and making the first exploration of the magnetic field in the upper chromosphere and transition region of the Sun. The CLASP instrument consists of a Cassegrain telescope, a rotating 1/2-wave plate, a dual-beam spectrograph assembly with a grating working as a beam splitter, and an identical pair of reflective polarization analyzers each equipped with a CCD camera. We propose to launch CLASP in December 2014. Title: Implications for Coronal Heating from Coronal Rain Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort, L.; Vissers, G.; Hansteen, V. Bibcode: 2012ASPC..454..171A Altcode: Coronal rain is a phenomenon above active regions in which cool plasma condensations fall down from coronal heights. Numerical simulations of loops have shown that such condensations can naturally form in the case of footpoint concentrated heating through the “catastrophic cooling” mechanism. In this work we analize high resolution limb observations in Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of SST and derive statistical properties. We further investigate the link between coronal rain and the coronal heating mechanisms by performing 1.5-D MHD simulations of a loop subject to footpoint heating and to Alfvén waves generated in the photosphere. It is found that if a loop is heated predominantly from Alfvén waves coronal rain is inhibited due to the characteristic uniform heating they produce. Hence coronal rain can point both to the spatial distribution of the heating and to the agent of the heating itself, thus acting as a marker for coronal heating mechanisms. Title: Non-local thermodynamic equilibrium inversions from a 3D magnetohydrodynamic chromospheric model Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.; Leenaarts, J. Bibcode: 2012A&A...543A..34D Altcode: 2012arXiv1205.3171D Context. The structure of the solar chromosphere is believed to be governed by magnetic fields, even in quiet-Sun regions that have a relatively weak photospheric field. During the past decade inversion methods have emerged as powerful tools for analyzing the chromosphere of active regions. The applicability of inversions to infer the stratification of the physical conditions in a dynamic 3D solar chromosphere has not yet been studied in detail.
Aims: This study aims to establish the diagnostic capabilities of non-local thermodynamical equilibrium (NLTE) inversion techniques of Stokes profiles induced by the Zeeman effect in the Ca ii λ8542 Å line.
Methods: We computed the Ca ii atomic level populations in a snapshot from a 3D radiation-MHD simulation of the quiet solar atmosphere in non-LTE using the 3D radiative transfer code Multi3d. These populations were used to compute synthetic full-Stokes profiles in the Ca ii λ8542 Å line using 1.5D radiative transfer and the inversion code Nicole. The profiles were then spectrally degraded to account for finite filter width, and Gaussian noise was added to account for finite photon flux. These profiles were inverted using Nicole and the results were compared with the original model atmosphere.
Results: Our NLTE inversions applied to quiet-Sun synthetic observations provide reasonably good estimates of the chromospheric magnetic field, line-of-sight velocities and somewhat less accurate, but still very useful, estimates of the temperature. Three-dimensional scattering of photons cause cool pockets in the chromosphere to be invisible in the line profile and consequently they are also not recovered by the inversions. To successfully detect Stokes linear polarization in this quiet snapshot, a noise level below 10-3.5 is necessary. Title: Ubiquitous Torsional Motions in Type II Spicules Authors: De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M.; Rutten, R. J.; Hansteen, V. H.; Watanabe, H. Bibcode: 2012ApJ...752L..12D Altcode: 2012arXiv1205.5006D Spicules are long, thin, highly dynamic features that jut out ubiquitously from the solar limb. They dominate the interface between the chromosphere and corona and may provide significant mass and energy to the corona. We use high-quality observations with the Swedish 1 m Solar Telescope to establish that so-called type II spicules are characterized by the simultaneous action of three different types of motion: (1) field-aligned flows of order 50-100 km s-1, (2) swaying motions of order 15-20 km s-1, and (3) torsional motions of order 25-30 km s-1. The first two modes have been studied in detail before, but not the torsional motions. Our analysis of many near-limb and off-limb spectra and narrowband images using multiple spectral lines yields strong evidence that most, if not all, type II spicules undergo large torsional modulation and that these motions, like spicule swaying, represent Alfvénic waves propagating outward at several hundred km s-1. The combined action of the different motions explains the similar morphology of spicule bushes in the outer red and blue wings of chromospheric lines, and needs to be taken into account when interpreting Doppler motions to derive estimates for field-aligned flows in spicules and determining the Alfvénic wave energy in the solar atmosphere. Our results also suggest that large torsional motion is an ingredient in the production of type II spicules and that spicules play an important role in the transport of helicity through the solar atmosphere. Title: A Sharp Look at Coronal Rain with Hinode/SOT and SST/CRISP Authors: Antolin, P.; Carlsson, M.; Rouppe van der Voort, L.; Verwichte, E.; Vissers, G. Bibcode: 2012ASPC..455..253A Altcode: 2012arXiv1202.0787A The tropical wisdom that when it is hot and dense we can expect rain might also apply to the Sun. Indeed, observations and numerical simulations have showed that strong heating at footpoints of loops, as is the case for active regions, puts their coronae out of thermal equilibrium, which can lead to a phenomenon known as catastrophic cooling. Following local pressure loss in the corona, hot plasma locally condenses in these loops and dramatically cools down to chromospheric temperatures. These blobs become bright in Hα and Ca ii H in time scales of minutes, and their dynamics seem to be subject more to internal pressure changes in the loop rather than to gravity. They thus become trackers of the magnetic field, which results in the spectacular coronal rain that is observed falling down coronal loops. In this work we report on high resolution observations of coronal rain with the Solar Optical Telescope (SOT) on Hinode and CRISP at the Swedish Solar Telescope (SST). A statistical study is performed in which properties such as velocities and accelerations of coronal rain are derived. We show how this phenomenon can constitute a diagnostic tool for the internal physical conditions inside loops. Furthermore, we analyze transverse oscillations of strand-like condensations composing coronal rain falling in a loop, and discuss the possible nature of the wave. This points to the important role that coronal rain can play in the fields of coronal heating and coronal seismology. Title: 4th Hinode Science Meeting: Unsolved Problems and Recent Insights Authors: Bellot Rubio, L.; Reale, F.; Carlsson, M. Bibcode: 2012ASPC..455.....B Altcode: 2012ASPC..455.....R No abstract at ADS Title: The Chromospheric Lyman-Alpha SpectroPolarimeter: CLASP Authors: Kobayashi, K.; Kano, R.; Trujillo-Bueno, J.; Asensio Ramos, A.; Bando, T.; Belluzzi, L.; Carlsson, M.; De Pontieu, R. C. B.; Hara, H.; Ichimoto, K.; Ishikawa, R.; Katsukawa, Y.; Kubo, M.; Manso Sainz, R.; Narukage, N.; Sakao, T.; Stepan, J.; Suematsu, Y.; Tsuneta, S.; Watanabe, H.; Winebarger, A. Bibcode: 2012ASPC..456..233K Altcode: The magnetic field plays a crucial role in the chromosphere and the transition region, and our poor empirical knowledge of the magnetic field in the upper chromosphere and transition region is a major impediment to advancing the understanding of the solar atmosphere. The Hanle effect promises to be a valuable alternative to Zeeman effect as a method of measuring the magnetic field in the chromosphere and transition region; it is sensitive to weaker magnetic fields, and also sensitive to tangled, unresolved field structures.

CLASP is a sounding rocket experiment that aims to observe the Hanle effect polarization of the Lyman α (1215.67Å) line in the solar chromosphere and transition region, and prove the usefulness of this technique in placing constraints on the magnetic field strength and orientation in the low plasma-β region of the solar atmosphere. The Ly-α line has been chosen because it is a chromospheric/transition-region line, and because the Hanle effect polarization of this line is predicted to be sensitive to 10-250 Gauss, encompassing the range of interest. The CLASP instrument is designed to measure linear polarization in the Ly-α line with a polarization sensitivity of 0.1%. The instrument is currently funded for development. The optical design of the instrument has been finalized, and an extensive series of component-level tests are underway to validate the design. Title: How Low-Quality Observations Affect Spicule Properties Authors: Pereira, Tiago M. D.; De Pontieu, B.; Carlsson, M. Bibcode: 2012AAS...22020306P Altcode: Spicules have been observed on the sun for more than 80 years, in several chromospheric lines such as H-alpha and Ca II H. Recent work has shown that spicules have the potential to drive the solar wind and heat the chromosphere, making them a hotly contested topic in solar physics. Despite the wealth of observations available, their properties are still a matter of debate. Difficulties in measuring their properties arise because spicules occur on short spatial and temporal scales, and are very abundant (superimposed) at the limb. Most of the older observations lacked either the spatial resolution or cadence necessary to measure spicules. This changed with Hinode/SOT, which has provided seeing-free observations with high cadence and spatial resolution. Using SOT observations, we find that in the quiet sun most spicules are shorter lived and can move much faster than previously measured. In this work we try to reconcile the recent results with results from older observations. We degrade SOT data to match the cadence and resolution of older data sets, and apply the same semi-automated method to detect and measure the properties of spicules to both the original and degraded data. We find that degrading the data has a significant effect on the measured properties of spicules. Most importantly, the results from the degraded data agree very well with older studies (e.g. mean spicule duration more than 5 minutes, and upward velocities of about 25 km/s). These results illustrate how the combination of spicule superposition, low spatial resolution, and cadence affect the measured properties of spicules, and that previous measurements can thus be unreliable. Title: The Formation of the Hα Line in the Solar Chromosphere Authors: Leenaarts, J.; Carlsson, M.; Rouppe van der Voort, L. Bibcode: 2012ApJ...749..136L Altcode: 2012arXiv1202.1926L We use state-of-the-art radiation-MHD simulations and three-dimensional (3D) non-LTE radiative transfer computations to investigate Hα line formation in the solar chromosphere and apply the results of this investigation to develop the potential of Hα as a diagnostic of the chromosphere. We show that one can accurately model Hα line formation assuming statistical equilibrium and complete frequency redistribution provided the computation of the model atmosphere included non-equilibrium ionization of hydrogen and the Lyα and Lyβ line profiles are described by Doppler profiles. We find that 3D radiative transfer is essential in modeling hydrogen lines due to the low photon destruction probability in Hα. The Hα opacity in the upper chromosphere is mainly sensitive to the mass density and only weakly sensitive to the temperature. We find that the Hα line-core intensity is correlated with the average formation height: The larger the average formation height is, the lower the intensity will be. The line-core width is a measure of the gas temperature in the line-forming region. The fibril-like dark structures seen in Hα line-core images computed from our model atmosphere are tracing magnetic field lines. These structures are caused by field-aligned ridges of enhanced chromospheric mass density that raise their average formation height, and therefore make them appear dark against their deeper-formed surroundings. We compare with observations, and find that the simulated line-core widths are very similar to the observed ones, without the need for additional microturbulence. Title: State-of-the-art of non-LTE diagnostics: observations and simulations Authors: Carlsson, Mats Bibcode: 2012decs.confE..32C Altcode: Advanced MHD simulations combined with non-LTE diagnostics are revolutionizing our view of chromospheric dynamics and heating. We will discuss how well synthetic observables compare with observations, how diagnostic codes can be used to derive physical information about the atmosphere, what is missing in current calculations and the consequences of current assumptions. Examples will focus on often used or to-be-used diagnostics of chromospheric lines, such as Ca II 8542, H-alpha and Mg II h/k. Title: Using 3D MHD realistic simulations of the solar corona to test plasma diagnostics Authors: Testa, P.; De Pontieu, B.; Martinez-Sykora, J.; Hansteen, V.; Carlsson, M. Bibcode: 2012decs.confE..27T Altcode: We synthesize coronal images and spectra from advanced 3D MHD simulations obtained from the state-of-the art Bifrost code, and explore how well they reproduce coronal observations with SDO/AIA and Hinode/EIS. We apply standard diagnostic techniques (e.g., density, and temperature diagnostics) to the synthetic observations and investigate how accurately the derived physical information matches the plasma parameters of the model. We discuss the limitations of the diagnostics and their implications. Title: NLTE inversions from a 3D MHD Chromospheric simulation Authors: de la Cruz Rodriguez, J.; Socas-Navarro, H.; Carlsson, M.; Leenaarts, J. Bibcode: 2012decs.confE..80D Altcode: The structure of the solar chromosphere is believed to be governed by magnetic fields, even in quiet Sun regions with a relatively weak field. Measuring the magnetic field of the solar chromosphere is an outstanding challenge for observers. Inversion codes allow for detailed interpretation of full-Stokes data from spectral lines formed in the chromosphere. However, the applicability of non-LTE inversions to infer physical conditions in the dynamic 3D solar chromosphere, has not yet been studied in detail. In this study, we use a snapshot from a 3D MHD simulation of quiet-sun, extending from the photosphere to the corona, to asses the reliability of non-LTE inversions to infer chromospheric quantities, especially the magnetic field. Title: The formation of the Halpha line in the solar chromosphere Authors: Leenaarts, J.; Carlsson; M.; Rouppe van der Voort, Rouppe, L. Bibcode: 2012decs.confE..14L Altcode: We use state-of-the-art radiation-MHD simulations and 3D non-LTE radiative transfer computations to investigate Halpha line formation in the solar chromosphere. We find that 3D radiative transfer is essential in modeling hydrogen lines due to the low photon destruction probability in Halpha. The Halpha opacity in the upper chromosphere is mainly sensitive to the mass density and only weakly sensitive to temperature. We find that the Halpha line-core intensity is correlated with the average formation height: the lower the intensity, the larger the average formation height. The line-core width is a measure of the gas temperature in the line-forming region. The fibril-like dark structures seen in Halpha line-core images computed from our model atmosphere are tracing magnetic field lines. These structures are caused by field-aligned ridges of enhanced chromospheric mass density that raise their average formation height, and therefore makes them appear dark against their deeper-formed surroundings. Title: Potential for diagnostics with IRIS and Mg II lines Authors: Pereira, Tiago M. D.; Carlsson, Mats; Leenaarts, Jorrit; Uitenbroek, Han; De Pontieu, Bart; Martinez-Sykora, Juan Bibcode: 2012decs.confE..13P Altcode: The IRIS mission will open up a new window into the solar chromosphere and transition region. An important diagnostic that IRIS will bring is the Mg II H and K lines. Radiation from these lines is believed to be come from a wide range of formation depths, from the higher photosphere to the onset of the transition region. With a complex formation mechanism, Mg II H and K suffer from departures from LTE and partial redistribution (PRD). In this preliminary analysis we will look into the potential for diagnostics of Mg II H and K. Using a new parallel version of the RH code we synthesised Mg II H and K spectra from 3D rMHD simulations of the solar atmosphere. We will discuss the relevance of several approximations on the final observables, and will compare the Mg II H and K filtergrams with those of Ca II H, a robust chromospheric diagnostic line widely used with Hinode/SOT/BFI. Title: The generation of shock waves traveling from the photosphere to the transition region within network magnetic elements Authors: Kato, Y.; Hansteen, V.; Steiner, O.; Carlsson, M. Bibcode: 2012decs.confE..54K Altcode: We investigate the generation of shock waves near the photosphere by convective downdrafts in the immediate surroundings of the magnetic flux concentration, using radiation magnetohydrodynamic (RMHD) 2D simulations of the solar atmosphere. The simulations comprise the layers from the upper convection zone to the lower corona. We call this the "magnetic pumping process". We find that the generated slow modes via magnetic pumping travel upward along the magnetic flux concentration, developing into a shock wave in chromospheric heights. The waves continue to propagate further up through the transition region and into the corona. In the course of propagation through the transition layer, a small fraction of the longitudinal slow mode is converted into a transverse wave mode. We report on how much energy is deposited by propagating shock waves through the transition region and we discuss the the dissipation process above the photosphere within the magnetic flux concentration.. Title: Approximations for radiative cooling and heating in the solar chromosphere Authors: Carlsson, M.; Leenaarts, J. Bibcode: 2012A&A...539A..39C Altcode: 2012arXiv1202.2996C Context. The radiative energy balance in the solar chromosphere is dominated by strong spectral lines that are formed out of LTE. It is computationally prohibitive to solve the full equations of radiative transfer and statistical equilibrium in 3D time dependent MHD simulations.
Aims: We look for simple recipes to compute the radiative energy balance in the dominant lines under solar chromospheric conditions.
Methods: We use detailed calculations in time-dependent and 2D MHD snapshots to derive empirical formulae for the radiative cooling and heating.
Results: The radiative cooling in neutral hydrogen lines and the Lyman continuum, the H and K and intrared triplet lines of singly ionized calcium and the h and k lines of singly ionized magnesium can be written as a product of an optically thin emission (dependent on temperature), an escape probability (dependent on column mass) and an ionization fraction (dependent on temperature). In the cool pockets of the chromosphere the same transitions contribute to the heating of the gas and similar formulae can be derived for these processes. We finally derive a simple recipe for the radiative heating of the chromosphere from incoming coronal radiation. We compare our recipes with the detailed results and comment on the accuracy and applicability of the recipes. Title: The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP)j Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Bando, T.; Belluzzi, L.; Casini, R.; Carlsson, M.; Cirtain, J. W.; De Pontieu, B.; Hara, H.; Ichimoto, K.; Ishikawa, R.; Kano, R.; Katsukawa, Y.; Kim, T.; Kubo, M.; Manso Sainz, R.; Narukage, N.; Asensio Ramos, A.; Robinson, B.; Sakao, T.; Shimizu, T.; Stepan, J.; Suematsu, Y.; Watanabe, H.; West, E.; Winebarger, A. R. Bibcode: 2011AGUFM.P14C..05K Altcode: We present an overview of the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) program. CLASP is a proposed sounding rocket experiment currently under development as collaboration between Japan, USA and Spain. The aim is to achieve the first measurement of magnetic field in the upper chromosphere and transition region of the Sun through the detection and measurement of Hanle effect polarization of the Lyman alpha line. The Hanle effect (i.e. the magnetic field induced modification of the linear polarization due to scattering processes in spectral lines) is believed to be a powerful tool for measuring the magnetic field in the upper chromosphere, as it is more sensitive to weaker magnetic fields than the Zeeman effect, and also sensitive to magnetic fields tangled at spatial scales too small to be resolved. The Lyman-alpha (121.567 nm) line has been chosen because it is a chromospheric/transition-region line, and because the Hanle effect polarization of the Lyman-alpha line is predicted to be sensitive to 10-250 Gauss, encompassing the range of interest. Hanle effect is predicted to be observable as linear polarization or depolarization, depending on the geometry, with a fractional polarization amplitude varying between 0.1% and 1% depending on the strength and orientation of the magnetic field. This quantification of the chromospheric magnetic field requires a highly sensitive polarization measurement. The CLASP instrument consists of a large aperture (287 mm) Cassegrain telescope mated to a polarizing beamsplitter and a matched pair of grating spectrographs. The polarizing beamsplitter consists of a continuously rotating waveplate and a linear beamsplitter, allowing simultaneous measurement of orthogonal polarizations and in-flight self-calibration. Development of the instrument is underway, and prototypes of all optical components have been tested using a synchrotron beamline. The experiment is proposed for flight in 2014. Title: Quantifying spicules Authors: Pereira, T. M.; De Pontieu, B.; Carlsson, M. Bibcode: 2011AGUFMSH34B..01P Altcode: Understanding the dynamic solar chromosphere is of paramount importance in solar physics. Spicules are an important feature of the chromosphere, connecting the photosphere to the corona, potentially mediating the transfer of energy and mass. While it is generally accepted that there is more than one type of spicule, their quick motions, small spatial scales, and short lifetimes have prevented a systematic study of their properties over different solar regions. In the present work we undertake such a study, using Ca H filtergrams from Hinode/SOT and a semi-automated method to detect and track the spicules. Looking at different magnetic field configurations (quiet Sun, coronal holes, active regions), we discuss how the properties of the spicules change, how the two spicule populations (type I and type II) are connected, and how spicules are related to other chromospheric phenomena such as dynamic fibrils. Title: Wave Propagation and Jet Formation in the Chromosphere Authors: Heggland, L.; Hansteen, V. H.; De Pontieu, B.; Carlsson, M. Bibcode: 2011ApJ...743..142H Altcode: 2011arXiv1112.0037H We present the results of numerical simulations of wave propagation and jet formation in solar atmosphere models with different magnetic field configurations. The presence in the chromosphere of waves with periods longer than the acoustic cutoff period has been ascribed to either strong inclined magnetic fields, or changes in the radiative relaxation time. Our simulations include a sophisticated treatment of radiative losses, as well as fields with different strengths and inclinations. Using Fourier and wavelet analysis techniques, we investigate the periodicity of the waves that travel through the chromosphere. We find that the velocity signal is dominated by waves with periods around 5 minutes in regions of strong, inclined field, including at the edges of strong flux tubes where the field expands, whereas 3 minute waves dominate in regions of weak or vertically oriented fields. Our results show that the field inclination is very important for long-period wave propagation, whereas variations in the radiative relaxation time have little effect. Furthermore, we find that atmospheric conditions can vary significantly on timescales of a few minutes, meaning that a Fourier analysis of wave propagation can be misleading. Wavelet techniques take variations with time into account and are more suitable analysis tools. Finally, we investigate the properties of jets formed by the propagating waves once they reach the transition region, and find systematic differences between the jets in inclined-field regions and those in vertical field regions, in agreement with observations of dynamic fibrils. Title: Generation and propagation of Alfvenic waves in spicules Authors: De Pontieu, B.; Okamoto, T. J.; Rouppe van der Voort, L.; Hansteen, V. H.; Carlsson, M. Bibcode: 2011AGUFMSH13B1956D Altcode: Both spicules and Alfven waves have recently been implicated in playing a role in the heating of the outer atmosphere. Yet we do not know how spicules or Alfven waves are generated. Here we focus on the properties of Alfvenic waves in spicules and their role in forming spicules. We use high-resolution observations taken with the Solar Optical Telescope onboard Hinode, and with the CRISP Fabry-Perot Interferometer at the Swedish Solar Telescope (SST) in La Palma to study the generation and propagation of Alfvenic waves in spicules and their disk counterparts. Using automated detection algorithms to identify propagating waves in limb spicules, we find evidence for both up- and downward propagating as well as standing waves. Our data suggests significant reflection of waves in and around spicules and provides constraints for theoretical models of spicules and wave propagation through the chromosphere. We also show observational evidence (using SST data) of the generation of Alfven waves and the role they play in forming spicules. Title: Testing coronal plasma diagnostics using 3D MHD models of the solar atmosphere Authors: Testa, P.; Martinez-Sykora, J.; Hansteen, V. H.; De Pontieu, B.; Carlsson, M. Bibcode: 2011AGUFMSH53C..06T Altcode: We synthesize coronal images and spectra from advanced 3D radiative MHD simulations obtained from the state-of-the-art Bifrost code, and explore how well they reproduce coronal observations with SDO/AIA and Hinode/EIS and XRT. We apply standard diagnostic techniques (e.g., density, temperature, abundance diagnostics) to the synthetic observations and investigate how accurately the derived physical information matches the plasma parameters of the model. We discuss the limitations of the diagnostics and their implications. Title: Overview of Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) Authors: Narukage, Noriyuki; Tsuneta, Saku; Bando, Takamasa; Kano, Ryouhei; Kubo, Masahito; Ishikawa, Ryoko; Hara, Hirohisa; Suematsu, Yoshinori; Katsukawa, Yukio; Watanabe, Hiroko; Ichimoto, Kiyoshi; Sakao, Taro; Shimizu, Toshifumi; Kobayashi, Ken; Robinson, Brian; Kim, Tony; Winebarger, Amy; West, Edward; Cirtain, Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier; Stepan, Jiri; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Carlsson, Mats Bibcode: 2011SPIE.8148E..0HN Altcode: 2011SPIE.8148E..16N The solar chromosphere is an important boundary, through which all of the plasma, magnetic fields and energy in the corona and solar wind are supplied. Since the Zeeman splitting is typically smaller than the Doppler line broadening in the chromosphere and transition region, it is not effective to explore weak magnetic fields. However, this is not the case for the Hanle effect, when we have an instrument with high polarization sensitivity (~ 0.1%). "Chromospheric Lyman- Alpha SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect linear polarization produced by the Hanle effect in Lyman-alpha line (121.567 nm) and to make the first direct measurement of magnetic fields in the upper chromosphere and lower transition region. To achieve the high sensitivity of ~ 0.1% within a rocket flight (5 minutes) in Lyman-alpha line, which is easily absorbed by materials, we design the optical system mainly with reflections. The CLASP consists of a classical Cassegrain telescope, a polarimeter and a spectrometer. The polarimeter consists of a rotating 1/2-wave plate and two reflecting polarization analyzers. One of the analyzer also works as a polarization beam splitter to give us two orthogonal linear polarizations simultaneously. The CLASP is planned to be launched in 2014 summer. Title: Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind Authors: McIntosh, Scott W.; de Pontieu, Bart; Carlsson, Mats; Hansteen, Viggo; Boerner, Paul; Goossens, Marcel Bibcode: 2011Natur.475..477M Altcode: Energy is required to heat the outer solar atmosphere to millions of degrees (refs 1, 2) and to accelerate the solar wind to hundreds of kilometres per second (refs 2-6). Alfvén waves (travelling oscillations of ions and magnetic field) have been invoked as a possible mechanism to transport magneto-convective energy upwards along the Sun's magnetic field lines into the corona. Previous observations of Alfvénic waves in the corona revealed amplitudes far too small (0.5kms-1) to supply the energy flux (100-200Wm-2) required to drive the fast solar wind or balance the radiative losses of the quiet corona. Here we report observations of the transition region (between the chromosphere and the corona) and of the corona that reveal how Alfvénic motions permeate the dynamic and finely structured outer solar atmosphere. The ubiquitous outward-propagating Alfvénic motions observed have amplitudes of the order of 20kms-1 and periods of the order of 100-500s throughout the quiescent atmosphere (compatible with recent investigations), and are energetic enough to accelerate the fast solar wind and heat the quiet corona. Title: Quiet-Sun imaging asymmetries in Na I D1 compared with other strong Fraunhofer lines Authors: Rutten, R. J.; Leenaarts, J.; Rouppe van der Voort, L. H. M.; de Wijn, A. G.; Carlsson, M.; Hansteen, V. Bibcode: 2011A&A...531A..17R Altcode: 2011arXiv1104.4307R Imaging spectroscopy of the solar atmosphere using the Na I D1 line yields marked asymmetry between the blue and red line wings: sampling a quiet-Sun area in the blue wing displays reversed granulation, whereas sampling in the red wing displays normal granulation. The Mg I b2 line of comparable strength does not show this asymmetry, nor does the stronger Ca II 8542 Å line. We demonstrate the phenomenon with near-simultaneous spectral images in Na I D1, Mg I b2, and Ca II 8542 Å from the Swedish 1-m Solar Telescope. We then explain it with line-formation insights from classical 1D modeling and with a 3D magnetohydrodynamical simulation combined with NLTE spectral line synthesis that permits detailed comparison with the observations in a common format. The cause of the imaging asymmetry is the combination of correlations between intensity and Dopplershift modulation in granular overshoot and the sensitivity to these of the steep profile flanks of the Na I D1 line. The Mg I b2 line has similar core formation but much wider wings due to larger opacity buildup and damping in the photosphere. Both lines obtain marked core asymmetry from photospheric shocks in or near strong magnetic concentrations, less from higher-up internetwork shocks that produce similar asymmetry in the spatially averaged Ca II 8542 Å profile. Title: The stellar atmosphere simulation code Bifrost. Code description and validation Authors: Gudiksen, B. V.; Carlsson, M.; Hansteen, V. H.; Hayek, W.; Leenaarts, J.; Martínez-Sykora, J. Bibcode: 2011A&A...531A.154G Altcode: 2011arXiv1105.6306G Context. Numerical simulations of stellar convection and photospheres have been developed to the point where detailed shapes of observed spectral lines can be explained. Stellar atmospheres are very complex, and very different physical regimes are present in the convection zone, photosphere, chromosphere, transition region and corona. To understand the details of the atmosphere it is necessary to simulate the whole atmosphere since the different layers interact strongly. These physical regimes are very diverse and it takes a highly efficient massively parallel numerical code to solve the associated equations.
Aims: The design, implementation and validation of the massively parallel numerical code Bifrost for simulating stellar atmospheres from the convection zone to the corona.
Methods: The code is subjected to a number of validation tests, among them the Sod shock tube test, the Orzag-Tang colliding shock test, boundary condition tests and tests of how the code treats magnetic field advection, chromospheric radiation, radiative transfer in an isothermal scattering atmosphere, hydrogen ionization and thermal conduction. Results.Bifrost completes the tests with good results and shows near linear efficiency scaling to thousands of computing cores. Title: On the minimum temperature of the quiet solar chromosphere Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Gudiksen, B. V. Bibcode: 2011A&A...530A.124L Altcode: 2011arXiv1104.5081L
Aims: We aim to provide an estimate of the minimum temperature of the quiet solar chromosphere.
Methods: We perform a 2D radiation-MHD simulation spanning the upper convection zone to the lower corona. The simulation includes non-LTE radiative transfer and an equation-of-state that includes non-equilibrium ionization of hydrogen and non-equilibrium H2 molecule formation. We analyze the reliability of the various assumptions made in our model in order to assess the realism of the simulation.
Results: Our simulation contains pockets of cool gas with down to 1660 K from 1 Mm up to 3.2 Mm height. It overestimates the radiative heating, and contains non-physical heating below 1660 K. Therefore we conclude that cool pockets in the quiet solar chromosphere might have even lower temperatures than in the simulation, provided that there exist areas in the chromosphere without significant magnetic heating. We suggest off-limb molecular spectroscopy to look for such cool pockets and 3D simulations including a local dynamo and a magnetic carpet to investigate Joule heating in the quiet chromosphere. Title: Helioseismic Studies With Multi-wavelength Data From HMI And AIA Onboard SDO Authors: Hill, Frank; Jain, K.; Tripathy, S.; Kholikov, S.; Gonzalez Hernandez, I.; Leibacher, J.; Howe, R.; Baudin, F.; Carlsson, M.; Chaplin, W.; Tarbell, T. Bibcode: 2011SPD....42.2111H Altcode: 2011BAAS..43S.2111H The successful launch of the Solar Dynamics Observatory (SDO) in February 2010 opens important, new possibilities for helioseismic exploration of the solar interior and atmosphere using multi-wavelength observations from multiple instruments. In order to better understand the solar interior and atmosphere, as well as the physics of the helioseismic modes and waves themselves, we exploit the potential of the Atmospheric Imaging Assembly (AIA) 1600 and 1700 Angstrom continuum measurements and the contemporaneous Helioseismic and Magnetic Imager (HMI) Fe I 6173.3 Angstrom velocity and intensity observations. Standard techniques of helioseismology e.g Sun-as-a-star, spherical harmonic analysis, ring diagrams, and time- distance analysis are applied to obtain acoustic mode parameters and other characteristics. Here we present our preliminary results, and interpret these in the context of the differences in the heights of formation of the lines. Title: A Sounding Rocket Experiment for Spectropolarimetric Observations with the Lyα Line at 121.6 nm (CLASP) Authors: Ishikawa, R.; Bando, T.; Fujimura, D.; Hara, H.; Kano, R.; Kobiki, T.; Narukage, N.; Tsuneta, S.; Ueda, K.; Wantanabe, H.; Kobayashi, K.; Trujillo Bueno, J.; Manso Sainz, R.; Stepan, J.; de Pontieu, B.; Carlsson, M.; Casini, R. Bibcode: 2011ASPC..437..287I Altcode: A team consisting of Japan, USA, Spain, and Norway is developing a high-throughput Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP), which is proposed to fly with a NASA sounding rocket in 2014. CLASP will explore the magnetism of the upper solar chromosphere and transition region via the Hanle effect of the Lyα line for the first time. This experiment requires spectropolarimetric observations with high polarimetric sensitivity (∼0.1%) and wavelength resolution (0.1 Å). The final spatial resolution (slit width) is being discussed taking into account the required high signal-to-noise ratio. We have demonstrated the performance of the Lyα polarimeter by extensively using the Ultraviolet Synchrotron ORbital Radiation Facility (UVSOR) at the Institute for Molecular Sciences. In this contribution, we report these measurements at UVSOR together with the current status of the CLASP project. 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: Solar velocity references from 3D HD photospheric models Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M. Bibcode: 2011A&A...528A.113D Altcode: 2011arXiv1101.2671D Context. The measurement of Doppler velocities in spectroscopic solar observations requires a reference for the local frame of rest. The rotational and radial velocities of the Earth and the rotation of the Sun introduce velocity offsets in the observations. Normally, good references for velocities are missing (e.g. telluric lines), especially in filter-based spectropolarimetric observations.
Aims: We determine an absolute reference for line-of-sight velocities measured from solar observations for any heliocentric angle, calibrating the convective line shift of spatially-averaged profiles on quiet sun from a 3D hydrodynamical simulation. This method works whenever there is quiet sun in the field-of-view, and it has the advantage of being relatively insensitive to uncertainties in the atomic data.
Methods: We carry out radiative transfer computations in LTE for selected C i and Fe i lines, whereas the Ca ii infrared lines are synthesized in non-LTE. Radiative transfer calculations are done with a modified version of Multi, using the snapshots of a non-magnetic 3D hydrodynamical simulation of the photosphere.
Results: The resulting synthetic profiles show the expected C-shaped bisector at disk center. The degree of asymmetry and the line shifts, however, show a clear dependence on the heliocentric angle and the properties of the lines. The profiles at μ = 1 are compared with observed profiles to prove their reliability, and they are tested against errors induced by the LTE calculations, inaccuracies in the atomic data and the 3D simulation.
Conclusions: Theoretical quiet-sun profiles of lines commonly used by solar observers are provided to the community. Those can be used as absolute references for line-of-sight velocities. The limb effect is produced by the projection of the 3D atmosphere along the line of sight. Non-LTE effects on Fe i lines are found to have a small impact on the convective shifts of the lines, reinforcing the usability of the LTE approximation in this case. We estimate the precision of the disk-center line shifts to be approximately 50 m s-1, but the off-center profiles remain to be tested against observations.

The spectral profiles are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A113 Title: VizieR Online Data Catalog: Synthetic lines in the Sun (de la Cruz Rodríguez+, 2011) Authors: de la Cruz Rodríguez, J.; Kiselman, D.; Carlsson, M. Bibcode: 2011yCat..35289113D Altcode: We have computed synthetic spectra from a realistic 3D numerical simulation of the solar photosphere. We provide the spatially averaged spectra for selected lines that are commonly used on solar applications. These data can be used to calibrate Doppler velocity measurements in the solar photosphere. The calculations are carried out along the solar disk from heliocentric angle mu=1.0 to mu=0.3.

(11 data files). Title: The Origins of Hot Plasma in the Solar Corona Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen, V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver, C. J.; Title, A. M. Bibcode: 2011Sci...331...55D Altcode: The Sun's outer atmosphere, or corona, is heated to millions of degrees, considerably hotter than its surface or photosphere. Explanations for this enigma typically invoke the deposition in the corona of nonthermal energy generated by magnetoconvection. However, the coronal heating mechanism remains unknown. We used observations from the Solar Dynamics Observatory and the Hinode solar physics mission to reveal a ubiquitous coronal mass supply in which chromospheric plasma in fountainlike jets or spicules is accelerated upward into the corona, with much of the plasma heated to temperatures between ~0.02 and 0.1 million kelvin (MK) and a small but sufficient fraction to temperatures above 1 MK. These observations provide constraints on the coronal heating mechanism(s) and highlight the importance of the interface region between photosphere and corona. Title: Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and Active Region Corona Authors: McIntosh, S. W.; de Pontieu, B.; Carlsson, M.; Hansteen, V. H.; Sdo/Aia Mission Team Bibcode: 2010AGUFMSH14A..01M Altcode: We use observations with AIA onboard SDO and report the discovery of ubiquitous Alfvenic oscillations in the corona of quiet Sun, active regions and coronal holes. These Alfvenic oscillations have significant power, and seem to be connected to the chromospheric Alfvenic oscillations previously reported with Hinode. We use Monte Carlo simulations to determine the strength and periods of the waves. Using unique joint observations of Hinode, the Solar Dynamics Observatory, and HAO's CoMP instrument we study the excitation of transverse oscillations as a function of space, time, and temperature. We will discuss the energetic impact and diagnostic capabilities of this ever-present process and how it can be used to build a more self-consistent picture of energy transport into the inner heliosphere. Transverse Oscillations Observed Above the Solar North Pole in the He II 304Å (bottom) and Fe IX 171Å (top) channels. Studying the progression of such points with altitude yields important information about wave propagation into the magnetically open corona. Title: The Chromospheric Lyman Alpha SpectroPolarimeter (CLASP) Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Cirtain, J. W.; Bando, T.; Kano, R.; Hara, H.; Fujimura, D.; Ueda, K.; Ishikawa, R.; Watanabe, H.; Ichimoto, K.; Sakao, T.; de Pontieu, B.; Carlsson, M.; Casini, R. Bibcode: 2010AGUFMSH11B1632K Altcode: Magnetic fields in the solar chromosphere play a key role in the energy transfer and dynamics of the solar atmosphere. Yet a direct observation of the chromospheric magnetic field remains one of the greatest challenges in solar physics. While some advances have been made for observing the Zeeman effect in strong chromospheric lines, the effect is small and difficult to detect outside sunspots. The Hanle effect offers a promising alternative; it is sensitive to weaker magnetic fields (e.g., 5-500 G for Ly-Alpha), and while its magnitude saturates at stronger magnetic fields, the linear polarization signals remain sensitive to the magnetic field orientation. The Hanle effect is not only limited to off-limb observations. Because the chromosphere is illuminated by an anisotropic radiation field, the Ly-Alpha line is predicted to show linear polarization for on-disk, near-limb regions, and magnetic field is predicted to cause a measurable depolarization. At disk center, the Ly-Alpha radiation is predicted to be negligible in the absence of magnetic field, and linearly polarized to an order of 0.3% in the presence of an inclined magnetic field. The proposed CLASP sounding rocket instrument is designed to detect 0.3% linear polarization of the Ly-Alpha line at 1.5 arcsecond spatial resolution (0.7’’ pixel size) and 10 pm spectral resolution. The instrument consists of a 30 cm aperture Cassegrain telescope and a dual-beam spectropolarimeter. The telescope employs a ``cold mirror’’ design that uses multilayer coatings to reflect only the target wavelength range into the spectropolarimeter. The polarization analyzer consists of a rotating waveplate and a polarizing beamsplitter that comprises MgF2 plates placed at Brewster’s Angle. Each output beam of the polarizing beamsplitter, representing two orthogonal linear polarizations, is dispersed and focused using a separate spherical varied-line-space grating, and imaged with a separate 512x512 CCD camera. Prototypes of key optical components have been fabricated and tested. Instrument design is being finalized, and the experiment will be proposed for a 2014 flight aboard a NASA sounding rocket. Title: The role of the chromosphere in filling the corona with hot plasma (Invited) Authors: de Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen, V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver, C. J.; Title, A. M. Bibcode: 2010AGUFMSH21C..03D Altcode: We use coordinated observations from the Solar Dynamics Observatory (SDO), Hinode and the Swedish Solar Telescope (SST) to show how plasma is heated to coronal temperatures from its source in the chromosphere. Our observations reveal a ubiquitous mass supply for the solar corona in which chromospheric plasma is accelerated upward into the corona with much of the plasma heated to transition region temperatures, and a small, but significant fraction heated to temperatures in excess of 1 million K. Our observations show, for the first time, how chromospheric spicules, fountain-like jets that have long been considered potential candidates for coronal heating, are directly associated with heating of plasma to coronal temperatures. These results provide strong physical constraints on the mechanism(s) responsible for coronal heating and do not seem compatible with current models. The association with chromospheric spicules highlights the importance of the interface region between the photosphere and corona to gain a full understanding of the coronal heating problem. Title: Radiative Hydrodynamic Simulations of Acoustic Waves in Sunspots Authors: Bard, S.; Carlsson, M. Bibcode: 2010ApJ...722..888B Altcode: We investigate the formation and evolution of the Ca II H line in a sunspot. The aim of our study is to establish the mechanisms underlying the formation of the frequently observed brightenings of small regions of sunspot umbrae known as "umbral flashes." We perform fully consistent NLTE radiation hydrodynamic simulations of the propagation of acoustic waves in sunspot umbrae and conclude that umbral flashes result from increased emission of the local solar material during the passage of acoustic waves originating in the photosphere and steepening to shock in the chromosphere. To quantify the significance of possible physical mechanisms that contribute to the formation of umbral flashes, we perform a set of simulations on a grid formed by different wave power spectra, different inbound coronal radiation, and different parameterized chromospheric heating. Our simulations show that the waves with frequencies in the range 4.5-7.0 mHz are critical to the formation of the observed blueshifts of umbral flashes while waves with frequencies below 4.5 mHz do not play a role despite their dominance in the photosphere. The observed emission in the Ca II H core between flashes only occurs in the simulations that include significant inbound coronal radiation and/or extra non-radiative chromospheric heating in addition to shock dissipation. Title: On Redshifts and Blueshifts in the Transition Region and Corona Authors: Hansteen, V. H.; Hara, H.; De Pontieu, B.; Carlsson, M. Bibcode: 2010ApJ...718.1070H Altcode: 2010arXiv1001.4769H Emission lines formed in the transition region (TR) of the Sun have long been known to show pervasive redshifts. Despite a variety of proposed explanations, these TR downflows (and the slight upflows in the low corona) remain poorly understood. We present results from comprehensive three-dimensional MHD models that span the upper convection zone up to the corona, 15 Mm above the photosphere. The TR and coronal heating in these models is caused by the stressing of the magnetic field by photospheric and convection "zone dynamics," but also in some models by the injection of emerging magnetic flux. We show that rapid, episodic heating, at low heights of the upper chromospheric plasma to coronal temperatures naturally produces downflows in TR lines, and slight upflows in low coronal lines, with similar amplitudes to those observed with EUV/UV spectrographs. We find that TR redshifts naturally arise in episodically heated models where the average volumetric heating scale height lies between that of the chromospheric pressure scale height of 200 km and the coronal scale height of 50 Mm. Title: On the Solar Chromosphere Observed at the LIMB with Hinode Authors: Judge, Philip G.; Carlsson, Mats Bibcode: 2010ApJ...719..469J Altcode: 2010arXiv1004.1398J Broadband images in the Ca II H line, from the Broadband Filter Imager (BFI) instrument on the Hinode spacecraft, show emission from spicules emerging from and visible right down to the observed limb. Surprisingly, little absorption of spicule light is seen along their lengths. We present formal solutions to the transfer equation for given (ad hoc) source functions, including a stratified chromosphere from which spicules emanate. The model parameters are broadly compatible with earlier studies of spicules. The visibility of Ca II spicules down to the limb in Hinode data seems to require that spicule emission be Doppler shifted relative to the stratified atmosphere, either by supersonic turbulent or organized spicular motion. The non-spicule component of the chromosphere is almost invisible in the broadband BFI data, but we predict that it will be clearly visible in high spectral resolution data. Broadband Ca II H limb images give the false impression that the chromosphere is dominated by spicules. Our analysis serves as a reminder that the absence of a signature can be as significant as its presence. Title: Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres. Numerical methods and application to the quiet, non-magnetic, surface of a solar-type star Authors: Hayek, W.; Asplund, M.; Carlsson, M.; Trampedach, R.; Collet, R.; Gudiksen, B. V.; Hansteen, V. H.; Leenaarts, J. Bibcode: 2010A&A...517A..49H Altcode: 2010arXiv1007.2760H
Aims: We present the implementation of a radiative transfer solver with coherent scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD) simulations of stellar surface convection. The code is fully parallelized using MPI domain decomposition, which allows for large grid sizes and improved resolution of hydrodynamical structures. We apply the code to simulate the surface granulation in a solar-type star, ignoring magnetic fields, and investigate the importance of coherent scattering for the atmospheric structure.
Methods: A scattering term is added to the radiative transfer equation, requiring an iterative computation of the radiation field. We use a short-characteristics-based Gauss-Seidel acceleration scheme to compute radiative flux divergences for the energy equation. The effects of coherent scattering are tested by comparing the temperature stratification of three 3D time-dependent hydrodynamical atmosphere models of a solar-type star: without scattering, with continuum scattering only, and with both continuum and line scattering.
Results: We show that continuum scattering does not have a significant impact on the photospheric temperature structure for a star like the Sun. Including scattering in line-blanketing, however, leads to a decrease of temperatures by about 350 K below log10 τ5000 ⪉ -4. The effect is opposite to that of 1D hydrostatic models in radiative equilibrium, where scattering reduces the cooling effect of strong LTE lines in the higher layers of the photosphere. Coherent line scattering also changes the temperature distribution in the high atmosphere, where we observe stronger fluctuations compared to a treatment of lines as true absorbers. Title: On the Origin of High-Frequency "Acoustic'' Power in Photospheric and Chromospheric Velocity Power Spectra Authors: Fleck, Bernard; Straus, T.; Carlsson, M.; Jefferies, S. M.; Severino, G.; Tarbell, T. D. Bibcode: 2010AAS...21640309F Altcode: 2010BAAS...41..879F In a recent paper (Fleck et al., 2010) we compared observed Dopplergram time series from Hinode with results from 3-D numerical simulations based on the Oslo "Stagger” and CO5BOLD codes. Given the rapid falloff of atmospheric modulation transfer functions at high frequencies due to the extended widths of typical velocity response functions, one would expect the high-frequency tail of Doppler power spectra to drop significantly below those of actual velocities at the corresponding heights in the simulations. Surprisingly, our analysis of power spectra of Doppler shifts of simulated line profiles did not reveal such a steep falloff at high frequencies. Instead, they are comparable to (and in some cases even larger than) those of the actual velocities, making estimates of the energy flux of high frequency acoustic waves questionable, in particular those that apply atmospheric MTF corrections. In this work we study the cause of this unexpected behavior in detail, with particular emphasis on the role of rapidly changing velocity response functions in a dynamic atmosphere with strong vertical velocity gradients. Title: New Frontiers in Solar Chromospheric Physics Authors: Carlsson, Mats P. O. Bibcode: 2010AAS...21620701C Altcode: The enigmatic chromosphere is the transition between the solar surface and the eruptive outer solar atmosphere. The chromosphere harbours and constrains the mass and energy loading processes that define the heating of the corona, the acceleration and the composition of the solar wind, and the energetics and triggering of solar outbursts (filament eruptions, flares, coronal mass ejections).

Magnetic fields break through the solar surface in a hierarchy of magnetic elements ranging from Earth-sized sunspots down to tiny concentrations that are barely resolved in the highest-resolution photospheric images. In the chromosphere they combine in intricate, highly dynamic, and continuously evolving fibrilar patterns. Movements of the photospheric field-line footpoints drive, guide, and control the flows of energy and mass into the corona, and trigger energy-releasing magnetic reconnection through relentless topological rearrangement. The conversion from convectively driven footpoint motion to outer-atmosphere outflows and loading takes place in the dynamic, fine-structured chromosphere.

The chromosphere is arguably the most difficult and least understood domain of solar physics. All at once it represents the transition from optically thick to thin radiation escape, from gas-pressure domination to magnetic-pressure domination, from neutral to ionised state, from MHD to plasma physics, and from near-equilibrium ("LTE") to non-equilibrium conditions.

A number of important facilities for observing the solar chromosphere have recently come on line (e.g. the Hinode satellite and ground-based Fabry-Perot interferometers) or will become operational in the near future (e.g. SDO and IRIS). The overwhelming complexity of the chromosphere makes it necessary to have numerical simulations for the interpretation of the observations. Such realistic simulations, spanning the solar atmosphere from the convection zone to the corona, are now becoming feasible.

This presentation will introduce the fascinating aspects of chromospheric physics and review recent results from both observations and numerical simulations. Title: Radiative Hydrodynamic Simulation of the Continuum Emission in Solar White-Light Flares Authors: Cheng, J. X.; Ding, M. D.; Carlsson, Mats Bibcode: 2010ApJ...711..185C Altcode: It is believed that solar white-light flares (WLFs) originate in the lower chromosphere and upper photosphere. In particular, some recently observed WLFs show a large continuum enhancement at 1.56 μm where the opacity reaches its minimum. Therefore, it is important to clarify how the energy is transferred to the lower layers responsible for the production of WLFs. Based on radiative hydrodynamic simulations, we study the role of non-thermal electron beams in increasing the continuum emission. We vary the parameters of the electron beam and disk positions and compare the results with observations. The electron beam heated model can explain most of the observational white-light enhancements. For the most energetic WLFs observed so far, however, a very large electron beam flux and a high low-energy cutoff, which are possibly beyond the parameter space in our simulations, are required in order to reproduce the observed white-light emission. Title: The Quiet Solar Atmosphere Observed and Simulated in Na I D1 Authors: Leenaarts, J.; Rutten, R. J.; Reardon, K.; Carlsson, M.; Hansteen, V. Bibcode: 2010ApJ...709.1362L Altcode: 2009arXiv0912.2206L The Na I D1 line in the solar spectrum is sometimes attributed to the solar chromosphere. We study its formation in quiet-Sun network and internetwork. We first present high-resolution profile-resolved images taken in this line with the imaging spectrometer Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope and compare these to simultaneous chromospheric images taken in Ca II 8542 Å and Hα. We then model Na I D1 formation by performing three-dimensional (3D) non-local thermodynamic equilibrium profile synthesis for a snapshot from a 3D radiation-magnetohydrodynamics simulation. We find that most Na I D1 brightness is not chromospheric but samples the magnetic concentrations that make up the quiet-Sun network in the photosphere, well below the height where they merge into chromospheric canopies, with aureoles from 3D resonance scattering. The line core is sensitive to magneto-acoustic shocks in and near magnetic concentrations, where shocks occur deeper than elsewhere, and may provide evidence of heating deep within magnetic concentrations. Title: On red-shifts in the transition region and corona . Authors: Hansteen, V. H.; Hara, H.; de Pontieu, B.; Carlsson, M. Bibcode: 2010MmSAI..81..729H Altcode: We present evidence that transition region red-shifts are naturally produced in episodically heated models where the average volumetric heating scale height lies between that of the chromospheric pressure scale height of 200 km and the coronal scale height of 50 Mm. In order to do so we present results from 3d MHD models spanning the upper convection zone up to the corona, 15 Mm above the photosphere. Transition region and coronal heating in these models is due both the stressing of the magnetic field by photospheric and convection `zone dynamics, but also in some models by the injection of emerging magnetic flux. Title: High frequency waves in the solar atmosphere?. Authors: Fleck, B.; Straus, T.; Carlsson, M.; Jefferies, S. M.; Severino, G.; Tarbell, T. D. Bibcode: 2010MmSAI..81..777F Altcode: 2010arXiv1002.3285F The present study addresses the following questions: How representative of the actual velocities in the solar atmosphere are the Doppler shifts of spectral lines? How reliable is the velocity signal derived from narrowband filtergrams? How well defined is the height of the measured Doppler signal? Why do phase difference spectra always pull to 0o phase lag at high frequencies? Can we actually observe high frequency waves (P< 70 s)? What is the atmospheric MTF of high frequency waves? How reliably can we determine the energy flux of high frequency waves? We address these questions by comparing observations obtained with Hinode/NFI with results from two 3D numerical simulations (Oslo Stagger and CO5BOLD). Our results suggest that the observed high frequency Doppler velocity signal is caused by rapid height variations of the velocity response function in an atmosphere with strong velocity gradients and cannot be interpreted as evidence of propagating high frequency acoustic waves. Estimates of the energy flux of high frequency waves should be treated with caution, in particular those that apply atmospheric MTF corrections. Title: On the detection of fast moving upflows in the quiet solar photosphere. Authors: Straus, Th.; Fleck, B.; Jefferies, S. M.; Carlsson, M.; Tarbell, T. D. Bibcode: 2010MmSAI..81..751S Altcode: 2010arXiv1002.3305S In our studies of the dynamics and energetics of the solar atmosphere, we have detected, in high-quality observations from Hinode SOT/NFI, ubiquitous small-scale upflows which move horizontally with supersonic velocities in the quiet Sun. We present the properties of these fast moving upflows (FMUs) and discuss different interpretations. Title: Chromospheric heating and structure as determined from high resolution 3D simulations . Authors: Carlsson, M.; Hansteen, V. H.; Gudiksen, B. V. Bibcode: 2010MmSAI..81..582C Altcode: 2010arXiv1001.1546C We have performed 3D radiation MHD simulations extending from the convection zone to the corona covering a box 16 Mm3 at 32 km spatial resolution. The simulations show very fine structure in the chromosphere with acoustic shocks interacting with the magnetic field. Magnetic flux concentrations have a temperature lower than the surroundings in the photosphere but higher in the low chromosphere. The heating is there mostly through ohmic dissipation preferentially at the edges of the flux concentrations. The magnetic field is often wound up around the flux concentrations. When acoustic waves travel up along the field this topology leads to swirling motions seen in chromospheric diagnostic lines such as the calcium infrared triplet. Title: The Interface Region Imaging Spectrograph (IRIS) Small Explorer Authors: de Pontieu, B.; Title, A. M.; Schryver, C. J.; Lemen, J. R.; Golub, L.; Kankelborg, C. C.; Carlsson, M. Bibcode: 2009AGUFMSH33B1499D Altcode: The Interface Region Imaging Spectrograph (IRIS) was recently selected as a small explorer mission by NASA. The primary goal of IRIS is to understand how the solar atmosphere is energized. The IRIS investigation combines advanced numerical modeling with a high resolution 20 cm UV imaging spectrograph that will obtain spectra covering temperatures from 4,500 to 10 MK in three wavelength ranges (1332-1358 Angstrom, 1390-1406 Angstrom and 2785-2835 Angstrom) and simultaneous images covering temperatures from 4,500 K to 65,000 K. IRIS will obtain UV spectra and images with high resolution in space (1/3 arcsec) and time (1s) focused on the chromosphere and transition region of the Sun, a complex dynamic interface region between the photosphere and corona. In this region, all but a few percent of the non-radiative energy leaving the Sun is converted into heat and radiation. IRIS fills a crucial gap in our ability to advance Sun-Earth connection studies by tracing the flow of energy and plasma through this foundation of the corona and heliosphere. The IRIS investigation is led by PI Alan Title (LMSAL) with major participation by the Harvard Smithsonian Astrophysical Observatory, Montana State University, NASA Ames Research Center, Stanford University and the University of Oslo (Norway). IRIS is scheduled for launch in late 2012, and will have a nominal two year mission lifetime. Title: MULTI3D: A Domain-Decomposed 3D Radiative Transfer Code Authors: Leenaarts, J.; Carlsson, M. Bibcode: 2009ASPC..415...87L Altcode: We present MULTI3D, a 3D radiative transfer code currently under development. It is optimized for computing NLTE problems based on (radiation-)MHD models of stellar atmospheres. MULTI3D is based on MULTI and includes most of the physics present in that code. MULTI3D was first written as a serial code by Botnen (1997) and has recently been upgraded to an MPI-parallelized, domain-decomposed version. The code has so far successfully been run on up to 64 processors, solving the NLTE radiative transfer for a six-level Ca II atom with 400 frequency points in an atmosphere of 256 × 128 × 108 grid points. Title: On-disk Counterparts of Type II Spicules in the Ca II 854.2 nm and Hα Lines Authors: Rouppe van der Voort, L.; Leenaarts, J.; de Pontieu, B.; Carlsson, M.; Vissers, G. Bibcode: 2009ApJ...705..272R Altcode: 2009arXiv0909.2115R Recently, a second type of spicules was discovered at the solar limb with the Solar Optical Telescope onboard the Japanese Hinode spacecraft. These previously unrecognized type II spicules are thin chromospheric jets that are shorter lived (10-60 s) and that show much higher apparent upward velocities (of order 50-100 km s-1) than the classical spicules. Since they have been implicated in providing hot plasma to coronal loops, their formation, evolution, and properties are important ingredients for a better understanding of the mass and energy balance of the low solar atmosphere. Here, we report on the discovery of the disk counterparts of type II spicules using spectral imaging data in the Ca II 854.2 nm and Hα lines with the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope in La Palma. We find rapid blueward excursions in the line profiles of both chromospheric lines that correspond to thin, jet-like features that show apparent velocities of order 50 km s-1. These blueward excursions seem to form a separate absorbing component with Doppler shifts of order 20 and 50 km s-1 for the Ca II 854.2 nm and Hα line, respectively. We show that the appearance, lifetimes, longitudinal and transverse velocities, and occurrence rate of these rapid blue excursions on the disk are very similar to those of the type II spicules at the limb. A detailed study of the spectral line profiles in these events suggests that plasma is accelerated along the jet, and plasma is being heated throughout the short lifetime of the event. Title: Twisted Flux Tube Emergence from the Convection Zone to the Corona. II. Later States Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats Bibcode: 2009ApJ...702..129M Altcode: 2009arXiv0906.5464M Three-dimensional numerical simulations of magnetic flux emergence are carried out in a computational domain spanning the upper layers of the convection zone to the lower corona. We use the Oslo Staggered Code to solve the full magnetohydrodynamic equations with non-gray and non-local thermodynamic equilibrium radiative transfer and thermal conduction along the magnetic field lines. In this paper, we concentrate on the later stages of the simulations and study the evolution of the structure of the rising flux in the upper chromosphere and corona, the interaction between the emerging flux and the weak coronal magnetic field initially present, and the associated dynamics. The flux tube injected at the bottom boundary rises to the photosphere where it largely remains. However, some parts of the flux tube become unstable and expand in patches into the upper chromosphere. The flux rapidly expands toward the corona, pushing the coronal and transition region material aside, and lifting and maintaining the transition region at heights greater than 5 Mm above the photosphere for extensive periods of time. The pre-existing magnetic field in the corona and transition region is perturbed by the incoming flux and reoriented by a series of high Joule heating events. Low-density structures form in the corona, while at later times a high-density filamentary structure appears in the lower part of the expanding flux. The dynamics of these and other structures is discussed. While Joule heating due to the expanding flux is episodic, it increases in relative strength as fresh magnetic field rises and becomes energetically important in the upper chromosphere and corona at later times. Chromospheric, transition region, and coronal lines are computed and their response to the perturbation caused by the expanding emerging flux is discussed. Title: Spicule-Like Structures Observed in Three-Dimensional Realistic Magnetohydrodynamic Simulations Authors: Martínez-Sykora, Juan; Hansteen, Viggo; De Pontieu, Bart; Carlsson, Mats Bibcode: 2009ApJ...701.1569M Altcode: 2009arXiv0906.4446M We analyze features that resemble type I spicules in two different three-dimensional numerical simulations in which we include horizontal magnetic flux emergence in a computational domain spanning the upper layers of the convection zone to the lower corona. The two simulations differ mainly in the pre-existing ambient magnetic field strength and in the properties of the inserted flux tube. We use the Oslo Staggered Code to solve the full magnetohydrodynamic equations with nongray and non-LTE radiative transfer and thermal conduction along the magnetic field lines. We find a multitude of features that show a spatiotemporal evolution that is similar to that observed in type I spicules, which are characterized by parabolic height versus time profiles, and are dominated by rapid upward motion at speeds of 10-30 km s-1, followed by downward motion at similar velocities. We measured the parameters of the parabolic profile of the spicules and find similar correlations between the parameters as those found in observations. The values for height (or length) and duration of the spicules found in the simulations are more limited in range than those in the observations. The spicules found in the simulation with higher pre-existing ambient field have shorter length and smaller velocities. From the simulations, it appears that these kinds of spicules can, in principle, be driven by a variety of mechanisms that include p-modes, collapsing granules, magnetic energy release in the photosphere and lower chromosphere, and convective buffeting of flux concentrations. Title: Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity Authors: Fabbian, D.; Asplund, M.; Barklem, P. S.; Carlsson, M.; Kiselman, D. Bibcode: 2009A&A...500.1221F Altcode: 2009arXiv0902.4472F Aims: A detailed study is presented, including estimates of the impact on elemental abundance analysis, of the non-local thermodynamic equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in model atmospheres representative of stars with spectral types F to K.
Methods: We have applied the statistical equilibrium code MULTI to a number of plane-parallel MARCS atmospheric models covering late-type stars (4500 ≤ T_eff ≤ 6500 K, 2 ≤ log g ≤ 5 [cgs], and -3.5 ≤ [Fe/H] ≤ 0). The atomic model employed includes, in particular, recent quantum-mechanical electron collision data.
Results: We confirm that the O i triplet lines form under non-LTE conditions in late-type stars, suffering negative abundance corrections with respect to LTE. At solar metallicity, the non-LTE effect, mainly attributed in previous studies to photon losses in the triplet itself, is also driven by an additional significant contribution from line opacity. At low metallicity, the very pronounced departures from LTE are due to overpopulation of the lower level (3s ^5S^o) of the transition. Large line opacity stems from triplet-quintet intersystem electron collisions, a form of coupling previously not considered or seriously underestimated. The non-LTE effects generally become severe for models (both giants and dwarfs) with higher T_eff. Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance corrections tend to rapidly become more severe towards lower metallicity. When neglecting H collisions, they amount to as much as |Δlog ɛ_O| ~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3 and [Fe/H] = -3.5. Even when such collisions are included, the LTE abundance remains a serious overestimate, correspondingly by |Δlog ɛ_O| ~ 0.5 dex and ~0.9 dex at such low metallicities. Although the poorly known inelastic hydrogen collisions thus remain an important uncertainty, the large metallicity-dependent non-LTE effects seem to point to a resulting “low” (compared to LTE) [O/Fe] in metal-poor halo stars.
Conclusions: Our results may be important in solving the long-standing [O/Fe] debate. When applying the derived non-LTE corrections, the LTE oxygen abundance inferred from the 777 nm permitted triplet will be decreased substantially at low metallicity. If the classical Drawin formula is employed for O+H collisions, the derived [O/Fe] trend becomes almost flat below [Fe/H] ~ -1, in better agreement with recent literature estimates generally obtained from other oxygen abundance indicators. A value of [O/Fe] ⪉ +0.5 may therefore be appropriate, as suggested by standard theoretical models of type II supernovae nucleosynthetic yields. If neglecting impacts with H atoms instead, [O/Fe] decreases towards lower [Fe/H], which would open new questions. Our tests using ATLAS model atmospheres show that, though non-LTE corrections for metal-poor dwarfs are smaller (by ~0.2 dex when adopting efficient H collisions) than in the MARCS case, our main conclusions are preserved, and that the LTE approach tends to seriously overestimate the O abundance at low metallicity. However, in order to finally reach consistency between oxygen abundances from the different available spectral features, it is of high priority to reduce the large uncertainty regarding H collisions, to undertake a full investigation of the interplay of non-LTE and 3D effects, and to clarify the issue of the temperature scale at low metallicity. Title: The Temperature Diagnostic Properties of the Mg I 457.1 nm Line Authors: Langangen, Øystein; Carlsson, Mats Bibcode: 2009ApJ...696.1892L Altcode: 2009arXiv0903.1492L We analyze the important formation processes for the Mg I 457.1 nm line. This line is an intercombination line and the source function is close to the local thermodynamic equilibrium (LTE) value. The strong coupling to the local temperature and the relatively high population of the lower level (the ground state of Mg I) makes this line an ideal candidate for temperature diagnostics in the lower chromosphere/temperature minimum region. Linking the temperature probed to an absolute physical height is nontrivial because of non-LTE (NLTE) ionization. We analyze the NLTE effects and find that photoionization from the lower energy levels together with cascading collisional recombination dominates the ionization balance. Taking properly into account the line blanketing in the UV is essential for obtaining the right photoionization rates. The identification of the main NLTE effects in the line allows us to construct a "quintessential" model atom, ideal for computationally demanding tasks, e.g., full three-dimensional and/or time-dependent radiative transfer. Furthermore, we analyze the diagnostic potential to temperature of this line in solarlike atmospheres, by synthesizing the line from a series of parameterized atmospheric models. These models have been constructed with fixed effective temperature, but with a variable heat term in the energy equation to obtain a chromospheric temperature rise at different heights. We conclude that the line has a significant potential in the diagnostics of the lower chromosphere temperature structure, especially for cooler atmospheres, such as sunspots. Title: Three-Dimensional Non-LTE Radiative Transfer Computation of the CA 8542 Infrared Line From a Radiation-MHD Simulation Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rouppe van der Voort, L. Bibcode: 2009ApJ...694L.128L Altcode: 2009arXiv0903.0791L The interpretation of imagery of the solar chromosphere in the widely used Ca II 854.2 nm infrared line is hampered by its complex, three-dimensional, and non-LTE formation. Forward modeling is required to aid understanding. We use a three-dimensional non-LTE radiative transfer code to compute synthetic Ca II 854.2 nm images from a radiation-MHD simulation of the solar atmosphere spanning from the convection zone to the corona. We compare the simulation with observations obtained with the CRISP filter at the Swedish 1 m Solar Telescope. We find that the simulation reproduces dark patches in the blue line wing caused by Doppler shifts, brightenings in the line core caused by upward-propagating shocks, and thin dark elongated structures in the line core that form the interface between upward and downward gas motion in the chromosphere. The synthetic line core is narrower than the observed one, indicating that the Sun exhibits both more vigorous large-scale dynamics as well as small scale motions that are not resolved within the simulation, presumably owing to a lack of spatial resolution. Title: Hydrodynamics and radiative transfer of 3D model atmospheres. Current status, limitations, and how to make headway Authors: Carlsson, Mats Bibcode: 2009MmSAI..80..606C Altcode: 3D MHD models are important tools for advancing our understanding of stellar atmospheres. A major computational challenge is the treatment of radiative transfer; both to get a realistic treatment of the energy transfer in the 3D modelling and for the diagnostic problem of calculating the emergent spectrum in more detail from such models. The current status, limitations and future directions of 3D MHD atmospheric modelling and the treatment of radiative transfer are here discussed. Title: Advancing our understanding of the chromosphere Authors: Hansteen, V. H.; Carlsson, M.; de Pontieu, B. Bibcode: 2008AGUFMSH51C..01H Altcode: Recent progress has shown the solar chromosphere to be fundamentally dynamic, where non-linear techniques must be used to understand its nature. It is also the region where the magnetic field grows to dominate the plasma and where the coupling between radiation and matter becomes becomes quite tenuous. Understanding the workings of the chromosphere is vital if one is to understand the flow of energy between the solar surface and its outer atmosphere and wind. Recent numerical developments have shown that it is feasible to model the chromosphere, even to the extent that newly available high resolution observations sometimes can be reproduced in detail. We will discuss the challenges facing numerical chromospheric models and the observations needed to validate or refute them. Title: Dynamics of the upper chromosphere Authors: de Pontieu, B.; Carlsson, M.; McIntosh, S.; Hansteen, V.; Tarbell, T. Bibcode: 2008AGUFMSH51C..05D Altcode: In the past few years, high-resolution observations with ground-based telescopes and the Broadband Filter Imager (BFI) and Narrowband Filter Imager (NFI) of the Solar Optical Telescope onboard Hinode have revolutionized our view of the dynamics and energetics of the chromosphere. We review some of these results, including the discovery of two different types of spicules and the finding that the chromosphere is riddled with strong Alfvenic waves. We describe how these observations, when combined with advanced numerical simulations, can help address important unresolved issues regarding the connection between the photosphere and corona, such as the role of waves and of reconnection in driving the dynamics and energetics of the upper chromosphere, and how chromospheric dynamics impact the transition region and corona. Title: 3D radiative transfer in stellar atmospheres Authors: Carlsson, M. Bibcode: 2008PhST..133a4012C Altcode: Three-dimensional (3D) radiative transfer in stellar atmospheres is reviewed with special emphasis on the atmospheres of cool stars and applications. A short review of methods in 3D radiative transfer shows that mature methods exist, both for taking into account radiation as an energy transport mechanism in 3D (magneto-) hydrodynamical simulations of stellar atmospheres and for the diagnostic problem of calculating the emergent spectrum in more detail from such models, both assuming local thermodynamic equilibrium (LTE) and in non-LTE. Such methods have been implemented in several codes, and examples of applications are given. Title: What do Spicules Tell us About the Chromosphere? Authors: de Pontieu, B.; Carlsson, M.; McIntosh, S.; Hansteen, V.; Tarbell, T. Bibcode: 2008ESPM...12.2.15D Altcode: In the past few years, high-resolution observations with ground-based telescopes and the Broadband Filter Imager (BFI) and Narrowband Filter Imager (NFI) of the Solar Optical Telescope onboard Hinode have revolutionized our view of spicules and their role in the chromosphere. We review some of these results, including the discovery of two different types of spicules with different dynamics and formation mechanisms, as well as the finding that the chromosphere is riddled with strong Alfvenic waves.

In an effort to determine the formation mechanism of spicules and their impact on the outer atmosphere, we further focus on the thermal evolution and velocities developed by spicules. We use Dopplergrams made in the Na D 589.6 nm, H-alpha 656.3 nm and Mg B 517.3 nm passbands, as well as filtergrams in the Ca H 396.8 nm passband to study the spatio-temporal relationship between the various spicular features. We compare those findings with synthesized images based on line profiles computed from high-resolution 3D MHD numerical simulations from the University of Oslo. We also use the Dopplergram data to investigate the velocities that develop in the two types of spicules that were reported previously. We perform statistical analysis of apparent velocities in the plane of the sky and line-of-sight velocities derived from Dopplergrams to disentangle the superposition of Alfvenic wave amplitudes and field-aligned flows. We study these properties for a variety of magnetic field configurations (coronal holes, quiet Sun, active region). Finally, we focus on the formation mechanism of spicules by analyzing spicular features in Dopplergrams on the disk that were taken simultaneously with SP magnetograms. Title: Constructing Computationally Tractable Models of Si I for the 1082.7 nm Transition Authors: Bard, S.; Carlsson, M. Bibcode: 2008ApJ...682.1376B Altcode: The Si I 1082.7 nm line is often observed together with the He I 1083.0 nm. The silicon line is assumed to give a measure of the photospheric driver of the chromospheric dynamics observed in the He I 1083.0 nm line. To exploit the diagnostic potential of the silicon line it is of interest to study its line formation. We here analyze physical processes in the formation of the Si I 1082.7 nm line and find that it is formed in non-LTE (NLTE). Its line core intensity is lower than the corresponding LTE value because the source function is below the Planck function due to photon losses in the line itself. In order to make possible full NLTE dynamic calculations of this line we develop a general method for reducing the size of atomic models used in NLTE calculations by representing several physically similar energy levels as one mean level. Our procedure preserves all the important physical properties of the atom that have a bearing on the observable spectrographic properties of the line under study. We apply our procedure to the Si I atom for the 1082.7 nm transition under solar conditions, and we are able to reduce the size of the atomic model from 238 levels and 3152 transitions to 23 levels and 171 transitions with no significant change in the calculated emergent intensity of the 1082.7 nm line. We calculate the response functions for the Si I 1082.7 nm line both in the quiet Sun and above sunspot umbrae. We find that the line-center Doppler shift has a mean response height to velocity perturbations of 541 km in a quiet-Sun atmosphere and 308 km in a sunspot atmosphere. These numbers depend on the model atmosphere employed. Title: Search for High Velocities in the Disk Counterpart of Type II Spicules Authors: Langangen, Ø.; De Pontieu, B.; Carlsson, M.; Hansteen, V. H.; Cauzzi, G.; Reardon, K. Bibcode: 2008ApJ...679L.167L Altcode: 2008arXiv0804.3256L Recently, De Pontieu and coworkers discovered a class of spicules that evolve more rapidly than previously known spicules, with rapid apparent motions of 50-150 km s-1, thickness of a few 100 km, and lifetimes of order 10-60 s. These so-called type II spicules have been difficult to study because of limited spatiotemporal and thermal resolution. Here we use the IBIS instrument to search for the high velocities in the disk counterpart of type II spicules. We have detected rapidly evolving events, with lifetimes that are less than a minute and often equal to the cadence of the instrument (19 s). These events are characterized by a Doppler shift that only appears in the blue wing of the Ca II IR line. Furthermore, the spatial extent, lifetime, and location near network all suggest a link to type II spicules. However, the magnitude of the measured Doppler velocity is significantly lower than the apparent motions seen at the limb. We use Monte Carlo simulations to show that this discrepancy can be explained by a forward model in which the visibility on the disk of the high-velocity flows in these events is limited by a combination of line-of-sight projection and reduced opacity in upward propelled plasma, especially in reconnection driven jets that are powered by a roughly constant energy supply. Title: Twisted Flux Tube Emergence From the Convection Zone to the Corona Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Carlsson, Mats Bibcode: 2008ApJ...679..871M Altcode: 2007arXiv0712.3854M Three-dimensional numerical simulations of a horizontal magnetic flux tube emergence with different twist are carried out in a computational domain spanning the upper layers of the convection zone to the lower corona. We use the Oslo Stagger Code to solve the full MHD equations with non-gray, non-LTE radiative transfer and thermal conduction along the magnetic lines. A magnetic flux tube is input at the bottom boundary into a weakly magnetized atmosphere. The photospheric and chromospheric response is described with magnetograms and synthetic continuum as well as Ca II H line images and velocity field distributions. In the photosphere the granular size increases when the flux tube approaches from below, as has been reported previously in the literature. In the convective overshoot region, some 200 km above the photosphere, adiabatic expansion produces cooling, darker regions with the structure of granulation cells. We also find evidence of collapsed granulation at the boundaries of the rising flux tube. Once the flux tube has crossed the photosphere, bright points related to concentrated magnetic field, vorticity, high vertical velocities, and heating by compressed material are found at heights up to 500 km above the photosphere. At greater heights, in the magnetized chromosphere, the rising flux tube produces a large, cool, magnetized bubble that tends to expel the usual chromospheric oscillations. In addition, the rising flux tube dramatically increases the chromospheric scale height, pushing the transition region and corona aside, such that the chromosphere extends up to 6 Mm above the photosphere. We find flux tube emergence through the photosphere to the lower corona to be a relatively slow process, taking of order 1 hr. Title: Velocities and thermal evolution of chromospheric spicules Authors: de Pontieu, B.; McIntosh, S. W.; Tarbell, T.; Carlsson, M. P.; Hansteen, V. H. Bibcode: 2008AGUSMSP53A..06D Altcode: We use the Broadband Filter Imager (BFI) and Narrowband Filter Imager (NFI) of the Solar Optical Telescope on Hinode to study the thermal evolution and velocities developed by chromospheric plasma in spicules. We use Dopplergrams made in the Na D 589.6 nm, Hα 656.3 nm and Mg B 517.3 nm passbands, as well as filtergrams in the Ca H 396.8 nm passband to study the spatio-temporal relationship between the various spicular features. We compare those findings with synthesized images based on line profiles computed from high-resolution 3D MHD numerical simulations from the University of Oslo. We also use the Dopplergram data to investigate the velocities that develop in the two types of spicules that were reported previously. We perform statistical analysis of apparent velocities in the plane of the sky and line-of-sight velocities derived from Dopplergrams to disentangle the superposition of Alfvenic wave amplitudes and field-aligned flows. We study these properties for a variety of magnetic field configurations (coronal holes, quiet Sun, active region). Finally, we focus on the formation mechanism of spicules by analyzing spicular features in Dopplergrams on the disk that were taken simultaneously with SP magnetograms. This work was supported by NASA contract NNM07AA01C. The Hinode mission is operated by ISAS/JAXA, NAOJ, NASA, STFC, ESA and NSC. Title: Chromospheric Flows in the Vicinity of Magnetic Features in the Quiet Sun Observed with Hinode SOT Authors: Tarbell, T.; de Pontieu, B.; Carlsson, M.; Hansteen, V.; McIntosh, S.; Ichimoto, K. Bibcode: 2008AGUSMSP41B..02T Altcode: The Narrowband Filter Imager of the Solar Optical Telescope on Hinode can measure Doppler shifts and line-of- sight magnetic fields in two lines with contributions from the low chromosphere: Na D 589.6 nm and Mg b 517.3 nm. The SOT Spectro-Polarimeter also measures very accurate vector magnetic fields and Doppler velocities in the photosphere. These observations have diffraction-limited spatial resolution and superb stability. We present examples of these measurements in quiet sun at various disk positions. In addition to the expected granulation and f- and p-modes, conspicuous longer-lived downflows are seen near strong network flux elements. Transient upflows are also detected, presumably the base of flows seen in spicules at the limb and H-alpha mottles on the disk. Velocity features associated with emerging and cancelling magnetic features are also described. The observations are compared with synthesized images made from line profiles computed from the University of Oslo 3-D MHD simulations. This work was supported by NASA contract NNM07AA01C. The Hinode mission is operated by ISAS/JAXA, NAOJ, NASA, STFC, ESA and NSC. Title: Spectroscopic Measurements of Dynamic Fibrils in the Ca II λ8662 Line Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo; De Pontieu, Bart Bibcode: 2008ApJ...673.1194L Altcode: 2007arXiv0710.0247L We present high spatial resolution spectroscopic measurements of dynamic fibrils (DFs) in the Ca II λ8662 line. These data show clear Doppler shifts in the identified DFs, which demonstrates that at least a subset of DFs are actual mass motions in the chromosphere. A statistical analysis of 26 DFs reveals a strong and statistically significant correlation between the maximal velocity and the deceleration. The range of the velocities and the decelerations are substantially lower, about a factor 2, in our spectroscopic observations compared to the earlier results based on proper motion in narrowband images. There are fundamental differences in the different observational methods; when DFs are observed spectroscopically, the measured Doppler shifts are a result of the atmospheric velocity, weighted with the response function to velocity over an extended height. When the proper motion of DFs is observed in narrowband images, the movement of the top of the DF is observed. This point is sharply defined because of the high contrast between the DF and the surroundings. The observational differences between the two methods are examined by several numerical experiments using both numerical simulations and a time series of narrowband Hα images. With basis in the simulations we conclude that the lower maximal velocity is explained by the low formation height of the Ca IR line. We conclude that the present observations support the earlier result that DFs are driven by magnetoacoustic shocks excited by convective flows and p-modes. Title: Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen, V. H.; Tarbell, T. D.; Schrijver, C. J.; Title, A. M.; Shine, R. A.; Tsuneta, S.; Katsukawa, Y.; Ichimoto, K.; Suematsu, Y.; Shimizu, T.; Nagata, S. Bibcode: 2007Sci...318.1574D Altcode: Alfvén waves have been invoked as a possible mechanism for the heating of the Sun's outer atmosphere, or corona, to millions of degrees and for the acceleration of the solar wind to hundreds of kilometers per second. However, Alfvén waves of sufficient strength have not been unambiguously observed in the solar atmosphere. We used images of high temporal and spatial resolution obtained with the Solar Optical Telescope onboard the Japanese Hinode satellite to reveal that the chromosphere, the region sandwiched between the solar surface and the corona, is permeated by Alfvén waves with strong amplitudes on the order of 10 to 25 kilometers per second and periods of 100 to 500 seconds. Estimates of the energy flux carried by these waves and comparisons with advanced radiative magnetohydrodynamic simulations indicate that such Alfvén waves are energetic enough to accelerate the solar wind and possibly to heat the quiet corona. Title: Hinode and the Corona's Lower Boundary: Spicules and Alfven Waves Authors: de Pontieu, B.; McIntosh, S. W.; Hansteen, V.; Carlsson, M. P. Bibcode: 2007AGUFMSH52C..08D Altcode: The lower boundary of the corona, or chromosphere, requires of order 100 times more energy than the corona itself, and provides the mass to fill coronal loops. Yet the chromosphere and its coupling to the corona is often overlooked. Recently, observations with the Solar Optical Telescope (SOT) onboard Hinode and ground-based telescopes combined with advanced numerical simulations have provided us with unprecedented views and a better understanding of the (spicular) dynamics of the chromosphere and how the lower boundary couples to the corona and solar wind. We analyze high-resolution, high-cadence Ca II and Hα observations of the solar chromosphere and find that the dynamics of the magnetized chromosphere are dominated by at least two different types of spicules. We show that the first type involves up- and downward motion that is driven by shock waves that form when global oscillations and convective flows leak into the chromosphere along magnetic field lines on on 3-7 minute timescales. The second type of spicules is much more dynamic: they form rapidly (in ~10s), are very thin (<200km wide), have lifetimes of 10-150s (at any one height) and seem to be rapidly heated to (at least) transition region temperatures, sending material through the chromosphere at speeds of order 50-150 km/s. The properties of Type II spicules suggest a formation process that is a consequence of magnetic reconnection. We discuss the impact of both spicules types on the coronal mass and energy balance. Our analysis of Hinode data also indicates that the chromosphere is permeated by strong Alfvén waves. Both types of spicules are observed to carry these Alfvén waves, which have significant amplitudes of order 20 km/s, transverse displacements of order 500-1,000 km and periods of 150-400 s. Estimates of the energy flux carried by these Alfvén waves and comparisons to advanced radiative MHD simulations indicate that these waves most likely play a significant role in the acceleration of the solar wind, and possibly the heating of the quiet Sun corona. We will discuss the implications of these waves on the energy balance of the lower atmosphere. Title: On Connecting the Dynamics of the Chromosphere and Transition Region with Hinode SOT and EIS Authors: Hansteen, Viggo H.; de Pontieu, Bart; Carlsson, Mats; McIntosh, Scott; Watanabe, Tetsuya; Warren, Harry P.; Harra, Louise K.; Hara, Hirohisa; Tarbell, Theodore D.; Shine, Dick; Title, Alan M.; Schrijver, Carolus J.; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto, Kiyoshi; Suematsu, Yoshinori; Shimizu, Toshifumi Bibcode: 2007PASJ...59S.699H Altcode: 2007arXiv0711.0487H We use coordinated Hinode SOT/EIS observations that include high-resolution magnetograms, chromospheric, and transition region (TR) imaging, and TR/coronal spectra in a first test to study how the dynamics of the TR are driven by the highly dynamic photospheric magnetic fields and the ubiquitous chromospheric waves. Initial analysis shows that these connections are quite subtle and require a combination of techniques including magnetic field extrapolations, frequency-filtered time-series, and comparisons with synthetic chromospheric and TR images from advanced 3D numerical simulations. As a first result, we find signatures of magnetic flux emergence as well as 3 and 5mHz wave power above regions of enhanced photospheric magnetic field in both chromospheric, transition region, and coronal emission. Title: Can High Frequency Acoustic Waves Heat the Quiet Sun Chromosphere? Authors: Carlsson, Mats; Hansteen, Viggo H.; de Pontieu, Bart; McIntosh, Scott; Tarbell, Theodore D.; Shine, Dick; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto, Kiyoshi; Suematsu, Yoshinori; Shimizu, Toshifumi; Nagata, Shin'ichi Bibcode: 2007PASJ...59S.663C Altcode: 2007arXiv0709.3462C We use Hinode/SOT Ca II H-line and blue continuum broadband observations to study the presence and power of high frequency acoustic waves at high spatial resolution. We find that there is no dominant power at small spatial scales; the integrated power using the full resolution of Hinode (0.05'' pixels, 0.16'' resolution) is larger than the power in the data degraded to 0.5'' pixels (TRACE pixel size) by only a factor of 1.2. At 20 mHz the ratio is 1.6. Combining this result with the estimates of the acoustic flux based on TRACE data of Fossum & Carlsson (2006), we conclude that the total energy flux in acoustic waves of frequency 5-40 mHz entering the internetwork chromosphere of the quiet Sun is less than 800 W m$^{-2}$, inadequate to balance the radiative losses in a static chromosphere by a factor of five. Title: A Tale of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere Authors: de Pontieu, Bart; McIntosh, Scott; Hansteen, Viggo H.; Carlsson, Mats; Schrijver, Carolus J.; Tarbell, Theodore D.; Title, Alan M.; Shine, Richard A.; Suematsu, Yoshinori; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Nagata, Shin'ichi Bibcode: 2007PASJ...59S.655D Altcode: 2007arXiv0710.2934D We use high-resolution observations of the Sun in CaIIH (3968Å) from the Solar Optical Telescope on Hinode to show that there are at least two types of spicules that dominate the structure of the magnetic solar chromosphere. Both types are tied to the relentless magnetoconvective driving in the photosphere, but have very different dynamic properties. ``Type-I'' spicules are driven by shock waves that form when global oscillations and convective flows leak into the upper atmosphere along magnetic field lines on 3--7minute timescales. ``Type-II'' spicules are much more dynamic: they form rapidly (in ∼ 10s), are very thin (≤ 200 km wide), have lifetimes of 10-150s (at any one height), and seem to be rapidly heated to (at least) transition region temperatures, sending material through the chromosphere at speeds of order 50--150kms-1. The properties of Type II spicules suggest a formation process that is a consequence of magnetic reconnection, typically in the vicinity of magnetic flux concentrations in plage and network. Both types of spicules are observed to carry Alfvén waves with significant amplitudes of order 20kms-1. Title: Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rutten, R. J. Bibcode: 2007A&A...473..625L Altcode: 2007arXiv0709.3751L Context: The ionization of hydrogen in the solar chromosphere and transition region does not obey LTE or instantaneous statistical equilibrium because the timescale is long compared with important hydrodynamical timescales, especially of magneto-acoustic shocks. Since the pressure, temperature, and electron density depend sensitively on hydrogen ionization, numerical simulation of the solar atmosphere requires non-equilibrium treatment of all pertinent hydrogen transitions. The same holds for any diagnostic application employing hydrogen lines.
Aims: To demonstrate the importance and to quantify the effects of non-equilibrium hydrogen ionization, both on the dynamical structure of the solar atmosphere and on hydrogen line formation, in particular Hα.
Methods: We implement an algorithm to compute non-equilibrium hydrogen ionization and its coupling into the MHD equations within an existing radiation MHD code, and perform a two-dimensional simulation of the solar atmosphere from the convection zone to the corona.
Results: Analysis of the simulation results and comparison to a companion simulation assuming LTE shows that: a) non-equilibrium computation delivers much smaller variations of the chromospheric hydrogen ionization than for LTE. The ionization is smaller within shocks but subsequently remains high in the cool intershock phases. As a result, the chromospheric temperature variations are much larger than for LTE because in non-equilibrium, hydrogen ionization is a less effective internal energy buffer. The actual shock temperatures are therefore higher and the intershock temperatures lower. b) The chromospheric populations of the hydrogen n = 2 level, which governs the opacity of Hα, are coupled to the ion populations. They are set by the high temperature in shocks and subsequently remain high in the cool intershock phases. c) The temperature structure and the hydrogen level populations differ much between the chromosphere above photospheric magnetic elements and above quiet internetwork. d) The hydrogen n = 2 population and column density are persistently high in dynamic fibrils, suggesting that these obtain their visibility from being optically thick in Hα also at low temperature.

Movie and Appendix A are only available in electronic form at http://www.aanda.org Title: High-Resolution Observations and Numerical Simulations of Chromospheric Fibrils and Mottles Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2007ASPC..368...65D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in Hα) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations.

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

\citep{ol-2006ApJ...647L..73H} requires compensation for several observational issues. Title: 3D Numerical Models of the Chromosphere, Transition Region, and Corona Authors: Hansteen, V. H.; Carlsson, M.; Gudiksen, B. Bibcode: 2007ASPC..368..107H Altcode: 2007arXiv0704.1511H A major goal in solar physics has during the last five decades been to find how energy flux generated in the solar convection zone is transported and dissipated in the outer solar layers. Progress in this field has been slow and painstaking. However, advances in computer hardware and numerical methods, vastly increased observational capabilities and growing physical insight seem finally to be leading towards understanding. Here we present exploratory numerical MHD models that span the entire solar atmosphere from the upper convection zone to the lower corona. These models include non-grey, non-LTE radiative transport in the photosphere and chromosphere, optically thin radiative losses as well as magnetic field-aligned heat conduction in the transition region and corona. Title: Modeling the Solar Chromosphere Authors: Carlsson, M. Bibcode: 2007ASPC..368...49C Altcode: 2007arXiv0704.1509C Spectral diagnostic features formed in the solar chromosphere are few and difficult to interpret --- they are neither formed in the optically thin regime nor in local thermodynamic equilibrium (LTE).

To probe the state of the chromosphere, both from observations and theory, it is therefore necessary with modeling.

I discuss both traditional semi-empirical modeling, numerical experiments illustrating important ingredients necessary for a self-consistent theoretical modeling of the solar chromosphere and the first results of such models. Title: Observational Evidence For The Ubiquity Of Strong Alfven Waves In The Magnetized Chromosphere Authors: De Pontieu, Bart; McIntosh, S. W.; Carlsson, M.; Hansteen, V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A.; SOT Team Bibcode: 2007AAS...210.9415D Altcode: 2007BAAS...39R.219D Hinode/SOT Ca II broadband observations show that Alfven waves with significant amplitudes of order 10-20 km/s and periods of 150-300 s are extremely ubiquitous in the magnetized middle to upper chromosphere. Our observations focus on spicules at the limb, and straw-like features associated with network and plage on the disk. We find that the weak straw-like features and most spicules all undergo significant transverse motions that are driven by Alfven waves. These waves are seen to propagate both up- and downward, and may carry an energy flux that is significant compared to both the local, coronal and solar wind energy balance. We will provide estimates of the energy flux carried by these waves, and will compare our observations with Alfven waves that are observed in 3D numerical simulations that include advanced radiative transfer treatment for the chromosphere.

This work was supported by NASA contract NNM07AA01C. Title: Connecting The Dynamics Of The Chromosphere And Transition Region With Hinode/sot And Eis Authors: Hansteen, Viggo H.; McIntosh, S.; De Pontieu, B.; Carlsson, M.; SOT Team Bibcode: 2007AAS...210.9430H Altcode: 2007BAAS...39..222H We will use coordinated Hinode SOT/EIS observations that include high-resolution magnetograms, chromospheric and TR imaging and TR/coronal spectra to study how the dynamics of the TR are driven by the higly dynamic photospheric magnetic fields and the ubiquitous chromospheric waves. Using travel time analysis, magnetic field extrapolations, frequency filtered timeseries and comparisons with synthetic chromospheric and TR images from advanced 3D numerical simulations, we will study and establish how the dynamics of the photosphere, chromosphere and TR are connected. Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen, V. H.; Carlsson, M.; van Noort, M. Bibcode: 2007ApJ...660L.169R Altcode: 2007astro.ph..3535R We present high spatial and high temporal resolution observations of the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on La Palma. We observe that many mottles, jetlike features in the quiet Sun, display clear up- and downward motions along their main axis. In addition, many mottles show vigorous transverse displacements. Unique identification of the mottles throughout their lifetime is much harder than for their active region counterpart, dynamic fibrils. This is because many seem to lack a sharply defined edge at their top, and significant fading often occurs throughout their lifetime. For those mottles that can be reliably tracked, we find that the mottle tops often undergo parabolic paths. We find a linear correlation between the deceleration these mottles undergo and the maximum velocity they reach, similar to what was found earlier for dynamic fibrils. Combined with an analysis of oscillatory properties, we conclude that at least part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed-polarity environment and vigorous dynamics suggest that reconnection may play a significant role in the formation of some quiet-Sun jets. Title: A Tale of Two Spicules Authors: McIntosh, Scott W.; De Pontieu, B.; Carlsson, M.; Hansteen, V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A. M.; SOT Team Bibcode: 2007AAS...210.9414M Altcode: 2007BAAS...39..219M Hinode/SOT Ca II broadband images and movies show that there are several different types of spicules at the limb. These different types are distinguished by dynamics on different timescales. The first type involves up- and downward motion on timescales of 3-5 minutes. The dynamics of these spicules are very similar to those of fibrils and mottles as observed on the disk. Recent work suggests that these are driven by slow-mode magnetoacoustic shocks that form when convective flows and global oscillations leak into the chromosphere along magnetic flux tubes. The second type is much more dynamic with typical lifetimes of 10-60 s. These spicules are characterized by sudden appearance and disappearance that may be indicative of rapid heating to TR temperatures. We will describe the properties of these spicules in various magnetic environments (coronal hole, quiet Sun, active region) and study the possible role of reconnection in driving the second type of spicules. In addition, we will perform detailed comparisons of these different types of jets with synthetic Ca images derived from advanced 3D numerical simulations that encompass the convection zone up through the corona. Title: Can High Frequency Acoustic Waves Heat the Quiet Sun Chromosphere? Authors: Carlsson, Mats P. O.; De Pontieu, B.; Tarbell, T.; Hansteen, V. H.; McIntosh, S.; SOT Team Bibcode: 2007AAS...210.6306C Altcode: 2007BAAS...39..172C We use Hinode/SOT Ca II, G-band and blue continuum broadband observations to study the presence and power of high frequency acoustic waves at high spatial resolution. Previous observations with TRACE, which were limited by the 1 arcsec resolution, and 1D numerical simulations (Fossum & Carlsson, 2005) have been used to constrain the possible role of high frequency waves in the heating of the quiet Sun chromosphere. We will use the higher spatial resolution Hinode data and comparisons with both 1D and 3D numerical models to study the amount of high frequency power at smaller scales, and whether that power is sufficient to heat the quiet Sun chromosphere. Title: Observations and Simulations of Fibrils and Mottles Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort, Luc; van Noort, Michiel; Carlsson, Mats Bibcode: 2007astro.ph..2081D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in H-alpha) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. In addition, we present some preliminary observations of quiet Sun jets or mottles. We find that the mechanism that produces fibrils in active regions is most likely also at work in quiet Sun regions, although it is modified by the weaker magnetic field and the presence of more mixed-polarity. A comparison with numerical simulations suggests that the weaker magnetic field in quiet Sun allows for significantly stronger (than in active regions) transverse motions that are superposed on the field-aligned, shock-driven motions. This leads to a more dynamic, and much more complex environment than in active region plage. In addition, our observations of the mixed polarity environment in quiet Sun regions suggest that other mechanisms, such as reconnection, may well play a significant role in the formation of some quiet Sun jets. Title: Velocities Measured in Small-Scale Solar Magnetic Elements Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort, Luc; Stein, R. F. Bibcode: 2007ApJ...655..615L Altcode: 2006astro.ph.11741L We have obtained high-resolution spectrograms of small-scale magnetic structures with the Swedish 1-m Solar Telescope. We present Doppler measurements at 0.2" spatial resolution of bright points, ribbons, and flowers, and their immediate surroundings, in the C I λ5380.3 line (formed in the deep photosphere) and the two Fe I lines at 5379.6 and 5386.3 Å. The velocity inside the flowers and ribbons are measured to be almost zero, while we observe downflows at the edges. These downflows are increasing with decreasing height. We also analyze realistic magnetoconvective simulations to obtain a better understanding of the interpretation of the observed signal. We calculate how the Doppler signal depends on the velocity field in various structures. Both the smearing effect of the nonnegligible width of this velocity response function along the line of sight and of the smearing from the telescope and atmospheric point-spread function are discussed. These studies lead us to the conclusion that the velocity inside the magnetic elements is really upflow of order 1-2 km s-1, while the downflows at the edges really are much stronger than observed, of order 1.5-3.3 km s-1. Title: High-Resolution Observations and Modeling of Dynamic Fibrils Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2007ApJ...655..624D Altcode: 2007astro.ph..1786D We present unprecedented high-resolution Hα observations, obtained with the Swedish 1 m Solar Telescope, that, for the first time, spatially and temporally resolve dynamic fibrils in active regions on the Sun. These jetlike features are similar to mottles or spicules in quiet Sun. We find that most of these fibrils follow almost perfect parabolic paths in their ascent and descent. We measure the properties of the parabolic paths taken by 257 fibrils and present an overview of the deceleration, maximum velocity, maximum length, and duration, as well as their widths and the thickness of a bright ring that often occurs above dynamic fibrils. We find that the observed deceleration of the projected path is typically only a fraction of solar gravity and incompatible with a ballistic path at solar gravity. We report on significant differences of fibril properties between those occurring above a dense plage region and those above a less dense plage region where the magnetic field seems more inclined from the vertical. We compare these findings to advanced numerical two-dimensional radiative MHD simulations and find that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. Detailed comparison of observed and simulated fibril properties shows striking similarities of the values for deceleration, maximum velocity, maximum length, and duration. We compare our results with observations of mottles and find that a similar mechanism is most likely at work in the quiet Sun. Title: High Spatial Resolution Observations of Solar Magnetic Structures Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.; Hansteen, V. Bibcode: 2006ASPC..354...37R Altcode: We present observations of the dynamic evolution of photospheric magnetic structures in the G-band, continuum, magnetograms and Dopplergrams. The observations were obtained with the Swedish one-m Solar Telescope on La Palma. Using adaptive optics and the Multi-Object Multi-Frame Blind Deconvolution image restoration technique, we obtained several datasets at close to the diffraction limit of the telescope (0.1 arcsec) over long periods of time. We show examples of the dynamical evolution of different magnetic structures: the advection of individual bright points by the granular flow, the formation and fragmentation of flux sheets, and the continuous transition between micro-pores, elongated ribbons and more circular ``flowers''. Narrow sheets with downdrafts are found right at the edges of magnetic field concentrations. Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2006AGUFMSH23B0359D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. We will present results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in Hα) above and about active region plage. These jets are similar to mottles and spicules in quiet Sun. Our analysis is based on a time series of extremely high-resolution (120 km) images taken in Hα linecenter at 1 second cadence, obtained by the Oslo group at the SST in October 2005. The 78 min long time series for the first time, spatially and temporally resolves dynamic fibrils in active regions. Our analysis shows that most of the fibrils follow almost perfect parabolic paths in their ascent and descent. We measure the properties of the parabolic paths taken by 257 different dynamic fibrils, and find that the observed deceleration of the projected path is typically only a fraction of solar gravity, and incompatible with a ballistic path at solar gravity. We report on significant differences of measured fibril properties between those occurring in association with a dense plage region, and those above a less dense plage region where the magnetic field seems more inclined away from the vertical. We compare these observational findings to advanced numerical 2D radiative MHD simulations, and find that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. Detailed comparison of the properties of fibrils found in our observations and those in our numerical simulations shows striking similarities of the values for deceleration, maximum velocity, maximum length and duration. The numerical simulations also reproduce the correlations we observe between various fibrils properties, as well as the regional differences, taking into account the different magnetic configuration for the various regions. We compare our results with observations of mottles and find that a similar mechanism is most likely at work in the quiet Sun. Title: Chromospheric Modeling Authors: Carlsson, M. Bibcode: 2006ASPC..354..291C Altcode: We discuss chromospheric modeling -- both semi-empirical models and theoretical models. The development of new computational schemes, new approximate treatments of time dependent hydrogen ionization and more powerful computers have paved the way for comprehensive models in three dimensions extending from the convection zone to the corona. The first results of such models have just appeared. Title: The non-LTE line formation of neutral carbon in late-type stars Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D. Bibcode: 2006A&A...458..899F Altcode: 2006astro.ph..8284F Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line formation of neutral carbon in late-type stars in order to remove some of the potential systematic errors in stellar abundance analyses employing C i features.
Methods: .The statistical equilibrium code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric models.
Results: .Within the parameter space explored, the high-excitation C i lines studied are stronger in non-LTE due to the combined effect of line source function drop and increased line opacity due to overpopulation of the lower level for the transitions considered; the relative importance of the two effects depends on the particular combination of T{eff}, log g, [Fe/H] and [C/Fe] and on the analysed C i line. As a consequence, the non-LTE abundance corrections are negative and can be substantially so, for example ∼ -0.4 dex in halo turn-off stars at [Fe/H]∼ -3. The magnitude of the non-LTE corrections is rather insensitive to whether inelastic H collisions are included or not.
Conclusions: .Our results have implications on studies of nucleosynthetic processes and on Galactic chemical evolution models. When applying our calculated corrections to recent observational data, the upturn in [C/O] at low metallicity might still be present (thus apparently still necessitating contributions from massive Pop. III stars for the carbon production), but at a lower level and possibly with a rather shallow trend of ∼ -0.2 dex/dex below [O/H]∼ -1. Title: Rapid Temporal Variability of Faculae: High-Resolution Observations and Modeling Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort, L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G. Bibcode: 2006ApJ...646.1405D Altcode: We present high-resolution G-band observations (obtained with the Swedish 1 m Solar Telescope) of the rapid temporal variability of faculae, which occurs on granular timescales. By combining these observations with magnetoconvection simulations of a plage region, we show that much of this variability is not intrinsic to the magnetic field concentrations that are associated with faculae, but rather a phenomenon associated with the normal evolution and splitting of granules. We also show examples of facular variability caused by changes in the magnetic field, with movies of dynamic behavior of the striations that dominate much of the facular appearance at 0.1" resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying. Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2006ApJ...647L..73H Altcode: 2006astro.ph..7332H The formation of jets such as dynamic fibrils, mottles, and spicules in the solar chromosphere is one of the most important, but also most poorly understood, phenomena of the Sun's magnetized outer atmosphere. We use extremely high resolution observations from the Swedish 1 m Solar Telescope combined with advanced numerical modeling to show that in active regions these jets are a natural consequence of upwardly propagating slow-mode magnetoacoustic shocks. These shocks form when waves generated by convective flows and global p-mode oscillations in the lower lying photosphere leak upward into the magnetized chromosphere. We find excellent agreement between observed and simulated jet velocities, decelerations, lifetimes, and lengths. Our findings suggest that previous observations of quiet-Sun spicules and mottles may also be interpreted in light of a shock-driven mechanism. Title: Determination of the Acoustic Wave Flux in the Lower Solar Chromosphere Authors: Fossum, Astrid; Carlsson, Mats Bibcode: 2006ApJ...646..579F Altcode: High-cadence observations of the quiet Sun have been obtained with the Transition Region And Coronal Explorer (TRACE) satellite in two UV passbands around 1600 and 1700 Å. The observational program has been optimized for the detection of high-frequency acoustic waves: the cadence is strictly regular, there is no data compression, and the exposure time is optimized for quiet-Sun regions. Significant intensity variations are detected up to 40 mHz frequency. Non-LTE radiation hydrodynamics simulations are performed in order to calculate the acoustic energy flux that the observed intensity variations correspond to. The derived acoustic energy flux spectrum at the formation height of the UV continua (about 400 km) is decreasing with frequency. For frequencies above 40 mHz we derive an upper limit. The integrated acoustic energy flux is 255 W m-2 in the frequency range 5-50 mHz. This is lower than what is needed to balance the radiative losses from the quiet-Sun chromosphere by at least a factor of 10. The major uncertainty in the analysis is the possibility of high-frequency power with spatial scales smaller than the resolution element of TRACE. We make estimates of this effect and find it unlikely that it is larger than a factor of 2. In the convection zone, where the waves are generated, the energy spectrum is rather flat and may have a peak at high frequencies. We also show that the sensitivity of the observations to high-frequency waves is directly given by the Fourier transform of the response function. Title: Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.; Carlsson, Mats Bibcode: 2006ApJ...644..484A Altcode: 2006astro.ph..3195A We report on radiative hydrodynamic simulations of M dwarf stellar flares and compare the model predictions to observations of several flares. The flares were simulated by calculating the hydrodynamic response of a model M dwarf atmosphere to a beam of nonthermal electrons. Radiative back-warming through numerous soft X-ray, extreme-ultraviolet, and ultraviolet transitions are also included. The equations of radiative transfer and statistical equilibrium are treated in non-LTE for many transitions of hydrogen, helium, and the Ca II ion, allowing the calculation of detailed line profiles and continuum radiation. Two simulations were carried out, with electron beam fluxes corresponding to moderate and strong beam heating. In both cases we find that the dynamics can be naturally divided into two phases: an initial gentle phase in which hydrogen and helium radiate away much of the beam energy and an explosive phase characterized by large hydrodynamic waves. During the initial phase, lower chromospheric material is evaporated into higher regions of the atmosphere, causing many lines and continua to brighten dramatically. The He II 304 line is especially enhanced, becoming the brightest line in the flaring spectrum. The hydrogen Balmer lines also become much brighter and show very broad line widths, in agreement with observations. We compare our predicted Balmer decrements to decrements calculated for several flare observations and find the predictions to be in general agreement with the observations. During the explosive phase both condensation and evaporation waves are produced. The moderate flare simulation predicts a peak evaporation wave of ~130 km s-1 and a condensation wave of ~30 km s-1. The velocity of the condensation wave matches velocities observed in several transition region lines. The optical continuum also greatly intensifies, reaching a peak increase of 130% (at 6000 Å) for the strong flare, but does not match observed white-light spectra. Title: A comparison of solar proxy-magnetometry diagnostics Authors: Leenaarts, J.; Rutten, R. J.; Carlsson, M.; Uitenbroek, H. Bibcode: 2006A&A...452L..15L Altcode: Aims.We test various proxy-magnetometry diagnostics, i.e., brightness signatures of small-scale magnetic elements, for studying magnetic field structures in the solar photosphere.
Methods: .Images are numerically synthesized from a 3D solar magneto-convection simulation for, respectively, the G band at 430.5 nm, the CN band at 388.3 nm, and the blue wings of the H α, H β, Ca ii H, and Ca ii 854.2 nm lines.
Results: .Both visual comparison and scatter diagrams of the computed intensity versus the magnetic field strength show that, in particular for somewhat spatially extended magnetic elements, the blue H α wing presents the best proxy-magnetometry diagnostic, followed by the blue wing of H β. The latter yields higher diffraction-limit resolution.
Conclusions: .We recommend using the blue H α wing to locate and track small-scale photospheric magnetic elements through their brightness appearance. Title: Simulation of Quiet-Sun Waves in the Ca II Infrared Triplet Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.; Carlsson, M.; Stein, R. F. Bibcode: 2006ApJ...640.1142P Altcode: 2005astro.ph.10744P The Ca II infrared triplet lines around 8540 Å are good candidates for observing chromospheric magnetism. Model spectra of these lines are obtained by combining a radiation hydrodynamic simulation with a Stokes synthesis code. The simulation shows interesting time-varying behavior of the Stokes V profiles as waves propagate through the formation region of the lines. Disappearing and reappearing lobes in the Stokes V profiles as well as profile asymmetries are closely related to the atmospheric velocity gradients. Title: DOT tomography of the solar atmosphere. VI. Magnetic elements as bright points in the blue wing of Hα Authors: Leenaarts, J.; Rutten, R. J.; Sütterlin, P.; Carlsson, M.; Uitenbroek, H. Bibcode: 2006A&A...449.1209L Altcode: High-resolution solar images taken in the blue wing of the Balmer H α line with the Dutch Open Telescope show intergranular magnetic elements as strikingly bright features, similar to, but with appreciably larger contrast over the surrounding granulation than their more familiar manifestation as G-band bright points. Part of this prominent appearance is due to low granular contrast, without granule/lane brightness reversal as, e.g., in the wings of Ca II H & K. We use 1D and 2D radiative transfer modeling and 3D solar convection and magnetoconvection simulations to reproduce and explain the H α wing images. We find that the blue H α wing obeys near-LTE line formation. It appears particularly bright in magnetic elements through low temperature gradients. The granulation observed in the blue wing of H α has low contrast because of the lack of H α opacity in the upper photosphere, Doppler cancellation, and large opacity sensitivity to temperature working against source function sensitivity. We conclude that the blue H α wing represents a promising proxy magnetometer to locate and track isolated intermittent magnetic elements, a better one than the G band and the wings of Ca II H & K although less sharp at given aperture. Title: Numerical modelling of MHD waves in the solar chromosphere Authors: Carlsson, Mats; Bogdan, Thomas J. Bibcode: 2006RSPTA.364..395C Altcode: No abstract at ADS Title: Ci Non-LTE Spectral Line Formation in Late-Type Stars Authors: Fabbian, D.; Asplund, M.; Carlsson, M. Bibcode: 2006cams.book...52F Altcode: We present the results from our non-LTE investigation for neutral carbon, which was carried out to remove potential systematic errors in stellar abundance analyses. The calculations were performed for late-type stars and give substantial negative non-LTE abundance corrections. When applied to observations of extremely metal-poor stars, which within the LTE framework seem to suggest a possible [C/O] uprise at low metallicities (Akerman et al. 2004), these improvements will have important implications, enabling us to understand if the standard chemical evolution model is adequate, with no need to invoke signatures by Pop. III stars for the carbon nucleosynthesis. Title: Simulated Solar Plages Authors: Stein, R. F.; Carlsson, M.; de Pontieu, B.; Scharmer, G.; Nordlund, Å.; Benson, D. Bibcode: 2006apri.meet...30S Altcode: No abstract at ADS Title: Are High Frequency Acoustic Waves Sufficient to Heat the Solar Chromosphere? Authors: Fossum, A.; Carlsson, M. Bibcode: 2005ESASP.600E..17F Altcode: 2005ESPM...11...17F; 2005dysu.confE..17F No abstract at ADS Title: The Diagnostic Potential of the MG i 4571.1 Å Line Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L. Bibcode: 2005ESASP.600E..65L Altcode: 2005ESPM...11...65L; 2005dysu.confE..65L No abstract at ADS Title: Chromospheric Waves Authors: Carlsson, M.; Hansteen, V. Bibcode: 2005ESASP.600E..16C Altcode: 2005dysu.confE..16C; 2005ESPM...11...16C No abstract at ADS Title: Chromospheric Waves Authors: Carlsson, M.; Hansteen, V. Bibcode: 2005ESASP.596E..39C Altcode: 2005ccmf.confE..39C No abstract at ADS Title: Small Scale Magnetic Elements as Bright Points in the Blue Hα Wing Authors: Leenaarts, J.; Sütterlin, P.; Rutten, R. J.; Carlsson, M.; Uitenbroek, H. Bibcode: 2005ESASP.596E..15L Altcode: 2005ccmf.confE..15L No abstract at ADS Title: Radiative Hydrodynamic Models of the Optical and Ultraviolet Emission from Solar Flares Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.; Carlsson, Mats Bibcode: 2005ApJ...630..573A Altcode: 2005astro.ph..7335A We report on radiative hydrodynamic simulations of moderate and strong solar flares. The flares were simulated by calculating the atmospheric response to a beam of nonthermal electrons injected at the apex of a one-dimensional closed coronal loop and include heating from thermal soft X-ray, extreme ultraviolet, and ultraviolet (XEUV) emission. The equations of radiative transfer and statistical equilibrium were treated in non-LTE and solved for numerous transitions of hydrogen, helium, and Ca II, allowing the calculation of detailed line profiles and continuum emission. This work improves on previous simulations by incorporating more realistic nonthermal electron beam models and includes a more rigorous model of thermal XEUV heating. We find that XEUV back-warming contributes less than 10% of the heating, even in strong flares. The simulations show elevated coronal and transition region densities resulting in dramatic increases in line and continuum emission in both the UV and optical regions. The optical continuum reaches a peak increase of several percent, which is consistent with enhancements observed in solar white-light flares. For a moderate flare (~M class), the dynamics are characterized by a long gentle phase of near balance between flare heating and radiative cooling, followed by an explosive phase with beam heating dominating over cooling and characterized by strong hydrodynamic waves. For a strong flare (~X class), the gentle phase is much shorter, and we speculate that for even stronger flares the gentle phase may be essentially nonexistent. During the explosive phase, synthetic profiles for lines formed in the upper chromosphere and transition region show blueshifts corresponding to a plasma velocity of ~120 km s-1, and lines formed in the lower chromosphere show redshifts of ~40 km s-1. Title: High-frequency acoustic waves are not sufficient to heat the solar chromosphere Authors: Fossum, Astrid; Carlsson, Mats Bibcode: 2005Natur.435..919F Altcode: One of the main unanswered questions in solar physics is why the Sun's outer atmosphere is hotter than its surface. Theory predicts abundant production of high-frequency (10-50mHz) acoustic waves in subsurface layers of the Sun, and such waves are believed by many to constitute the dominant heating mechanism of the chromosphere (the lower part of the outer solar atmosphere) in non-magnetic regions. Such high-frequency waves are difficult to detect because of high-frequency disturbances in Earth's atmosphere (seeing) and other factors. Here we report the detection of high-frequency waves, and we use numerical simulations to show that the acoustic energy flux of these waves is too low, by a factor of at least ten, to balance the radiative losses in the solar chromosphere. Acoustic waves therefore cannot constitute the dominant heating mechanism of the solar chromosphere. Title: Response Functions of the Ultraviolet Filters of TRACE and the Detectability of High-Frequency Acoustic Waves Authors: Fossum, Astrid; Carlsson, Mats Bibcode: 2005ApJ...625..556F Altcode: We have used detailed non-LTE radiation hydrodynamic simulations to investigate the detectability of high-frequency acoustic waves with the Transition Region And Coronal Explorer (TRACE). A broad spectrum of acoustic waves are fed into the computational domain at the lower boundary of the model atmosphere, and TRACE UV continuum intensities are calculated by folding the derived intensities with the TRACE filter transmission functions for the 1700 and 1600 filters. Power spectra, phase diagrams, and intensity response functions are calculated, and intensity formation heights are derived. The simulations show that the average response height of the 1700 and 1600 passbands are 360 and 430 km, with widths of 325 and 185 km. The width of the TRACE intensity response functions reduces the power of the intensity oscillations considerably, but if waves are present with power enough to be of importance for the energy balance of the chromosphere, they should be detectable at least up to 40 mHz in the absence of instrumental noise, especially in the 1600 passband. The phase difference between the synthesized 1600 and 1700 TRACE intensities follows the curve expected for propagating acoustic waves up to 15 mHz. For higher frequencies the phase difference decreases and approaches zero before the coherence drops down, similar to the observed behavior. This is explained by the double-peaked nature of the response function for the 1700 intensities. Title: Multi-dimensional Radiation MHD as a Tool to Understanding Waves in Realistic Magnetic Topologies of the Solar Chromosphere Authors: Carlsson, M. Bibcode: 2005AGUSMSH12A..01C Altcode: Advances in computing capabilities have made it possible to perform multi-dimensional radiation MHD simulations with a realistic description of the physics in the photosphere. In the chromosphere non-LTE becomes important and the choice is between realistic physics in one dimension and approximations to the non-local radiation in multi-dimensions. We will here discuss such simulations from a methodological vantage point. We will discuss how we can make such simulations to study how waves are excited in the solar convection zone, how they propagate through the chromosphere and transition region and undergo mode conversion, refraction and reflection in various magnetic field topologies. We will also discuss the limitations of observational diagnostics with respect to wide response functions, non-locality and non-equilibrium effects. Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical evolution Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson, M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E. Bibcode: 2005A&A...435..327R Altcode: Small magnetic structures can be seen in G-band filtergrams as isolated bright points, strings of bright points and dark micro-pores. At a resolution of 0.1 arcsec, new forms of magnetic structures are found in strong field areas: elongated “ribbons” and more circular “flowers”. In this paper we study the temporal evolution of these small scale magnetic structures. In strong-field regions the time-evolution is more that of a magnetic fluid than that of collections of flux-tubes that keep their identity. We find that the granular flow concentrates the magnetic field into flux sheets that are visible as thin bright features in the filtergrams. Weak upflows are found in the flux sheets and downflows in the immediate surroundings. The flux sheets often become unstable to a fluting instability and the edges buckle. The sheets tend to break up into strings of bright points, still with weak upflows in the magnetic elements and zero velocity or downflows between them. Where there are larger flux concentrations we find ribbons, flowers and micro-pores. There is a continuous transition between these forms and they evolve from one form to another. The appearance is mostly determined by the horizontal size - larger structures are dark (micro-pores), narrower structures are ribbon shaped and the flowers are the smallest in extent. All these structures have darker inner parts and a bright edge. The plasma is found to be at rest in the ribbons, with small concentrations of weak upflow sites. Narrow sheets with downdrafts are found right at the edges of the magnetic field concentrations. Title: Detectability of high frequency acoustic waves with TRACE Authors: Fossum, A.; Carlsson, M. Bibcode: 2005ASSL..320..239F Altcode: 2005smp..conf..239F No abstract at ADS Title: CI non LTE spectral line formation in late-type stars Authors: Fabbian, D.; Asplund, M.; Carlsson, M.; Kiselman, D. Bibcode: 2005IAUS..228..255F Altcode: 2005astro.ph..8063F We present non-Local Thermodynamic Equilibrium (non-LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non-LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al (2004), the effects are of the order of ΔlogɛC ≃ -0.35ldots-0.45 (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis. Title: Chromospheric Heating and Dynamics Authors: Carlsson, M.; Stein, R. F. Bibcode: 2004ASPC..325..243C Altcode: We review observations of the dynamics and energetics of the solar chromosphere. The observations are interpreted with the help of detailed radiation hydrodynamic modelling. It is concluded that acoustic waves play an important role for the dynamics and energetics of the chromosphere but additional heating is necessary, even for the internetwork regions. This additional heating is strongly correlated with the observed magnetic field strength. Title: Solar magnetic elements at 0.1 arcsec resolution. General appearance and magnetic structure Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.; Title, A.; Scharmer, G. Bibcode: 2004A&A...428..613B Altcode: New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were obtained at the recently commissioned Swedish 1-m Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.1 arcsec) structures and find new forms of magnetic structures in this particular region. A cotemporal Ca II H-line image is used to examine the low-chromosphere of network elements. A cotemporal Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx cm-2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s-1. We find that magnetic flux in this region as seen in both the magnetogram and the G-band image is typically structured into larger, amorphous, ``ribbons'' which are not resolved into individual flux tubes. The measured magnetic flux density in the ribbon structures ranges from 300 to 1500 Mx cm-2, the higher values occurring at localized concentrations embedded within the ribbons. The Dopplergram indicates relative downflows associated with all magnetic elements with some indication that higher downflows occur adjacent to the peak magnetic flux location. The mean absolute magnetic flux density of the remnant plage network is about 130 Mx cm-2; in the lowest flux regions of the field-of-view, the mean absolute flux density is approximately 60 Mx cm-2. Within these quiet regions we do not find evidence of pervasive kilo-gauss strength magnetic elements as seen in recent high resolution internetwork studies. In general, the observations confirm recent 3-dimensional numerical simulations which show that the magnetic field in high-density regions such as plage is concentrated in complex structures that are not generally composed of discrete magnetic flux tubes.

Appendices are only available in electronic form at http://www.edpsciences.org Title: Mode Conversion in Magneto-Atmospheres Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Heggland, L.; Leer, E.; McMurry, A. D.; Stein, R. F. Bibcode: 2004AGUFMSH13A1162B Altcode: Numerical simulations of wave propagation in a simple magneto-atmosphere are employed to illustrate the complex nature of wave transformation and conversion taking place in solar and stellar atmospheres. An isothermal atmosphere threaded by a potential poloidal magnetic field, and a superposed uniform toroidal field, is treated in a local cartesian approximation. Spatial variations are restricted to the two poloidal dimensions, but the toroidal field ensures that all three MHD waves are present in the simulation. As in our previous purely two-dimensional simulations (Bogdan et al. ApJ 599, 626-60, 2003), mode mixing and transformation take place at surfaces where the magnetic and thermal pressures are equal. In the present case, the upward propagating acoustic-gravity (MAG) wave is converted into roughly equal parts transmitted fast, intermediate (Alfven), and slow magneto-acoustic-gravity waves in passing through this mixing layer. Unlike the fast and slow waves, the Alfven wave is weakly damped, and is able to deposit its energy and momentum in the upper chromosphere and corona. The fast and slow MAG waves are decoupled on either side of mixing layer owing to their disparate propagation speeds. Under certain fortuitous circumstances, the Alfven wave also decouples from the fast and slow MAG waves. Title: Observational Manifestations of Solar Magnetoconvection: Center-to-Limb Variation Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer, Göran B. Bibcode: 2004ApJ...610L.137C Altcode: 2004astro.ph..6160C We present the first center-to-limb G-band images synthesized from high-resolution simulations of solar magnetoconvection. Toward the limb the simulations show ``hilly'' granulation with dark bands on the far side, bright granulation walls, and striated faculae, similar to observations. At disk center G-band bright points are flanked by dark lanes. The increased brightness in magnetic elements is due to their lower density compared with the surrounding intergranular medium. One thus sees deeper layers where the temperature is higher. At a given geometric height, the magnetic elements are cooler than the surrounding medium. In the G band, the contrast is further increased by the destruction of CH in the low-density magnetic elements. The optical depth unity surface is very corrugated. Bright granules have their continuum optical depth unity 80 km above the mean surface, the magnetic elements 200-300 km below. The horizontal temperature gradient is especially large next to flux concentrations. When viewed at an angle, the deep magnetic elements' optical surface is hidden by the granules and the bright points are no longer visible, except where the ``magnetic valleys'' are aligned with the line of sight. Toward the limb, the low density in the strong magnetic elements causes unit line-of-sight optical depth to occur deeper in the granule walls behind than for rays not going through magnetic elements, and variations in the field strength produce a striated appearance in the bright granule walls. Title: A Comparison of the Outer Atmosphere of the ``Flat Activity'' Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V) Authors: Judge, Philip G.; Saar, Steven H.; Carlsson, Mats; Ayres, Thomas R. Bibcode: 2004ApJ...609..392J Altcode: Driven by the desire to understand the roles of acoustic and magnetic mechanisms in heating the outer atmospheres of Sun-like stars, we compare solar UV spectra with archival STIS spectra from the Hubble Space Telescope of α Cen A (G2 V) and new STIS spectra of the extremely inactive dwarf star τ Cet (G8 V, V=3.5). The activity of τ Cet shows occasional rotational modulations but no long-term cyclic variation. It may be in a phase analogous to the solar Maunder minimum. Solar disk center intensities from both the HRTS instrument and the SUMER instrument on SOHO were converted to Sun-as-a-star fluxes by using center-to-limb data from Dammasch and colleagues. The derived solar flux spectrum represents conditions near the minimum of the solar magnetic activity cycle. We find that the τ Cet line profiles differ systematically from those of the Sun and α Cen A. While lines formed in the middle chromospheres appear similar, the entire emission from the upper chromosphere to the middle transition region of τ Cet has lower flux densities by factors of ~2, the line widths are significantly narrower, and, uniquely, the transition region lines are not significantly redshifted. The soft X-ray surface flux of τ Cet, measured between 0.1 and 2.4 keV, is ~9×103 ergs cm-2 s-1, several times smaller than the median solar value. We also find that the UV spectrum of α Cen serves as a proxy for a Sun-as-a-star spectrum when the Sun is in an intermediate phase of its activity cycle but that its coronal emission may be somewhat smaller. We discuss the implications of these results for magnetic fields and heating mechanisms in the stars and speculate that τ Cet's UV spectrum might represent the solar spectrum during a grand minimum phase. Title: Solar Physics at Evergreen Authors: Zita, E. J.; Bogdan, T. J.; Carlsson, M.; Judge, P.; Heller, N.; Johnson, M.; Petty, S. Bibcode: 2004APS..NWS.C1005Z Altcode: We have recently established a solar physics research program at The Evergreen State College. Famed for its cloudy skies, the Pacific Northwest is an ideal location for solar physics research activities that do not require local observations. Collaborators from the High Altitude Observatory (HAO) at the National Center for Atmospheric Research have shared solar data from satellite-borne instruments such as TRACE and SUMER. HAO colleagues also share data from computer simulations of magneto-hydrodynamics (MHD) in the chromosphere, generated by the Institute for Theoretical Astrophysics (ITA) at the University of Oslo. Evergreen students and faculty learned to analyze data from satellites and simulations, in Boulder and Oslo, and established an infrastructure for continuing our analyses in Olympia. We are investigating the role of magnetic waves in heating the solar atmosphere. Comparing data from satellites and simulations shows that acoustic oscillations from the photosphere cannot effectively propagate into the chromosphere, but that magnetic waves can carry energy up toward the hot, thin corona. We find that acoustic waves can change into magnetic waves, especially near the magnetic "canopy," a region where the sound speed is comparable to magnetic wave speeds. Understanding MHD wave transformations and their role in energy transport can help answer outstanding questions about the anomalous heating of the solar atmosphere. Ref: Waves in the magnetized solar atmosphere II: Waves from localized sources in magnetic flux concentrations. Bogdan et al., 2003, ApJ 597 Title: Millimeter observations and chromospheric dynamics Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; Stein, R. F. Bibcode: 2004A&A...419..747L Altcode: The intensities of submillimeter and millimeter continua, which are formed in LTE and depend linearly on temperature, may be able to provide a test of models of the Solar chromosphere. We have taken a collection of submillimeter and millimeter wave observed brightness temperatures Tb of the quiet Sun from the literature and compared it with brightness temperatures computed from the standard static models of Fontenla, Avrett and Loeser (FAL) and the dynamic simulations of Carlsson & Stein (CS). The analysis of the dynamic simulations of Carlsson & Stein reveals that radio emission at millimeter wavelengths is extremely sensitive to dynamic processes in the chromosphere, if these are spatially and temporally resolved. The most striking result is that the dynamic picture of the solar internetwork chromosphere is consistent with currently available millimeter and submillimeter brightness observations. The spectrum obtained by averaging over the spectra from all time-steps of CS simulations provides a good fit to observed temporally and spatially averaged millimeter data in spite of the absence of a permanent temperature rise at low chromospheric heights in the simulations. This does not by itself rule out the presence of a chromospheric temperature rise as present in the FAL models, since a combination of such models also reproduces the (low resolution) data relatively well. Millimeter observations indicate that using radio techniques it is possible to extend observations of the solar oscillatory component to the heights above those previously observed in the photospheric and low chromospheric spectral lines and submillimeter continuum. For more precise diagnostics of chromospheric dynamics, high temporal and spatial resolution interferometric observations in the millimeter-wavelength region would be particularly useful.

Table \ref{tab:table} is only available in electronic form at http://www.edpsciences.org Title: G-band Images from MHD Convection Simulations Authors: Stein, R. F.; Carlsson, M.; Nordlund, A.; Scharmer, G. Bibcode: 2004AAS...204.8804S Altcode: 2004BAAS...36..820S High resolution magneto-convection simulations are used to calculate G-band and G-continuum images at various angles. Towards the limb the simulations show "hilly" granulation, bright granulation walls, intergranular striations and "sticking out" G-band bright features similar to observations. The increased brightness in magnetic elements is due to their lower density compared with the surrounding intergranular medium, so that one sees deeper layers where the temperature is higher. At a given geometric height, the magnetic elements are not hotter than the surrounding medium. In the G-band, the contrast is further increased by the destruction of CH in the low density magnetic elements. The optical depth unity surface is very corrugated. Bright granules have their continuum optical depth unity 80 km above the mean surface, the magnetic elements 200-300 km below. At large angles, the deep lying magnetic elements are hidden by the granules and the bright points are no longer visible. Where the "magnetic valleys" are aligned with the line of sight, they are visible as elongated structures seemingly "sticking out". Even when the deep hot surface is hidden, the low density in the strong magnetic elements causes unit line-of-sight optical depth to occur deeper in the granule walls behind then for rays not going through magnetic elements. Flux concentrations in intergranular lanes therefore cause a striped intensity pattern. This work is funded by NSF grants AST 0205500 and ATM 99881112 and NASA grants NAG 5 12450 and NNGO4GB92G. Title: Observations of solar magnetic elements with 0.1" resolution Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl, M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.; Title, A. M.; Scharmer, G. Bibcode: 2004AAS...204.2005B Altcode: 2004BAAS...36..686B New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were taken with the Swedish 1-meter Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.''1) structures and find new forms of magnetic structures in this particular region. A simultaneous Ca II H-line image is used to examine the low-chromosphere of network elements. A simultaneous Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.''18) FWHM with a flux sensitivity of approximately 130 Mx cm-2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s-1. Magnetic flux in this region as seen in both the magnetogram and the G-band image is typically structured into larger, amorphous, ``ribbons'' with a wide range of flux density values, rather than isolated kilogauss flux tubes. We also present filtergrams and magnetograms of magnetic elements at the solar limb showing that solar faculae are resolved into bright granular walls that appear to project 350 to 500 km above the photosphere. Title: Detectability of High Frequency Acoustic Waves with TRACE Authors: Fossum, A.; Carlsson, M. Bibcode: 2004ESASP.547..125F Altcode: 2004soho...13..125F High frequency acoustic waves have been proposed as a mechanism to heat the Solar chromosphere in internetwork regions. Such waves are difficult to detect using ground based observations because of seeing. Space based Solar observatories like SOHO and TRACE are not hampered by such high frequency disturbances caused by the Earth's atmosphere. We have used detailed NLTE radiation hydrodynamic simulations to investigate the detectability of high frequency acoustic waves with TRACE. A broad spectrum of acoustic waves are fed into the computational domain at the lower boundary of the model atmosphere and TRACE UV continuum intensities are calculated by folding the derived intensities with the TRACE filter functions for the 1600 and 1700 filters. Power spectra, phase diagrams and intensity response functions are calculated and intensity formation heights are derived. The simulations show that the width of the TRACE intensity response functions sets an upper frequency limit of 40 mHz for the detection of high frequency waves even in the absence of instrumental noise. Furthermore, Poisson noise for typical TRACE exposure times in a 30 minute time-series limits the detectability to below 25 mHz. Title: Observations at 0{. ^{primeprime}}1 Resolution of the Dynamic Evolution of Magnetic Elements Authors: Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo Bibcode: 2004IAUS..223..207C Altcode: 2005IAUS..223..207C We present observations of the dynamic evolution of photospheric magnetic elements in the G-band, magnetograms and Dopplergrams. The observations were obtained with the Swedish 1m Solar Telescope on La Palma at close to the diffraction limit of 0{. ^{primeprime}}1. In the most quiet regions we observe individual bright points in the G-band with corresponding magnetic signal in the magnetograms. Where the filling factor of the magnetic field is larger, the bright points interact when advected by the granular and super-granular flow-fields, flux sheets form and fragment. The plage region of the decaying active region is filled with more complex topologies like ribbon structures with darker interior and bright, knotted edges. These change into flower-like shape when small in extent and into micro-pores when the flux region is larger in extent. The magnetic elements in the plage region are associated with upflows with strong downflows in the immediate vicinity in the low-field region. Title: Radiative hydrodynamic simulations of acoustic waves in sunspots Authors: Bard, S.; Carlsson, M. Bibcode: 2004IAUS..223..445B Altcode: 2005IAUS..223..445B We describe fully consistent NLTE radiation hydrodynamic simulations of the propagation of acoustic waves in sunspot umbrae. In contrast to the case of the quiet internetwork Sun, we find that the observed behavior of the resonance lines of singly ionized calcium (H and K lines) cannot be explained without adding non-acoustic heating. The best agreement with observations is obtained with an extra heating term as a function of height that is constant per volume. Title: High resolution limb images synthesized from 3D MHD simulations Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer, Göran B. Bibcode: 2004IAUS..223..233C Altcode: 2005IAUS..223..233C We present the first center-to-limb G-band images synthesized from high resolution simulations of solar magneto-convection. Towards the limb the simulations show "hilly" granulation with dark bands on the far side, bright granulation walls and striated faculae, similar to observations. At disk center G-band bright points are flanked by dark lanes. The increased brightness in magnetic elements is due to their lower density compared with the surrounding intergranular medium. One thus sees deeper layers where the temperature is higher. At a given geometric height, the magnetic elements are cooler than the surrounding medium. In the G-band, the contrast is further increased by the destruction of CH in the low density magnetic elements. The optical depth unity surface is very corrugated. Bright granules have their continuum optical depth unity 80 km above the mean surface, the magnetic elements 200-300 km below. The horizontal temperature gradient is especially large next to flux concentrations. When viewed at an angle, the deep magnetic elements optical surface is hidden by the granules and the bright points are no longer visible, except where the "magnetic valleys" are aligned with the line of sight. Towards the limb, the low density in the strong magnetic elements causes unit line-of-sight optical depth to occur deeper in the granule walls behind than for rays not going through magnetic elements and variations in the field strength produce a striated appearance in the bright granule walls. Title: Theory and Simulations of Solar Atmosphere Dynamics Authors: Stein, R. F.; Bogdan, T. J.; Carlsson, M.; Hansteen, V.; McMurry, A.; Rosenthal, C. S.; Nordlund, Å. Bibcode: 2004ESASP.547...93S Altcode: 2004soho...13...93S Numerical simulations are used to study the generation and propagation of waves in the solar atmosphere. Solar p-mode oscillations are excited by turbulent pressure work and entropy fluctuations (non-adiabatic gas pressure work) near the solar surface. Interactions between short and long period waves and radiative energy transfer control the formation of shocks. The magnetic structure of the atmosphere induces coupling among various MHD wave modes, with intense coupling and wave transformation at the beta equal one surface, which likely is the location of the so-called "magnetic canopy". Title: Waves in the Magnetized Solar Atmosphere. II. Waves from Localized Sources in Magnetic Flux Concentrations Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V. H.; McMurry, A.; Rosenthal, C. S.; Johnson, M.; Petty-Powell, S.; Zita, E. J.; Stein, R. F.; McIntosh, S. W.; Nordlund, Å. Bibcode: 2003ApJ...599..626B Altcode: Numerical simulations of wave propagation in a two-dimensional stratified magneto-atmosphere are presented for conditions that are representative of the solar photosphere and chromosphere. Both the emergent magnetic flux and the extent of the wave source are spatially localized at the lower photospheric boundary of the simulation. The calculations show that the coupling between the fast and slow magneto-acoustic-gravity (MAG) waves is confined to thin quasi-one-dimensional atmospheric layers where the sound speed and the Alfvén velocity are comparable in magnitude. Away from this wave conversion zone, which we call the magnetic canopy, the two MAG waves are effectively decoupled because either the magnetic pressure (B2/8π) or the plasma pressure (p=NkBT) dominates over the other. The character of the fluctuations observed in the magneto-atmosphere depend sensitively on the relative location and orientation of the magnetic canopy with respect to the wave source and the observation point. Several distinct wave trains may converge on and simultaneously pass through a given location. Their coherent superposition presents a bewildering variety of Doppler and intensity time series because (1) some waves come directly from the source while others emerge from the magnetic canopy following mode conversion, (2) the propagation directions of the individual wave trains are neither co-aligned with each other nor with the observer's line of sight, and (3) the wave trains may be either fast or slow MAG waves that exhibit different characteristics depending on whether they are observed in high-β or low-β plasmas (β≡8πp/B2). Through the analysis of four numerical experiments a coherent and physically intuitive picture emerges of how fast and slow MAG waves interact within two-dimensional magneto-atmospheres. Title: MHD Waves in Magnetic Flux Concentrations Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Zita, E. J.; Stein, R. F.; McIntosh, S. W. Bibcode: 2003AGUFMSH42B0535B Altcode: Results from 2D MHD simulations of waves in a stratified isothermal atmosphere will be presented and analyzed. The waves are generated by a localized piston source situated on the lower, photospheric, boundary of the computational domain. A combination of fast and slow magneto-atmospheric waves propagates with little mutual interaction until they encounter the surface where the sound speed and the Alfven speed are comparable in magnitude. The waves couple strongly in this region and emerge with different amplitudes and phases. Owing to this mode mixing and the large variation in the Alfven speed in the magneto-atmosphere, the fluctuations observed at a given location are often a superposition of both fast and slow waves which have traversed different paths and have undergone different transformations during their journies. Title: On the Origin of the Basal Emission from Stellar Atmospheres: Analysis of Solar C II Lines Authors: Judge, Philip G.; Carlsson, Mats; Stein, Robert F. Bibcode: 2003ApJ...597.1158J Altcode: Combining a variety of data with radiation hydrodynamic simulations, we examine the heating of the Sun's internetwork chromosphere and the hypothesis that the chromospheric ``basal'' emission arises because of acoustic wave dissipation. We focus on the 2s2p22D-2s22p2Po multiplet of C II near 1335 Å, whose basal level of chromospheric emission has been reliably determined for stars and the Sun by Schrijver and colleagues. By accounting for center-to-limb variations and the different spectral bandpasses of the instruments used, we find that Schrijver's C II solar basal intensity substantially exceeds stellar values, and that it can be identified with intensities seen in typical internetwork regions with the SUMER instrument on the SOHO spacecraft. Some time-series data sets of internetwork regions are then examined and compared with simulations made specifically for a typical observational data set, with vertical velocities at the lower boundaries fixed from observations with the MDI instrument on SOHO. The simulations can qualitatively account for the observed internetwork UV continuum fluctuations seen with SUMER, formed 0.6-0.85 Mm above the photosphere. However, they fail to capture almost any property of the observed internetwork C II multiplet, which is formed substantially higher. The time-averaged simulations can account for between 1/7 and 1/4 of the C II basal intensities; they predict oscillatory power between 5 and 10 mHz, whereas internetwork observations are dominated by low-frequency (<2 mHz) power of solar origin. The average simulated C II intensities, which have a large contribution from the transition region heated by conduction down from a coronal upper boundary, fall short even of the smaller stellar basal intensities by a factor of >=2. Together with known properties of weak, internetwork photospheric magnetic fields, we conclude that the internetwork upper chromosphere is probably dominated by magnetic heating. Thus, the solar basal (and internetwork) intensities of the C II 1335 Å multiplet originate from magnetic, and not acoustic, mechanisms, in contradiction to the commonly accepted picture Title: Dynamic Modelling of the Outer Atmosphere of α Tau Authors: McMurry, A. D.; Carlsson, M.; Stein, R. F. Bibcode: 2003csss...12..323M Altcode: Using one-dimensional radiation-hydrodynamics simulations a model of the outer atmosphere of α Tau is created. The reaction of the model to acoustic waves is explored. It is found that high frequency waves are radiatively damped out in the photosphere. The lower frequency waves above the Hydrodynamic acoustic cutoff frequency do produce some chromospheric heating. Title: Nonequilibrium CO Chemistry in the Solar Atmosphere Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Carlsson, M.; Cernicharo, J. Bibcode: 2003ApJ...588L..61A Altcode: 2003astro.ph..3460R; 2003astro.ph..3460A Investigating the reliability of the assumption of instantaneous chemical equilibrium (ICE) for calculating the CO number density in the solar atmosphere is of crucial importance for the resolution of the long-standing controversy over the existence of ``cool clouds'' in the chromosphere and for determining whether the cool gas owes its existence to CO radiative cooling or to a hydrodynamical process. Here we report the first results of such an investigation in which we have carried out time-dependent gas-phase chemistry calculations in radiation hydrodynamical simulations of solar chromospheric dynamics. We show that while the ICE approximation turns out to be suitable for modeling the observed infrared CO lines at the solar disk center, it may substantially overestimate the ``heights of formation'' of strong CO lines synthesized close to the edge of the solar disk, especially concerning vigorous dynamic cases resulting from relatively strong photospheric disturbances. This happens because during the cool phases of the hydrodynamical simulations, the CO number density in the outer atmospheric regions is smaller than what is stipulated by the ICE approximation, resulting in decreased CO opacity in the solar chromosphere. As a result, the cool CO-bearing gas that produces the observed molecular lines must be located at atmospheric heights not greater than ~700 km. We conclude that taking into account the nonequilibrium chemistry improves the agreement with the available on-disk and off-limb observations but that the hydrodynamical simulation model has to be even cooler than anticipated by the ICE approximation, and this has to be the case at the ``new'' (i.e., deeper) formation regions of the rovibrational CO lines. Title: Multi-level 3D non-LTE computations of lithium lines in the metal-poor halo stars HD 140283 and HD 84937 Authors: Asplund, M.; Carlsson, M.; Botnen, A. V. Bibcode: 2003A&A...399L..31A Altcode: 2003astro.ph..2406A The lithium abundances in metal-poor halo stars are of importance for cosmology, galaxy evolution and stellar structure. In an attempt to study possible systematic errors in the derived Li abundances, the line formation of Li I lines has been investigated by means of realistic 3D hydrodynamical model atmospheres of halo stars and 3D non-LTE radiative transfer calculations. These are the first detailed 3D non-LTE computations reported employing a multi-level atomic model showing that such problems are now computationally tractable. The detailed computations reveal that the Li I population has a strong influence from the radiation field rather than the local gas temperature, indicating that the low derived Li abundances found by Asplund et al. (\cite{Asplund1999}) are an artifact of their assumption of LTE. Relative to 3D LTE, the detailed calculations show pronounced over-ionization. In terms of abundances the 3D non-LTE values are within 0.05 dex of the 1D non-LTE results for the particular cases of HD 140283 and HD 84937, which is a consequence of the dominance of the radiation in determining the population density of Li I. Although 3D non-LTE can be expected to give results rather close (~+/- 0.1 dex) to 1D non-LTE for this reason, there may be systematic trends with metallicity and effective temperature. Title: 3D NLTE Radiative Transfer - Current Status and Future Prospects Authors: Carlsson, M. Bibcode: 2003IAUS..210...51C Altcode: No abstract at ADS Title: Towards 3D NLTE Radiation Magneto-Hydrodynamics Authors: Carlsson, M.; Stein, R. F. Bibcode: 2003ASPC..288..505C Altcode: 2003sam..conf..505C The problem of 3D Radiation Magneto-Hydrodynamics is too complex to solve numerically in the general case; approximations are needed to bring the numerical complexity to tractable levels. These approximations are problem dependent. We will use the case of the Solar chromosphere to illustrate these issues. The implementation of a 1D Radiation Hydrodynamics code with a rather detailed and realistic treatment of the coupling between radiation and matter is described. Scaling properties and parallelization issues are discussed. Various strategies and on-going work for the implementation of a 3D Radiation Magneto-Hydrodynamics code are described. Title: Modelling Acoustic Shocks in Outer Atmospheres of Cool Giant Stars Authors: McMurry, A. D.; Carlsson, M.; Stein, R. F. Bibcode: 2003IAUS..210P..B7M Altcode: No abstract at ADS Title: Wave processes in the solar upper atmosphere Authors: Carlsson, Mats; Stein, Robert F. Bibcode: 2002ESASP.505..293C Altcode: 2002IAUCo.188..293C; 2002solm.conf..293C The existence of a wide variety of wave-like phenomena are inferred from observations of the solar upper atmosphere. Acoustic waves play an important role for the dynamics and energetics of the chromosphere but additional heating seems necessary even for the internetwork regions. We show that it is unlikely that this extra heating is due to high frequency acoustic waves due to the lack of their preferential excitation and the strong radiative damping of high frequency waves in the photosphere. When acoustic waves reach the height where the magnetic pressure equals the gas pressure they will undergo mode conversion, refraction and reflection. We discuss these processes and show that the critical quantity is the angle between the magnetic field and the velocity polarization; at angles smaller than 30 degrees much of the acoustic, fast mode from the photosphere is transmitted as an acoustic, slow mode propagating along the field lines. At larger angles, most of the energy is refracted/reflected and returns as a fast mode creating an interference pattern between the upward and downward propagating waves. In 3D, this interference between waves at small angles creates patterns with large horizontal phase speeds, especially close to magnetic field concentrations. When damping from shock dissipation and radiation is taken into account, the waves in the low-mid chromosphere have mostly the character of upward propagating acoustic waves and it is only close to the reflecting layer we get similar amplitudes for the upward propagating and refracted/reflected waves. Title: Waves in magnetic flux concentrations: The critical role of mode mixing and interference Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; Hansteen, V.; McMurry, A.; Zita, E. J.; Johnson, M.; Petty-Powell, S.; McIntosh, S. W.; Nordlund, Å.; Stein, R. F.; Dorch, S. B. F. Bibcode: 2002AN....323..196B Altcode: Time-dependent numerical simulations of nonlinear wave propagation in a two-dimensional (slab) magnetic field geometry show wave mixing and interference to be important aspects of oscillatory phenomena in starspots and sunspots. Discrete sources located within the umbra generate both fast and slow MHD waves. The latter are compressive acoustic waves which are guided along the magnetic field lines and steepen into N-waves with increasing height in the spot atmosphere. The former are less compressive, and accelerate rapidly upward through the overlying low-beta portion of the umbral photosphere and chromosphere (beta equiv 8pi p/ B2). As the fast wave fronts impinge upon the beta ~ 1 penumbral ``magnetic canopy" from above, they interfere with the outward-propagating field-guided slow waves, and they also mode convert to (non-magnetic) acoustic-gravity waves as they penetrate into the weak magnetic field region which lies between the penumbral canopy and the base of the surrounding photosphere. In a three-dimensional situation, one expects additional generation, mixing and interference with the remaining torsional Alfvén waves. Title: Dynamics in the solar chromosphere as a function of the magnetic field topology Authors: Karlsen, N.; Carlsson, M. Bibcode: 2002ESASP.508..303K Altcode: 2002soho...11..303K We have looked at the coupling between the magnetic field and chromospheric dynamics. Observations with the SUMER spectrograph of the continuum radiation at 1319 Å have been correlated with simultaneous MDI magnetograms and dopplergrams in high resolution mode. We have used 7 different observing runs for our analysis, all from 1996. The absolute value of the magnetic field crossing the SUMER slit lies in the range 0-100 gauss. We observe a correlation between continuum intensity and magnetic field strength all the way to the sensitivity limit of MDI (about 2 G as 3σ in the mean value). Relative intensity fluctuations at frequencies corresponding to propagating acoustic waves (>4.5 mHz) have smaller amplitudes with increasing radiation temperature (or magnetic field strength). The absolute intensity fluctuations show an increase with increasing radiation temperature. These findings are consistent with a picture where a basic intensity level is set by a magnetic heating process even in the darkest internetwork areas with superimposed intensity variations caused by acoustic waves. Title: Dynamic Hydrogen Ionization Authors: Carlsson, Mats; Stein, R. F. Bibcode: 2002ApJ...572..626C Altcode: 2002astro.ph..2313C We investigate the ionization of hydrogen in a dynamic solar atmosphere. The simulations include a detailed non-LTE treatment of hydrogen, calcium, and helium but lack other important elements. Furthermore, the omission of magnetic fields and the one-dimensional approach make the modeling unrealistic in the upper chromosphere and higher. We discuss these limitations and show that the main results remain valid for any reasonable chromospheric conditions. As in the static case, we find that the ionization of hydrogen in the chromosphere is dominated by collisional excitation in the Lyα transition followed by photoionization by Balmer continuum photons-the Lyman continuum does not play any significant role. In the transition region, collisional ionization from the ground state becomes the primary process. We show that the timescale for ionization/recombination can be estimated from the eigenvalues of a modified rate matrix where the optically thick Lyman transitions that are in detailed balance have been excluded. We find that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state. Throughout the chromosphere the timescale is long (103-105 s), except in shocks where the increased temperature and density shorten the timescale for ionization/recombination, especially in the upper chromosphere. Because the relaxation timescale is much longer than dynamic timescales, hydrogen ionization does not have time to reach its equilibrium value and its fluctuations are much smaller than the variation of its statistical equilibrium value appropriate for the instantaneous conditions. Because the ionization and recombination rates increase with increasing temperature and density, ionization in shocks is more rapid than recombination behind them. Therefore, the ionization state tends to represent the higher temperature of the shocks, and the mean electron density is up to a factor of 6 higher than the electron density calculated in statistical equilibrium from the mean atmosphere. The simulations show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there. Title: Dynamics and energetics of the solar chromosphere Authors: Carlsson, Mats; Stein, Robert F. Bibcode: 2002ESASP.508..245C Altcode: 2002soho...11..245C We present a summary of results from a number of observational programs carried out with the SUMER instrument on board SOHO. Most datasets show clear quasi-periodic dynamic behavior ("grains") in contiunuum intensities with frequencies 3-10 mHz. Corresponding grains are seen in intensities and velocities in neutral lines, normally with phase differences consistent with upward propagating sound-waves. We compare the observations with 1D radiation hydrodynamic simulations using MDI Doppler-shifts to set the lower boundary. For continua formed in the mid-chromosphere we find that the simulations give a good match to the intensity fluctuations but that the minimum intensity is too low. We find that high frequency acoustic waves (missing from the current simulations) are unlikely to give the extra heating necessary because of the strong radiative damping (90-99%) of such waves in the photosphere. In continua formed in the low chromosphere the mean intensity is similar in the simulations and the observations but the simulated fluctuations are too large. The reported findings are consistent with a picture where a basic intensity level is set by a magnetic heating process even in the darkest internetwork areas with superimposed intensity variations caused by acoustic waves. Title: Waves in the Magnetized Solar Atmosphere. I. Basic Processes and Internetwork Oscillations Authors: Rosenthal, C. S.; Bogdan, T. J.; Carlsson, M.; Dorch, S. B. F.; Hansteen, V.; McIntosh, S. W.; McMurry, A.; Nordlund, Å.; Stein, R. F. Bibcode: 2002ApJ...564..508R Altcode: We have modeled numerically the propagation of waves through magnetic structures in a stratified atmosphere. We first simulate the propagation of waves through a number of simple, exemplary field geometries in order to obtain a better insight into the effect of differing field structures on the wave speeds, amplitudes, polarizations, direction of propagation, etc., with a view to understanding the wide variety of wavelike and oscillatory processes observed in the solar atmosphere. As a particular example, we then apply the method to oscillations in the chromospheric network and internetwork. We find that in regions where the field is significantly inclined to the vertical, refraction by the rapidly increasing phase speed of the fast modes results in total internal reflection of the waves at a surface whose altitude is highly variable. We conjecture a relationship between this phenomenon and the observed spatiotemporal intermittancy of the oscillations. By contrast, in regions where the field is close to vertical, the waves continue to propagate upward, channeled along the field lines but otherwise largely unaffected by the field. Title: Men-Invasion of Space Authors: Carlsson, M. E. Bibcode: 2002iaf..confE.649C Altcode: Last year I was given the fantastic opportunity by ESA to go to the IAF congress in Toulouse. This was one of the most exciting experiences of my life. I loved every minute of it, listening to the brilliant men from different areas of space research, telling us wonderfully interesting things about space. After a couple of sessions I understood there were very few women standing up there and giving presentations. Except of course, the Russian women translators and the students that were invited. This struck me and I started to think about it and wondered why? This encouraged me to investigate. I realized that I had to turn to the children, the ones that are going to take over after us, our new faces in space. When we were children we all wanted to become astronauts, both boys and girls. But after a couple of years it changed, why? So I started my research by asking questions to the children in different ages concerning their lives and upbringing. I was curious to find out when and how their personal goals changed in life, compared to the men. Could it depend on what our parents did when we were growing up or maybe where we were raised geographically? Perhaps the encouragement of our teachers in our first years in school or our social background, genes, manners and customs. When we have the answers we also must ask ourselves how we can make women choose a more engineering and scientific profile. Should we try to convert them to engineers or should we use their other special abilities and try to integrate them into the space program for example as medical doctors? I think that ESA and other space organizations should use their university students in a new outreach project. The students should visit their local schools and inform the children about space and the wonderful opportunities it can give us. This would give the boys, but mostly the girls, a chance to discover space. Title: Wave Propagation in a Magnetized Atmosphere Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; McIntosh, S.; Dorch, S.; Hansteen, V.; McMurry, A.; Nordlund, Å; Stein, R. F. Bibcode: 2001AGUSM..SH41A01B Altcode: Numerical simulations of MHD wave propagation in plane-parallel atmospheres threaded by non-trivial potential magnetic fields will be presented, and their implications for understanding distinctions between intranetwork and internetwork oscillations will be discussed. Our findings basically confirm the conjecture of McIntosh et al. (2001, ApJ 548, L237), that the two-dimensional surface where the Alfvén and sound speeds coincide (i.e., where the plasma-β , the ratio of gas to magnetic pressure, is of order unity) plays a fundamental role in mediating the conversion between the fast-, intermediate- (Alfvén), and slow-Magneto-Atmospheric-Gravity (MAG) waves. For example, upward-propagating acoustic waves generated at the base of the internetwork photosphere suffer significant downward reflection when they encounter this β ≈ 1 surface. Close to the network, this surface descends from the upper chromosphere and low corona (which pertains in the internetwork cell interiors) down into the photosphere, and so chromospheric oscillation `shadows' are predicted to surround the network. In the network, strong vertical magnetic fields further depress the β ≈ 1 surface below the surface layers where the (magnetic field-aligned) acoustic waves (i.e., slow MAG-waves) are generated. For frequencies in excess of the cutoff frequency, these acoustic waves suffer little reflection from the overlying atmosphere and they steepen as they progress upward. Title: An Observational Manifestation of Magnetoatmospheric Waves in Internetwork Regions of the Chromosphere and Transition Region Authors: McIntosh, S. W.; Bogdan, T. J.; Cally, P. S.; Carlsson, M.; Hansteen, V. H.; Judge, P. G.; Lites, B. W.; Peter, H.; Rosenthal, C. S.; Tarbell, T. D. Bibcode: 2001ApJ...548L.237M Altcode: We discuss an observational signature of magnetoatmospheric waves in the chromosphere and transition region away from network magnetic fields. We demonstrate that when the observed quantity, line or continuum emission, is formed under high-β conditions, where β is the ratio of the plasma and magnetic pressures, we see fluctuations in intensity and line-of-sight (LOS) Doppler velocity consistent with the passage of the magnetoatmospheric waves. Conversely, if the observations form under low-β conditions, the intensity fluctuation is suppressed, but we retain the LOS Doppler velocity fluctuations. We speculate that mode conversion in the β~1 region is responsible for this change in the observational manifestation of the magnetoatmospheric waves. Title: Waves in the Magnetised Solar Atmosphere Authors: Rosenthal, C. S.; Carlsson, M.; Hansteen, V.; McMurry, A.; Bogdan, T. J.; McIntosh, S.; Nordlund, A.; Stein, R. F.; Dorch, S. B. F. Bibcode: 2001IAUS..203..170R Altcode: We have simulated the propagation of magneto-acoustic disturbances through various magneto-hydrostatic structures constructed to mimic the solar magnetic field. As waves propagate from regions of strong to weak magnetic field and vice-versa different types of wave modes (transverse and longitudinal) are coupled. In closed-field geometries we see the trapping of wave energy within loop-like structures. In open-field regions we see wave energy preferentially focussed away from strong-field regions. We discuss these oscillations in terms of various wave processes seen on the Sun - umbral oscillations, penumbral running waves, internetwork oscillations etc. Title: Solar Chromospheric Oscillations Authors: Carlsson, M. Bibcode: 2000eaa..bookE2261C Altcode: By the term solar chromospheric oscillations we here refer to any periodic or quasi-periodic dynamic phenomena seen in the solar CHROMOSPHERE. We thus include both propagating and standing waves.... Title: Rapid intensity and velocity variations in solar transition region lines Authors: Hansteen, V. H.; Betta, R.; Carlsson, M. Bibcode: 2000A&A...360..742H Altcode: We have obtained short exposure (3 s) time series of strong upper chromospheric and transition region emission lines from the quiet Sun with the SUMER instrument onboard SOHO during two 1 hour periods in 1996. With a Nyqvist frequency of 167 mHz and relatively high count rates the dataset is uniquely suited for searching for high frequency variations in intensity and Doppler velocity. From Monte-Carlo experiments taking into account the photon-counting statistics we estimate our detection limit to correspond to a wave-packet of four periods coherent over 3'' with a Doppler-shift amplitude of 2.5km s-1 in the darkest internetwork areas observed in C III. In the network the detection limit is estimated to be 1.5km s-1. Above 50 mHz we detect wave-packet amplitudes above 3km s-1 less than 0.5% of the time. Between 20 and 50 mHz we detect some wave-packets with a typical duration of four periods and amplitudes up to 8km s-1. At any given internetwork location these wave-packets are present 1% of the time. In the 10-20 mHz range we see amplitudes above 3km s-1 12% of the time. At lower frequencies our dataset is consistent with other SUMER datasets reported in the literature. The chromospheric 3-7 mHz signal is discernible in the line emission. In the internetwork this is the dominant oscillation frequency but higher frequencies (7-10 mHz) are often present and appear coherent in Doppler velocity over large spatial regions (≍ 40"). Wavelet analysis implies that these oscillations have typical durations of 1000s. The network emission also shows a 5 mHz signal but is dominated by low frequency variations (of < 4 mHz) in both intensity and velocity. The oscillations show less power in intensity than in velocity. We find that while both red and blue shifted emission is observed, the transition region lines are on average red shifted between 5-10km s-1 in the network. A net red shift is also found in the internetwork emission but it is smaller (< 4km s-1). The line widths do not differ much between the internetwork and network, the non-thermal line widths increase with increasing temperature of line formation from 30km s-1 for the C II 1334 Å line to 45km s-1 for the O VI 1032 Å line. By constructing scatterplots of velocity versus intensity we find that in the network a mean redshift is correlated with a high mean intensity. In the internetwork regions we do not find any correlation between the intensity and the Doppler velocity. Title: The Dynamic Solar Chromosphere Authors: Carlsson, M.; Stein, R. F. Bibcode: 2000SPD....31.1102C Altcode: 2000BAAS...32..843C The natural state of the Solar chromosphere is dynamic. Any photospheric disturbance will grow and naturally form shocks over the twenty scale-heights in density between the photosphere and the corona. Observations in the resonance lines from singly ionized calcium and in the ultraviolet region of the spectrum observed with the Solar and Heliospheric Observatory satellite also show a dynamic chromosphere. The dynamic picture is further supported by numerical simulations. Static and dynamic pictures of the chromosphere are fundamentally different. The simulations also show that time variations are crucial for our understanding of the chromosphere itself and for the spectral features formed there. Title: Chromospheric and Transition Region Internetwork Oscillations: A Signature of Upward-propagating Waves Authors: Wikstøl, Ø.; Hansteen, V. H.; Carlsson, M.; Judge, P. G. Bibcode: 2000ApJ...531.1150W Altcode: We analyze spectral time series obtained on 1997 April 25 with the SUMER instrument on SOHO. Line and continuum data near 1037 Å were acquired at a cadence of 16 s. This spectral region was chosen because it contains strong emission lines of C II, formed in the upper chromosphere/lower transition region; O VI, formed in the upper transition region; and neighboring continuum emission formed in the middle chromosphere. The time series reveal oscillatory behavior. Subsonic (3-5 km s-1 amplitude) Doppler velocity oscillations in the C II and O VI lines, with periods between 120 and 200 s, are prominent. They are seen as large-scale coherent oscillations, typically of 3-7 Mm length scale, occasionally approaching 15 Mm, visible most clearly in internetwork regions. The Doppler velocity oscillations are related to oscillations seen in the continuum intensity, which precede upward velocity in C II by 40-60 s. The C II Doppler shift precedes the O VI Doppler shift by 3-10 s. Oscillations are also present in the line intensities, but the intensity amplitudes associated with the oscillations are small. The continuum intensity precedes the C II intensity by 30-50 s. Phase difference analysis shows that there is a preponderance of upward-propagating waves in the upper chromosphere that drive an oscillation in the transition region plasma, thus extending the evidence for upward-propagating waves from the photosphere up to the base of the corona. Title: Oxygen Line Formation in 3D Hydrodynamical Model Atmospheres Authors: Asplund, M.; Carlsson, M.; Garcia Perez, A. E.; Kiselman, D. Bibcode: 2000IAUJD...8E...8A Altcode: 2000astro.ph.11043A The new generation of realistic 3-dimensional, time-dependent, hydrodynamical model atmospheres have been applied to the line formation of {O}{I}, [{O}{I}] and OH lines. Additionally detailed 3D non-LTE calculations have been performed for {O}{I} in order to study the influence of temperature inhomogeneities on the line formation process. Implications in terms of the evolution of oxygen abundance with metallicity will be discussed, partly based on new VLT/UVES observations of metal-poor stars. Title: Chromospheric Dynamics as seen by SUMER Authors: Carlsson, M. Bibcode: 1999ESASP.448..183C Altcode: 1999ESPM....9..183C; 1999mfsp.conf..183C No abstract at ADS Title: Transition Region Oscillations Authors: Hansteen, V. H.; Wisktol, O.; Carlsson, M.; Judge, P. G. Bibcode: 1999ESASP.446..351H Altcode: 1999soho....8..351H We analyze time series data obtained April 25th 1997 with the SUMER instrument on SOHO. Line and continuum data near 1037 were acquired at a cadence of 16s. This spectral region was chosen because it contains strong emission lines of C II, formed in the upper chromosphere/lower transition region, O VI, formed in the upper transition region, as well as neighboring continuum emission formed in the middle chromosphere. The dataset reveals oscillatory behavior containing valuable information on the physical structure of the chromosphere and transition region. Prominent in the data are subsonic (3-5 km/s amplitude) velocity oscillations with periods between 120 and 200 sec. They are seen as large scale coherent oscillations, typically of 3--7 Mm length scale but sometimes approaching 15Mm, visible most clearly in internetwork regions. The oscillations are present in C II and O VI velocities, as well as in the continuum intensity. The continuum intensity precedes upward velocity in the C II line by 40-60s and the C III velocity precedes the O VI velocity by 3-10s. The oscillations are also present in the intensities of the two lines, but the intensity amplitudes associated with the oscillations are small. We find that the continuum intensity precedes the C II intensity by 30-50s. These phase shifts indicate that there are upward propagating waves in the upper chromosphere that drive an oscillation in the transition region plasma. The oscillations seem to be present in most internetwork areas at any time, thus they are the dominant resolvable dynamical feature of the internetwork chromosphere and transition region. Title: Wave Modes in a Chromospheric Cavity Authors: Carlsson, M.; Stein, R. S. Bibcode: 1999ASPC..184..206C Altcode: Observations show enhanced power near 5.3 mHz and 7 mHz and phase jumps near 7 mHz. Interpretations have varied from standing waves, resonant wakes, shock merging and source interference. We investigate these processes using a non-LTE radiation hydrodynamics code. It is shown that the enhanced power at certain frequencies is caused by partial reflections from steep temperature gradients in the photosphere and the transition region. Due to a phase-speed that is very dependent on frequency close to the acoustic cut-off frequency, the lowest mode frequencies are rather insensitive to the location of the upper reflecting layer. Temperature jumps across shocks introduce additional reflection and modify the mode structure. A corrugated and moving transition region washes out the higher frequency modes but does not affect the lowest modes much. The modal structure is superimposed on a wave field that is dominated by propagating waves in the lower chromosphere. Title: The dynamic solar chromosphere and the ionization of hydrogen Authors: Carlsson, Mats; Stein, Robert F. Bibcode: 1999AIPC..471...23C Altcode: 1999sowi.conf...23C Basic physical considerations, observations and numerical simulations show that the solar chromosphere is very dynamic. The enhanced chromospheric emission, which corresponds to an outwardly increasing semiempirical temperature structure, can even be produced by wave motion without any increase in the mean gas temperature. Hence, the sun may not have a classical chromosphere in magnetic field free internetwork regions. This dynamic picture of the solar internetwork chromosphere is consistent with ground based observations of the Call resonance lines and with observations from the SOHO satellite. The simulations also show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there. Whether the dynamic nature of the chromosphere is important for solar wind models depends on their sensitivity to chromospheric conditions. Contrary to some claims in the literature, the ionization of hydrogen in the upper chromosphere is dominated by collisional excitation in the Lyman line followed by photo-ionization by Balmer continuum photons-the Lyman continuum does not play any significant role. In the transition region, collisional ionization takes over as the major process. Ionization/recombination time-scales can be on the order of hundreds of seconds causing the ionization balance in the chromosphere to be significantly out of equilibrium with higher ionization than the equilibrium value. The hydrogen ionization zone is also considerably thicker than is claimed from dimension analyses; the ionization fraction goes from 1% to 40% over a height range of 600 km. Title: Multi3D, 3D Non-LTE Radiative Transfer Authors: Botnen, A.; Carlsson, M. Bibcode: 1999ASSL..240..379B Altcode: 1999numa.conf..379B No abstract at ADS Title: The new chromosphere Authors: Carlsson, M.; Stein, R. F. Bibcode: 1998IAUS..185..435C Altcode: Numerical simulations have shown that enhanced chromospheric emission, which corresponds to an outwardly increasing semiempirical temperature structure, can be produced by wave motion without any increase in the mean gas temperture. Hence, the sun may not have a classical chromosphere in magnetic field free internetwork regions. This dynamic picture of the solar internetwork chromosphere is also consistent with ground based observations of the CaII resonance lines and of CO absorption lines and with observations from the SOHO satellite. The simulations also show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and for the spectral features formed there. Title: SUMER observations in transition region lines Authors: Betta, R.; Hansteen, V.; Carlsson, M.; Wilhelm, K. Bibcode: 1998MmSAI..69..699B Altcode: No abstract at ADS Title: Radiative Transfer and Radiation Hydrodynamics Authors: Carlsson, Mats Bibcode: 1998LNP...507..163C Altcode: 1998sspt.conf..163C Radiation plays an important role, firstly in determining the structure of stars through the dominant role radiation plays in the energy balance (in some objects also in the momentum balance), secondly because we diagnose astrophysical plasma through the emitted electromagnetic radiation. Title: SUMER Observations of the Quiet Solar Atmosphere: The Network Chromosphere and Lower Transition Region Authors: Judge, Philip; Carlsson, Mats; Wilhelm, Klaus Bibcode: 1997ApJ...490L.195J Altcode: We examine spectral properties of the network chromosphere and lower transition region from the SUMER instrument on the SOHO spacecraft, using time-series data sets discussed in an accompanying Letter by Carlsson, Judge, & Wilhelm. The data were obtained early in the mission with no tracking of solar features and so cannot generally be used to examine intrinsic variations in features on timescales in excess of 383 s. Upon examination of the temporal variations and some preliminary power spectrum analysis, we find the following: (1) Transition region lines show more redshift in network regions than in internetwork regions and also a correlation between line intensity brightenings and increased redshift. (2) The internetwork ``Ca II grain'' phenomenon is not seen in He I λ584 or in lines of Si III and C III. (3) Very rapid changes are seen in the network for transition region lines with no obvious correspondence with the underlying chromosphere. (4) He I λ584 line profiles show very slow time variations. (5) Small-amplitude (2-5 km s-1) coherent oscillations of 5"-10" scale length and ~130 s period are seen in Doppler shifts of Si III between regions of bright network elements. (6) Essentially all blueshifts or redshifts are substantially less than line widths. We conclude that upward-propagating acoustic shock waves do not contribute significantly to the heating of the lower transition region, and that ionization equilibrium is likely to fail for the interpretation of certain emission lines. The spatial coherency of the Si III velocity oscillations indicates that the quiet Sun's magnetic field topology is more uniform than emission-line intensity data alone might suggest. Title: Dynamic Behavior of the Solar Atmosphere Authors: Stein, R. F.; Carlsson, M. Bibcode: 1997ASSL..225..261S Altcode: 1997scor.proc..261S We have studied the dynamics of acoustic and MHD waves in the solar atmosphere using a one-dimensional, non-LTE, radiation magneto-hydrodynamic code, with 6 level model atoms for hydrogen and singly ionized calcium. We drive waves by a piston through an initial atmosphere in radiative equilibrium. We report on the effects of radiative energy loss on the waves, the effects of shocks on line formation, and the behavior of typical diagnostics in a dynamic atmosphere. Title: SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Internetwork Chromosphere Authors: Carlsson, Mats; Judge, P. G.; Wilhelm, K. Bibcode: 1997ApJ...486L..63C Altcode: 1997astro.ph..6226C On 1996 March 12, during the commissioning phase of the SOHO mission, we obtained observations of the quiet-Sun with the SUMER instrument. The observations were sequences of 15-20 s exposures of ultraviolet emission-line profiles and of the neighboring continua. These data contain signatures of the dynamics of the solar chromosphere that are uniquely useful because of wavelength coverage, moderate signal-to-noise ratios, and image stability.

We focus on data for the internetwork chromosphere. The dominant observed phenomenon is an oscillatory behavior that is analogous to the 3 minute oscillations seen in CaII lines. The oscillations appear to be coherent over 3"-8" diameter areas. At any time they occur over about 50% of the area studied, and they appear as large perturbations in the intensities of lines and continua. The oscillations are most clearly seen in intensity variations in the ultraviolet (λ > 912 Å) continua, and they are also seen in the intensities and velocities of chromospheric lines of CI, NI, and OI. Intensity brightenings are accompanied by blueshifts of typically 5 km s-1. Phase differences between continuum and line intensities also indicate the presence of upward propagating waves. The detailed behavior is different between different lines, sometimes showing phase lags. The 3 minute intensity oscillations are occasionally seen in second spectra (CII λ1335) but never in third spectra (CIII and SiIII). Third spectra and HeI λ584 show oscillations in velocity that are not simply related to the 3 minute oscillations. The continuum intensity variations are consistent with recent simulations of chromospheric dynamics (Carlsson and Stein), while the line observations indicate that important ingredients are missing at higher layers in the simulations.

The data show that time variations are crucial for our understanding of the chromosphere itself and for the spectral features formed there--the quiet-Sun's chromosphere is very dynamic and not ``quiet.'' The implications of these data should be considered when planning chromospheric work with instruments such as those on SOHO. Title: Formation of Solar Calcium H and K Bright Grains Authors: Carlsson, Mats; Stein, Robert F. Bibcode: 1997ApJ...481..500C Altcode: We have simulated the generation of Ca II H2V bright grains by acoustic shocks. We employ a one-dimensional, non-LTE radiation-hydrodynamic code, with six-level model atoms for hydrogen and singly ionized calcium. We drive acoustic waves through a stratified radiative equilibrium atmosphere by a piston, whose velocity is chosen to match the Doppler shift observed in the Fe I 396.68 nm line in the H line wing, formed at about 260 km above τ500 = 1.

The simulations closely match the observed behavior of Ca II H2V bright grains down to the level of individual grains. The bright grains are produced by shocks near 1 Mm above τ500 = 1. Shocks in the mid-chromosphere produce a large source function (and therefore high emissivity) because the density is high enough for collisions to couple the Ca II populations to the local conditions. The asymmetry of the line profile is due to velocity gradients near 1 Mm. Material motion Doppler-shifts the frequency at which atoms emit and absorb photons, so the maximum opacity is located at--and the absorption profile is symmetric about--the local fluid velocity, which is shifted to the blue behind shocks. The optical depth depends upon the velocity structure higher up. Shocks propagate generally into downflowing material, so there is little matter above to absorb the Doppler-shifted radiation. The corresponding red peak is absent because of small opacity at the source function maximum and large optical depth due to overlying material. The bright grains are produced primarily by waves from the photosphere that are slightly above the acoustic cutoff frequency. The precise time and strength of a grain depend upon the interference between these waves near the acoustic cutoff frequency and higher frequency waves. When waves near the acoustic cutoff frequency are weak, then higher frequency waves may produce grains. The ``5 minute'' trapped p-mode oscillations are not the source of the grains, although they can slightly modify the behavior of higher frequency waves. Title: Chromospheric Dynamics - What Can Be Learnt from Numerical Simulations Authors: Carlsson, M.; Stein, R. F. Bibcode: 1997LNP...489..159C Altcode: 1997shpp.conf..159C Observations of the solar chromosphere are often interpreted using methods derived from static modeling (e.g., the Vernazza et al. 1981 model atmospheres and work based on such models) or linear theory (e.g., phase relations). Recent numerical simulations have shown that such an analysis can be very misleading. It is found that enhanced chromospheric emission, which corresponds to an outwardly increasing semi-empirical temperature structure, can be produced by wave motions without any increase in the mean gas temperature. Thus, despite long held beliefs, the Sun may not have a classical chromosphere in magnetic field free internetwork regions. This dynamic picture is consistent with observations in CO lines and the calcium H and K bright grains. More opaque lines, on the other hand, seem to show emission all of the time. This indicates the existence of a hotter, magnetic, component that increases in importance with height. Title: Shock Signature in Sunspots Authors: Bard, S.; Carlsson, M. Bibcode: 1997ESASP.404..189B Altcode: 1997cswn.conf..189B No abstract at ADS Title: Numerical Simulations Can Lead to New Insights Authors: Stein, Robert F.; Carlsson, Mats; Nordlund, Ake Bibcode: 1997ASPC..123...72S Altcode: 1997taca.conf...72S No abstract at ADS Title: The non-magnetic solar chromosphere. Authors: Carlsson, M.; Stein, R. F. Bibcode: 1997smf..conf...59C Altcode: The authors summarize recent results form self-consistent non-LTE radiation hydrodynamics simulations of the propagation of acoustic waves through the non-magnetic solar chromosphere. References to more detailed write-ups of the work are given. Title: Intensity and Velocity Variations in Transition Region Lines Observed with SUMER Authors: Betta, R.; Hansteen, V.; Carlsson, M.; Wilhelm, K. Bibcode: 1997ESASP.404..205B Altcode: 1997cswn.conf..205B No abstract at ADS Title: The NLTE formation of neutral-boron lines in cool stars. Authors: Kiselman, D.; Carlsson, M. Bibcode: 1996A&A...311..680K Altcode: 1996astro.ph..1144K We study the formation of BI lines in a grid of cool stellar model atmospheres without the assumption of local thermodynamic equilibrium (LTE). The non-LTE modelling includes the effect of other lines blending with the BI resonance lines. Except for the cases where the BI lines are very strong, the departures from LTE relevant for the resonance lines can be described as an overionisation effect and an optical-pumping effect. This causes the lines to be weaker than in LTE so that an abundance analysis assuming LTE will underestimate stellar boron abundances. We present non-LTE abundance corrections useful to improve on abundances derived from the Bi250nm and 209nm lines under the LTE assumption. Application of the results on literature data indicates that the B/Fe ratio in metal-poor stars is constant. Title: Solar chromospheric dynamics - Results from numerical simulations Authors: Carlsson, M.; Stein, R. F. Bibcode: 1996ASPC..109..119C Altcode: 1996csss....9..119C No abstract at ADS Title: Does a Nonmagnetic Solar Chromosphere Exist? Authors: Carlsson, Mats; Stein, Robert F. Bibcode: 1995ApJ...440L..29C Altcode: 1994astro.ph.11036C Enhanced chromospheric emission which corresponds to an outwardly increasing semiempirical temperature structure can be produced by wave motion without any increase in the mean gas temperture. Hence, the sun may not have a classical chromosphere in magnetic field free internetwork regions. Other significant differences between the properties of dynamic and static atmospheres should be considered when analyzing chromospheric observations. Title: Stellar Atmospheres Authors: Carlsson, M. Bibcode: 1995fras.conf..181C Altcode: No abstract at ADS Title: Infrared lines as probes of solar magnetic features. VIII. MgI 12μm diagnostics of sunspots. Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson, M. Bibcode: 1995A&A...293..225B Altcode: Due to their large Zeeman sensitivity, the MgI lines at 12μm are important diagnostics of solar magnetism. The formation of their central emission features is now understood, enabling quantitative modeling and diagnostic application of these lines. We supply the first systematic analysis of solar MgI 12μm Stokes profiles employing detailed line-profile synthesis. We compute Stokes profiles of MgI 12.32μm for the quiet Sun, for sunspot penumbrae and for the extended ("superpenumbral") magnetic canopies surrounding sunspots. We use these computations to analyze recent MgI 12μm observations by Hewagama et al. (1993). Our results are the following: (1) -Saha-Boltzmann temperature sensitivity explains that the emission peaks are stronger in penumbrae than in the quiet Sun, and that they disappear in umbrae. (2) -The formation heights of the emission features are approximately the same in penumbrae and in the quiet Sun, namely τ_500_=~10^-3^. (3) -The simple Seares formula allows relatively accurate determinations of field strength and magnetic inclination. (4) -The observed excess broadening of the σ-component peaks compared with the π component in penumbrae is well explained by primarily horizontal, smooth radial variation of the magnetic field strength. Additional small-scale variations are less than {DELTA}B =~200G. (5) -The vertical field gradients dB/dz in penumbrae range from 0.7G/km to 3G/km; the larger gradients occur near the umbra, the smaller ones near the outer edge of the penumbra. (6) -The MgI 12μm lines are well-suited to measure the base heights of superpenumbral magnetic canopies. These heights range between 300km and 500km above τ_500_=1 out to twice the sunspot radius, in excellent agreement with determinations from other infrared lines. Title: No Magnetic Field - No Chromosphere (Abstract only) Authors: Carlsson, M.; Stein, R. Bibcode: 1995itsa.conf..325C Altcode: No abstract at ADS Title: Non-LTE Effects on Be and B Abundance Determinations in Cool Stars Authors: Kiselman, D.; Carlsson, M. Bibcode: 1995lea..conf..372K Altcode: 1994astro.ph..9064K We discuss the nature of non-LTE effects affecting abundance analysis of cool stars. The departures from LTE of importance for the B I lines in solar-type stars are described and some new results are presented. Boron abundances derived under the LTE assumption have significant systematic errors, especially for metal-poor stars. For beryllium, current results suggest that departures from LTE will not affect abundance analysis significantly. Title: The Formation of the Solar He II 1640.4 Angstrom Emission Line Authors: Wahlstrom, Cathrine; Carlsson, Mats Bibcode: 1994ApJ...433..417W Altcode: We explain the formation of the solar He II 1640.4 A Balmer-alpha emission line. Non-LTE (non-local thermodynamic equilibrium) modeling is performed including the effects of an incoming coronal radiation field, the overlapping emission profile of He II 304 A with the He I continuum and the pumping effect of hydrogen Lyman-alpha. We find that the line is formed by the photoionization from the ground state of He II, mainly by the incident coronal radiation, followed by cascade recombination into the n = 3 levels. The peak of the contribution functions is at low temperatures, 7000-10,000 K, with a very small contribution from collisional excitation at a temperature of 70,000 K. We obtain good agreement with the observed line widths, the observed fine structure and the relative strength of the fine structure components with the blue peak about 10% stronger than the red peak. It is shown that these results are insensitive to the assumptions of geometry in the line forming regions and to the filling factor. The relative intensities only depend on an optically thin 1640.4 A line and optically thick He II Lyman lines. The observed asymmetry also imposes an upper limit to the density of 30 times the density at 8000 K in a plane parallel model. It is shown that the total intensity is very sensitive to the assumed geometry and filling factors. This sensitivity is parameterized in terms of a multiplicative factor to the assumed coronal radiation field. In a plane-parallel model the total intensity comes out a factor of 4 too low with the adopted coronal radiation field but already an enhancement factor of 2 of the incident radiation field gives a total intensity in agreement with observations. Note that this enhancement factor is only a quantification of the disagreement with observations. Title: The non-LTE formation of Li I lines in cool stars Authors: Carlsson, M.; Rutten, R. J.; Bruls, J. H. M. J.; Shchukina, N. G. Bibcode: 1994A&A...288..860C Altcode: We study the non-LTE (non local thermodynamic equilibrium) formation of Li I lines in the spectra of cool stars for a grid of radiative-equilibrium model atmospheres with variation in effective temperature, gravity, metallicity and lithium abundance. We analyze the mechanisms by which departures from LTE (local thermodynamic equilibrium) arise for Li I lines, first for the young sun (prior to its lithium depletion) and then across the cool-star grid. There are various mechanisms which compete in their effects on emergent Li I line strengths. Their neglect produces errors in lithium abundance determinations that vary in sign as well as size, both across the stellar grid and between different Li I lines (Figs). The errors are appreciable for all cooler stars and largest for cool lithium-rich metal-poor giants. They reverse sign between lithium-rich stars and lithium-poor stars for the λ=670.8nm resonance line, but not for the λ=610.4nm subordinate line. The non-LTE corrections are large enough that they should be taken into account in ongoing debates on lithium synthesis and depletion. We provide convenient numerical approximations of our results (Table 1) to this purpose. We end the paper with some examples in which non-LTE corrections change the slope of published relationships. Title: The Formation of Infrared Rydberg Lines Authors: Rutten, R. J.; Carlsson, M. Bibcode: 1994IAUS..154..309R Altcode: No abstract at ADS Title: Computation of Infrared Hydrogen Lines Authors: Carlsson, M.; Rutten, R. J. Bibcode: 1994IAUS..154..341C Altcode: No abstract at ADS Title: The Non-LTE Formation of Li I Lines from Cool Stars Authors: Carlsson, M.; Rutten, R. J.; Bruls, J. H. M. J.; Shchukina, N. G. Bibcode: 1994ASPC...64..270C Altcode: 1994csss....8..270C No abstract at ADS Title: Radiation shock dynamics in the solar chromosphere - results of numerical simulations Authors: Carlsson, M.; Stein, R. F. Bibcode: 1994chdy.conf...47C Altcode: No abstract at ADS Title: Chromospheric dynamics Authors: Carlsson, Mats Bibcode: 1994chdy.conf.....C Altcode: No abstract at ADS Title: Calcium II phase relations and chromospheric dynamics Authors: Skartlien, R.; Carlsson, M.; Stein, R. F. Bibcode: 1994chdy.conf...79S Altcode: No abstract at ADS Title: MgI 12 μm diagnostics of sunspot penumbrae Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson, M. Bibcode: 1994smf..conf..191B Altcode: No abstract at ADS Title: On i Lines in the Sun and Stars. I. Understanding the Resonance Lines Authors: Carlsson, M.; Judge, P. G. Bibcode: 1993ApJ...402..344C Altcode: The use of O I lines as spectral diagnostics of conditions in the chromospheres of the sun and cool stars is addressed, focusing on the resonance lines. The important processes influencing these lines are identified and an attempt it made to understand the detailed line transfer calculations in terms of simpler analytical models. How the lines respond to changes in uncertain atomic parameters is illustrated, and it is shown how the O I line flux densities depend simply and sensitively on the radiative transfer solution for hydrogen. Approximate analytical formulas are derived for the O I line flux densities in terms of the hydrogen number densities. Title: Non-LTE radiating shocks and the formation of Ca II lines in the solar chromosphere. Authors: Carlsson, M.; Stein, R. F. Bibcode: 1993wpst.conf...21C Altcode: The authors present self-consistent solutions of the time dependent one-dimensional equations of non-LTE radiation-hydrodynamics in solar chromospheric conditions. The vertical propagation of acoustic waves is calculated. Title: Non-LTE Radiating Acoustic Shocks and CA II K2V Bright Points Authors: Carlsson, Mats; Stein, Robert F. Bibcode: 1992ApJ...397L..59C Altcode: We present, for the first time, a self-consistent solution of the time-dependent 1D equations of non-LTE radiation hydrodynamics in solar chromospheric conditions. The vertical propagation of sinusoidal acoustic waves with periods of 30, 180, and 300 s is calculated. We find that departures from LTE and ionization recombination determine the temperature profiles of the shocks that develop. In LTE almost all the thermal energy goes into ionization, so the temperature rise is very small. In non-LTE, the finite transition rates delay the ionization to behind the shock front. The compression thus goes into thermal energy at the shock front leading to a high temperature amplitude. Further behind the shock front, the delayed ionization removes energy from the thermal pool, which reduces the temperature, producing a temperature spike. The 180 s waves reproduce the observed temporal changes in the calcium K line profiles quite well. The observed wing brightening pattern, the violet/red peak asymmetry and the observed line center behavior are all well reproduced. The short-period waves and the 5 minute period waves fail especially in reproducing the observed behavior of the wings. Title: Solar hydrogen lines in the infrared Authors: Carlsson, M.; Rutten, R. J. Bibcode: 1992A&A...259L..53C Altcode: We study recently observed H I lines in the infrared solar spectrum, employing detailed NLTE modeling to explain their formation and to evaluate their diagnostic merits. The solar infrared H I lines vary much in character, depending on opacity and wavelength; our computations reproduce the observations closely. The line wings are primarily set by Stark broadening due to metal ions and protons; the line cores are sensitive to NLTE population departure divergence which is driven by Balmer-continuum photoionization. The formation heights of the H I lines range from the deep photosphere for near-infrared line wings to the chromosphere for line cores with wavelengths greater than 10 microns; these features provide valuable diagnostics of the thermal structure of the solar atmosphere. Title: The calcium infrared triplet lines in stellar spectra. Authors: Jorgensen, U. G.; Carlsson, M.; Johnson, H. R. Bibcode: 1992A&A...254..258J Altcode: Observations of the infrared triplet lines of ionized calcium are often used as diagnostics of surface gravity among the stars contributing to the integrated light of distant galaxies. We have calculated the equivalent widths of these lines for a series of models with a broad range of input parameters to test their sensitivity to surface gravity, temperature, and calcium abundance. Over a larger range of parameter space than in earlier investigations, we find the sensitivity to be more complex than previously thought. We derive theoretical relations between equivalent width and log(g) for different values of Teff and metallicity and compare our results with observations. By actual NLTE calculations, we show that departures from LTE are small. Comparison of our results with observations of stars in the Galaxy suggests that the Ca/Fe ratio is an increasing function of metallicity. Title: The formation of the MG I emission features near 12 microns Authors: Carlsson, M.; Rutten, R. J.; Shchukina, N. G. Bibcode: 1992A&A...253..567C Altcode: The formation of two Mg I 12-micron emission features in the solar spectrum, the existence of which was reported by Murcray et al. (1981), is explained using plane-parallel nonlocal thermodynamic equilibrium modeling with a radiative-equilibrium model atmosphere without chromosphere. It is shown that these emissions are a natural consequence of population depletion by line photon losses followed by population replenishment from the ionic reservoir in the highly excited levels. The results confirm the suggestion by Lemke and Holweger (1987) that the 12-micron lines are formed in the photosphere and disprove the claim by Zirin and Popp (1989) that the temperature minimum occurs much deeper than in standard models of the solar atmosphere. Title: Formation of the MG 112 TTM Lines Authors: Carlsson, M.; Rutten, R. J.; Shchukina, N. G. Bibcode: 1992ASPC...26..518C Altcode: 1992csss....7..518C No abstract at ADS Title: CA II K2V Bright Grains Formed by Acoustic Waves Authors: Carlsson, M.; Stein, R. Bibcode: 1992ASPC...26..515C Altcode: 1992csss....7..515C No abstract at ADS Title: The MULTI Non-LTE Program (Invited Review) Authors: Carlsson, M. Bibcode: 1992ASPC...26..499C Altcode: 1992csss....7..499C No abstract at ADS Title: Formation of the KI 7699A Line in Sunspots Authors: Caccin, B.; Carlsson, M.; Gomez, M. T.; Severino, G. Bibcode: 1991ASIC..341..415C Altcode: 1991sabc.conf..415C No abstract at ADS Title: Global and Local Methods for One-Dimensional Problems - Implementation Aspects and Cpu-Time and Memory Scalings Authors: Carlsson, M. Bibcode: 1991ASIC..341...39C Altcode: 1991sabc.conf...39C No abstract at ADS Title: Shock Amplification by Radiation (With 1 Figure) Authors: Carlsson, M.; Stein, R. Bibcode: 1991mcch.conf..366C Altcode: No abstract at ADS Title: Deviation from the Wilson-Bappu relationship in faint red dwarf stars. Authors: Elgaroy, O.; Engvold, O.; Carlsson, M. Bibcode: 1990A&A...234..308E Altcode: New observations of the Mg II h and k lines on red dwarf stars have been carried out with the International Ultraviolet Explorer observatory. It is found that several of the dwarf stars in the sample have Mg II h and k lines which are narrower than expected from the Wilson-Bappu relationship, that is established for stars brighter than M(v) about 7. The deviations may depend on the Mg II h and k line opacity. Evidence is presented in favor of the suggestion that the optical thickness of the Mg II lines in faint red dwarfs depends on the level of activity of the stars. Calculations based on models for the sun and YZ CMi (Mv = 12.3) support the observational results. Title: The Formation of the Mg I 12-Micron Emission Lines Authors: Carlsson, M.; Rutten, R. J.; Shchukina, N. G. Bibcode: 1990PDHO....7..260C Altcode: 1990dysu.conf..260C; 1990ESPM....6..260C Contents: The Mg I 12 μm line, LTE or NLTE, chromospheric formation, photospheric formation, collisional NLTE; departure diffusion. Title: Non-LTE radiative hydrodynamic interactions in the solar chromosphere. Authors: Carlsson, M.; Stein, R. Bibcode: 1990ppst.conf..177C Altcode: Strong, optically thick lines from iron and from ionized calcium and magnesium dominate the radiative losses of the solar chromosphere. This radiative loss cannot be approximated in the optically thin limit or by a grey approximation. In order to properly calculate the effects of waves in the chromosphere it is necessary to solve simultaneously the equations of hydrodynamics, radiative transfer and statistical equilibrium. Efficient methods in radiative transfer are here being combined with a treatment of the dynamical equations capable of resolving shocks. The authors present the first results showing that radiative hydrodynamic interactions may have a significant effect on the heating by acoustic waves. Title: Waves and radiation in stellar atmospheres. Authors: Carlsson, M. Bibcode: 1990ppst.conf..115C Altcode: The author discusses the problem of calculating the interactions between radiation and hydrodynamics in various density regimes. The possible approximations are outlined and diagnostic problems are discussed. The interactions between waves and radiation in the intermediate density regime (applicable to e.g. the solar chromosphere) are reviewed in some detail. Title: A 1-D code for radiation hydrodynamics problems. Authors: Carlsson, M. Bibcode: 1989ftsa.conf...93C Altcode: In order to properly calculate the effects of waves in the chromosphere it is necessary to solve simultaneously the equation of hydrodynamics, radiative transfer and statistical equilibrium. Efficient methods in radiative transfer are beeing combined with methods to treat the dynamical equations capable of resolving shocks. There is no complete code finished yet and this report is more of a progress report centered on methodology rather than a presentation of physically relevant results. Title: A Computer Program for Solving Multi-Level Non-Lte Radiative Transfer Problems in Moving or Static Atmospheres Authors: Carlsson, M. Bibcode: 1988ASSL..138..273C Altcode: 1988pffl.proc..273C; 1988IAUCo..94..273C A summary of the characteristics of a non-LTE computer program is presented. The code can be used to solve non-LTE radiative transfer problems in semi-infinite, plane-parallel one-dimensional atmospheres with a prescribed velocity field. The model atom can contain many atomic levels and several ionization stages. The lines are assumed to be formed with complete redistribution over the line profile which is assumed to be a Voigt function. Title: Stokes Profile Analysis and Vector Magnetic Fields. III. Extended Temperature Minima of Sunspot Umbrae as Inferred from Stokes Profiles of MG i lambda 4571 Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.; Carlsson, M. Bibcode: 1987ApJ...318..930L Altcode: Observed Stokes profiles of Mg I 4571 A are analyzed as a diagnostic of the magnetic field and thermal structure at the temperature minimum of sunspot umbrae. Multilevel non-LTE transfer calculations of the Mg I-II-III excitation and ionization balance in model umbral atmospheres show: (1) Mg I to be far less ionized in sunspot umbrae than in the quiet sun, leading to greatly enhanced opacity in 4571 A, and (2) LTE excitation of 4571 A. Existing umbral models predict emission cores of the Stokes I profile due to the chromospheric temperature rise. This feature is not present in observed umbral profiles. Moreover, such an emission reversal causes similar anomalous features in the Stokes Q, U, V profiles, which are also not observed. Umbral atmospheres with extended temperature minima are suggested. Implications for chromospheric heating mechanisms and the utility of this line for solar vector magnetic field measurements are discussed. Title: Radiative transfer : an operator perturbation method and the application to mesoturbulence Authors: Carlsson, Mats Per-Olof Bibcode: 1987PhDT.......250C Altcode: No abstract at ADS Title: A New Sunspot Umbral Model and Its Variation with the Solar Cycle Authors: Maltby, P.; Avrett, E. H.; Carlsson, M.; Kjeldseth-Moe, O.; Kurucz, R. L.; Loeser, R. Bibcode: 1986ApJ...306..284M Altcode: Semiempirical model atmospheres are presented for the darkest parts of large sunspot umbrae, regions have called umbral cores. The approach is based on general-purpose computational procedures that are applicable to different types of stellar atmospheres. It is shown that recent umbral intensity measurements of the spectral energy distribution may be accounted for by an umbral core atmospheric model that varies with time during the solar cycle; the observed center-limb variation can be accounted for by the properties of the model. Three umbral core models are presented, corresponding to the early, middle, and late phases of the solar cycle. These three models also may be regarded as having the properties of dark, average, and bright umbral cores respectively. The effects of atomic, opacity, and abundance data uncertainties on the model calculations are briefly discussed. For comparison, a new reference model for the average quiet solar photosphere is given. Title: The outer atmosphere of the carbon star TX Piscium. Authors: Eriksson, K.; Gustafsson, B.; Johnson, H. R.; Querci, F.; Querci, M.; Baumert, J. H.; Carlsson, M.; Olofsson, H. Bibcode: 1986A&A...161..305E Altcode: A high-resolution LWP IUE spectrum of the bright N-type carbon star TX Psc demonstrates that the Mg II h and k emission profiles are strongly affected by absorption from Mg II, Mn I, probably Fe I, and possibly from molecules. The indication that the absorbing matter has a column density of not less than 10 to the 20th H atoms or molecules per sq cm is consistent with absorption in a slowly expanding envelope. The integrated Mg II line flux is found to be much greater than in 1981, and the radio CO (J = 1 - 0) line from the circumstellar shell is detected. Results for a column density of not larger than 10 to the 22nd H2 molecules/sq cm, and a radial velocity close to that of the star, are in agreement with those obtained from UV data. Some dust emission from carbon grains is suggested by the far infrared flux distribution, and a mass-loss rate estimation for the star of 10 to the -6th to 10 to the -8th solar masses is obtained. Title: Extended Temperature Minima of Sunspot Umbrae Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.; Carlsson, M. Bibcode: 1986BAAS...18..662L Altcode: No abstract at ADS Title: A computer program for solving multi-level non-LTE radiative transferproblems in moving or static atmospheres. Authors: Carlsson, M. Bibcode: 1986UppOR..33.....C Altcode: A computer program for solving multi-level non-LTE radiative transfer problems is presented. The method used is that developed by Scharmer (1981) and Scharmer & Carlsson (1985a, 1985b). The code can be used to solve non-LTE radiative transfer problems in semi-infinite, plane-parallel one-dimensional atmospheres with a prescribed macroscopic velocity field. The model atom can contain many atomic levels and several ionization stages. The lines are assumed to be formed with complete redistribution over the profile function which is assumed to be a Voigt function.

Some aspects of the numerical solution of radiative transfer problems are also discussed; in particular the choice of a starting approximation, instability problems due to the optically thin radiation field and the accuracy of methods for solving the transfer equation with a known source function. Title: Radiative Transfer and Turbulent Atmospheres Authors: Carlsson, M. Bibcode: 1985tphr.conf...67C Altcode: No abstract at ADS Title: A new approach to multi-level non-LTE radiative transfer problems. Authors: Scharmer, G. B.; Carlsson, M. Bibcode: 1985JCoPh..59...56S Altcode: A new approach to the numerical solution of multilevel, non-LTE problems is described. The standard non-LTE multilevel problem is formulated, and the statistical equilibrium equations and the radiative transfer equation are linearized. It is shown how to precondition the statistical equilibrium equations and the radiative transfer equation in a way which enables the solution of problems with strong numerical cancellation which arise from 'passive' scatterings at large optical depth. Simplifications in the numerical representation of radiative transfer processes are introduced which lead to rapid methods for setting up and solving the statistical equilibrium equations. Some calculations which have been made to test the convergence properties of the present scheme are described, and some generalizations of the present methods are briefly outlined. Title: A proposal for the LEST control system. Authors: Engvold, O.; Andersen, T. E.; Carlsson, M.; Jensen, J. R.; Klim, K. Bibcode: 1985LFTR...15.....E Altcode: Contents: Trends in development of computers and peripherals. Trends in telescope control and operation. Control system requirements for LEST. Proposed LEST control system. Remote control of LEST. Cost estimate. Estimated total cost of control and data reduction system. Title: Radiative transfer and turbulent atmospheres. Authors: Carlsson, M. Bibcode: 1985MPARp.212...67C Altcode: The paper is divided into two parts - in the first part some trends in radiative transfer methodology towards three dimensional modeling are reviewed. Special emphasis is given to operator perturbation techniques and to the impact of increased parallelism in new computer architecture. The second part illustrates some effects of velocity fields on the radiative transfer in spectral lines. Title: Effects of meso-scale velocity fields on the solar Ca II spectral lines. Authors: Carlsson, M.; Scharmer, G. B. Bibcode: 1985cdm..proc..137C Altcode: The effects of random mesoscale velocity fields on the solar Ca II K-line and the infrared 8542 A line are discussed, and average profiles and radiative cooling functions are compared with those obtained from a combination of microturbulence and macroturbulence. Profiles in the mesoscale regime could not be reproduced with any combination of micro- and macroturbulence owing to the fact that mesoscale velocity fields give core intensities that are much higher than those of static atmospheres. Calculations of radiative cooling in the two lines were different when mesoscale velocity fields were taken into account; this difference was due mainly to a redistribution of the radiative cooling between the lines. Title: A new method for solving multi-level non-LTE problems. Authors: Scharmer, G. B.; Carlsson, M. Bibcode: 1985ASIC..152..189S Altcode: 1985pssl.proc..189S A new scheme for solving multi-level non-LTE problems is described. This method uses an approximate operator for the relation between the intensity and the source function. This operator results in a matrix equation for the population numbers which has a simple and characteristic structure. Solutions are obtained such that the results are "exact", irrespective of the choice of the approximate operator. Title: The outer atmosphere of the carbon star TX Piscium. Authors: Eriksson, K.; Gustafsson, B.; Johnson, H. R.; Querci, F.; Querci, M.; Baumert, J. H.; Carlsson, M.; Olofsson, H. Bibcode: 1985BETSP...2....5E Altcode: No abstract at ADS Title: The lightcurve and phase relation of the asteroid 133 Cyrene Authors: Harris, A. W.; Carlsson, M.; Young, J. W.; Lagerkvist, C. I. Bibcode: 1984Icar...58..377H Altcode: The asteroid 133 Cyrene was observed photometrically on 17 nights during oppositions in 1979 and 1980. The synodic period of rotation was found to be 12. h708 ± 0. h001 with an amplitude of ∼0. m30 during both oppositions. At large phase angles, the phase relation is quite ordinary ( βv ≈ 0.025 mag/degree); however, the low phase angle observations reveal a dramatic opposition brightening, ∼0.2 mag/degree near zero phase angle. The absolute magnitude, V(1,0), extrapolated with the above linear phase coefficient, is 8.40. The following color indicates were also measured: B- V = 0.90, U-B = 0.51. Title: Physical studies of asteroids. XI - Photoelectric observations of the asteroids 2, 161, 216 and 276 Authors: Carlsson, M.; Lagerkvist, C. -I. Bibcode: 1983A&AS...53..157C Altcode: Lightcurves and UBV-colours are presented for the asteroids 2 Pallas, 161 Athor, 216 Kleopatra and 276 Adelheid. For asteroid 216 a lightcurve amplitude of lm.1 was observed. Title: Physical studies of asteroids. IV - Photoelectric observations of the asteroids 47, 95, 431 Authors: Carlsson, M.; Lagerkvist, C. -I. Bibcode: 1981A&AS...45....1C Altcode: Photoelectric lightcurves are presented. The synodic period of rotation of asteroid 95 was found to be 0d.3620 ± 0d.0005. The asteroids 47 and 431 showed no significant brightness variation throughout the observing runs of 8 hours. Title: Physical studies of asteroids. I - Photoelectric observations of the asteroids 38, 218, 268, 344, 485, 683, 690 and 792 Authors: Carlsson, M.; Lagerkvist, C. -I. Bibcode: 1981A&AS...44...15C Altcode: Lightcurves, rotation periods, maximum amplitudes, absolute magnitudes and UBV colours are presented. The following synodic periods of rotation were derived : Asteroid 218: 0d.268 ± 0d.006, asteroid 485: 0d.7331 ± 0d.0002, asteroid 683: 0d.1801 ± 0d.0004, asteroid 792: 0.d3823 ± 0d.0004. Title: Positions of asteroids obtained with the Schmidt telescope at the Uppsala Southern Station Authors: Carlsson, M.; Hahn, G.; Lagerkvist, C. -I. Bibcode: 1980A&AS...41..117C Altcode: Photographic positions of asteroids obtained at the Uppsala Southern Station are presented. Positions are presented for asteroids marked X or XX in Ephemerides of Minor Planets (1978), ordinary main-belt asteroids, Trojans and unnumbered asteroids. The IRIS plate measuring machine (Aslund et al., 1975) was used, and the means of the absolute differences between the catalog positions of the AGK3 stars and the observed positions were found to be 0.46 and 0.43 arc sec in right ascension (RA) and declination, respectively. For the SAO stars the corresponding values were 0.76 and 0.84 arc sec in RA and declination.