leenaarts

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Author name code: leenaarts
ADS astronomy entries on 2022-09-14
author:Leenaarts, Jorrit

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Title: Quiet Sun Center to Limb Variation of the Linear Polarization
Observed by CLASP2 Across the Mg II h and k Lines
Authors: Rachmeler, L. A.; Bueno, J. Trujillo; McKenzie, D. E.;
Ishikawa, R.; Auchère, F.; Kobayashi, K.; Kano, R.; Okamoto,
T. J.; Bethge, C. W.; Song, D.; Ballester, E. Alsina; Belluzzi,
L.; Pino Alemán, T. del; Ramos, A. Asensio; Yoshida, M.; Shimizu,
T.; Winebarger, A.; Kobelski, A. R.; Vigil, G. D.; Pontieu, B. De;
Narukage, N.; Kubo, M.; Sakao, T.; Hara, H.; Suematsu, Y.; Štěpán,
J.; Carlsson, M.; Leenaarts, J.
2022ApJ...936...67R Altcode: 2022arXiv220701788R
The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket
mission was launched on 2019 April 11. CLASP2 measured the four Stokes
parameters of the Mg II h and k spectral region around 2800 Å along a
200″ slit at three locations on the solar disk, achieving the first
spatially and spectrally resolved observations of the solar polarization
in this near-ultraviolet region. The focus of the work presented here
is the center-to-limb variation of the linear polarization across these
resonance lines, which is produced by the scattering of anisotropic
radiation in the solar atmosphere. The linear polarization signals of
the Mg II h and k lines are sensitive to the magnetic field from the
low to the upper chromosphere through the Hanle and magneto-optical
effects. We compare the observations to theoretical predictions
from radiative transfer calculations in unmagnetized semiempirical
models, arguing that magnetic fields and horizontal inhomogeneities
are needed to explain the observed polarization signals and spatial
variations. This comparison is an important step in both validating and
refining our understanding of the physical origin of these polarization
signatures, and also in paving the way toward future space telescopes
for probing the magnetic fields of the solar upper atmosphere via
ultraviolet spectropolarimetry.

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Title: Radiative losses in the chromosphere during a C-class flare
Authors: Yadav, R.; de la Cruz Rodríguez, J.; Kerr, G. S.; Díaz Baso,
C. J.; Leenaarts, J.
2022A&A...665A..50Y Altcode: 2022arXiv220702840Y
Context. Solar flares release an enormous amount of energy
(∼1032 erg) into the corona. A substantial fraction of
this energy is transported to the lower atmosphere, which results in
chromospheric heating. The mechanisms that transport energy to the lower
solar atmosphere during a flare are still not fully understood. 
Aims: We aim to estimate the temporal evolution of the radiative losses
in the chromosphere at the footpoints of a C-class flare, in order
to set observational constraints on the electron beam parameters of
a RADYN flare simulation. Methods: We estimated the radiative
losses from hydrogen, and singly ionized Ca and Mg using semiempirical
model atmospheres, which were inferred from a multiline inversion of
observed Stokes profiles obtained with the CRISP and CHROMIS instruments
on the Swedish 1-m Solar Telescope. The radiative losses were computed
taking into account the effect of partial redistribution and non-local
thermodynamic equilibrium. To estimate the integrated radiative losses
in the chromosphere, the net cooling rates were integrated between the
temperature minimum and the height where the temperature reaches 10
kK. We also compared our time series of radiative losses with those
from the RADYN flare simulations. Results: We obtained a high
spatial-resolution map of integrated radiative losses around the flare
peak time. The stratification of the net cooling rate suggests that the
Ca IR triplet lines are responsible for most of the radiative losses in
the flaring atmosphere. During the flare peak time, the contribution
from Ca II H and K and Mg II h and k lines are strong and comparable
to the Ca IR triplet (∼32 kW m−2). Since our flare is a
relatively weak event, the chromosphere is not heated above 11 kK, which
in turn yields a subdued Lyα contribution (∼7 kW m−2)
in the selected limits of the chromosphere. The temporal evolution
of total integrated radiative losses exhibits sharply rising losses
(0.4 kW m−2 s−1) and a relatively slow decay
(0.23 kW m−2 s−1). The maximum value of total
radiative losses is reached around the flare peak time, and can go up to
175 kW m−2 for a single pixel located at footpoint. After
a small parameter study, we find the best model-data consistency in
terms of the amplitude of radiative losses and the overall atmospheric
structure with a RADYN flare simulation in the injected energy flux
of 5 × 1010 erg s−1 cm−2.

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Title: Formation and heating of chromospheric fibrils in a
radiation-MHD simulation
Authors: Druett, M. K.; Leenaarts, J.; Carlsson, M.; Szydlarski, M.
2022A&A...665A...6D Altcode: 2021arXiv211208245D
 Aims: We examine the movements of mass elements within dense
fibrils using passive tracer particles (corks) in order to understand
the creation and destruction processes of fibrils. Methods:
Simulated fibrils were selected at times when they were visible in a
Hα image proxy. The corks were selected within fibril Hα formation
regions. From this set, we selected a cork and constructed the field
line passing through it. Other fibrilar corks close to this field
line were also selected and pathlines were constructed, revealing the
locations of the mass elements forwards and backwards in time. Finally,
we analysed the forces acting on these mass elements. Results:
The main process of fibrilar loading in the simulation is different
to the mass loading scenario in which waves steepen into shocks and
push material upwards along the field lines from locations near their
footpoints. The twisted, low-lying field lines were destabilised
and then they untwisted, lifting the material trapped above their
apexes via the Lorentz force. Subsequently, the majority of the mass
drained down the field lines towards one or both footpoints under the
influence of gravity. Material with large horizontal velocities could
also be elevated in rising field lines, creating somewhat parabolic
motions, but the material was not generally moving upward along a
stationary magnetic field line during loading. Conclusions:
The processes observed in the simulation are additional scenarios that
are plausible. The criteria for observing such events are described
in this work. We note that it is desirable for our simulations to
also be able to form more densely packed fibrils from material fed
from the base of field footpoints. The experimental parameters
required to achieve this are also discussed in this paper. Movies associated to Figs. 1, 4, 9, 14 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142399/olm">https://www.aanda.org</A>

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Title: Spatio-temporal analysis of chromospheric heating in a
plage region
Authors: Morosin, R.; de la Cruz Rodríguez, J.; Díaz Baso, C. J.;
Leenaarts, J.
2022A&A...664A...8M Altcode: 2022arXiv220301688M
Context. Our knowledge of the heating mechanisms that are at work
in the chromosphere of plage regions remains highly unconstrained
from observational studies. While many heating candidates have been
proposed in theoretical studies, the exact contribution from each of
them is still unknown. The problem is rather difficult because there
is no direct way of estimating the heating terms from chromospheric
observations. Aims: The purpose of our study is to estimate the
chromospheric heating terms from a multi-line high-spatial-resolution
plage dataset, characterize their spatio-temporal distribution
and set constraints on the heating processes that are at work in
the chromosphere. Methods: We used nonlocal thermodynamical
equilibrium inversions in order to infer a model of the photosphere
and chromosphere of a plage dataset acquired with the Swedish 1-m
Solar Telescope (SST). We used this model atmosphere to calculate
the chromospheric radiative losses from the main chromospheric cooler
from H I, Ca II, and Mg II atoms. In this study, we approximate the
chromospheric heating terms by the net radiative losses predicted
by the inverted model. In order to make the analysis of time-series
over a large field of view computationally tractable, we made use
of a neural network which is trained from the inverted models of
two non-consecutive time-steps. We have divided the chromosphere
in three regions (lower, middle, and upper) and analyzed how the
distribution of the radiative losses is correlated with the physical
parameters of the model. Results: In the lower chromosphere,
the contribution from the Ca II lines is dominant and predominantly
located in the surroundings of the photospheric footpoints. In the
upper chromosphere, the H I contribution is dominant. Radiative
losses in the upper chromosphere form a relatively homogeneous
patch that covers the entire plage region. The Mg II also peaks in
the upper chromosphere. Our time analysis shows that in all pixels,
the net radiative losses can be split in a periodic component with
an average amplitude of amp̅Q = 7.6 kW m−2
and a static (or very slowly evolving) component with a mean value of
−26.1 kW m−2. The period of the modulation present in
the net radiative losses matches that of the line-of-sight velocity
of the model. Conclusions: Our interpretation is that in the
lower chromosphere, the radiative losses are tracing the sharp lower
edge of the hot magnetic canopy that is formed above the photosphere,
where the electric current is expected to be large. Therefore, Ohmic
current dissipation could explain the observed distribution. In the
upper chromosphere, both the magnetic field and the distribution of net
radiative losses are room-filling and relatively smooth, whereas the
amplitude of the periodic component is largest. Our results suggest that
acoustic wave heating may be responsible for one-third of the energy
deposition in the upper chromosphere, whereas other heating mechanisms
must be responsible for the rest: turbulent Alfvén wave dissipation
or ambipolar diffusion could be among them. Given the smooth nature
of the magnetic field in the upper chromosphere, we are inclined to
rule out Ohmic dissipation of current sheets in the upper chromosphere.

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Title: Chromospheric extension of the MURaM code
Authors: Przybylski, D.; Cameron, R.; Solanki, S. K.; Rempel, M.;
Leenaarts, J.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.
2022A&A...664A..91P Altcode: 2022arXiv220403126P
Context. Detailed numerical models of the chromosphere and corona are
required to understand the heating of the solar atmosphere. An accurate
treatment of the solar chromosphere is complicated by the effects
arising from non-local thermodynamic equilibrium (NLTE) radiative
transfer. A small number of strong, highly scattering lines dominate the
cooling and heating in the chromosphere. Additionally, the recombination
times of ionised hydrogen are longer than the dynamical timescales,
requiring a non-equilibrium (NE) treatment of hydrogen ionisation. Aims: We describe a set of necessary additions to the MURaM code that
allow it to handle some of the important NLTE effects. We investigate
the impact on solar chromosphere models caused by NLTE and NE effects in
radiation magnetohydrodynamic simulations of the solar atmosphere. Methods: The MURaM code was extended to include the physical
process required for an accurate simulation of the solar chromosphere,
as implemented in the Bifrost code. This includes a time-dependent
treatment of hydrogen ionisation, a scattering multi-group radiation
transfer scheme, and approximations for NLTE radiative cooling. 
Results: The inclusion of NE and NLTE physics has a large impact on the
structure of the chromosphere; the NE treatment of hydrogen ionisation
leads to a higher ionisation fraction and enhanced populations in
the first excited state throughout cold inter-shock regions of the
chromosphere. Additionally, this prevents hydrogen ionisation from
buffering energy fluctuations, leading to hotter shocks and cooler
inter-shock regions. The hydrogen populations in the ground and first
excited state are enhanced by 102-103 in the
upper chromosphere and by up to 109 near the transition
region. Conclusions: Including the necessary NLTE physics
leads to significant differences in chromospheric structure and
dynamics. The thermodynamics and hydrogen populations calculated using
the extended version of the MURaM code are consistent with previous
non-equilibrium simulations. The electron number and temperature
calculated using the non-equilibrium treatment of the chromosphere
are required to accurately synthesise chromospheric spectral
lines. Movies associated to Fig. 2 are only available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141230/olm">https://www.aanda.org</A>

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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: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
M.; the EST team
2022arXiv220710905Q Altcode:
The European Solar Telescope (EST) is a project aimed at studying
the magnetic connectivity of the solar atmosphere, from the deep
photosphere to the upper chromosphere. Its design combines the knowledge
and expertise gathered by the European solar physics community during
the construction and operation of state-of-the-art solar telescopes
operating in visible and near-infrared wavelengths: the Swedish 1m Solar
Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR,
the French Télescope Héliographique pour l'Étude du Magnétisme
et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope
(DOT). With its 4.2 m primary mirror and an open configuration, EST will
become the most powerful European ground-based facility to study the Sun
in the coming decades in the visible and near-infrared bands. EST uses
the most innovative technological advances: the first adaptive secondary
mirror ever used in a solar telescope, a complex multi-conjugate
adaptive optics with deformable mirrors that form part of the optical
design in a natural way, a polarimetrically compensated telescope design
that eliminates the complex temporal variation and wavelength dependence
of the telescope Mueller matrix, and an instrument suite containing
several (etalon-based) tunable imaging spectropolarimeters and several
integral field unit spectropolarimeters. This publication summarises
some fundamental science questions that can be addressed with the
telescope, together with a complete description of its major subsystems.

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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.
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. 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. 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. 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. 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.

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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.
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. 
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). 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. 
Results: We find enhanced heating rates in the upper chromosphere of up
to ∼5 kW m−2, 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 ∼104 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. 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. The movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>

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Title: Subarcsecond Imaging of a Solar Active Region Filament With
ALMA and IRIS
Authors: da Silva Santos, J. M.; White, S. M.; Reardon, K.; Cauzzi,
G.; Gunár, S.; Heinzel, P.; Leenaarts, J.
2022FrASS...9.8115D Altcode: 2022arXiv220413178D
Quiescent filaments appear as absorption features on the solar disk
when observed in chromospheric lines and at continuum wavelengths
in the millimeter (mm) range. Active region (AR) filaments are their
small-scale, low-altitude analogues, but they could not be resolved
in previous mm observations. This spectral diagnostic can provide
insight into the details of the formation and physical properties of
their fine threads, which are still not fully understood. Here, we shed
light on the thermal structure of an AR filament using high-resolution
brightness temperature (Tb) maps taken with ALMA Band 6 complemented by
simultaneous IRIS near-UV spectra, Hinode/SOT photospheric magnetograms,
and SDO/AIA extreme-UV images. Some of the dark threads visible in the
AIA 304 Å passband and in the core of Mg ii resonance lines have dark
(Tb &lt; 5,000 K) counterparts in the 1.25 mm maps, but their visibility
significantly varies across the filament spine and in time. These
opacity changes are possibly related to variations in temperature and
electron density in filament fine structures. The coolest Tb values
(&lt; 5,000 K) coincide with regions of low integrated intensity in the
Mg ii h and k lines. ALMA Band 3 maps taken after the Band 6 ones do not
clearly show the filament structure, contrary to the expectation that
the contrast should increase at longer wavelengths based on previous
observations of quiescent filaments. The ALMA maps are not consistent
with isothermal conditions, but the temporal evolution of the filament
may partly account for this.

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Title: Solar oxygen abundance
Authors: Bergemann, Maria; Hoppe, Richard; Semenova, Ekaterina;
Carlsson, Mats; Yakovleva, Svetlana A.; Voronov, Yaroslav V.;
Bautista, Manuel; Nemer, Ahmad; Belyaev, Andrey K.; Leenaarts, Jorrit;
Mashonkina, Lyudmila; Reiners, Ansgar; Ellwarth, Monika
2021MNRAS.508.2236B Altcode: 2021arXiv210901143B; 2021MNRAS.tmp.1964B
Motivated by the controversy over the surface metallicity of the
Sun, we present a re-analysis of the solar photospheric oxygen (O)
abundance. New atomic models of O and Ni are used to perform non-local
thermodynamic equilibrium (NLTE) calculations with 1D hydrostatic
(MARCS) and 3D hydrodynamical (Stagger and Bifrost) models. The
Bifrost 3D MHD simulations are used to quantify the influence of
the chromosphere. We compare the 3D NLTE line profiles with new
high-resolution, R$\approx 700\, 000$, spatially resolved spectra
of the Sun obtained using the IAG FTS instrument. We find that the
O I lines at 777 nm yield the abundance of log A(O) = 8.74 ± 0.03
dex, which depends on the choice of the H-impact collisional data and
oscillator strengths. The forbidden [O I] line at 630 nm is less model
dependent, as it forms nearly in LTE and is only weakly sensitive to
convection. However, the oscillator strength for this transition is more
uncertain than for the 777 nm lines. Modelled in 3D NLTE with the Ni I
blend, the 630 nm line yields an abundance of log A(O) = 8.77 ± 0.05
dex. We compare our results with previous estimates in the literature
and draw a conclusion on the most likely value of the solar photospheric
O abundance, which we estimate at log A(O) = 8.75 ± 0.03 dex.

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Title: Line formation of He I D3 and He I 10 830 Å in
a small-scale reconnection event
Authors: Libbrecht, Tine; Bjørgen, Johan P.; Leenaarts, Jorrit;
de la Cruz Rodríguez, Jaime; Hansteen, Viggo; Joshi, Jayant
2021A&A...652A.146L Altcode: 2020arXiv201015946L
Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection
events that occur in the region of the upper photosphere to the
chromosphere. It has recently been discovered that these events can
have emission signatures in the He I D3 and He I 10 830 Å
lines, suggesting that their temperatures are higher than previously
expected. Aims: We aim to explain the line formation of He I
D3 and He I 10 830 Å in small-scale reconnection events. Methods: We used a simulated EB in a Bifrost-generated radiative
magnetohydrodynamics snapshot. The resulting He I D3 and He
I 10 830 Å line intensities were synthesized in 3D using the non-local
thermal equilibrium (non-LTE) Multi3D code. The presence of coronal
extreme UV (EUV) radiation was included self-consistently. We compared
the synthetic helium spectra with observed raster scans of EBs in He I
10 830 Å and He I D3 obtained at the Swedish Solar Telescope
with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. 
Results: Emission in He I D3 and He I 10 830 Å is formed
in a thin shell around the EB at a height of ∼0.8 Mm, while the He I
D3 absorption is formed above the EB at ∼4 Mm. The height
at which the emission is formed corresponds to the lower boundary of the
EB, where the temperature increases rapidly from 6 × 103 K
to 106 K. The synthetic line profiles at a heliocentric angle
of μ = 0.27 are qualitatively similar to the observed profiles at the
same μ-angle in dynamics, broadening, and line shape: emission in the
wing and absorption in the line core. The opacity in He I D3
and He I 10 830 Å is generated through photoionization-recombination
driven by EUV radiation that is locally generated in the EB at
temperatures in the range of 2 × 104 − 2 × 106
K and electron densities between 1011 and 1013
cm−3. The synthetic emission signals are a result of
coupling to local conditions in a thin shell around the EB, with
temperatures between 7 × 103 and 104 K and
electron densities ranging from ∼1012 to 1013
cm−3. This shows that both strong non-LTE and thermal
processes play a role in the formation of He I D3 and
He I 10 830 Å in the synthetic EB/UV burst that we studied. 
Conclusions: In conclusion, the synthetic He I D3 and He I 10
830 Å emission signatures are an indicator of temperatures of at least
2 × 104 K; in this case, as high as ∼106 K.

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Title: Mapping of Solar Magnetic Fields from the Photosphere to the
Top of the Chromosphere with CLASP2
Authors: McKenzie, D.; Ishikawa, R.; Trujillo Bueno, J.; Auchere, F.;
del Pino Aleman, T.; Okamoto, T.; Kano, R.; Song, D.; Yoshida, M.;
Rachmeler, L.; Kobayashi, K.; Narukage, N.; Kubo, M.; Ishikawa, S.;
Hara, H.; Suematsu, Y.; Sakao, T.; Bethge, C.; De Pontieu, B.; Vigil,
G.; Winebarger, A.; Alsina Ballester, E.; Belluzzi, L.; Stepan, J.;
Asensio Ramos, A.; Carlsson, M.; Leenaarts, J.
2021AAS...23810603M Altcode:
Coronal heating, chromospheric heating, and the heating &amp;
acceleration of the solar wind, are well-known problems in solar
physics. Additionally, knowledge of the magnetic energy that
powers solar flares and coronal mass ejections, important drivers
of space weather, is handicapped by imperfect determination of the
magnetic field in the sun's atmosphere. Extrapolation of photospheric
magnetic measurements into the corona is fraught with difficulties and
uncertainties, partly due to the vastly different plasma beta between
the photosphere and the corona. Better results in understanding
the coronal magnetic field should be derived from measurements of
the magnetic field in the chromosphere. To that end, we are pursuing
quantitative determination of the magnetic field in the chromosphere,
where plasma beta transitions from greater than unity to less than
unity, via ultraviolet spectropolarimetry. The CLASP2 mission, flown
on a sounding rocket in April 2019, succeeded in measuring all four
Stokes polarization parameters in UV spectral lines formed by singly
ionized Magnesium and neutral Manganese. Because these ions produce
spectral lines under different conditions, CLASP2 thus was able to
quantify the magnetic field properties at multiple heights in the
chromosphere simultaneously, as shown in the recent paper by Ishikawa
et al. In this presentation we will report the findings of CLASP2,
demonstrating the variation of magnetic fields along a track on
the solar surface and as a function of height in the chromosphere;
and we will illustrate what is next for the CLASP missions and the
demonstration of UV spectropolarimetry in the solar chromosphere.

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Title: An observationally constrained model of strong magnetic
reconnection in the solar chromosphere. Atmospheric stratification
and estimates of heating rates
Authors: Díaz Baso, C. J.; de la Cruz Rodríguez, J.; Leenaarts, J.
2021A&A...647A.188D Altcode: 2020arXiv201206229D
Context. The evolution of the photospheric magnetic field plays
a key role in the energy transport into the chromosphere and the
corona. In active regions, newly emerging magnetic flux interacts
with the pre-existent magnetic field, which can lead to reconnection
events that convert magnetic energy into thermal energy. Aims:
We aim to study the heating caused by a strong reconnection event
that was triggered by magnetic flux cancelation. Methods: We
use imaging and spectropolarimetric data in the Fe I 6301&amp; 6302
Å, Ca II 8542 Å, and Ca II K spectral lines obtained with the CRISP
and CHROMIS instruments at the Swedish 1-m Solar Telescope. These data
were inverted with the STiC code by performing multi-atom, multi-line,
non-local thermodynamic equilibrium inversions. These inversions yielded
a three-dimensional model of the reconnection event and surrounding
atmosphere, including temperature, velocity, microturbulence, magnetic
field, and radiative loss rate. Results: The model atmosphere
shows the emergence of magnetic loops with a size of several arcseconds
into a pre-existing predominantly unipolar field. Where the reconnection
region is expected to be, we see an increase in the chromospheric
temperature of roughly 2000 K as well as bidirectional flows of the
order of 10 km s−1 emanating from there. We see bright
blobs of roughly 0.2 arcsec in diameter in the Ca II K, moving at a
plane-of-the-sky velocity of the order of 100 km s−1
and a blueshift of 100 km s−1, which we interpret as
ejected plasmoids from the same region. This scenario is consistent
with theoretical reconnection models, and therefore provides evidence
of a reconnection event taking place. The chromospheric radiative
losses at the reconnection site are as high as 160 kW m−2,
providing a quantitative constraint on theoretical models that aim to
simulate reconnection caused by flux emergence in the chromosphere.

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Title: Mapping solar magnetic fields from the photosphere to the
base of the corona
Authors: Ishikawa, Ryohko; Bueno, Javier Trujillo; del Pino Alemán,
Tanausú; Okamoto, Takenori J.; McKenzie, David E.; Auchère,
Frédéric; Kano, Ryouhei; Song, Donguk; Yoshida, Masaki; Rachmeler,
Laurel A.; Kobayashi, Ken; Hara, Hirohisa; Kubo, Masahito; Narukage,
Noriyuki; Sakao, Taro; Shimizu, Toshifumi; Suematsu, Yoshinori; Bethge,
Christian; De Pontieu, Bart; Dalda, Alberto Sainz; Vigil, Genevieve D.;
Winebarger, Amy; Ballester, Ernest Alsina; Belluzzi, Luca; Štěpán,
Jiří; Ramos, Andrés Asensio; Carlsson, Mats; Leenaarts, Jorrit
2021SciA....7.8406I Altcode: 2021arXiv210301583I
Routine ultraviolet imaging of the Sun's upper atmosphere shows the
spectacular manifestation of solar activity; yet we remain blind to
its main driver, the magnetic field. Here we report unprecedented
spectropolarimetric observations of an active region plage and
its surrounding enhanced network, showing circular polarization in
ultraviolet (Mg II $h$ &amp; $k$ and Mn I) and visible (Fe I) lines. We
infer the longitudinal magnetic field from the photosphere to the
very upper chromosphere. At the top of the plage chromosphere the
field strengths reach more than 300 gauss, strongly correlated with
the Mg II $k$ line core intensity and the electron pressure. This
unique mapping shows how the magnetic field couples the different
atmospheric layers and reveals the magnetic origin of the heating in
the plage chromosphere.

---------------------------------------------------------
Title: 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.
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. 
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. 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. 
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. 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.
2020AGUFMSH0010001D Altcode:
Observations at visible and ultraviolet wavelengths have shown that
solar active regions host different kinds of small-scale, transient,
bright structures that are believed to be heating events resulting
from the release of magnetic energy in the low atmosphere of the Sun,
especially at the early stages of flux emergence. It is of great
scientific interest to be able to accurately infer temperatures and
formation heights of the most localized events, which are still
matter of debate, in the hope of learning about the evolution of
active regions where occasionally more energetic phenomena lead to
much larger outbursts that propagate across the Solar System. The
millimeter (mm) continuum is a new complementary diagnostic for
chromospheric heating that is now available thanks to the Atacama
Large Millimeter/submillimeter Array (ALMA). We report on the
first ALMA 3 mm observations of small-scale heating events in a
solar active region. In contrast with the low-amplitude brightness
temperature variations in the quiet-Sun, the interferometric maps show
that the active region consists of long, warm, fibril-like structures
that connect magnetic concentrations of opposite polarity and often
flare up along with compact, flickering mm-bursts -- reminiscent of
ultraviolet bursts -- with brightness temperatures of up to 14000 K at
1.2" scales. These events also show simultaneous EUV emission observed
by the Solar Dynamics Observatory (SDO). We find a weak correlation
between the photospheric bright patches and the 3 mm continuum
brightness and, in particular, we do not detect any mm counterpart of
Ellerman bombs which confirms that they are photospheric phenomena. Our observations and modelling highlight the diagnostic capabilities
of ALMA for local heating in solar active regions and emphasize the
need for coordinated observations with IRIS and DKIST in the future.

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Title: ALMA observations of transient heating in a solar active region
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.
2020A&A...643A..41D Altcode: 2020arXiv200614564D
 Aims: We aim to investigate the temperature enhancements and
formation heights of solar active-region brightenings such as Ellerman
bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region
fibrils (FAFs) using interferometric observations in the millimeter
(mm) continuum provided by the Atacama Large Millimeter/submillimeter
Array (ALMA). Methods: We examined 3 mm signatures of heating
events identified in Solar Dynamics Observatory observations of an
active region and compared the results with synthetic spectra from a 3D
radiative magnetohydrodynamic simulation. We estimated the contribution
from the corona to the mm brightness using differential emission measure
analysis. Results: We report the null detection of EBs in the 3
mm continuum at ∼1.2″ spatial resolution, which is evidence that
they are sub-canopy events that do not significantly contribute to
heating the upper chromosphere. In contrast, we find the active region
to be populated with multiple compact, bright, flickering mm-bursts -
reminiscent of UVBs. The high brightness temperatures of up to ∼14 200
K in some events have a contribution (up to ∼7%) from the corona. We
also detect FAF-like events in the 3 mm continuum. These events show
rapid motions of &gt; 10 kK plasma launched with high plane-of-sky
velocities (37 - 340 km s-1) from bright kernels. The mm FAFs
are the brightest class of warm canopy fibrils that connect magnetic
regions of opposite polarities. The simulation confirms that ALMA
should be able to detect the mm counterparts of UVBs and small flares
and thus provide a complementary diagnostic for localized heating in the
solar chromosphere. Movie associated to Fig. 5 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038755/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: New Light on an Old Problem of the Cores of Solar Resonance
Lines
Authors: Judge, Philip G.; Kleint, Lucia; Leenaarts, Jorrit;
Sukhorukov, Andrii V.; Vial, Jean-Claude
2020ApJ...901...32J Altcode: 2020arXiv200801250J
We reexamine a 50+ yr old problem of deep central reversals predicted
for strong solar spectral lines, in contrast to the smaller reversals
seen in observations. We examine data and calculations for the resonance
lines of H I, Mg II, and Ca II, the self-reversed cores of which form
in the upper chromosphere. Based on 3D simulations, as well as data for
the Mg II lines from the Interface Region Imaging Spectrograph (IRIS),
we argue that the resolution lies not in velocity fields on scales in
either of the micro- or macroturbulent limits. Macroturbulence is ruled
out using observations of optically thin lines formed in the upper
chromosphere, and by showing that it would need to have unreasonably
special properties to account for critical observations of the Mg
II resonance lines from the IRIS mission. The power in "turbulence"
in the upper chromosphere may therefore be substantially lower than
earlier analyses have inferred. Instead, in 3D calculations horizontal
radiative transfer produces smoother source functions, smoothing out
intensity gradients in wavelength and in space. These effects increase
in stronger lines. Our work will have consequences for understanding
the onset of the transition region, for understanding the energy in
motions available for heating the corona, and for the interpretation
of polarization data in terms of the Hanle effect applied to resonance
line profiles.

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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
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. 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. 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. 
Results: The line profiles in bright fibrils have a higher intensity in
their K2 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(τ500 nm)
= -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-1 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. 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: Radiation hydrodynamics in simulations of the solar atmosphere
Authors: Leenaarts, Jorrit
2020LRSP...17....3L Altcode: 2020arXiv200203623L
Nearly all energy generated by fusion in the solar core is ultimately
radiated away into space in the solar atmosphere, while the remaining
energy is carried away in the form of neutrinos. The exchange of energy
between the solar gas and the radiation field is thus an essential
ingredient of atmospheric modeling. The equations describing these
interactions are known, but their solution is so computationally
expensive that they can only be solved in approximate form in
multi-dimensional radiation-MHD modeling. In this review, I discuss
the most commonly used approximations for energy exchange between gas
and radiation in the photosphere, chromosphere, and corona.

---------------------------------------------------------
Title: Ion-neutral Interactions and Nonequilibrium Ionization in
the Solar Chromosphere
Authors: Martínez-Sykora, Juan; Leenaarts, Jorrit; De Pontieu,
Bart; Nóbrega-Siverio, Daniel; Hansteen, Viggo H.; Carlsson, Mats;
Szydlarski, Mikolaj
2020ApJ...889...95M Altcode: 2019arXiv191206682M
The thermal structure of the chromosphere is regulated through a
complex interaction of various heating processes, radiative cooling,
and the ionization degree of the plasma. Here, we study the impact on
the thermal properties of the chromosphere when including the combined
action of nonequilibrium ionization (NEI) of hydrogen and helium and
ion-neutral interaction effects. We have performed a 2.5D radiative
magnetohydrodynamic simulation using the Bifrost code. This model
includes ion-neutral interaction effects by solving the generalized
Ohm' s law (GOL) as well as NEI for hydrogen and helium. The GOL
equation includes ambipolar diffusion and the Hall term. We compare
this simulation with another simulation that computes the ionization in
local thermodynamic equilibrium (LTE) including ion-neutral interaction
effects. Our numerical models reveal substantial thermal differences
in magneto-acoustic shocks, the wake behind the shocks, spicules,
low-lying magnetic loops, and the transition region. In particular,
we find that heating through ambipolar diffusion in shock wakes is
substantially less efficient, while in the shock fronts themselves it
is more efficient, under NEI conditions than when assuming LTE.

---------------------------------------------------------
Title: The multi-thermal chromosphere. Inversions of ALMA and
IRIS data
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; Leenaarts,
J.; Chintzoglou, G.; De Pontieu, B.; Wedemeyer, S.; Szydlarski, M.
2020A&A...634A..56D Altcode: 2019arXiv191209886D
Context. Numerical simulations of the solar chromosphere predict a
diverse thermal structure with both hot and cool regions. Observations
of plage regions in particular typically feature broader and brighter
chromospheric lines, which suggests that they are formed in hotter
and denser conditions than in the quiet Sun, but also implies a
nonthermal component whose source is unclear. Aims: We revisit
the problem of the stratification of temperature and microturbulence
in plage and the quiet Sun, now adding millimeter (mm) continuum
observations provided by the Atacama Large Millimiter Array (ALMA) to
inversions of near-ultraviolet Interface Region Imaging Spectrograph
(IRIS) spectra as a powerful new diagnostic to disentangle the
two parameters. We fit cool chromospheric holes and track the fast
evolution of compact mm brightenings in the plage region. 
Methods: We use the STiC nonlocal thermodynamic equilibrium (NLTE)
inversion code to simultaneously fit real ultraviolet and mm spectra
in order to infer the thermodynamic parameters of the plasma. 
Results: We confirm the anticipated constraining potential of ALMA
in NLTE inversions of the solar chromosphere. We find significant
differences between the inversion results of IRIS data alone compared to
the results of a combination with the mm data: the IRIS+ALMA inversions
have increased contrast and temperature range, and tend to favor lower
values of microturbulence (∼3-6 km s-1 in plage compared
to ∼4-7 km s-1 from IRIS alone) in the chromosphere. The
average brightness temperature of the plage region at 1.25 mm is 8500
K, but the ALMA maps also show much cooler (∼3000 K) and hotter
(∼11 000 K) evolving features partially seen in other diagnostics. To
explain the former, the inversions require the existence of localized
low-temperature regions in the chromosphere where molecules such as CO
could form. The hot features could sustain such high temperatures due to
non-equilibrium hydrogen ionization effects in a shocked chromosphere
- a scenario that is supported by low-frequency shock wave patterns
found in the Mg II lines probed by IRIS.

---------------------------------------------------------
Title: Observational constraints on the origin of the elements. II. 3D
non-LTE formation of Ba II lines in the solar atmosphere
Authors: Gallagher, A. J.; Bergemann, M.; Collet, R.; Plez, B.;
Leenaarts, J.; Carlsson, M.; Yakovleva, S. A.; Belyaev, A. K.
2020A&A...634A..55G Altcode: 2019arXiv191003898G
Context. The pursuit of more realistic spectroscopic modelling
and consistent abundances has led us to begin a new series of
papers designed to improve current solar and stellar abundances of
various atomic species. To achieve this, we have begun updating the
three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE)
radiative transfer code, MULTI3D, and the equivalent one-dimensional
(1D) non-LTE radiative transfer code, MULTI 2.3. Aims: We
examine our improvements to these codes by redetermining the solar
barium abundance. Barium was chosen for this test as it is an important
diagnostic element of the s-process in the context of galactic chemical
evolution. New Ba II + H collisional data for excitation and charge
exchange reactions computed from first principles had recently become
available and were included in the model atom. The atom also includes
the effects of isotopic line shifts and hyperfine splitting. 
Methods: A grid of 1D LTE barium lines were constructed with MULTI 2.3
and fit to the four Ba II lines available to us in the optical region
of the solar spectrum. Abundance corrections were then determined in
1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium
abundance was computed from these corrections. Results: We
present for the first time the full 3D non-LTE barium abundance of
A(Ba) = 2.27 ± 0.02 ± 0.01, which was derived from four individual
fully consistent barium lines. Errors here represent the systematic
and random errors, respectively.

---------------------------------------------------------
Title: 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.
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: Observational constraints on the origin of the elements. I. 3D
NLTE formation of Mn lines in late-type stars
Authors: Bergemann, Maria; Gallagher, Andrew J.; Eitner, Philipp;
Bautista, Manuel; Collet, Remo; Yakovleva, Svetlana A.; Mayriedl,
Anja; Plez, Bertrand; Carlsson, Mats; Leenaarts, Jorrit; Belyaev,
Andrey K.; Hansen, Camilla
2019A&A...631A..80B Altcode: 2019arXiv190505200B
Manganese (Mn) is a key Fe-group element, commonly employed in
stellar population and nucleosynthesis studies to explore the role
of SN Ia. We have developed a new non-local thermodynamic equilibrium
(NLTE) model of Mn, including new photo-ionisation cross-sections and
new transition rates caused by collisions with H and H-
atoms. We applied the model in combination with one-dimensional (1D)
LTE model atmospheres and 3D hydrodynamical simulations of stellar
convection to quantify the impact of NLTE and convection on the line
formation. We show that the effects of NLTE are present in Mn I and, to
a lesser degree, in Mn II lines, and these increase with metallicity and
with the effective temperature of a model. Employing 3D NLTE radiative
transfer, we derive a new abundance of Mn in the Sun, A(Mn) = 5.52 ±
0.03 dex, consistent with the element abundance in C I meteorites. We
also applied our methods to the analysis of three metal-poor benchmark
stars. We find that 3D NLTE abundances are significantly higher than 1D
LTE. For dwarfs, the differences between 1D NLTE and 3D NLTE abundances
are typically within 0.15 dex, however, the effects are much larger in
the atmospheres of giants owing to their more vigorous convection. We
show that 3D NLTE successfully solves the ionisation and excitation
balance for the RGB star HD 122563 that cannot be achieved by 1D LTE or
1D NLTE modelling. For HD 84937 and HD 140283, the ionisation balance is
satisfied, however, the resonance Mn I triplet lines still show somewhat
lower abundances compared to the high-excitation lines. Our results for
the benchmark stars confirm that 1D LTE modelling leads to significant
systematic biases in Mn abundances across the full wavelength range
from the blue to the IR. We also produce a list of Mn lines that are
not significantly biased by 3D and can be reliably, within the 0.1 dex
uncertainty, modelled in 1D NLTE. The new cross-sections and rate
coefficients are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A80">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A80</A>The
atomic model is available at <A
href="https://keeper.mpdl.mpg.de/f/1ce2a838074b49fc9424/?dl=1">https://keeper.mpdl.mpg.de/f/1ce2a838074b49fc9424/?dl=1</A>

---------------------------------------------------------
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. 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. 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. 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. 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: VizieR Online Data Catalog: Mn lines 3D NLTE formation in
late-type stars (Bergemann+, 2019)
Authors: Bergemann, M.; Gallagher, A. G.; Eitner, P.; Bautista, M.;
Collet, R.; Yakovleva, S.; Mayriedl, A.; Plez, B.; Carlsson, M.;
Leenaarts, J.; Belyaev, A. K.; Hansen, C.
2019yCat..36310080B Altcode:
Model rate coefficients in cm3/s for neutralization and
de-exci processes in collisions of MnI+H and MnII+H-, as
well as MnII+H and MnIII+H- for temperatures from T=1000K to
T=10000K. Total photoionization cross section for MnI levels. (24 data files).

---------------------------------------------------------
Title: 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.
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: Recent advancements in the EST project
Authors: Jurčák, Jan; Collados, Manuel; Leenaarts, Jorrit; van Noort,
Michiel; Schlichenmaier, Rolf
2019AdSpR..63.1389J Altcode: 2018arXiv181100851J
The European Solar Telescope (EST) is a project of a new-generation
solar telescope. It has a large aperture of 4 m, which is necessary for
achieving high spatial and temporal resolution. The high polarimetric
sensitivity of the EST will allow to measure the magnetic field in the
solar atmosphere with unprecedented precision. Here, we summarise the
recent advancements in the realisation of the EST project regarding
the hardware development and the refinement of the science requirements.

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Title: Chromospheric observations and magnetic configuration of a
supergranular structure
Authors: Robustini, Carolina; Esteban Pozuelo, Sara; Leenaarts,
Jorrit; de la Cruz Rodríguez, Jaime
2019A&A...621A...1R Altcode: 2018A&A...621A...1R; 2018arXiv181010762R
Context. Unipolar magnetic regions are often associated with
supergranular cells. The chromosphere above these regions is regulated
by the magnetic field, but the field structure is poorly known. In
unipolar regions, the fibrillar arrangement does not always coincide
with magnetic field lines, and polarimetric observations are needed
to establish the chromospheric magnetic topology. Aims: In an
active region close to the limb, we observed a unipolar annular network
of supergranular size. This supergranular structure harbours a radial
distribution of the fibrils converging towards its centre. We aim to
improve the description of this structure by determining the magnetic
field configuration and the line-of-sight velocity distribution in both
the photosphere and the chromosphere. Methods: We observed the
supergranular structure at different heights by taking data in the Fe I
6301-6302 Å, Hα, Ca II 8542 Å, and the Ca II H&amp;K spectral lines
with the CRisp Imaging SpectroPolarimeter (CRISP) and CHROMospheric
Imaging Spectrometer (CHROMIS) at the Swedish 1-m Solar Telescope. We
performed Milne-Eddington inversions of the spectropolarimetric data
of Fe I 6301-6302 Å and applied the weak field approximation to Ca
II 8542 Å data to retrieve the magnetic field in the photosphere
and chromosphere. We used photospheric magnetograms of CRISP, Hinode
Solar Optical Telescope spectropolarimeter, and Helioseismic and
Magnetic Imager to calculate the magnetic flux. We investigated the
velocity distribution using the line-of-sight velocities computed
from the Milne-Eddington inversion and from the Doppler shift of the
K3 feature in the Ca II K spectral line. To describe the
typical spectral profiles characterising the chromosphere above the
inner region of the supergranular structure, we performed a K-mean
clustering of the spectra in Ca II K. Results: The photospheric
magnetic flux shows that the supergranular boundary has an excess
of positive polarity and the whole structure is not balanced. The
magnetic field vector at chromospheric heights, retrieved by the
weak field approximation, indicates that the field lines within the
supergranular cell tend to point inwards, and might form a canopy
above the unipolar region. In the centre of the supergranular cell
hosting the unipolar region, we observe a persistent chromospheric
brightening coinciding with a strong gradient in the line-of-sight
velocity. The movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833246/olm">https://www.aanda.org</A>

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Title: Chromospheric condensations and magnetic field in a C3.6-class
flare studied via He I D3 spectro-polarimetry
Authors: Libbrecht, Tine; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Leenaarts, Jorrit; Pazira, Hiva
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. Aims:
We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D3 line. Methods: We present the first SST/CRISP spectro-polarimetric
observations of He I D3. We analyzed the data using the
inversion code HAZEL, and estimate the line-of-sight velocity and
the magnetic field vector. Results: Strong He I D3
emission at the flare footpoints, as well as strong He I D3
absorption profiles tracing the flaring loops are observed during the
flare. The He I D3 traveling emission kernels at the flare
footpoints exhibit strong chromospheric condensations of up to ∼60
km s-1 at their leading edge. Our observations suggest that
such condensations result in shocking the deep chromosphere, causing
broad and modestly blueshifted He I D3 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 D3 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 D3 inversions. We propose a
fan-spine configuration as the flare magnetic field topology. 
Conclusions: The He I D3 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.

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Title: Temperature constraints from inversions of synthetic solar
optical, UV, and radio spectra
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.;
Leenaarts, J.
2018A&A...620A.124D Altcode: 2018arXiv180606682D
Context. High-resolution observations of the solar chromosphere
at millimeter wavelengths are now possible with the Atacama Large
Millimeter Array (ALMA), bringing with them the promise of tackling
many open problems in solar physics. Observations from other ground
and space-based telescopes will greatly benefit from coordinated
endeavors with ALMA, yet the diagnostic potential of combined optical,
ultraviolet and mm observations has remained mostly unassessed. 
Aims: In this paper we investigate whether mm-wavelengths could aid
current inversion schemes to retrieve a more accurate representation
of the temperature structure of the solar atmosphere. Methods:
We performed several non-LTE inversion experiments of the emergent
spectra from a snapshot of 3D radiation-MHD simulation. We included
common line diagnostics such as Ca II H, K, 8542 Å and Mg II h and k,
taking into account partial frequency redistribution effects, along
with the continuum around 1.2 mm and 3 mm. Results: We find
that including the mm-continuum in inversions allows a more accurate
inference of temperature as function of optical depth. The addition
of ALMA bands to other diagnostics should improve the accuracy of the
inferred chromospheric temperatures between log τ ∼ [-6, -4.5]
where the Ca II and Mg II lines are weakly coupled to the local
conditions. However, we find that simultaneous multiatom, non-LTE
inversions of optical and UV lines present equally strong constraints
in the lower chromosphere and thus are not greatly improved by the
1.2 mm band. Nonetheless, the 3 mm band is still needed to better
constrain the mid-upper chromosphere.

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Title: STiC: Stockholm inversion code
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
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.

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Title: Tracing the evolution of radiation-MHD simulations of solar
and stellar atmospheres in the Lagrangian frame
Authors: Leenaarts, Jorrit
2018A&A...616A.136L Altcode: 2018arXiv180506666L
Context. Radiation magnetohydrodynamics (radiation-MHD) simulations
have become a standard tool for investigating the physics of solar and
stellar atmospheres. Aims: The aim of this paper is to present
a method that allows the efficient and accurate analysis of flows in
such simulations in the Lagrangian frame. Methods: This paper
presents a method that allows the construction of pathlines given a
seed point that can be chosen freely at any location and at any time
during the simulation where the simulation state is stored. The method
is based on passive tracer particles. Through injection of particles in
expanding regions the occurrence of particle-free volumes is avoided,
even in the case of strongly compressive flows. Results: The
method was implemented in the solar and stellar atmosphere simulation
code Bifrost. It is efficient and accurate. As examples I present
an analysis of a gas parcel in the convection zone and a particle in
the solar transition region. The movie associated to Fig. 2 is
available at <A href="https://www.aanda.org">http://www.aanda.org</A>

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Title: Disentangling flows in the solar transition region
Authors: Zacharias, P.; Hansteen, V. H.; Leenaarts, J.; Carlsson,
M.; Gudiksen, B. V.
2018A&A...614A.110Z Altcode: 2018arXiv180407513Z
Context. The measured average velocities in solar and stellar spectral
lines formed at transition region temperatures have been difficult
to interpret. The dominant redshifts observed in the lower transition
region naturally leads to the question of how the upper layers of the
solar (and stellar) atmosphere can be maintained. Likewise, no ready
explanation has been made for the average blueshifts often found in
upper transition region lines. However, realistic three-dimensional
radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere
are able to reproduce the observed dominant line shifts and may thus
hold the key to resolve these issues. Aims: These new 3D rMHD
simulations aim to shed light on how mass flows between the chromosphere
and corona and on how the coronal mass is maintained. These simulations
give new insights into the coupling of various atmospheric layers
and the origin of Doppler shifts in the solar transition region and
corona. Methods: The passive tracer particles, so-called corks,
allow the tracking of parcels of plasma over time and thus the study of
changes in plasma temperature and velocity not only locally, but also
in a co-moving frame. By following the trajectories of the corks, we
can investigate mass and energy flows and understand the composition
of the observed velocities. Results: Our findings show that
most of the transition region mass is cooling. The preponderance of
transition region redshifts in the model can be explained by the higher
percentage of downflowing mass in the lower and middle transition
region. The average upflows in the upper transition region can be
explained by a combination of both stronger upflows than downflows
and a higher percentage of upflowing mass. The most common combination
at lower and middle transition region temperatures are corks that are
cooling and traveling downward. For these corks, a strong correlation
between the pressure gradient along the magnetic field line and the
velocity along the magnetic field line has been observed, indicating a
formation mechanism that is related to downward propagating pressure
disturbances. Corks at upper transition region temperatures are
subject to a rather slow and highly variable but continuous heating
process. Conclusions: Corks are shown to be an essential tool
in 3D rMHD models in order to study mass and energy flows. We have
shown that most transition region plasma is cooling after having been
heated slowly to upper transition region temperatures several minutes
before. Downward propagating pressure disturbances are identified as
one of the main mechanisms responsible for the observed redshifts at
transition region temperatures. The movie associated to Fig. 3
is available at <A href="https://www.aanda.org">http://www.aanda.org</A>

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Title: Erratum: “A First Comparison of Millimeter Continuum and
Mg II Ultraviolet Line Emission from the Solar Chromosphere”
(<A href="http://doi.org/10.3847/2041-8213/aa844c">2017, ApJL,
845, L19</A>)
Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo,
M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M.
2018ApJ...860L..16B Altcode:
No abstract at ADS

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Title: Chromospheric heating during flux emergence in the solar
atmosphere
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Scharmer, Göran; Carlsson, Mats
2018A&A...612A..28L Altcode: 2017arXiv171200474L
Context. The radiative losses in the solar chromosphere vary from
4 kW m-2 in the quiet Sun, to 20 kW m-2 in
active regions. The mechanisms that transport non-thermal energy to
and deposit it in the chromosphere are still not understood. Aim. We
aim to investigate the atmospheric structure and heating of the solar
chromosphere in an emerging flux region. Methods: We have used
observations taken with the CHROMIS and CRISP instruments on the
Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα,
and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line
profiles and in addition perform multi-line, multi-species, non-local
thermodynamic equilibrium (non-LTE) inversions to estimate the spatial
and temporal variation of the chromospheric structure. Results:
We investigate which spectral features of Ca II K contribute to the
frequency-integrated Ca II K brightness, which we use as a tracer
of chromospheric radiative losses. The majority of the radiative
losses are not associated with localised high-Ca II K-brightness
events, but instead with a more gentle, spatially extended, and
persistent heating. The frequency-integrated Ca II K brightness
correlates strongly with the total linear polarization in the Ca II
854.2 nm, while the Ca II K profile shapes indicate that the bulk
of the radiative losses occur in the lower chromosphere. Non-LTE
inversions indicate a transition from heating concentrated around
photospheric magnetic elements below log τ500 = -3 to a more
space-filling and time-persistent heating above log τ500
= -4. The inferred gas temperature at log τ500 = -3.8
correlates strongly with the total linear polarization in the Ca
II 854.2 nm line, suggesting that that the heating rate correlates
with the strength of the horizontal magnetic field in the low
chromosphere. Movies attached to Figs. 1 and 4 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201732027/olm">https://www.aanda.org/</A>

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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. Methods: We model the synthetic
spectrum of Ca II using the radiative transfer code Multi3D in three
different radiation-magnetohydrodynamic model atmospheres computed with
the Bifrost code. We classify synthetic intensity profiles according
to their shapes and study how their features are related to the
physical properties in the model atmospheres. We investigate whether
the synthetic data reproduce the observed spatially-averaged line
shapes, center-to-limb variation and compare this data with SST/CHROMIS
images. Results: The spatially-averaged synthetic line profiles
show too low central emission peaks, and too small separation between
the peaks. The trends of the observed center-to-limb variation of
the profiles properties are reproduced by the models. The Ca II H and
K line profiles provide a temperature diagnostic of the temperature
minimum and the temperature at the formation height of the emission
peaks. The Doppler shift of the central depression is an excellent
probe of the velocity in the upper chromosphere.

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Title: The chromosphere above a δ-sunspot in the presence of
fan-shaped jets
Authors: Robustini, Carolina; Leenaarts, Jorrit; de la Cruz Rodríguez,
Jaime
2018A&A...609A..14R Altcode: 2017A&A...609A..14R; 2017arXiv170903864R
Context. Delta-sunspots are known to be favourable locations for
fast and energetic events like flares and coronal mass ejections. The
photosphere of this sunspot type has been thoroughly investigated in
the past three decades. The atmospheric conditions in the chromosphere
are not as well known, however. Aims: This study is focused on
the chromosphere of a δ-sunspot that harbours a series of fan-shaped
jets in its penumbra. The aim of this study is to establish the
magnetic field topology and the temperature distribution in the
presence of jets in the photosphere and the chromosphere. 
Methods: We use data from the Swedish 1m Solar Telescope (SST) and
the Solar Dynamics Observatory. We invert the spectropolarimetric
Fe I 6302 Å and Ca II 8542 Å data from the SST using the non-LTE
inversion code NICOLE to estimate the magnetic field configuration,
temperature, and velocity structure in the chromosphere. 
Results: A loop-like magnetic structure is observed to emerge
in the penumbra of the sunspot. The jets are launched from this
structure. Magnetic reconnection between this emerging field and the
pre-existing vertical field is suggested by hot plasma patches on the
interface between the two fields. The height at which the reconnection
takes place is located between log τ500 = -2 and log
τ500 = -3. The magnetic field vector and the atmospheric
temperature maps show a stationary configuration during the whole
observation. Movies associated to Figs. 3-5 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201731504/olm">http://www.aanda.org</A>

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Title: Comparison of Solar Fine Structure Observed Simultaneously
in Lyα and Mg II h
Authors: Schmit, D.; Sukhorukov, A. V.; De Pontieu, B.; Leenaarts,
J.; Bethge, C.; Winebarger, A.; Auchère, F.; Bando, T.; Ishikawa,
R.; Kano, R.; Kobayashi, K.; Narukage, N.; Trujillo Bueno, J.
2017ApJ...847..141S Altcode: 2017arXiv170900035S
The Chromospheric Lyman Alpha Spectropolarimeter (CLASP) observed the
Sun in H I Lyα during a suborbital rocket flight on 2015 September
3. The Interface Region Imaging Telescope (IRIS) coordinated with the
CLASP observations and recorded nearly simultaneous and co-spatial
observations in the Mg II h and k lines. The Mg II h and Lyα lines
are important transitions, energetically and diagnostically, in the
chromosphere. The canonical solar atmosphere model predicts that these
lines form in close proximity to each other and so we expect that the
line profiles will exhibit similar variability. In this analysis, we
present these coordinated observations and discuss how the two profiles
compare over a region of quiet Sun at viewing angles that approach the
limb. In addition to the observations, we synthesize both line profiles
using a 3D radiation-MHD simulation. In the observations, we find that
the peak width and the peak intensities are well correlated between the
lines. For the simulation, we do not find the same relationship. We
have attempted to mitigate the instrumental differences between IRIS
and CLASP and to reproduce the instrumental factors in the synthetic
profiles. The model indicates that formation heights of the lines
differ in a somewhat regular fashion related to magnetic geometry. This
variation explains to some degree the lack of correlation, observed
and synthesized, between Mg II and Lyα. Our analysis will aid in the
definition of future observatories that aim to link dynamics in the
chromosphere and transition region.

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Title: The Formation of IRIS Diagnostics. IX. The Formation of the
C I 135.58 NM Line in the Solar Atmosphere
Authors: Lin, Hsiao-Hsuan; Carlsson, Mats; Leenaarts, Jorrit
2017ApJ...846...40L Altcode: 2017arXiv170809426L
The C I 135.58 nm line is located in the wavelength range of NASA’s
Interface Region Imaging Spectrograph (IRIS) small explorer mission. We
study the formation and diagnostic potential of this line by means of
non local-thermodynamic-equilibrium modeling, employing both 1D and 3D
radiation-magnetohydrodynamic models. The C I/C II ionization balance is
strongly influenced by photoionization by Lyα emission. The emission
in the C I 135.58 nm line is dominated by a recombination cascade and
the line forming region is optically thick. The Doppler shift of the
line correlates strongly with the vertical velocity in its line forming
region, which is typically located at 1.5 Mm height. With IRIS, the C
I 135.58 nm line is usually observed together with the O I 135.56 nm
line, and from the Doppler shift of both lines, we obtain the velocity
difference between the line forming regions of the two lines. From
the ratio of the C I/O I line core intensity, we can determine the
distance between the C I and the O I forming layers. Combined with the
velocity difference, the velocity gradient at mid-chromospheric heights
can be derived. The C I/O I total intensity line ratio is correlated
with the inverse of the electron density in the mid-chromosphere. We
conclude that the C I 135.58 nm line is an excellent probe of the
middle chromosphere by itself, and together with the O I 135.56 nm
line the two lines provide even more information, which complements
other powerful chromospheric diagnostics of IRIS such as the Mg II h
and k lines and the C II lines around 133.5 nm.

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Title: A First Comparison of Millimeter Continuum and Mg II
Ultraviolet Line Emission from the Solar Chromosphere
Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo,
M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M.
2017ApJ...845L..19B Altcode: 2017arXiv170604532B
We present joint observations of the Sun by the Atacama Large
Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging
Spectrograph (IRIS). Both millimeter/submillimeter-λ continuum emission
and ultraviolet (UV) line emission originate from the solar chromosphere
and both have the potential to serve as powerful and complementary
diagnostics of physical conditions in this enigmatic region of the solar
atmosphere. The observations were made of a solar active region on 2015
December 18 as part of the ALMA science verification effort. A map of
the Sun’s continuum emission was obtained by ALMA at a wavelength of
1.25 mm (239 GHz). A contemporaneous map was obtained by IRIS in the
Mg II h doublet line at 2803.5 Å. While a clear correlation between
the 1.25 mm brightness temperature TB and the Mg II h
line radiation temperature Trad is observed, the slope
is &lt;1, perhaps as a result of the fact that these diagnostics
are sensitive to different parts of the chromosphere and that the
Mg II h line source function includes a scattering component. There
is a significant difference (35%) between the mean TB
(1.25 mm) and mean Trad (Mg II). Partitioning the maps
into “sunspot,” “quiet areas,” and “plage regions” we
find the relation between the IRIS Mg II h line Trad and
the ALMA TB region-dependent. We suggest this may be the
result of regional dependences of the formation heights of the IRIS
and ALMA diagnostics and/or the increased degree of coupling between
the UV source function and the local gas temperature in the hotter,
denser gas in plage regions.

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Title: Simulating the Mg II NUV Spectra &amp; C II Resonance Lines
During Solar Flares
Authors: Kerr, Graham Stewart; Allred, Joel C.; Leenaarts, Jorrit;
Butler, Elizabeth; Kowalski, Adam
2017SPD....48.0102K Altcode:
The solar chromosphere is the origin of the bulk of the enhanced
radiative output during solar flares, and so comprehensive understanding
of this region is important if we wish to understand energy transport in
solar flares. It is only relatively recently, however, with the launch
of IRIS that we have routine spectroscopic flarea observations of the
chromsphere and transition region. Since several of the spectral lines
observed by IRIS are optically thick, it is necessary to use forward
modelling to extract the useful information that these lines carry about
the flaring chromosphere and transition region. We present the results
of modelling the formation properties Mg II resonance lines &amp;
subordinate lines, and the C II resonance lines during solar flares. We
focus on understanding their relation to the physical strucutre of the
flaring atmosphere, exploiting formation height differences to determine
if we can extract information about gradients in the atmosphere. We
show the effect of degrading the profiles to the resolution of the
IRIS, and that the usual observational techniques used to identify
the line centroid do a poor job in the early stages of the flare
(partly due to multiple optically thick line components). Finally,
we will tentatively comment on the effects that 3D radiation transfer
may have on these lines.

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Title: Solar off-limb emission of the O I 7772 Å line
Authors: Pazira, H.; Kiselman, D.; Leenaarts, J.
2017A&A...604A..49P Altcode: 2017arXiv170506459P
 Aims: The aim of this paper is to understand the formation of
the O I line at 7772 Å in the solar chromosphere. Methods: We
used SST/CRISP observations to observe O I 7772 Å in several places
around the solar limb. We compared the observations with synthetic
spectra calculated with the RH code in the one-dimension spherical
geometry mode. New accurate hydrogen collisional rates were included
for the RH calculations. Results: The observations reveal a
dark gap in the lower chromosphere, which is caused by variations in
the line opacity as shown by our models. The lower level of the 7772
Å transition is populated by a downward cascade from the continuum. We
study the effect of Lyman-β pumping and hydrogen collisions between the
triplet and quintet system in O I. Both have a small but non-negligible
influence on the line intensity.

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Title: Non-LTE line formation of Fe in late-type stars - IV. Modelling
of the solar centre-to-limb variation in 3D
Authors: Lind, K.; Amarsi, A. M.; Asplund, M.; Barklem, P. S.;
Bautista, M.; Bergemann, M.; Collet, R.; Kiselman, D.; Leenaarts,
J.; Pereira, T. M. D.
2017MNRAS.468.4311L Altcode: 2017arXiv170304027L
Our ability to model the shapes and strengths of iron lines in the solar
spectrum is a critical test of the accuracy of the solar iron abundance,
which sets the absolute zero-point of all stellar metallicities. We use
an extensive 463-level Fe atom with new photoionization cross-sections
for Fe I and quantum mechanical calculations of collisional excitation
and charge transfer with neutral hydrogen; the latter effectively remove
a free parameter that has hampered all previous line formation studies
of Fe in non-local thermodynamic equilibrium (NLTE). For the first
time, we use realistic 3D NLTE calculations of Fe for a quantitative
comparison to solar observations. We confront our theoretical line
profiles with observations taken at different viewing angles across
the solar disc with the Swedish 1-m Solar Telescope. We find that
3D modelling well reproduces the observed centre-to-limb behaviour
of spectral lines overall, but highlight aspects that may require
further work, especially cross-sections for inelastic collisions with
electrons. Our inferred solar iron abundance is log (ɛFe)
= 7.48 ± 0.04 dex.

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Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun
Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson,
M.; Rouppe van der Voort, L.; Leenaarts, J.
2017ApJ...839...22H Altcode: 2017arXiv170402872H
A spectacular manifestation of solar activity is the appearance of
transient brightenings in the far wings of the Hα line, known as
Ellerman bombs (EBs). Recent observations obtained by the Interface
Region Imaging Spectrograph have revealed another type of plasma
“bombs” (UV bursts) with high temperatures of perhaps up to 8 ×
104 K within the cooler lower solar atmosphere. Realistic
numerical modeling showing such events is needed to explain
their nature. Here, we report on 3D radiative magnetohydrodynamic
simulations of magnetic flux emergence in the solar atmosphere. We
find that ubiquitous reconnection between emerging bipolar magnetic
fields can trigger EBs in the photosphere, UV bursts in the mid/low
chromosphere and small (nano-/micro-) flares (106 K) in
the upper chromosphere. These results provide new insights into the
emergence and build up of the coronal magnetic field and the dynamics
and heating of the solar surface and lower atmosphere.

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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(n2), with n the number of grid
points. When the grid becomes finer, the computational cost increases
quadratically. Aims: We aim to develop a 3D non-LTE radiative
transfer code that is more efficient than MALI. 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. 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. 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.

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Title: Observations of Ellerman bomb emission features in He I
D3 and He I 10 830 Å
Authors: Libbrecht, Tine; Joshi, Jayant; de la Cruz Rodríguez, Jaime;
Leenaarts, Jorrit; Ramos, Andrés Asensio
2017A&A...598A..33L Altcode: 2016arXiv161001321L
Context. Ellerman bombs (EBs) are short-lived emission features,
characterised by extended wing emission in hydrogen Balmer lines. Until
now, no distinct signature of EBs has been found in the He I 10 830 Å
line, and conclusive observations of EBs in He I D3 have
never been reported. Aims: We aim to study the signature of
EBs in neutral helium triplet lines. Methods: The observations
consisted of ten consecutive SST/TRIPPEL raster scans close to the
limb, featuring the Hβ, He I D3 and He I 10 830 Å spectral
regions. We also obtained raster scans with IRIS and made use of the
SDO/AIA 1700 Å channel. We used Hazel to invert the neutral helium
triplet lines. Results: Three EBs in our data show distinct
emission signatures in neutral helium triplet lines, most prominently
visible in the He I D3 line. The helium lines have two
components: a broad and blueshifted emission component associated with
the EB, and a narrower absorption component formed in the overlying
chromosphere. One of the EBs in our data shows evidence of strong
velocity gradients in its emission component. The emission component of
the other two EBs could be fitted using a constant slab. Our analysis
hints towards thermal Doppler motions having a large contribution to
the broadening for helium and IRIS lines. We conclude that the EBs
must have high temperatures to exhibit emission signals in neutral
helium triplet lines. An order of magnitude estimate places our
observed EBs in the range of T 2 × 104-105
K. Movies associated to Figs. 3-5 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629266/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Helium lines in the solar spectrum: spatial structure in He
I 10830 and the anomalous intensity of the resonance lines
Authors: Leenaarts, Jorrit
2017psio.confE..25L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Formation of the helium extreme-UV resonance lines
Authors: Golding, T. P.; Leenaarts, J.; Carlsson, M.
2017A&A...597A.102G Altcode: 2016arXiv161000352G
Context. While classical models successfully reproduce intensities
of many transition region lines, they predict helium extreme-UV
(EUV) line intensities roughly an order of magnitude lower than the
observed value. Aims: Our aim is to determine the relevant
formation mechanism(s) of the helium EUV resonance lines capable of
explaining the high intensities under quiet Sun conditions. 
Methods: We synthesised and studied the emergent spectra from a 3D
radiation-magnetohydrodynamics simulation model. The effects of coronal
illumination and non-equilibrium ionisation of hydrogen and helium
are included self-consistently in the numerical simulation. 
Results: Radiative transfer calculations result in helium EUV line
intensities that are an order of magnitude larger than the intensities
calculated under the classical assumptions. The enhanced intensity
of He Iλ584 is primarily caused by He II recombination cascades. The
enhanced intensity of He IIλ304 and He IIλ256 is caused primarily by
non-equilibrium helium ionisation. Conclusions: The analysis
shows that the long standing problem of the high helium EUV line
intensities disappears when taking into account optically thick
radiative transfer and non-equilibrium ionisation effects.

---------------------------------------------------------
Title: Partial redistribution in 3D non-LTE radiative transfer in
solar-atmosphere models
Authors: Sukhorukov, Andrii V.; Leenaarts, Jorrit
2017A&A...597A..46S Altcode: 2016A&A...597A..46S; 2016arXiv160605180S
Context. Resonance spectral lines such as H I Ly α, Mg II
H&amp;K, and Ca II H&amp;K that form in the solar chromosphere, are
influenced by the effects of 3D radiative transfer as well as partial
redistribution (PRD). So far no one has modeled these lines including
both effects simultaneously owing to the high computing demands of
existing algorithms. Such modeling is, however, indispensable for
accurate diagnostics of the chromosphere. Aims: We present
a computationally tractable method to treat PRD scattering in 3D
model atmospheres using a 3D non-local thermodynamic equilibrium
(non-LTE) radiative transfer code. Methods: To make the
method memory-friendly, we use the hybrid approximation for the
redistribution integral. To make the method fast, we use linear
interpolation on equidistant frequency grids. We verify our algorithm
against computations with the RH code and analyze it for stability,
convergence, and usefulness of acceleration using model atoms of Mg
II with the H&amp;K lines and H I with the Ly α line treated in
PRD. Results: A typical 3D PRD solution can be obtained in
a model atmosphere with 252 × 252 × 496 coordinate points in 50
000-200 000 CPU hours, which is a factor ten slower than computations
assuming complete redistribution. We illustrate the importance of the
joint action of PRD and 3D effects for the Mg II H&amp;K lines for
disk-center intensities, as well as the center-to-limb variation. Conclusions: The proposed method allows for the simulation of
PRD lines in a time series of radiation-magnetohydrodynamic models,
in order to interpret observations of chromospheric lines at high
spatial resolution.

---------------------------------------------------------
Title: 3D NLTE analysis of the most iron-deficient star, SMSS0313-6708
Authors: Nordlander, T.; Amarsi, A. M.; Lind, K.; Asplund, M.; Barklem,
P. S.; Casey, A. R.; Collet, R.; Leenaarts, J.
2017A&A...597A...6N Altcode: 2016arXiv160907416N; 2016A&A...597A...6N
Context. Models of star formation in the early universe require
a detailed understanding of accretion, fragmentation and radiative
feedback in metal-free molecular clouds. Different simulations predict
different initial mass functions of the first stars, ranging from
predominantly low-mass (0.1-10 M⊙), to massive (10-100
M⊙), or even supermassive (100-1000 M⊙). The
mass distribution of the first stars should lead to unique chemical
imprints on the low-mass second and later generation metal-poor
stars still in existence. The chemical composition of SMSS0313-6708,
which has the lowest abundances of Ca and Fe of any star known,
indicates it was enriched by a single massive supernova. Aims:
The photospheres of metal-poor stars are relatively transparent in
the UV, which may lead to large three-dimensional (3D) effects as
well as departures from local thermodynamical equilibrium (LTE),
even for weak spectral lines. If 3D effects and departures from LTE
(NLTE) are ignored or treated incorrectly, errors in the inferred
abundances may significantly bias the inferred properties of the
polluting supernovae. We redetermine the chemical composition of
SMSS0313-6708by means of the most realistic methods available, and
compare the results to predicted supernova yields. Methods:
A 3D hydrodynamical Staggermodel atmosphere and 3D NLTE radiative
transfer were applied to obtain accurate abundances for Li, Na, Mg, Al,
Ca and Fe. The model atoms employ realistic collisional rates, with
no calibrated free parameters. Results: We find significantly
higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5
dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03
dex. In particular, our upper limit for [Fe/H] is now a factor ten
larger, at [Fe/H] &lt; -6.53 (3σ), than previous estimates based on
⟨ 3D ⟩NLTE (I.e., using averaged 3D models). This higher estimate
is due to a conservative upper limit estimation, updated NLTE data,
and 3D-⟨ 3D ⟩NLTE differences, all of which lead to a higher
abundance determination. Conclusions: We find that supernova
yields for models in a wide range of progenitor masses reproduce the
revised chemical composition. In addition to massive progenitors of
20-60 M⊙ exploding with low energies (1-2 B, where 1
B = 1051 erg), we also find good fits for progenitors of
10 M⊙, with very low explosion energies (&lt;1 B). We
cannot reconcile the new abundances with supernovae or hypernovae with
explosion energies above 2.5 B, nor with pair-instability supernovae.

---------------------------------------------------------
Title: Non-LTE Inversions of the Mg II h &amp; k and UV Triplet Lines
Authors: de la Cruz Rodríguez, Jaime; Leenaarts, Jorrit; Asensio
Ramos, Andrés
2016ApJ...830L..30D Altcode: 2016arXiv160909527D
The Mg II h &amp; k lines are powerful diagnostics for studying the
solar chromosphere. They have become particularly popular with the
launch of the Interface Region Imaging Spectrograph (IRIS) satellite,
and a number of studies that include these lines have lead to great
progress in understanding chromospheric heating, in many cases thanks
to the support from 3D MHD simulations. In this study, we utilize
another approach to analyze observations: non-LTE inversions of
the Mg II h &amp; k and UV triplet lines including the effects of
partial redistribution. Our inversion code attempts to construct a
model atmosphere that is compatible with the observed spectra. We have
assessed the capabilities and limitations of the inversions using the
FALC atmosphere and a snapshot from a 3D radiation-MHD simulation. We
find that Mg II h &amp; k allow reconstructing a model atmosphere from
the middle photosphere to the transition region. We have also explored
the capabilities of a multi-line/multi-atom setup, including the Mg
II h &amp; k, the Ca II 854.2 nm, and the Fe I 630.25 lines to recover
the full stratification of physical parameters, including the magnetic
field vector, from the photosphere to the chromosphere. Finally, we
present the first inversions of observed IRIS spectra from quiet-Sun,
plage, and sunspot, with very promising results.

---------------------------------------------------------
Title: The cause of spatial structure in solar He I 1083 nm multiplet
images
Authors: Leenaarts, Jorrit; Golding, Thomas; Carlsson, Mats; Libbrecht,
Tine; Joshi, Jayant
2016A&A...594A.104L Altcode: 2016arXiv160800838L
Context. The He I 1083 nm is a powerful diagnostic for inferring
properties of the upper solar chromosphere, in particular for the
magnetic field. The basic formation of the line in one-dimensional
models is well understood, but the influence of the complex
three-dimensional structure of the chromosphere and corona has however
never been investigated. This structure must play an essential role
because images taken in He I 1083 nm show structures with widths
down to 100 km. Aims: We aim to understand the effect of
the three-dimensional temperature and density structure in the
solar atmosphere on the formation of the He I 1083 nm line. 
Methods: We solved the non-LTE radiative transfer problem assuming
statistical equilibrium for a simple nine-level helium atom that
nevertheless captures all essential physics. As a model atmosphere we
used a snapshot from a 3D radiation-MHD simulation computed with the
Bifrost code. Ionising radiation from the corona was self-consistently
taken into account. Results: The emergent intensity in the He
I 1083 nm is set by the source function and the opacity in the upper
chromosphere. The former is dominated by scattering of photospheric
radiation and does not vary much with spatial location. The latter
is determined by the photonionisation rate in the He I ground state
continuum, as well as the electron density in the chromosphere. The
spatial variation of the flux of ionising radiation is caused
by the spatially-structured emissivity of the ionising photons
from material at T ≈ 100 kK in the transition region. The hotter
coronal material produces more ionising photons, but the resulting
radiation field is smooth and does not lead to small-scale variation
of the UV flux. The corrugation of the transition region further
increases the spatial variation of the amount of UV radiation in the
chromosphere. Finally we find that variations in the chromospheric
electron density also cause strong variation in He I 1083 nm
opacity. We compare our findings to observations using SST, IRIS and
SDO/AIA data. A movie associated to Fig. 4 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201628490/olm">http://www.aanda.org</A>

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Title: Chromospheric LAyer SpectroPolarimeter (CLASP2)
Authors: Narukage, Noriyuki; McKenzie, David E.; Ishikawa, Ryoko;
Trujillo-Bueno, Javier; De Pontieu, Bart; Kubo, Masahito; Ishikawa,
Shin-nosuke; Kano, Ryouhei; Suematsu, Yoshinori; Yoshida, Masaki;
Rachmeler, Laurel A.; Kobayashi, Ken; Cirtain, Jonathan W.; Winebarger,
Amy R.; Asensio Ramos, Andres; del Pino Aleman, Tanausu; Štępán,
Jiri; Belluzzi, Luca; Larruquert, Juan Ignacio; Auchère, Frédéric;
Leenaarts, Jorrit; Carlsson, Mattias J. L.
2016SPIE.9905E..08N Altcode:
The sounding rocket Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP) was launched on September 3rd, 2015, and successfully detected
(with a polarization accuracy of 0.1 %) the linear polarization signals
(Stokes Q and U) that scattering processes were predicted to produce
in the hydrogen Lyman-alpha line (Lyα 121.567 nm). Via the Hanle
effect, this unique data set may provide novel information about the
magnetic structure and energetics in the upper solar chromosphere. The
CLASP instrument was safely recovered without any damage and we have
recently proposed to dedicate its second flight to observe the four
Stokes profiles in the spectral region of the Mg II h and k lines
around 280 nm; in these lines the polarization signals result from
scattering processes and the Hanle and Zeeman effects. Here we describe
the modifications needed to develop this new instrument called the
"Chromospheric LAyer SpectroPolarimeter" (CLASP2).

---------------------------------------------------------
Title: Fan-shaped jets above the light bridge of a sunspot driven
by reconnection
Authors: Robustini, Carolina; Leenaarts, Jorrit; de la Cruz Rodriguez,
Jaime; Rouppe van der Voort, Luc
2016A&A...590A..57R Altcode: 2015arXiv150807927R
We report on a fan-shaped set of high-speed jets above a strongly
magnetized light bridge (LB) of a sunspot observed in the Hα line. We
study the origin, dynamics, and thermal properties of the jets using
high-resolution imaging spectroscopy in Hα from the Swedish 1m Solar
Telescope and data from the Solar Dynamics Observatory and Hinode. The
Hα jets have lengths of 7-38 Mm, are impulsively accelerated to a speed
of ~100 km s-1 close to photospheric footpoints in the LB,
and exhibit a constant deceleration consistent with solar effective
gravity. They are predominantly launched from one edge of the light
bridge, and their footpoints appear bright in the Hα wings. Atmospheric
Imaging Assembly data indicates elongated brightenings that are nearly
co-spatial with the Hα jets. We interpret them as jets of transition
region temperatures. The magnetic field in the light bridge has a
strength of 0.8-2 kG and it is nearly horizontal. All jet properties
are consistent with magnetic reconnection as the driver. Movies
associated to Figs. 1 and 2 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201528022/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Non-LTE oxygen line formation in 3D hydrodynamic model
stellar atmospheres
Authors: Amarsi, A. M.; Asplund, M.; Collet, R.; Leenaarts, J.
2016MNRAS.455.3735A Altcode: 2015arXiv151101155A
The O I 777 nm lines are among the most commonly used diagnostics for
the oxygen abundances in the atmospheres of FGK-type stars. However,
they form in conditions that are far from local thermodynamic
equilibrium (LTE). We explore the departures from LTE of atomic
oxygen, and their impact on O I lines, across the STAGGER-grid of
three-dimensional hydrodynamic model atmospheres. For the O I 777 nm
triplet, we find significant departures from LTE. These departures are
larger in stars with larger effective temperatures, smaller surface
gravities, and larger oxygen abundances. We present grids of predicted
3D non-LTE based equivalent widths for the O I 616 nm, [O I] 630 nm,
[O I] 636 nm, and O I 777 nm lines, as well as abundance corrections
to 1D LTE based results.

---------------------------------------------------------
Title: Non-equilibrium Helium Ionization in an MHD Simulation of
the Solar Atmosphere
Authors: Golding, Thomas Peter; Leenaarts, Jorrit; Carlsson, Mats
2016ApJ...817..125G Altcode: 2015arXiv151204738G
The ionization state of the gas in the dynamic solar chromosphere can
depart strongly from the instantaneous statistical equilibrium commonly
assumed in numerical modeling. We improve on earlier simulations of
the solar atmosphere that only included non-equilibrium hydrogen
ionization by performing a 2D radiation-magnetohydrodynamics
simulation featuring non-equilibrium ionization of both hydrogen
and helium. The simulation includes the effect of hydrogen Lyα and
the EUV radiation from the corona on the ionization and heating
of the atmosphere. Details on code implementation are given. We
obtain helium ion fractions that are far from their equilibrium
values. Comparison with models with local thermodynamic equilibrium
(LTE) ionization shows that non-equilibrium helium ionization leads to
higher temperatures in wavefronts and lower temperatures in the gas
between shocks. Assuming LTE ionization results in a thermostat-like
behavior with matter accumulating around the temperatures where the
LTE ionization fractions change rapidly. Comparison of DEM curves
computed from our models shows that non-equilibrium ionization leads
to more radiating material in the temperature range 11-18 kK, compared
to models with LTE helium ionization. We conclude that non-equilibrium
helium ionization is important for the dynamics and thermal structure
of the upper chromosphere and transition region. It might also help
resolve the problem that intensities of chromospheric lines computed
from current models are smaller than those observed.

---------------------------------------------------------
Title: A publicly available simulation of an enhanced network region
of the Sun
Authors: Carlsson, Mats; Hansteen, Viggo H.; Gudiksen, Boris V.;
Leenaarts, Jorrit; De Pontieu, Bart
2016A&A...585A...4C Altcode: 2015arXiv151007581C
Context. The solar chromosphere is the interface between the
solar surface and the solar corona. Modelling of this region is
difficult because it represents the transition from optically
thick to thin radiation escape, from gas-pressure domination to
magnetic-pressure domination, from a neutral to an ionised state,
from MHD to plasma physics, and from near-equilibrium (LTE) to
non-equilibrium conditions. Aims: Our aim is to provide the
community with realistic simulations of the magnetic solar outer
atmosphere. This will enable detailed comparison of existing and
upcoming observations with synthetic observables from the simulations,
thereby elucidating the complex interactions of magnetic fields and
plasma that are crucial for our understanding of the dynamic outer
atmosphere. Methods: We used the radiation magnetohydrodynamics
code Bifrost to perform simulations of a computational volume
with a magnetic field topology similar to an enhanced network
area on the Sun. Results: The full simulation cubes are
made available from the Hinode Science Data Centre Europe. The
general properties of the simulation are discussed, and limitations
are discussed. The Hinode Science Data Centre Europe (<A
href="http://www.sdc.uio.no/search/simulations">http://www.sdc.uio.no/search/simulations</A>).

---------------------------------------------------------
Title: The Galactic chemical evolution of oxygen inferred from 3D
non-LTE spectral-line-formation calculations.
Authors: Amarsi, A. M.; Asplund, M.; Collet, R.; Leenaarts, J.
2015MNRAS.454L..11A Altcode: 2015arXiv150804857A
We revisit the Galactic chemical evolution of oxygen, addressing the
systematic errors inherent in classical determinations of the oxygen
abundance that arise from the use of one-dimensional (1D) hydrostatic
model atmospheres and from the assumption of local thermodynamic
equilibrium (LTE). We perform detailed 3D non-LTE radiative-transfer
calculations for atomic oxygen lines across a grid of 3D hydrodynamic
STAGGER model atmospheres for dwarfs and subgiants. We apply our grid
of predicted line strengths of the [O I] 630 nm and O I 777 nm lines
using accurate stellar parameters from the literature. We infer a steep
decay in [O/Fe] for [Fe/H] ≳ -1.0, a plateau [O/Fe] ≈ 0.5 down to
[Fe/H] ≈ -2.5, and an increasing trend for [Fe/H] ≲ -2.5. Our 3D
non-LTE calculations yield overall concordant results from the two
oxygen abundance diagnostics.

---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. VI. The Diagnostic Potential
of the C II Lines at 133.5 nm in the Solar Atmosphere
Authors: Rathore, Bhavna; Carlsson, Mats; Leenaarts, Jorrit; De
Pontieu, Bart
2015ApJ...811...81R Altcode: 2015arXiv150804423R
We use 3D radiation magnetohydrodynamic models to investigate how the
thermodynamic quantities in the simulation are encoded in observable
quantities, thus exploring the diagnostic potential of the C ii 133.5
nm lines. We find that the line core intensity is correlated with the
temperature at the formation height but the correlation is rather weak,
especially when the lines are strong. The line core Doppler shift is a
good measure of the line-of-sight velocity at the formation height. The
line width is both dependent on the width of the absorption profile
(thermal and non-thermal width) and an opacity broadening factor of
1.2-4 due to the optically thick line formation with a larger broadening
for double peak profiles. The C ii 133.5 nm lines can be formed both
higher and lower than the core of the Mg ii k line depending on the
amount of plasma in the 14-50 kK temperature range. More plasma in
this temperature range gives a higher C ii 133.5 nm formation height
relative to the Mg ii k line core. The synthetic line profiles have been
compared with Interface Region Imaging Spectrograph observations. The
derived parameters from the simulated line profiles cover the parameter
range seen in observations but, on average, the synthetic profiles are
too narrow. We interpret this discrepancy as a combination of a lack
of plasma at chromospheric temperatures in the simulation box and too
small non-thermal velocities. The large differences in the distribution
of properties between the synthetic profiles and the observed ones
show that the C ii 133.5 nm lines are powerful diagnostics of the
upper chromosphere and lower transition region.

---------------------------------------------------------
Title: Observed Variability of the Solar Mg II h Spectral Line
Authors: Schmit, D.; Bryans, P.; De Pontieu, B.; McIntosh, S.;
Leenaarts, J.; Carlsson, M.
2015ApJ...811..127S Altcode: 2015arXiv150804714S
The Mg ii h&amp;k doublet are two of the primary spectral lines observed
by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These
lines are tracers of the magnetic and thermal environment that spans
from the photosphere to the upper chromosphere. We use a double-Gaussian
model to fit the Mg ii h profile for a full-Sun mosaic data set taken
on 2014 August 24. We use the ensemble of high-quality profile fits to
conduct a statistical study on the variability of the line profile as
it relates the magnetic structure, dynamics, and center-to-limb viewing
angle. The average internetwork profile contains a deeply reversed
core and is weakly asymmetric at h2. In the internetwork, we find a
strong correlation between h3 wavelength and profile asymmetry as well
as h1 width and h2 width. The average reversal depth of the h3 core
is inversely related to the magnetic field. Plage and sunspots exhibit
many profiles that do not contain a reversal. These profiles also occur
infrequently in the internetwork. We see indications of magnetically
aligned structures in plage and network in statistics associated with
the line core, but these structures are not clear or extended in the
internetwork. The center-to-limb variations are compared to predictions
of semi-empirical model atmospheres. We measure a pronounced limb
darkening in the line core that is not predicted by the model. The
aim of this work is to provide a comprehensive measurement baseline
and preliminary analysis on the observed structure and formation of
the Mg ii profiles observed by IRIS.

---------------------------------------------------------
Title: What Do IRIS Observations of Mg II k Tell Us about the Solar
Plage Chromosphere?
Authors: Carlsson, Mats; Leenaarts, Jorrit; De Pontieu, Bart
2015ApJ...809L..30C Altcode: 2015arXiv150804888C
We analyze observations from the Interface Region Imaging Spectrograph
of the Mg ii k line, the Mg ii UV subordinate lines, and the O i
135.6 {nm} line to better understand the solar plage chromosphere. We
also make comparisons with observations from the Swedish 1-m Solar
Telescope of the Hα line, the Ca ii 8542 line, and Solar Dynamics
Observatory/Atmospheric Imaging Assembly observations of the coronal
19.3 {nm} line. To understand the observed Mg ii profiles, we compare
these observations to the results of numerical experiments. The
single-peaked or flat-topped Mg ii k profiles found in plage imply a
transition region at a high column mass and a hot and dense chromosphere
of about 6500 K. This scenario is supported by the observed large-scale
correlation between moss brightness and filled-in profiles with very
little or absent self-reversal. The large wing width found in plage
also implies a hot and dense chromosphere with a steep chromospheric
temperature rise. The absence of emission in the Mg ii subordinate
lines constrain the chromospheric temperature and the height of the
temperature rise while the width of the O i 135.6 {nm} line sets a
limit to the non-thermal velocities to around 7 km s-1.

---------------------------------------------------------
Title: Three-dimensional Radiative Transfer Simulations of
the Scattering Polarization of the Hydrogen Lyα Line in a
Magnetohydrodynamic Model of the Chromosphere-Corona Transition Region
Authors: Štěpán, J.; Trujillo Bueno, J.; Leenaarts, J.; Carlsson, M.
2015ApJ...803...65S Altcode: 2015arXiv150106382S
Probing the magnetism of the upper solar chromosphere requires measuring
and modeling the scattering polarization produced by anisotropic
radiation pumping in UV spectral lines. Here we apply PORTA (a novel
radiative transfer code) to investigate the hydrogen Lyα line in
a three-dimensional model of the solar atmosphere resulting from a
state of the art magnetohydrodynamic (MHD) simulation. At full spatial
resolution the linear polarization signals are very significant all
over the solar disk, with a large fraction of the field of view (FOV)
showing line-center amplitudes well above the 1% level. Via the Hanle
effect the line-center polarization signals are sensitive to the
magnetic field of the model's transition region, even when its mean
field strength is only 15 G. The breaking of the axial symmetry of the
radiation field produces significant forward-scattering polarization
in Lyα, without the need of an inclined magnetic field. Interestingly,
the Hanle effect tends to decrease such forward-scattering polarization
signals in most of the points of the FOV. When the spatial resolution is
degraded, the line-center polarization of Lyα drops below the 1% level,
reaching values similar to those previously found in one-dimensional
(1D) semi-empirical models (i.e., up to about 0.5 %). The center to
limb variation (CLV) of the spatially averaged polarization signals
is qualitatively similar to that found in 1D models, with the largest
line-center amplitudes at μ =cos θ ≈ 0.4 (θ being the heliocentric
angle). These results are important, both for designing the needed
space-based instrumentation and for a reliable interpretation of future
observations of the Lyα polarization.

---------------------------------------------------------
Title: On Fibrils and Field Lines: the Nature of Hα Fibrils in the
Solar Chromosphere
Authors: Leenaarts, Jorrit; Carlsson, Mats; Rouppe van der Voort, Luc
2015ApJ...802..136L Altcode: 2015arXiv150200295L
Observations of the solar chromosphere in the line core of the Hα
line show dark elongated structures called fibrils that show swaying
motion. We performed a three-dimensional radiation-MHD simulation of a
network region and computed synthetic Hα images from this simulation
to investigate the relation between fibrils and the magnetic field
lines in the chromosphere. The periods, amplitudes, and phase speeds
of the simulated fibrils are consistent with observations. We find that
some fibrils trace out the same field line along the fibril’s length,
while other fibrils sample different field lines at different locations
along their length. Fibrils sample the same field lines on a timescale
of ∼200 s. This is shorter than their own lifetime. Fibril-threading
field lines carry slow-mode waves, as well as transverse waves
propagating with the Alfvén speed. Transverse waves propagating
in opposite directions cause an interference pattern with complex
apparent phase speeds. The relationship between fibrils and field lines
is governed by constant migration and swaying of the field lines,
their mass loading and draining, and their visibility in Hα. Field
lines are visible where they lie close to the optical depth unity
surface. The location of the latter is at a height at which the column
mass reaches a certain fixed value. The visibility of the field line
is thus determined by its own mass density and by the mass density of
the material above it. Using the swaying motion of fibrils as a tracer
of chromospheric transverse oscillations must be done with caution.

---------------------------------------------------------
Title: Observables of Ion-Neutral Interaction Effects in the Solar
Chromosphere
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
Pereira, T. M. D.; Leenaarts, J.; Carlsson, M.
2014AGUFMSH51C4176M Altcode:
The chromosphere and transition region constitute the interface
between the solar surface and the corona and modulate the flow of
mass and energy into the upper atmosphere. IRIS was launched in 2013
to study the chromosphere and transition region. The complexity of the
chromosphere is due to various regime changes that take place across it,
like: Hydrogen goes from predominantly neutral to predominantly ionized;
the plasma behavior changes from collisional to collision-less; it goes
from gas-pressure dominated to magnetically driven, etc. Consequently,
the interpretation of chromospheric observations in general and those
from IRIS, in particular, is a challenging task. It is thus crucial
to combine IRIS observations with advanced radiative-MHD numerical
modeling. Because the photosphere, chromosphere and transition region
are partially ionized, the interaction between ionized and neutral
particles has important consequences on the magneto-thermodynamics of
these regions. We implemented the effects of partial ionization using
generalized Ohm's law in the Bifrost code (Gudiksen et al. 2011) which
solves the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along magnetic field lines. The
implementation of partial ionization effects impact our modeled
radiative-MHD atmosphere, such as producing chromospheric heating and
diffusion of photospheric magnetic field into the upper-chromosphere. We
will focus on which observables of these processes can be revealed
with IRIS.

---------------------------------------------------------
Title: Wave Propagation in the Internetwork Chromosphere: Comparing
IRIS Observations of Mg II h and k with Simulations
Authors: Fleck, B.; De Pontieu, B.; Leenaarts, J.; Pereira, T. M. D.;
Straus, T.
2014AGUFMSH51C4174F Altcode:
The objective of this study is to explore the dynamics of the
upper internetwork chromosphere using high-resolution spectroscopic
"sit-and-stare" time series obtained with the Interface Region Imaging
Spectrogragh (IRIS) in the Mg II h and k lines. The Mg II h and k lines
reveal a particularly complex spatio-temporal behavior, which strongly
depends on the magnetic field topology. We focus on six parameters in
both the h and k line: the Doppler shift and intensity of the central
reversal (h3 and k3) and the blue and red emission peaks (h2v, h2r,
k2v, k2r). In an effort to better understand what physical parameters
can be extracted from these lines and to put our interpretation of
the observations on more solid grounds, we extend our analysis to
synthetic spectra obtained from numerical simulations and compare the
results to the observations.

---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
2014SoPh..289.2733D Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
The Interface Region Imaging Spectrograph (IRIS) small explorer
spacecraft provides simultaneous spectra and images of the photosphere,
chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
spatial resolution, two-second temporal resolution, and 1 km
s−1 velocity resolution over a field-of-view of up to
175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
19-cm UV telescope that feeds a slit-based dual-bandpass imaging
spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
images in four different passbands (C II 1330, Si IV 1400, Mg II k
2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will
advance our understanding of the flow of mass and energy through an
interface region, formed by the chromosphere and transition region,
between the photosphere and corona. This highly structured and dynamic
region not only acts as the conduit of all mass and energy feeding
into the corona and solar wind, it also requires an order of magnitude
more energy to heat than the corona and solar wind combined. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data
(after compression) are acquired by IRIS each day and made available
for unrestricted use within a few days of the observation.

---------------------------------------------------------
Title: On the Signatures of Waves and Oscillations in IRIS
Observations
Authors: Fleck, Bernard; Straus, Thomas; De Pontieu, Bart; Leenaarts,
Jorrit; Pereira, Tiago M. D.
2014AAS...22432305F Altcode:
The objective of this study is to explore the signatures of acoustic
waves and oscillations in a variety of magnetic field configurations
in the Sun’s atmosphere using high-resolution spectroscopic
“sit-an-stare” time series obtained with the Interface Region
Imaging Spectrogragh (IRIS) in lines formed in the chromosphere and
lower transition region (C II 1335 &amp; 1336, C I 1352, O I 1356,
Si IV 1394 &amp; 1403 and Mg h and k). The occurrence of oscillations
in the transition region is found to strongly depend on the magnetic
field topology. The Mg h and k lines reveal a particularly complex
spatio-temporal behavior. In an effort to better understand what
physical parameters can be extracted from these lines, we extend our
analysis to synthetic spectra obtained from numerical simulations and
compare the results to observations.

---------------------------------------------------------
Title: Detailed and Simplified Nonequilibrium Helium Ionization in
the Solar Atmosphere
Authors: Golding, Thomas Peter; Carlsson, Mats; Leenaarts, Jorrit
2014ApJ...784...30G Altcode: 2014arXiv1401.7562G
Helium ionization plays an important role in the energy balance of the
upper chromosphere and transition region. Helium spectral lines are also
often used as diagnostics of these regions. We carry out one-dimensional
radiation-hydrodynamics simulations of the solar atmosphere and
find that the helium ionization is set mostly by photoionization and
direct collisional ionization, counteracted by radiative recombination
cascades. By introducing an additional recombination rate mimicking
the recombination cascades, we construct a simplified three-level
helium model atom consisting of only the ground states. This model
atom is suitable for modeling nonequilibrium helium ionization in
three-dimensional numerical models. We perform a brief investigation of
the formation of the He I 10830 and He II 304 spectral lines. Both lines
show nonequilibrium features that are not recovered with statistical
equilibrium models, and caution should therefore be exercised when
such models are used as a basis for interpretating observations.

---------------------------------------------------------
Title: The Effect of Isotopic Splitting on the Bisector and Inversions
of the Solar Ca II 854.2 nm Line
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Kochukhov,
Oleg; Carlsson, Mats
2014ApJ...784L..17L Altcode: 2014arXiv1401.5019L
The Ca II 854.2 nm spectral line is a common diagnostic of the solar
chromosphere. The average line profile shows an asymmetric core,
and its bisector shows a characteristic inverse-C shape. The line
actually consists of six components with slightly different wavelengths
depending on the isotope of calcium. This isotopic splitting of the
line has been taken into account in studies of non-solar stars, but
never for the Sun. We performed non-LTE radiative transfer computations
from three models of the solar atmosphere and show that the line-core
asymmetry and inverse C-shape of the bisector of the 854.2 nm line
can be explained by isotopic splitting. We confirm this finding by
analyzing observations and showing that the line asymmetry is present
irrespective of conditions in the solar atmosphere. Finally, we show
that inversions based on the Ca II 854.2 nm line should take the
isotopic splitting into account, otherwise the inferred atmospheres
will contain erroneous velocity gradients and temperatures.

---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. III. Near-ultraviolet
Spectra and Images
Authors: Pereira, T. M. D.; Leenaarts, J.; De Pontieu, B.; Carlsson,
M.; Uitenbroek, H.
2013ApJ...778..143P Altcode: 2013arXiv1310.1926P
The Mg II h&amp;k lines are the prime chromospheric diagnostics
of NASA's Interface Region Imaging Spectrograph (IRIS). In the
previous papers of this series, we used a realistic three-dimensional
radiative magnetohydrodynamics model to calculate the h&amp;k lines
in detail and investigated how their spectral features relate to the
underlying atmosphere. In this work, we employ the same approach to
investigate how the h&amp;k diagnostics fare when taking into account
the finite resolution of IRIS and different noise levels. In addition,
we investigate the diagnostic potential of several other photospheric
lines and near-continuum regions present in the near-ultraviolet
(NUV) window of IRIS and study the formation of the NUV slit-jaw
images. We find that the instrumental resolution of IRIS has a small
effect on the quality of the h&amp;k diagnostics; the relations
between the spectral features and atmospheric properties are mostly
unchanged. The peak separation is the most affected diagnostic, but
mainly due to limitations of the simulation. The effects of noise
start to be noticeable at a signal-to-noise ratio (S/N) of 20, but we
show that with noise filtering one can obtain reliable diagnostics at
least down to a S/N of 5. The many photospheric lines present in the
NUV window provide velocity information for at least eight distinct
photospheric heights. Using line-free regions in the h&amp;k far wings,
we derive good estimates of photospheric temperature for at least
three heights. Both of these diagnostics, in particular the latter,
can be obtained even at S/Ns as low as 5.

---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. II. The Formation of the
Mg II h&amp;k Lines in the Solar Atmosphere
Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek,
H.; De Pontieu, B.
2013ApJ...772...90L Altcode: 2013arXiv1306.0671L
NASA's Interface Region Imaging Spectrograph (IRIS) small explorer
mission will study how the solar atmosphere is energized. IRIS
contains an imaging spectrograph that covers the Mg II h&amp;k lines
as well as a slit-jaw imager centered at Mg II k. Understanding
the observations requires forward modeling of Mg II h&amp;k line
formation from three-dimensional (3D) radiation-magnetohydrodynamic
(RMHD) models. This paper is the second in a series where we undertake
this modeling. We compute the vertically emergent h&amp;k intensity
from a snapshot of a dynamic 3D RMHD model of the solar atmosphere,
and investigate which diagnostic information about the atmosphere is
contained in the synthetic line profiles. We find that the Doppler
shift of the central line depression correlates strongly with the
vertical velocity at optical depth unity, which is typically located
less than 200 km below the transition region (TR). By combining the
Doppler shifts of the h and k lines we can retrieve the sign of the
velocity gradient just below the TR. The intensity in the central line
depression is anti-correlated with the formation height, especially
in subfields of a few square Mm. This intensity could thus be used to
measure the spatial variation of the height of the TR. The intensity
in the line-core emission peaks correlates with the temperature at
its formation height, especially for strong emission peaks. The peaks
can thus be exploited as a temperature diagnostic. The wavelength
difference between the blue and red peaks provides a diagnostic of the
velocity gradients in the upper chromosphere. The intensity ratio of
the blue and red peaks correlates strongly with the average velocity
in the upper chromosphere. We conclude that the Mg II h&amp;k lines
are excellent probes of the very upper chromosphere just below the
TR, a height regime that is impossible to probe with other spectral
lines. They also provide decent temperature and velocity diagnostics
of the middle chromosphere.

---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. I. A Quintessential
Model Atom of Mg II and General Formation Properties of the Mg II
h&amp;k Lines
Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek,
H.; De Pontieu, B.
2013ApJ...772...89L Altcode: 2013arXiv1306.0668L
NASA's Interface Region Imaging Spectrograph (IRIS) space mission will
study how the solar atmosphere is energized. IRIS contains an imaging
spectrograph that covers the Mg II h&amp;k lines as well as a slit-jaw
imager centered at Mg II k. Understanding the observations will require
forward modeling of Mg II h&amp;k line formation from three-dimensional
(3D) radiation-MHD models. This paper is the first in a series where we
undertake this forward modeling. We discuss the atomic physics pertinent
to h&amp;k line formation, present a quintessential model atom that can
be used in radiative transfer computations, and discuss the effect of
partial redistribution (PRD) and 3D radiative transfer on the emergent
line profiles. We conclude that Mg II h&amp;k can be modeled accurately
with a four-level plus continuum Mg II model atom. Ideally radiative
transfer computations should be done in 3D including PRD effects. In
practice this is currently not possible. A reasonable compromise is to
use one-dimensional PRD computations to model the line profile up to
and including the central emission peaks, and use 3D transfer assuming
complete redistribution to model the central depression.

---------------------------------------------------------
Title: A Detailed Comparison between the Observed and Synthesized
Properties of a Simulated Type II Spicule
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit;
Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie
V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc
2013ApJ...771...66M Altcode: 2013arXiv1305.2397M
We have performed a three-dimensional radiative MHD simulation of the
solar atmosphere. This simulation shows a jet-like feature that shows
similarities to the type II spicules observed for the first time with
Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the
solar disk are associated with these spicules. Observational results
suggest they may contribute significantly in supplying the corona
with hot plasma. We perform a detailed comparison of the properties
of the simulated jet with those of type II spicules (observed with
Hinode) and RBEs (with ground-based instruments). We analyze a wide
variety of synthetic emission and absorption lines from the simulations
including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition
region and coronal temperatures (10,000 K to several million K). We
compare their synthetic intensities, line profiles, Doppler shifts,
line widths, and asymmetries with observations from Hinode/SOT and
EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many
properties of the synthetic observables resemble the observations,
and we describe in detail the physical processes that lead to these
observables. Detailed analysis of the synthetic observables provides
insight into how observations should be analyzed to derive information
about physical variables in such a dynamic event. For example, we
find that line-of-sight superposition in the optically thin atmosphere
requires the combination of Doppler shifts and spectral line asymmetry
to determine the velocity in the jet. In our simulated type II spicule,
the lifetime of the asymmetry of the transition region lines is shorter
than that of the coronal lines. Other properties differ from the
observations, especially in the chromospheric lines. The mass density
of the part of the spicule with a chromospheric temperature is too low
to produce significant opacity in chromospheric lines. The synthetic
Ca II 8542 Å and Hα profiles therefore do not show signal resembling
RBEs. These and other discrepancies are described in detail, and we
discuss which mechanisms and physical processes may need to be included
in the MHD simulations to mimic the thermodynamic processes of the
chromosphere and corona, in particular to reproduce type II spicules.

---------------------------------------------------------
Title: How realistic are solar model atmospheres?
Authors: Pereira, T. M. D.; Asplund, M.; Collet, R.; Thaler, I.;
Trampedach, R.; Leenaarts, J.
2013A&A...554A.118P Altcode: 2013arXiv1304.4932P
Context. Recently, new solar model atmospheres have been developed
to replace classical 1D local thermodynamical equilibrium (LTE)
hydrostatic models and used to for example derive the solar chemical
composition. Aims: We aim to test various models against key
observational constraints. In particular, a 3D model used to derive
the solar abundances, a 3D magnetohydrodynamical (MHD) model (with an
imposed 10 mT vertical magnetic field), 1D NLTE and LTE models from
the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical
model of Holweger &amp; Müller. Methods: We confronted the
models with observational diagnostics of the temperature profile:
continuum centre-to-limb variations (CLVs), absolute continuum fluxes,
and the wings of hydrogen lines. We also tested the 3D models for the
intensity distribution of the granulation and spectral line shapes. Results: The predictions from the 3D model are in excellent agreement
with the continuum CLV observations, performing even better than
the Holweger &amp; Müller model (constructed largely to fulfil such
observations). The predictions of the 1D theoretical models are worse,
given their steeper temperature gradients. For the continuum fluxes,
predictions for most models agree well with the observations. No
model fits all hydrogen lines perfectly, but again the 3D model comes
ahead. The 3D model also reproduces the observed continuum intensity
fluctuations and spectral line shapes very well. Conclusions:
The excellent agreement of the 3D model with the observables reinforces
the view that its temperature structure is realistic. It outperforms
the MHD simulation in all diagnostics, implying that recent claims
for revised abundances based on MHD modelling are premature. Several
weaknesses in the 1D hydrostatic models (theoretical and semi-empirical)
are exposed. The differences between the PHOENIX LTE and NLTE models
are small. We conclude that the 3D hydrodynamical model is superior
to any of the tested 1D models, which gives further confidence in the
solar abundance analyses based on it.

---------------------------------------------------------
Title: Chromospheric Magnetic Fields: Observations, Simulations and
their Interpretation
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012ASPC..463...15D Altcode: 2012arXiv1203.4577D
The magnetic field of the quiet-Sun chromosphere remains a mystery
for solar physicists. The reduced number of chromospheric lines are
intrinsically hard to model and only a few of them are magnetically
sensitive. In this work, we use a 3D numerical simulation of the outer
layers of the solar atmosphere, to asses the reliability of non-LTE
inversions, in this case applied to the Ca II λ8542 Å line. We show
that NLTE inversions provide realistic estimates of physical quantities
from synthetic observations.

---------------------------------------------------------
Title: The Hanle Effect of Lyα in a Magnetohydrodynamic Model of
the Solar Transition Region
Authors: Štěpán, J.; Trujillo Bueno, J.; Carlsson, M.; Leenaarts, J.
2012ApJ...758L..43S Altcode: 2012arXiv1208.4929S
In order to understand the heating of the solar corona it is crucial
to obtain empirical information on the magnetic field in its lower
boundary (the transition region). To this end, we need to measure
and model the linear polarization produced by scattering processes in
strong UV lines, such as the hydrogen Lyα line. The interpretation
of the observed Stokes profiles will require taking into account that
the outer solar atmosphere is highly structured and dynamic, and that
the height of the transition region may well vary from one place in
the atmosphere to another. Here, we report on the Lyα scattering
polarization signals we have calculated in a realistic model of an
enhanced network region, resulting from a state-of-the-art radiation
magnetohydrodynamic simulation. This model is characterized by spatially
complex variations of the physical quantities at transition region
heights. The results of our investigation lead us to emphasize that
scattering processes in the upper solar chromosphere should indeed
produce measurable linear polarization in Lyα. More importantly,
we show that via the Hanle effect the model's magnetic field produces
significant changes in the emergent Q/I and U/I profiles. Therefore, we
argue that by measuring the polarization signals produced by scattering
processes and the Hanle effect in Lyα and contrasting them with those
computed in increasingly realistic atmospheric models, we should be
able to decipher the magnetic, thermal, and dynamic structure of the
upper chromosphere and transition region of the Sun.

---------------------------------------------------------
Title: Fast approximation of angle-dependent partial redistribution
in moving atmospheres
Authors: Leenaarts, J.; Pereira, T.; Uitenbroek, H.
2012A&A...543A.109L Altcode: 2012arXiv1205.5110L
 Aims: Radiative transfer modeling of spectral lines including
partial redistribution (PRD) effects requires the evaluation of
the ratio of the emission to the absorption profile. This quantity
requires a large amount of computational work if one employs the
angle-dependent redistribution function, which prohibits its use in
3D radiative transfer computations with model atmospheres containing
velocity fields. We aim to provide a method to compute the emission
to absorption profile ratio that requires less computational work but
retains the effect of angle-dependent scattering in the resulting line
profiles. Methods: We present a method to compute the profile
ratio that employs the angle-averaged redistribution function and
wavelength transforms to and from the rest frame of the scattering
particles. We compare the emergent line profiles of the Mg II k and
Lyα lines computed with angle-dependent PRD, angle-averaged PRD and
our new method in two representative test atmospheres. Results:
The new method yields a good approximation of true angle-dependent
profile ratio and the resulting emergent line profiles while keeping
the computational speed and simplicity of angle-averaged PRD theory.

---------------------------------------------------------
Title: Non-local thermodynamic equilibrium inversions from a 3D
magnetohydrodynamic chromospheric model
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012A&A...543A..34D Altcode: 2012arXiv1205.3171D
Context. The structure of the solar chromosphere is believed to
be governed by magnetic fields, even in quiet-Sun regions that
have a relatively weak photospheric field. During the past decade
inversion methods have emerged as powerful tools for analyzing the
chromosphere of active regions. The applicability of inversions to
infer the stratification of the physical conditions in a dynamic 3D
solar chromosphere has not yet been studied in detail. Aims:
This study aims to establish the diagnostic capabilities of non-local
thermodynamical equilibrium (NLTE) inversion techniques of Stokes
profiles induced by the Zeeman effect in the Ca ii λ8542 Å line. Methods: We computed the Ca ii atomic level populations in a snapshot
from a 3D radiation-MHD simulation of the quiet solar atmosphere in
non-LTE using the 3D radiative transfer code Multi3d. These populations
were used to compute synthetic full-Stokes profiles in the Ca ii
λ8542 Å line using 1.5D radiative transfer and the inversion code
Nicole. The profiles were then spectrally degraded to account for
finite filter width, and Gaussian noise was added to account for
finite photon flux. These profiles were inverted using Nicole and
the results were compared with the original model atmosphere. 
Results: Our NLTE inversions applied to quiet-Sun synthetic observations
provide reasonably good estimates of the chromospheric magnetic field,
line-of-sight velocities and somewhat less accurate, but still very
useful, estimates of the temperature. Three-dimensional scattering
of photons cause cool pockets in the chromosphere to be invisible in
the line profile and consequently they are also not recovered by the
inversions. To successfully detect Stokes linear polarization in this
quiet snapshot, a noise level below 10-3.5 is necessary.

---------------------------------------------------------
Title: The Formation of the Hα Line in the Solar Chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Rouppe van der Voort, L.
2012ApJ...749..136L Altcode: 2012arXiv1202.1926L
We use state-of-the-art radiation-MHD simulations and three-dimensional
(3D) non-LTE radiative transfer computations to investigate Hα
line formation in the solar chromosphere and apply the results of
this investigation to develop the potential of Hα as a diagnostic
of the chromosphere. We show that one can accurately model Hα line
formation assuming statistical equilibrium and complete frequency
redistribution provided the computation of the model atmosphere
included non-equilibrium ionization of hydrogen and the Lyα and
Lyβ line profiles are described by Doppler profiles. We find that
3D radiative transfer is essential in modeling hydrogen lines due
to the low photon destruction probability in Hα. The Hα opacity
in the upper chromosphere is mainly sensitive to the mass density
and only weakly sensitive to the temperature. We find that the Hα
line-core intensity is correlated with the average formation height:
The larger the average formation height is, the lower the intensity
will be. The line-core width is a measure of the gas temperature
in the line-forming region. The fibril-like dark structures seen in
Hα line-core images computed from our model atmosphere are tracing
magnetic field lines. These structures are caused by field-aligned
ridges of enhanced chromospheric mass density that raise their average
formation height, and therefore make them appear dark against their
deeper-formed surroundings. We compare with observations, and find that
the simulated line-core widths are very similar to the observed ones,
without the need for additional microturbulence.

---------------------------------------------------------
Title: NLTE inversions from a 3D MHD Chromospheric simulation
Authors: de la Cruz Rodriguez, J.; Socas-Navarro, H.; Carlsson, M.;
Leenaarts, J.
2012decs.confE..80D Altcode:
The structure of the solar chromosphere is believed to be governed
by magnetic fields, even in quiet Sun regions with a relatively weak
field. Measuring the magnetic field of the solar chromosphere is an
outstanding challenge for observers. Inversion codes allow for detailed
interpretation of full-Stokes data from spectral lines formed in the
chromosphere. However, the applicability of non-LTE inversions to infer
physical conditions in the dynamic 3D solar chromosphere, has not yet
been studied in detail. In this study, we use a snapshot from a 3D MHD
simulation of quiet-sun, extending from the photosphere to the corona,
to asses the reliability of non-LTE inversions to infer chromospheric
quantities, especially the magnetic field.

---------------------------------------------------------
Title: The formation of the Halpha line in the solar chromosphere
Authors: Leenaarts, J.; Carlsson; M.; Rouppe van der Voort, Rouppe, L.
2012decs.confE..14L Altcode:
We use state-of-the-art radiation-MHD simulations and 3D non-LTE
radiative transfer computations to investigate Halpha line formation
in the solar chromosphere. We find that 3D radiative transfer is
essential in modeling hydrogen lines due to the low photon destruction
probability in Halpha. The Halpha opacity in the upper chromosphere
is mainly sensitive to the mass density and only weakly sensitive to
temperature. We find that the Halpha line-core intensity is correlated
with the average formation height: the lower the intensity, the larger
the average formation height. The line-core width is a measure of
the gas temperature in the line-forming region. The fibril-like dark
structures seen in Halpha line-core images computed from our model
atmosphere are tracing magnetic field lines. These structures are
caused by field-aligned ridges of enhanced chromospheric mass density
that raise their average formation height, and therefore makes them
appear dark against their deeper-formed surroundings.

---------------------------------------------------------
Title: Potential for diagnostics with IRIS and Mg II lines
Authors: Pereira, Tiago M. D.; Carlsson, Mats; Leenaarts, Jorrit;
Uitenbroek, Han; De Pontieu, Bart; Martinez-Sykora, Juan
2012decs.confE..13P Altcode:
The IRIS mission will open up a new window into the solar chromosphere
and transition region. An important diagnostic that IRIS will bring
is the Mg II H and K lines. Radiation from these lines is believed
to be come from a wide range of formation depths, from the higher
photosphere to the onset of the transition region. With a complex
formation mechanism, Mg II H and K suffer from departures from LTE
and partial redistribution (PRD). In this preliminary analysis we will
look into the potential for diagnostics of Mg II H and K. Using a new
parallel version of the RH code we synthesised Mg II H and K spectra
from 3D rMHD simulations of the solar atmosphere. We will discuss
the relevance of several approximations on the final observables,
and will compare the Mg II H and K filtergrams with those of Ca II H,
a robust chromospheric diagnostic line widely used with Hinode/SOT/BFI.

---------------------------------------------------------
Title: Using non-LTE diagnostic tools: Multi3d
Authors: Leenaarts, J.
2012decs.confE..15L Altcode:
I will give a tutorial session on the use of the 3D NLTE radiative
transfer code Multi3d. The code uses MPI-parallelization and can handle
large 3D input atmospheres such as those provided by radiation-MHD
models. I'll show how to set up a run and discuss some of the commonly
used input options and show how to analyze the results with IDL. I'll
provide a web location where the code can be downloaded together with
a manual, the IDL analysis package and a test problem. I'll show some
results obtained by combining radiation-MHD models with radiative
transfer computations done with Multi3d.

---------------------------------------------------------
Title: Approximations for radiative cooling and heating in the
solar chromosphere
Authors: Carlsson, M.; Leenaarts, J.
2012A&A...539A..39C Altcode: 2012arXiv1202.2996C
Context. The radiative energy balance in the solar chromosphere is
dominated by strong spectral lines that are formed out of LTE. It
is computationally prohibitive to solve the full equations of
radiative transfer and statistical equilibrium in 3D time dependent
MHD simulations. Aims: We look for simple recipes to compute
the radiative energy balance in the dominant lines under solar
chromospheric conditions. Methods: We use detailed calculations
in time-dependent and 2D MHD snapshots to derive empirical formulae
for the radiative cooling and heating. Results: The radiative
cooling in neutral hydrogen lines and the Lyman continuum, the H
and K and intrared triplet lines of singly ionized calcium and the h
and k lines of singly ionized magnesium can be written as a product
of an optically thin emission (dependent on temperature), an escape
probability (dependent on column mass) and an ionization fraction
(dependent on temperature). In the cool pockets of the chromosphere
the same transitions contribute to the heating of the gas and similar
formulae can be derived for these processes. We finally derive a simple
recipe for the radiative heating of the chromosphere from incoming
coronal radiation. We compare our recipes with the detailed results
and comment on the accuracy and applicability of the recipes.

---------------------------------------------------------
Title: Quiet-Sun imaging asymmetries in Na I D1 compared
with other strong Fraunhofer lines
Authors: Rutten, R. J.; Leenaarts, J.; Rouppe van der Voort, L. H. M.;
de Wijn, A. G.; Carlsson, M.; Hansteen, V.
2011A&A...531A..17R Altcode: 2011arXiv1104.4307R
Imaging spectroscopy of the solar atmosphere using the Na I
D1 line yields marked asymmetry between the blue and
red line wings: sampling a quiet-Sun area in the blue wing displays
reversed granulation, whereas sampling in the red wing displays normal
granulation. The Mg I b2 line of comparable strength does
not show this asymmetry, nor does the stronger Ca II 8542 Å line. We
demonstrate the phenomenon with near-simultaneous spectral images in
Na I D1, Mg I b2, and Ca II 8542 Å from the
Swedish 1-m Solar Telescope. We then explain it with line-formation
insights from classical 1D modeling and with a 3D magnetohydrodynamical
simulation combined with NLTE spectral line synthesis that permits
detailed comparison with the observations in a common format. The
cause of the imaging asymmetry is the combination of correlations
between intensity and Dopplershift modulation in granular overshoot
and the sensitivity to these of the steep profile flanks of the Na
I D1 line. The Mg I b2 line has similar core
formation but much wider wings due to larger opacity buildup and
damping in the photosphere. Both lines obtain marked core asymmetry
from photospheric shocks in or near strong magnetic concentrations,
less from higher-up internetwork shocks that produce similar asymmetry
in the spatially averaged Ca II 8542 Å profile.

---------------------------------------------------------
Title: The stellar atmosphere simulation code Bifrost. Code
description and validation
Authors: Gudiksen, B. V.; Carlsson, M.; Hansteen, V. H.; Hayek, W.;
Leenaarts, J.; Martínez-Sykora, J.
2011A&A...531A.154G Altcode: 2011arXiv1105.6306G
Context. Numerical simulations of stellar convection and photospheres
have been developed to the point where detailed shapes of observed
spectral lines can be explained. Stellar atmospheres are very complex,
and very different physical regimes are present in the convection zone,
photosphere, chromosphere, transition region and corona. To understand
the details of the atmosphere it is necessary to simulate the whole
atmosphere since the different layers interact strongly. These physical
regimes are very diverse and it takes a highly efficient massively
parallel numerical code to solve the associated equations. Aims:
The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the
convection zone to the corona. Methods: The code is subjected
to a number of validation tests, among them the Sod shock tube test,
the Orzag-Tang colliding shock test, boundary condition tests and
tests of how the code treats magnetic field advection, chromospheric
radiation, radiative transfer in an isothermal scattering atmosphere,
hydrogen ionization and thermal conduction. Results.Bifrost completes
the tests with good results and shows near linear efficiency scaling
to thousands of computing cores.

---------------------------------------------------------
Title: On the minimum temperature of the quiet solar chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Gudiksen, B. V.
2011A&A...530A.124L Altcode: 2011arXiv1104.5081L
 Aims: We aim to provide an estimate of the minimum temperature
of the quiet solar chromosphere. Methods: We perform a 2D
radiation-MHD simulation spanning the upper convection zone to the
lower corona. The simulation includes non-LTE radiative transfer
and an equation-of-state that includes non-equilibrium ionization
of hydrogen and non-equilibrium H2 molecule formation. We
analyze the reliability of the various assumptions made in our model
in order to assess the realism of the simulation. Results:
Our simulation contains pockets of cool gas with down to 1660 K from
1 Mm up to 3.2 Mm height. It overestimates the radiative heating,
and contains non-physical heating below 1660 K. Therefore we conclude
that cool pockets in the quiet solar chromosphere might have even
lower temperatures than in the simulation, provided that there exist
areas in the chromosphere without significant magnetic heating. We
suggest off-limb molecular spectroscopy to look for such cool pockets
and 3D simulations including a local dynamo and a magnetic carpet to
investigate Joule heating in the quiet chromosphere.

---------------------------------------------------------
Title: CO5BOLD: COnservative COde for the COmputation of COmpressible
COnvection in a BOx of L Dimensions with l=2,3
Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven;
Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner;
Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga;
Steiner, Oskar
2010ascl.soft11014F Altcode:
CO5BOLD - nickname COBOLD - is the short form of "COnservative
COde for the COmputation of COmpressible COnvection in a BOx of L
Dimensions with l=2,3". It is used to model solar and stellar
surface convection. For solar-type stars only a small fraction of the
stellar surface layers are included in the computational domain. In
the case of red supergiants the computational box contains the entire
star. Recently, the model range has been extended to sub-stellar objects
(brown dwarfs). CO5BOLD solves the coupled non-linear equations
of compressible hydrodynamics in an external gravity field together
with non-local frequency-dependent radiation transport. Operator
splitting is applied to solve the equations of hydrodynamics (including
gravity), the radiative energy transfer (with a long-characteristics
or a short-characteristics ray scheme), and possibly additional 3D
(turbulent) diffusion in individual sub steps. The 3D hydrodynamics
step is further simplified with directional splitting (usually). The 1D
sub steps are performed with a Roe solver, accounting for an external
gravity field and an arbitrary equation of state from a table. The radiation transport is computed with either one of three
modules: MSrad module: It uses long characteristics. The lateral
boundaries have to be periodic. Top and bottom can be closed or open
("solar module"). LHDrad module: It uses long characteristics
and is restricted to an equidistant grid and open boundaries at all
surfaces (old "supergiant module"). SHORTrad module: It uses
short characteristics and is restricted to an equidistant grid and
open boundaries at all surfaces (new "supergiant module"). The
code was supplemented with an (optional) MHD version [Schaffenberger
et al. (2005)] that can treat magnetic fields. There are also modules
for the formation and advection of dust available. The current version
now contains the treatment of chemical reaction networks, mostly used
for the formation of molecules [Wedemeyer-Böhm et al. (2005)], and
hydrogen ionization [Leenaarts &amp; Wedemeyer-Böhm (2005)], too. CO5BOLD is written in Fortran90. The parallelization is done with
OpenMP directives.

---------------------------------------------------------
Title: Radiative transfer with scattering for domain-decomposed 3D
MHD simulations of cool stellar atmospheres. Numerical methods and
application to the quiet, non-magnetic, surface of a solar-type star
Authors: Hayek, W.; Asplund, M.; Carlsson, M.; Trampedach, R.; Collet,
R.; Gudiksen, B. V.; Hansteen, V. H.; Leenaarts, J.
2010A&A...517A..49H Altcode: 2010arXiv1007.2760H
 Aims: We present the implementation of a radiative
transfer solver with coherent scattering in the new BIFROST
code for radiative magneto-hydrodynamical (MHD) simulations of
stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and
improved resolution of hydrodynamical structures. We apply the code
to simulate the surface granulation in a solar-type star, ignoring
magnetic fields, and investigate the importance of coherent scattering
for the atmospheric structure. Methods: A scattering term
is added to the radiative transfer equation, requiring an iterative
computation of the radiation field. We use a short-characteristics-based
Gauss-Seidel acceleration scheme to compute radiative flux divergences
for the energy equation. The effects of coherent scattering are
tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star:
without scattering, with continuum scattering only, and with both
continuum and line scattering. Results: We show that continuum
scattering does not have a significant impact on the photospheric
temperature structure for a star like the Sun. Including scattering in
line-blanketing, however, leads to a decrease of temperatures by about
350 K below log10 τ5000 ⪉ -4. The effect is
opposite to that of 1D hydrostatic models in radiative equilibrium,
where scattering reduces the cooling effect of strong LTE lines in
the higher layers of the photosphere. Coherent line scattering also
changes the temperature distribution in the high atmosphere, where
we observe stronger fluctuations compared to a treatment of lines as
true absorbers.

---------------------------------------------------------
Title: The Quiet Solar Atmosphere Observed and Simulated in Na
I D1
Authors: Leenaarts, J.; Rutten, R. J.; Reardon, K.; Carlsson, M.;
Hansteen, V.
2010ApJ...709.1362L Altcode: 2009arXiv0912.2206L
The Na I D1 line in the solar spectrum is sometimes
attributed to the solar chromosphere. We study its formation in
quiet-Sun network and internetwork. We first present high-resolution
profile-resolved images taken in this line with the imaging
spectrometer Interferometric Bidimensional Spectrometer at the Dunn
Solar Telescope and compare these to simultaneous chromospheric images
taken in Ca II 8542 Å and Hα. We then model Na I D1
formation by performing three-dimensional (3D) non-local
thermodynamic equilibrium profile synthesis for a snapshot from a
3D radiation-magnetohydrodynamics simulation. We find that most Na I
D1 brightness is not chromospheric but samples the magnetic
concentrations that make up the quiet-Sun network in the photosphere,
well below the height where they merge into chromospheric canopies,
with aureoles from 3D resonance scattering. The line core is sensitive
to magneto-acoustic shocks in and near magnetic concentrations, where
shocks occur deeper than elsewhere, and may provide evidence of heating
deep within magnetic concentrations.

---------------------------------------------------------
Title: Numerical simulations of the quiet chromosphere.
Authors: Leenaarts, J.
2010MmSAI..81..576L Altcode: 2010arXiv1001.0888L
Numerical simulations of the solar chromosphere have become increasingly
realistic over the past 5 years. However, many observed chromospheric
structures and their behavior are not reproduced. Current models do not
show fibrils in Ca II 8542 Å, and neither reproduce the Ca II 8542
Å bisector. The emergent Hα line core intensity computed from the
models show granulation instead of chromospheric shocks or fibrils. I
discuss these deficiencies and speculate about what physics should be
included to alleviate these shortcomings.

---------------------------------------------------------
Title: MULTI3D: A Domain-Decomposed 3D Radiative Transfer Code
Authors: Leenaarts, J.; Carlsson, M.
2009ASPC..415...87L Altcode:
We present MULTI3D, a 3D radiative transfer code currently under
development. It is optimized for computing NLTE problems based on
(radiation-)MHD models of stellar atmospheres. MULTI3D is based on
MULTI and includes most of the physics present in that code. MULTI3D was
first written as a serial code by Botnen (1997) and has recently been
upgraded to an MPI-parallelized, domain-decomposed version. The code
has so far successfully been run on up to 64 processors, solving the
NLTE radiative transfer for a six-level Ca II atom with 400 frequency
points in an atmosphere of 256 × 128 × 108 grid points.

---------------------------------------------------------
Title: On-disk Counterparts of Type II Spicules in the Ca II 854.2
nm and Hα Lines
Authors: Rouppe van der Voort, L.; Leenaarts, J.; de Pontieu, B.;
Carlsson, M.; Vissers, G.
2009ApJ...705..272R Altcode: 2009arXiv0909.2115R
Recently, a second type of spicules was discovered at the solar
limb with the Solar Optical Telescope onboard the Japanese Hinode
spacecraft. These previously unrecognized type II spicules are thin
chromospheric jets that are shorter lived (10-60 s) and that show much
higher apparent upward velocities (of order 50-100 km s-1)
than the classical spicules. Since they have been implicated in
providing hot plasma to coronal loops, their formation, evolution,
and properties are important ingredients for a better understanding
of the mass and energy balance of the low solar atmosphere. Here, we
report on the discovery of the disk counterparts of type II spicules
using spectral imaging data in the Ca II 854.2 nm and Hα lines with
the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope in
La Palma. We find rapid blueward excursions in the line profiles of
both chromospheric lines that correspond to thin, jet-like features
that show apparent velocities of order 50 km s-1. These
blueward excursions seem to form a separate absorbing component with
Doppler shifts of order 20 and 50 km s-1 for the Ca II 854.2
nm and Hα line, respectively. We show that the appearance, lifetimes,
longitudinal and transverse velocities, and occurrence rate of these
rapid blue excursions on the disk are very similar to those of the type
II spicules at the limb. A detailed study of the spectral line profiles
in these events suggests that plasma is accelerated along the jet,
and plasma is being heated throughout the short lifetime of the event.

---------------------------------------------------------
Title: Three-Dimensional Non-LTE Radiative Transfer Computation of
the CA 8542 Infrared Line From a Radiation-MHD Simulation
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rouppe van der
Voort, L.
2009ApJ...694L.128L Altcode: 2009arXiv0903.0791L
The interpretation of imagery of the solar chromosphere in the
widely used Ca II 854.2 nm infrared line is hampered by its complex,
three-dimensional, and non-LTE formation. Forward modeling is
required to aid understanding. We use a three-dimensional non-LTE
radiative transfer code to compute synthetic Ca II 854.2 nm images
from a radiation-MHD simulation of the solar atmosphere spanning
from the convection zone to the corona. We compare the simulation
with observations obtained with the CRISP filter at the Swedish 1 m
Solar Telescope. We find that the simulation reproduces dark patches
in the blue line wing caused by Doppler shifts, brightenings in the
line core caused by upward-propagating shocks, and thin dark elongated
structures in the line core that form the interface between upward and
downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the Sun exhibits both
more vigorous large-scale dynamics as well as small scale motions that
are not resolved within the simulation, presumably owing to a lack of
spatial resolution.

---------------------------------------------------------
Title: Non-equilibrium hydrogen ionization in 2D simulations of the
solar atmosphere
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rutten, R. J.
2007A&A...473..625L Altcode: 2007arXiv0709.3751L
Context: The ionization of hydrogen in the solar chromosphere and
transition region does not obey LTE or instantaneous statistical
equilibrium because the timescale is long compared with important
hydrodynamical timescales, especially of magneto-acoustic shocks. Since
the pressure, temperature, and electron density depend sensitively on
hydrogen ionization, numerical simulation of the solar atmosphere
requires non-equilibrium treatment of all pertinent hydrogen
transitions. The same holds for any diagnostic application employing
hydrogen lines. Aims: To demonstrate the importance and to
quantify the effects of non-equilibrium hydrogen ionization, both
on the dynamical structure of the solar atmosphere and on hydrogen
line formation, in particular Hα. Methods: We implement an
algorithm to compute non-equilibrium hydrogen ionization and its
coupling into the MHD equations within an existing radiation MHD code,
and perform a two-dimensional simulation of the solar atmosphere from
the convection zone to the corona. Results: Analysis of the
simulation results and comparison to a companion simulation assuming
LTE shows that: a) non-equilibrium computation delivers much smaller
variations of the chromospheric hydrogen ionization than for LTE. The
ionization is smaller within shocks but subsequently remains high in
the cool intershock phases. As a result, the chromospheric temperature
variations are much larger than for LTE because in non-equilibrium,
hydrogen ionization is a less effective internal energy buffer. The
actual shock temperatures are therefore higher and the intershock
temperatures lower. b) The chromospheric populations of the hydrogen
n = 2 level, which governs the opacity of Hα, are coupled to the
ion populations. They are set by the high temperature in shocks
and subsequently remain high in the cool intershock phases. c)
The temperature structure and the hydrogen level populations differ
much between the chromosphere above photospheric magnetic elements
and above quiet internetwork. d) The hydrogen n = 2 population and
column density are persistently high in dynamic fibrils, suggesting
that these obtain their visibility from being optically thick in Hα
also at low temperature. Movie and Appendix A are only available
in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Inter-network regions of the Sun at millimetre wavelengths
Authors: Wedemeyer-Böhm, S.; Ludwig, H. G.; Steffen, M.; Leenaarts,
J.; Freytag, B.
2007A&A...471..977W Altcode: 2007arXiv0705.2304W
Aims:The continuum intensity at wavelengths around 1 mm provides an
excellent way to probe the solar chromosphere and thus valuable input
for the ongoing controversy on the thermal structure and the dynamics of
this layer. The synthetic continuum intensity maps for near-millimetre
wavelengths presented here demonstrate the potential of future
observations of the small-scale structure and dynamics of internetwork
regions on the Sun. Methods: The synthetic intensity/brightness
temperature maps are calculated on basis of three-dimensional
radiation (magneto-)hydrodynamic (MHD) simulations. The assumption
of local thermodynamic equilibrium (LTE) is valid for the source
function. The electron densities are also treated in LTE for most maps
but also in non-LTE for a representative model snapshot. Quantities
like intensity contrast, intensity contribution functions, spatial
and temporal scales are analysed in dependence on wavelength and
heliocentric angle. Results: While the millimetre continuum
at 0.3 mm originates mainly from the upper photosphere, the longer
wavelengths considered here map the low and middle chromosphere. The
effective formation height increases generally with wavelength and
also from disk-centre towards the solar limb. The average intensity
contribution functions are usually rather broad and in some cases they
are even double-peaked as there are contributions from hot shock waves
and cool post-shock regions in the model chromosphere. The resulting
shock-induced thermal structure translates to filamentary brightenings
and fainter regions in between. Taking into account the deviations from
ionisation equilibrium for hydrogen gives a less strong variation of
the electron density and with it of the optical depth. The result is
a narrower formation height range although the intensity maps still
are characterised by a highly complex pattern. The average brightness
temperature increases with wavelength and towards the limb although
the wavelength-dependence is reversed for the MHD model and the NLTE
brightness temperature maps. The relative contrast depends on wavelength
in the same way as the average intensity but decreases towards the
limb. The dependence of the brightness temperature distribution on
wavelength and disk-position can be explained with the differences in
formation height and the variation of temperature fluctuations with
height in the model atmospheres. The related spatial and temporal
scales of the chromospheric pattern should be accessible by future
instruments. Conclusions: Future high-resolution millimetre
arrays, such as the Atacama Large Millimeter Array (ALMA), will
be capable of directly mapping the thermal structure of the solar
chromosphere. Simultaneous observations at different wavelengths
could be exploited for a tomography of the chromosphere, mapping its
three-dimensional structure, and also for tracking shock waves. The
new generation of millimetre arrays will be thus of great value for
understanding the dynamics and structure of the solar atmosphere.

---------------------------------------------------------
Title: Numerical simulations of the solar atmosphere
Authors: Leenaarts, J.
2007PhDT.......304L Altcode:
In this thesis several aspects of the solar atmosphere are investigated
using numerical simulations. Simulations and observations of reversed
solar granulation are compared. It is concluded that reversed
granulation is a hydrodynamical process and is a consequence
of convection reversal. Images are synthesized from simulations
of solar magnetoconvection to study the quality of different
proxy-magnetometry diagnostics. It is concluded that the blue wing of
the Halpha line is the best proxy-magnetometry diagnostic, followed
by the blue wing of Hbeta, which has slightly smaller contrast but
higher diffraction-limited resolution. The effect of non-equilibrium
ionization of hydrogen on the structure of the solar chromosphere is
investigated. An algorithm for computation of non-equilibrium ionization
is described, and is used in multi-dimensional numerical simulations of
the solar atmosphere. It is concluded that the ionization fraction in
the chromosphere is rather constant at about 0.1%-1% , irrespective
of the gas temperature. Because the population of the lower level
of the Halpha line is coupled to the continuum population, Halpha
opacity is retained in cool parts of the model chromosphere. This
is in stark contrast to models employing instantaneous equilibrium
hydrogen ionization.

---------------------------------------------------------
Title: Non-equilibrium Hydrogen Ionization in the Solar Atmosphere
Authors: Leenaarts, J.; Wedemeyer-Böhm, S.; Carlsson, M.; Hansteen,
V. H.
2007ASPC..368..103L Altcode:
The assumption of statistical equilibrium for atomic level populations
of hydrogen does not hold under the conditions of the chromosphere
due to the low density and the short dynamic timescale. In order to
calculate the hydrogen ionization balance and the electron density one
has to solve the time-dependent rate equations. We present results
from 2D and 3D radiation-magneto-hydrodynamics simulations of the
solar atmosphere incorporating the time-dependent rate equations
for hydrogen. Both the hydrogen ionization degree and the electron
density in our models are much more constant than LTE and statistical
equilibrium theory predict. These simulations provide multi-dimensional
model atmospheres with realistic electron densities and hydrogen level
populations that can be used in detailed radiative transfer modeling.

---------------------------------------------------------
Title: Numerical simulations of the solar atmosphere
Authors: Leenaarts, Jorrit
2007PhDT.......331L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamic Hydrogen Ionization in Simulations of the Solar
Chromosphere
Authors: Leenaarts, J.; Wedemeyer-Böhm, S.
2006ASPC..354..306L Altcode:
Since the assumption of statistical equilibrium does not hold under the
conditions of the dynamical solar chromosphere, the time dependence
of the rate equations has to be taken into account when calculating
ionization stages of elements. We present a method based on the
work by Sollum (1999) to calculate the dynamic hydrogen ionization
degree and electron density in the 3-D radiation-hydrodynamics code
CO^5BOLD. In our model chromosphere, both quantities are more constant
over time and horizontal position than LTE theory predicts. We compare
synthetic brightness temperature images at λ=1~mm calculated with LTE
and time-dependent NLTE electron densities. Both formation height and
average brightness temperature change significantly compared to LTE
when using time-dependent electron densities.

---------------------------------------------------------
Title: Time-dependent hydrogen ionisation in 3D simulations of the
solar chromosphere. Methods and first results
Authors: Leenaarts, J.; Wedemeyer-Böhm, S.
2006A&A...460..301L Altcode: 2006astro.ph..8620L
Context: .The hydrogen ionisation degree deviates substantially
from statistical equilibrium under the conditions of the solar
chromosphere. A realistic description of this atmospheric layer
thus must account for time-dependent non-equilibrium effects. 
Aims: .Advancing the realism of numerical simulations of the solar
chromosphere by improved numerical treatment of the relevant physics
will provide more realistic models that are essential for interpretation
of existing and future observations. Methods: .An approximate
method for solving the rate equations for the hydrogen populations
was extended and implemented in the three-dimensional radiation
(magneto-)hydrodynamics code CO^5BOLD. The method is based on a
model atom with six energy levels and fixed radiative rates. It has
been tested extensively in one-dimensional simulations. The extended
method has been used to create a three-dimensional model that extends
from the upper convection zone to the chromosphere. Results:
.The ionisation degree of hydrogen in our time-dependent simulation is
comparable to the corresponding equilibrium value up to 500 km above
optical depth unity. Above this height, the non-equilibrium ionisation
degree is fairly constant over time and space, and tends to be at a
value set by hot propagating shock waves. The hydrogen level populations
and electron density are much more constant than the corresponding
values for statistical equilibrium, too. In contrast, the equilibrium
ionisation degree varies by more than 20 orders of magnitude between
hot, shocked regions and cool, non-shocked regions. Conclusions:
.The simulation shows for the first time in 3D that the chromospheric
hydrogen ionisation degree and electron density cannot be calculated
in equilibrium. Our simulation can provide realistic values of those
quantities for detailed radiative transfer computations.

---------------------------------------------------------
Title: Star cluster disruption by giant molecular clouds
Authors: Gieles, M.; Portegies Zwart, S. F.; Baumgardt, H.;
Athanassoula, E.; Lamers, H. J. G. L. M.; Sipior, M.; Leenaarts, J.
2006MNRAS.371..793G Altcode: 2006MNRAS.tmp..808G; 2006astro.ph..6451G
We investigate encounters between giant molecular clouds (GMCs)
and star clusters. We propose a single expression for the energy
gain of a cluster due to an encounter with a GMC, valid for all
encounter distances and GMC properties. This relation is verified
with N-body simulations of cluster-GMC encounters, where the
GMC is represented by a moving analytical potential. Excellent
agreement is found between the simulations and the analytical
work for fractional energy gains of ΔE/|E0| &lt; 10,
where |E0| is the initial total cluster energy. The
fractional mass loss from the cluster scales with the fractional
energy gain as (ΔM/M0) = f(ΔE/|E0|), where
f ~= 0.25. This is because a fraction 1 - f of the injected energy
goes to the velocities of escaping stars, that are higher than the
escape velocity. We therefore suggest that the disruption time of
clusters, tdis, is best defined as the time needed to bring
the cluster mass to zero, instead of the time needed to inject the
initial cluster energy. We derive an expression for tdis
based on the mass loss from the simulations, taking into account
the effect of gravitational focusing by the GMC. Assuming spatially
homogeneous distributions of clusters and GMCs with a relative velocity
dispersion of σcn, we find that clusters lose most of
their mass in relatively close encounters with high relative velocities
(~2σcn). The disruption time depends on the cluster mass
(Mc) and half-mass radius (rh) as tdis
=2.0S(Mc/104Msolar)(3.75pc/rh)3
Gyr, with S ≡ 1 for the solar neighbourhood and
S scales with the surface density of individual GMCs
(Σn) and the global GMC density (ρn) as S ~
(Σnρn)-1. Combined with the observed
relation between rh and Mc, that is, rh
~ Mλc, tdis depends on Mc
as tdis ~ Mγc. The index γ is then
defined as γ = 1 - 3λ. The observed shallow relation between cluster
radius and mass (e.g. λ ~= 0.1), makes the value of the index γ =
0.7 similar to that found from observations and from simulations of
clusters dissolving in tidal fields (γ ~= 0.62). The constant of 2.0
Gyr, which is the disruption time of a 104 Msolar
cluster in the solar neighbourhood, is about a factor of 3.5 shorter
than that found from earlier simulations of clusters dissolving under
the combined effect of Galactic tidal field and stellar evolution. It
is somewhat higher than the observationally determined value of 1.3
Gyr. It suggests, however, that the combined effect of tidal field and
encounters with GMCs can explain the lack of old open clusters in the
solar neighbourhood. GMC encounters can also explain the (very) short
disruption time that was observed for star clusters in the central
region of M51, since there ρn is an order of magnitude
higher than that in the solar neighbourhood.

---------------------------------------------------------
Title: A comparison of solar proxy-magnetometry diagnostics
Authors: Leenaarts, J.; Rutten, R. J.; Carlsson, M.; Uitenbroek, H.
2006A&A...452L..15L Altcode:
Aims.We test various proxy-magnetometry diagnostics, i.e., brightness
signatures of small-scale magnetic elements, for studying magnetic
field structures in the solar photosphere. Methods: .Images are
numerically synthesized from a 3D solar magneto-convection simulation
for, respectively, the G band at 430.5 nm, the CN band at 388.3 nm,
and the blue wings of the H α, H β, Ca ii H, and Ca ii 854.2 nm
lines. Results: .Both visual comparison and scatter diagrams of
the computed intensity versus the magnetic field strength show that,
in particular for somewhat spatially extended magnetic elements, the
blue H α wing presents the best proxy-magnetometry diagnostic, followed
by the blue wing of H β. The latter yields higher diffraction-limit
resolution. Conclusions: .We recommend using the blue H α wing
to locate and track small-scale photospheric magnetic elements through
their brightness appearance.

---------------------------------------------------------
Title: DOT tomography of the solar atmosphere. VI. Magnetic elements
as bright points in the blue wing of Hα
Authors: Leenaarts, J.; Rutten, R. J.; Sütterlin, P.; Carlsson, M.;
Uitenbroek, H.
2006A&A...449.1209L Altcode:
High-resolution solar images taken in the blue wing of the Balmer H
α line with the Dutch Open Telescope show intergranular magnetic
elements as strikingly bright features, similar to, but with
appreciably larger contrast over the surrounding granulation than
their more familiar manifestation as G-band bright points. Part of
this prominent appearance is due to low granular contrast, without
granule/lane brightness reversal as, e.g., in the wings of Ca II H
&amp; K. We use 1D and 2D radiative transfer modeling and 3D solar
convection and magnetoconvection simulations to reproduce and explain
the H α wing images. We find that the blue H α wing obeys near-LTE
line formation. It appears particularly bright in magnetic elements
through low temperature gradients. The granulation observed in the blue
wing of H α has low contrast because of the lack of H α opacity in the
upper photosphere, Doppler cancellation, and large opacity sensitivity
to temperature working against source function sensitivity. We conclude
that the blue H α wing represents a promising proxy magnetometer to
locate and track isolated intermittent magnetic elements, a better one
than the G band and the wings of Ca II H &amp; K although less sharp
at given aperture.

---------------------------------------------------------
Title: Small Scale Magnetic Elements as Bright Points in the Blue
Hα Wing
Authors: Leenaarts, J.; Sütterlin, P.; Rutten, R. J.; Carlsson, M.;
Uitenbroek, H.
2005ESASP.596E..15L Altcode: 2005ccmf.confE..15L
No abstract at ADS

---------------------------------------------------------
Title: DOT tomography of the solar atmosphere. III. Observations
and simulations of reversed granulation
Authors: Leenaarts, J.; Wedemeyer-Böhm, S.
2005A&A...431..687L Altcode:
We compare high-quality image sequences from the Dutch Open
Telescope (DOT) with synthetic image sequences obtained from 3D
radiation-hydrodynamics simulations of the solar granulation. In
particular, we study the subsonic brightness pattern observed in the
wings of Ca II HK. The simulations reproduce the observed intensity
contrast, time scales, and Fourier behaviour rather well. Most
differences can be attributed to the resolution difference between the
observations and the simulation and the small geometrical extent of
the simulation. We conclude that magnetic fields play no major role
in the formation of reversed granulation.

---------------------------------------------------------
Title: The Dutch Open Telescope on La Palma
Authors: Rutten, R. J.; Bettonvil, F. C. M.; Hammerschlag, R. H.;
Jägers, A. P. L.; Leenaarts, J.; Snik, F.; Sütterlin, P.; Tziotziou,
K.; de Wijn, A. G.
2004IAUS..223..597R Altcode: 2005IAUS..223..597R
The Dutch Open Telescope (DOT) on La Palma is an innovative solar
telescope combining open telescope structure and an open support tower
with a multi-wavelength imaging assembly and with synchronous speckle
cameras to generate high-resolution movies which sample different
layers of the solar atmosphere simultaneously and co-spatially at high
resolution over long durations. The DOT test and development phase is
nearly concluded. The installation of an advanced speckle processor
enables full science utilization including "Open-DOT" time allocation
to the international community. Co-pointing with spectropolarimeters
at other Canary Island telescopes and with TRACE furnishes valuable
Solar-B precursor capabilities.