Author name code: smitha ADS astronomy entries on 2022-09-14 =author:"Smitha, H.N." ------------------------------------------------------------------------ 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. Bibcode: 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.
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.
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.
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.
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. 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. Bibcode: 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.
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.
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.
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).
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. Title: Solar Photosphere Authors: Chitta, L. P.; Smitha, H. N.; Solanki, S. K. Bibcode: 2020orep.bookE...1C Altcode: No abstract at ADS 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. Bibcode: 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.
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 (vLOS), 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.
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.
Results: Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in vLOS 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.
Conclusions: We provide an overview of how neglecting NLTE effects influences the values of T, vLOS, 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. Title: Using the infrared iron lines to probe solar subsurface convection Authors: Milić, I.; Smitha, H. N.; Lagg, A. Bibcode: 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.
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.
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.
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.
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. Title: Observations of solar chromospheric heating at sub-arcsec spatial resolution Authors: Smitha, H. N.; Chitta, L. P.; Wiegelmann, T.; Solanki, S. K. Bibcode: 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.

The movie associated to Fig. 2 is available at https://www.aanda.org/ Title: Probing photospheric magnetic fields with new spectral line pairs Authors: Smitha, H. N.; Solanki, S. K. Bibcode: 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.
Aims: The aim of the present work is to identify new line pairs useful for the MLR technique and to test their reliability.
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.
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.
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 τ5000 = 1), both the new line pairs are well suited for probing magnetic fields in the lower photosphere. 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. Bibcode: 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 × 1014 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}15{--}{10}18 Mx, we measure an FER of 1100 {Mx} {{cm}}-2 {{day}}-1. The smaller features with fluxes ≤slant {10}16 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. Title: Flux emergence rate in the quiet Sun from Sunrise data Authors: Smitha, H. N.; Anusha, L. S.; Solanki, S. K.; Riethmüller, T. L. Bibcode: 2017psio.confE.106S Altcode: No abstract at ADS 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. Bibcode: 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. Title: A revisit to model the Cr i triplet at 5204-5208 Å and the Ba ii D2 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. Bibcode: 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 D2 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 D2, 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. Title: The Role of Quantum Interference and Partial Redistribution in the Solar Ba <font size=2>II D2 4554 Å Line Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M. Bibcode: 2014ASPC..489..213S Altcode: 2014arXiv1409.0465S The Ba <font size=2>II D2 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. 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. Bibcode: 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 D2 and Ba II D2, 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. 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. Bibcode: 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. Title: Modeling the Quantum Interference Signatures of the Ba II D2 4554 Å Line in the Second Solar Spectrum Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M. Bibcode: 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 D2 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 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 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 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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 Is = 3/2. We confine our attention to the non-magnetic scattering in the collisionless regime. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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.
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.
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.
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. 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. Bibcode: 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. Bibcode: 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. 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. Bibcode: 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.