arXiv subjects

M. Sampoorna

Publications and source records attributed to M. Sampoorna.

At least 19 recordsLinked to original sources

arXiv2026

Application of the coherent structure tracking to solar Doppler maps to determine horizontal velocity fields at the Sun's surface

Coherent Structure Tracking (CST) is a technique for determining the solar surface horizontal flows at high spatial and temporal resolution by tracking the proper motion of granules. CST has been traditionally applied to solar intensity images in the continuum, which clearly depict the granular patterns. However, solar granulation is also visible in the Dopplergrams. We aim to show that CST can be applied to solar Dopplergrams to derive the solar surface horizontal velocity fields with the same level of confidence as those determined by CST on intensity images. For this purpose, we apply the CST to continuum intensity images and Dopplergrams obtained from SDO/HMI and also from a numerical simulation of granulation. We then compare the resulting solar surface horizontal velocity fields and their derivatives for different time windows. Pearson's linear global correlation coefficient (GCC) between the horizontal velocity fields determined from CST on Doppler and on intensity images of a relatively less active Sun is about 73% for a 30 min time average, while the corresponding local correlation coefficient (LCC) near the disk center is about 80%. For the divergence of the horizontal velocity field, we obtain a GCC of 72% and a near disk center LCC of 84%. The curl of the horizontal velocity field being more noisy exhibits somewhat reduced GCC and LCC. These coefficients increase with increasing time window. A similar trend is exhibited by Spearman's and Kendall's rank-order correlation coefficients, although they are somewhat smaller in value. The different correlation coefficients slightly decrease for magnetically more active Sun with sunspots or emerging pores in a plage region. A high correlation is obtained between the horizontal flows derived by applying CST to intensity and vertical velocity maps from a numerical simulation.

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arXiv2026

Full non-LTE multi-level radiative transfer II. The case of a 5-level Ca ii atom with broadened excited levels

The so-called full non-local thermodynamic equilibrium (FNLTE) radiative transfer problem allows us to take into account not only deviations of the radiation field from the Planckian but also deviations of the densities and velocity distributions of massive particles from Maxwell-Boltzmann statistics. This article discusses the extension of this formalism to physically realistic multi-level atoms, including natural broadening of the excited levels. In practice, we must solve self-consistently a coupled set of kinetic equations and determine, for each line, an emission and absorption profile by convolving a non-Lorentzian atomic profile with a non-Maxwellian velocity distribution at each iteration. To solve this numerically challenging problem, we have developed a new efficient iterative method based on well-known approximate operator techniques. After validating our numerical strategy, we present the results obtained for the H & K lines and the infrared triplet of the Ca II. Under the conditions studied, for this particular atomic model and for a simplified atmosphere, we find that the standard NLTE with partial redistribution is sufficient to describe the formation of Ca II spectral lines. The more exact treatment of FNLTE is unnecessary in the case of Ca II H & K, and infrared triplet lines, even when accounting for velocity-changing collisions.

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arXiv2024

Full non-LTE spectral line formation III. The case of a two-level atom with broadened upper level

In the present paper we consider the full nonlocal thermodynamic equilibrium (non-LTE) radiation transfer problem. This formalism allows us to account for deviation from equilibrium distribution of both the radiation field and the massive particles. In the present study two-level atoms with broadened upper level represent the massive particles. In the absence of velocity-changing collisions, we demonstrate the analytic equivalence of the full non-LTE source function with the corresponding standard non-LTE partial frequency redistribution (PFR) model. We present an iterative method based on operator splitting techniques to numerically solve the problem at hand. We benchmark it against the standard non-LTE transfer problem for a two-level atom with PFR. We illustrate the deviation of the velocity distribution function of excited atoms from the equilibrium distribution. We also discuss the dependence of the emission profile and the velocity distribution function on elastic collisions and velocity-changing collisions.

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arXiv2023

Full non-LTE spectral line formation II. Two-distribution radiation transfer with coherent scattering in the atom's frame

In the present article, we discuss a numerical method of solution for the so-called "full non-LTE" radiation transfer problem, basic formalism of which was revisited by Paletou & Peymirat (2021; see also Oxenius 1986). More specifically, usual numerical iterative methods for non-LTE radiation transfer are coupled with the above-mentioned formalism. New numerical additions are explained in detail. We benchmark the whole process with the standard non-LTE transfer problem for a two-level atom with Hummer's (1962, 1969) $R_{\rm I-A}$ partial frequency redistribution function. We finally display new quantities such as the spatial distribution of the velocity distribution function of excited atoms, that can only be accessed to by adopting this more general frame for non-LTE radiation transfer.

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arXiv2020

Importance of Angle-dependent Partial Frequency Redistribution in Hyperfine Structure Transitions Under Incomplete Paschen-Back Effect Regime

Angle-frequency coupling in scattering of polarized light on atoms is represented by the angle-dependent (AD) partial frequency redistribution (PRD) matrices. There are several lines in the linearly polarized solar spectrum, for which PRD combined with quantum interference between hyperfine structure states play a significant role. Here we present the solution of the polarized line transfer equation including the AD-PRD matrix for scattering on a two-level atom with hyperfine structure splitting (HFS) and an unpolarized lower level. We account for the effects of arbitrary magnetic fields (including the incomplete Paschen-Back effect regime) and elastic collisions. For exploratory purposes we consider a self-emitting isothermal planar atmosphere and use atomic parameters that represent an isolated Na\,{\sc i} D$_2$ line. For this case we show that the AD-PRD effects are significant for field strengths below about 30G, but that the computationally much less demanding approximation of angle-averaged (AA) PRD may be used for stronger fields.

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arXiv2020

Resonance Line Polarization in Spherically Symmetric Moving Media: a Parametric Study

In the present paper we consider the problem of resonance line polarization formed in the spherically symmetric expanding atmospheres. For the solution of the concerned polarized transfer equation we use the comoving frame formulation, and apply the Accelerated Lambda Iteration (ALI) method. We restrict ourselves to the non-relativistic regime of velocities wherein mainly Doppler shift effects are significant. For our studies, we consider the scattering on a two-level atom, including the effects of partial frequency redistribution (PFR). We present the dependence of linearly polarized profiles on different atmospheric and atomic parameters.

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arXiv2019

Polarized Line Formation in Arbitrary Strength Magnetic Fields: the case of a two-level atom with hyperfine structure splitting

Quantum interference effects together with partial frequency redistribution (PFR) in line scattering produce subtle signatures in the so called Second Solar Spectrum (the linearly polarized spectrum of the Sun). These signatures are modified in the presence of arbitrary strength magnetic fields via the Hanle, Zeeman, and Paschen-Back effects. In the present paper we solve the problem of polarized line formation in a magnetized atmosphere taking into account scattering in a two-level atom with hyperfine structure splitting together with PFR. To this end we incorporate the collisionless PFR matrix derived in Sowmya et al. (2014) in the polarized transfer equation. We apply the scattering expansion method to solve this transfer equation. We study the combined effects of PFR and Paschen-Back effect on polarized line profiles formed in an isothermal one-dimensional planar atmosphere. For this purpose, we consider the cases of D$_2$ lines of Li\,{\sc i} and Na\,{\sc i}.

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arXiv2015

Polarized scattering with Paschen-Back effect, hyperfine structure, and partial frequency redistribution in magnetized stellar atmospheres

$F$-state interference significantly modifies the polarization produced by scattering processes in the solar atmosphere. Its signature in the emergent Stokes spectrum in the absence of magnetic fields is depolarization in the line core. In the present paper, we derive the partial frequency redistribution (PRD) matrix that includes interference between the upper hyperfine structure states of a two-level atom in the presence of magnetic fields of arbitrary strengths. The theory is applied to the Na I D$_2$ line that is produced by the transition between the lower $J=1/2$ and upper $J=3/2$ states which split into $F$ states because of the coupling with the nuclear spin $I_s=3/2$. The properties of the PRD matrix for the single-scattering case is explored, in particular, the effects of the magnetic field in the Paschen--Back regime and their usefulness as a tool for the diagnostics of solar magnetic fields.

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arXiv2015

Polarized scattering of light for arbitrary magnetic fields with level-crossings from the combination of hyperfine and fine structure splittings

Interference between magnetic substates of the hyperfine structure states belonging to different fine structure states of the same term influences the polarization for some of the diagnostically important lines of the Sun's spectrum, like the sodium and lithium doublets. The polarization signatures of this combined interference contain information on the properties of the solar magnetic fields. Motivated by this, in the present paper, we study the problem of polarized scattering on a two-term atom with hyperfine structure by accounting for the partial redistribution in the photon frequencies arising due to the Doppler motions of the atoms. We consider the scattering atoms to be under the influence of a magnetic field of arbitrary strength and develop a formalism based on the Kramers--Heisenberg approach to calculate the scattering cross section for this process. We explore the rich polarization effects that arise from various level-crossings in the Paschen--Back regime in a single scattering case using the lithium atomic system as a concrete example that is relevant to the Sun.

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arXiv2015

Blend lines in the polarized spectrum of the Sun

Blend lines form an integral part of the theoretical analysis and modeling of the polarized spectrum of the Sun. Their interaction with other spectral lines needs to be explored and understood before we can properly use the main spectral lines to diagnose the Sun. They are known to cause a decrease in the polarization in the wings of the main line on which they superpose, or in the polarization of the continuum, when they are assumed to be formed either under the local thermodynamic equilibrium (LTE) conditions, or when their intrinsic polarizability factor is zero. In this paper, we describe the theoretical framework to include the blend lines formed under non-LTE conditions, in the radiative transfer equation, and the numerical techniques to solve it. The properties of a blend line having an intrinsic polarization of its own and its interaction with the main line are discussed. The results of our analysis show that the influence of the blend lines on the main spectral lines, though small in the present context, is important and needs to be considered when interpreting the polarized spectral lines in the second solar spectrum.

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arXiv2014

The role of quantum interference and partial redistribution in the solar Ba II D2 4554 A line

The Ba II D2 line at 4554 A 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 (PRD), which is shown to be essential to model this line. This is because complete frequency redistribution (CRD) 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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arXiv2014

Polarized light scattering with Paschen-Back effect, level-crossing of fine structure states and partial frequency redistribution

The quantum interference between the fine structure states of an atom modifies the shapes of the emergent Stokes profiles in the Second Solar Spectrum. This phenomenon has been studied in great detail both in the presence and absence of magnetic fields. By assuming a flat-spectrum for the incident radiation, the signatures of this effect have been explored for arbitrary field strengths. Even though the theory which takes into account the frequency dependence of the incident radiation is well developed, it is restricted to the regime in which the magnetic splitting is much smaller than the fine structure splitting. In the present paper, we carry out a generalization of our scattering matrix formalism including the effects of partial frequency redistribution (PRD) for arbitrary magnetic fields. We test the formalism using available benchmarks for special cases. In particular we apply it to the Li\,{\sc i} 6708\,Å D$_1$ and D$_2$ line system, for which observable effects from the Paschen-Back regime are expected in the Sun's spectrum.

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arXiv2013

An efficient decomposition technique to solve angle-dependent Hanle scattering problems

Hanle scattering is an important diagnostic tool to study weak solar magnetic fields. Partial frequency redistribution (PRD) is necessary to interpret the linear polarization observed in strong resonance lines. Usually angle-averaged PRD functions are used to analyze linear polarization. However it is established that angle-dependent PRD functions are often necessary to interpret polarization profiles formed in the presence of weak magnetic fields. Our aim is to present an efficient decomposition technique, and the numerical method to solve the concerned angle-dependent line transfer problem. Together with the standard Stokes decomposition technique we employ Fourier expansion over the outgoing azimuth angle to express in a more convenient form, the angle-dependent PRD function for the Hanle effect. It allows the use of angle-dependent frequency domains of Bommier to solve the Hanle transfer problem. Such an approach is self-consistent and accurate compared to a recent approach where angle-averaged frequency domains were used to solve the same problem. We show that it is necessary to incorporate angle-dependent frequency domains instead of angle-averaged frequency domains to solve the Hanle transfer problem accurately, especially for the Stokes U parameter. The importance of using angle-dependent domains has been highlighted by taking the example of Hanle effect in the case of line transfer with vertical magnetic fields in a slab atmosphere. We have also studied the case of polarized line formation when micro-turbulent magnetic fields are present. The difference between angle-averaged and angle-dependent solutions is enhanced by the presence of micro-turbulent fields.

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arXiv2013

Modeling the quantum interference signatures of the Ba II D2 4554 A line in the second solar spectrum

Quantum interference effects play a vital role in shaping the linear polarization profiles of solar spectral lines. The Ba II D2 line at 4554 A 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 D2 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 (CRD) alone fails to produce polarization profiles that have any resemblance with 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 D2 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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arXiv2012

Polarized line formation with J-state interference in the presence of magnetic fields: A heuristic treatment of collisional frequency redistribution

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 an L=0 to 1 to 0 scattering transition with S=1/2 (a hypothetical doublet centered at 5000 A and 5001 A). 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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arXiv2012

Polarized line transfer with F-state interference in a non-magnetic medium: Partial frequency redistribution effects in the collisionless regime

Quantum interference phenomena manifests itself 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 PRD 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 (HFS) of the upper J=3/2 and lower J=1/2 levels of a two-level atom model with nuclear spin I_s=3/2. We confine our attention to the non-magnetic scattering in the collisionless regime.

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arXiv2012

J-state interference signatures in the Second Solar Spectrum: Modeling the Cr I triplet at 5204-5208 A

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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arXiv2011

Observations of the forward scattering Hanle effect in the Ca i 4227 Å line

Chromospheric magnetic fields are notoriously diffcult 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 10830 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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