55 research outputs found
Numerical non-LTE 3D radiative transfer using a multigrid method
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 to the method of choice
to perform high-resolution computations in multidimensional problems. The
disadvantage of MALI is that its computing time scales as ,
with the number of grid points. When the grid gets finer, the computational
cost increases quadratically. We aim to develop a 3D non-LTE radiative transfer
code that is more efficient than MALI. 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-MHD simulations as input atmospheres. With
multigrid, we obtain a factor 3.3-4.5 speedup compared to MALI. With
full-multigrid the speed-up increases to a factor 6. The speedup is expected to
increase for input atmospheres with more grid points and finer grid spacing.
Solving 3D non-LTE radiative transfer problems using non-linear multigrid
methods can be applied to realistic atmospheres with a substantial speed-up.Comment: Accepted for publication by A&
What do iris observations of Mg II k tell us about the solar plage chromosphere?
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 H{\alpha} 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
Detailed and simplified non-equilibrium helium ionization in the solar atmosphere
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 1D radiation-hydrodynamics
simulations of the solar atmosphere and find that the helium ionization is
mostly set 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 3 level
helium model atom consisting of only the ground states. This model atom is
suitable for modeling non-equilibrium helium ionization in 3D numerical models.
We perform a brief investigation of the formation of the He I 10830 and He II
304 spectral lines. Both lines show non-equilibrium features that are not
recovered with statistical equilibrium models, and caution should therefore be
exercised when such models are used as a basis in the interpretation of
observations.Comment: 11 pages, 9 figures. Accepted for publication in Ap
The Formation of IRIS Diagnostics. IX. The Formation of the C I 135.58 Line in the Solar Atmosphere
The C I 135.58 line is located in the wavelength range of NASA's Interface
Region Imagin Spectrograph (IRIS) small explorer mission. We here 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 Ly-alpha emission. The emission in
the C I 135.58 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 line is usually
observed together with the O I 135.56 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 line is an excellent probe of the middle chromosphere by
itself, and together with the O I 135.56 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
The chromosphere above a -sunspot in the presence of fan-shaped jets
-sunspots are known to be favourable locations for fast and energetic
events like flares and CMEs. The photosphere of this type of sunspots has been
thoroughly investigated in the past three decades. The atmospheric conditions
in the chromosphere are not so well known, however. his study is focused on the
chromosphere of a -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. We use data from the Swedish 1-m Solar
Telescope (SST) and the Solar Dynamics Observatory. We invert the
spectropolarimetric FeI 6302~\AA\ and CaII ~8542~\AA\ data from the SST using
the the non-LTE inversion code NICOLE to estimate the magnetic field
configuration, temperature and velocity structure in the chromosphere. A
loop-like magnetic structure is observed to emerge in the penumbra of the
sunspot. The jets are launched from the loop-like 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 and . The magnetic field vector and the
atmospheric temperature maps show a stationary configuration during the whole
observation.Comment: 12 pages, 15 figures. Recommended for publication in A&
The cause of spatial structure in solar He I 1083 nm multiplet images
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 3D 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. 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 solve the non-LTE radiative transfer problem assuming
statistical equilibrium for a simple 9-level helium atom that nevertheless
captures all essential physics. As a model atmosphere we use a snapshot from a
3D radiation-MHD simulation computed with the Bifrost code. Ionising radiation
from the corona is 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.Comment: Accepted for publication by A&
The effect of isotopic splitting on the bisector and inversions of the solar Ca II 854.2 nm line
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 asymmetric line-core and inverse C-shape of the bisector of the
854.2 nm line can be explained by isotopic splitting. We confirm this finding
by analysing 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.Comment: Accepted for ApJ
Recent advancements in the EST project
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.Comment: accepted to Advances in Space Researc
Observed Variability of the Solar Mg II h Spectral Line
The Mg II h&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 dataset taken 24-Aug-2014. 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 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 which 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 with
predictions of semi-empirical model atmospheres. We measure a pronounced limb
darkening in the line core which 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.Comment: Accepted for publicatio
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