Author name code: hansteen ADS astronomy entries on 2022-09-14 author:"Hansteen, Viggo H." ------------------------------------------------------------------------ Title: A novel inversion method to determine the coronal magnetic field including the impact of bound-free absorption Authors: Martinez-Sykora, Juan; Hansteen, Viggo H.; De Pontieu, Bart; Landi, Enrico Bibcode: 2022arXiv220813984M Altcode: The magnetic field governs the corona; hence it is a crucial parameter to measure. Unfortunately, existing techniques for estimating its strength are limited by strong assumptions and limitations. These techniques include photospheric or chromospheric field extrapolation using potential or non-linear-force-free methods, estimates based on coronal seismology, or by direct observations via, e.g., the Cryo-NIRSP instrument on DKIST which will measure the coronal magnetic field, but only off the limb. Alternately, in this work we investigate a recently developed approach based on the magnetic-field-induced (MIT) transition of the \fex~257.261~Å. In order to examine this approach, we have synthesized several \fex\ lines from two 3D magnetohydrodynamic simulations, one modeling an emerging flux region and the second an established mature active region. In addition, we take bound-free absorption from neutral hydrogen and helium and singly ionised helium into account. The absorption from cool plasma that occurs at coronal heights has a significant impact on determining the magnetic field. We investigate in detail the challenges of using these \fex\ lines to measure the field, considering their density and temperature dependence. We present a novel approach to deriving the magnetic field from the MIT using inversions of the differential emission measure as a function of the temperature, density, and magnetic field. This approach successfully estimates the magnetic field strength (up to \%18 relative error) in regions that do not suffer from significant absorption and that have relatively strong coronal magnetic fields ($>250$~G). This method allows the masking of regions where absorption is significant. Title: Fine-scale, Dot-like, Brightenings in an Emerging Flux Region: SolO/EUI Observations, and Bifrost MHD Simulations Authors: Tiwari, Sanjiv Kumar; Berghmans, David; De Pontieu, Bart; Hansteen, Viggo; Panesar, Navdeep Kaur Bibcode: 2022cosp...44.2529T Altcode: Numerous tiny bright dots are observed in SolO's EUI/\hri\ data of an emerging flux region (a coronal bright point) in 174 \AA, emitted by the coronal plasma at $\sim$1 MK. These dots are roundish, with a diameter of 675$\pm$300 km, a lifetime of 50$\pm$35 seconds, and an intensity enhancement of 30% $\pm$10% from their immediate surroundings. About half of the dots remain isolated during their evolution and move randomly and slowly ($<$10 \kms). The other half show extensions, appearing as a small loop or surge/jet, with intensity propagations below 30\,\kms. Some dots form at the end of a fine-scale explosion. Many of the bigger and brighter EUI/HRI dots are discernible in SDO/AIA 171 \AA\ channel, have significant EM in the temperature range of 1--2 MK, and are often located at polarity inversion lines observed in HMI LOS magnetograms. Bifrost MHD simulations of an emerging flux region do show dots in synthetic Fe IX/X images, although dots in simulations are not as pervasive as in observations. The dots in simulations show distinct Doppler signatures -- blueshifts and redshifts coexist, or a redshift of the order of 10 \kms\ is followed by a blueshift of similar or higher magnitude. The synthetic images of O V/VI and Si IV lines, which form in the transition region, also show the dots that are observed in Fe IX/X images, often expanded in size, or extended as a loop, and always with stronger Doppler velocities (up to 100 \kms) than that in Fe IX/X lines. Our results, together with the field geometry of dots in the simulations, suggest that most dots in emerging flux regions form in the lower solar atmosphere (at $\approx$1 Mm) by magnetic reconnection between emerging and pre-existing/emerged magnetic field. The dots are smaller in Fe IX/X images (than in O V/VI & Si IV lines) most likely because only the hottest counterpart of the magnetic reconnection events is visible in the hotter emission. Some dots might be manifestations of magneto-acoustic shocks (from the lower atmosphere) through the line formation region of Fe IX/X. A small number of dots could also be a response of supersonic downflows impacting transition-region/chromospheric density. 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: Detailed Description of the Collision Frequency in the Solar Atmosphere Authors: Wargnier, Q. M.; Martínez-Sykora, J.; Hansteen, V. H.; De Pontieu, B. Bibcode: 2022ApJ...933..205W Altcode: This work aims to provide an accurate description and calculations of collision frequencies in conditions relevant to the solar atmosphere. To do so, we focus on the detailed description of the collision frequency in the solar atmosphere based on a classical formalism with Chapman-Cowling collision integrals, as described by Zhdanov. These collision integrals allow linking the macroscopic transport fluxes of multifluid models to the kinetic scales involved in the Boltzmann equations. In this context, the collision frequencies are computed accurately while being consistent at the kinetic level. We calculate the collision frequencies based on this formalism and compare them with approaches commonly used in the literature for conditions typical of the solar atmosphere. To calculate the collision frequencies, we focus on the collision integral data provided by Bruno et al., which is based on a multicomponent hydrogen-helium mixture used for conditions typical for the atmosphere of Jupiter. We perform a comparison with the classical formalism of Vranjes & Krstic and Leake & Linton. We highlight the differences obtained in the distribution of the cross sections as functions of the temperature. Then, we quantify the disparities obtained in numerical simulations of a 2.5D solar atmosphere by calculating collision frequencies and ambipolar diffusion. This strategy allows us to validate and assess the accuracy of these collision frequencies for conditions typical of the solar atmosphere. Title: SolO/EUI Observations of Ubiquitous Fine-scale Bright Dots in an Emerging Flux Region: Comparison with a Bifrost MHD Simulation Authors: Tiwari, Sanjiv K.; Hansteen, Viggo H.; De Pontieu, Bart; Panesar, Navdeep K.; Berghmans, David Bibcode: 2022ApJ...929..103T Altcode: 2022arXiv220306161T We report on the presence of numerous tiny bright dots in and around an emerging flux region (an X-ray/coronal bright point) observed with SolO's EUI/HRIEUV in 174 Å. These dots are roundish and have a diameter of 675 ± 300 km, a lifetime of 50 ± 35 s, and an intensity enhancement of 30% ± 10% above their immediate surroundings. About half of the dots remain isolated during their evolution and move randomly and slowly (<10 km s-1). The other half show extensions, appearing as a small loop or surge/jet, with intensity propagations below 30 km s-1. Many of the bigger and brighter HRIEUV dots are discernible in the SDO/AIA 171 Å channel, have significant emissivity in the temperature range of 1-2 MK, and are often located at polarity inversion lines observed in SDO/HMI LOS magnetograms. Although not as pervasive as in observations, a Bifrost MHD simulation of an emerging flux region does show dots in synthetic Fe IX/X images. These dots in the simulation show distinct Doppler signatures-blueshifts and redshifts coexist, or a redshift of the order of 10 km s-1 is followed by a blueshift of similar or higher magnitude. The synthetic images of O V/VI and Si IV lines, which represent transition region radiation, also show the dots that are observed in Fe IX/X images, often expanded in size, or extended as a loop, and always with stronger Doppler velocities (up to 100 km s-1) than that in Fe IX/X lines. Our observation and simulation results, together with the field geometry of dots in the simulation, suggest that most dots in emerging flux regions form in the lower solar atmosphere (at ≍ 1 Mm) by magnetic reconnection between emerging and preexisting/emerged magnetic field. Some dots might be manifestations of magnetoacoustic shocks through the line formation region of Fe IX/X emission. 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). 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, 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: Description of collisional frequencies for multifluid MHD models with Chapman-Cowling collision integrals Authors: Wargnier, Q.; Martinez-Sykora, Juan; Hansteen, Viggo; Magin, Thierry Bibcode: 2021AGUFMSH45B2362W Altcode: We focus on the detailed description of the collisional frequency in the solar atmosphere based on a classical formalism with Chapman-Cowling collision integrals, as described by Zhdanov (2002) in the context of the 13N-moment model derived with a method of Grad (Grad 1949). These collision integrals allow linking the macroscopic transport fluxes of multifluid models to the kinetic scales involved in the Boltzmann equations. In this context, the collisional frequencies are computed accurately while being consistent at the kinetic level. We calculate the collisional frequencies based on this formalism and compare them with approaches commonly used in the literature in solar atmosphere conditions. To calculate the collisional frequencies, we focus on the collision integrals data provided by Bruno et al. (2010), which is based on a multicomponent hydrogen-helium mixture used in Jupiter atmosphere conditions. We propose a comparison with the classical formalism of Vranjes & Krstic (2013) and Leake & Linton (2013). We compare it with the formalism used in the three approaches and highlight the differences obtained in the distribution of the cross sections as functions of the temperature. Then, we quantify the disparities obtained in postprocessed simulations of a 2.5D solar atmosphere with the Bifrost code (see Gudiksen et al. 2011). Finally, we assess the impact of the collisional frequency in a simulated 2.5D solar atmosphere with a single-fluid radiative MHD model with ambipolar diffusion to consider ion-neutral interactions. Significant disparities in the cross sections have been obtained between these three formalisms. or instance, we note that Vranjes & Krstic 2013 did no integrate the transport cross sections. We will describe the impact of these discrepancies from previous results and the importance of doing these calculations properly. Title: Linking the Sun to the Heliosphere Using Composition Data and Modelling Authors: Parenti, Susanna; Chifu, Iulia; Del Zanna, Giulio; Edmondson, Justin; Giunta, Alessandra; Hansteen, Viggo H.; Higginson, Aleida; Laming, J. Martin; Lepri, Susan T.; Lynch, Benjamin J.; Rivera, Yeimy J.; von Steiger, Rudolf; Wiegelmann, Thomas; Wimmer-Schweingruber, Robert F.; Zambrana Prado, Natalia; Pelouze, Gabriel Bibcode: 2021SSRv..217...78P Altcode: 2021arXiv211006111P Our understanding of the formation and evolution of the corona and the heliosphere is linked to our capability of properly interpret the data from remote sensing and in-situ observations. In this respect, being able to correctly connect in-situ observations with their source regions on the Sun is the key for solving this problem. In this work we aim at testing a diagnostics method for this connectivity. Title: Line formation of He I D3 and He I 10 830 Å in a small-scale reconnection event Authors: Libbrecht, Tine; Bjørgen, Johan P.; Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Joshi, Jayant Bibcode: 2021A&A...652A.146L Altcode: 2020arXiv201015946L Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection events that occur in the region of the upper photosphere to the chromosphere. It has recently been discovered that these events can have emission signatures in the He I D3 and He I 10 830 Å lines, suggesting that their temperatures are higher than previously expected.
Aims: We aim to explain the line formation of He I D3 and He I 10 830 Å in small-scale reconnection events.
Methods: We used a simulated EB in a Bifrost-generated radiative magnetohydrodynamics snapshot. The resulting He I D3 and He I 10 830 Å line intensities were synthesized in 3D using the non-local thermal equilibrium (non-LTE) Multi3D code. The presence of coronal extreme UV (EUV) radiation was included self-consistently. We compared the synthetic helium spectra with observed raster scans of EBs in He I 10 830 Å and He I D3 obtained at the Swedish Solar Telescope with the TRI-Port Polarimetric Echelle-Littrow Spectrograph.
Results: Emission in He I D3 and He I 10 830 Å is formed in a thin shell around the EB at a height of ∼0.8 Mm, while the He I D3 absorption is formed above the EB at ∼4 Mm. The height at which the emission is formed corresponds to the lower boundary of the EB, where the temperature increases rapidly from 6 × 103 K to 106 K. The synthetic line profiles at a heliocentric angle of μ = 0.27 are qualitatively similar to the observed profiles at the same μ-angle in dynamics, broadening, and line shape: emission in the wing and absorption in the line core. The opacity in He I D3 and He I 10 830 Å is generated through photoionization-recombination driven by EUV radiation that is locally generated in the EB at temperatures in the range of 2 × 104 − 2 × 106 K and electron densities between 1011 and 1013 cm−3. The synthetic emission signals are a result of coupling to local conditions in a thin shell around the EB, with temperatures between 7 × 103 and 104 K and electron densities ranging from ∼1012 to 1013 cm−3. This shows that both strong non-LTE and thermal processes play a role in the formation of He I D3 and He I 10 830 Å in the synthetic EB/UV burst that we studied.
Conclusions: In conclusion, the synthetic He I D3 and He I 10 830 Å emission signatures are an indicator of temperatures of at least 2 × 104 K; in this case, as high as ∼106 K. 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: ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.; Sainz Dalda, Alberto Bibcode: 2021ApJ...906...83C Altcode: 2020arXiv201205970C We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate a plage from its surrounding regions than other methods commonly used. We caution that isolating a plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that λ = 1.25 mm free-free emission in plage regions observed with the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition-region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph (IRIS)/Si IV at 1393 Å. We also show that IRIS/Mg II h and k are not as well correlated with ALMA/Band6 as was previously thought, and we discuss discrepancies with previous works. Lastly, we report indications of chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations. Title: ALMA and IRIS Observations of the Solar Chromosphere. I. An On-disk Type II Spicule Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.; Sainz Dalda, Alberto Bibcode: 2021ApJ...906...82C Altcode: 2020arXiv200512717C We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph. The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers nonequilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature. Title: Probing chromospheric heating with millimeter interferometry Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White, S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S. Bibcode: 2020AGUFMSH0010001D Altcode: Observations at visible and ultraviolet wavelengths have shown that solar active regions host different kinds of small-scale, transient, bright structures that are believed to be heating events resulting from the release of magnetic energy in the low atmosphere of the Sun, especially at the early stages of flux emergence. It is of great scientific interest to be able to accurately infer temperatures and formation heights of the most localized events, which are still matter of debate, in the hope of learning about the evolution of active regions where occasionally more energetic phenomena lead to much larger outbursts that propagate across the Solar System. The millimeter (mm) continuum is a new complementary diagnostic for chromospheric heating that is now available thanks to the Atacama Large Millimeter/submillimeter Array (ALMA).

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

Our observations and modelling highlight the diagnostic capabilities of ALMA for local heating in solar active regions and emphasize the need for coordinated observations with IRIS and DKIST in the future. Title: ALMA and IRIS Observations Highlighting the Dynamics and Structure of Chromospheric Plage Authors: Chintzoglou, G.; De Pontieu, B.; Martinez-Sykora, J.; Hansteen, V. H.; de la Cruz Rodriguez, J.; Szydlarski, M.; Jafarzadeh, S.; Wedemeyer, S.; Bastian, T.; Sainz Dalda, A. Bibcode: 2020AGUFMSH0010009C Altcode: We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers non-equilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature. In addition, we discuss the strikingly high degree of similarity between chromospheric plage features observed in ALMA/Band6 and IRIS/\ion{Si}{4} (also reproduced in our model) suggesting that ALMA/Band6 does not observe in the low chromosphere as previously thought but rather observes the upper chromospheric parts of structures such as spicules and other bright structures above plage at geometric heights near transition region temperatures. We also show that IRIS/\ion{Mg}{2} is not as well correlated with ALMA/Band6 as was previously thought. For these comparisons, we propose and employ a novel empirical method for the determination of plage regions, which seems to better isolate plage from its surrounding regions as compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care to mitigate statistical bias in quantitative comparisons between different chromospheric observables. Lastly, we report indications for chromospheric heating due to traveling shocks supported by the ALMA/Band6 observations. Title: On the velocity drift between ions in the solar atmosphere Authors: Martínez-Sykora, J.; Szydlarski, M.; Hansteen, V. H.; De Pontieu, B. Bibcode: 2020AGUFMSH0010017M Altcode: Very recent results that compare ALMA and IRIS observations with 2D radiative MHD including non-equilibrium ionization and ambipolar diffusion models of the type II spicules reveal that these models may underestimate the energy dissipated in the chromosphere. The solar atmosphere is composed of many species that are populated at different ionization and excitation levels. The upper chromosphere, transition region, and corona are nearly collisionless. Consequently, slippage between, for instance, ions and neutral particles, or interactions between separate species, may play an important role in the local momentum and energy balance. The interaction between species is missing in the 2D radiative MHD model. We have developed a 3D multi-fluid and multi-species numerical code (Ebysus) to investigate such effects. Ebysus is capable of treating species (e.g., hydrogen, helium, etc) and fluids (neutrals, excited and ionized elements) separately. Treating different species as different fluids leads to drifts between different ions and an electric field that couple these motions. Different ionized species and momentum exchange can dissipate this velocity drift, i.e., convert wave kinetic energy into thermal energy. High-frequency Alfven waves, driven for instance by reconnection, thought to occur in the solar atmosphere, can drive such multi-ion velocity drifts. Title: The correlation between chromospheric and coronal heating in active region plage Authors: De Pontieu, B.; Hansteen, V. H.; Winebarger, A. R. Bibcode: 2020AGUFMSH0010010D Altcode: The upper transition region at the footpoints of the hottest loops in active regions is known as moss, highly structured and dynamic 1 MK plasma that is formed at the same heights as dynamic chromospheric jets emanating from the underlying plage regions. Moss provides an excellent laboratory to disentangle the complex interface between chromosphere and corona and to study how chromospheric and coronal heating mechanisms are spatio-temporally correlated (if at all). This is because moss is very sensitive to changes in the local heating rate and, since it is formed in a thin, corrugated layer, avoids the confusion introduce by line-of-sight superposition taht affects optically thin coronal diagnostics. Previous results based on lower-resolution instruments (e.g., TRACE, SDO/AIA) suggested a puzzling mismatch between low chromospheric and upper TR emission. We will present results based on analysis of a unique coordinated dataset from IRIS and the sounding rocket HiC. The HiC 2.1 flight took place in 2018 and obtained several minutes of sub-arcsecond resolution images of the upper TR in Fe IX 171A, while IRIS obtained high-resolution rasters in the Mg II h & k lines at high cadence. Our analysis will focus on spatio-temporal correlations between the properties of the optically thick Mg II h & k lines, and the intensities of the HiC 2.1 images. We will also exploit the recently developed IRIS2 database to invert the Mg II h & k profiles and study correlations between the derived chromospheric temperature, density, and micro-turbulence (as a function of height in the chromosphere) and the overlying upper TR and coronal emission. Our analysis provides insight and constraints on the nature and (dis)similarities of the heating mechanisms in both the chromosphere and corona. Title: A Multi-Fluid Multi-Species (MFMS) numerical code for simulating the solar atmosphere Authors: Wargnier, Q.; Martínez-Sykora, J.; Hansteen, V. H.; Szydlarski, M.; Evans, S. Bibcode: 2020AGUFMSH0370007W Altcode: The solar atmosphere is characterized by multiple periods and sizes involving a large spectrum of temporal and spatial scales. It is regulated through complex interactions between different species and chemical reactions, amongst other physical processes . Because of this complexity, an accurate description of all of these multi-scale phenomena in the solar atmosphere is out of the reach of standard single-fluid MHD models (Hartlep et al. 2012). Furthermore, the enrichment of low first ionization potential elements in the outer layer of the solar atmosphere (the FIP effect) is not fully described by the current theoretical models (e.g. Laming et al. 2017), since they employ semi-empirical static atmospheres. Title: Chromospheric and TR diagnostics in a large scale numerical simulation of flux emergence: Synthetic vs Real observables Authors: Hansteen, V. H.; De Pontieu, B.; Testa, P.; Gosic, M.; Martinez-Sykora, J. Bibcode: 2020AGUFMSH0010021H Altcode: Field stored just below or rising to the photosphere will break through the surface and enter the upper atmosphere once the gradient of the subphotospheric field strength becomes sufficiently large. Opposite polarity flux bundles will reconnect in the photosphere and above, to form steadily longer loops that expand into the outer solar atmosphere, forming the corona. Some of the emerging flux is likely due to a local dynamo, but also the direct emergence of large scale magnetic structures from below is important, even in the quiet Sun. A significant proportion of this field likely reaches the chromosphere and may leave imprint on chromospheric dynamics and energetics. Using large scale numerical models (72x72x60) Mm and the high resolution spectra and slit jaw images from IRIS, as well as photospheric data from Hinode/SOT, and SDO/HMI we study the interactions between the magnetic flux caught in the granular flow field and the chromosphere and chromospheric field above. We will compare synthetic observables of the photospheric Fe I 617.3 nm line, the chromospheric Mg II h&k lines, and the transition region Si IV lines, with their observational counterparts. We will also generate synthetic ALMA band 3 images. The comparison of synthetic observational data will let us draw conclusions as to the validity of the numerical modeling and the importance of flux emergence for the dynamics and energetics of the outer solar atmosphere. Title: ALMA observations of transient heating in a solar active region Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White, S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H. Bibcode: 2020A&A...643A..41D Altcode: 2020arXiv200614564D
Aims: We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA).
Methods: We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis.
Results: We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts - reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 - 340 km s-1) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

Movie associated to Fig. 5 is available at https://www.aanda.org Title: IRIS Observations of the Low-atmosphere Counterparts of Active Region Outflows Authors: Polito, Vanessa; De Pontieu, Bart; Testa, Paola; Brooks, David H.; Hansteen, Viggo Bibcode: 2020ApJ...903...68P Altcode: 2020arXiv201015945P Active region (AR) outflows have been studied in detail since the launch of Hinode/EIS and are believed to provide a possible source of mass and energy to the slow solar wind. In this work, we investigate the lower atmospheric counterpart of AR outflows using observations from the Interface Region Imaging Spectrograph (IRIS). We find that the IRIS Si IV, C II> and Mg II transition region (TR) and chromospheric lines exhibit different spectral features in the outflows as compared to neighboring regions at the footpoints ("moss") of hot AR loops. The average redshift of Si IV in the outflow region (≍5.5 km s-1) is smaller than typical moss (≍12-13 km s-1) and quiet Sun (≍7.5 km s-1) values, while the C II line is blueshifted (≍-1.1-1.5 km s-1), in contrast to the moss where it is observed to be redshifted by about ≍2.5 km s-1. Further, we observe that the low atmosphere underneath the coronal outflows is highly structured, with the presence of blueshifts in Si IV and positive Mg II k2 asymmetries (which can be interpreted as signatures of chromospheric upflows) which are mostly not observed in the moss. These observations show a clear correlation between the coronal outflows and the chromosphere and TR underneath, which has not been shown before. Our work strongly suggests that these regions are not separate environments and should be treated together, and that current leading theories of AR outflows, such as the interchange reconnection model, need to take into account the dynamics of the low atmosphere. 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: On the Velocity Drift between Ions in the Solar Atmosphere Authors: Martínez-Sykora, Juan; Szydlarski, Mikolaj; Hansteen, Viggo H.; De Pontieu, Bart Bibcode: 2020ApJ...900..101M Altcode: 2020arXiv200800069M The solar atmosphere is composed of many species, which are populated at different ionization and excitation levels. The upper chromosphere, transition region, and corona are nearly collisionless. Consequently, slippage between, for instance, ions and neutral particles, or interactions between separate species, may play important roles. We have developed a 3D MFMS numerical code (Ebysus) to investigate such effects. Ebysus is capable of treating species (e.g., hydrogen, helium, etc.) and fluids (neutrals, excited and ionized elements) separately, including nonequilibrium ionization, momentum exchange, radiation, thermal conduction, and other complex processes in the solar atmosphere. Treating different species as different fluids leads to drifts between different ions and an electric field that couples these motions. The coupling for two ionized fluids can lead to an anti-phase rotational motion between them. Different ionized species and momentum exchange can dissipate this velocity drift, i.e., convert wave kinetic energy into thermal energy. High-frequency Alfvén waves driven by, e.g., reconnection thought to occur in the solar atmosphere, can drive such multi-ion velocity drifts. 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: Self-consistent 3D radiative magnetohydrodynamic simulations of coronal rain formation and evolution Authors: Kohutova, P.; Antolin, P.; Popovas, A.; Szydlarski, M.; Hansteen, V. H. Bibcode: 2020A&A...639A..20K Altcode: 2020arXiv200503317K Context. Coronal rain consists of cool and dense plasma condensations formed in coronal loops as a result of thermal instability.
Aims: Previous numerical simulations of thermal instability and coronal rain formation have relied on the practice of artificially adding a coronal heating term to the energy equation. To reproduce large-scale characteristics of the corona, the use of more realistic coronal heating prescription is necessary.
Methods: We analysed coronal rain formation and evolution in a three-dimensional radiative magnetohydrodynamic simulation spanning from convection zone to corona which is self-consistently heated by magnetic field braiding as a result of convective motions.
Results: We investigate the spatial and temporal evolution of energy dissipation along coronal loops which become thermally unstable. Ohmic dissipation in the model leads to the heating events capable of inducing sufficient chromospheric evaporation into the loop to trigger thermal instability and condensation formation. The cooling of the thermally unstable plasma occurs on timescales that are comparable to the duration of the individual impulsive heating events. The impulsive heating has sufficient duration to trigger thermal instability in the loop but does not last long enough to lead to coronal rain limit cycles. We show that condensations can either survive and fall into the chromosphere or be destroyed by strong bursts of Joule heating associated with a magnetic reconnection events. In addition, we find that condensations can also form along open magnetic field lines.
Conclusions: We modelled, for the first time, coronal rain formation in a self-consistent 3D radiative magnetohydrodynamic simulation, in which the heating occurs mainly through the braiding and subsequent Ohmic dissipation of the magnetic field. The heating is stratified enough and lasts for long enough along specific field lines to produce the necessary chromospheric evaporation that triggers thermal instability in the corona.

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

The movies are available at https://www.aanda.org Title: The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE) Authors: De Pontieu, Bart; Martínez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo; Cheung, Mark C. M.; Antolin, Patrick Bibcode: 2020ApJ...888....3D Altcode: 2019arXiv190908818D The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed at understanding the physical mechanisms driving the heating of the solar corona and the eruptions that are at the foundation of space weather. MUSE contains two instruments, a multi-slit extreme ultraviolet (EUV) spectrograph and a context imager. It will simultaneously obtain EUV spectra (along 37 slits) and context images with the highest resolution in space (0.″33-0.″4) and time (1-4 s) ever achieved for the transition region (TR) and corona. The MUSE science investigation will exploit major advances in numerical modeling, and observe at the spatial and temporal scales on which competing models make testable and distinguishable predictions, thereby leading to a breakthrough in our understanding of coronal heating and the drivers of space weather. By obtaining spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX 108Å, Fe XXI 108 Å) covering a wide range of TR and coronal temperatures along 37 slits simultaneously, MUSE will be able to “freeze” the evolution of the dynamic coronal plasma. We describe MUSE’s multi-slit approach and show that the optimization of the design minimizes the impact of spectral lines from neighboring slits, generally allowing line parameters to be accurately determined. We also describe a Spectral Disambiguation Code to resolve multi-slit ambiguity in locations where secondary lines are bright. We use simulations of the corona and eruptions to perform validation tests and show that the multi-slit disambiguation approach allows accurate determination of MUSE observables in locations where significant multi-slit contamination occurs. Title: Ebysus: a multi-fluid and multi-species numerical code: on coupling between ionized species Authors: Martínez-Sykora, J.; Szydlarski, M.; Hansteen, V. H. Bibcode: 2019AGUFMSH33D3412M Altcode: The solar atmosphere is composed of many species which are populated at different ionized/excited levels. The upper chromosphere, transition region and corona are nearly collisionless. Consequently, ion-neutral interaction effects or interactions between species may play a role. We have developed a 3D multi-fluid and multi-species numerical code (Ebysus) to investigate this scenario. Ebysus is capable of treating species (e.g., hydrogen, helium etc) and fluids (neutrals, excited and ionized elements) separately including non-equilibrium ionization, momentum exchange, radiation, thermal conduction, and other complex processes in the solar atmosphere. The development of the Ebysus code started from the already existing and advanced 3D radiative MHD Bifrost code designed to model the upper convection zone and outer solar atmosphere. Treating different species as different fluids leads to the presence of an electric coupling due to drifts on different ion velocities. This coupling leads to anti-correlated high frequency waves between the different ionized species. Momentum exchange can dissipate this kinetic energy. Alfven waves and reconnection can produce such high frequency waves and thus result in reconnection instabilities, and in addition reduce the reconnection rate. Title: Ellerman bombs and UV bursts: reconnection at different atmospheric layers Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Nobrega, D. E.; Rouppe van der Voort, L. Bibcode: 2019AGUFMSH13B..06H Altcode: The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst many other dynamical phenomena, the appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed in the wings of H(alpha) and are highly likely to be due to reconnection in the photosphere, below the chromospheric canopy. Signs of the reconnection process are also observed in several other spectral lines, typical of the chromosphere or the transition region. An example are the UV bursts observed in the transition region lines of Si IV and the upper chromospheric lines of Mg II. In this work we analyze high cadence, high resolution coordinated observations between the Swedish 1-m Solar Telescope (SST) and the Interface Region Ima ging Spectrograph (IRIS) spacecraft in order to study the possible relationship between reconnection events at different layers in the atmosphere and, in particular, the timing history between them. H(alpha) images from the SST provide us with the positions, timings and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 1330 (C II, transition region), 1400 (Si IV, transition region) and 2796 (Mg II k, core, upper chromosphere) Ã…, as well as spectroscopy in the far and near ultraviolet from the fast spectrograph raster, allow us to study the possible chromospheric/transition region counterparts of those photospheric EBs. Our main goal is to study whether there is a temporal and spatial relationship between the appearance of an EB and the appearance of a UV burst and the connection of these dynamical phenomena to the appearance of surges in the chromosphere. We also investigate in detail the properties of an extended UV burst and their v ariations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. This scenario is in agreement with the most recent 3D numerical experiments modeling flux emergence. Title: Unfolding Overlappogram Data: Preparing for the COOL-AID instrument on Hi-C FLARE Authors: Winebarger, A. R.; De Pontieu, B.; Cheung, C. M. M.; Martinez-Sykora, J.; Hansteen, V. H.; Testa, P.; Golub, L.; Savage, S. L.; Samra, J.; Reeves, K. Bibcode: 2019AGUFMSH33A..06W Altcode: During a solar flare, energy released in the corona streams to the solar chromosphere, where plasma is heated and then evaporated upward. The magnitude of these velocities and their evolution as a function of time can provide quantitative information on the magnitude of energy released and the method by which it is transported in a solar flare. Measuring these velocities, however, is quite challenging. Typically, they are measured with single slit spectrometers, where light passing through a long but narrow slit is dispersed and emission lines formed across a range of temperatures are observed. The main issue with using single slit spectrometers to make this measurement is that they are rarely pointed at the right place at the right time. Additionally, their fields of view are limited by narrow slit widths, and although rastering can effectively expand the field of view, it does so at the cost of time. This combination means that single slit spectrometers cannot adequately capture the evolution of the flare velocities. On the contrary, slitless spectrometers can make "overlappograms'', which provide both imaging and spectral information over a large field of view. However, spatial information from different spectral lines can overlap in the dispersion direction, making the data difficult to interpret. Furthermore, the spectral resolution of slitless spectrometers are limited and typically worse than single-slit spectrometers, since no line fitting (and hence sub-pixel sampling) is possible.

For the next generation of the High-resolution Coronal Imager (Hi-C) Rocket Experiment, which we are proposing to launch during a solar flare, we are including the COronal OverLapagram - Ancillary Imaging Diagnostics (COOL-AID) instrument. COOL-AID is a slitless spectrometer based on the COronal Spectrographic Imager in the EUV (COSIE) design, but with a narrow passband coating around 12.9 nm (the same passband as the primary Hi-C telescope), a spatial resolution of ~1"x2", and a velocity resolution of ~5 km/s. The goal of the COOL-AID instrument is to determine the velocity associated with the Fe XXI 12.9 nm spectral line during a solar flare. In this talk, we will demonstrate the unfolding method developed by Cheung et al (2019) to determine the velocity information from a simulated COOL-AID data set. 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: Principles Of Heliophysics: a textbook on the universal processes behind planetary habitability Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer, Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David; Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen, Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry; Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George; Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo; Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert; Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp, Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel; Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav; Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George; Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley, Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan; Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian; Woods, Tom; Zapp, Neal Bibcode: 2019arXiv191014022S Altcode: This textbook gives a perspective of heliophysics in a way that emphasizes universal processes from a perspective that draws attention to what provides Earth (and similar (exo-)planets) with a relatively stable setting in which life as we know it can thrive. The book is intended for students in physical sciences in later years of their university training and for beginning graduate students in fields of solar, stellar, (exo-)planetary, and planetary-system sciences. Title: Multi-component Decomposition of Astronomical Spectra by Compressed Sensing Authors: Cheung, Mark C. M.; De Pontieu, Bart; Martínez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo; Antolin, Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young, Peter; MUSE Team Bibcode: 2019ApJ...882...13C Altcode: 2019arXiv190203890C The signal measured by an astronomical spectrometer may be due to radiation from a multi-component mixture of plasmas with a range of physical properties (e.g., temperature, Doppler velocity). Confusion between multiple components may be exacerbated if the spectrometer sensor is illuminated by overlapping spectra dispersed from different slits, with each slit being exposed to radiation from a different portion of an extended astrophysical object. We use a compressed sensing method to robustly retrieve the different components. This method can be adopted for a variety of spectrometer configurations, including single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer mission), and slot spectrometers (which produce overlappograms). 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: 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: Multi-component Decomposition of Astronomical Spectra by Compressed Sensing Authors: Cheung, Mark; De Pontieu, Bart; Martinez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian N.; Hansteen, Viggo; Antolin, Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young, Peter R. Bibcode: 2019AAS...23411603C Altcode: The signal measured by an astronomical spectrometer may be due to radiation from a multi-component mixture of plasmas with a range of physical properties (e.g. temperature, Doppler velocity). Confusion between multiple components may be exacerbated if the spectrometer sensor is illuminated by overlapping spectra dispersed from different slits, with each slit being exposed to radiation from a different portion of an extended astrophysical object. We use a compressed sensing method to robustly retrieve the different components. This method can be adopted for a variety of spectrometer configurations, including single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer mission; MUSE) and slot spectrometers (which produce overlappograms). Title: Linking the Sun to the heliosphere using composition data and modelling: coronal jets as a test case Authors: Wimmer-Schweingruber, Robert F.; Parenti, Susanna; Del Zanna, G.; Edmondson, J.; Giunta, A.; Hansteen, V. H.; Higginson, A.; Lepri, S.; Laming, M.; Lynch, B. J.; von Steiger, R. E.; Wiegelmann, T.; Zambrana Prado, N. Bibcode: 2019shin.confE.231W Altcode: Understanding the formation and evolution of the solar wind is still a priority in the Solar and Heliospheric communities. We expect a significant progress in terms of observations with the upcoming Solar Orbiter mission (launch in 2020), which will provide detailed in-situ measurements of the solar wind and several remote-sensing observations. However, real progress will only be possible if we improve our understanding of the physical link between what measured in-situ and its source regions on the Sun. In this respect, the plasma chemical and charge-state compositions are considered good diagnostic tools. In this paper we present results obtained from an extensive team work aiming at providing solid diagnostics for linking the in-situ and the remote sensing measurements. For our test cases, we selected two periods when a single active region produced, close to its sunspot, jets which had a counterpart signature in the Heliosphere in the form of type-III radio bursts. These jets therefore marked magnetically open regions expanding in the heliosphere. Firstly, we looked for signatures of the open field associated with the active regions in in-situ data from ACE and WIND, finding potential tracers. Secondly, we studied the magnetic topology of the full Sun and Heliosphere with extrapolations of photospheric data and MHD modeling. We found that the open field area is consistent with the source and evolution of the jets, as observed with EUV imagers (SDO/AIA, STEREO/EUVI). Thirdly, we analysed remote sensing EUV spectroscopic observations to measure the plasma conditions (densities, temperatures and chemical composition) whenever available. We then modeled the solar wind and charge state evolution with the solar distance along the open fields to establish a link between the in-situ signatures and the remote sensing observations. We discuss the various difficulties associated with such studies, and highlight how Solar Orbiter measurements can improve them. Title: Ellerman bombs and UV bursts: reconnection at different atmospheric layers? Authors: Hansteen, Viggo Bibcode: 2018csc..confE..71H Altcode: The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst many other phenomena, the appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed in the wings of H(alpha) and are highly likely to be due to reconnection in the photosphere, below the chromospheric canopy. But signs of the reconnection process are also observed in several other spectral lines, typical of the chromosphere or transition region. An example are the UV bursts observed in the transition region lines of Si IV. In this work we analyze high cadence coordinated observations between the 1-m Swedish Solar Telescope and the IRIS spacecraft in order to study the possible relationship between reconnection events at different layers in the atmosphere, and in particular, the timing history between them. High cadence, high resolution H-alpha images from the SST provide us with the positions, timings and trajectories of Ellerman bombs in an emerging flux region. Simultaneous co-aligned IRIS slit-jaw images at 1400 and 1330 A and detailed Si IV spectra from the fast spectrograph raster allow us to study the possible transition region counterparts of those photospheric Ellerman bombs. We complement these observations with numerical models of Ellerman bombs and UV bursts. Our main goal is to study whether there is a temporal and spatial relationship between the appearance of an EB and the appearance of a UV burst. 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: Small-scale Magnetic Flux Emergence in the Quiet Sun Authors: Moreno-Insertis, F.; Martinez-Sykora, J.; Hansteen, V. H.; Muñoz, D. Bibcode: 2018ApJ...859L..26M Altcode: 2018arXiv180600489M Small bipolar magnetic features are observed to appear in the interior of individual granules in the quiet Sun, signaling the emergence of tiny magnetic loops from the solar interior. We study the origin of those features as part of the magnetoconvection process in the top layers of the convection zone. Two quiet-Sun magnetoconvection models, calculated with the radiation-magnetohydrodynamic (MHD) Bifrost code and with domain stretching from the top layers of the convection zone to the corona, are analyzed. Using 3D visualization as well as a posteriori spectral synthesis of Stokes parameters, we detect the repeated emergence of small magnetic elements in the interior of granules, as in the observations. Additionally, we identify the formation of organized horizontal magnetic sheets covering whole granules. Our approach is twofold, calculating statistical properties of the system, like joint probability density functions (JPDFs), and pursuing individual events via visualization tools. We conclude that the small magnetic loops surfacing within individual granules in the observations may originate from sites at or near the downflows in the granular and mesogranular levels, probably in the first 1 or 1.5 Mm below the surface. We also document the creation of granule-covering magnetic sheet-like structures through the sideways expansion of a small subphotospheric magnetic concentration picked up and pulled out of the interior by a nascent granule. The sheet-like structures that we found in the models may match the recent observations of Centeno et al. Title: Small-scale heating events in the solar atmosphere: lifetime, total energy and magnetic properties Authors: Guerreiro, Nuno; Haberreiter, Margit; Hansteen, Viggo; Schmutz, Werner Bibcode: 2018EGUGA..2014516G Altcode: Small-scale heating events (SSHEs) are believed to play a fundamental role in the heating of the solar corona, the pervading redshifts in the transition region, and the acceleration of spicules. We present a method to identify and track SSHEs over their lifetime and apply it to two simulation models. We identify the locations where the energy dissipation is maximum inside the SSHEs volume and we trace the SSHEs by following the spatial and temporal evolution of the maximum energy dissipation inside the SSHEs volume. The method is effective in following the SSHE and allows us to determine their lifetime, total energy and properties of the plasma as well as the magnetic field orientation in the vicinity of the SSHEs. We conclude that the SSHEs that have the potential to heat the corona live less than 4 minutes. Moreover, the typically energy release ranges from 1020 erg to 1024 erg. In addition, the directional change of the magnetic field lines on both sides of the current sheet constituting the SSHEs at the time of the absolute maximum energy dissipation ranges from 5 to 15 degree. This work is very relevant for the interpretation and the potential observational evidence of the SSHEs from upcoming data from the Spectral Imaging of the Coronal Environment instrument (SPICE) and the Extreme UV imager (EUI) onboard the Solar Orbiter Mission. Title: Chromospheric counterparts of solar transition region unresolved fine structure loops Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Hansteen, Viggo H.; De Pontieu, Bart Bibcode: 2018A&A...611L...6P Altcode: 2018arXiv180304415P Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models.

Two movies associated to Fig. 1 are available at https://www.aanda.org Title: 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: Ellerman bombs and UV bursts: reconnection at different atmospheric layers? Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Rouppe van der Voort, L. Bibcode: 2017AGUFMSH43A2801H Altcode: The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst many other phenomena, the appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed in the wings of H(alpha) and are highly likely to be due to reconnection in the photosphere, below the chromospheric canopy. However, signs of the reconnection process are also observed in several other spectral lines, typical of the chromosphere or transition region. An example are the UV bursts observed in the transition region lines of Si IV. In this work we analyze high cadence coordinated observations between the 1-m Swedish Solar Telescope and the IRIS spacecraft in order to study the possible relationship between reconnection events at different layers in the atmosphere, and in particular, the timing history between them. High cadence, high resolution H-alpha images from the SST provide us with the positions, timings and trajectories of Ellerman bombs in an emerging flux region. Simultaneous co-aligned IRIS slit-jaw images at 1400 and 1330 A and detailed Si IV spectra from the fast spectrograph raster allow us to study the transition region counterparts of those photospheric Ellerman bombs. Our main goal is to study whether there is a temporal relationship between the appearance of an EB and the appearance of a UV burst. Eventually we would like to investigate whether reconnection happens at discrete heights, or as a reconnection sheet spanning several layers at the same time. Title: Mass and energy flows between the Solar chromosphere, transition region, and corona Authors: Hansteen, V. H. Bibcode: 2017AGUFMSH41C..01H Altcode: A number of increasingly sophisticated numerical simulations spanning the convection zone to corona have shed considerable insight into the role of the magnetic field in the structure and energetics of the Sun's outer atmosphere. This development is strengthened by the wealth of observational data now coming on-line from both ground based and space borne observatories. We discuss what numerical models can tell us about the mass and energy flows in the region of the upper chromosphere and lower corona, using a variety of tools, including the direct comparison with data and the use of passive tracer particles (so-called 'corks') inserted into the simulated flows. 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: 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: Small-scale heating events in the solar atmosphere. II. Lifetime, total energy, and magnetic properties Authors: Guerreiro, N.; Haberreiter, M.; Hansteen, V.; Schmutz, W. Bibcode: 2017A&A...603A.103G Altcode: Context. Small-scale heating events (SSHEs) are believed to play a fundamental role in understanding the process responsible for heating of the solar corona, the pervading redshifts in the transition region, and the acceleration of spicules.
Aims: We determine the properties of the SSHEs and the atmospheric response to them in 3D magnetohydrodynamics (3D-MHD) simulations of the solar atmosphere.
Methods: We developed a method for identifying and following SSHEs over their lifetime, and applied it to two simulation models. We identified the locations where the energy dissipation is greatest inside the SSHEs volume, and we traced the SSHEs by following the spatial and temporal evolution of the maximum energy dissipation inside the SSHEs volume.
Results: The method is effective in following the SSHEs. We can determine their lifetime, total energy, and properties of the plasma, as well as the magnetic field orientation in the vicinity of the SSHEs.
Conclusions: We determine that the SSHEs that have the potential to heat the corona live less than 4 min, and typically the energy they release ranges from 1020 to 1024 erg. In addition, the directional change of the field lines on the two sides of the current sheet constituting the SSHEs ranges from 5° to 15° at the moment of the absolute maximum energy dissipation. 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: 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: Bombs, jets and flares at the surface and lower atmosphere of the Sun Authors: Hansteen, Viggo Bibcode: 2017psio.confE..58H Altcode: No abstract at ADS Title: Fundamental Physics of the Slow Solar Wind - What do we Know? Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.; Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.; von Steiger, R.; Wang, Y. M. Bibcode: 2016AGUFMSH42A..01O Altcode: Fundamental physical properties of the slow solar wind (SSW), such as density, temperature, outflow speed, heavy ion abundances and charges states were obtained from in-situ measurements at 1AU in the past from WIND, ACE, and other spacecraft. Plasma and magnetic field measurement are available as close as 0.3 AU from Helios data, Spektr-R, and MESSENGER spacecraft. Remote sensing spectroscopic measurements are available in the corona and below from SOHO/UVCS, Hinode, and other missions. One of the major objectives of the Solar Orbiter and Solar Probe Plus missions is to study the sources of the SSW close to the Sun. The present state of understanding of the physics of the SSW is based on the combination of the existing observations, theoretical and numerical 3D MHD and multi-fluid models, that connect between the SSW sources in the corona and the heliosphere. Recently, hybrid models that combine fluid electrons and kinetic ions of the expanding solar wind were developed, and provide further insights of the local SSW plasma heating processes that related to turbulent magnetic fluctuations spectra and kinetic ion instabilities observed in the SSW plasma. These models produce the velocity distribution functions (VDFs) of the protons and heavier ions as well as the ion anisotropic temperatures. I will discuss the results of the above observations and models, and review the current status of our understanding of the fundamental physics of the SSW. I will review the open questions, and discuss how they could be addressed with near future observations and models. Title: Slow Solar Wind: Observations and Modeling Authors: Abbo, L.; Ofman, L.; Antiochos, S. K.; Hansteen, V. H.; Harra, L.; Ko, Y. -K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.; von Steiger, R.; Wang, Y. -M. Bibcode: 2016SSRv..201...55A Altcode: 2016SSRv..tmp...34A While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations. 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: Ellerman Bombs and IRIS Bombs; In the photosphere and above Authors: Hansteen, Viggo; Archontis, V. Bibcode: 2016usc..confE.118H Altcode: The lower Solar atmosphere, consisting of the photosphere and chromosphere, can occasionally show violent activity more often associated with the magnetically dominated outer layers of the Sun; the upper chromosphere, transition region and corona. However, in regions of strong flux emergence, where Solar active regions are being formed, one can see evidence of photospheric reconnection as the field struggles to emerge through the non-buoyant photosphere and expand into the atmosphere above. Ellerman bombs, short lived, brightness enhancements in the outer wings of strong optical lines are thought to be a result of such reconnection. Observations made with the NASA's Interface Region Imaging Spectrograph, showed similar 'UV bursts' in lines usually associated with the outer Solar atmosphere, while at the same time clearly being situated below much cooler gas. We here present a numerical model of flux emergence in which both Ellerman bombs and perhaps IRIS bombs (UV bursts) are naturally and copiously produced. Title: High Spatial Resolution Fe XII Observations of Solar Active Regions Authors: Testa, Paola; De Pontieu, Bart; Hansteen, Viggo Bibcode: 2016ApJ...827...99T Altcode: 2016arXiv160604603T We use UV spectral observations of active regions with the Interface Region Imaging Spectrograph (IRIS) to investigate the properties of the coronal Fe xii 1349.4 Å emission at unprecedented high spatial resolution (∼0.33″). We find that by using appropriate observational strategies (I.e., long exposures, lossless compression), Fe xii emission can be studied with IRIS at high spatial and spectral resolution, at least for high-density plasma (e.g., post-flare loops and active region moss). We find that upper transition region (TR; moss) Fe xii emission shows very small average Doppler redshifts ({v}{{D}} ∼ 3 km s-1) as well as modest non-thermal velocities (with an average of ∼24 km s-1 and the peak of the distribution at ∼15 km s-1). The observed distribution of Doppler shifts appears to be compatible with advanced three-dimensional radiative MHD simulations in which impulsive heating is concentrated at the TR footpoints of a hot corona. While the non-thermal broadening of Fe xii 1349.4 Å peaks at similar values as lower resolution simultaneous Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) measurements of Fe xii 195 Å, IRIS observations show a previously undetected tail of increased non-thermal broadening that might be suggestive of the presence of subarcsecond heating events. We find that IRIS and EIS non-thermal line broadening measurements are affected by instrumental effects that can only be removed through careful analysis. Our results also reveal an unexplained discrepancy between observed 195.1/1349.4 Å Fe xii intensity ratios and those predicted by the CHIANTI atomic database. 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: Detection and characterization of small-scale heating events in the solar atmosphere from 3D-MHD simulations and their potential role in coronal heating Authors: Guerreiro, Nuno; Haberreiter, Margit; Schmutz, Werner; Hansteen, Viggo Bibcode: 2016cosp...41E.750G Altcode: Aiming at better understanding the mechanism(s) responsible for the coronal heating we focus on analyzing the properties of the magnetically generated small-scale heating events (SSHEs) in the solar atmosphere. We present a comprehensive method to detect and follow SSHEs over time in 3D-MHD simulations of the solar atmosphere. Applying the method we are able to better understand the properties of the SSHEs and how the plasma in their vicinity respond to them. We study the lifetime, energy and spectral signatures and show that the energy flux dissipated by them is enough to heat the corona. Ultimately, these results will be important for the coordinated scientific exploration of SPICE and EUI along with other instruments on board solar orbiter. 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: 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: Characterization of small-scale heating events in the solar atmosphere from 3D-MHD simulations and their potential role in coronal heating Authors: Guerreiro, Nuno; Haberreiter, Margit; Hansteen, Viggo; Schmutz, Werner Bibcode: 2016EGUGA..1814407G Altcode: Aiming at better understanding the mechanism(s) responsible for the coronal heating and the ubiquitous redshifts observed in the lower transition region we focus on analyzing the properties of small-scale heating events (SSHEs) in the solar atmosphere. We present a comprehensive method to follow SSHEs over time in 3D-MHD simulations of the solar atmosphere. Applying the method we are able to better understand the properties of the SSHEs and how the plasma in their vicinity respond to them. We present results for the lifetime, energy and spectral signatures of the SSHEs. Ultimately, these results will be important for the coordinated scientific exploration of SPICE and EUI along with other interments on board solar orbiter. ​ 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: 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: Time Dependent Nonequilibrium Ionization of Transition Region Lines Observed with IRIS Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo H.; Gudiksen, Boris Bibcode: 2016ApJ...817...46M Altcode: 2015arXiv151200865M The properties of nonstatistical equilibrium ionization of silicon and oxygen ions are analyzed in this work. We focus on five solar targets (quiet Sun; coronal hole; plage; quiescent active region, AR; and flaring AR) as observed with the Interface Region Imaging Spectrograph (IRIS). IRIS is best suited for this work owing to the high cadence (up to 0.5 s), high spatial resolution (up to 0.″32), and high signal-to-noise ratios for O IV λ1401 and Si IV λ1402. We find that the observed intensity ratio between lines of three times ionized silicon and oxygen ions depends on their total intensity and that this correlation varies depending on the region observed (quiet Sun, coronal holes, plage, or active regions) and on the specific observational objects present (spicules, dynamic loops, jets, microflares, or umbra). In order to interpret the observations, we compare them with synthetic profiles taken from 2D self-consistent radiative MHD simulations of the solar atmosphere, where the statistical equilibrium or nonequilibrium treatment of silicon and oxygen is applied. These synthetic observations show vaguely similar correlations to those in the observations, I.e., between the intensity ratios and their intensities, but only in the nonequilibrium case do we find that (some of) the observations can be reproduced. We conclude that these lines are formed out of statistical equilibrium. We use our time-dependent nonequilibrium ionization simulations to describe the physical mechanisms behind these observed properties. Title: Slow Solar Wind: Observable Characteristics for Constraining Modelling Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.; Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.; von Steiger, R.; Wang, Y. M. Bibcode: 2015AGUFMSH11F..03O Altcode: The Slow Solar Wind (SSW) origin is an open issue in the post SOHO era and forms a major objective for planned future missions such as the Solar Orbiter and Solar Probe Plus.Results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in-situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW.Advances models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.Nevertheless, there are still debated questions such as:What are the source regions of SSW? What are their contributions to the SSW?Which is the role of the magnetic topology in corona for the origin, acceleration and energy deposition of SSW?Which are the possible acceleration and heating mechanisms for the SSW?The aim of this study is to present the insights on the SSW origin and formationarisen during the discussions at the International Space Science Institute (ISSI) by the Team entitled ''Slowsolar wind sources and acceleration mechanisms in the corona'' held in Bern (Switzerland) in March2014--2015. The attached figure will be presented to summarize the different hypotheses of the SSW formation. Title: Properties of moss emission from joint FeXII IRIS and Hinode observations of active region plasma Authors: Testa, P.; De Pontieu, B.; Hansteen, V. H. Bibcode: 2015AGUFMSH31D..06T Altcode: IRIS provides unprecedented high resolution observations of the solar chromosphere and transition region. Joint with Hinode XRT and EIS, and SDO/AIA, these observations cover from the upper photosphere to the corona and provide tight constraints on the mechanisms of energy transport and heating of the plasma to coronal temperatures. We present new IRIS and Hinode coronal studies of the corona in non-flaring conditions, and compare the spectral line properties of FeXII emission observed with EIS and IRIS in active region moss. We will discuss the implications for the heating of hot coronal loops in the core of active regions. Title: Characterisation of small-scale heating events in the solar atmosphere from 3D MHD simulations and their potential role in coronal heating Authors: Haberreiter, M.; Guerreiro, N.; Hansteen, V. H.; Schmutz, W. K. Bibcode: 2015AGUFMSH31C2442H Altcode: The physical mechanism that heats the solar corona is one of the still open science questions in solar physics. One of the proposed mechanism for coronal heating are nanoflares. To investigate their role in coronal heating we study the properties of the small-scale heating events in the solar atmosphere using 3D MHD simulations. We present a method to identify and track these heating events in time which allows us to study their life time, energy, and spectral signatures. These spectal signatures will be compared with available spectrosopic observations obtained with IRIS and SUMER. Ultimately, these results will be important for the coordinated scientific exploitation of SPICE and EUI along with other instruments onboard Solar Orbiter to address the coronal heating problem. 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: Small-scale Heating Events in the Solar Atmosphere. I. Identification, Selection, and Implications for Coronal Heating Authors: Guerreiro, N.; Haberreiter, M.; Hansteen, V.; Schmutz, W. Bibcode: 2015ApJ...813...61G Altcode: We present a comprehensive method to analyze small-scale heating events in detail in a 3D magnetohydrodynamics simulation for quiet-Sun conditions. The method determines the number, volume, and some general geometric properties of the small-scale heating events at different instants in a simulation with a volume of 16 × 8 × 16 Mm3, spanning from the top of the convection zone to the corona. We found that there are about 104 small-scale heating events at any instant above the simulated area of 128 Mm2. They occur mainly at heights between 1.5 and 3.0 Mm. We determine the average value of their projected vertical extent, which ranges from 375 to 519 km over time, and we show that height, volume, and energy distribution of the events at any instant resemble power laws. Finally, we demonstrate that larger heating structures are a combination of much smaller heating events and that small-scale heating events dissipate enough energy to maintain the coronal energetic balance at any instant. 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: 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: Emergence of Granular-sized Magnetic Bubbles through the Solar Atmosphere. II. Non-LTE Chromospheric Diagnostics and Inversions Authors: de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Bellot-Rubio, Luis; Ortiz, Ada Bibcode: 2015ApJ...810..145D Altcode: 2015arXiv150303846D Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photosphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca ii 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain information on the line-of-sight stratifications of temperature, velocity, and the magnetic field. Our analysis explains the peculiar Ca ii 8542 Å profiles observed in the flux emerging region. Additionally, we derive detailed temperature and velocity maps describing the ascent of a magnetic bubble from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere. 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: 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: 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: Synthesized Spectra of Optically Thin Emission Lines Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H.; De Pontieu, B. Bibcode: 2015ApJ...802....5O Altcode: In recent years realistic 3D numerical models of the solar atmosphere have become available. The models attempt to recreate the solar atmosphere and mimic observations in the best way, in order to make it possible to couple complicated observations with physical properties such as the temperatures, densities, velocities, and magnetic fields. We here present a study of synthetic spectra created using the Bifrost code in order to assess how well they fit with previously taken solar data. A study of the synthetic intensity, nonthermal line widths, Doppler shifts, and correlations between any two of these three components of the spectra first assuming statistical equilibrium is made, followed by a report on some of the effects nonequilibrium ionization will have on the synthesized spectra. We find that the synthetic intensities compare well with the observations. The synthetic observations depend on the assumed resolution and point-spread function (PSF) of the instrument, and we find a large effect on the results, especially for intensity and nonthermal line width. The Doppler shifts produce the reported persistent redshifts for the transition region (TR) lines and blueshifts for the upper TR and corona lines. The nonthermal line widths reproduce the well-known turnoff point around (2-3) × 105 K, but with much lower values than those observed. The nonthermal line widths tend to increase with decreasing assumed instrumental resolution, also when nonequilibrium ionization is included. Correlations between the nonthermal line width of any two TR line studies as reported by Chae et al. are reproduced, while the correlations of intensity to line width are reproduced only after applying a PSF to the data. Doppler shift correlations reported by Doschek for the TR lines and correlations of Doppler shift to nonthermal line width of the Fe xii 19.5 line reported by Doschek et al. are reproduced. 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: 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: Solar Flaring Activity and Coronal Heating. Authors: Archontis, V.; Hansteen, V. H. Bibcode: 2014AGUFMSH53D..02A Altcode: We report on the formation of small solar flares produced by patchy magnetic reconnection between interacting magnetic loops. Three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiments were performed, where a uniform magnetic flux sheet was injected into a fully developed convective layer. The gradual emergence of the field into the solar atmosphere results in a network of magnetic loops, which interact dynamically forming current layers at their interfaces. The formation and ejection of plasmoids out of the current layers leads to patchy reconnection and the spontaneous formation of several small (size ≈1-2 Mm) flares. We find that these flares are short-lived (30 s-3 minutes) bursts of energy in the range O(1025-1027) erg, which is basically the nanoflare-microflare range. Their persistent formation and co-operative action and evolution leads to recurrent emission of fast EUV/X-ray jets and considerable plasma heating in the active corona. 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: 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: 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: Clusters of Small Eruptive Flares Produced by Magnetic Reconnection in the Sun Authors: Archontis, V.; Hansteen, V. Bibcode: 2014ApJ...788L...2A Altcode: 2014arXiv1405.6420A We report on the formation of small solar flares produced by patchy magnetic reconnection between interacting magnetic loops. A three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiment was performed, where a uniform magnetic flux sheet was injected into a fully developed convective layer. The gradual emergence of the field into the solar atmosphere results in a network of magnetic loops, which interact dynamically forming current layers at their interfaces. The formation and ejection of plasmoids out of the current layers leads to patchy reconnection and the spontaneous formation of several small (size ≈1-2 Mm) flares. We find that these flares are short-lived (30 s-3 minutes) bursts of energy in the range O(1025-1027) erg, which is basically the nanoflare-microflare range. Their persistent formation and co-operative action and evolution leads to recurrent emission of fast EUV/X-ray jets and considerable plasma heating in the active corona. 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: Emergence of Granular-sized Magnetic Bubbles through the Solar Atmosphere. I. Spectropolarimetric Observations and Simulations Authors: Ortiz, Ada; Bellot Rubio, Luis R.; Hansteen, Viggo H.; de la Cruz Rodríguez, Jaime; Rouppe van der Voort, Luc Bibcode: 2014ApJ...781..126O Altcode: 2013arXiv1312.5735O We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.''14. Simultaneous full Stokes observations of the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca II 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca II 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s-1 and expands at a horizontal speed of 4 km s-1. In about 3.5 minutes it travels some 1100 km to reach the mid chromosphere, implying an average ascent speed of 5.2 km s-1. The maximum separation attained by the magnetic legs is 6.''6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90° in the magnetic bubble interior, along with temperature deficits of up to 250 K at log τ = -2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes. Title: Realistic 3D simulations of a small flare resulting from flux emergence Authors: Hansteen, Viggo; Archontis, Vasilis Bibcode: 2014cosp...40E1151H Altcode: We have performed three-dimensional (3d) magnetohydrodynamic simulations of magnetic flux emergence in a model that spans the convection zone and into the outer solar atmosphere with the Bifrost code. This is a ``realistic'' model, in the sense that the parameters and physical effects that control the atmosphere can be used to produce diagnostics that can be directly compared with observations. The emerging flux leads to the formation of several current sheets as it rises into the modeled corona. Multiple plasmoids are ejected from the current sheets. Reconnection occurs impulsively, producing heating and fast outflows near or in the current sheet, arranged in a manner reminiscent of the CSHKP flare model. This includes a cusp like arcade and a flux rope in the lower atmospere underneath the current sheet. We discuss the evolution of the model and several synthetic observables. 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: Identification and characterization of small-scale heating events in the solar atmosphere from 3D MHD simulations Authors: Guerreiro, Nuno; Haberreiter, Margit; Schmutz, Werner; Hansteen, Viggo Bibcode: 2014cosp...40E1095G Altcode: We studied the properties of small scale heating events (in the nano and micro-flare regime) in the solar atmosphere using existing 3D MHD simulations. We put forward a method for event identification and categorization. We determine the spatial extend of the events and their frequency and energy distributions. These results aim at improving the understanding of small scale heating events and their role for the heating of the solar corona. 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: IRIS observations of transition region unresolved fine structure Authors: Hansteen, Viggo; De Pontieu, Bart Bibcode: 2014cosp...40E1152H Altcode: The Interface Region Imaging Spectrograph was launched on 28-June-2013 and has been obtaining high resolution images and spectra in the far and near ultraviolet since 17-July-2013 covering temperatures from the photosphere into the corona. We analyze the presence of a multitude of short, relatively cool transition region loops as visible at the solar limb in slit jaw images dominated by C II 1335 Angstrom and Si IV 1402 Angstrom emission. We study the dynamical nature and temperature evolution of these loops and investigate how they relate to the so-called "unresolved fine structure" (UFS) that has been proposed as a dominant source of transition region emission, but that has not yet been directly observed to date. We will also discuss the physical nature of this emission. 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: Detecting Nanoflare Heating Events in Subarcsecond Inter-moss Loops Using Hi-C Authors: Winebarger, Amy R.; Walsh, Robert W.; Moore, Ronald; De Pontieu, Bart; Hansteen, Viggo; Cirtain, Jonathan; Golub, Leon; Kobayashi, Ken; Korreck, Kelly; DeForest, Craig; Weber, Mark; Title, Alan; Kuzin, Sergey Bibcode: 2013ApJ...771...21W Altcode: The High-resolution Coronal Imager (Hi-C) flew aboard a NASA sounding rocket on 2012 July 11 and captured roughly 345 s of high-spatial and temporal resolution images of the solar corona in a narrowband 193 Å channel. In this paper, we analyze a set of rapidly evolving loops that appear in an inter-moss region. We select six loops that both appear in and fade out of the Hi-C images during the short flight. From the Hi-C data, we determine the size and lifetimes of the loops and characterize whether these loops appear simultaneously along their length or first appear at one footpoint before appearing at the other. Using co-aligned, co-temporal data from multiple channels of the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, we determine the temperature and density of the loops. We find the loops consist of cool (~105 K), dense (~1010 cm-3) plasma. Their required thermal energy and their observed evolution suggest they result from impulsive heating similar in magnitude to nanoflares. Comparisons with advanced numerical simulations indicate that such dense, cold and short-lived loops are a natural consequence of impulsive magnetic energy release by reconnection of braided magnetic field at low heights in the solar atmosphere. 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: Observing Coronal Nanoflares in Active Region Moss Authors: Testa, Paola; De Pontieu, Bart; Martínez-Sykora, Juan; DeLuca, Ed; Hansteen, Viggo; Cirtain, Jonathan; Winebarger, Amy; Golub, Leon; Kobayashi, Ken; Korreck, Kelly; Kuzin, Sergey; Walsh, Robert; DeForest, Craig; Title, Alan; Weber, Mark Bibcode: 2013ApJ...770L...1T Altcode: 2013arXiv1305.1687T The High-resolution Coronal Imager (Hi-C) has provided Fe XII 193Å images of the upper transition region moss at an unprecedented spatial (~0.''3-0.''4) and temporal (5.5 s) resolution. The Hi-C observations show in some moss regions variability on timescales down to ~15 s, significantly shorter than the minute-scale variability typically found in previous observations of moss, therefore challenging the conclusion of moss being heated in a mostly steady manner. These rapid variability moss regions are located at the footpoints of bright hot coronal loops observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly in the 94 Å channel, and by the Hinode/X-Ray Telescope. The configuration of these loops is highly dynamic, and suggestive of slipping reconnection. We interpret these events as signatures of heating events associated with reconnection occurring in the overlying hot coronal loops, i.e., coronal nanoflares. We estimate the order of magnitude of the energy in these events to be of at least a few 1023 erg, also supporting the nanoflare scenario. These Hi-C observations suggest that future observations at comparable high spatial and temporal resolution, with more extensive temperature coverage, are required to determine the exact characteristics of the heating mechanism(s). Title: The Cycling of Material between the Solar Corona and Chromosphere Authors: Guerreiro, N.; Hansteen, Viggo; De Pontieu, B. Bibcode: 2013ApJ...769...47G Altcode: Observations of transition region emission lines reveal the presence of redshifts in lines formed from the top of the chromosphere up to temperatures of about 2.5 × 105 K and blueshifts for temperatures above that. However, it is doubtful that the apparent large downward flows in the lower transition region represents an emptying of the corona, so some mechanism must be responsible for maintaining the mass balance between the corona and the lower atmospheric layers. We use a three-dimensional magnetohydrodynamics code to study the cycling of mass between the corona, transition region, and chromosphere by adding a tracer fluid to the simulation in various temperature intervals in the transition region. We find that most of the material seen in transition region emission lines formed at temperatures below 3 × 105 K is material that has been rapidly heated from chromospheric temperatures and thereafter is pushed down as it cools. This implies that the bulk of transition region material resides in small loops. In these loops, the density is high and radiative cooling is efficient. Title: Non-equilibrium Ionization Effects on the Density Line Ratio Diagnostics of O IV Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H. Bibcode: 2013ApJ...767...43O Altcode: The dynamic timescales in the solar atmosphere are shorter than the ionization and recombination times of many ions used for line ratio diagnostics of the transition region and corona. The long ionization and recombination times for these ions imply that they can be found far from their equilibrium temperatures, and spectroscopic investigations require more care before being trusted in giving correct information on local quantities, such as density and temperature. By solving the full time-dependent rate equations for an oxygen model atom in the three-dimensional numerical model of the solar atmosphere generated by the Bifrost code, we are able to construct synthetic intensity maps and study the emergent emission. We investigate the method of electron density diagnostics through line ratio analysis of the O IV 140.1 nm to the 140.4 nm ratio, the assumptions made in carrying out the diagnostics, and the different interpretations of the electron density. The results show big discrepancies between emission in statistical equilibrium and emission where non-equilibrium (NEQ) ionization is treated. Deduced electron densities are up to an order of magnitude higher when NEQ effects are accounted for. The inferred electron density is found to be a weighted mean average electron density along the line of sight and has no relation to the temperature of emission. This study shows that numerical modeling is essential for electron density diagnostics and is a valuable tool when the ions used for such studies are expected to be out of ionization equilibrium. Though this study has been performed on the O IV ion, similar results are also expected for other transition region ions. 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: Non-equilibrium Ionization in the Bifrost Stellar Atmosphere Code Authors: Olluri, K.; Gudiksen, B. V.; Hansteen, V. H. Bibcode: 2013AJ....145...72O Altcode: The chromosphere and transition region have for the last 20 years been known to be quite dynamic layers of the solar atmosphere, characterized by timescales shorter than the ionization equilibrium timescales of many of the ions dominating emission in these regions. Due to the fast changes in the properties of the atmosphere, long ionization and recombination times can lead these ions to being found far from their equilibrium temperatures. A number of the spectral lines that we observe can therefore not be expected a priori to reflect information about local quantities such as the density or temperature, and interpreting observations requires numerical modeling. Modeling the ionization balance is computationally expensive and has earlier only been done in one dimension. However, one-dimensional models can primarily be used to investigate the possible importance of a physical effect, but cannot verify or disprove the importance of that effect in the fully three-dimensional solar atmosphere. Here, using the atomic database package DIPER, we extend one-dimensional methods and implement a solver for the rate equations of the full three-dimensional problem, using the numerical code Bifrost. We present our implementation and report on a few test cases. We also report on studies of the important C IV and Fe XII ions in a semi-realistic two-dimensional solar atmosphere model, focusing on differences between statistical equilibrium and non-equilibrium ionization results. Title: Solar Wind Models from the Chromosphere to 1 AU Authors: Hansteen, Viggo H.; Velli, Marco Bibcode: 2013mspc.book...89H Altcode: No abstract at ADS Title: Sources of Solar Wind at Solar Minimum: Constraints from Composition Data Authors: Zurbuchen, Thomas H.; von Steiger, Rudolf; Gruesbeck, Jacob; Landi, Enrico; Lepri, Susan T.; Zhao, Liang; Hansteen, Viggo Bibcode: 2013mspc.book...41Z Altcode: No abstract at ADS 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: Solar Wind Models from the Chromosphere to 1 AU Authors: Hansteen, Viggo H.; Velli, Marco Bibcode: 2012SSRv..172...89H Altcode: 2012SSRv..tmp...35H Recent models of the fast solar wind are characterized by low coronal electron temperatures while proton, α-particle, and minor ion temperatures are expected to be quite high and generally anisotropic, including large temperatures perpendicular to the magnetic field and parallel beams. This entails that the electric field should be relatively unimportant and that solar wind outflows with both high asymptotic flow speeds but maintaining a low mass flux should be a natural outcome of plasma expansion along open polar magnetic field lines. In this chapter we will explain why such changes with respect to the classical, electron thermally driven solar wind have come about and outline the most important remaining concerning the astrophysics of coronal winds. The progress we have seen in the last decade is largely due observations made with instruments onboard Ulysses (McComas et al. in Space Sci. Rev. 72:93, 1995) and SOHO (Fleck et al. in The SOHO Mission, Kluwer, Dordrecht, 1995). These observations have spawned a new understanding of solar wind energetics, and the consideration of the chromosphere, corona, and solar wind as a unified system. We will begin by giving our own, highly biased, "pocket history" of solar wind theory highlighting the problems that had to be resolved in order to make the original Parker formulation of thermally driven winds conform with observational results. Central to this discussion are questions of how the wind's asymptotic flow speed and mass flux are set, but we will also touch upon higher order moments such as the ion and electron temperatures and heat fluxes as well as the possible role of Alfvén waves and particle effects in driving the solar wind outflow. Solar wind scaling laws will be discussed in the context of the origin of slow and fast wind streams. Title: Sources of Solar Wind at Solar Minimum: Constraints from Composition Data Authors: Zurbuchen, Thomas H.; von Steiger, Rudolf; Gruesbeck, Jacob; Landi, Enrico; Lepri, Susan T.; Zhao, Liang; Hansteen, Viggo Bibcode: 2012SSRv..172...41Z Altcode: 2012SSRv..tmp...25Z In this discussion of observational constraints on the source regions and acceleration processes of solar wind, we will focus on the ionic composition of the solar wind and the distribution of charge states of heavy elements such as oxygen and iron. We first focus on the now well-known bi-modal nature of solar wind, which dominates the heliosphere at solar minimum: Compositionally cool solar wind from polar coronal holes over-expands, filling a much larger solid angle than the coronal holes on the Sun. We use a series of remote and in-situ characteristics to derive a global geometric expansion factor of ∼5. Slower, streamer-associated wind is located near the heliospheric current sheet with a width of 10-20°, but in a well-defined band with a geometrically small transition width. We then compute charge states under the assumption of thermal electron distributions and temperature, velocity, and density profiles predicted by a recent solar wind model, and conclude that the solar wind originates from a hot source at around 1 million K, characteristic of the closed corona. 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: 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: Spicules type I and type II: numerical simulations and their role in coronal heating Authors: Hansteen, Viggo Bibcode: 2012cosp...39..719H Altcode: 2012cosp.meet..719H When viewed on the limb the chromosphere shows a great amount of structure, both horizontally and vertically: the chromospheric plasma is clearly ordered by the magnetic field. This is most particularly true for the upper chromosphere where one of the most characteristic features are the so called spicules, thin jets of chromspheric plasma reaching up to heights of 10,000 km or more above the photosphere. Though spicules were recognized as early as 1877 by Angelo Secchi, progress on understanding their physical basis has proved slow. Progress has been hampered by their small physical scale, short lifetimes, and the large number of superpositions when viewed on the limb. Recently however, the launch of the Hinode satellite with its large optical telescope SOT and the application of new reduction techniques such as MOMFBD to ground based solar data, has led to a much improved observational situation and thus revived the field. This is also true of numerical simulations which finally are approaching a level of sophistication good enough to study the coronal heating problem as well as spicule dynamics and energetics. In this talk we will discuss recent observations and simulations of spicules and comment on their relevance for coronal heating and solar wind acceleration. Title: Two-dimensional Radiative Magnetohydrodynamic Simulations of the Importance of Partial Ionization in the Chromosphere Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo Bibcode: 2012ApJ...753..161M Altcode: 2012arXiv1204.5991M The bulk of the solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles likely have significant consequences for the thermodynamics of the chromospheric plasma. We investigate the importance of introducing neutral particles into the MHD equations using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span the solar atmosphere from the upper layers of the convection zone to the low corona, and solve the full MHD equations with non-gray and non-LTE radiative transfer, and thermal conduction along the magnetic field. The effects of partial ionization are implemented using the generalized Ohm's law, i.e., we consider the effects of the Hall term and ambipolar diffusion in the induction equation. The approximations required in going from three fluids to the generalized Ohm's law are tested in our simulations. The Ohmic diffusion, Hall term, and ambipolar diffusion show strong variations in the chromosphere. These strong variations of the various magnetic diffusivities are absent or significantly underestimated when, as has been common for these types of studies, using the semi-empirical VAL-C model as a basis for estimates. In addition, we find that differences in estimating the magnitude of ambipolar diffusion arise depending on which method is used to calculate the ion-neutral collision frequency. These differences cause uncertainties in the different magnetic diffusivity terms. In the chromosphere, we find that the ambipolar diffusion is of the same order of magnitude or even larger than the numerical diffusion used to stabilize our code. As a consequence, ambipolar diffusion produces a strong impact on the modeled atmosphere. Perhaps more importantly, it suggests that at least in the chromospheric domain, self-consistent simulations of the solar atmosphere driven by magnetoconvection can accurately describe the impact of the dominant form of resistivity, i.e., ambipolar diffusion. This suggests that such simulations may be more realistic in their approach to the lower solar atmosphere (which directly drives the coronal volume) than previously assumed. 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: Non-equilibrium ionization in 3D numerical models Authors: Olluri, Kosovare; Gudiksen, Boris; Hansteen, Viggo Bibcode: 2012decs.confE.118O Altcode: The dynamic timescales in the chromosphere and transition region have been observed to be much smaller then the ionization equilibration timescales of many ions found in the region. Due to the fast changes in the properties of the atmosphere, long ionization- and recombination times may lead to ions being found far from their equilibrium temperatures. Spectroscopic investigations therefore needs to be interpreted with the help of numerical modeling in order to produce reliable results. By solving the rate equations within a realistic MHD simulation of the solar atmosphere, we are able to follow the ionization balance, and study the non equilibrium effects of the emitting gas. Due top lack of computation power, this has previously been done in simple 1D, but because of the many free parameters in these models, their conclusions are not free of uncertainties. The resent development in computing technology and atmospheric modeling makes it possible to study the full 3D effect of non equilibrium ionization. With the solar atmosphere model Bifrost, we have a 3D platform for calculating and following the ionization degree of important atoms of high abundances in the solar atmosphere. We will present our implementation, and a study of the carbon IV 1549 Å , Iron XII 195 Å, Oxygen IV 1399 Å and 1401 Å lines in 2D. 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: Importance of the partial ionization in the chromosphere using 2D radiative-MHD simulations Authors: Martinez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo H. Bibcode: 2012decs.confE..81M Altcode: The bulk of the solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles will have significant consequences for the thermodynamics of the chromospheric plasma. We investigate the importance of introducing neutral particles into the MHD equations using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span the solar atmosphere from upper layers of the convection zone to the low corona, and solve the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along the magnetic field. The effects of partial ionization are implemented using the generalized Ohm's law, i.e., we consider the effects of the Hall and ambipolar diffusion in the induction equation. The ohmic, Hall, and ambipolar diffusivities show variations of several orders of magnitude in the chromosphere. These strong variations of the various magnetic diffusivities are absent and significantly underestimated when using the semi-empirical VAL-C model as a basis for estimates. We find that in the chromosphere, the ambipolar diffusion is of the same order of magnitude or even larger than the numerical diffusion used to stabilize our code. As result of this, we can study the effects of it in the simulations. The ambipolar diffusion produces strong impact on the chromosphere changing the thermal properties, dynamics and magnetic field evolution. 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: Overview of simulations and observations of the coupling between solar regions Authors: Hansteen, Viggo Bibcode: 2012decs.confE..30H Altcode: We will discuss simulations and observations of the outer solar atmosphere, in particular the chromosphere, transition region and corona, as a coupled system. Focus will be on how the atmosphere is energized, how mass and energy is transferred between various regions, and what the mechanisms are that couple the various regions. Current simulations are described with an eye towards the physics included, and a discussion of what is missing, e.g. the topology of magnetic fields studied, large physical scales, non-equilibrium ionization and generalized Ohm's law. The impact of the physical assumptions is assessed. Specific synthetic observables from the IRIS instrument are presented and their diagnostic value is discussed. Title: Forward Modeling of Emission in Solar Dynamics Observatory/Atmospheric Imaging Assembly Passbands from Dynamic Three-dimensional Simulations Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Testa, Paola; Hansteen, Viggo Bibcode: 2011ApJ...743...23M Altcode: 2011arXiv1109.0704M It is typically assumed that emission in the passbands of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) is dominated by single or several strong lines from ions that under equilibrium conditions are formed in a narrow range of temperatures. However, most SDO/AIA channels also contain contributions from lines of ions that have formation temperatures that are significantly different from the "dominant" ion(s). We investigate the importance of these lines by forward modeling the emission in the SDO/AIA channels with three-dimensional radiative MHD simulations of a model that spans the upper layer of the convection zone to the low corona. The model is highly dynamic. In addition, we pump a steadily increasing magnetic flux into the corona, in order to increase the coronal temperature through the dissipation of magnetic stresses. As a consequence, the model covers different ranges of coronal temperatures as time progresses. The model covers coronal temperatures that are representative of plasma conditions in coronal holes and quiet Sun. The 131, 171, and 304 Å AIA passbands are found to be the least influenced by the so-called non-dominant ions, and the emission observed in these channels comes mostly from plasma at temperatures near the formation temperature of the dominant ion(s). On the other hand, the other channels are strongly influenced by the non-dominant ions, and therefore significant emission in these channels comes from plasma at temperatures that are different from the "canonical" values. We have also studied the influence of non-dominant ions on the AIA passbands when different element abundances are assumed (photospheric and coronal), and when the effects of the electron density on the contribution function are taken into account. 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: On the Origin of the Type II Spicules: Dynamic Three-dimensional MHD Simulations Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Moreno-Insertis, Fernando Bibcode: 2011ApJ...736....9M Altcode: 2010arXiv1011.4703M Recent high temporal and spatial resolution observations of the chromosphere have forced the definition of a new type of spicule, "type II's," that are characterized by rising rapidly, having short lives, and by fading away at the end of their lifetimes. Here, we report on features found in realistic three-dimensional simulations of the outer solar atmosphere that resemble the observed type II spicules. These features evolve naturally from the simulations as a consequence of the magnetohydrodynamical evolution of the model atmosphere. The simulations span from the upper layer of the convection zone to the lower corona and include the emergence of a horizontal magnetic flux. The state-of-art Oslo Staggered Code is used to solve the full MHD equations with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field lines. We describe in detail the physics involved in a process which we consider a possible candidate for the driver mechanism that produces type II spicules. The modeled spicule is composed of material rapidly ejected from the chromosphere that rises into the corona while being heated. Its source lies in a region with large field gradients and intense electric currents, which lead to a strong Lorentz force that squeezes the chromospheric material, resulting in a vertical pressure gradient that propels the spicule along the magnetic field, as well as Joule heating, which heats the jet material, forcing it to fade. 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: What do Spectral Line Profile Asymmetries Tell us About the Solar Atmosphere? Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Hansteen, Viggo; McIntosh, Scott W. Bibcode: 2011ApJ...732...84M Altcode: Recently, analysis of solar spectra obtained with the EUV Imaging Spectrograph (EIS) onboard the Hinode satellite has revealed the ubiquitous presence of asymmetries in transition region (TR) and coronal spectral line profiles. These asymmetries have been observed especially at the footpoints of coronal loops and have been associated with strong upflows that may play a significant role in providing the corona with hot plasma. Here, we perform a detailed study of the various processes that can lead to spectral line asymmetries, using both simple forward models and state-of-the-art three-dimensional radiative MHD simulations of the solar atmosphere using the Bifrost code. We describe a novel technique to determine the presence and properties of faint secondary components in the wings of spectral line profiles. This method is based on least-squares fitting of observed so-called R(ed)B(lue) asymmetry profiles with pre-calculated RB asymmetry profiles for a wide variety of secondary component properties. We illustrate how this method could be used to perform reliable double Gaussian fits that are not over- or under-constrained. We also find that spectral line asymmetries appear in TR and coronal lines that are synthesized from our three-dimensional MHD simulations. Our models show that the spectral asymmetries are a sensitive measure of the velocity gradient with height in the TR of coronal loops. The modeled TR shows a large gradient of velocity that increases with height: this occurs as a consequence of ubiquitous, episodic heating at low heights in the model atmosphere. We show that the contribution function of spectral lines as a function of temperature is critical for sensitivity to velocity gradients and thus line asymmetries: lines that are formed over a temperature range that includes most of the TR are the most sensitive. As a result, lines from lithium-like ions (e.g., O VI) are found to be the most sensitive to line asymmetries. We compare the simulated line profiles directly with line profiles observed in the quiet Sun with SOHO/SUMER and Hinode/EIS and find that the shape of the profiles is very similar. In addition, the simulated profiles with the strongest blueward asymmetry occur in footpoint regions of coronal loops, which is similar to what we observe with SUMER and EIS. There is however a significant discrepancy between the simulations and observations: the simulated RB asymmetries are an order of magnitude smaller than the observations. We discuss the possible reasons for this discrepancy. In summary, our analysis shows that observations of spectral line asymmetries can provide a powerful new diagnostic to help constrain coronal heating models. 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 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: Line profile asymmetries in the transition region: models and observations Authors: Martinez-Sykora, J.; de Pontieu, B.; Hansteen, V. H.; McIntosh, S. W. Bibcode: 2010AGUFMSH31A1784M Altcode: Asymmetries in spectral line profiles provide a wealth of information on the properties of the emitting plasma along the line-of-sight. Asymmetries can be produced by the superposition of profiles with different line-of-sight velocities and/or widths resulting from the variation of the velocity and/or temperature from emission sources along the line of sight. Spectral line asymmetries from synthetic transition region and coronal lines constructed from realistic 3D models appear similar to those observed with Hinode/EIS. The simulations span the upper layer of the convection zone to the lower corona and include horizontal magnetic flux emergence. We use the state of the art Bifrost code to solve the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along the magnetic field line. Here, we perform a detailed study of the various physical, dynamical and observational processes that can lead to spectral line asymmetries at the transition region footpoints of loops in 3D radiative MHD simulations of the solar atmosphere and compare these with observations. Our models show that the spectral asymmetries are a sensitive measure of the velocity gradient with height in the transition region of coronal loops. In our models the TR shows a large gradient of velocity that increases with height: this occurs as a natural consequence of ubiquitous, episodic heating at low heights in the model atmosphere. Title: Forward modeling of emission in AIA passbands from advanced radiative MHD simulations Authors: de Pontieu, B.; Martinez-Sykora, J.; Hansteen, V. H. Bibcode: 2010AGUFMSH11A1597D Altcode: The emission from many of the passbands observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is dominated by single or several lines from ions that are formed in a narrow range of temperatures (under equilibrium conditions). However, most AIA passbands contain contributions from lines of ions that have formation temperatures that are significantly different from the dominant ion. We investigate the importance of these lines by forward modeling of the AIA passband emission from advanced radiative 3D MHD simulations calculated with the state of the art Bifrost code. We use simulations that span the upper layer of the convection zone to the low corona and solve the full magnetohydrodynamic equations with non-grey and non-LTE radiative transfer and thermal conduction along the magnetic field lines. We find that several of the AIA passbands often include significant contributions from plasma at different temperatures than the canonical temperature values. We describe under which solar conditions in the simulations these discrepancies can typically be expected to occur. Title: Modeling of EIS Spectrum Drift from Instrumental Temperatures Authors: Kamio, S.; Hara, H.; Watanabe, T.; Fredvik, T.; Hansteen, V. H. Bibcode: 2010SoPh..266..209K Altcode: 2010SoPh..tmp..137K; 2010arXiv1003.3540K An empirical model has been developed to reproduce the drift of the spectrum recorded by the EIS on Hinode using instrumental temperatures and relative motion of the spacecraft. The EIS spectrum shows an artificial drift in wavelength dimension in sync with the revolution of the spacecraft, which is caused by temperature variations inside the spectrometer. The drift amounts to 70 km s−1 in Doppler velocity and introduces difficulties in velocity measurements. An artificial neural network is incorporated to establish a relationship between the instrumental temperatures and the spectral drift. This empirical model reproduces observed spectrum shift with an rms error of 4.4 km s−1. This procedure is robust and applicable to any spectrum obtained with EIS, regardless of the observing field. In addition, spectral curvatures and spatial offset in the north - south direction are determined to compensate for instrumental effects. 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: 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: Numerical Simulations of the Chromosphere Authors: Hansteen, Viggo H. Bibcode: 2010AAS...21630505H Altcode: The solar chromosphere forms the all important link between the photosphere's and convection zone's excess mechanical energy and the magnetically dominated corona and the solar wind. We still do not know which modes of nonthermal energy power the chromosphere and overlying layers. We know that waves, electrical currents, and magnetic reconnection all may release substantial energy, and that non-thermal particles, resistive dissipation, and wave damping occur. Shedding light on these issues requires that one use a variety of approaches, both theoretical and observational. In this presentation we will focus on sol called `realistic' numerical modelling and on comparing the results of numerical modelling with present and upcoming observations.

To model the chromosphere with neighboring regions from the convection zone to the corona in a realistic manner a 3D radiation magnetohydrodynamic code `Bifrost' has been developend using a sixth-order finite difference compact scheme. Radiation is treated with multi-group opacities. Conduction along the magnetic field is treated implicitly using a multi-grid approach. Novel aspects of the code are the extension of the multi-group opacity method to include scattering and the treatment of the radiative exchange in strong lines in the middle and upper chromosphere. We have there used the detailed 1D radiation-hydrodynamic simulations to develop recipes that contain the essentials of the physics while keeping the computational expenses at tractable levels. Currently we are extending the code to include the effects of partial time-dependent ionization of Hydrogen and a generalized Ohm's law. Title: Comparison Of Observations And Advanced Numerical Simulations Of Type II Spicules Authors: Martinez-Sykora, Juan; De Pontieu, B.; Hansteen, V.; Moreno-Insertis, F. Bibcode: 2010AAS...21640306M Altcode: 2010BAAS...41..878M We have performed realistic 3D radiation MHD simulations of the solar atmosphere. These simulations show jet-like features that are similar to the type II spicules discovered with Hinode's Solar Optical Telescope. These type II spicules have been associated with so-called rapid blueshifted events (RBE's) on the solar disk, and with significant blueward asymmetries in transition region and coronal lines at the footpoints of coronal loops (discovered with Hinode's EIS). These observational results and their ubiquity suggest they may play a significant role in providing the corona with hot plasma. We will present a detailed comparison of the properties of the simulated jets, with those of type II spicules (observed with Hinode) and RBE's (with ground-based instruments). We will present analysis of a wide variety of synthetic emission lines from the simulations covering temperatures from 10,000 K to several million K, and compare their intensities, velocities, line widths and asymmetry with those of the observed phenomena. We will also show how the formation mechanism of these jets (reconnection at tangential discontinuities) complicates efforts to establish a firm link between observations of magnetic fields and of chromospheric flows, and suggests that magnetic field observations at chromospheric heights may be crucial to establish from observations how these jets are formed. 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: 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: Observing the Roots of Coronal Heating - in the Chromosphere Authors: McIntosh, S. W.; de Pontieu, B.; Hansteen, V. H.; Schrjver, K. Bibcode: 2009AGUFMSH44A..01M Altcode: I will discuss recent results using Hinode/SOT-EIS-XRT, SOHO/SUMER, CRISP (at the Swedish Solar Telescope) and TRACE that provide a direct connection between coronal dynamics and those of the lower atmosphere. We use chromospheric measurements (H-alpha and Ca II 8542 spectral imaging, and Ca II H images), as well as UV spectra (EIS and SUMER), and EUV/X-ray images (XRT and TRACE) to show that faint, high-speed upflows at velocities of 50-100 km/s across a wide range of temperatures from chromospheric (10,000 K), through lower and upper transition region (0.1 to 0.7 MK) and coronal temperatures (2 MK) are associated with significant mass-loading of the corona with hot plasma. Our observations are incompatible with current models in which coronal heating occurs as a result of nanoflares at coronal heights. Instead we suggest that a significant fraction of heating of plasma to coronal temperatures may occur at chromospheric heights in association with jets driven from below (the recently discovered type II spicules). Illustrating the mass and energy transport between the chromosphere, transition region and corona, as deduced from Hinode observations. Convective flows and oscillations in the convection zone and photosphere of the Sun buffet the magnetic field of the Sun. This leads to at least two different kinds of jets in the chromosphere: Type I, and II spicules. Type II spicules drive matter upward violently and likely form when magnetic field reconnects because of stresses introduced by convective flows. A significant fraction of the plasma in type II spicules is heated to coronal temperatures (>1MK), providing the corona with hot plasma. The correlation between the chromospheric and coronal parts of the spicules depends greatly on the viewing angle between the line-of-sight and the direction of the upward flows. Order of magnitude estimates indicate that the mass supplied by type II spicules plays a significant role in supplying the corona with hot plasma. 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: Estimating the Chromospheric Absorption of Transition Region Moss Emission Authors: De Pontieu, Bart; Hansteen, Viggo H.; McIntosh, Scott W.; Patsourakos, Spiros Bibcode: 2009ApJ...702.1016D Altcode: 2009arXiv0907.1883D Many models for coronal loops have difficulty explaining the observed EUV brightness of the transition region, which is often significantly less than theoretical models predict. This discrepancy has been addressed by a variety of approaches including filling factors and time-dependent heating, with varying degrees of success. Here, we focus on an effect that has been ignored so far: the absorption of EUV light with wavelengths below 912 Å by the resonance continua of neutral hydrogen and helium. Such absorption is expected to occur in the low-lying transition region of hot, active region loops that is colocated with cool chromospheric features and called "moss" as a result of the reticulated appearance resulting from the absorption. We use cotemporal and cospatial spectroheliograms obtained with the Solar and Heliospheric Observatory/SUMER and Hinode/EIS of Fe XII 1242 Å, 195 Å, and 186.88 Å, and compare the density determination from the 186/195 Å line ratio to that resulting from the 195/1242 Å line ratio. We find that while coronal loops have compatible density values from these two line pairs, upper transition region moss has conflicting density determinations. This discrepancy can be resolved by taking into account significant absorption of 195 Å emission caused by the chromospheric inclusions in the moss. We find that the amount of absorption is generally of the order of a factor of 2. We compare to numerical models and show that the observed effect is well reproduced by three-dimensional radiative MHD models of the transition region and corona. We use STEREO A/B data of the same active region and find that increased angles between line of sight and local vertical cause additional absorption. Our determination of the amount of chromospheric absorption of TR emission can be used to better constrain coronal heating models. Title: Observational Signatures of Simulated Reconnection Events in the Solar Chromosphere and Transition Region Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H. Bibcode: 2009ApJ...702....1H Altcode: 2009arXiv0902.0977H We present the results of numerical simulations of wave-induced magnetic reconnection in a model of the solar atmosphere. In the magnetic field geometry we study in this paper, the waves, driven by a monochromatic piston and a driver taken from Hinode observations, induce periodic reconnection of the magnetic field, and this reconnection appears to help drive long-period chromospheric jets. By synthesizing spectra for a variety of wavelengths that are sensitive to a wide range of temperatures, we shed light on the often confusing relationship between the plethora of jet-like phenomena in the solar atmosphere, e.g., explosive events, spicules, and other phenomena thought to be caused by reconnection. Our simulations produce spicule-like jets with lengths and lifetimes that match observations, and the spectral signatures of several reconnection events are similar to observations of explosive events. We also find that in some cases, absorption from overlying neutral hydrogen can hide emission from matter at coronal temperatures. 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: Stellar winds and magnetic fields Authors: Hansteen, Viggo H. Bibcode: 2009hppl.book..225H Altcode: No abstract at ADS Title: Observing the Roots of Solar Coronal Heating—in the Chromosphere Authors: De Pontieu, Bart; McIntosh, Scott W.; Hansteen, Viggo H.; Schrijver, Carolus J. Bibcode: 2009ApJ...701L...1D Altcode: 2009arXiv0906.5434D The Sun's corona is millions of degrees hotter than its 5000 K photosphere. This heating enigma is typically addressed by invoking the deposition at coronal heights of nonthermal energy generated by the interplay between convection and magnetic field near the photosphere. However, it remains unclear how and where coronal heating occurs and how the corona is filled with hot plasma. We show that energy deposition at coronal heights cannot be the only source of coronal heating by revealing a significant coronal mass supply mechanism that is driven from below, in the chromosphere. We quantify the asymmetry of spectral lines observed with Hinode and SOHO and identify faint but ubiquitous upflows with velocities that are similar (50-100 km s-1) across a wide range of magnetic field configurations and for temperatures from 100,000 to several million degrees. These upflows are spatiotemporally correlated with and have similar upward velocities as recently discovered, cool (10,000 K) chromospheric jets or (type II) spicules. We find these upflows to be pervasive and universal. Order of magnitude estimates constrained by conservation of mass and observed emission measures indicate that the mass supplied by these spicules can play a significant role in supplying the corona with hot plasma. The properties of these events are incompatible with coronal loop models that include only nanoflares at coronal heights. Our results suggest that a significant part of the heating and energizing of the corona occurs at chromospheric heights, in association with chromospheric jets. Title: The solar atmosphere Authors: Hansteen, Viggo H. Bibcode: 2009hppl.book..195S Altcode: No abstract at ADS Title: Evidence from the Extreme-Ultraviolet Imaging Spectrometer for Axial Filament Rotation before a Large Flare Authors: Williams, David R.; Harra, Louise K.; Brooks, David H.; Imada, Shinsuke; Hansteen, Viggo H. Bibcode: 2009PASJ...61..493W Altcode: In this article, we present observations made with the Extreme-ultraviolet Imaging Spectrometer on-board the Hinode solar satellite, of an active region filament in the HeII emission line at 256.32Å. The host active region AR 10930 produces an X-class flare during these observations. We measure Doppler shifts with apparent velocities of up to 20km s-1, which are antisymmetric about the filament length and occur several minutes before the flare's impulsive phase. This is indicative of a rotation of the filament, which is in turn consistent with expansion of a twisted flux rope due to the MHD helical kink instability. This is the first time that such an observation has been possible in this transition-region line, and we note that the signature observed occurs before the first indications of pre-flare activity in the GOES solar soft X-ray flux, suggesting that the filament begins to destabilise in tandem with a reorganization of the local magnetic field. We suggest that this expansion is triggered by the decrease of magnetic tension around, and/or total pressure above, the filament. Title: Observing the Roots of Solar Coronal Heating in the Chromosphere Authors: McIntosh, Scott W.; De Pontieu, B.; Hansteen, V.; Schrijver, C. J. Bibcode: 2009SPD....40.2602M Altcode: The Sun's atmosphere or corona is millions of degrees hotter than its 5,000 K surface or photosphere. This heating enigma is typically addressed by invoking the deposition at coronal heights of non-thermal energy generated by the interplay between convection and magnetic field near the photosphere. However, it remains unclear how and where coronal heating occurs and how the corona is filled with hot plasma. Here, we show that energy deposition at coronal heights cannot be the only source of coronal heating, by revealing a significant coronal mass supply mechanism that is driven from below, in the chromosphere, the interface between photosphere and corona. We quantify the asymmetry of spectral lines observed with Hinode and SOHO and identify faint but ubiquitous upflows with velocities that are similar (50-100 km/s) across a wide range of magnetic field configurations and for temperatures from 100,000 to several million degrees. These upflows are correlated with and have similar upward velocities as the very fine and dynamic chromospheric jets, or spicules, discovered by Hinode. As these phenomena are incompatible with models of coronal loops that only include nanoflare heating at coronal heights, we conclude that a significant fraction of the energy needed to heat coronal plasma is deposited at chromospheric heights in association with spicular jets driven from below. 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: 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: 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: 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: The Structure and Dynamics of the Quiet Corona from Observations with the Extreme ultraviolet Imaging Spectrometer Authors: Dere, K. P.; Doschek, G. A.; Mariska, J. T.; Harra, L. K.; Matsuzaki, K.; Hansteen, V.; Thomas, R. J. Bibcode: 2007AGUFMSH53A1046D Altcode: The goal of the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode satellite is to measure such physical parameters as the velocity and density of the solar corona in order to provide an observational basis to understand how coronal plasmas are heated and accelerated. On 2007 January 20, EIS performed a raster of a 128 x 512 arc-sec. area of a quiet region near Sun center. The observing program recorded spectra of He II λ256, formed at 9 × 104 K, and lines of Fe VIII-XV, formed at temperatures spanning the range from 5 × 105 through 2 × 106 K. Maps of intensities, velocities and electron densities derived from these observations are presented and discussed. Intensity maps in He II λ256 show the chromospheric network. Line intensities of Fe X-XIV show small-scale bright points and more extended structures. The intensity map of Fe VIII shows a transition between the two temperatures. The coronal lines reveal regions of high outflow velocities on the order of 100 km s-1 in a compact region and 12 km s-1 in an extended region. The presence of these high velocities in the quiet corona is an entirely new and unexpected result. Electron densities derived from density sensitive line ratios of Fe XII and XIII are typically about 3 - 20×108 cm-3. The highest densities are found in bright, compact areas. For the first time, explosive events in the quiet sun have been observed in the extreme-ultraviolet in He II λ256 profiles and have properties similar to those previously reported. Title: The Structure and Dynamics of the Quiet Corona from Observations with the Extreme Ultraviolet Imaging Spectrometer on Hinode Authors: Dere, Kenneth P.; Doschek, George A.; Mariska, John T.; Hansteen, Viggo H.; Harra, Louise K.; Matsuzaki, Keiichi; Thomas, Roger J. Bibcode: 2007PASJ...59S.721D Altcode: The goal of the Hinode mission is to provide an observational basis for understanding the heating and acceleration of coronal plasmas. On 2007 January 20, the Extreme ultraviolet Imaging Spectrometer performed a raster of a quiet region near Sun center. Maps of intensities, velocities, and electron densities derived from these observations are presented and discussed. Intensity maps in HeII λ 256, formed at 9 × 104 K, show the chromospheric network. Line intensities of FeX-XIV, formed at temperatures from 1-2 × 106 K, show small-scale bright points and more extended structures. The intensity map of FeVIII shows a transition between the two temperatures. The coronal lines reveal regions of high outflow velocities on the order of 100kms-1 in a compact region and 12kms-1 in an extended region. The presence of such high velocities in the quiet corona is an entirely new and unexpected result. Electron densities derived from density sensitive line ratios of FeXII and XIII are typically 3-20 × 108cm-3. The highest densities are found in bright, compact areas. For the first time, explosive events in the quiet sun have been observed in the extreme-ultraviolet in HeII λ 256 profiles. Title: Hinode EUV Study of Jets in the Sun's South Polar Corona Authors: Culhane, Len; Harra, Louise K.; Baker, Deborah; van Driel-Gesztelyi, Lidia; Sun, Jian; Doschek, George A.; Brooks, David H.; Lundquist, Loraine L.; Kamio, Suguru; Young, Peter R.; Hansteen, Viggo H. Bibcode: 2007PASJ...59S.751C Altcode: A number of coronal bright points and associated plasma jet features were seen in an observation of the South polar coronal hole during 2007 January. The 40" wide slot was used at the focus of the Hinode EUV Imaging Spectrometer to provide spectral images for two of these events. Light curves are plotted for a number of emission lines that include He II 256Å (0.079MK) and cover the temperature interval from 0.4MK to 5.0MK. Jet speed measurements indicate values less than the escape velocity. The light curves show a post-jet enhancement in a number of the cooler coronal lines indicating that after a few minutes cooling, the plasma fell back to its original acceleration site. This behavior has not been previously observed by e.g., the Yohkoh Soft X-ray Telescope due to the comparatively high temperature cut-off in its response. The observations are consistent with the existing models that involve magnetic reconnection between emerging flux and the ambient open field lines in the polar coronal hole. However we do not have sufficient coverage of lines from lower temperature ion species to register the Hα-emitting surge material that is associated with some of these models. 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: Waves and Shocks in the Solar Atmosphere Authors: Hansteen, V. H. Bibcode: 2007ASPC..369..193H Altcode: There is a large volume of observations showing wave phenomena in the solar chromosphere and corona. These waves affect the very structure of the atmosphere, and may contribute to the heating of both the chromosphere, transition region and corona. They also provide powerful diagnostics using seismological techniques. In this talk we will review the observations of waves and compare these with simulations of wave excitation in the solar convection zone and of wave propagation and mode conversion in realistic magnetic field topologies. 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: Numerical Simulations of Shock Wave-driven Chromospheric Jets Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H. Bibcode: 2007ApJ...666.1277H Altcode: 2007astro.ph..3498H We present the results of numerical simulations of shock wave-driven jets in the solar atmosphere. The dependence of observable quantities such as maximum velocity and deceleration on parameters such as the period and amplitude of initial disturbances and the inclination of the magnetic field is investigated. Our simulations show excellent agreement with observations, and shed new light on the correlation between velocity and deceleration and on the regional differences found in observations. Title: The EUV Imaging Spectrometer for Hinode Authors: Culhane, J. L.; Harra, L. K.; James, A. M.; Al-Janabi, K.; Bradley, L. J.; Chaudry, R. A.; Rees, K.; Tandy, J. A.; Thomas, P.; Whillock, M. C. R.; Winter, B.; Doschek, G. A.; Korendyke, C. M.; Brown, C. M.; Myers, S.; Mariska, J.; Seely, J.; Lang, J.; Kent, B. J.; Shaughnessy, B. M.; Young, P. R.; Simnett, G. M.; Castelli, C. M.; Mahmoud, S.; Mapson-Menard, H.; Probyn, B. J.; Thomas, R. J.; Davila, J.; Dere, K.; Windt, D.; Shea, J.; Hagood, R.; Moye, R.; Hara, H.; Watanabe, T.; Matsuzaki, K.; Kosugi, T.; Hansteen, V.; Wikstol, Ø. Bibcode: 2007SoPh..243...19C Altcode: The EUV Imaging Spectrometer (EIS) on Hinode will observe solar corona and upper transition region emission lines in the wavelength ranges 170 - 210 Å and 250 - 290 Å. The line centroid positions and profile widths will allow plasma velocities and turbulent or non-thermal line broadenings to be measured. We will derive local plasma temperatures and densities from the line intensities. The spectra will allow accurate determination of differential emission measure and element abundances within a variety of corona and transition region structures. These powerful spectroscopic diagnostics will allow identification and characterization of magnetic reconnection and wave propagation processes in the upper solar atmosphere. We will also directly study the detailed evolution and heating of coronal loops. The EIS instrument incorporates a unique two element, normal incidence design. The optics are coated with optimized multilayer coatings. We have selected highly efficient, backside-illuminated, thinned CCDs. These design features result in an instrument that has significantly greater effective area than previous orbiting EUV spectrographs with typical active region 2 - 5 s exposure times in the brightest lines. EIS can scan a field of 6×8.5 arc min with spatial and velocity scales of 1 arc sec and 25 km s−1 per pixel. The instrument design, its absolute calibration, and performance are described in detail in this paper. EIS will be used along with the Solar Optical Telescope (SOT) and the X-ray Telescope (XRT) for a wide range of studies of the solar atmosphere. 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: Hinode Euv Study Of Jets In The Sun’s South Polar Corona Authors: Culhane, J. L.; Brooks, D. H.; Doschek, G. A.; Harra, L. K.; van Driel-Gesztelyi, L.; Baker, D.; Lundquist, L. L.; Hansteen, V. H.; Kamio, S. Bibcode: 2007AAS...210.7201C Altcode: 2007BAAS...39..178C Using the Hinode EUV Imaging Spectrometer coronal jets were observed on 20-JAN-2007 over a range of emission lines and corresponding plasma temperatures using the 40 arc sec wide slot images. In this preliminary analysis, jet plasma temperature and emissivity have been estimated while, based on assumptions about the jet morphology, electron density estimates are given and jet velocity measured. The evolution of the jets will be followed in a number of different EUV emission lines and jet energy input as a function of time will be assessed with reference to the magnetic field topologies involved. 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: 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: Magneto-acoustic Waves And Their Role In The Energetics And Dynamics Of The Solar Chromosphere Authors: Jefferies, Stuart; De Pontieu, B.; McIntosh, S.; Hansteen, V. H. Bibcode: 2007AAS...21012004J Altcode: 2007BAAS...39..245J We analyze a diverse set of observations obtained with SOHO and TRACE, as well as with MOTH and the Swedish 1 m Solar Telescope to show that sound waves play an important role in shaping the structure and energetics of the magnetized chromosphere. Travel time analysis of TRACE, MOTH and SST observations and comparisons with numerical simulations show that normally evanescent 5 minute p-mode oscillations leak into the chromosphere along flux tubes that are inclined with the vertical. Comparisons of SST data of fibril-like jets above active region plage and quiet Sun mottles with advanced radiative MHD simulations show how these oscillations develop into slow mode magnetoacoustic shocks that drive spicule-like chromospheric jets up to coronal heights.

The leaking waves not only drive much of the dynamics of the magnetized chromosphere: Doppler measurements from the MOTH instrument at several heights in the atmosphere show that the total energy flux carried by these leaking waves may play a significant role in the energy balance of the magnetized chromosphere.

We describe first approaches to determine more precisely how and where the wave energy is deposited in the low atmosphere. 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: 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: 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: Radiative transfer aspects of the magnetic coupling Authors: Hansteen, V. H. Bibcode: 2006cosp...36.3543H Altcode: 2006cosp.meet.3543H We present numerical simulations of the quiet sun chromosphere and corona These models span the entire solar atmosphere from the upper convection zone to the lower corona Included are non-grey non-lte radiative transport in the photosphere and lower chromosphere effectively thin radiative losses in the upper chromosphere transition region and corona as well as magnetic field-aligned heat conduction Coronal heating is effected through the dissipation of currents generated by photospheric motions We will discuss radiative diagnostics of various phenomena in the chromosphere transition region and corona as they occur in the simulations and compare these with observations Title: a Multi-Wavelength View on Coronal Rain Authors: Müller, D. A. N.; de Groof, A.; de Pontieu, B.; Hansteen, V. H. Bibcode: 2005ESASP.600E..30M Altcode: 2005dysu.confE..30M; 2005ESPM...11...30M 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: a Multi-Wavelength View on Coronal Rain Authors: Müller, D. A. N.; de Groof, A.; de Pontieu, B.; Hansteen, V. H. Bibcode: 2005ESASP.596E..37M Altcode: 2005ccmf.confE..37M 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: 3D Numerical Models of Quiet Sun Coronal Heating Authors: Hansteen, V. H.; Gudiksen, B. Bibcode: 2005ESASP.592..483H Altcode: 2005soho...16E..87H; 2005ESASP.592E..87H No abstract at ADS Title: High-speed coronal rain Authors: Müller, D. A. N.; De Groof, A.; Hansteen, V. H.; Peter, H. Bibcode: 2005A&A...436.1067M Altcode: At high spatial and temporal resolution, coronal loops are observed to have a highly dynamic nature. Recent observations with SOHO and TRACE frequently show localized brightenings "raining" down towards the solar surface. What is the origin of these features? Here we present for the first time a comparison of observed intensity enhancements from an EIT shutterless campaign with non-equilibrium ionization simulations of coronal loops in order to reveal the physical processes governing fast flows and localized brightenings. We show that catastrophic cooling around the loop apex as a consequence of footpoint-concentrated heating offers a simple explanation for these observations. An advantage of this model is that no external driving mechanism is necessary as the dynamics result entirely from the non-linear character of the problem. Title: Heating of the Quiet Solar Corona Authors: Hansteen, V. H.; Gudiksen, B. W. Bibcode: 2005AGUSMSP41A..01H Altcode: 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 capababilities and growing physical insight seem finally to be leading towards understanding. We present numerical simulations of quiet sun heating 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. The relation between the mean magnetic field strength and structure and the heating of the corona is discussed. Title: Release of Helium from Closed-Field Regions of the Sun Authors: Endeve, Eirik; Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil Bibcode: 2005ApJ...624..402E Altcode: Using a numerical model that extends from the chromosphere into the supersonic solar wind, we study the dynamics of previously closed coronal flux tubes that open, allowing plasma to be expelled from the corona. In particular, we study whether the opening of flux tubes may provide a source of helium-rich material for the solar wind. We use higher order moment fluid equations to describe the plasma, which consists of hydrogen (neutral and protons), helium (neutral, singly ionized, and α-particles), and electrons. The helium abundance decreases rapidly with altitude in a closed flux tube, caused by the collisional coupling between α-particles and protons leading to a small α-particle scale height. When the flux tube is rapidly opened, protons escape from the Sun immediately. The coronal α-particles leave the corona only much later, 10-20 hr after the protons, when the collisional coupling to protons has eased, allowing their temperature to become sufficiently high for them to escape. Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical evolution Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson, M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E. Bibcode: 2005A&A...435..327R Altcode: Small magnetic structures can be seen in G-band filtergrams as isolated bright points, strings of bright points and dark micro-pores. At a resolution of 0.1 arcsec, new forms of magnetic structures are found in strong field areas: elongated “ribbons” and more circular “flowers”. In this paper we study the temporal evolution of these small scale magnetic structures. In strong-field regions the time-evolution is more that of a magnetic fluid than that of collections of flux-tubes that keep their identity. We find that the granular flow concentrates the magnetic field into flux sheets that are visible as thin bright features in the filtergrams. Weak upflows are found in the flux sheets and downflows in the immediate surroundings. The flux sheets often become unstable to a fluting instability and the edges buckle. The sheets tend to break up into strings of bright points, still with weak upflows in the magnetic elements and zero velocity or downflows between them. Where there are larger flux concentrations we find ribbons, flowers and micro-pores. There is a continuous transition between these forms and they evolve from one form to another. The appearance is mostly determined by the horizontal size - larger structures are dark (micro-pores), narrower structures are ribbon shaped and the flowers are the smallest in extent. All these structures have darker inner parts and a bright edge. The plasma is found to be at rest in the ribbons, with small concentrations of weak upflow sites. Narrow sheets with downdrafts are found right at the edges of the magnetic field concentrations. Title: Solar 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: Thermal Instability as the Origin of High Speed Coronal Rain Authors: Müller, D. A. N.; de Groof, A.; Hansteen, V. H.; Peter, H. Bibcode: 2004ESASP.575..291M Altcode: 2004soho...15..291M No abstract at ADS Title: Dynamics of solar coronal loops. II. Catastrophic cooling and high-speed downflows Authors: Müller, D. A. N.; Peter, H.; Hansteen, V. H. Bibcode: 2004A&A...424..289M Altcode: 2004astro.ph..5538M This work addresses the problem of plasma condensation and ``catastrophic cooling'' in solar coronal loops. We have carried out numerical calculations of coronal loops and find several classes of time-dependent solutions (static, periodic, irregular), depending on the spatial distribution of a temporally constant energy deposition in the loop. Dynamic loops exhibit recurrent plasma condensations, accompanied by high-speed downflows and transient brightenings of transition region lines, in good agreement with features observed with TRACE. Furthermore, these results also offer an explanation for the recent EIT observations of \cite{DeGroof+al2004AA} of moving bright blobs in large coronal loops. In contrast to earlier models, we suggest that the process of catastrophic cooling is not initiated by a drastic decrease of the total loop heating but rather results from a loss of equilibrium at the loop apex as a natural consequence of heating concentrated at the footpoints of the loop, but constant in time. 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: 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: Thermal non-equilibrium in coronal loops: A road to complex evolution Authors: Müller, Daniel; de Groof, A.; Hansteen, V. H.; Peter, H. Bibcode: 2004IAUS..223..289M Altcode: 2005IAUS..223..289M At high spatial and temporal resolution, coronal loops are observed to have a highly dynamic nature. Recent observations with SOHO and TRACE frequently show localized brightening "raining" down towards the solar surface. What is the origin of these features? Here we present for the first time a comparison of observed intensity enhancements from an EIT shutterless campaign with non-equilibrium ionization simulations of coronal loops in order to reveal the physical processes governing fast flows and localized brightening. We show that catastrophic cooling around the loop apex as a consequence of footpoint-concentrated heating offers a simple explanation for these observations. An advantage of this model is that no external driving mechanism is necessary as the dynamics result entirely from the non-linear character of the system. Title: Catastrophic Cooling and High-Speed Downflows in Solar Coronal Loops Authors: Müller, D.; Peter, H.; Hansteen, V. Bibcode: 2004IAUS..219..765M Altcode: 2003IAUS..219E..48M We report numerical simulations of the condensation of plasma in short solar coronal loops which has several interesting physical consequences. Firstly we propose a connection between small cool loops which presumably constitute the solar transition region and prominences in the sense that the same physical mechanism governs their dynamics namely the onset of instability and runaway cooling due to strong radiative losses. Secondly we show that the temporal evolution of these loop models exhibit a cyclic pattern of chromospheric evaporation condensation formation motion of the condensation region to either side of the loop and finally loop reheating with a period of 4000 - 6000 s for a loop of 10 Mm length. Thirdly we have synthesized transition region lines from these simulations which show strong periodic intensity variations making condensation processes in loops a candidate to account for the observed temporal variability of these lines. Title: Plasma Condensation in Solar Coronal Loops: II. "Catastrophic Cooling" and High-Speed Downflows Authors: Müller, D. A. N.; Peter, H.; Hansteen, V. H. Bibcode: 2004ESASP.547..199M Altcode: 2004soho...13..199M The second part of this work focuses on the application of the concept of plasma condensation to large coronal loops. In contrast to the short loops analyzed in Müller et al. (2003a), these models can more easily be compared to SOHO and TRACE observations. From our numerical calculations of coronal loops we find several classes of time-dependent solutions (static, periodic, irregular), depending on the spatial dependence of a temporally constant energy deposition in the loop. One of these classes is in remarkably close agreement with the features observed with TRACE, described by Schrijver (2001): Emission in C IV (154.8 nm), developing initially near the loop tops, cool plasma sliding down on both sides of the loop, downflow velocities of up to 100 km/s, and a downward acceleration which is substantially reduced with respect to the solar surface gravity. Furthermore, these results also offer an explanation for the observations of De Groof et al. (2003a,b). In contrast to earlier models, we suggest that the process of catastrophic cooling does not have to be initiated by a drastic decrease of the loop heating. It can also result from a loss of equilibrium at the loop apex which is a natural consequence if the loop is heated predominantly at the footpoints, but constant in time. Title: Initial simulations spanning the upper convection zone to the corona Authors: Hansteen, Viggo H. Bibcode: 2004IAUS..223..385H Altcode: 2005IAUS..223..385H 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 capababilities 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: 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: Plasma Condensation in Solar Coronal Loops -- I. Basic Processes Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2004ESASP.547..285M Altcode: 2004soho...13..285M In the first part of this work, we report numerical calculations of the condensation of plasma in short coronal loops, which has several interesting physical consequences. We propose a connection between small, cool loops, which presumably constitute the solar transition region, and prominences in the sense that the same physical mechanism governs their dynamics, namely the onset of instability and runaway cooling due to strong radiative losses. We show that the temporal evolution of these loop models exhibits a cyclic pattern of chromospheric evaporation, condensation, motion of the condensation region to either side of the loop, and finally loop reheating with a period of 4000 - 8000 s for a loop of 10 Mm length. Furthermore, we have synthesized transition region lines from these calculations which show strong periodic intensity variations, making condensation in loops a candidate to account for observed transient brightenings of solar transition region lines. Remarkably, all these dynamic processes take place for a heating function which is constant in time and has a simple exponential height dependence. In the second part of this work (Müller et al., 2003b), we apply this concept to large coronal loops. 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: Dynamics of solar coronal loops. I. Condensation in cool loops and its effect on transition region lines Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2003A&A...411..605M Altcode: We report numerical calculations of the condensation of plasma in short coronal loops, which have several interesting physical consequences. Firstly, we propose a connection between small, cool loops (T < 106 K), which constitute one of the basic components of the solar transition region, and prominences, in the sense that the same physical mechanism governs their dynamics: Namely the onset of instability and runaway cooling due to strong radiative losses. Secondly, we show that the temporal evolution of these loop models exhibits a cyclic pattern of chromospheric evaporation, condensation, motion of the condensation region to either side of the loop, and finally loop reheating with a period of 4000-8000 s for a loop of 10 Mm length. Thirdly, we have synthesized transition region lines from these calculations which show strong periodic intensity variations, making condensation in loops a candidate to account for observed transient brightenings of solar transition region lines. Remarkably, all these dynamic processes take place for a heating function which is constant in time and has a simple exponential height dependence. Title: Helium Abundance in the Corona and Solar Wind: Gyrotropic Modeling from the Chromosphere to 1 AU Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil Bibcode: 2003ApJ...596..621L Altcode: We have developed a solar wind model including helium that extends from the chromosphere to 1 AU. The model is based on the gyrotropic approximation to the 16-moment set of fluid transport equations, which allows it to accommodate temperature anisotropies, as well as nonclassical heat transport. We find that, as in a pure electron-proton solar wind, the flow geometry close to the Sun also has a large impact on helium. In a radially expanding flow, downward proton heat conduction from the corona leads to a high transition region pressure and a large thermal force that pulls helium ions into the corona. In this case α-particles may easily become the dominant species in the corona, resulting in a polar wind type of solar wind in which the light protons are accelerated outward in the electric field set up by the α-particles and electrons. By contrast, applying the same form for the coronal heating in a rapidly expanding geometry intended to simulate a coronal hole, protons become collisionless closer to the Sun, and therefore the downward proton heat flux is smaller, resulting in a lower transition region pressure and a lower thermal force on helium. In this case the helium abundance is low everywhere and helium is unimportant for the acceleration of the solar wind. For the low coronal proton and α-particle densities found in the rapidly expanding flow, where asymptotic flow speeds are typically significantly higher than the gravitational escape speed at the solar surface, the solar wind helium mass flux is determined by the amount of helium available at the top of the chromosphere. In the radially expanding flow, with asymptotic flow speeds lower than the escape speed, the helium mass flux depends on the amount of energy available in the corona to lift helium out of the gravitational potential. In both cases the frictional coupling between helium and hydrogen in the chromosphere, using currently accepted elastic cross sections, is too weak to pull a sufficient number of helium atoms up to the top of the chromosphere and thus obtain a mass flux in agreement with observations. A better understanding of the chromosphere is therefore called for. Title: Thermal forces and the coronal helium abundance Authors: Hansteen, V. H.; Lie-Svendsen, Ø.; Leer, E. Bibcode: 2003AIPC..679..620H Altcode: The interaction between protons and minor ions in the chromosphere-corona transition region produces an upward force on the minor ions and an enhanced coronal abundance. In this presentation we compare a ``classical'' hydrodynamical model of a hydrogen - helium solar wind and a model based on a 16-moment fluid description where the heat flux is treated in a self-consistent manner. Title: The effect of time-dependent coronal heating on the solar wind from coronal holes Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil Bibcode: 2003AIPC..679..299L Altcode: We have modelled the solar wind response to a time-dependent energy input in the corona. The model, which extends from the upper chromosphere to 1 AU, solves the time-dependent transport equations based on the gyrotropic approximation to the 16-moment set of transport equations, which allow for temperature anisotropies. Protons are heated perpendicularly to the magnetic field, assuming a coronal heating function that varies sinusoidally in time. We find that heating with periods less than about 3 hours does not leave visible manifestations in the solar wind (the oscillations are efficiently damped near the Sun) heating with periods of order 10 hours leads to perturbations comparable to Ulysses observations; while heating with periods of order 100 hours results in a series of steady-state solutions. Mass flux perturbations tend to be larger than perturbations in wind speed. Heating in coronal holes with periods of order 30 hours leads to large mass flux perturbations near Earth, even when the amplitude of the change in heating rate in the corona is small. Title: Dynamics of Coronal loops: "Catastrophic Cooling" and High-speed Downflows Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2003ANS...324...13M Altcode: 2003ANS...324..B09M No abstract at ADS Title: Condensation in Cool Coronal Loops and its Effect on Transition Region Lines Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2003ANS...324..108M Altcode: 2003ANS...324..P14M No abstract at ADS Title: On the nature of the 3 minute oscillations above sunspots Authors: Hansteen, V. H.; Wikstøl, Ø.; Brynildsen, N.; Maltby, P.; Fredvik, T.; Kjeldseth-Moe, O. Bibcode: 2002ESASP.505..183H Altcode: 2002IAUCo.188..183H; 2002solm.conf..183H Oscillations in the sunspot transition region and corona are observed simultaneously with the Coronal Diagnostic Spectrometer - CDS on SOHO and the Transition Region And Coronal Explorer - TRACE. Observations of thirteen sunspots in 2001 and 2002 show that the amplitude in the relative integrated line intensity increases with increasing temperature, reaches a maximum for emission lines formed close to 1-2×105K, and decreases at higher temperatures. Part of the wave energy penetrates into the corona, channeled into small areas that appear to coincide with the endpoints of sunspot coronal loops. The observed power spectra show one dominating peak close to 6 mHz. To explore the nature of the waves we calculate upwardly propagating acoustic waves confined to a magnetic flux tube and compare the results with the observations. Title: Effect of time-dependent coronal heating on the solar wind Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil Bibcode: 2002JGRA..107.1325L Altcode: We have used a higher-order fluid model to study the effect of time-dependent coronal heating of protons on the solar wind, both for the fast wind from rapidly expanding coronal holes and for slower winds from a radially expanding geometry. The model extends from the chromosphere to 1 AU in order to capture the coupling between the chromosphere and corona and therefore the supply of plasma to the solar wind. The protons are heated in the perpendicular direction (relative to the magnetic field), assuming a simple sinusoidal variation with time. With a short time period for the heating, less than about 3 hours, the oscillations are efficiently damped in the inner part of the solar wind, even when the amplitude of the change in heating rate is large, leading to essentially steady state solutions near Earth. Heating with a period of 10 hours or so leads to perturbations in the wind near Earth that are comparable to Ulysses observations. Heating with a period of order 100 hours leads to essentially a series of steady state solutions. The mass flux perturbations are in general larger than the perturbations in wind speed, and for heating of coronal holes with periods of order 30 hours the mass flux perturbations are amplified in the wind by the interaction between the fast and slow components of the wind. In this case even moderate changes in the heating rate in the corona can produce solar wind interaction regions with forward and reversed shocks and large mass flux perturbations near Earth. Title: Neutral Atom Diffusion in a Partially Ionized Prominence Plasma Authors: Gilbert, Holly R.; Hansteen, Viggo H.; Holzer, Thomas E. Bibcode: 2002ApJ...577..464G Altcode: The support of solar prominences is normally described in terms of a magnetic force on the prominence plasma that balances the solar gravitational force. Because the prominence plasma is only partially ionized, this support needs to be understood in terms of the frictional coupling between the neutral and ionized components of the prominence plasma, the efficacy of which depends directly on the ion density. More specifically, the frictional force is proportional to the relative flow of neutral and ion species, and for a plasma with a sufficiently small vertical ion column density, this flow must be relatively large to produce a frictional force that balances gravity. A large relative flow, of course, implies significant draining of neutral particles from the prominence. We evaluate the importance of this draining effect for a hydrogen-helium plasma and consider the variation of the draining with a variety of prominence parameters. Our calculations show that the loss timescale for hydrogen is much longer than that for helium, which for typical prominence parameters is about one day. 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: The Effect of Transition Region Heating on the Solar Wind from Coronal Holes Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil; Holzer, Thomas E. Bibcode: 2002ApJ...566..562L Altcode: Using a 16 moment solar wind model extending from the chromosphere to 1 AU, we study how the solar wind is affected by direct deposition of energy in the transition region, in both radially expanding geometries and rapidly expanding coronal holes. Energy is required in the transition region to lift the plasma up to the corona, where additional coronal heating takes place. The amount of energy deposited determines the transition region pressure and the number of particles reaching the corona and, hence, how the solar wind energy flux is divided between gravitational potential and kinetic energy. We find that when only protons are heated perpendicularly to the magnetic field in a rapidly expanding coronal hole, the protons quickly become collisionless and therefore conduct very little energy into the transition region, leading to a wind much faster than what is observed. Only by additional deposition of energy in the transition region can a reasonable mass flux and flow speed at 1 AU be obtained. Radiative loss in the transition region is negligible in these low-mass flux solutions. In a radially expanding geometry the same form of coronal heating results in a downward heat flux to the transition region substantially larger than what is needed to heat the upwelling plasma, resulting in a higher transition region pressure, a slow, massive solar wind, and radiative loss playing a dominant role in the transition region energy budget. No additional energy input is needed in the transition region in this case. In the coronal hole geometry the solar wind response to transition region heating is highly nonlinear, and even a tiny input of energy can have a very large influence on the asymptotic properties of the wind. By contrast, the radially expanding wind is quite insensitive to additional deposition of energy in the transition region. 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: Key issues of the solar transition region to be addressed by the Solar Orbiter Authors: Hansteen, V. H. Bibcode: 2001ESASP.493...51H Altcode: 2001sefs.work...51H The transition region is the thermal contact between the dense and cold chromosphere and the hot and tenuous corona. As such, the spectral emission lines formed in the region potentially contain information on the phenomena occurring in both. This diagnostic potential is increased since the transition region is geometrically quite small and since the radiative transfer is relatively simple for these lines. The transition region thus presents us with the possibility of making observations that can elucidate the still not understood energetics and dynamics of the corona and the chromosphere. On the other hand, the nearly discontinuous rise in temperature and the fact that plasma beta passes through β = 1 close to the transition region practically ensure that wave phenomena such as reflection and refraction will occur. It is also certain that time-dependent phenomena and the topology of the magnetic field are factors that contribute to the complications in unraveling the nature of the dynamics of these regions. There is a large contrast between network and internetwork emission throughout the outer layers of the Sun (indeed it is not certain that any material at coronal temperatures is in thermal contact with the internetwork at all), while at any given location the emission varies with time on several different time scales. In the internetwork there is, perhaps, general agreement that acoustic waves play an important role in powering the emission stemming from the upper chromosphere. In the network the acoustic power seems suppressed and in the upper regions of the solar atmosphere magnetic phenomena, such as rapid reconnection or the dissipation of high frequency Alfvén waves surely contribute to the heating. We will discuss the various phenomena expected in the transition region in the context of observations possible with the Solar Orbiter. Title: Neutral Atom Diffusion in a Paritially Ionized Prominence Plasma Authors: Gilbert, H. R.; Hansteen, V. H.; Holzer, T. E. Bibcode: 2001AGUSM..SP61A09G Altcode: The support of solar prominences is normally described in terms of a magnetic force on the prominence plasma that balances the solar gravitational force. Because the prominence plasma is only partially ionized, it is necessary to consider in addition the support of the neutral component of the prominence plasma. This support is accomplished through a frictional interaction between the neutral and ionized components of the plasma, and its efficacy depends strongly on the degree of ionization of the plasma. More specifically, the frictional force is proportional to the relative flow of neutral and ion species, and for a sufficiently weakly ionized plasma, this flow must be relatively large to produce a frictional force that balances gravity. A large relative flow, of course, implies significant draining of neutral particles from the prominence. We evaluate the importance of this draining effect for a hydrogen-helium plasma, and consider the variation of the draining with the degree of heat input to the prominence plasma, which determines the ionization state of the prominence. Title: Coupling Between Chromosphere and Corona: Why it Matters for the Solar Wind Authors: Lie-Svendsen, {; Hansteen, V. H.; Leer, E. Bibcode: 2001AGUSM..SH21B02L Altcode: The solar wind is driven by energy input which must be deposited mainly in the corona. In some sense, therefore, the solar wind ``starts'' in the corona, and most solar wind models have their lower boundary here. However, the underlying chromosphere and transition region is not only a ``passive'' supplier of solar wind plasma. Energy must be supplied as well in the upper chromosphere and transition region to ionize and heat the outflowing gas from chromospheric to coronal temperatures. While this energy input is usually small compared to the energy deposited in the corona, it nevertheless can have a crucial influence on the solar wind because the amount of energy deposited in the transition region limits the mass flux of the wind. Using a higher-order (16-moment) fluid solar wind model that extends from the chromosphere to 1~AU, we find that when protons are heated in the corona they may quickly become collisionless so that the heat flux deviates strongly from classical heat conduction, making it difficult to transport sufficient amounts of energy downwards to the transition region. The problem is most acute in the rapidly expanding magnetic field of coronal holes, where we find that adding even a small energy input in the transition region therefore can increase the solar wind mass flux by a factor ten or more, and not doing so leads to a wind much faster than what is observed even in high-speed streams. 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: A 16-moment solar wind model: From the chromosphere to 1 AU Authors: Lie-Svendsen, Øystein; Leer, Egil; Hansteen, Viggo H. Bibcode: 2001JGR...106.8217L Altcode: We present a solar wind fluid model extending from the chromosphere to Earth. The model is based on the gyrotropic approximation to the 16-moment set of transport equations, in which we solve for the density, drift speed, temperature parallel and perpendicular to the magnetic field, and transport of parallel and perpendicular thermal energy along the magnetic field (heat flux). The solar wind plasma is created dynamically through (photo) ionization in the chromosphere, and the plasma density in the transition region and corona is computed dynamically, dependent on the type of coronal heating applied, rather than being set arbitrarily. The model improves the description of proton energy transport in the transition region, where classical heat conduction is only retrieved in the collision-dominated limit. This model can serve as a ``test bed'' for any coronal heating mechanism. We consider heating of protons by a turbulent cascade of Alfvén waves in rapidly expanding coronal holes. The resulting high coronal proton temperatures lead to a downward proton energy flux from the corona which is much smaller than what classical transport theory predicts, causing a very low coronal density and an extremely fast solar wind with a small mass flux. Only when some of the wave energy is forcibly deposited in the lower transition region can a realistic solar wind be obtained. Because of the poor proton heat transport, in order to produce a realistic solar wind any viable heating mechanism must deposit some energy in the transition region, either directly or via explicit heating of coronal electrons. 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: Transition Region Models Authors: Hansteen, V. Bibcode: 2000eaa..bookE2267H Altcode: The SOLAR TRANSITION REGION comprises the PLASMA between the CHROMOSPHERE and the CORONA. In both of these regions the temperature is fairly uniform. The transition region, by contrast, is believed to be characterized by a very steep temperature rise from a chromospheric temperature of slightly less than 104 K to coronal temperatures on the order of 106 K. The goal of modeling the transition regi... 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: Spectral diagnostics and transition region structure Authors: Hansteen, V. H. Bibcode: 2000SPD....31.1202H Altcode: 2000BAAS...32..844H Observations of spectral lines formed in the upper chromosphere, transition region, and corona paint a complicated picture open to several interpretations. It seems certain that time-dependent phenomena and the topology of the magnetic field are factors that contribute to the complications: There is a large contrast between network and internetwork emission throughout the region, while at any given location the emission varies with time on several different timescales. There is, perhaps, general agreement that acoustic waves play an important role in powering the upper chromosphere while in the upper regions of the Solar atmosphere magnetic phenomena also contribute to the heating. Given this framework we suggest that the best method of progressing towards untangling the complicated picture drawn by the observations is to proceed by comparing the results of simple physical models with the observations. We will present various models to support this point of view. This work has been supported by the Norwegian Research Coucil's grant 121076/420, "Modelling of Astrophysical Plasmas" 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: Advances in Modelling the Fast Solar Wind Authors: Hansteen, V. H.; Leer, E.; Lie-Svendsen, Ø. Bibcode: 1999ESASP.448.1091H Altcode: 1999mfsp.conf.1091H; 1999ESPM....9.1091H 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: The origin of the high speed solar wind Authors: Hansteen, Viggo H.; Leer, Egil; Holzer, Thomas E. Bibcode: 1999AIPC..471...17H Altcode: 1999sowi.conf...17H The outflow of coronal plasma into interplanetary space is a consequence of the coronal heating process. Therefore the formation of the corona and the acceleration of the solar wind should be treated as a single problem. The deposition of energy into the corona through some mechanical or electromagnetic energy flux is balanced by the various sinks available to the corona, and the sum of these processes determines the coronal structure, i.e. its temperature and density. Heating of the extended solar corona leads to high proton and ion temperatures and relatively low electron temperatures. This is due to the low heat conductivity in the proton (ion) gas as compared to the electrons. To a fairly good approximation we can say that most of the energy flux deposited in the protons and ions is lost as kinetic and gravitational energy flux in the solar wind flow, whereas a large fraction of the energy flux added to the electrons is conducted back into the transition region and lost as radiation. In order to drive high speed wind most of the energy must be deposited in the ions. Title: Evidence in Support of the ``Nanoflare'' Picture of Coronal Heating from SUMER Data Authors: Judge, P. G.; Hansteen, V.; Wikstøl, Ø.; Wilhelm, K.; Schühle, U.; Moran, T. Bibcode: 1998ApJ...502..981J Altcode: We study high signal-to-noise profiles of O IV emission lines obtained using the SUMER instrument on SOHO. Data for the quiet Sun obtained close to the disk center and at the solar limb were acquired. After careful data processing in which disk data were analyzed differentially against limb data, we find a systematic correlation between a density-sensitive emission-line ratio and Doppler shift across the same emission-line profiles. While unidentified blended lines cannot be completely discounted, the data suggest that the effects of such blends are small. Based on theoretical results in an earlier paper, we argue that if wave motions are responsible for the observed behavior, then the data reveal evidence for compressive waves propagating downward from the corona to the chromosphere. This analysis naturally lends support to the dominance of the ``nanoflare'' mechanism for coronal heating over other theories that invoke upward wave propagation, but other mechanisms capable of generating downward-propagating waves cannot be discounted. If, instead, steady flows are the cause of the observed behavior, such as return flows from spicules, then they must be such that the density is higher in the downflowing plasma. While these particular data do not allow us to discriminate between waves or steady flows, additional data from SOHO should be able to address this problem. This work required and achieved very accurate wavelength calibrations (better than 1/5 of a pixel on the detectors), taking SUMER close to its observational capabilities. We therefore present the elements of the analysis and calibration of SUMER data that may be of interest to other users. Title: On Inferring the Properties of Dynamic Plasmas from Their Emitted Spectra: The Case of the Solar Transition Region Authors: Wikstøl, Øivind; Judge, P. G.; Hansteen, Viggo Bibcode: 1998ApJ...501..895W Altcode: We reexamine the issue of inferring physical properties of solar plasmas using EUV and UV observations. We focus on the question of whether one can determine if typical structures seen as bright in typical ``transition-region'' lines are formed in the thermal interface between the coronal and chromospheric plasmas. Since 1983, Feldman and colleagues have proposed, based upon Skylab and other data, that much of the transition-region emission is formed in so-called unresolved fine structures (UFS) that are magnetically and thermally disconnected from the corona. This has led others to consider theoretical models of the transition region that differ from classical models. We examine the evidence cited in support of the UFS picture, specifically by relaxing the implicit assumption of a static atmospheric structure. Noting that observational data alone do not contain the information necessary to infer essential properties of the emitting plasmas, we argue that additional information must be added through forward calculations using physical models. MHD models of coronal flux tubes are then examined with explicit assumptions and boundary conditions, not as an attempt to ``fit'' observed data, but in order to study the formation of emission lines in dynamically evolving plasmas that are unresolved in space and time. We show that incorrect conclusions can be drawn by applying reasonable and traditional diagnostic methods to spectral data when unresolved dynamic evolution of the emitting plasma is important but not accounted for. In the particular case of the transition region, we show that the UFS interpretation is not unique, and is likely to be incorrect in the presence of unresolved dynamics. Most or all of the evidence for UFS is amenable to a different, equally reasonable interpretation, in which the transition-region emission is at all times formed in the time-varying thermal interface between the corona and the chromosphere. This work is likely to be important for a wider range of astrophysical plasmas than simply in the solar transition region. At stake is our basic ability to correctly diagnose physical conditions of plasmas for which heating mechanisms are not yet understood, but which are likely to be time dependent. Title: Acceleration of the Solar Wind: A New View Authors: Hansteen, V. H.; Leer, E. Bibcode: 1998HiA....11..838H Altcode: No abstract at ADS 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: Understanding the Solar Wind Authors: Leer, E.; Hansteen, V. H.; Holzer, T. E. Bibcode: 1998cvsw.conf..263L Altcode: No abstract at ADS Title: Signatures of Acoustic and Magnetic Waves in Solar and Stellar Coronae Authors: Wikstøl, Øivind; Judge, Philip G.; Hansteen, Viggo H. Bibcode: 1997ApJ...483..972W Altcode: Time-dependent dynamical calculations of the radiating gas in solar coronal flux tubes are used to identify features of UV spectral-line profiles that can reveal the direction in which wave energy flows through the solar transition region. The profile features survive spatial and temporal averaging through nonlinear dependencies of line emission coefficients on thermal properties of the plasma that are correlated with fluid velocities. This approach can be applied to stellar and other unresolved sources, as well as the solar atmosphere. It can be regarded as a new angle of attack on the long-standing problem of determining coronal heating mechanisms. The approach requires low noise data of high spectral resolution. Therefore, it can take advantage of some unique properties of the SUMER instrument on SOHO. We make specific predictions for SUMER data that, in principle, can test whether energy propagates upward or downward in coronal flux tubes, thus allowing one to discriminate between competing theories of coronal heating. We are acquiring SUMER data in an attempt to do this. Title: The Role of Helium in the Outer Solar Atmosphere Authors: Hansteen, V. H.; Leer, E.; Holzer, T. E. Bibcode: 1997ApJ...482..498H Altcode: We construct models of the outer solar atmosphere comprising the region from the mid-chromosphere and into the solar wind in order to study the force and energy balance in models with a significant helium abundance. The corona is created by dissipation of an energy flux from the Sun. The energy flux is lost as radiation from the top of the chromosphere and as gravitational and kinetic solar wind energy flux. We find that in models with significant ion heating of the extended corona most of the energy flux is lost in the solar wind. The ion temperatures are higher than the electron temperature in these models, and the α-particle temperature is much higher than the proton temperature, so there is energy transfer from the α-particle fluid to the protons and electrons, but this energy exchange between the different species is relatively small. To a fairly good approximation we can say that the energy flux deposited in the protons and α-particles is lost as kinetic and gravitational energy flux in the proton and α-particle flow. How this energy flux is divided between gravitational and kinetic energy flux (i.e., how large the particle fluxes and flow speeds are) depends upon details of the heating process. We also find that mixing processes in the chromosphere play an important role in determining the coronal helium abundance and the relative solar wind proton and α-particle fluxes. Roughly speaking, we find that the relative α-particle and proton fluxes are set by the degree of chromospheric mixing, while the speeds are set by the details of the coronal heating process. Title: Evidence in Support of the ``Nanoflare Picture'' of Coronal Heating from SUMER Data Authors: Judge, P.; Wikstol, O.; Hansteen, V. Bibcode: 1997SPD....28.0505J Altcode: 1997BAAS...29..909J We study high signal\--to\--noise profiles of O 4 emission lines obtained using the SUMER instrument on SoHO. Data for the quiet Sun obtained close to disk center and at the solar limb were acquired. After careful data processing in which disk data were analyzed differentially against limb data, we find a systematic correlation between a density sensitive emission line ratio and Doppler shift across the same emission line profiles. Based upon theoretical results in an earlier paper, we argue that if wave motions are responsible for the observed behavior, then the data reveal evidence for compressive waves propagating downwards from the corona to the chromosphere. This analysis therefore lends support to the dominance of the ``nanoflare'' mechanism for coronal heating (\cite{Parker1988}) over other theories that invoke upward wave propagation. If instead steady flows are the cause of the observed behavior, such as return flows from spicules, then they must be such that the density is higher in the downflowing plasma. We present time series data of other transition region lines from SoHO that suggest that episodic heating such as may occur through nanoflares is a more reasonable picture of heating mechanisms than models based upon steady flows. Title: The Coronal Helium Abundance and the Solar Wind Authors: Hansteen, Viggo H.; Hassler, Donald M.; Leer, Egil; Holzer, Thomas E.; Woods, Thomas N. Bibcode: 1997SPD....28.0154H Altcode: 1997BAAS...29Q.889H The coronal Helium abundance depends on the rate Helium is brought into the corona via the transition region from the chromosphere and on the rate that Helium is removed from the corona in the Solar wind. Recent multi-fluid models of the combined chromosphere, corona, solar wind system show that the corona may have a significant Helium abundance; perhaps even exceeding 50% of the Hydrogen number density. These models also indicate that in order to reproduce the Solar wind at 1AU ions (alpha -particles as well as protons) may be required to be the most important recipient of the coronal heating process. In these models the role of electrons in the energetics of the Solar wind is much reduced compared to the standard thermally driven winds. A measurement of the coronal Helium abundance will serve to fix these theoretical ideas and may give important clues as to mixing processes in the chromosphere as well as to coronal heating processes. Consequently, we will discuss the current state of observations (i.e. SOHO), and their limitations, as well as plans for future observations (i.e. sounding rocket, Spartan). Title: Kinetic electrons in high-speed solar wind streams: Formation of high-energy tails Authors: Lie-Svendsen, Øystein; Hansteen, Viggo H.; Leer, Egil Bibcode: 1997JGR...102.4701L Altcode: We study the evolution of the electron velocity distribution function in high-speed solar wind streams from the collision-dominated corona and into the collisionless interplanetary space. The model we employ solves the kinetic transport equation with the Fokker-Planck collision operator to describe Coulomb collisions between electrons. We use a test particle approach, where test electrons are injected into a prescribed solar wind background. The density, temperature, and electric field associated with the background are computed from fluid models. The test electrons are in thermal equilibrium with the background at the base of the corona, and we study the evolution of the velocity distribution of the test electrons as a function of altitude. We find that velocity filtration, due to the energy dependence of the Coulomb cross section, is a small effect and is not capable of producing significant beams in the distribution or a temperature moment that increases with altitude. The distribution function is mainly determined by the electric field and the expanding geometry and consists of a population with an almost isotropic core which is bound in the electrostatic potential and a beam-like high-energy tail which escapes. The trapped electrons contribute significantly to the even moments of the distribution function but almost nothing to the odd moments; the drift speed and energy flux moments are carried solely by the tail. In order to describe the high-speed solar wind observed near 0.3 AU by the Helios spacecraft, we use a multifluid model where ions are heated preferentially. The resulting test electron distribution at 0.3 AU, in this background, is in very good agreement with the velocity distributions observed by the Helios spacecraft. Title: Waves, Shocks and Non-Stationary Phenomena in the Outer Solar Atmosphere Authors: Hansteen, V. H. Bibcode: 1997ESASP.404...45H Altcode: 1997soho....5...45H; 1997cswn.conf...45H No abstract at ADS Title: SUMER Observations Detecting Downward Propagating Waves in the Solar Transition Region Authors: Wiskstøl, Ø.; Judge, P. G.; Hansteen, V.; Wilhelm, K.; Schühle, U.; Moran, T. Bibcode: 1997ESASP.404..731W Altcode: 1997cswn.conf..731W No abstract at ADS Title: Temporal Variability in the Quiet Sun Transition Region Authors: Wikstoøl, Ø.; Hansteen, V. H.; Brynildsen, N.; Maltby, P.; Kyeldseth-Moe, O.; Harrison, R. A.; Wilhelm, K.; Tarbell, T. D.; Scherrer, P. H. Bibcode: 1997ESASP.404..733W Altcode: 1997cswn.conf..733W No abstract at ADS 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: Transition region lineshifts and nanoflare heating of the corona. Authors: Hansteen, V. H. Bibcode: 1997smf..conf...87H Altcode: After briefly reviewing previous data and new observations taken by the SUMER instrument aboard the SOHO spacecraft the author pursues the interpretation that the observed pervasive transition region line shifts are caused by MHD waves propagating along the magnetic field lines down from the corona towards the chromosphere. He argues that a likely source of such coronally generated MHD waves are the episodic magnetic reconection events that are believed to heat the corona. He also presents an alternate method of observation based on densitive sensitive line pairs that may give further evidence of the processes heating the corona. Title: Solar magnetic fields Authors: Hansteen, V. H. Bibcode: 1997smf..conf.....H Altcode: No abstract at ADS Title: Helium in the outer solar atmosphere Authors: Hansteen, V. H.; Leer, E.; Holzer, T. E. Bibcode: 1997AIPC..385..197H Altcode: 1997recs.conf..197H We construct models of the outer solar atmosphere comprising the region from the mid chromosphere and into the solar wind in order to study the force and energy balance in models with a significant helium abundance. The corona is created by dissipation of an energy flux from the Sun. The energy flux is lost as radiation from the top of the chromosphere and as gravitational and kinetic solar wind energy flux. We find that in models with significant ion heating of the extended corona most of the energy flux is lost in the solar wind. The ion temperatures are higher than the electron temperature in these models, and the α-particle temperature is much higher than the proton temperature. Roughly speaking we find that the relative α-particle and proton fluxes are set by the degree of chromospheric mixing while the speeds are set by the details of the coronal heating process. Title: Acceleration of the Solar Wind Authors: Holzer, T. E.; Hansteen, V. H.; Leer, E. Bibcode: 1997cwh..conf..239H Altcode: 2006mslp.conf..239H No abstract at ADS Title: Observational signatures of nanoflare heated solar stellar coronae Authors: Wikstøl, Ø.; Judge, P. G.; Hansteen, V. H. Bibcode: 1997AdSpR..20.2289W Altcode: Using time-dependent dynamical models of the radiating gas in coronal flux tubes, we identify features in UV spectral line profiles that can reveal the direction in which energy flows through the solar transition region, in observations without temporal or spatial resolution. The profile features survive spatial and temporal averaging through non-linear dependencies of the line emission coefficients on thermal properties of the plasma that are correlated with the material velocity. This approach requires only low noise data of high spectral resolution and could naturally be applied to stars as well as the solar corona. We make predictions for the SUMER instrument that can in principle test whether energy propagates upwards or downwards in coronal flux tubes, suggesting a new angle of attack on the long standing problem of determining coronal heating mechanisms. Title: Outflow of He+ from the polar ionosphere: Comparison of hydrodynamic and kinetic descriptions Authors: Leer, Egil; Lie-Svendsen, Øystein; Olsen, Espen Lyngdal; Hansteen, Viggo H. Bibcode: 1996JGR...10117207L Altcode: Singly ionized helium, a minor species in the polar ionosphere, is being propelled out along open magnetic field lines by an outward polarization electric field. In the present study we compare the kinetic and hydrodynamic descriptions of this transonic outflow treating the singly ionized helium as a test particle population in a static background of singly ionized oxygen and electrons. We find that the resultant He+ particle fluxes are equal in both the eight-moment hydrodynamic description and in the kinetic description based on the Fokker-Planck equation. A five-moment hydrodynamic description gives a flux that is some 40% lower. The increase of the He+ energy flux with altitude is also equal in the eight-moment hydrodynamic and in the kinetic descriptions. Title: Solar wind theory and modeling Authors: Hansteen, Viggo H. Bibcode: 1996AIPC..382...44H Altcode: No abstract at ADS Title: Are the Redshifts Observed in Transition Region Lines Caused by Magnetic Reconnection? Authors: Hansteen, Viggo; Maltby, Per; Malagoli, Andrea Bibcode: 1996ASPC..111..116H Altcode: 1997ASPC..111..116H The detection of differential redshift between transition region and chromospheric line emission, both in spectra of late type stars and in solar spectra has been a puzzle for two decades. The fact that solar observations are inconsistent with a continuous downflow calls for another interpretation. The authors pursue the interpretation that the observed pervasive transition region line shifts are caused by MHD waves propagating along the magnetic field lines from the corona downward towards the chromosphere. Title: Non-Equilibrium Effects on the Optically Thin Radiative Loss Function Authors: Wikstol, O.; Hansteen, V. H. Bibcode: 1996mpsa.conf..465W Altcode: 1996IAUCo.153..465W No abstract at ADS Title: Coronal heating, densities, and temperatures and solar wind acceleration Authors: Hansteen, Viggo H.; Leer, Egil Bibcode: 1995JGR...10021577H Altcode: The outflow of coronal plasma into interplanetary space is a consequence of the coronal heating process. Therefore the formation of the corona and the acceleration of the solar wind should be treated as a single problem. The deposition of energy into the corona through some ``mechanical'' energy flux is balanced by the various energy sinks available to the corona, and the sum of these processes determines the coronal structure, i.e., its temperature and density. The corona loses energy through heat conduction into the transition region and through the gravitational potential energy and kinetic energy put into the solar wind. We show from a series of models of the chromosphere-transition region-corona-solar wind system that most of the energy deposited in a magnetically open region goes into the solar wind. The transition region pressures and the coronal density and temperature structure may vary considerably with the mode and location of energy deposition, but the solar wind mass flux is relatively insensitive to these variations; it is determined by the amplitude of the energy flux. In these models the transition region pressure decreases in accordance with the increasing coronal density scale height such that the solar wind mass loss is consistent with the energy flux deposited in the corona. On the basis of the present study we can conclude that the exponential increase of solar wind mass flux with coronal temperature, found in most thermally driven solar wind models, is a consequence of fixing the transition region pressure. Title: Solar wind theory and modelling Authors: Hansteen, Viggo H. Bibcode: 1995sowi.conf...29H Altcode: The outflow of coronal plasma into interplanetary space is a consequence of the coronal heating process. Therefore the formation of the corona and the acceleration of the solar wind should be treated as a single problem. Traditionally the mass or particle flux emanating from the extended corona has been thought of as being determined by the coronal temperature or scale height and the coronal (base) density. This argument follows from considerations of the momentum balance of the corona-wind system from which one obtains models of a close to hydrostatic corona out to the critical point where the flow becomes supersonic. With this approach to the acceleration of the wind is has been difficult to reconcile the relatively small variation observed in the proton flux at 1 AU with the predicted exponential dependence of the proton flux on the coronal temperature. In this talk we would like to emphasize another approach in which coronal energetics play the primary role. The deposition of energy into the corona through some 'mechanical' energy flux is balanced by the various energy sinks available to the corona and the sum of these processes determine the coronal structure, i.e. its temperature and density. The corona loses energy through heat conduction into the transition region, through radiative losses, and through the gravitational potential energy and kinetic energy put into the solar wind itself. We will show from a series of models of the chromosphere transition region-corona-solar wind system that most of the energy deposited in a magnetically open region will go into the solar wind, with roughly half going into kinetic energy and half into lifting the plasma out of the solar gravity field. The coronal base density will adjust itself in such a way that the heat conductive flux flowing into the transition region is radiated away in the upper chromosphere. The coronal temperature is set by the requirements that most of the deposited energy goes into accelerating the solar wind; the coronal scale height will adjust itself so that the solar wind energy losses conform to the amplitude of the input energy. These processes are modified by the 'mode' of energy deposition, and we will show the effects on coronal structure of changing the parameters describing coronal heating as well as the effects of including a helium fluid in the models. However, the location, scale height and/or form of the energy deposition (i.e. heating or direct acceleration) are not too important for the solar wind, the coronal density and temperature structure will vary with the 'mode' of energy deposition, but the solar wind mass flux depends mainly on the amplitude of the energy flux. Title: On the Relation between Coronal Heating, Flux Tube Divergence, and the Solar Wind Proton Flux and Flow Speed Authors: Sandbaek, Onulf; Leer, Egil; Hansteen, Viggo H. Bibcode: 1994ApJ...436..390S Altcode: A one-fluid solar wind model is used to investigate some relations between coronal heating, the flux tube divergence near the Sun, and the solar wind proton flux and flow speed. The effects of energy addition to the supersonic region of the flow are also studied. We allow for a mechanical energy flux that heats the corona, and an Alfven wave energy flux that adds energy, mainly to the supersonic flow, both as momentum and as heat. We find that the mechanical energy flux determines the solar wind mass flux, and in order to keep an almost constant proton flux at the orbit of Earth with changing flow geometry, that the mechanical energy flux must vary linearly with the magnetic field in the inner corona. This thermally driven wind generally has a low asymptotic flow speed. When Alfven waves are added to the thermally driven flow, the asymptotic flow speed is increased and is determined by the ratio of the Alfven wave and the mechanical energy fluxes at the coronal base. Flow speeds characteristic of recurrent high-speed solar wind streams can be obtained only when the Alfven wave energy flux, deposited in the supersonic flow, is larger than the mechanical energy flux heating the corona. Title: Transition region lineshifts in the rebound shock spicule model. Authors: Hansteen, V. H.; Wikstol, O. Bibcode: 1994A&A...290..995H Altcode: Spectral emission lines created in the solar chromosphere-corona transition region show Doppler shifts indicating widespread plasma down-flows. It has been proposed that this may be the result of the return of spicular material. We simulate a spicule numerically using the rebound shock model and find that the resulting hydrodynamic evolution leads to a perceived up-flow in transition region spectral lines even though the average velocity in the line forming region is directed downward. The explanation for this apparent paradox is found in the correlation between density and velocity in the waves generated by the rebound shock `spicule'. Title: Transition region line-shifts in the rebound shock spicule model Authors: Hansteen, V. H.; Wikstøl, Ø. Bibcode: 1994SSRv...70..103H Altcode: Spectral emission lines created in the solar chromosphere — corona transition region show net red-shifts. It has been proposed that this may be the result of the return of spicular material. We simulate a spicule numerically using the rebound shock model and find that the resulting hydrodynamic evolution leads to a perceived up-flow in transition region spectral lines even though the average velocity in the line forming region is directed downward. The explanation for this apparent paradox is found in the correlation between density and velocity in the waves generated by the rebound shock ‘spicule’. Title: Coupling of the coronal He abundance to the solar wind Authors: Hansteen, V. H.; Leer, E.; Holzer, T. E. Bibcode: 1994SSRv...70..347H Altcode: Models of the transition region — corona — solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the the solar wind proton flux. The thermal force on α-particles in the transition region sets the flow of helium into the corona. The frictional coupling between α-particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content. Title: Coupling of the Coronal Helium Abundance to the Solar Wind Authors: Hansteen, Viggo H.; Leer, Egil; Holzer, Thomas E. Bibcode: 1994ApJ...428..843H Altcode: Models of the transition region-corona-solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the solar wind proton flux. The thermal force on alpha-particles in the transition region sets the flow of helium into the corona. The frictional coupling between alpha-particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content. The models are constructed by solving the time-dependent population and momentum equations for all species of hydrogen and helium in an atmosphere with a given temperature profile. Several temperature profiles are considered in order to very the roles of frictional coupling and electric polarization field in the solar wind, and the thermal force in the transition region. Steady-state solutions are found for coronae with a hydrogen flux at 1 AU of 1.0 x 109/cm2/sec or larger. For coronae with lower hydrogen fluxes, the helium flux into the corona is larger than the flux 'pulled out' by the solar wind protons, and solutions with increasing coronal helium content are found. The timescale for forming a helium-filled corona, that may allow for a steady outflow, is long compared to the mixing time for the corona. Title: Redshifted transition region lines explained Authors: Hansteen, V.; Maltby, P. Bibcode: 1994AdSpR..14d..57H Altcode: 1994AdSpR..14...57H The discovery of differential redshift between transition region and chromospheric line emissions, both in the spectra of late-type stars and in the solar spectra has been a puzzle for more than a decade. The fact that the solar observations appear to be inconsistent with a continuous downflow calls for another interpretation. We propose that this pervasive redshift observed in transition region spectral lines is caused by conduction modified MHD waves propagating along the magnetic field lines from the corona towards the chromosphere. The waves are assumed generated in the corona by nanoflares, i.e.by an episodic heating mechanism. The calculations for acoustic waves show line profiles with many of the same characteristics as the observations. Title: Observational signatures of acoustic wave propagation in the solar transition region Authors: Wikstøl, Ø.; Hansteen, V. H. Bibcode: 1994chdy.conf...91W Altcode: No abstract at ADS Title: Solar Wind Acceleration (Invited) Authors: Hansteen, V. H. Bibcode: 1994scs..conf..453H Altcode: 1994IAUCo.144..453H The general aspects of solar wind acceleration are well described by considering the thermally driven outflow from an electron-proton corona. However, two puzzling observations remain to be explained: 1) The predicted asymptotic flow velocity is much lower than that observed in high speed streams, and 2) The proton flux observed at 1 AU varies considerably less than expected when considering the sensitivity of the proton flux to the coronal temperature predicted by thermally driven models. The solution of the first problem rests upon finding a mechanism which can deposit energy and/or momentum beyond the critical point of the flow. The invariance of the proton flux requires that a mechanism for maintaining a relatively constant proton density scale height in the subsonic region of the flow is found. One such possibility lies in considering the effects of an enhanced coronal helium abundance on the force balance of the subsonic flow. This scenario is discussed in some depth. Title: A New Interpretation of the Redshift Observed in Optically Thin Transition Region Lines Authors: Hansteen, Viggo Bibcode: 1993ApJ...402..741H Altcode: It is proposed that the pervasive redshift observed in transition region spectral lines is caused by downward propagating acoustic waves. The dynamic response of a coronal loop to energy released as heat near the loop apex is examined by solving the hydrodynamic equations numerically, consistently including the effects of nonequilibrium ionization on the radiative losses and on the internal energy. It is found that the radiative loss curve may change by a factor of 2 during the loop evolution as a result of flows and waves. A simple analytical analysis is performed to isolate the physical effects relevant to the line formation process. The amplitude of the line shift is found to depend on the characteristic time scale for ionization of the radiating ion as well as the global loop parameters, the time scales for loop cooling, the maximum temperature, and the base pressure. Title: Diffusion Effects on the Helium Abundance of the Solar Transition Region and Corona Authors: Hansteen, Viggo H.; Holzer, Thomas E.; Leer, Egil Bibcode: 1993ApJ...402..334H Altcode: The diffusion of helium in the solar transition region is studied by solving the mass and momentum conservation equations for a hydrogen-helium plasma given a representative temperature profile. Steady state solutions show that two distinct atmospheres may result. In cases where the thermal force on alpha-particles is balanced by the partial pressure gradient force, helium is the dominant coronal species. On the other hand, if it is the frictional force between protons and alpha-particles which balances the thermal force on alpha-particles then hydrogen is the major coronal component. In order to explore which of these solutions are attainable within reasonable time scales, the time-dependent equations are solved, starting from an initial state with a uniform helium abundance of 10 percent. The atmosphere as a whole is close to hydrostatic equilibrium, but due the thermal forces the individual elements are not. This force inbalance leads to a differential flow between species. It is found that this differential flow leads to a significant enhancement of the coronal helium abundance. Even for the relatively shallow temperature gradient used the helium abundance in the lower corona increases to 30 percent over a 24 hr period. Title: On Redshifts in Stellar Ultraviolet Emission Lines Authors: Hansteen, V.; Maltby, P. Bibcode: 1992ComAp..16..137H Altcode: No abstract at ADS Title: Signatures of episodic coronal heating in solar transition region spectral lines. Authors: Hansteen, V.; Maltby, P. Bibcode: 1992mrpa.work..127H Altcode: A brief introduction to the heating mechanisms in the solar atmosphere and to the observed redshifts in transition region lines is given. The authors discuss a new interpretation of the observed redshift and present calculations that support the new explanation. Title: The Effect of Waves on Optically Thin Transition Region Lines (With 2 Figures) Authors: Hansteen, V. Bibcode: 1991mcch.conf..347H Altcode: No abstract at ADS Title: Diffusion effects on the helium abundance of the solar transition region and corona. Authors: Hansteen, V. Bibcode: 1991cwlt.conf..171H Altcode: The mass and momentum conservation equations for a hydrogen-helium plasma are solved, given representative temperature profiles of the transition region and corona. Steady state solutions show that two distinct atmospheres may result depending on which term of the momentum equation balances the thermal force. If the partial pressure gradient dominates a helium filled corona occurs, while if the frictional force is dominant hydrogen is the major coronal constituent. The time-dependent problem is then solved, from an initially uniform helium abundance corona, to explore which of these solutions are attainable within reasonable timescales. The author has found that, even for the relatively shallow temperature gradients given by the observed emission measure, a significant enhancement of the coronal helium abundance occurs. Title: The Effect of MHD-Waves on Transition Region Spectral Lines Authors: Hansteen, V.; Maltby, P. Bibcode: 1990BAAS...22.1234H Altcode: No abstract at ADS Title: An Ultraviolet Spectral Atlas of a Prominence Authors: Engvold, Oddbjorn; Hansteen, V.; Kjeldseth-Moe, O.; Brueckner, G. E. Bibcode: 1990Ap&SS.170..179E Altcode: The ultraviolet spectrum of a large prominence has been observed with the High Resolution Telescope and Spectrograph (HRTS) on SPACELAB 2, 5 August, 1985. The spectrum covers the wavelength range λλ1335 1670 Å and shows numerous emission lines from gas at chromospheric and transition region temperatures. An Atlas of the prominence spectrum has been made showing absolute calibrated intensities on an accurate wavelength scale. The Atlas includes for comparison the corresponding UV-spectrum from an average quiet solar region. Title: Propagation of magnetohydrodynamic (MHD) waves in the solar transition region and corona. Authors: Leer, E.; Hansteen, V. Bibcode: 1990ppst.conf...81L Altcode: The propagation of MHD waves in the solar transition region and the corona is discussed. It is shown that the non-compressive Alfvén mode is the best candidate for energy transport into the corona and in the solar wind. Title: The Prominence/corona Transition Region Analyzed from SL-2 HRTS Authors: Engvold, O.; Hansteen, V.; Kjeldseth-Moe, O.; Brueckner, G. E. Bibcode: 1990LNP...363..250E Altcode: 1990IAUCo.117..250E; 1990doqp.coll..250E The ultraviolet spectrum of a large prominence has been observed with the High Resolution Telescope and Spectrograph (HRTS) on Spacelab 2 August 5, 1985. The spectrum covers the wavelength range 1335-1670Å and shows numerous emission lines from gas at chromospheric and transition region temperatures. A spectral atlas of these data is available. The data reveals a variation with height of the line intensities. The prominence becomes hotter with height. A value of ~ 0.12 dyn cm-2 for the gas pressure in the prominence-corona transition region is obtained from line ratios. The resolved fine structure of the He II 1640.400Å line indicates that a major part of this emission comes from cold gas. A broad Fe XI 1467.080Å suggests high velocities in the coronal cavity region. The Fe XI line in the cavity region is a factor ~ 5 less bright in the normal corona at the same height. Assuming that the temperature is the same in the two regions the present obervations suggest that the pressure in the cavity region is lower by a similar factor. Title: The effect of waves on optically thin transition region lines. Authors: Hansteen, V. Bibcode: 1990ppst.conf..185H Altcode: Numerical simulations of acoustic waves passing through plasma at transition region temperature and density are performed. The resultant effects on optically thin resonance line profiles are computed. The ionization balance of the emitting ion is solved consistently with the hydrodynamic equations. The results of the calculations show that acoustic waves can produce average line shifts on the order of the amplitude of the wave velocity. Title: The ionization and energy balance of the chromosphere-corona transition region. Authors: Hansteen, V. Bibcode: 1989ftsa.conf...35H Altcode: The hypothesis of heating the lower transition region by thermal conduction from the corona has been shown to contradict the observations. Emission measure analysis implies that heating and radiative losses are nearly in balance for the lower transition region, explaining this balance may require time dependent modeling. The geometry of the transition region is not known, though theoretical and observational arguments imply that it is fine structured, and further that not all gas at transition region temperatures is in thermal contact with the corona. Observations show that flows are prevalent throughout the transition region. This is important to the energy balance through the enthalpy flux. Furthermore, flows will impact the ionization balance and therefore the energy balance through the radiative loss rate. The ionization balance should also be considered in emission line diagnostics when flows are present, the author has shown the contribution function for emission lines to be sensitive to variations in the ionization balance. Title: The limb effect of the K i resonance line, 769.9 n m Authors: Andersen, B. N.; Barth, S.; Hansteen, V.; Leifsen, T.; Lilje, P. B.; Vikanes, F. Bibcode: 1985SoPh...99...17A Altcode: Low-noise observations have been obtained to search for a possible limb effect in the K I 769.9 nm resonance line. The observations were carried out along the north/south diameter of the solar disc. The data were individually corrected for the effects of straylight on the velocity measurements. A small, but significant limb effect is detected. The total shift in the line core from center to limb corresponds to 125 m s−1 with an uncertainty of < 30 m s−1. Title: Zwei Schreiben des Herrn Professors Hansteen an den Herausgeber Authors: Hansteen Bibcode: 1848AN.....27..161H Altcode: No abstract at ADS