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Author name code: bjorgen
ADS astronomy entries on 2022-09-14
author:"Bjorgen, Johan P."
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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.
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.
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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
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.
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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.
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&k, and Ca II H&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.
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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.
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>
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Title: The synthetic chromosphere: Results and techniques with a
numerical approach
Authors: Bjørgen, Johan Pires
2019PhDT.......176B Altcode:
No abstract at ADS
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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.
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.
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Title: Numerical non-LTE 3D radiative transfer using a multigrid
method
Authors: Bjørgen, Johan P.; Leenaarts, Jorrit
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.
