explanation blue bibcodes open ADS page with paths to full text
Author name code: smitha
ADS astronomy entries on 2022-09-14
=author:"Smitha, H.N."
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Title: Ti I lines at 2.2 μm as probes of the cooler regions of
sunspots
Authors: Smitha, H. N.; Castellanos Durán, J. S.; Solanki, S. K.;
Tiwari, S. K.
2021A&A...653A..91S Altcode: 2021arXiv210701247S
Context. The sunspot umbra harbours the coolest plasma on the solar
surface due to the presence of strong magnetic fields. The atomic
lines that are routinely used to observe the photosphere have weak
signals in the umbra and are often swamped by molecular lines. This
makes it harder to infer the properties of the umbra, especially in
the darkest regions. <BR /> Aims: The lines of the Ti I multiplet
at 2.2 μm are formed mainly at temperatures ≤4500 K and are not
known to be affected by molecular blends in sunspots. Since the first
systematic observations in the 1990s, these lines have been seldom
observed due to the instrumental challenges involved at these longer
wavelengths. We revisit these lines and investigate their formation
in different solar features. <BR /> Methods: We synthesized the
Ti I multiplet using a snapshot from 3D magnetohydrodynamic (MHD)
simulations of a sunspot and explored the properties of two of its
lines in comparison with two commonly used iron lines, at 6302.5 Å and
1.5648 μm. <BR /> Results: We find that the Ti I lines have stronger
signals than the Fe I lines in both intensity and polarization in the
sunspot umbra and in penumbral spines. They have little to no signal
in the penumbral filaments and the quiet Sun, at μ = 1. Their strong
and well-split profiles in the dark umbra are less affected by stray
light. Consequently, inside the sunspot, it is easier to invert these
lines and to infer the atmospheric properties as compared to the iron
lines. <BR /> Conclusions: The Cryo-NIRSP instrument at the DKIST will
provide the first-ever high-resolution observations in this wavelength
range. In this preparatory study, we demonstrate the unique temperature
and magnetic sensitivities of the Ti multiplet by probing the Sun's
coolest regions, which are not favourable for the formation of other
commonly used spectral lines. We thus expect such observations to
advance our understanding of sunspot properties.
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Title: The influence of NLTE effects in Fe I lines on an inverted
atmosphere. II. 6301 Å and 6302 Å lines formed in 3D NLTE
Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K.
2021A&A...647A..46S Altcode: 2021arXiv210100506S
Context. This paper forms the second part of our study of how
neglecting non-local thermodynamic equilibrium (NLTE) conditions in
the formation of Fe I 6301.5 Å and the 6302.5 Å lines affects the
atmosphere that is obtained by inverting the Stokes profiles of these
lines in LTE. The main cause of NLTE effects in these lines is the line
opacity deficit that is due to the excess ionisation of Fe I atoms by
ultraviolet (UV) photons in the Sun. <BR /> Aims: In the first paper,
these photospheric lines were assumed to have formed in 1D NLTE and the
effects of horizontal radiation transfer (RT) were neglected. In the
present paper, the iron lines are computed by solving the RT in 3D. We
investigate the effect of horizontal RT on the inverted atmosphere and
how it can enhance or reduce the errors that are due to neglecting 1D
NLTE effects. <BR /> Methods: The Stokes profiles of the iron lines
were computed in LTE, 1D NLTE, and 3D NLTE. They were all inverted
using an LTE inversion code. The atmosphere from the inversion of
LTE profiles was taken as the reference model. The atmospheres from
the inversion of 1D NLTE profiles (testmodel-1D) and 3D NLTE profiles
(testmodel-3D) were compared with it. Differences between reference and
testmodels were analysed and correspondingly attributed to NLTE and 3D
effects. <BR /> Results: The effects of horizontal RT are evident in
regions surrounded by strong horizontal temperature gradients. That is,
along the granule boundaries, regions surrounding magnetic elements,
and its boundaries with intergranular lanes. In some regions, the 3D
effects enhance the 1D NLTE effects, and in some, they weaken these
effects. In the small region analysed in this paper, the errors due to
neglecting the 3D effects are lower than 5% in temperature. In most of
the pixels, the errors are lower than 20% in both velocity and magnetic
field strength. These errors also persist when the Stokes profiles
are spatially and spectrally degraded to the resolution of the Swedish
Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST). <BR
/> Conclusions: Neglecting horizontal RT introduces errors not only in
the derived temperature, but also in other atmospheric parameters. The
error sizes depend on the strength of the local horizontal temperature
gradients. Compared to the 1D NLTE effect, the 3D effects are more
localised in specific regions in the atmosphere and are weaker overall.
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Title: Solar Photosphere
Authors: Chitta, L. P.; Smitha, H. N.; Solanki, S. K.
2020orep.bookE...1C Altcode:
No abstract at ADS
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Title: The influence of NLTE effects in Fe I lines on an inverted
atmosphere. I. 6301 Å and 6302 Å lines formed in 1D NLTE
Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K.
2020A&A...633A.157S Altcode: 2019arXiv191207007S
Context. Ultraviolet overionisation of iron atoms in the solar
atmosphere leads to deviations in their level populations based on
Saha-Boltzmann statistics. This causes their line profiles to form in
non-local thermodynamic equilibrium (NLTE) conditions. When inverting
such profiles to determine atmospheric parameters, the NLTE effects
are often neglected and other quantities are tweaked to compensate for
deviations from the LTE. <BR /> Aims: We investigate how the routinely
employed LTE inversion of iron lines formed in NLTE underestimates
or overestimates atmospheric quantities, such as temperature (T),
line-of-sight velocity (v<SUB>LOS</SUB>), magnetic field strength (B),
and inclination (γ) while the earlier papers have focused mainly
on T. Our findings has wide-ranging consequences since many results
derived in solar physics are based on inversions of Fe I lines carried
out in LTE. <BR /> Methods: We synthesized the Stokes profiles of Fe I
6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of
a 3D magnetohydrodynamic simulation. The profiles were then inverted
in LTE. We considered the atmosphere inferred from the inversion of
LTE profiles as the fiducial model and compared it to the atmosphere
resulting from the inversion of NLTE profiles. The observed differences
have been attributed to NLTE effects. <BR /> Results: Neglecting the
NLTE effects introduces errors in the inverted atmosphere. While the
errors in T can go up to 13%, in v<SUB>LOS</SUB> and B, the errors can
go as high as 50% or above. We find these errors to be present at all
three inversion nodes. Importantly, they survive degradation from the
spatial averaging of the profiles. <BR /> Conclusions: We provide an
overview of how neglecting NLTE effects influences the values of T,
v<SUB>LOS</SUB>, B, and γ that are determined by inverting the Fe I
6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors
are found at the sites of granules, intergranular lanes, magnetic
elements, and basically in every region susceptible to NLTE effects. For
an accurate determination of the atmospheric quantities and their
stratification, it is, therefore, important to take the NLTE effects
into account.
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Title: Using the infrared iron lines to probe solar subsurface
convection
Authors: Milić, I.; Smitha, H. N.; Lagg, A.
2019A&A...630A.133M Altcode: 2019arXiv190407306M
Context. Studying the properties of solar convection using
high-resolution spectropolarimetry began in the early 1990s with
the focus on observations in the visible wavelength regions. Its
extension to the infrared (IR) remains largely unexplored. <BR />
Aims: The IR iron lines around 15 600 Å, most commonly known for
their high magnetic sensitivity, also have a non-zero response to
line-of-sight (LOS) velocity below log(τ) = 0.0. In this paper we
explore the possibility of using these lines to measure subsurface
convective velocities. <BR /> Methods: By assuming a snapshot of a
three-dimensional magnetohydrodynamic simulation to represent the quiet
Sun, we investigate how well the iron IR lines can reproduce the LOS
velocity in the cube and to what depth. We use the recently developed
spectropolarimetric inversion code SNAPI and discuss the optimal node
placements for the retrieval of reliable results from these spectral
lines. <BR /> Results: We find that the IR iron lines can measure the
convective velocities down to log(τ) = 0.5, below the photosphere,
not only at the original resolution of the cube, but also when degraded
with a reasonable spectral and spatial PSF and stray light. Instead, the
commonly used Fe I 6300 Å line pair performs significantly worse. <BR
/> Conclusions: Our investigation reveals that the IR iron lines can
probe the subsurface convection in the solar photosphere. This paper
is a first step towards exploiting this diagnostic potential.
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Title: Observations of solar chromospheric heating at sub-arcsec
spatial resolution
Authors: Smitha, H. N.; Chitta, L. P.; Wiegelmann, T.; Solanki, S. K.
2018A&A...617A.128S Altcode: 2018arXiv180701078S
A wide variety of phenomena such as gentle but persistent
brightening, dynamic slender features (∼100 km), and compact
(∼1″) ultraviolet (UV) bursts are associated with the heating of
the solar chromosphere. High spatio-temporal resolution is required to
capture the finer details of the likely magnetic reconnection-driven,
rapidly evolving bursts. Such observations are also needed to reveal
their similarities to large-scale flares, which are also thought to be
reconnection driven, and more generally their role in chromospheric
heating. Here we report observations of chromospheric heating in
the form of a UV burst obtained with the balloon-borne observatory
SUNRISE. The observed burst displayed a spatial morphology similar
to that of a large-scale solar flare with a circular ribbon. While
the co-temporal UV observations at 1.5″ spatial resolution and
24 s cadence from the Solar Dynamics Observatory showed a compact
brightening, the SUNRISE observations at diffraction-limited spatial
resolution of 0.1″ at 7 s cadence revealed a dynamic substructure
of the burst that it is composed of an extended ribbon-like feature
and a rapidly evolving arcade of thin (∼0.1″) magnetic loop-like
features, similar to post-flare loops. Such a dynamic substructure
reveals the small-scale nature of chromospheric heating in these
bursts. Furthermore, based on magnetic field extrapolations, this
heating event is associated with a complex fan-spine magnetic
topology. Our observations strongly hint at a unified picture of
magnetic heating in the solar atmosphere from some large-scale
flares to small-scale bursts, all associated with such a magnetic
topology. <P />The movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833276/olm">https://www.aanda.org/</A>
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Title: Probing photospheric magnetic fields with new spectral
line pairs
Authors: Smitha, H. N.; Solanki, S. K.
2017A&A...608A.111S Altcode: 2017arXiv170908926S
Context. The magnetic line ratio (MLR) method has been extensively
used in the measurement of photospheric magnetic field strength. It
was devised for the neutral iron line pair at 5247.1 Å and 5250.2 Å
(5250 Å pair). Other line pairs as well-suited as this pair have not
been reported in the literature. <BR /> Aims: The aim of the present
work is to identify new line pairs useful for the MLR technique and to
test their reliability. <BR /> Methods: We used a three-dimensional
magnetohydrodynamic 3D MHD simulation representing the quiet Sun
atmosphere to synthesize the Stokes profiles. Then, we applied the MLR
technique to the Stokes V profiles to recover the fields in the MHD
cube both at original resolution and after degrading with a point spread
function. In both these cases, we aim to empirically represent the field
strengths returned by the MLR method in terms of the field strengths
in the MHD cube. <BR /> Results: We have identified two new line pairs
that are very well adapted to be used for MLR measurements. The first
pair is in the visible, Fe I 6820-6842 Å, whose intensity profiles
have previously been used to measure stellar magnetic fields, and
the other pair is in the infrared (IR), Fe I 15 534-15 542 Å. The
lines in these pairs reproduce the magnetic fields in the MHD cube
rather well and, in fact, somewhat better than the original 5250 Å
pair. <BR /> Conclusions: The newly identified line pairs complement
the old pairs. The lines in the new IR pair, because of their higher
Zeeman sensitivity, are ideal for the measurement of weak fields. The
new visible pair works best above 300 G. The new IR pair, due to its
large Stokes V signal samples more fields in the MHD cube than the old
IR pair at 1.56 μm, even in the presence of noise, and hence likely
also on the real Sun. Owing to their low formation heights (100-200 km
above τ<SUB>5000</SUB> = 1), both the new line pairs are well suited
for probing magnetic fields in the lower photosphere.
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Title: Estimation of the Magnetic Flux Emergence Rate in the Quiet
Sun from Sunrise Data
Authors: Smitha, H. N.; Anusha, L. S.; Solanki, S. K.; Riethmüller,
T. L.
2017ApJS..229...17S Altcode: 2016arXiv161106432S
Small-scale internetwork (IN) features are thought to be the major
source of fresh magnetic flux in the quiet Sun. During its first science
flight in 2009, the balloon-borne observatory Sunrise captured images of
the magnetic fields in the quiet Sun at a high spatial resolution. Using
these data we measure the rate at which the IN features bring magnetic
flux to the solar surface. In a previous paper it was found that the
lowest magnetic flux in small-scale features detected using the Sunrise
observations is 9 × 10<SUP>14</SUP> Mx. This is nearly an order of
magnitude smaller than the smallest fluxes of features detected in
observations from the Hinode satellite. In this paper, we compute the
flux emergence rate (FER) by accounting for such small fluxes, which
was not possible before Sunrise. By tracking the features with fluxes
in the range {10}<SUP>15</SUP>{--}{10}<SUP>18</SUP> Mx, we measure an
FER of 1100 {Mx} {{cm}}<SUP>-2</SUP> {{day}}<SUP>-1</SUP>. The smaller
features with fluxes ≤slant {10}<SUP>16</SUP> Mx are found to be the
dominant contributors to the solar magnetic flux. The FER found here
is an order of magnitude higher than the rate from Hinode, obtained
with a similar feature tracking technique. A wider comparison with the
literature shows, however, that the exact technique of determining the
rate of the appearance of new flux can lead to results that differ by
up to two orders of magnitude, even when applied to similar data. The
causes of this discrepancy are discussed and first qualitative
explanations proposed.
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Title: Flux emergence rate in the quiet Sun from Sunrise data
Authors: Smitha, H. N.; Anusha, L. S.; Solanki, S. K.; Riethmüller,
T. L.
2017psio.confE.106S Altcode:
No abstract at ADS
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Title: Modeling the center-to-limb variation of the Ca i 4227 Å
line using FCHHT models
Authors: Supriya, H. D.; Smitha, H. N.; Nagendra, K. N.; Stenflo,
J. O.; Bianda, M.; Ravindra, B.; Ramelli, R.; Anusha, L. S.
2015IAUS..305..381S Altcode:
The Ca i 4227 Å is a chromospheric line exhibiting the largest degree
of linear polarization near the limb, in the visible spectrum of the
Sun. Modeling the observations of the center-to-limb variations (CLV)
of different lines in the Second Solar Spectrum helps to sample the
height dependence of the magnetic field, as the observations made
at different lines of sight sample different heights in the solar
atmosphere. Supriya et al. (2014) attempted to simultaneously model
the CLV of the (I, Q/I) spectra of the Ca i 4227 Å line using the
standard 1-D FAL model atmospheres. They found that the standard FAL
model atmospheres and also any appropriate combination of them, fail
to simultaneously fit the observed Stokes (I, Q/I) profiles at all the
limb distances (μ) satisfying at the same time all the observational
constraints. This failure of 1-D modeling approach can probably be
overcome by using multi-dimensional modeling which is computationally
expensive. To eliminate an even wider choice of 1-D models, we attempt
here to simultaneously model the CLV of the (I, Q/I) spectra using the
FCHHT solar model atmospheres which are updated and recent versions
of the FAL models. The details of our modeling efforts and the results
are presented.
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Title: A revisit to model the Cr i triplet at 5204-5208 Å and the
Ba ii D<SUB>2</SUB> line at 4554 Å in the Second Solar Spectrum
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Sampoorna, M.; Ramelli, R.
2015IAUS..305..372S Altcode:
In our previous attempt to model the Stokes profiles of the Cr i triplet
at 5204-5208 Å and the Ba ii D<SUB>2</SUB> at 4554 Å, we found it
necessary to slightly modify the standard FAL model atmospheres to fit
the observed polarization profiles. In the case of Cr i triplet, this
modification was done to reduce the theoretical continuum polarization,
and in the case of Ba ii D<SUB>2</SUB>, it was needed to reproduce the
central peak in Q/I. In this work, we revisit both these cases using
different standard model atmospheres whose temperature structures
closely resemble those of the modified FAL models, and explore the
possibility of synthesizing the line profiles without the need for
small modifications of the model atmosphere.
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Title: The Role of Quantum Interference and Partial Redistribution
in the Solar Ba <font size=2>II D<SUB>2</SUB> 4554 Å Line
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.
2014ASPC..489..213S Altcode: 2014arXiv1409.0465S
The Ba <font size=2>II D<SUB>2</SUB> line at 4554 Å is a good
example, where the F-state interference effects due to the odd isotopes
produce polarization profiles, which are very different from those
of the even isotopes that do not exhibit F-state interference. It is
therefore necessary to account for the contributions from the different
isotopes to understand the observed linear polarization profiles of
this line. In this paper we present radiative transfer modeling with
partial frequency redistribution, which is shown to be essential to
model this line. This is because complete frequency redistribution
cannot reproduce the observed wing polarization. We present the observed
and computed Q/I profiles at different limb distances. The theoretical
profiles strongly depend on limb distance (μ) and the model atmosphere
which fits the limb observations fails at other μ positions.
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Title: The Quantum Interference Effects in the Sc II 4247 Å Line
of the Second Solar Spectrum
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Ramelli, R.
2014ApJ...794...30S Altcode: 2014arXiv1408.4247S
The Sc II 4247 Å line formed in the chromosphere is one of the lines
well known, like the Na I D<SUB>2</SUB> and Ba II D<SUB>2</SUB>,
for its prominent triple-peak structure in Q/I and the underlying
quantum interference effects governing it. In this paper, we try
to study the nature of this triple-peak structure using the theory
of F-state interference including the effects of partial frequency
redistribution (PRD) and radiative transfer (RT). We compare our
results with the observations taken in a quiet region near the
solar limb. In spite of accounting for PRD and RT effects, it has
not been possible to reproduce the observed triple-peak structure in
Q/I. While the two wing PRD peaks (on either side of central peak)
and the near wing continuum can be reproduced, the central peak is
completely suppressed by the enhanced depolarization resulting from
the hyperfine structure splitting. This suppression remains for all
the tested widely different one-dimensional model atmospheres or for
any multi-component combinations of them. While multidimensional RT
effects may improve the fit to the intensity profiles, they do not
appear capable of explaining the enigmatic central Q/I peak. This
leads us to suspect that some aspect of quantum physics is missing.
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Title: Center-to-limb Observations and Modeling of the Ca I 4227
Å Line
Authors: Supriya, H. D.; Smitha, H. N.; Nagendra, K. N.; Stenflo,
J. O.; Bianda, M.; Ramelli, R.; Ravindra, B.; Anusha, L. S.
2014ApJ...793...42S Altcode: 2014arXiv1407.5461S
The observed center-to-limb variation (CLV) of the scattering
polarization in different lines of the Second Solar Spectrum can be used
to constrain the height variation of various atmospheric parameters, in
particular the magnetic fields, via the Hanle effect. Here we attempt
to model the nonmagnetic CLV observations of the Q/I profiles of the
Ca I 4227 Å line recorded with the Zurich Imaging Polarimeter-3
at IRSOL. For modeling, we use the polarized radiative transfer
with partial frequency redistribution with a number of realistic
one-dimensional (1D) model atmospheres. We find that all the standard
Fontenla-Avrett-Loeser (FAL) model atmospheres, which we used, fail
to simultaneously fit the observed (I, Q/I) at all the limb distances
(μ). However, an attempt is made to find a single model which can
provide a fit to at least the CLV of the observed Q/I instead of a
simultaneous fit to the (I, Q/I) at all μ. To this end we construct a
new 1D model by combining two of the standard models after modifying
their temperature structures in the appropriate height ranges. This
new combined model closely reproduces the observed Q/I at all μ but
fails to reproduce the observed rest intensity at different μ. Hence
we find that no single 1D model atmosphere succeeds in providing a
good representation of the real Sun. This failure of 1D models does
not, however, cause an impediment to the magnetic field diagnostic
potential of the Ca I 4227 Å line. To demonstrate this we deduce the
field strength at various μ positions without invoking the use of
radiative transfer.
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Title: Modeling the Quantum Interference Signatures of the Ba II
D<SUB>2</SUB> 4554 Å Line in the Second Solar Spectrum
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.
2013ApJ...768..163S Altcode: 2013arXiv1303.7304S
Quantum interference effects play a vital role in shaping the linear
polarization profiles of solar spectral lines. The Ba II D<SUB>2</SUB>
line at 4554 Å is a prominent example, where the F-state interference
effects due to the odd isotopes produce polarization profiles,
which are very different from those of the even isotopes that have
no F-state interference. It is therefore necessary to account for the
contributions from the different isotopes to understand the observed
linear polarization profiles of this line. Here we do radiative
transfer modeling with partial frequency redistribution (PRD) of such
observations while accounting for the interference effects and isotope
composition. The Ba II D<SUB>2</SUB> polarization profile is found
to be strongly governed by the PRD mechanism. We show how a full PRD
treatment succeeds in reproducing the observations, while complete
frequency redistribution alone fails to produce polarization profiles
that have any resemblance to the observed ones. However, we also find
that the line center polarization is sensitive to the temperature
structure of the model atmosphere. To obtain a good fit to the line
center peak of the observed Stokes Q/I profile, a small modification
of the FALX model atmosphere is needed, by lowering the temperature
in the line-forming layers. Because of the pronounced temperature
sensitivity of the Ba II D<SUB>2</SUB> line it may not be a suitable
tool for Hanle magnetic-field diagnostics of the solar chromosphere,
because there is currently no straightforward way to separate the
temperature and magnetic-field effects from each other.
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Title: Quantum interference with angle-dependent partial frequency
redistribution: solution of the polarized line transfer in the
non-magnetic case
Authors: Supriya, H. D.; Smitha, H. N.; Nagendra, K. N.; Ravindra,
B.; Sampoorna, M.
2013MNRAS.429..275S Altcode:
Angle-dependent partial frequency redistribution (PRD) matrices
represent the physical redistribution in the process of light scattering
on atoms. For the purpose of numerical simplicity, it is a common
practice in astrophysical literature to use the angle-averaged versions
of these matrices, in the line transfer computations. The aim of this
paper is to study the combined effects of angle-dependent PRD and the
quantum interference phenomena arising either between the fine structure
(J) states of a two-term atom or between the hyperfine structure (F)
states of a two-level atom. We restrict our attention to the case
of non-magnetic and collisionless line scattering on atoms. A rapid
method of solution based on Neumann series expansion is developed to
solve the angle-dependent PRD problem including quantum interference in
an atomic system. We discuss the differences that occur in the Stokes
profiles when angle-dependent PRD mechanism is taken into account.
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Title: Polarized line formation with J-state interference in the
presence of magnetic fields: A Heuristic treatment of collisional
frequency redistribution
Authors: Smitha, H. N.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
2013JQSRT.115...46S Altcode: 2012arXiv1209.0243S
An expression for the partial frequency redistribution (PRD) matrix
for line scattering in a two-term atom, which includes the J-state
interference between its fine structure line components is derived. The
influence of collisions (both elastic and inelastic) and an external
magnetic field on the scattering process is taken into account. The
lower term is assumed to be unpolarized and infinitely sharp. The
linear Zeeman regime in which the Zeeman splitting is much smaller than
the fine structure splitting is considered. The inelastic collision
rates between the different levels are included in our treatment. We
account for the depolarization caused by the collisions coupling the
fine structure states of the upper term, but neglect the polarization
transfer between the fine structure states. When the fine structure
splitting goes to zero, we recover the redistribution matrix that
represents the scattering on a two-level atom (which exhibits only
m-state interference—namely the Hanle effect). The way in which the
multipolar index of the scattering atom enters into the expression
for the redistribution matrix through the collisional branching
ratios is discussed. The properties of the redistribution matrix are
explored for a single scattering process for a L=0→1→0 scattering
transition with S=1/2 (a hypothetical doublet centered at 5000 Å and
5001 Å). Further, a method for solving the Hanle radiative transfer
equation for a two-term atom in the presence of collisions, PRD, and
J-state interference is developed. The Stokes profiles emerging from
an isothermal constant property medium are computed.
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Title: Polarized Line Transfer with F-state Interference in a
Non-magnetic Medium: Partial Frequency Redistribution Effects in
the Collisionless Regime
Authors: Smitha, H. N.; Sowmya, K.; Nagendra, K. N.; Sampoorna, M.;
Stenflo, J. O.
2012ApJ...758..112S Altcode: 2012arXiv1208.6369S
Quantum interference phenomena manifest themselves in several ways
in the polarized solar spectrum formed due to coherent scattering
processes. One such effect arises due to interference between the fine
structure (J) states giving rise to multiplets. Another effect is that
which arises due to interference between the hyperfine structure (F)
states. We extend the redistribution matrix derived for the J-state
interference to the case of F-state interference. We then incorporate
it into the polarized radiative transfer equation and solve it for
isothermal constant property slab atmospheres. The relevant transfer
equation is solved using a polarized approximate lambda iteration (PALI)
technique based on operator perturbation. An alternative method derived
from the Neumann series expansion is also proposed and is found to be
relatively more efficient than the PALI method. The effects of partial
frequency redistribution and the F-state interference on the shapes
of the linearly polarized Stokes profiles are discussed. The emergent
Stokes profiles are computed for hypothetical line transitions arising
due to hyperfine structure splitting of the upper J = 3/2 and lower J =
1/2 levels of a two-level atom model with nuclear spin I<SUB>s</SUB>
= 3/2. We confine our attention to the non-magnetic scattering in the
collisionless regime.
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Title: Forward-scattering Hanle effect in the solar Ca I 4227 Å line
Authors: Frisch, H.; Anusha, L. S.; Bianda, M.; Holzreuter, R.;
Nagendra, K. N.; Ramelli, R.; Sampoorna, M.; Smitha, H. N.; Stenflo,
J. O.
2012EAS....55...59F Altcode:
High sensitivity spectropolarimetric observations of the four Stokes
parameters of the solar Ca I 4227 Å line have been performed in
October 2010 at IRSOL with the ZIMPOL polarimeter, near the disk center,
outside an active region (Bianda et al. 2011). They were analyzed in
Anusha et al. 2011 with a combination of detailed radiative transfer
modelling of the Hanle effect for the linear polarization and weak
field Zeeman approximation for the circular polarization. This approach
made possible a unique determination of the magnetic field vector at
various positions along the slit of the spectrograph. A summary of
the observations and of their analysis is presented here.
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Title: J-state interference signatures in the second solar
spectrum. Modeling the Cr i triplet at 5204-5208 Å
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Sampoorna, M.; Ramelli, R.; Anusha, L. S.
2012A&A...541A..24S Altcode: 2012arXiv1203.4934S
The scattering polarization in the solar spectrum is traditionally
modeled with each spectral line treated separately, but this is
generally inadequate for multiplets where J-state interference
plays a significant role. Through simultaneous observations of all
the 3 lines of a Cr i triplet, combined with realistic radiative
transfer modeling of the data, we show that it is necessary to include
J-state interference consistently when modeling lines with partially
interacting fine structure components. Polarized line formation theory
that includes J-state interference effects together with partial
frequency redistribution for a two-term atom is used to model the
observations. Collisional frequency redistribution is also accounted
for. We show that the resonance polarization in the Cr i triplet is
strongly affected by the partial frequency redistribution effects in
the line core and near wing peaks. The Cr i triplet is quite sensitive
to the temperature structure of the photospheric layers. Our complete
frequency redistribution calculations in semi-empirical models of the
solar atmosphere cannot reproduce the observed near wing polarization or
the cross-over of the Stokes Q/I line polarization about the continuum
polarization level that is due to the J-state interference. When
however partial frequency redistribution is included, a good fit to
these features can be achieved. Further, to obtain a good fit to the
far wings, a small temperature enhancement of the FALF model in the
photospheric layers is necessary.
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Title: Radiative transfer with J-state interference in a two-term
atom. Partial frequency redistribution in the non-magnetic case
Authors: Smitha, H. N.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
2011A&A...535A..35S Altcode:
Context. Quantum interference phenomena play a fundamental role in
the formation of linear polarization that arises from scattering
processes in multiplets of the solar spectrum. In particular,
the J-state interference between different line components of a
multiplet (arising from transitions in a two-term atom) produces
significant effects in the linearly polarized spectra. <BR /> Aims:
We aim to solve the polarized radiative transfer equation for a
two-term atom with the unpolarized lower term in isothermal slabs,
including the effect of the interference between the upper J-states
and partial frequency redistribution (PRD). We consider only the case
of non-magnetic scattering. <BR /> Methods: The PRD matrix for the
J-state interference derived in previous works is incorporated into
the polarized transfer equation. The standard form of the two-level
atom transfer equation is extended to a two-term atom. The transfer
problem is then solved using a traditional polarized approximate lambda
iteration method. <BR /> Results: We show how the PRD and the J-state
interference together affect the shapes of the (I,Q/I) profiles. We
present the benchmark solutions for isothermal, constant-property
slabs of a given optical thickness. We consider a hypothetical doublet
produced by an L = 0 → 1 → 0 scattering transition with spin S =
1/2. We present the results in the form of Stokes (I,Q/I) profiles for
different values of (i) the line separation, (ii) optical thickness,
(iii) thermalization parameter, and (iv) the continuum opacity.
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Title: Analysis of the Forward-scattering Hanle Effect in the Ca I
4227 Å Line
Authors: Anusha, L. S.; Nagendra, K. N.; Bianda, M.; Stenflo, J. O.;
Holzreuter, R.; Sampoorna, M.; Frisch, H.; Ramelli, R.; Smitha, H. N.
2011ApJ...737...95A Altcode:
Coherent scattering of limb-darkened radiation is responsible for the
generation of the linearly polarized spectrum of the Sun (the Second
Solar Spectrum). This Second Solar Spectrum is usually observed near the
limb of the Sun, where the polarization amplitudes are largest. At the
center of the solar disk the linear polarization is zero for an axially
symmetric atmosphere. Any mechanism that breaks the axial symmetry (like
the presence of an oriented magnetic field, or resolved inhomogeneities
in the atmosphere) can generate a non-zero linear polarization. In the
present paper we study the linear polarization near the disk center
in a weakly magnetized region, where the axisymmetry is broken. We
present polarimetric (I, Q/I, U/I, and V/I) observations of the Ca
I 4227 Å line recorded around μ = cos θ = 0.9 (where θ is the
heliocentric angle) and a modeling of these observations. The high
sensitivity of the instrument (ZIMPOL-3) makes it possible to measure
the weak polarimetric signals with great accuracy. The modeling of
these high-quality observations requires the solution of the polarized
radiative transfer equation in the presence of a magnetic field. For
this we use standard one-dimensional model atmospheres. We show that the
linear polarization is mainly produced by the Hanle effect (rather than
by the transverse Zeeman effect), while the circular polarization is due
to the longitudinal Zeeman effect. A unique determination of the full
\bm {B} vector may be achieved when both effects are accounted for. The
field strengths required for the simultaneous fitting of Q/I, U/I, and
V/I are in the range 10-50 G. The shapes and signs of the Q/I and U/I
profiles are highly sensitive to the orientation of the magnetic field.
---------------------------------------------------------
Title: Observations of the forward scattering Hanle effect in the
Ca I 4227 Å line
Authors: Bianda, M.; Ramelli, R.; Anusha, L. S.; Stenflo, J. O.;
Nagendra, K. N.; Holzreuter, R.; Sampoorna, M.; Frisch, H.; Smitha,
H. N.
2011A&A...530L..13B Altcode: 2011arXiv1105.2157B
Chromospheric magnetic fields are notoriously difficult to measure. The
chromospheric lines are broad, while the fields are producing
a minuscule Zeeman-effect polarization. A promising diagnostic
alternative is provided by the forward-scattering Hanle effect, which
can be recorded in chromospheric lines such as the He i 10 830 Å
and the Ca i 4227 Å lines. We present a set of spectropolarimetric
observations of the full Stokes vector obtained near the center of the
solar disk in the Ca i 4227 Å line with the ZIMPOL polarimeter at the
IRSOL observatory. We detect a number of interesting forward-scattering
Hanle effect signatures, which we model successfully using polarized
radiative transfer. Here we focus on the observational aspects, while
a separate companion paper deals with the theoretical modeling.
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Title: Polarized Line Formation with J-state Interference in the
Presence of Magnetic Fields. I. Partial Frequency Redistribution in
the Collisionless Regime
Authors: Smitha, H. N.; Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
2011ApJ...733....4S Altcode:
Quantum interference phenomena play a fundamental role in astrophysical
spectra that are formed by coherent scattering processes. Here we
derive a partial frequency redistribution (PRD) matrix that includes
J-state interference in the presence of magnetic fields of arbitrary
strength. The paper focuses on PRD in the collisionless regime, which
in the traditional PRD terminology is referred to as Hummer's type-II
scattering. By limiting the treatment to the linear Zeeman regime,
for which the Zeeman splitting is much smaller than the fine-structure
splitting, it is possible to formulate analytical expressions for
the PRD matrices. In the special case of non-magnetic scattering we
recover the redistribution matrix derived from an independent quantum
electrodynamic formulation based on the metalevel theory.
