Author name code: danilovic
ADS astronomy entries on 2022-09-14
author:"Danilovic, Sanja" 
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Title: Modeling of Chromospheric Features and Dynamics in Solar Plage
Authors: Danilovic, Sanja
Bibcode: 2022arXiv220803744D
Altcode:
  The chromosphere is a dynamic and complex layer where all the relevant
  physical processes happen on very small spatio-temporal scales. A
  few spectral lines that can be used as chromospheric diagnostics
  give us convoluted information that is hard to interpret without
  realistic theoretical models. What are the key ingredients that these
  models need to contain? The magnetic field has a paramount effect
  on chromospheric structuring. This is obvious from the ubiquitous
  presence of chromospheric dynamic fibrilar structures visible on the
  solar disk and at the limb. The numerical experiments presented in this
  manuscript illustrate the present state of modeling. They showcase
  to what extent our models reproduce various chromospheric features
  and their dynamics. The publication describes the effect different
  ingredients have on chromospheric models and provides a recipe for
  building one-to-one models. Combining these models with observations
  will provide insight into the physical processes that take place in
  the solar atmosphere.

Title: Rapid Blue- and Red-shifted Excursions in H$\alpha$ line
    profiles synthesized from realistic 3D MHD simulations
Authors: Danilovic, S.; Bjørgen, J. P.; Leenaarts, J.; Rempel, M.
Bibcode: 2022arXiv220813749D
Altcode:
  Rapid blue- and red-shifted events (RBEs/RREs) may have an important
  role in mass-loading and heating the solar corona, but their nature
  and origin are still debatable. We aim to model these features to
  learn more about their properties, formation and origin. A realistic
  three-dimensional (3D) magneto-hydrodynamic (MHD) model of a solar
  plage region is created. Synthetic H$\alpha$ spectra are generated
  and the spectral signatures of these features are identified. The
  magnetic field lines associated with these events are traced and the
  underlying dynamic is studied. The model reproduces well many properties
  of RBEs and RREs, such as spatial distribution, lateral movement,
  length and lifetimes. Synthetic H$\alpha$ line profiles, similarly to
  observed ones, show strong blue- or red-shift and asymmetries. These
  line profiles are caused by the vertical component of velocity with
  magnitudes larger than $30-40$ km/s that appear mostly in the height
  range of $2-4$ Mm. By tracing magnetic field lines, we show that the
  vertical velocity that causes the appearance of RBE/RREs to appear is
  always associated with the component of velocity perpendicular to the
  magnetic field line. The study confirms the hypothesis that RBEs and
  RREs are signs of Alfv{é}nic waves with, in some cases, a significant
  contribution from slow magneto-acoustic mode.

Title: Active region chromospheric magnetic fields. Observational
    inference versus magnetohydrostatic modelling
Authors: Vissers, G. J. M.; Danilovic, S.; Zhu, X.; Leenaarts, J.;
   Díaz Baso, C. J.; da Silva Santos, J. M.; de la Cruz Rodríguez,
   J.; Wiegelmann, T.
Bibcode: 2022A&A...662A..88V
Altcode: 2021arXiv210902943V
  Context. A proper estimate of the chromospheric magnetic fields is
  thought to improve modelling of both active region and coronal mass
  ejection evolution. However, because the chromospheric field is not
  regularly obtained for sufficiently large fields of view, estimates
  thereof are commonly obtained through data-driven models or field
  extrapolations, based on photospheric boundary conditions alone and
  involving pre-processing that may reduce details and dynamic range in
  the magnetograms. <BR /> Aims: We investigate the similarity between
  the chromospheric magnetic field that is directly inferred from
  observations and the field obtained from a magnetohydrostatic (MHS)
  extrapolation based on a high-resolution photospheric magnetogram. <BR
  /> Methods: Based on Swedish 1-m Solar Telescope Fe I 6173 Å and
  Ca II 8542 Å observations of NOAA active region 12723, we employed
  the spatially regularised weak-field approximation (WFA) to derive
  the vector magnetic field in the chromosphere from Ca II, as well as
  non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca
  II to infer a model atmosphere for selected regions. Milne-Eddington
  inversions of Fe I serve as photospheric boundary conditions for the
  MHS model that delivers the three-dimensional field, gas pressure,
  and density self-consistently. <BR /> Results: For the line-of-sight
  component, the MHS chromospheric field generally agrees with the
  non-LTE inversions and WFA, but tends to be weaker by 16% on average
  than these when larger in magnitude than 300 G. The observationally
  inferred transverse component is systematically stronger, up to an
  order of magnitude in magnetically weaker regions, but the qualitative
  distribution with height is similar to the MHS results. For either
  field component, the MHS chromospheric field lacks the fine structure
  derived from the inversions. Furthermore, the MHS model does not
  recover the magnetic imprint from a set of high fibrils connecting
  the main polarities. <BR /> Conclusions: The MHS extrapolation and
  WFA provide a qualitatively similar chromospheric field, where the
  azimuth of the former is better aligned with Ca II 8542 Å fibrils than
  that of the WFA, especially outside strong-field concentrations. The
  amount of structure as well as the transverse field strengths are,
  however, underestimated by the MHS extrapolation. This underscores the
  importance of considering a chromospheric magnetic field constraint in
  data-driven modelling of active regions, particularly in the context
  of space weather predictions.

Title: Heating of the solar chromosphere through current dissipation
Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de
   la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.;
   Rempel, M.
Bibcode: 2022A&A...661A..59D
Altcode: 2022arXiv220203955D
  Context. The solar chromosphere is heated to temperatures higher than
  predicted by radiative equilibrium. This excess heating is greater
  in active regions where the magnetic field is stronger. <BR />
  Aims: We aim to investigate the magnetic topology associated with an
  area of enhanced millimeter (mm) brightness temperatures in a solar
  active region mapped by the Atacama Large Millimeter/submillimeter
  Array (ALMA) using spectropolarimetric co-observations with the 1-m
  Swedish Solar Telescope (SST). <BR /> Methods: We used Milne-Eddington
  inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions,
  and a magnetohydrostatic extrapolation to obtain constraints on the
  three-dimensional (3D) stratification of temperature, magnetic field,
  and radiative energy losses. We compared the observations to a snapshot
  of a magnetohydrodynamics simulation and investigate the formation
  of the thermal continuum at 3 mm using contribution functions. <BR />
  Results: We find enhanced heating rates in the upper chromosphere of up
  to ∼5 kW m<SUP>−2</SUP>, where small-scale emerging loops interact
  with the overlying magnetic canopy leading to current sheets as shown
  by the magnetic field extrapolation. Our estimates are about a factor
  of two higher than canonical values, but they are limited by the ALMA
  spatial resolution (∼1.2″). Band 3 brightness temperatures reach
  about ∼10<SUP>4</SUP> K in the region, and the transverse magnetic
  field strength inferred from the non-LTE inversions is on the order
  of ∼500 G in the chromosphere. <BR /> Conclusions: We are able to
  quantitatively reproduce many of the observed features including the
  integrated radiative losses in our numerical simulation. We conclude
  that the heating is caused by dissipation in current sheets. However,
  the simulation shows a complex stratification in the flux emergence
  region where distinct layers may contribute significantly to the
  emission in the mm continuum. <P />The movie is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>

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 (&lt;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
  (&lt;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: What drives peacock jets?
Authors: Danilovic, Sanja
Bibcode: 2021AGUFMSH43A..04D
Altcode:
  What drives peacock jets? Peacock jets are remarkable features
  that appear above light bridges or generally in newly emerging
  regions. Observed in chromospheric lines like Halpha and Ca II K,
  they show fine structures that resemble extended peacock tails. The
  size and dynamics of these features can vary, but the common property
  is a clear intrusion into a strong magnetic field region e.g. umbra or
  pore. By using MURaM code and simulating the observed magnetic field
  configuration, we report here on the first realistic model of such a
  feature. We follow its evolution from the formation till the destruction
  and discuss the acceleration and collimation mechanisms. We compare
  synthetic chromospheric and coronal observables with real observations
  and discuss similarities and differences.

Title: Non-LTE inversions of a confined X2.2 flare. I. The vector
    magnetic field in the photosphere and chromosphere
Authors: Vissers, G. J. M.; Danilovic, S.; de la Cruz Rodríguez,
   J.; Leenaarts, J.; Morosin, R.; Díaz Baso, C. J.; Reid, A.; Pomoell,
   J.; Price, D. J.; Inoue, S.
Bibcode: 2021A&A...645A...1V
Altcode: 2020arXiv200901537V
  Context. Obtaining an accurate measurement of magnetic field vector
  in the solar atmosphere is essential for studying changes in field
  topology during flares and reliably modelling space weather. <BR />
  Aims: We tackle this problem by applying various inversion methods to a
  confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017
  and comparing the photospheric and chromospheric magnetic field vector
  with the results of two numerical models of this event. <BR /> Methods:
  We obtained the photospheric magnetic field from Milne-Eddington
  and (non-)local thermal equilibrium (non-LTE) inversions of Hinode
  SOT/SP Fe I 6301.5 Å and 6302.5 Å. The chromospheric field was
  obtained from a spatially regularised weak-field approximation (WFA)
  and non-LTE inversions of Ca II 8542 Å observed with CRISP at the
  Swedish 1 m Solar Telescope. We investigated the field strengths
  and photosphere-to-chromosphere shear in the field vector. <BR />
  Results: The LTE- and non-LTE-inferred photospheric magnetic field
  components are strongly correlated across several optical depths in
  the atmosphere, with a tendency towards a stronger field and higher
  temperatures in the non-LTE inversions. For the chromospheric field,
  the non-LTE inversions correlate well with the spatially regularised
  WFA, especially in terms of the line-of-sight field strength and field
  vector orientation. The photosphere exhibits coherent strong-field
  patches of over 4.5 kG, co-located with similar concentrations
  exceeding 3 kG in the chromosphere. The obtained field strengths
  are up to two to three times higher than in the numerical models,
  while the photosphere-to-chromosphere shear close to the polarity
  inversion line is more concentrated and structured. <BR /> Conclusions:
  In the photosphere, the assumption of LTE for Fe I line formation
  does not yield significantly different magnetic field results in
  comparison to the non-LTE case, while Milne-Eddington inversions
  fail to reproduce the magnetic field vector orientation where Fe
  I is in emission. In the chromosphere, the non-LTE-inferred field
  is excellently approximated by the spatially regularised WFA. Our
  inversions confirm the locations of flux rope footpoints that have
  been predicted by numerical models. However, pre-processing and lower
  spatial resolution lead to weaker and smoother field in the models than
  what our data indicate. This highlights the need for higher spatial
  resolution in the models to better constrain pre-eruptive flux ropes.

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). <P />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. <P
  />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: Tracing the origin of spicules in 3D radiative MHD models
Authors: Danilovic, S.
Bibcode: 2020AGUFMSH004..07D
Altcode:
  Are the flux emergence and ambipolar diffusion essential for the
  production of solar spicules? What is the role of vorticity and magnetic
  reconnection? Are RBEs and RREs traces of plasma flows? We try to give
  answers to these questions by analyzing 3D model of a solar plage.

Title: Physical properties of bright Ca II K fibrils in the solar
    chromosphere
Authors: Kianfar, Sepideh; Leenaarts, Jorrit; Danilovic, Sanja;
   de la Cruz Rodríguez, Jaime; Díaz Baso, Carlos José
Bibcode: 2020A&A...637A...1K
Altcode: 2020arXiv200311302K
  Context. Broad-band images of the solar chromosphere in the Ca
  II H&amp;K line cores around active regions are covered with fine
  bright elongated structures called bright fibrils. The mechanisms
  that form these structures and cause them to appear bright are still
  unknown. <BR /> Aims: We aim to investigate the physical properties,
  such as temperature, line-of-sight velocity, and microturbulence,
  in the atmosphere that produces bright fibrils and to compare those
  to the properties of their surrounding atmosphere. <BR /> Methods:
  We used simultaneous observations of a plage region in Fe I 6301-2
  Å, Ca II 8542 Å, Ca II K, and Hα acquired by the CRISP and CHROMIS
  instruments on the Swedish 1 m Solar Telescope. We manually selected
  a sample of 282 Ca II K bright fibrils. We compared the appearance
  of the fibrils in our sample to the Ca II 8542 Å and Hα data. We
  performed non-local thermodynamic equilibrium inversions using the
  inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, and Ca II
  K lines to infer the physical properties of the atmosphere. <BR />
  Results: The line profiles in bright fibrils have a higher intensity in
  their K<SUB>2</SUB> peaks compared to profiles formed in the surrounding
  atmosphere. The inversion results show that the atmosphere in fibrils is
  on average -100 K hotter at an optical depth log(τ<SUB>500 nm</SUB>)
  = -4.3 compared to their surroundings. The line-of-sight velocity
  at chromospheric heights in the fibrils does not show any preference
  towards upflows or downflows. The microturbulence in the fibrils is on
  average 0.5 km s<SUP>-1</SUP> higher compared to their surroundings. Our
  results suggest that the fibrils have a limited extent in height, and
  they should be viewed as hot threads pervading the chromosphere. <P
  />Movies associated to Figs. 9, 11, and 15 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202037572/olm">https://www.aanda.org</A>

Title: Science Requirement Document (SRD) for the European Solar
    Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
   Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
   Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
   K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
   Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
   T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
   J.; Utz, D.; Zuccarello, F.
Bibcode: 2019arXiv191208650S
Altcode:
  The European Solar Telescope (EST) is a research infrastructure
  for solar physics. It is planned to be an on-axis solar telescope
  with an aperture of 4 m and equipped with an innovative suite of
  spectro-polarimetric and imaging post-focus instrumentation. The EST
  project was initiated and is driven by EAST, the European Association
  for Solar Telescopes. EAST was founded in 2006 as an association
  of 14 European countries. Today, as of December 2019, EAST consists
  of 26 European research institutes from 18 European countries. The
  Preliminary Design Phase of EST was accomplished between 2008 and
  2011. During this phase, in 2010, the first version of the EST Science
  Requirement Document (SRD) was published. After EST became a project
  on the ESFRI roadmap 2016, the preparatory phase started. The goal
  of the preparatory phase is to accomplish a final design for the
  telescope and the legal governance structure of EST. A major milestone
  on this path is to revisit and update the Science Requirement Document
  (SRD). The EST Science Advisory Group (SAG) has been constituted by
  EAST and the Board of the PRE-EST EU project in November 2017 and has
  been charged with the task of providing with a final statement on the
  science requirements for EST. Based on the conceptual design, the SRD
  update takes into account recent technical and scientific developments,
  to ensure that EST provides significant advancement beyond the current
  state-of-the-art. The present update of the EST SRD has been developed
  and discussed during a series of EST SAG meetings. The SRD develops
  the top-level science objectives of EST into individual science
  cases. Identifying critical science requirements is one of its main
  goals. Those requirements will define the capabilities of EST and the
  post-focus instrument suite. The technical requirements for the final
  design of EST will be derived from the SRD.

Title: Three-dimensional modeling of chromospheric spectral lines
    in a simulated active region
Authors: Bjørgen, Johan P.; Leenaarts, Jorrit; Rempel, Matthias;
   Cheung, Mark C. M.; Danilovic, Sanja; de la Cruz Rodríguez, Jaime;
   Sukhorukov, Andrii V.
Bibcode: 2019A&A...631A..33B
Altcode: 2019arXiv190601098B
  Context. Because of the complex physics that governs the formation of
  chromospheric lines, interpretation of solar chromospheric observations
  is difficult. The origin and characteristics of many chromospheric
  features are, because of this, unresolved. <BR /> Aims: We focus on
  studying two prominent features: long fibrils and flare ribbons. To
  model these features, we use a 3D magnetohydrodynamic simulation of
  an active region, which self-consistently reproduces both of these
  features. <BR /> Methods: We modeled the Hα, Mg II k, Ca II K,
  and Ca II 8542 Å lines using the 3D non-LTE radiative transfer
  code Multi3D. To obtain non-LTE electron densities, we solved the
  statistical equilibrium equations for hydrogen simultaneously with the
  charge conservation equation. We treated the Ca II K and Mg II k lines
  with partially coherent scattering. <BR /> Results: This simulation
  reproduces long fibrils that span between the opposite-polarity
  sunspots and go up to 4 Mm in height. They can be traced in all lines
  owing to density corrugation. In contrast to previous studies, Hα,
  Mg II h&amp;k, and Ca II H&amp;K are formed at similar height in this
  model. Although some of the high fibrils are also visible in the Ca II
  8542 Å line, this line tends to sample loops and shocks lower in the
  chromosphere. Magnetic field lines are aligned with the Hα fibrils,
  but the latter holds to a lesser extent for the Ca II 8542 Å line. The
  simulation shows structures in the Hα line core that look like flare
  ribbons. The emission in the ribbons is caused by a dense chromosphere
  and a transition region at high column mass. The ribbons are visible in
  all chromospheric lines, but least prominent in Ca II 8542 Å line. In
  some pixels, broad asymmetric profiles with a single emission peak
  are produced similar to the profiles observed in flare ribbons. They
  are caused by a deep onset of the chromospheric temperature rise
  and large velocity gradients. <BR /> Conclusions: The simulation
  produces long fibrils similar to what is seen in observations. It
  also produces structures similar to flare ribbons despite the lack
  of nonthermal electrons in the simulation. The latter suggests that
  thermal conduction might be a significant agent in transporting flare
  energy to the chromosphere in addition to nonthermal electrons.

Title: Solar image denoising with convolutional neural networks
Authors: Díaz Baso, C. J.; de la Cruz Rodríguez, J.; Danilovic, S.
Bibcode: 2019A&A...629A..99D
Altcode: 2019arXiv190802815D
  The topology and dynamics of the solar chromosphere are greatly
  affected by the presence of magnetic fields. The magnetic field
  can be inferred by analyzing polarimetric observations of spectral
  lines. Polarimetric signals induced by chromospheric magnetic fields
  are, however, particularly weak, and in most cases very close to
  the detection limit of current instrumentation. Because of this,
  there are only few observational studies that have successfully
  reconstructed the three components of the magnetic field vector in the
  chromosphere. Traditionally, the signal-to-noise ratio of observations
  has been improved by performing time-averages or spatial averages, but
  in both cases, some information is lost. More advanced techniques, like
  principal-component analysis, have also been employed to take advantage
  of the sparsity of the observations in the spectral direction. In the
  present study, we use the spatial coherence of the observations to
  reduce the noise using deep-learning techniques. We designed a neural
  network that is capable of recovering weak signals under a complex
  noise corruption (including instrumental artifacts and non-linear
  post-processing). The training of the network is carried out without
  a priori knowledge of the clean signals, or an explicit statistical
  characterization of the noise or other corruption. We only use the same
  observations as our generative model. The performance of this method
  is demonstrated on both synthetic experiments and real data. We show
  examples of the improvement in typical signals obtained in current
  telescopes such as the Swedish 1 m Solar Telescope. The presented
  method can recover weak signals equally well no matter what spectral
  line or spectral sampling is used. It is especially suitable for cases
  when the wavelength sampling is scarce.

Title: STiC: A multiatom non-LTE PRD inversion code for full-Stokes
    solar observations
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
   Uitenbroek, H.
Bibcode: 2019A&A...623A..74D
Altcode: 2018arXiv181008441D
  The inference of the underlying state of the plasma in the solar
  chromosphere remains extremely challenging because of the nonlocal
  character of the observed radiation and plasma conditions in this
  layer. Inversion methods allow us to derive a model atmosphere that
  can reproduce the observed spectra by undertaking several physical
  assumptions. The most advanced approaches involve a depth-stratified
  model atmosphere described by temperature, line-of-sight velocity,
  turbulent velocity, the three components of the magntic field vector,
  and gas and electron pressure. The parameters of the radiative transfer
  equation are computed from a solid ground of physical principles. In
  order to apply these techniques to spectral lines that sample the
  chromosphere, nonlocal thermodynamical equilibrium effects must be
  included in the calculations. We developed a new inversion code STiC
  (STockholm inversion Code) to study spectral lines that sample the
  upper chromosphere. The code is based on the RH forward synthesis code,
  which we modified to make the inversions faster and more stable. For
  the first time, STiC facilitates the processing of lines from multiple
  atoms in non-LTE, also including partial redistribution effects (PRD)
  in angle and frequency of scattered photons. Furthermore, we include
  a regularization strategy that allows for model atmospheres with a
  complex depth stratification, without introducing artifacts in the
  reconstructed physical parameters, which are usually manifested in
  the form of oscillatory behavior. This approach takes steps toward
  a node-less inversion, in which the value of the physical parameters
  at each grid point can be considered a free parameter. In this paper
  we discuss the implementation of the aforementioned techniques, the
  description of the model atmosphere, and the optimizations that we
  applied to the code. We carry out some numerical experiments to show
  the performance of the code and the regularization techniques that we
  implemented. We made STiC publicly available to the community.

Title: Chromospheric condensations and magnetic field in a C3.6-class
    flare studied via He I D<SUB>3</SUB> spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
   Sanja; Leenaarts, Jorrit; Pazira, Hiva
Bibcode: 2019A&A...621A..35L
Altcode: 2018arXiv180606880L
  Context. Magnetic reconnection during flares takes place in the
  corona, but a substantial part of flare energy is deposited in
  the chromosphere. However, high-resolution spectro-polarimetric
  chromospheric observations of flares are very rare. The most used
  observables are Ca II 8542 Å and He I 10830 Å. <BR /> Aims:
  We aim to study the chromosphere during a C3.6 class flare via
  spectro-polarimetric observations of the He I D<SUB>3</SUB> line. <BR
  /> Methods: We present the first SST/CRISP spectro-polarimetric
  observations of He I D<SUB>3</SUB>. We analyzed the data using the
  inversion code HAZEL, and estimate the line-of-sight velocity and
  the magnetic field vector. <BR /> Results: Strong He I D<SUB>3</SUB>
  emission at the flare footpoints, as well as strong He I D<SUB>3</SUB>
  absorption profiles tracing the flaring loops are observed during the
  flare. The He I D<SUB>3</SUB> traveling emission kernels at the flare
  footpoints exhibit strong chromospheric condensations of up to ∼60
  km s<SUP>-1</SUP> at their leading edge. Our observations suggest that
  such condensations result in shocking the deep chromosphere, causing
  broad and modestly blueshifted He I D<SUB>3</SUB> profiles indicating
  subsequent upflows. A strong and rather vertical magnetic field of up
  to ∼2500 G is measured in the flare footpoints, confirming that the He
  I D<SUB>3</SUB> line is likely formed in the deep chromosphere at those
  locations. We provide chromospheric line-of-sight velocity and magnetic
  field maps obtained via He I D<SUB>3</SUB> inversions. We propose a
  fan-spine configuration as the flare magnetic field topology. <BR />
  Conclusions: The He I D<SUB>3</SUB> line is an excellent diagnostic to
  study the chromosphere during flares. The impact of strong condensations
  on the deep chromosphere has been observed. Detailed maps of the flare
  dynamics and the magnetic field are obtained.

Title: Solar Ultraviolet Bursts
Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.;
   Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal
   J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria
   S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark
   C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr
Bibcode: 2018SSRv..214..120Y
Altcode: 2018arXiv180505850Y
  The term "ultraviolet (UV) burst" is introduced to describe small,
  intense, transient brightenings in ultraviolet images of solar active
  regions. We inventorize their properties and provide a definition
  based on image sequences in transition-region lines. Coronal signatures
  are rare, and most bursts are associated with small-scale, canceling
  opposite-polarity fields in the photosphere that occur in emerging flux
  regions, moving magnetic features in sunspot moats, and sunspot light
  bridges. We also compare UV bursts with similar transition-region
  phenomena found previously in solar ultraviolet spectrometry and
  with similar phenomena at optical wavelengths, in particular Ellerman
  bombs. Akin to the latter, UV bursts are probably small-scale magnetic
  reconnection events occurring in the low atmosphere, at photospheric
  and/or chromospheric heights. Their intense emission in lines with
  optically thin formation gives unique diagnostic opportunities
  for studying the physics of magnetic reconnection in the low solar
  atmosphere. This paper is a review report from an International Space
  Science Institute team that met in 2016-2017.

Title: STiC: Stockholm inversion code
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
   Uitenbroek, H.
Bibcode: 2018ascl.soft10014D
Altcode:
  STiC is a MPI-parallel non-LTE inversion code for observed full-Stokes
  observations. The code processes lines from multiple atoms in non-LTE,
  including partial redistribution effects of scattered photons in
  angle and frequency of scattered photons (PRD), and can be used with
  model atmospheres that have a complex depth stratification without
  introducing artifacts.

Title: Chromospheric heating during flux emergence in the solar
    atmosphere
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Danilovic,
   Sanja; Scharmer, Göran; Carlsson, Mats
Bibcode: 2018A&A...612A..28L
Altcode: 2017arXiv171200474L
  Context. The radiative losses in the solar chromosphere vary from
  4 kW m<SUP>-2</SUP> in the quiet Sun, to 20 kW m<SUP>-2</SUP> in
  active regions. The mechanisms that transport non-thermal energy to
  and deposit it in the chromosphere are still not understood. Aim. We
  aim to investigate the atmospheric structure and heating of the solar
  chromosphere in an emerging flux region. <BR /> Methods: We have used
  observations taken with the CHROMIS and CRISP instruments on the
  Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα,
  and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line
  profiles and in addition perform multi-line, multi-species, non-local
  thermodynamic equilibrium (non-LTE) inversions to estimate the spatial
  and temporal variation of the chromospheric structure. <BR /> Results:
  We investigate which spectral features of Ca II K contribute to the
  frequency-integrated Ca II K brightness, which we use as a tracer
  of chromospheric radiative losses. The majority of the radiative
  losses are not associated with localised high-Ca II K-brightness
  events, but instead with a more gentle, spatially extended, and
  persistent heating. The frequency-integrated Ca II K brightness
  correlates strongly with the total linear polarization in the Ca II
  854.2 nm, while the Ca II K profile shapes indicate that the bulk
  of the radiative losses occur in the lower chromosphere. Non-LTE
  inversions indicate a transition from heating concentrated around
  photospheric magnetic elements below log τ<SUB>500</SUB> = -3 to a more
  space-filling and time-persistent heating above log τ<SUB>500</SUB>
  = -4. The inferred gas temperature at log τ<SUB>500</SUB> = -3.8
  correlates strongly with the total linear polarization in the Ca
  II 854.2 nm line, suggesting that that the heating rate correlates
  with the strength of the horizontal magnetic field in the low
  chromosphere. <P />Movies attached to Figs. 1 and 4 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201732027/olm">https://www.aanda.org/</A>

Title: Simulating Ellerman bomb-like events
Authors: Danilovic, S.
Bibcode: 2017A&A...601A.122D
Altcode: 2017arXiv170102112D
  Context. Ellerman bombs (EB) seem to be a part of a whole spectrum
  of phenomena that might have the same underlying physical mechanism:
  magnetic reconnection. <BR /> Aims: The aim of this study is to
  investigate whether the proposed mechanism, applied to the circumstances
  of EBs, produces the observed characteristics. <BR /> Methods:
  To this end, comprehensive three-dimensional magnetohydrodynamic
  (3D MHD) simulations were used. Two different cases are presented:
  the quiet Sun and an active region. <BR /> Results: Both runs confirm
  that EB-like brightenings coincide with hot and dense plasma, which is
  in agreement with predictions of 1D and 2D modellings. The simulated
  EB-like phenomena assume the observed flame-like form which depends on
  the complexity of the ongoing reconnection and the viewing angle. At
  the layers sampled by Hα-wings, near temperature minimum and below,
  the magnetic field topology seems always to be the same. The field
  lines there trace the base of the current sheet and the reconnected
  Ω-loops. <BR /> Conclusions: The EB features are caused by reconnection
  of strong-field patches of opposite polarity in the regions where the
  surface flows are the strongest. The weakest cases among them can be
  reproduced quantitatively by the current simulations.

Title: High-frequency Oscillations in Small Magnetic Elements Observed
    with Sunrise/SuFI
Authors: Jafarzadeh, S.; Solanki, S. K.; Stangalini, M.; Steiner,
   O.; Cameron, R. H.; Danilovic, S.
Bibcode: 2017ApJS..229...10J
Altcode: 2016arXiv161109302J
  We characterize waves in small magnetic elements and investigate
  their propagation in the lower solar atmosphere from observations at
  high spatial and temporal resolution. We use the wavelet transform to
  analyze oscillations of both horizontal displacement and intensity
  in magnetic bright points found in the 300 nm and the Ca II H 396.8
  nm passbands of the filter imager on board the Sunrise balloon-borne
  solar observatory. Phase differences between the oscillations at the
  two atmospheric layers corresponding to the two passbands reveal
  upward propagating waves at high frequencies (up to 30 mHz). Weak
  signatures of standing as well as downward propagating waves are also
  obtained. Both compressible and incompressible (kink) waves are found
  in the small-scale magnetic features. The two types of waves have
  different, though overlapping, period distributions. Two independent
  estimates give a height difference of approximately 450 ± 100 km
  between the two atmospheric layers sampled by the employed spectral
  bands. This value, together with the determined short travel times of
  the transverse and longitudinal waves provide us with phase speeds of 29
  ± 2 km s<SUP>-1</SUP> and 31 ± 2 km s<SUP>-1</SUP>, respectively. We
  speculate that these phase speeds may not reflect the true propagation
  speeds of the waves. Thus, effects such as the refraction of fast
  longitudinal waves may contribute to an overestimate of the phase speed.

Title: The Second Flight of the Sunrise Balloon-borne Solar
Observatory: Overview of Instrument Updates, the Flight, the Data,
    and First Results
Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic,
   S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott,
   D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.;
   Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez,
   J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta,
   J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.;
   Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati,
   L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.;
   Knölker, M.; Lecinski, A.
Bibcode: 2017ApJS..229....2S
Altcode: 2017arXiv170101555S
  The Sunrise balloon-borne solar observatory, consisting of a 1 m
  aperture telescope that provides a stabilized image to a UV filter
  imager and an imaging vector polarimeter, carried out its second science
  flight in 2013 June. It provided observations of parts of active regions
  at high spatial resolution, including the first high-resolution images
  in the Mg II k line. The obtained data are of very high quality, with
  the best UV images reaching the diffraction limit of the telescope
  at 3000 Å after Multi-Frame Blind Deconvolution reconstruction
  accounting for phase-diversity information. Here a brief update is
  given of the instruments and the data reduction techniques, which
  includes an inversion of the polarimetric data. Mainly those aspects
  that evolved compared with the first flight are described. A tabular
  overview of the observations is given. In addition, an example time
  series of a part of the emerging active region NOAA AR 11768 observed
  relatively close to disk center is described and discussed in some
  detail. The observations cover the pores in the trailing polarity of
  the active region, as well as the polarity inversion line where flux
  emergence was ongoing and a small flare-like brightening occurred in
  the course of the time series. The pores are found to contain magnetic
  field strengths ranging up to 2500 G, and while large pores are clearly
  darker and cooler than the quiet Sun in all layers of the photosphere,
  the temperature and brightness of small pores approach or even exceed
  those of the quiet Sun in the upper photosphere.

Title: Photospheric Response to an Ellerman Bomb-like Event—An
    Analogy of Sunrise/IMaX Observations and MHD Simulations
Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer,
   A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.;
   Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.;
   Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229....5D
Altcode: 2016arXiv160903817D
  Ellerman Bombs are signatures of magnetic reconnection, which is an
  important physical process in the solar atmosphere. How and where they
  occur is a subject of debate. In this paper, we analyze Sunrise/IMaX
  data, along with 3D MHD simulations that aim to reproduce the exact
  scenario proposed for the formation of these features. Although
  the observed event seems to be more dynamic and violent than the
  simulated one, simulations clearly confirm the basic scenario for the
  production of EBs. The simulations also reveal the full complexity of
  the underlying process. The simulated observations show that the Fe I
  525.02 nm line gives no information on the height where reconnection
  takes place. It can only give clues about the heating in the aftermath
  of the reconnection. However, the information on the magnetic field
  vector and velocity at this spatial resolution is extremely valuable
  because it shows what numerical models miss and how they can be
  improved.

Title: Are Internetwork Magnetic Fields in the Solar Photosphere
    Horizontal or Vertical?
Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S.
Bibcode: 2017ApJ...835...14L
Altcode:
  Using many observations obtained during 2007 with the
  Spectro-Polarimeter of the Hinode Solar Optical Telescope, we explore
  the angular distribution of magnetic fields in the quiet internetwork
  regions of the solar photosphere. Our work follows from the insight of
  Stenflo, who examined only linear polarization signals in photospheric
  lines, thereby avoiding complications of the analysis arising from the
  differing responses to linear and circular polarization. We identify
  and isolate regions of a strong polarization signal that occupy only
  a few percent of the observed quiet Sun area yet contribute most to
  the net linear polarization signal. The center-to-limb variation of
  the orientation of linear polarization in these strong signal regions
  indicates that the associated magnetic fields have a dominant vertical
  orientation. In contrast, the great majority of the solar disk is
  occupied by much weaker linear polarization signals. The orientation of
  the linear polarization in these regions demonstrates that the field
  orientation is dominantly horizontal throughout the photosphere. We
  also apply our analysis to Stokes profiles synthesized from the
  numerical MHD simulations of Rempel as viewed at various oblique
  angles. The analysis of the synthetic data closely follows that of
  the observations, lending confidence to using the simulations as a
  guide for understanding the physical origins of the center-to-limb
  variation of linear polarization in the quiet Sun area.

Title: Observed and simulated power spectra of kinetic and magnetic
    energy retrieved with 2D inversions
Authors: Danilovic, S.; Rempel, M.; van Noort, M.; Cameron, R.
Bibcode: 2016A&A...594A.103D
Altcode: 2016arXiv160706242D
  Context. Information on the origin of internetwork magnetic field is
  hidden at the smallest spatial scales. <BR /> Aims: We try to retrieve
  the power spectra with certainty to the highest spatial frequencies
  allowed by current instrumentation. <BR /> Methods: To accomplish this,
  we use a 2D inversion code that is able to recover information up to
  the instrumental diffraction limit. <BR /> Results: The retrieved power
  spectra have shallow slopes that extend further down to much smaller
  scales than has been found before. They do not seem to show any power
  law. The observed slopes at subgranular scales agree with those obtained
  from recent local dynamo simulations. Small differences are found for
  the vertical component of kinetic energy that suggest that observations
  suffer from an instrumental effect that is not taken into account. <BR
  /> Conclusions: Local dynamo simulations quantitatively reproduce the
  observed magnetic energy power spectra on the scales of granulation
  down to the resolution limit of Hinode/SP, within the error bars
  inflicted by the method used and the instrumental effects replicated.

Title: Internetwork magnetic field as revealed by two-dimensional
    inversions
Authors: Danilovic, S.; van Noort, M.; Rempel, M.
Bibcode: 2016A&A...593A..93D
Altcode: 2016arXiv160700772D
  Context. Properties of magnetic field in the internetwork regions
  are still fairly unknown because of rather weak spectropolarimetric
  signals. <BR /> Aims: We address the matter by using the two-dimensional
  (2D) inversion code, which is able to retrieve the information on
  smallest spatial scales up to the diffraction limit, while being less
  susceptible to noise than most of the previous methods used. <BR />
  Methods: Performance of the code and the impact of various effects
  on the retrieved field distribution is tested first on the realistic
  magneto-hydrodynamic (MHD) simulations. The best inversion scenario
  is then applied to the real data obtained by Spectropolarimeter (SP)
  on board Hinode. <BR /> Results: Tests on simulations show that: (1)
  the best choice of node position ensures a decent retrieval of all
  parameters; (2) the code performs well for different configurations
  of magnetic field; (3) slightly different noise levels or slightly
  different defocus included in the spatial point spread function
  (PSF) produces no significant effect on the results; and (4)
  temporal integration shifts the field distribution to a stronger,
  more horizontally inclined field. <BR /> Conclusions: Although the
  contribution of the weak field is slightly overestimated owing to noise,
  2D inversions are able to recover well the overall distribution of the
  magnetic field strength. Application of the 2D inversion code on the
  Hinode SP internetwork observations reveals a monotonic field strength
  distribution. The mean field strength at optical depth unity is ~
  130 G. At higher layers, field strength drops as the field becomes
  more horizontal. Regarding the distribution of the field inclination,
  tests show that we cannot directly retrieve it with the observations
  and tools at hand, however, the obtained distributions are consistent
  with those expected from simulations with a quasi-isotropic field
  inclination after accounting for observational effects.

Title: Variation of the Mn I 539.4 nm line with the solar cycle
Authors: Danilovic, S.; Solanki, S. K.; Livingston, W.; Krivova, N.;
   Vince, I.
Bibcode: 2016A&A...587A..33D
Altcode: 2015arXiv151101286D
  Context. As a part of the long-term program at Kitt Peak National
  Observatory (KPNO), the Mn I 539.4 nm line has been observed for
  nearly three solar cycles using the McMath telescope and the 13.5 m
  spectrograph in double-pass mode. These full-disk spectrophotometric
  observations revealed an unusually strong change of this line's
  parameters over the solar cycle. <BR /> Aims: Optical pumping by the Mg
  II k line was originally proposed to explain these variations. More
  recent studies have proposed that this is not required and that
  the magnetic variability (I.e., the changes in solar atmospheric
  structure due to faculae) might explain these changes. Magnetic
  variability is also the mechanism that drives the changes in total
  solar irradiance variations (TSI). With this work we investigate this
  proposition quantitatively by using the same model that was earlier
  successfully employed to reconstruct the irradiance. <BR /> Methods:
  We reconstructed the changes in the line parameters using the model
  SATIRE-S, which takes only variations of the daily surface distribution
  of the magnetic field into account. We applied exactly the same model
  atmospheres and value of the free parameter as were used in previous
  solar irradiance reconstructions to now model the variation in the Mn
  I 539.4 nm line profile and in neighboring Fe I lines. We compared
  the results of the theoretical model with KPNO observations. <BR />
  Results: The changes in the Mn I 539.4 nm line and a neighbouring Fe
  I 539.52 nm line over approximately three solar cycles are reproduced
  well by the model without additionally tweaking the model parameters,
  if changes made to the instrument setup are taken into account. The
  model slightly overestimates the change for the strong Fe I 539.32 nm
  line. <BR /> Conclusions: Our result confirms that optical pumping
  of the Mn II 539.4 nm line by Mg II k is not the main cause of its
  solar cycle change. It also provides independent confirmation of solar
  irradiance models which are based on the assumption that irradiance
  variations are caused by the evolution of the solar surface magnetic
  flux. The result obtained here also supports the spectral irradiance
  variations computed by these models.

Title: Simulated magnetic flows in the solar photosphere
Authors: Danilovic, S.; Cameron, R. H.; Solanki, S. K.
Bibcode: 2015A&A...574A..28D
Altcode: 2014arXiv1408.6159D
  Context. Recent Sunrise/IMaX observations have revealed supersonic
  magnetic flows. <BR /> Aims: Our aim is to determine the origin of
  these flows by using realistic magnetohydrodynamics simulations. <BR
  /> Methods: We simulated cancellation and emergence of magnetic
  flux through the solar photosphere. Our first numerical experiment
  started with a magnetic field of both polarities. To simulate
  emergence into a region with pre-existing field, we introduced a
  large-scale horizontally uniform sheet of a horizontal field. We
  followed the subsequent evolution and created synthetic polarimetric
  observations, including known instrumental effects of the Sunrise/IMaX
  and Hinode/SP instruments. We compared the simulated and observed
  spectropolarimetric signals. <BR /> Results: Strongly blue- and
  redshifted Stokes V signals are produced in locations where strong
  line-of-sight velocities coincide with the strong line-of-sight
  component of the magnetic field. The size and strength of simulated
  events is smaller than observed, and they are mostly associated with
  downflows, contrary to observations. In a few cases where they appear
  above a granule, single blue-lobed Stokes V are produced by strong
  gradients in magnetic field and velocity. No change of magnetic field
  sign is detected along the line of sight in these instances. More
  high-speed magnetised flows occurred when an emergence was simulated
  than when no horizontal field was added. <BR /> Conclusions: The
  simulations indicate that the observed events result from magnetic flux
  emergences in which reconnection may take place, but does not seem to
  be necessary. <P />The movies are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201423779/olm">http://www.aanda.org</A>

Title: Centre-to-limb properties of small, photospheric quiet-Sun jets
Authors: Rubio da Costa, F.; Solanki, S. K.; Danilovic, S.; Hizberger,
   J.; Martínez-Pillet, V.
Bibcode: 2015A&A...574A..95R
Altcode: 2014arXiv1412.1620R
  Context. Strongly Doppler-shifted Stokes V profiles have been detected
  in the quiet Sun with the IMaX instrument on-board the SUNRISE
  stratospheric balloon-borne telescope. High velocities are required
  to produce such signals, hence these events have been interpreted as
  jets, although other sources are also possible. <BR /> Aims: We aim
  to characterize the variation of the main properties of these events
  (occurrence rate, lifetime, size, and velocities) with their position on
  the solar disk between disk centre and the solar limb. <BR /> Methods:
  These events were identified in SUNRISE/IMaX data according to the same
  objective criteria at all available positions on the solar disk. Their
  properties were determined using standard techniques. <BR /> Results:
  Our study yielded a number of new insights into this phenomenon. Most
  importantly, the number density of these events is independent of
  the heliocentric angle, meaning that the investigated supersonic
  flows are nearly isotropically distributed. Size and lifetime are
  also nearly independent of the heliocentric angle, while their
  intensity contrast increases towards the solar limb. The Stokes V
  jets are associated with upflow velocities deduced from Stokes I,
  which are stronger towards the limb. Their intensity decreases with
  time, while their line-of-sight velocity does not display a clear
  temporal evolution. Their association with linear polarization signals
  decreases towards the limb. <BR /> Conclusions: The density of events
  appears to be independent of heliocentric angle, establishing that they
  are directed nearly randomly. If these events are jets triggered by
  magnetic reconnection between emerging magnetic flux and the ambient
  field, then our results suggest that there is no preferred geometry
  for the reconnection process.

Title: Inclinations of small quiet-Sun magnetic features based on
    a new geometric approach
Authors: Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; Bellot Rubio,
   L. R.; van Noort, M.; Feller, A.; Danilovic, S.
Bibcode: 2014A&A...569A.105J
Altcode: 2014arXiv1408.2443J
  Context. High levels of horizontal magnetic flux have been reported
  in the quiet-Sun internetwork, often based on Stokes profile
  inversions. <BR /> Aims: Here we introduce a new method for deducing
  the inclination of magnetic elements and use it to test magnetic field
  inclinations from inversions. <BR /> Methods: We determine accurate
  positions of a set of small, bright magnetic elements in high spatial
  resolution images sampling different photospheric heights obtained by
  the Sunrise balloon-borne solar observatory. Together with estimates
  of the formation heights of the employed spectral bands, these provide
  us with the inclinations of the magnetic features. We also compute
  the magnetic inclination angle of the same magnetic features from the
  inversion of simultaneously recorded Stokes parameters. <BR /> Results:
  Our new, geometric method returns nearly vertical fields (average
  inclination of around 14° with a relatively narrow distribution
  having a standard deviation of 6°). In strong contrast to this, the
  traditionally used inversions give almost horizontal fields (average
  inclination of 75 ± 8°) for the same small magnetic features,
  whose linearly polarised Stokes profiles are adversely affected by
  noise. We show that for such magnetic features inversions overestimate
  the flux in horizontal magnetic fields by an order of magnitude. <BR />
  Conclusions: The almost vertical field of bright magnetic features from
  our geometric method is clearly incompatible with the nearly horizontal
  magnetic fields obtained from the inversions. This indicates that the
  amount of magnetic flux in horizontal fields deduced from inversions is
  overestimated in the presence of weak Stokes signals, in particular if
  Stokes Q and U are close to or under the noise level. Inversions should
  be used with great caution when applied to data with no clear Stokes Q
  and no U signal. By combining the proposed method with inversions we are
  not just improving the inclination, but also the field strength. This
  technique allows us to analyse features that are not reliably treated
  by inversions, thus greatly extending our capability to study the
  complete magnetic field of the quiet Sun.

Title: Interpreting the Helioseismic and Magnetic Imager (HMI)
    Multi-Height Velocity Measurements
Authors: Nagashima, Kaori; Löptien, Björn; Gizon, Laurent; Birch,
   Aaron C.; Cameron, Robert; Couvidat, Sebastien; Danilovic, Sanja;
   Fleck, Bernhard; Stein, Robert
Bibcode: 2014SoPh..289.3457N
Altcode: 2014arXiv1404.3569N; 2014SoPh..tmp...84N
  The Solar Dynamics Observatory/Helioseismic and Magnetic Imager
  (SDO/HMI) filtergrams, taken at six wavelengths around the Fe I 6173.3
  Å line, contain information about the line-of-sight velocity over
  a range of heights in the solar atmosphere. Multi-height velocity
  inferences from these observations can be exploited to study wave
  motions and energy transport in the atmosphere. Using realistic
  convection-simulation datasets provided by the STAGGER and MURaM
  codes, we generate synthetic filtergrams and explore several methods
  for estimating Dopplergrams. We investigate at which height each
  synthetic Dopplergram correlates most strongly with the vertical
  velocity in the model atmospheres. On the basis of the investigation,
  we propose two Dopplergrams other than the standard HMI-algorithm
  Dopplergram produced from HMI filtergrams: a line-center Dopplergram
  and an average-wing Dopplergram. These two Dopplergrams correlate most
  strongly with vertical velocities at the heights of 30 - 40 km above
  (line center) and 30 - 40 km below (average wing) the effective height
  of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity
  information from two layers separated by about a half of a scale height
  in the atmosphere, at best. The phase shifts between these multi-height
  Dopplergrams from observational data as well as those from the simulated
  data are also consistent with the height-difference estimates in the
  frequency range above the photospheric acoustic-cutoff frequency.

Title: Vigorous convection in a sunspot granular light bridge
Authors: Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel;
   Danilovic, Sanja
Bibcode: 2014A&A...568A..60L
Altcode: 2014arXiv1407.1202L
  Context. Light bridges are the most prominent manifestation of
  convection in sunspots. The brightest representatives are granular
  light bridges composed of features that appear to be similar to
  granules. <BR /> Aims: An in-depth study of the convective motions,
  temperature stratification, and magnetic field vector in and around
  light bridge granules is presented with the aim of identifying
  similarities and differences to typical quiet-Sun granules. <BR />
  Methods: Spectropolarimetric data from the Hinode Solar Optical
  Telescope were analyzed using a spatially coupled inversion technique
  to retrieve the stratified atmospheric parameters of light bridge and
  quiet-Sun granules. <BR /> Results: Central hot upflows surrounded by
  cooler fast downflows reaching 10 km s<SUP>-1</SUP> clearly establish
  the convective nature of the light bridge granules. The inner part
  of these granules in the near surface layers is field free and is
  covered by a cusp-like magnetic field configuration. We observe
  hints of field reversals at the location of the fast downflows. The
  quiet-Sun granules in the vicinity of the sunspot are covered by a
  low-lying canopy field extending radially outward from the spot. <BR
  /> Conclusions: The similarities between quiet-Sun and light bridge
  granules point to the deep anchoring of granular light bridges in
  the underlying convection zone. The fast, supersonic downflows are
  most likely a result of a combination of invigorated convection
  in the light bridge granule due to radiative cooling into the
  neighboring umbra and the fact that we sample deeper layers, since the
  downflows are immediately adjacent to the slanted walls of the Wilson
  depression. <P />The two movies are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201424071/olm">http://www.aanda.org</A>

Title: Comparison between Mg II k and Ca II H Images Recorded by
    SUNRISE/SuFI
Authors: Danilovic, S.; Hirzberger, J.; Riethmüller, T. L.; Solanki,
   S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.; Knölker,
   M.; Schmidt, W.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.
Bibcode: 2014ApJ...784...20D
Altcode:
  We present a comparison of high-resolution images of the solar surface
  taken in the Mg II k and Ca II H channels of the Filter Imager on the
  balloon-borne solar observatory SUNRISE. The Mg and Ca lines are sampled
  with 0.48 nm and 0.11 nm wide filters, respectively. The two channels
  show remarkable qualitative and quantitative similarities in the quiet
  Sun, in an active region plage and during a small flare. However, the Mg
  filtergrams display 1.4-1.7 times higher intensity contrast and appear
  more smeared and smoothed in the quiet Sun. In addition, the fibrils
  in a plage are wider. Although the exposure time is 100 times longer
  for Mg images, the evidence suggests that these differences cannot be
  explained only with instrumental effects or the evolution of the solar
  scene. The differences at least partially arise because of different
  line-formation heights, the stronger response of Mg k emission peaks
  to the higher temperatures, and the larger height range sampled by
  the broad Mg filter used here. This is evidently manifested during
  the flare when a surge in Mg evolves differently than in Ca.

Title: Point spread function of SDO/HMI and the effects of stray
    light correction on the apparent properties of solar surface phenomena
Authors: Yeo, K. L.; Feller, A.; Solanki, S. K.; Couvidat, S.;
   Danilovic, S.; Krivova, N. A.
Bibcode: 2014A&A...561A..22Y
Altcode: 2013arXiv1310.4972Y
  <BR /> Aims: We present a point spread function (PSF) for the
  Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
  Observatory (SDO) and discuss the effects of its removal on the
  apparent properties of solar surface phenomena in HMI data. <BR />
  Methods: The PSF was retrieved from observations of Venus in transit
  by matching it to the convolution of a model of the Venusian disc and
  solar background with a guess PSF. We described the PSF as the sum of
  five Gaussian functions, the amplitudes of which vary sinusoidally with
  azimuth. This relatively complex functional form was required by the
  data. Observations recorded near in time to the transit of Venus were
  corrected for instrumental scattered light by the deconvolution with the
  PSF. We also examined the variation in the shape of the solar aureole in
  daily data, as an indication of PSF changes over time. <BR /> Results:
  Granulation contrast in restored HMI data is greatly enhanced relative
  to the original data and exhibit reasonable agreement with numerical
  simulations. Image restoration enhanced the apparent intensity and
  pixel averaged magnetic field strength of photospheric magnetic features
  significantly. For small-scale magnetic features, restoration enhanced
  intensity contrast in the continuum and core of the Fe I 6173 Å line
  by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For
  sunspots and pores, the enhancement varied strongly within and between
  features, being more acute for smaller features. Magnetic features are
  also rendered smaller, as signal smeared onto the surrounding quiet
  Sun is recovered. Image restoration increased the apparent amount of
  magnetic flux above the noise floor by a factor of about 1.2, most
  of the gain coming from the quiet Sun. Line-of-sight velocity due to
  granulation and supergranulation is enhanced by a factor of 1.4 to 2.1,
  depending on position on the solar disc. The shape of the solar aureole
  varied, with time and between the two CCDs. There are also indications
  that the PSF varies across the FOV. However, all these variations were
  found to be relatively small, such that a single PSF can be applied to
  HMI data from both CCDs, over the period examined without introducing
  significant error. <BR /> Conclusions: Restoring HMI observations
  with the PSF presented here returns a reasonable estimate of the stray
  light-free intensity contrast. Image restoration affects the measured
  radiant, magnetic and dynamic properties of solar surface phenomena
  sufficiently to significantly impact interpretation.

Title: First High-resolution Images of the Sun in the 2796 Å Mg II
    k Line
Authors: Riethmüller, T. L.; Solanki, S. K.; Hirzberger, J.;
   Danilovic, S.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.;
   Knölker, M.; Schmidt, W.; Del Toro Iniesta, J. C.
Bibcode: 2013ApJ...776L..13R
Altcode: 2013arXiv1309.5213R
  We present the first high-resolution solar images in the Mg II k 2796
  Å line. The images, taken through a 4.8 Å broad interference filter,
  were obtained during the second science flight of Sunrise in 2013 June
  by the Sunrise Filter Imager (SuFI) instrument. The Mg II k images
  display structures that look qualitatively very similar to images taken
  in the core of Ca II H. The Mg II images exhibit reversed granulation
  (or shock waves) in the internetwork regions of the quiet Sun, at
  intensity contrasts that are similar to those found in Ca II H. Very
  prominent in Mg II are bright points, both in the quiet Sun and in plage
  regions, particularly near the disk center. These are much brighter than
  at other wavelengths sampled at similar resolution. Furthermore, Mg II k
  images also show fibril structures associated with plage regions. Again,
  the fibrils are similar to those seen in Ca II H images, but tend to
  be more pronounced, particularly in weak plage.

Title: On the structure and dynamics of Ellerman bombs. Detailed
    study of three events and modelling of Hα
Authors: Bello González, N.; Danilovic, S.; Kneer, F.
Bibcode: 2013A&A...557A.102B
Altcode:
  <BR /> Aims: We study the structure and dynamics of three Ellerman
  bombs (EBs) observed in an evolving active region. <BR /> Methods: The
  active region NOAA 11271 was observed with the Vacuum Tower Telescope
  at Observatorio del Teide/Tenerife on August 18, 2011. We used the
  two-dimensional Triple Etalon SOlar Spectrometer (TESOS) to obtain time
  sequences of the active region and of EBs in Hα at a cadence of 15
  s. Simultaneously, we obtained full Stokes profiles with the Tenerife
  Infrared Polarimeter (TIP II) in the two magnetically sensitive Fe i
  infrared lines (IR) at 1.56 μ, scanning spatial sections of the area
  with cadences of 28-46 s. The Hα data were reconstructed with speckle
  methods to study the evolution of the atmospheric stratification. Two
  methods were used to extract magnetic field information from the IR
  Stokes profiles: 1) fitting of the (Q,U,V) profiles by Gaussians; and
  2) applying the Milne-Eddington approximation, assuming two separate
  magnetic structures in the resolution element and fitting by trial and
  error some profiles from the EB areas. Data from SDO-HMI and -AIA were
  also used. We performed two-dimensional (2D) non-LTE radiative transfer
  calculations of Hα in parameterised models of EBs. <BR /> Results:
  The three EBs studied in detail occurred in a complex active region near
  sunspots. They were very bright with a factor of 1.5-2.8 brighter than
  the nearby area. They lived for 1/2 h and longer. They were related to
  broadband faculae, but the latter were not the brightest features in the
  field of view. The EBs occurred in magnetic field configurations with
  opposite polarity close together. One EB was located at the outskirts
  of a penumbra of a complex sunspot and showed repeated "flaring" in
  SDO-AIA data. Another was close to a strong field patch and moved into
  this during the end of its lifetime. The third EB showed clear changes
  of field structure during the time it was observed. We obtained from
  the 2D modelling that heating and increase in Hα opacity are likely
  to occur at heights of 300-800 km. Line shifts and asymmetries can
  well be reproduced by velocities at these heights and also at much
  larger heights. <BR /> Conclusions: The three EBs occurred at sites
  with magnetic fields of opposite polarity, which were likely the cause
  of the Hα brightening upon reconnection.

Title: On the relation between continuum brightness and magnetic
    field in solar active regions
Authors: Danilovic, S.; Röhrbein, D.; Cameron, R. H.; Schüssler, M.
Bibcode: 2013A&A...550A.118D
Altcode:
  Context. Variations of solar irradiance are mainly determined
  by the changing coverage of the visible solar disk with magnetic
  flux concentrations. The relationship between brightness and field
  strength is an important ingredient for models and reconstructions of
  irradiance variations. <BR /> Aims: We assess the effect of limited
  observational resolution on the relationship between brightness
  and magnetic field by comparing comprehensive MHD simulations with
  observational results. <BR /> Methods: Simulations of magnetoconvection
  representing the near-surface layers of a plage region were used to
  determine maps of the continuum brightness and Stokes profiles for the
  Fe i line at 630.22 nm. After convolving with instrumental profiles,
  synthetic observations of the magnetic field were generated by applying
  a Stokes inversion code. We compare the resulting relation between
  brightness and apparent vertical magnetic field to the corresponding
  outcome derived from real observations of a plage region with the
  Hinode satellite. <BR /> Results: Consideration of the image smearing
  effects due to the limited resolution of the observations transform the
  largely monotonic relation between brightness and field strength at the
  original resolution of the simulations into a profile with a maximum
  at intermediate field strength, which is in good agreement with the
  observations. <BR /> Conclusions: Considering the effect of limited
  observational resolution renders the relation between brightness and
  magnetic field from comprehensive MHD simulations consistent with
  observational results. This is a necessary prerequisite for the
  utilization of simulations for models and reconstruction of solar
  irradiance variations.

Title: Structure and dynamics of isolated internetwork Ca II H bright
    points observed by SUNRISE
Authors: Jafarzadeh, S.; Solanki, S. K.; Feller, A.; Lagg, A.;
   Pietarila, A.; Danilovic, S.; Riethmüller, T. L.; Martínez Pillet, V.
Bibcode: 2013A&A...549A.116J
Altcode: 2012arXiv1211.4836J
  <BR /> Aims: We aim to improve our picture of the low chromosphere in
  the quiet-Sun internetwork by investigating the intensity, horizontal
  velocity, size and lifetime variations of small bright points (BPs;
  diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 Å
  passband along with their magnetic field parameters, derived from
  photospheric magnetograms. <BR /> Methods: Several high-quality
  time series of disc-centre, quiet-Sun observations from the Sunrise
  balloon-borne solar telescope, with spatial resolution of around 100
  km on the solar surface, have been analysed to study the dynamics
  of BPs observed in the Ca II H passband and their dependence on the
  photospheric vector magnetogram signal. <BR /> Results: Parameters such
  as horizontal velocity, diameter, intensity and lifetime histograms of
  the isolated internetwork and magnetic Ca II H BPs were determined. Mean
  values were found to be 2.2 km s<SUP>-1</SUP>, 0.2 arcsec (≈150 km),
  1.48 ⟨ I<SUB>Ca</SUB> ⟩ and 673 s, respectively. Interestingly, the
  brightness and the horizontal velocity of BPs are anti-correlated. Large
  excursions (pulses) in horizontal velocity, up to 15 km s<SUP>-1</SUP>,
  are present in the trajectories of most BPs. These could excite kink
  waves travelling into the chromosphere and possibly the corona, which we
  estimate to carry an energy flux of 310 W m<SUP>-2</SUP>, sufficient to
  heat the upper layers, although only marginally. <BR /> Conclusions:
  The stable observing conditions of Sunrise and our technique for
  identifying and tracking BPs have allowed us to determine reliable
  parameters of these features in the internetwork. Thus we find, e.g.,
  that they are considerably longer lived than previously thought. The
  large velocities are also reliable, and may excite kink waves. Although
  these wave are (marginally) energetic enough to heat the quiet corona,
  we expect a large additional contribution from larger magnetic elements
  populating the network and partly also the internetwork.

Title: First Results from the SUNRISE Mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
   Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller,
   T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González,
   M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta,
   J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González,
   N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt,
   W.; Steiner, O.; Title, A. M.
Bibcode: 2012ASPC..455..143S
Altcode:
  The SUNRISE balloon-borne solar observatory consists of a 1m aperture
  Gregory telescope, a UV filter imager, an imaging vector polarimeter,
  an image stabilization system, and further infrastructure. The first
  science flight of SUNRISE yielded high-quality data that reveal the
  structure, dynamics, and evolution of solar convection, oscillations,
  and magnetic fields at a resolution of around 100 km in the quiet
  Sun. Here we describe very briefly the mission and the first results
  obtained from the SUNRISE data, which include a number of discoveries.

Title: Observational signatures of simulated reconnection in solar
    photosphere
Authors: Danilovic, S.
Bibcode: 2012decs.confE..61D
Altcode:
  Recent IMaX/Sunrise observations reveal many short-lived high
  velocity flows that appear at very small scales everywhere in
  the quiet Sun. The flows usually appear close to the patches of
  opposite magnetic polarities. In some cases, the inversion result show
  localised temperature increase and strong downflows. Here, we confirm
  the hypothesis that some of the observed events are in fact produced
  by the reconnection of the emerging with the preexisting field. We
  concentrate on individual reconnection events in the realistic 3D MHD
  simulations and describe observational signatures that are likely to
  arise. The comparison of simulated with observed cases suggests that
  there might be currents sheets forming very low in the atmosphere. The
  retrieved temperatures and velocities imply that the observed events
  are much more energetic than the simulated cases.

Title: The Sun at high resolution: first results from the Sunrise
    mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
   A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
   Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
   Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
   Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
   Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
Bibcode: 2011IAUS..273..226S
Altcode:
  The Sunrise balloon-borne solar observatory consists of a 1m aperture
  Gregory telescope, a UV filter imager, an imaging vector polarimeter,
  an image stabilization system and further infrastructure. The first
  science flight of Sunrise yielded high-quality data that reveal the
  structure, dynamics and evolution of solar convection, oscillations
  and magnetic fields at a resolution of around 100 km in the quiet
  Sun. Here we describe very briefly the mission and the first results
  obtained from the Sunrise data, which include a number of discoveries.

Title: Transport of Magnetic Flux from the Canopy to the Internetwork
Authors: Pietarila, A.; Cameron, R. H.; Danilovic, S.; Solanki, S. K.
Bibcode: 2011ApJ...729..136P
Altcode: 2011arXiv1102.1397P
  Recent observations have revealed that 8% of linear polarization
  patches in the internetwork (INW) quiet Sun are fully embedded in
  downflows. These are not easily explained with the typical scenarios for
  the source of INW fields which rely on flux emergence from below. Using
  radiative MHD simulations, we explore a scenario where magnetic flux
  is transported from the magnetic canopy overlying the INW into the
  photosphere by means of downward plumes associated with convective
  overshoot. We find that if a canopy-like magnetic field is present in
  the simulation, the transport of flux from the canopy is an important
  process for seeding the photospheric layers of the INW with magnetic
  field. We propose that this mechanism is relevant for the Sun as well,
  and it could naturally explain the observed INW linear polarization
  patches entirely embedded in downflows.

Title: SUNRISE: Instrument, Mission, Data, and First Results
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
   Gandorfer, A.; Hirzberger, J.; Riethmüller, T. L.; Schüssler, M.;
   Bonet, J. A.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo,
   V.; Palacios, J.; Knölker, M.; Bello González, N.; Berkefeld, T.;
   Franz, M.; Schmidt, W.; Title, A. M.
Bibcode: 2010ApJ...723L.127S
Altcode: 2010arXiv1008.3460S
  The SUNRISE balloon-borne solar observatory consists of a 1 m aperture
  Gregory telescope, a UV filter imager, an imaging vector polarimeter,
  an image stabilization system, and further infrastructure. The first
  science flight of SUNRISE yielded high-quality data that revealed the
  structure, dynamics, and evolution of solar convection, oscillations,
  and magnetic fields at a resolution of around 100 km in the quiet
  Sun. After a brief description of instruments and data, the first
  qualitative results are presented. In contrast to earlier observations,
  we clearly see granulation at 214 nm. Images in Ca II H display narrow,
  short-lived dark intergranular lanes between the bright edges of
  granules. The very small-scale, mixed-polarity internetwork fields
  are found to be highly dynamic. A significant increase in detectable
  magnetic flux is found after phase-diversity-related reconstruction
  of polarization maps, indicating that the polarities are mixed right
  down to the spatial resolution limit and probably beyond.

Title: Transverse Component of the Magnetic Field in the Solar
    Photosphere Observed by SUNRISE
Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.;
   Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta,
   J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
   Knölker, M.; Schmidt, W.; Title, A. M.
Bibcode: 2010ApJ...723L.149D
Altcode: 2010arXiv1008.1535D
  We present the first observations of the transverse component of
  a photospheric magnetic field acquired by the imaging magnetograph
  SUNRISE/IMaX. Using an automated detection method, we obtain statistical
  properties of 4536 features with significant linear polarization
  signal. We obtain a rate of occurrence of 7 × 10<SUP>-4</SUP>
  s<SUP>-1</SUP> arcsec<SUP>-2</SUP>, which is 1-2 orders of magnitude
  larger than the values reported by previous studies. We show that
  these features have no characteristic size or lifetime. They appear
  preferentially at granule boundaries with most of them being caught
  in downflow lanes at some point. Only a small percentage are entirely
  and constantly embedded in upflows (16%) or downflows (8%).

Title: Probing quiet Sun magnetism using MURaM simulations and
Hinode/SP results: support for a local dynamo
Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K.
Bibcode: 2010A&A...513A...1D
Altcode: 2010arXiv1001.2183D
  Context. Owing to the limited spatial resolution and the weak
  polarization signal coming from the quietest regions on the Sun, the
  organization of the magnetic field on the smallest scales is largely
  unknown. <BR /> Aims: We obtain information about the magnetic flux
  present in the quiet Sun by comparing radiative MHD simulations with
  observations, with particular emphasis on the role of surface dynamo
  action. <BR /> Methods: We synthesized Stokes profiles on the basis of
  the MHD simulation results. The profiles are degraded by taking the
  properties of the spectropolarimeter (SP) into account onboard the
  Hinode satellite. We used simulation runs with different magnetic
  Reynolds numbers (R<SUB>m</SUB>) and observations at different
  heliocentric angles with different levels of noise. <BR /> Results:
  Simulations with an imposed mixed-polarity field and R<SUB>m</SUB>
  below the threshold for dynamo action reproduce the observed vertical
  flux density, but do not display a high enough horizontal flux
  density. Surface dynamo simulations at the highest R<SUB>m</SUB>
  feasible at the moment yield a ratio of the horizontal and vertical
  flux density consistent with observational results, but the overall
  amplitudes are too low. Based on the properties of the local dynamo
  simulations, a tentative scaling of the magnetic field strength
  by a factor 2-3 reproduces the signal observed in the internetwork
  regions. <BR /> Conclusions: We find agreement with observations at
  different heliocentric angles. The mean field strength in internetwork
  implied by our analysis is roughly 170 G at the optical depth unity. Our
  study shows that surface dynamo could be responsible for most of the
  magnetic flux in the quiet Sun outside the network, given that the
  extrapolation to higher R<SUB>m</SUB> is valid.

Title: The small-scale solar surface dynamo
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schuessler,
   Manfred
Bibcode: 2010arXiv1003.0347P
Altcode:
  The existence of a turbulent small-scale solar surface dynamo is likely,
  considering existing numerical and laboratory experiments, as well as
  comparisons of a small-scale dynamo in MURaM simulations with Hinode
  observations. We find the observed peaked probability distribution
  function (PDF) from Stokes-V magnetograms is consistent with a monotonic
  PDF of the actual vertical field strength. The cancellation function
  of the vertical flux density from a Hinode SP observation is found to
  follow a self-similar power law over two decades in length scales down
  to the ~200 km resolution limit. This provides observational evidence
  that the scales of magnetic structuring in the photosphere extend
  at least down to 20 km. From the power law, we determine a lower
  bound for the true quiet-Sun mean vertical unsigned flux density of
  ~43 G, consistent with our numerically-based estimates that 80% or
  more of the vertical unsigned flux should be invisible to Stokes-V
  observations at a resolution of 200 km owing to cancellation. Our
  estimates significantly reduce the order-of-magnitude discrepancy
  between Zeeman- and Hanle-based estimates.

Title: Magnetic field intensification: comparison of 3D MHD
    simulations with Hinode/SP results
Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K.
Bibcode: 2010A&A...509A..76D
Altcode: 2009arXiv0910.1211D
  Context. Recent spectro-polarimetric observations have provided detailed
  measurements of magnetic field, velocity and intensity during events of
  magnetic field intensification in the solar photosphere. <BR /> Aims:
  By comparing with synthetic observations derived from MHD simulations,
  we investigate the physical processes underlying the observations,
  as well as verify the simulations and the interpretation of the
  observations. <BR /> Methods: We consider the temporal evolution of
  the relevant physical quantities for three cases of magnetic field
  intensification in a numerical simulation. In order to compare with
  observations, we calculate Stokes profiles and take into account the
  spectral and spatial resolution of the spectropolarimeter (SP) on
  board Hinode. We determine the evolution of the intensity, magnetic
  flux density and zero-crossing velocity derived from the synthetic
  Stokes parameters, using the same methods as applied to the Hinode/SP
  observations to derive magnetic field and velocity information from
  the spectro-polarimetric data. <BR /> Results: The three events
  considered show a similar evolution: advection of magnetic flux to a
  granular vertex, development of a strong downflow, evacuation of the
  magnetic feature, increase of the field strength and the appearance
  of the bright point. The magnetic features formed have diameters of
  0.1-0.2´´. The downflow velocities reach maximum values of 5-10
  km s<SUP>-1</SUP> at τ = 1. In the largest feature, the downflow
  reaches supersonic speed in the lower photosphere. In the same case,
  a supersonic upflow develops approximately 200 s after the formation of
  the flux concentration. We find that synthetic and real observations
  are qualitatively consistent and, for one of the cases considered,
  also agree very well quantitatively. The effect of finite resolution
  (spatial smearing) is most pronounced in the case of small features,
  for which the synthetic Hinode/SP observations miss the bright point
  formation and also the high-velocity downflows during the formation
  of the smaller magnetic features. <BR /> Conclusions: The observed
  events are consistent with the process of field intensification by
  flux advection, radiative cooling, and evacuation by strong downflow
  found in MHD simulations. The quantitative agreement of synthetic and
  real observations indicates the validity of both the simulations and
  the interpretations of the spectro-polarimetric observations.

Title: Relation between the Sunrise photospheric magnetic field and
    the Ca II H bright features
Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.;
   Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.;
   Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez
   Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A.
Bibcode: 2010cosp...38.2856J
Altcode: 2010cosp.meet.2856J
  Recent observations from the Sunrise balloon-borne solar telescope
  have enabled us to reach an unprecedented high spatial resolution
  on the solar surface with the near-ultraviolet photo-spheric and
  chromospheric images as well as the magnetograms. We use these high
  resolution observations to investigate the structure of the solar
  upper photosphere and lower chromosphere as well as their temporal
  evolutions. We study the relation between the inter-granular Ca II
  397 nm bright structures in images obtained by the Sunrise Filter
  Imager (SuFI) and their corresponding photospheric vector magnetic
  field computed from the Imaging Magnetogram eXperiment (IMaX)
  observations. The targets under study are in a quiet Sun region and
  close to disc-centre.

Title: The Small-Scale Solar Surface Dynamo (Keynote)
Authors: Pietarila Graham, J.; Danilovic, S.; Schüssler, M.
Bibcode: 2009ASPC..415...43P
Altcode:
  The existence of a turbulent small-scale solar surface dynamo is likely,
  considering existing numerical and laboratory experiments, as well as
  comparisons of a small-scale dynamo in MURaM simulations with Hinode
  observations. We find the observed peaked probability distribution
  function (PDF) from Stokes-V magnetograms is consistent with a monotonic
  PDF of the actual vertical field strength. The cancellation function
  of the vertical flux density from a Hinode SP observation is found
  to follow a self-similar power law over two decades in length scales
  down to the ≈200 km resolution limit. This provides observational
  evidence that the scales of magnetic structuring in the photosphere
  extend at least down to 20 km. From the power law, we determine a
  lower bound for the true quiet-Sun mean vertical unsigned flux density
  of ≈43 G, consistent with our numerically-based estimates that 80%
  or more of the vertical unsigned flux should be invisible to Stokes-V
  observations at a resolution of 200 km owing to cancellation. Our
  estimates significantly reduce the order-of-magnitude discrepancy
  between Zeeman- and Hanle-based estimates.

Title: Turbulent Magnetic Fields in the Quiet Sun: Implications of
    Hinode Observations and Small-Scale Dynamo Simulations
Authors: Pietarila Graham, Jonathan; Danilovic, Sanja; Schüssler,
   Manfred
Bibcode: 2009ApJ...693.1728P
Altcode: 2008arXiv0812.2125P
  Using turbulent MHD simulations (magnetic Reynolds numbers up to
  ≈8000) and Hinode observations, we study effects of turbulence on
  measuring the solar magnetic field outside active regions. First,
  from synthetic Stokes V profiles for the Fe I lines at 6301 and
  6302 Å, we show that a peaked probability distribution function
  (PDF) for observationally derived field estimates is consistent
  with a monotonic PDF for actual vertical field strengths. Hence, the
  prevalence of weak fields is greater than would be naively inferred from
  observations. Second, we employ the fractal self-similar geometry of the
  turbulent solar magnetic field to derive two estimates (numerical and
  observational) of the true mean vertical unsigned flux density. We also
  find observational evidence that the scales of magnetic structuring in
  the photosphere extend at least down to an order of magnitude smaller
  than 200 km: the self-similar power-law scaling in the signed measure
  from a Hinode magnetogram ranges (over two decades in length scales
  and including the granulation scale) down to the ≈200 km resolution
  limit. From the self-similar scaling, we determine a lower bound for
  the true quiet-Sun mean vertical unsigned flux density of ~50 G. This
  is consistent with our numerically based estimates that 80% or more of
  the vertical unsigned flux should be invisible to Stokes V observations
  at a resolution of 200 km owing to the cancellation of signal from
  opposite magnetic polarities. Our estimates significantly reduce
  the order-of-magnitude discrepancy between Zeeman- and Hanle-based
  estimates.

Title: Magnetic fine structure in the solar photosphere: observations
    and MHD simulations
Authors: Danilović, Sanja
Bibcode: 2009PhDT.......244D
Altcode:
  No abstract at ADS

Title: How Well Do Zeeman Measurements Reflect the Turbulent Solar
    Magnetic Field?
Authors: Pietarila, J. Graham; Danilovic, S.; Schüssler, M.
Bibcode: 2008ESPM...12.3.13P
Altcode:
  We employ the turbulent nature of the magnetic field in the solar
  photosphere to constrain interpretations of Zeeman-polarimetry-based
  observations. Using higher Reynolds number, more turbulent, simulations
  of the solar convection zone than have previously been reported,
  we compare the distribution and cancellation statistics of the
  magnetic field itself with the statistics derived from simulated
  Stokes profiles. A favorable comparison between the cancellation
  statistics of the observables and the field allow for a prediction
  at the approximated magnetic Reynolds number of the sun, ReM = 1e6,
  and a comparison with Hinode observations. The difference between the
  probability distribution functions (PDFs) from simulations, Zeeman-based
  polarimetry, and interpretations of the Hanle effect are also examined
  and a possible explanation is suggested.

Title: The intensity contrast of solar granulation: comparing Hinode
    SP results with MHD simulations
Authors: Danilovic, S.; Gandorfer, A.; Lagg, A.; Schüssler, M.;
   Solanki, S. K.; Vögler, A.; Katsukawa, Y.; Tsuneta, S.
Bibcode: 2008A&A...484L..17D
Altcode: 2008arXiv0804.4230D
  Context: The contrast of granulation is an important quantity
  characterizing solar surface convection. <BR />Aims: We compare the
  intensity contrast at 630 nm, observed using the Spectro-Polarimeter
  (SP) aboard the Hinode satellite, with the 3D radiative MHD simulations
  of Vögler &amp; Schüssler (2007, A&amp;A, 465, L43). <BR />Methods:
  A synthetic image from the simulation is degraded using a theoretical
  point-spread function of the optical system, and by considering other
  important effects. <BR />Results: The telescope aperture and the
  obscuration by the secondary mirror and its attachment spider, reduce
  the simulated contrast from 14.4% to 8.5%. A slight effective defocus
  of the instrument brings the simulated contrast down to 7.5%, close to
  the observed value of 7.0%. <BR />Conclusions: A proper consideration
  of the effects of the optical system and a slight defocus, lead to
  sufficient degradation of the synthetic image from the MHD simulation,
  such that the contrast reaches almost the observed value. The remaining
  small discrepancy can be ascribed to straylight and slight imperfections
  of the instrument, which are difficult to model. Hence, Hinode SP data
  are consistent with a granulation contrast which is predicted by 3D
  radiation MHD simulations.

Title: Magnetic source of the solar cycle variation of the Mn I
    539.4 nm line
Authors: Danilović, S.; Solanki, S. K.; Livingston, W.; Krivova,
   N.; Vince, I.
Bibcode: 2007msfa.conf..189D
Altcode:
  As a part of the long term program at KPNO, the Mn I 539.4 nm line has
  been observed for nearly three solar cycles using the McMath telescope
  and the 13.5 m spectrograph in double pass mode. These full-disk
  spectrophotometric observations revealed an unusually large amplitude
  change of its parameters over the solar cycle and its correlation with
  Ca II K intensity. One of the proposed explanations for this phenomenon
  is the optical pumping by the Mg II k line. With this work we would like
  to show that this may not be the main mechanism behind the change. We
  reconstructed the changes of the line parameters using a model that
  takes into account only changes of the daily surface distributions of
  magnetic field. This model has already been used to successfully model
  total solar irradiance. We now apply it for modelling the Mn I line,
  as well as its neighboring Fe I line using exactly the same value
  of the free parameter as used for the reconstruction of total solar
  irradiance. We reproduce well the Mn I and Fe I line changes over the
  cycle purely with LTE modelling. This indicates that optical pumping
  of the Mn I line by Mg II k is not the main cause of its solar cycle
  change and sets an independent constraint on solar irradiance models.

Title: Formation of Neutral Manganese Lines Potentially Suitable
    for Plasma Diagnostics
Authors: Vitas, N.; Danilović, S.; Atanacković-Vukmanović, O.;
   Vince, Ištvan
Bibcode: 2005ESASP.600E..73V
Altcode: 2005dysu.confE..73V; 2005ESPM...11...73V
  No abstract at ADS

Title: Time Series Analysis of Long Term Full Disk Observations Of
    The Mn I 539.4 nm Solar Line
Authors: Danilovic, S.; Vince, I.; Vitas, N.; Jovanovic, P.
Bibcode: 2005SerAJ.170...79D
Altcode:
  The equivalent width and central depth data of Mn 539.4 nm solar
  spectral line, observed in the period from 1979 to 1992 at Kitt
  Peak Observatory, was analyzed in pursuit for periodic changes. As
  the observations are highly unevenly sampled, test if the periods
  really exist in the observed data was needed. Two different methods
  for spectral analysis were applied to synthesized data sampled in the
  same way as observations. Comparation of these results with results
  obtained from the observed data showed that the parameters display
  at least three periodic changes with the periods of: 11-years,
  quasi-biannual and 27-days.

Title: Variability of the Mn I 539.4 nm solar spectral line parameters
    with solar activity.
Authors: Danilovic, S.; Vince, I.
Bibcode: 2005MmSAI..76..949D
Altcode:
  The time analysis of Mn I 539.4 nm parameters, observed in solar
  flux spectrum, shows high degree of correlation with sunspot area
  and Mg II c/w index time spectra. The more pronounced correlation
  between equivalent width of this line with Mg II index implies that
  bright magnetic elements are important source of the line parameters
  variation. We synthesized line profiles, taking into account the
  coverage of solar surface with different features, at various solar
  activity levels and compared them with observations. The result shows
  very good correspondence between modelled and observed change.

Title: Sensitivity of the MnI 539.47 nm Spectral Line to Solar
    Activity
Authors: Danilovic, S.; Vince, I.
Bibcode: 2004SerAJ.169...47D
Altcode: 2005SerAJ.169...47D
  CCD observations of the MnI 539.47 nm spectral line in the solar flux
  made at the Astronomical Observatory in Belgrade between 1994 and 2003
  have been reduced and certain line parameters -the equivalent width,
  full width at half maximum and central depth have been derived. The
  variation of these parameters with the solar activity has been
  evaluated. The relative variation of the equivalent width from minimum
  to maximum of solar activity was found to be 1.4%, while the variation
  of the central depth is 2.3%. The full width at half maximum remains
  practically constant.

