arXiv subjects

K. Nagaraju

Publications and source records attributed to K. Nagaraju.

18 recordsLinked to original sources

arXiv2025

The H$α$ line as a probe of chromospheric magnetic fields

We explore the diagnostic potential of the H$α$ line for probing the chromospheric magnetic field using a realistic 3D radiative magnetohydrodynamic (rMHD) model. The Stokes profiles of the H$α$ line are synthesized through full 3D radiative transfer under the field-free approximation, alongside the Ca II 8542 Å and Fe I 6173 Å lines for comparison. The line-of-sight (LOS) magnetic fields are inferred using the weak field approximation (WFA) for theH$α$ and Ca II 8542 Å lines, while the Fe I 6173 Å line is analyzed through Milne-Eddington inversion techniques. The comparison between the inferred LOS magnetic field maps and the magnetic fields in the rMHD model revealed that the H$α$ line core primarily probes the chromospheric magnetic field at log tau_500 = -5.7, which corresponds to higher layers than the Ca II 8542 Å line core, which is most sensitive to conditions at log tau_500 = -5.1. On average, the Stokes V profiles of the H$α$ line core form 500 km higher than those of the Ca II 8542 Å line core. The H$α$ polarization signals persist after adding noise, and with noise at the level of 10^-3 Ic, most simulated magnetic structures remain visible. These findings suggest that spectropolarimetric observations of the H$α$ line can provide complementary insights into the stratification of the magnetic field at higher altitudes, especially when recorded simultaneously with widely used chromospheric diagnostics such as the Ca II 8542 Å line.

astro-ph.SR↗

arXiv2025

Visible Emission Line Coronagraph (VELC) onboard ADITYA-L1

Aditya-L1, India's first dedicated mission to study the Sun and its atmosphere from the Sun-Earth Lagrangian L1 location was successfully launched on 2023 September 2. It carries seven payloads. The Visible Emission Line Coronagraph (VELC) is a major payload on Aditya-L1. VELC is designed to carry out imaging and spectroscopic observations (the latter in three emission lines of the corona), simultaneously. Images of the solar corona in the continuum at 5000 A, with a field of view (FoV) from 1.05 Ro to 3 Ro can be obtained at variable intervals depending on the data volume that can be downloaded. Spectroscopic observations of the solar corona in three emission lines, namely 5303 A Fexiv, 7892 A Fexi, and 10747 A Fexiii are possible simultaneously, with different exposure times and cadence. Four slits, each of width 50 um, separated by 3.75 mm help to simultaneously obtain spectra at four positions in the solar corona at all the aforementioned lines. A Linear Scan Mechanism (LSM) makes it possible to scan the solar corona up to +/-1.5 Ro. The instrument has the facility to carry out spectropolarimetric observations at 10747 A also in the FoV range 1.05 - 1.5 Ro. Various components of the instrument were tested interferometrically on the optical bench before installation. The individual components were aligned and performance of the payload was checked in the laboratory using a laser source and tungsten lamp. Wavelength calibration of the instrument was verified using Sun as a light source. All the detectors were calibrated for different parameters such as dark current and its variation with exposure time. Here, we discuss the various features of the VELC, alignment, calibration, performance, possible observations, initial data analysis and results of initial tests conducted in-orbit.

astro-ph.SR↗

arXiv2025

Test and Calibration of the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1

The Solar Ultraviolet Imaging Telescope (SUIT) on board the AdityaL1 mission observes the Sun in the 200-400 nm wavelength range. This paper presents the results of various on ground and on board tests and their comparison with the specifications. Moreover, we also present the scheme for data calibration. We demonstrate that the test results are compliant with the specified figures, except the spatial resolution. Such discrepancy will limit the photometric measurements only, at a scale of 2.2" instead of 1.4" as originally envisioned. The results obtained here show that SUIT observations open up a new window for solar observations.

astro-ph.IM↗

arXiv2024

EUV polarimetric diagnostics of the solar corona: the Hanle effect of Ne VIII 770 Å

Magnetic fields are the primary driver of the plasma thermodynamics in the upper solar atmosphere, especially in the corona. However, magnetic field measurements in the solar corona are sporadic, thereby limiting us from the complete understanding of physical processes occurring in the coronal plasma. In this paper, we explore the diagnostic potential of a coronal emission line in the extreme-ultraviolet (EUV), i.e., Ne VIII 770 Åto probe the coronal magnetic fields. We utilize 3D 'Magneto-hydrodynamic Algorithm outside a Sphere' (MAS) models as input to the FORWARD code to model polarization in Ne VIII line produced due to resonance scattering, and interpret its modification due to collisions and the magnetic fields through the Hanle effect. The polarization maps are synthesized both on the disk as well as off-the-limb. The variation of this polarization signal through the different phases of solar cycle 24 and the beginning phase of solar cycle 25 is studied in order to understand the magnetic diagnostic properties of this line owing to different physical conditions in the solar atmosphere. The detectability of the linear polarization signatures of the Hanle effect significantly improves with increasing solar activity, consistently with the increase in the magnetic field strength and the intensity of the mean solar brightness at these wavelengths. We finally discuss the signal-to-noise ratio (SNR) requirements by considering realistic instrument designs.

astro-ph.SR↗

arXiv2024

Simultaneous spectropolarimetric observations in the H$α$ and Ca II 8662 Å lines of an active region

We present spectropolarimetric observations of an active region recorded simultaneously in the H$α$ Ca II 8662 Å lines. The sunspot exhibits multiple structures, including a lightbridge and a region where Ca II 8662 Å line core is in emission. Correspondingly, the H$α$ line core image displays brightening in the emission region, with the spectral profiles showing elevated line cores. The stratification of the line-of-sight magnetic field is inferred through non-LTE multiline inversions of the Ca II 8662 Å line and the weak field approximation over the H$α$ line. The field strength inferred from the H$α$ line core is consistently smaller than that inferred from inversions at $\log τ_{500}$ = $-$4.5. However, the study finds no correlation between the WFA over the core of the H$α$ line and that inferred from inversions at $\log τ_{500}$ = $-$4.5. In regions exhibiting emission features, the morphology of the magnetic field at $\log τ_{500}$ = $-$4.5 resembles that at $\log τ_{500}$ = $-$1, with slightly higher or comparable field strengths. The magnetic field morphology inferred from the core of the H$α$ line is also similar to that inferred from the full spectral range of the H$α$ line in the emission region. The field strength inferred in the lightbridge at $\log τ_{500}$ = $-$1 is smaller than the surrounding umbral regions and comparable at $\log τ_{500}$ = $-$4.5. Similarly, the field strength inferred in the lightbridge from the WFA over the H$α$ line appears lower compared to the surrounding umbral regions.

astro-ph.SR↗

arXiv2023

Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1

ADITYA-L1 is India's first dedicated mission to observe the sun and its atmosphere from a halo orbit around L1 point. Visible emission line coronagraph (VELC) is the prime payload on board at Aditya-L1 to observe the sun's corona. VELC is designed as an internally occulted reflective coronagraph to meet the observational requirements of wide wavelength band and close to the solar limb (1.05 Ro). Images of the solar corona in continuum and spectra in three emission lines 5303Å [Fe xiv], 7892Å [Fe xi] and 10747 [Fe xiii] obtained with high cadence to be analyzed using software algorithms automatically. A reasonable part of observations will be made in synoptic mode, those, need to be analyzed and results made available for public use. The procedure involves the calibration of instrument and detectors, converting the images into fits format, correcting the images and spectra for the instrumental effects, align the images etc. Then, develop image processing algorithms to detect the occurrence of energetic events using continuum images. Also derive physical parameters, such as temperature and velocity structure of solar corona using emission line observations. Here, we describe the calibration of detectors and the development of software algorithms to detect the occurrence of CMEs and analyze the spectroscopic data.

astro-ph.SR↗

arXiv2023

Does H$α$ Stokes~$V$ profiles probe the chromospheric magnetic field? An observational perspective

We investigated the diagnostic potential of the Stokes $V$ profile of the H$α$ line to probe the chromospheric line-of-sight (LOS) magnetic field ($B_{\mathrm{LOS}}$) by comparing the $B_{\mathrm{LOS}}$ inferred from the weak field approximation (WFA) with that of inferred from the multi-line inversions of the Ca II 8542 Å, Si I 8536 Å and Fe I 8538 Å lines using the STiC inversion code. Simultaneous spectropolarimetric observations of a pore in the Ca II 8542 Å and H$α$ spectral lines obtained from the SPINOR at the Dunn Solar Telescope on the 4th of December, 2008 are used in this study. The WFA was applied on the Stokes $I$ and $V$ profiles of H$α$ line over three wavelength ranges viz.: around line core ($Δλ=\pm0.35$ Å), line wings ($Δλ=[-1.5, -0.6]$ and $[+0.6, +1.5]$ Å) and full spectral range of the line ($Δλ=\pm1.5$ Å) to derive the $B_{\mathrm{LOS}}$. We found the maximum $B_{\mathrm{LOS}}$ strengths of $\sim+800$ and $\sim+600$ G at $\logτ_{\mathrm{500}}$ = $-$1 and $-$4.5, respectively in the pore. The morphological map of the $B_{\mathrm{LOS}}$ inferred from the H$α$ line core is similar to the $B_{\mathrm{LOS}}$ map at $\logτ_{\mathrm{500}}$ = $-$4.5 inferred from multi-line inversions. The $B_{\mathrm{LOS}}$ map inferred from the H$α$ line wings and full spectral range have a similar morphological structure to the $B_{\mathrm{LOS}}$ map inferred at $\logτ_{\mathrm{500}}$ = $-$1. The $B_{\mathrm{LOS}}$ estimated from H$α$ using WFA is weaker by a factor of $\approx 0.53$ than that of inferred from the multi-line inversions.

astro-ph.SR↗

arXiv2023

Solar Mean Magnetic Field of the Chromosphere

The Solar Mean Magnetic Field (SMMF) is the mean value of the line of sight (LOS) component of the solar vector magnetic field averaged over the visible hemisphere of the Sun. So far, the studies on SMMF have mostly been confined to the magnetic field measurements at the photosphere. In this study, we calculate and analyse the SMMF using magnetic field measurements at the chromosphere, in conjunction with that of photospheric measurements. For this purpose, we have used full disk LOS magnetograms derived from spectropolarimetric observations carried out in Fe I 630.15 nm and Ca II 854.2 nm by the Synoptic Optical Long term Investigations of the Sun (SOLIS)/Vector Spectromagnetograph (VSM) instrument during 2010 to 2017. It is found from this study that the SMMF at the chromosphere is weaker by a factor of 0.60 compared to the SMMF at the upper photosphere. The correlation analysis between them gives a Pearson correlation coefficient of 0.80. The similarity and reduced intensity of the chromospheric SMMF with respect to the photospheric SMMF corroborate the idea that it is the source of the Interplanetary Magnetic Field (IMF).

astro-ph.SR↗

arXiv2022

Properties of shock waves in the quiet Sun chromosphere

Short-lived (100s or less), sub-arcsec to a couple of arcsec size features of enhanced brightenings in the narrowband images at the $\mathrm{H_{2V}}$ and $\mathrm{K_{2V}}$ positions of the Ca II H and K lines in the quiet Sun are known as bright grains. With simultaneous observations of a quiet Sun internetwork region in the Fe I 6173 Å, Ca II 8542 {AA}, and Ca II K lines acquired by the CRisp Imaging Spectro-Polarimeter and the CHROMospheric Imaging Spectrometer instruments on the Swedish 1-m Solar Telescope, we performed multi-line non-local thermodynamic equilibrium inversions using the STockholm inversion Code to infer the time-varying stratified atmosphere's physical properties such as the temperature, line-of-sight (LOS) velocity, and microturbulence. The Ca II K profiles of bright grains show enhancement in the $\mathrm{K_{2V}}$ peak intensities with absence of the $\mathrm{K_{2R}}$ features. At the time of maximum enhancement in the $\mathrm{K_{2V}}$ peak intensities, we found average enhancements in temperature at lower chromospheric layers (at $\logτ_{500}$ $\simeq$ $-$4.2) of about 1.1 kK with maximum enhancement of about 4.5 kK. These temperature enhancements are colocated with upflows, as strong as $-$6 $\mathrm{km\;s^{-1}}$, in the direction of the LOS. The LOS velocities at upper chromospheric layers at $\logτ_{500}$ < $-$4.2 show consistent downflows greater than $+$8 $\mathrm{km\;s^{-1}}$. The retrieved value of microturbulence in the atmosphere of bright grains is negligible at chromospheric layers. The study provides observational evidence to support the interpretation that the bright grains observed in narrowband images at the $\mathrm{H_{2V}}$ and $\mathrm{K_{2V}}$ positions of the Ca II H and K lines are manifestations of upward propagating acoustic shocks against a background of downflowing atmospheres.

astro-ph.SR↗

arXiv2022

Spectral lines in FUV and EUV for diagnosing coronal magnetic field

The diagnostic capabilities of spectral lines in far ultraviolet (FUV) and extreme ultraviolet (EUV) wavelength range are explored in terms of their Hanle and Zeeman sensitivity to probe vector magnetic field in the solar corona. The temperature range covered is log$_{10}(T)=5.5-6.3$. The circular polarization signal due to longitudinal Zeeman effect is estimated for spectral lines in the wavelength range of 500 to 1600 Å. The Stokes $V/I$ signal for a FUV line is found to be in the order of 10$^{-4}$ for a longitudinal field strength of 10 Gauss, which further reduces to 10$^{-5}$ for wavelengths below 1200 Å. Due to such low signals, the present study aims to find combination of spectral lines having different Hanle sensitivity but with identical peak formation temperature to probe coronal magnetic field vector. The combination of Hanle sensitive lines is better suited because the Hanle signals are stronger by at least an order of magnitude compared to Zeeman signals. The linear polarization signals due to Hanle effect from at least two spectral lines are required to derive information on the full vector. It is found from this study that there is always a pair of Hanle sensitive lines for a given temperature range suitable for probing coronal vector magnetic field and they are located in close proximity with each other in terms of their wavelength.

astro-ph.SR↗

arXiv2021

Spectropolarimeter on-board the Aditya-L1: Polarization Modulation and Demodulation

One of the major science goals of the Visible Emission Line Coronagraph (VELC) payload aboard the Aditya-L1 mission is to map the coronal magnetic field topology and the quantitative estimation of longitudinal magnetic field on routine basis. The infrared (IR) channel of VELC is equipped with a polarimeter to carry out full Stokes spectropolarimetric observations in the Fe XIII line at 1074.7~nm. The polarimeter is in dual-beam setup with continuously rotating waveplate as the polarization modulator. Detection of circular polarization due to Zeeman effect and depolarization of linear polarization in the presence of magnetic field due to saturated Hanle effect in the Fe~{\sc xiii} line require high signal-to-noise ratio (SNR). Due to limited number of photons, long integration times are expected to build the required SNR. In other words signal from a large number of modulation cycles are to be averaged to achieve the required SNR. This poses several difficulties. One of them is the increase in data volume and the other one is the change in modulation matrix in successive modulation cycles. The latter effect arises due to a mismatch between the retarder's rotation period and the length of the signal detection time in the case of VELC spectropolarimeter (VELC/SP). It is shown in this paper that by appropriately choosing the number of samples per half rotation the data volume can be optimized. A potential solution is suggested to account for modulation matrix variation from one cycle to the other.

astro-ph.IM↗

arXiv2018

Full Stokes polarimetry using Dual-Frequency Liquid Crystals

In a dual-frequency liquid crystal (DFLC), when the frequency of the applied voltage is more than a critical value ($f_c$), the dielectric anisotropy of the material changes from positive to negative. This causes the director to switch its orientation from parallel to the field (for $f < f_c$), to perpendicular to it ($f > f_c$). Hence DFLC can be used in modulating the light by switching the frequency of an externally applied voltage. We present in this work about application of DFLCs in full Stokes polarimetery. A polarization modulator has been worked out based on two DFLCs and two static retarders. The combination of DFLCs' switching and static retarders are chosen such that more or less equal weightage is given to all the Stokes parameters. Initial results on the optimization of position angles of the modulators are presented towards the goal of achieving polychromatic modulator in the wavelength range 600-900 nm.

astro-ph.IM↗

arXiv2018

On the variability of the solar mean magnetic field: contributions from various magnetic features on the surface of the Sun

The solar mean magnetic field (SMMF) is referred to as the disc-averaged line-of-sight (LOS) magnetic field that also reflects the polarity imbalance of the magnetic field on the Sun. The origin of the SMMF has been debated over the past few decades, with one school of thought suggesting that the contribution to the SMMF is mostly due to the large scale magnetic field structure, also termed as the background magnetic field, whereas other and more recent studies have indicated that active regions have a major contribution to the observed SMMF. In this paper, we re-investigate the issue of the origin of the SMMF by decomposing the solar disk into plages, networks, sunspots and background regions, thereby calculating the variation in the observed SMMF due to each of these features. We have used full disk images from SDO/{\it AIA} recorded at 1600~Å\, for earmarking plages, networks and background regions and 4500~Å\, images for separating the sunspots. The LOS fields corresponding to each of these regions are estimated from the co-temporal SDO/{\it HMI} full disk magnetograms. The temporal variation of the SMMF shows a near one-to-one correspondence with that of the background field regions suggesting that they constitute the major component of the observed SMMF. A linear regression analysis based on the coefficient of determination shows that the background field dominates, and accounts for 89\% of the variation in the SMMF whereas the magnetic field from the other features accounts for the rest 11\%.

astro-ph.SR↗

arXiv2016

High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter

Context. Remote sensing of weak and small-scale solar magnetic fields is of utmost relevance for a number of important open questions in solar physics. This requires the acquisition of spectropolarimetric data with high spatial resolution (0.1 arcsec) and low noise (1e-3 to 1e-5 of the continuum intensity). The main limitations to obtain these measurements from the ground, are the degradation of the image resolution produced by atmospheric seeing and the seeing-induced crosstalk (SIC). Aims. We introduce the prototype of the Fast Solar Polarimeter (FSP), a new ground-based, high-cadence polarimeter that tackles the above-mentioned limitations by producing data that are optimally suited for the application of post-facto image restoration, and by operating at a modulation frequency of 100 Hz to reduce SIC. Results. The pnCCD camera reaches 400 fps while keeping a high duty cycle (98.6 %) and very low noise (4.94 erms). The modulator is optimized to have high (> 80%) total polarimetric efficiency in the visible spectral range. This allows FSP to acquire 100 photon-noise-limited, full-Stokes measurements per second. We found that the seeing induced signals present in narrow-band, non-modulated, quiet-sun measurements are (a) lower than the noise (7e-5) after integrating 7.66 min, (b) lower than the noise (2.3e-4) after integrating 1.16 min and (c) slightly above the noise (4e-3) after restoring case (b) by means of a multi-object multi-frame blind deconvolution. In addition, we demonstrate that by using only narrow-band images (with low SNR of 13.9) of an active region, we can obtain one complete set of high-quality restored measurements about every 2 s.

astro-ph.IM↗

arXiv2015

Smear correction of highly-variable, frame-transfer-CCD images with application to polarimetry

Image smear, produced by the shutter-less operation of frame transfer CCD detectors, can be detrimental for many imaging applications. Existing algorithms used to numerically remove smear, do not contemplate cases where intensity levels change considerably between consecutive frame exposures. In this report we reformulate the smearing model to include specific variations of the sensor illumination. The corresponding desmearing expression and its noise properties are also presented and demonstrated in the context of fast imaging polarimetry.

astro-ph.IM↗

arXiv2012

Precision in ground based solar polarimetry: Simulating the role of adaptive optics

Accurate measurement of polarization in spectral lines is important for the reliable inference of magnetic fields on the Sun. For ground based observations, polarimetric precision is severely limited by the presence of Earth's atmosphere. Atmospheric turbulence (seeing) produces signal fluctuations which combined with the non-simultaneous nature of the measurement process cause intermixing of the Stokes parameters known as seeing induced polarization cross-talk. Previous analysis of this effect (Judge et al., 2004) suggests that cross-talk is reduced not only with increase in modulation frequency but also by compensating the seeing induced image aberrations by an Adaptive Optics (AO) system. However, in those studies the effect of higher order image aberrations than those corrected by the AO system was not taken into account. We present in this paper an analysis of seeing induced cross-talk in the presence of higher order image aberrations through numerical simulation. In this analysis we find that the amount of cross-talk among Stokes parameters is practically independent of the degree of image aberration corrected by an AO system. However, higher order AO corrections increase the signal-to-noise ratio by reducing the seeing caused image smearing.

physics.ins-det↗

arXiv2008

On the Weakening of Chromospheric Magnetic Field in Active Regions

Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields at the photosphere and chromosphere are presented. Fe I line at $\lambda6569$ and $H_α$ at $\lambda6563$ are used respectively for deriving the physical parameters at photospheric and chromospheric heights. The LOS magnetic field obtained through the center-of-gravity method show a linear relation between photospheric and chromospheric field for field strengths less than 700 G. But in strong field regions, the LOS magnetic field values derived from $H_α$ are much weaker than what one gets from the linear relationship and also from those expected from the extrapolation of the photospheric magnetic field. We discuss in detail the properties of magnetic field observed in $H_α$ from the point of view of observed velocity gradients. The bisector analysis of $H_α$ Stokes $I$ profiles show larger velocity gradients in those places where strong photospheric magnetic fields are observed. These observations may support the view that the stronger fields diverge faster with height compared to weaker fields.

astro-ph↗

arXiv2007

An efficient modulation scheme for dual beam polarimetry

An eight stage balanced modulation scheme for dual beam polarimetry is presented in this paper. The four Stokes parameters are weighted equally in all the eight stages of modulation resulting in total polarimetric efficiency of unity. The gain table error inherent in dual beam system is reduced by using the well known beam swapping technique. The wavelength dependent polarimetric efficiencies of Stokes parameters due to the chromatic nature of the waveplates are presented. The proposed modulation scheme produces better Stokes $Q$ and $V$ efficiencies for wavelengths larger than the design wavelength whereas Stokes $U$ has better efficiency in the shorter wavelength region. Calibration of the polarimeter installed as a backend instrument of the Kodaikanal Tower Telescope is presented. It is found through computer simulation that a 14% sky transparency variation during calibration of the polarimeter can introduce $\approx 1.8%$ uncertainty in the determination of its response matrix.

astro-ph↗