259 research outputs found
Effect of cross-redistribution on the resonance scattering polarization of O {\sc i} line at 1302 \AA\,
Oxygen is the most abundant element on the Sun after Hydrogen and Helium. The
intensity spectrum of resonance lines of neutral Oxygen namely O {\sc i} (1302,
1305 and 1306 \AA\,) has been studied in the literature for chromospheric
diagnostics. In this paper we study the resonance scattering polarization in
the O {\sc i} line at 1302 \AA\, using two-dimensional radiative transfer in a
composite atmosphere constructed using a two-dimensional magneto-hydrodynamical
snapshot in the photosphere and columns of the one-dimensional FALC atmosphere
in the chromosphere. The methods developed by us recently in a series of papers
to solve multi-dimensional polarized radiative transfer have been incorporated
in our new code POLY2D which we use for our analysis. We find that
multi-dimensional radiative transfer including XRD effects is important in
reproducing the amplitude and shape of scattering polarization signals of the O
{\sc i} line at 1302 \AA\,
Fast approximation of angle-dependent partial redistribution in moving atmospheres
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.
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 \MgII\,k and \Lyalpha\ lines computed with angle-dependent PRD,
angle-averaged PRD and our new method in two representative test atmospheres.
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.Comment: Accepted for publication in A&
On Molecular Hydrogen Formation and the Magnetohydrostatic Equilibrium of Sunspots
We have investigated the problem of sunspot magnetohydrostatic equilibrium
with comprehensive IR sunspot magnetic field survey observations of the highly
sensitive Fe I lines at 15650 \AA\ and nearby OH lines. We have found that some
sunspots show isothermal increases in umbral magnetic field strength which
cannot be explained by the simplified sunspot model with a single-component
ideal gas atmosphere assumed in previous investigations. Large sunspots
universally display non-linear increases in magnetic pressure over temperature,
while small sunspots and pores display linear behavior. The formation of
molecules provides a mechanism for isothermal concentration of the umbral
magnetic field, and we propose that this may explain the observed rapid
increase in umbral magnetic field strength relative to temperature. Existing
multi-component sunspot atmospheric models predict that a significant amount of
molecular hydrogen (H2) exists in the sunspot umbra. The formation of H2 can
significantly alter the thermodynamic properties of the sunspot atmosphere and
may play a significant role in sunspot evolution. In addition to the survey
observations, we have performed detailed chemical equilibrium calculations with
full consideration of radiative transfer effects to establish OH as a proxy for
H2, and demonstrate that a significant population of H2 exists in the coolest
regions of large sunspots.Comment: 17 pages, 19 figures, accepted for publication in Ap
The contrast of magnetic elements in synthetic CH- and CN-band images of solar magnetoconvection
We present a comparative study of the intensity contrast in synthetic CH-band
and violet CN-band filtergrams computed from a high-resolution simulation of
solar magnetoconvection. The underlying simulation has an average vertical
magnetic field of 250 G with kG fields concentrated in its intergranular lanes,
and is representative of a plage region. To simulate filtergrams typically
obtained in CH- and CN-band observations we computed spatially resolved spectra
in both bands and integrated these spectra over 1 nm FWHM filter functions
centred at 430.5 nm and 388.3 nm, respectively. We find that the average
contrast of magnetic bright points in the simulated filtergrams is lower in the
CN-band by a factor of 0.96. This result strongly contradicts earlier
semi-empirical modeling and recent observations, which both etimated that the
bright-point contrast in the CN-band is \emph{higher} by a factor of 1.4. We
argue that the near equality of the bright-point contrast in the two bands in
the present simulation is a natural consequence of the mechanism that causes
magnetic flux elements to be particularly bright in the CN and CH filtergrams,
namely the partial evacuation of these elements and the concomitant weakening
of molecular spectral lines in the filter passbands. We find that the RMS
intensity contrast in the whole field-of-view of the filtergrams is 20.5% in
the G band and 22.0% in the CN band and conclude that this slight difference in
contrast is caused by the shorter wavelength of the latter. Both the
bright-point and RMS intensity contrast in the CN band are sensitive to the
precise choice of the central wavelength of the filter.Comment: 24 pages, 9 figures, submitted to Ap
Narrow-band Imaging in the CN Band at 388.33 nm
We promote the use of narrow-band (0.05 -- 0.20 nm FWHM) imaging in the molecularvCN band head at 388.33 nm as an effective method for monitoring small-scale magnetic field elements because it renders them with exceptionally high contrast. We create synthetic narrow-band CN filtergrams from spectra computed from a three-dimensional snapshot of a magnetohydrodynamic simulation of the solar convection to illustrate the expected high contrast and explain its nature. In addition, we performed observations with the horizontal slit spectrograph at the Dunn Solar Tower at 388.3 nm to experimentally confirm the high bright-point contrast, and to characterize and optimize the transmission profile of a narrow-band (0.04 FWHM) Lyot filter, which was built by Lyot and tailored to the CN band at Sacramento Peak in the early 70's. The presented theoretical computations predict that bright-point contrast in narrow-band (0.04 FWHM) CN filtergrams is more than 3 times higher than in CN filtergrams taken with 1 nm FWHM wide filters, and in typical G-band filtergrams
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
Morphology and Dynamics of the Low Solar Chromosphere
The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn
Solar Telescope of the NSO/SP is used to investigate the morphology and
dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere
below. The study addresses in particular the structure of magnetic elements
that extend into these layers. We choose different quiet Sun regions in and
outside coronal holes. In inter-network regions with no significant magnetic
flux contributions above the detection limit of IBIS, we find intensity
structures with the characteristics of a shock wave pattern. The magnetic flux
elements in the network are long lived and seem to resemble the spatially
extended counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight magnetic
field strength and explain some of the difficulties in deriving the magnetic
field vector from observations of the fluctosphere.Comment: accepted by ApJ, 16 pages, 8 figure
The formation of IRIS diagnostics I. A quintessential model atom of Mg II and general formation properties of the Mg II h&k lines
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&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&k
line formation from 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&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&k can be modeled accurately with a 4-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 1D 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.Comment: 13 pages, 13 figures, accepted for Ap
- …