254 research outputs found
Numerical simulations of the quiet chromosphere
Numerical simulations of the solar chromosphere have become increasingly
realistic over the past 5 years. However, many observed chromospheric
structures and behavior are not reproduced. Current models do not show fibrils
in Ca II 8542, and neither reproduce the Ca II 8542 bisector. The emergent
H-alpha 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.Comment: 6 pages, 2 Figures. To appear in Proceedings of the 25th NSO
Workshop: Chromospheric Structure and Dynamic
Solar off-limb emission of the OI 7772 \AA\ line
The aim of this paper is to understand the formation of the OI line at 7772
\AA\ in the solar chromosphere. We used SST/CRISP observations to observe OI
7772 \AA\ 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. 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 \AA\ transition is populated by a downward
cascade from the continuum. We study the effect of Lyman- pumping and
hydrogen collisions between the triplet and quintet system in OI. Both have a
small but non-negligible influence on the line intensity.Comment: 9 pages, 12 figures, Accepted for publication in A&
Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
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. 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. 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 Halpha, 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
Halpha also at low temperature.Comment: 10 pages, 4 figure
Time-dependent hydrogen ionisation in the solar chromosphere. I: Methods and first results
An approximate method for solving the rate equations for the hydrogen
populations was extended and implemented in the three-dimensional radiation
(magneto-)hydrodynamics code CO5BOLD. 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. 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. 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.Comment: 8 pages, 7 figure
STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations
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 magnetic field vector, and gas and electron pressure. The
parameters of the radiative transfer equation are computed from a solid ground
of physical principles. To apply these techniques to spectral lines that sample
the chromosphere, NLTE effects must be included in the calculations.
We developed a new inversion code STiC to study spectral lines that sample
the upper chromosphere. The code is based the RH synthetis 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. Furthermore, we include a
regularization strategy that allows for model atmospheres with a complex
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.Comment: Accepted for publication in Astronomy & Astrophysic
Non-local thermodynamic equilibrium inversions from a 3D MHD chromospheric model
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.
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 8542 line.
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 8542 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.
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.Comment: Accepted for publication in Astronomy & Astrophysic
The formation of IRIS diagnostics II. The formation of the Mg II h&k lines in the solar atmosphere
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&k lines as well as a slit-jaw imager
centered at Mg II k. Understanding the observations requires forward modeling
of Mg II h&k line formation from 3D radiation-MHD models.
We compute the vertically emergent h&k intensity from a snapshot of a dynamic
3D radiation-MHD 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 the k line we can retrieve the sign
of the velocity gradient just below the TR. The intensity in the central line
depression is anticorrelated 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 transition region. 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&k lines are excellent probes of the very upper chromosphere just below the
transition region, a height regime that is impossible to probe with other
spectral lines.Comment: 15 pages, 12 figures, accepted for ApJ, astro-ph abstract shortened
to confirm to submission requirement
Quiet-Sun imaging asymmetries in NaI D1 compared with other strong Fraunhofer lines
Imaging spectroscopy of the solar atmosphere using the NaI 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 MgI b2 line of comparable strength does
not show this asymmetry, nor does the stronger CaII 8542 line. We demonstrate
the phenomenon with near-simultaneous spectral images in NaI D1, MgI b2, and
CaII 8542 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 NaI D1 line. The MgI 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 CaII 8542 profile.Comment: Accepted by Astron & Astrophys. In each in-text citation the year
links to the corresponding ADS abstract pag
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