Author name code: bjorgen
ADS astronomy entries on 2022-09-14
author:"Bjorgen, Johan P."
------------------------------------------------------------------------  

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: Line formation of He I D<SUB>3</SUB> and He I 10 830 Å in
    a small-scale reconnection event
Authors: Libbrecht, Tine; Bjørgen, Johan P.; Leenaarts, Jorrit;
   de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Joshi, Jayant
Bibcode: 2021A&A...652A.146L
Altcode: 2020arXiv201015946L
  Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection
  events that occur in the region of the upper photosphere to the
  chromosphere. It has recently been discovered that these events can
  have emission signatures in the He I D<SUB>3</SUB> and He I 10 830 Å
  lines, suggesting that their temperatures are higher than previously
  expected. <BR /> Aims: We aim to explain the line formation of He I
  D<SUB>3</SUB> and He I 10 830 Å in small-scale reconnection events. <BR
  /> Methods: We used a simulated EB in a Bifrost-generated radiative
  magnetohydrodynamics snapshot. The resulting He I D<SUB>3</SUB> and He
  I 10 830 Å line intensities were synthesized in 3D using the non-local
  thermal equilibrium (non-LTE) Multi3D code. The presence of coronal
  extreme UV (EUV) radiation was included self-consistently. We compared
  the synthetic helium spectra with observed raster scans of EBs in He I
  10 830 Å and He I D<SUB>3</SUB> obtained at the Swedish Solar Telescope
  with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. <BR />
  Results: Emission in He I D<SUB>3</SUB> and He I 10 830 Å is formed
  in a thin shell around the EB at a height of ∼0.8 Mm, while the He I
  D<SUB>3</SUB> absorption is formed above the EB at ∼4 Mm. The height
  at which the emission is formed corresponds to the lower boundary of the
  EB, where the temperature increases rapidly from 6 × 10<SUP>3</SUP> K
  to 10<SUP>6</SUP> K. The synthetic line profiles at a heliocentric angle
  of μ = 0.27 are qualitatively similar to the observed profiles at the
  same μ-angle in dynamics, broadening, and line shape: emission in the
  wing and absorption in the line core. The opacity in He I D<SUB>3</SUB>
  and He I 10 830 Å is generated through photoionization-recombination
  driven by EUV radiation that is locally generated in the EB at
  temperatures in the range of 2 × 10<SUP>4</SUP> − 2 × 10<SUP>6</SUP>
  K and electron densities between 10<SUP>11</SUP> and 10<SUP>13</SUP>
  cm<SUP>−3</SUP>. The synthetic emission signals are a result of
  coupling to local conditions in a thin shell around the EB, with
  temperatures between 7 × 10<SUP>3</SUP> and 10<SUP>4</SUP> K and
  electron densities ranging from ∼10<SUP>12</SUP> to 10<SUP>13</SUP>
  cm<SUP>−3</SUP>. This shows that both strong non-LTE and thermal
  processes play a role in the formation of He I D<SUB>3</SUB> and
  He I 10 830 Å in the synthetic EB/UV burst that we studied. <BR />
  Conclusions: In conclusion, the synthetic He I D<SUB>3</SUB> and He I 10
  830 Å emission signatures are an indicator of temperatures of at least
  2 × 10<SUP>4</SUP> K; in this case, as high as ∼10<SUP>6</SUP> K.

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: Ellerman bombs and UV bursts: transient events in chromospheric
    current sheets
Authors: Hansteen, V.; Ortiz, A.; Archontis, V.; Carlsson, M.; Pereira,
   T. M. D.; Bjørgen, J. P.
Bibcode: 2019A&A...626A..33H
Altcode: 2019arXiv190411524H
  Context. Ellerman bombs (EBs), observed in the photospheric wings
  of the Hα line, and UV bursts, observed in the transition region Si
  IV line, are both brightenings related to flux emergence regions and
  specifically to magnetic flux of opposite polarity that meet in the
  photosphere. These two reconnection-related phenomena, nominally formed
  far apart, occasionally occur in the same location and at the same
  time, thus challenging our understanding of reconnection and heating
  of the lower solar atmosphere. <BR /> Aims: We consider the formation
  of an active region, including long fibrils and hot and dense coronal
  plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5
  Mm below the photosphere, is studied as it pierces the photosphere
  and interacts with the preexisting ambient field. Specifically, we
  aim to study whether EBs and UV bursts are generated as a result of
  such flux emergence and examine their physical relationship. <BR />
  Methods: The Bifrost radiative magnetohydrodynamics code was used
  to model flux emerging into a model atmosphere that contained a
  fairly strong ambient field, constraining the emerging field to
  a limited volume wherein multiple reconnection events occur as
  the field breaks through the photosphere and expands into the outer
  atmosphere. Synthetic spectra of the different reconnection events were
  computed using the 1.5D RH code and the fully 3D MULTI3D code. <BR
  /> Results: The formation of UV bursts and EBs at intensities and
  with line profiles that are highly reminiscent of observed spectra
  are understood to be a result of the reconnection of emerging flux
  with itself in a long-lasting current sheet that extends over several
  scale heights through the chromosphere. Synthetic spectra in the Hα
  and Si IV 139.376 nm lines both show characteristics that are typical
  of the observations. These synthetic diagnostics suggest that there
  are no compelling reasons to assume that UV bursts occur in the
  photosphere. Instead, EBs and UV bursts are occasionally formed at
  opposite ends of a long current sheet that resides in an extended bubble
  of cool gas. <P />The movie associated to Fig. 3 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201935376/olm">https://www.aanda.org</A>

Title: The synthetic chromosphere: Results and techniques with a
    numerical approach
Authors: Bjørgen, Johan Pires
Bibcode: 2019PhDT.......176B
Altcode:
  No abstract at ADS

Title: Three-dimensional modeling of the Ca II H and K lines in the
    solar atmosphere
Authors: Bjørgen, Johan P.; Sukhorukov, Andrii V.; Leenaarts, Jorrit;
   Carlsson, Mats; de la Cruz Rodríguez, Jaime; Scharmer, Göran B.;
   Hansteen, Viggo H.
Bibcode: 2018A&A...611A..62B
Altcode: 2017arXiv171201045B
  Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar
  Telescope (SST), can observe the chromosphere in the H and K lines of
  Ca II at high spatial and spectral resolution. Accurate modeling as
  well as an understanding of the formation of these lines are needed to
  interpret the SST/CHROMIS observations. Such modeling is computationally
  challenging because these lines are influenced by strong departures from
  local thermodynamic equilibrium, three-dimensional radiative transfer,
  and partially coherent resonance scattering of photons. Aim. We aim to
  model the Ca II H and K lines in 3D model atmospheres to understand
  their formation and to investigate their diagnostic potential for
  probing the chromosphere. <BR /> Methods: We model the synthetic
  spectrum of Ca II using the radiative transfer code Multi3D in three
  different radiation-magnetohydrodynamic model atmospheres computed with
  the Bifrost code. We classify synthetic intensity profiles according
  to their shapes and study how their features are related to the
  physical properties in the model atmospheres. We investigate whether
  the synthetic data reproduce the observed spatially-averaged line
  shapes, center-to-limb variation and compare this data with SST/CHROMIS
  images. <BR /> Results: The spatially-averaged synthetic line profiles
  show too low central emission peaks, and too small separation between
  the peaks. The trends of the observed center-to-limb variation of
  the profiles properties are reproduced by the models. The Ca II H and
  K line profiles provide a temperature diagnostic of the temperature
  minimum and the temperature at the formation height of the emission
  peaks. The Doppler shift of the central depression is an excellent
  probe of the velocity in the upper chromosphere.

Title: Numerical non-LTE 3D radiative transfer using a multigrid
    method
Authors: Bjørgen, Johan P.; Leenaarts, Jorrit
Bibcode: 2017A&A...599A.118B
Altcode: 2017arXiv170101607B
  Context. 3D non-LTE radiative transfer problems are computationally
  demanding, and this sets limits on the size of the problems that can
  be solved. So far, multilevel accelerated lambda iteration (MALI)
  has been the method of choice to perform high-resolution computations
  in multidimensional problems. The disadvantage of MALI is that its
  computing time scales as O(n<SUP>2</SUP>), with n the number of grid
  points. When the grid becomes finer, the computational cost increases
  quadratically. <BR /> Aims: We aim to develop a 3D non-LTE radiative
  transfer code that is more efficient than MALI. <BR /> Methods: We
  implement a non-linear multigrid, fast approximation storage scheme,
  into the existing Multi3D radiative transfer code. We verify our
  multigrid implementation by comparing with MALI computations. We show
  that multigrid can be employed in realistic problems with snapshots
  from 3D radiative magnetohydrodynamics (MHD) simulations as input
  atmospheres. <BR /> Results: With multigrid, we obtain a factor
  3.3-4.5 speed-up compared to MALI. With full-multigrid, the speed-up
  increases to a factor 6. The speed-up is expected to increase for
  input atmospheres with more grid points and finer grid spacing. <BR
  /> Conclusions: Solving 3D non-LTE radiative transfer problems using
  non-linear multigrid methods can be applied to realistic atmospheres
  with a substantial increase in speed.

