Measuring the linear polarization signal in extreme-ultraviolet (EUV) spectral lines, produced by the Hanle effect, offers a promising technique for studying magnetic fields in the solar corona. The required signal-to-noise ratio for detecting the Hanle polarization signals is on the order of 101 (off-limb) to 106 (disk center). Measuring such low signals in the photon starved observations demands highly efficient instruments. In this paper, we present the design of an instrument, SpectroPOLarimeter for Extreme-ultraviolet Observations (SPOLEO), which utilizes reflective components with suitable mirror coatings and thicknesses to minimize the throughput losses. We analyze the system performance within the spectral range from 740 to 800 & Aring;. The K-mirror-based polarimeter model provides a polarizing power of 20%-40% in this wavelength range. Based on the system throughput and polarizing power, we discuss various possibilities for achieving the required signal-to-noise ratio, along with their limitations. Due to lack of facilities for fabrication and testing in the EUV, we have calibrated a prototype of the reflection-based polarimeter setup in the laboratory at the visible wavelength of 700 nm. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

We present measurements of seeing-induced crosstalk using spectropolarimetric observations of sunspots recorded simultaneously in the H alpha and Ca II 8662 & Aring; lines with the Kodaikanal Tower Tunnel (KTT) telescope. The Kodaikanal Tower Tunnel telescope is integrated and installed with an image stabilization system consisting of a tip-tilt and an autoguider system. Additionally, the spectropolarimeter at KTT is upgraded to allow for the simultaneous recording of spectropolarimetric observations in three spectral lines. The tip-tilt system is shown to have a cutoff frequency of 80 Hz, effectively reducing the seeing induced crosstalk in the measured Stokes parameters by at least a factor of 2. (c) 2024 Optica Publishing Group

We present spectropolarimetric observations of an active region recorded simultaneously in the H alpha and the Ca ii 8662 & Aring; lines. The sunspot exhibits multiple structures, including a light bridge and a region where the Ca ii 8662 & Aring; line core is in emission. Correspondingly, the H alpha 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 nonlocal thermodynamic equilibrium multiline inversions of the Ca ii 8662 & Aring; line and the weak field approximation (WFA) over the H alpha line. The field strength inferred from the H alpha line core is consistently smaller than that inferred from inversions at log tau(500) = -4.5. However, the study finds no correlation between the WFA over the core of the H alpha line and that inferred from inversions at log tau(500) = -4.5. In regions exhibiting emission features, the morphology of the magnetic field at log tau(500) = -4.5 resembles that at log tau(500) = -1, with slightly higher or comparable field strengths. The magnetic field morphology inferred from the core of the H alpha line is also similar to that inferred from the full spectral range of the H alpha line in the emission region. The field strength inferred in the light bridge at log tau(500) = -1 is smaller than the surrounding umbral regions and comparable at log tau(500) = -4.5. Similarly, the field strength inferred in the light bridge from the WFA over the H alpha line appears lower compared to the surrounding umbral regions.

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 6301.5 Å and Ca ii 8542 Å by the Synoptic Optical Long-term Investigations of the Sun (SOLIS)/Vector Spectromagnetograph (VSM) instrument during 2010–2017. 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).

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 LOS ) by comparing the B LOS inferred from the weak field approximation (WFA) with that inferred from the multiline 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 2008 December 4 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 (Δ λ = ±0.35 Å), line wings (Δ λ = [−1.5, −0.6], and [+0.6, +1.5] Å), and full spectral range of the line (Δ λ = ± 1.5 Å) to derive the B LOS . We found the maximum B LOS strengths of ∼+800 and ∼+600 G at log τ 500 = −1 and −4.5, respectively, in the pore. The morphological map of the B LOS inferred from the H α line core is similar to the B LOS map at log τ 500 = −4.5 inferred from multiline inversions. The B LOS map inferred from the H α line wings and full spectral range have a similar morphological structure to the B LOS map inferred at log τ 500 = −1. The B LOS estimated from H α using WFA is weaker by a factor of ≈0.53 than that of inferred from the multiline inversions.

The Kodaikanal Tower Tunnel Telescope (KTT) is a three-mirror Coelostat-based Solar telescope. An inherent variable image drift is present in the KTT which is detrimental to observations particularly to slit-based spectroscopic and spectropolarimetric observations. An image Auto Guider system has recently been installed at KTT to arrest the image drift based on sensing the image limb. The image’s limb position on the image plane is sensed using a two-segment photodiode sensor in North–South and East–West directions. Feedback is given to the second mirror of Coelostat, which tilts, causing the image to move on the image plane. This paper discusses instrumental setup, calibration, and testing of the Auto Guider system at KTT. The Auto Guider system is robust and capable of arresting image drifts up to 55[Formula: see text]arcsec[Formula: see text][Formula: see text], which is much higher than the typically observed drifts (maximum 7[Formula: see text]arcsec[Formula: see text][Formula: see text]) at KTT with a precision of [Formula: see text][Formula: see text]arcsec.

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 {\AA}, 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 $\log\tau_{500}$ $\simeq$ $-$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 $\log\tau_{500}$ < $-$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.

High time cadence Spectro Polarimetry allows the feasibility of studying magnetic field evolution coupled with the plasma flows. Such a high cadence solar spectropolarimetry if developed will allow one to study magnetic field evolution in eruptive processes like solar flares, prominence eruptions, etc. A single shot solar spectroscopy was recently demonstrated at Multi Application Solar Telescope (MAST) at Udaipur Solar Observatory. The snapshot spectroscopy is performed by sampling the pupil plane using the lenslet array to get multiple images of the field of view (FOV), which are then collimated and the collimated beam is made to pass through an FP Etalon in collimated mode. As the distance from the FP axis increases, the peak transmitted wavelength shift towards the bluer side. Using a prefilter with a full width half maximum (FWHM) less than the free spectral range (FSR) of FP, combined with an imaging lens, we can get multiple images of FOV on image plane with a blue shift in spectra as we move radially outwards from the optical axis.

Stuart J. Mumford∗1, 2, 3, Nabil Freij4, Steven Christe5, Jack Ireland5, Florian Mayer6, V. Keith Hughitt7, Albert Y. Shih5, Daniel F. Ryan8, 5, Simon Liedtke6, David Pérez-Suárez9, Pritish Chakraborty10, Vishnunarayan K I.6, Andrew Inglis11, Punyaslok Pattnaik12, Brigitta Sipőcz13, Rishabh Sharma6, Andrew Leonard3, David Stansby14, Russell Hewett15, Alex Hamilton6, Laura Hayes5, Asish Panda6, Matt Earnshaw6, Nitin Choudhary16, Ankit Kumar6, Prateek Chanda17, Md Akramul Haque18, Michael S Kirk11, Michael Mueller6, Sudarshan Konge6, Rajul Srivastava6, Yash Jain19, Samuel Bennett6, Ankit Baruah6, Will Barnes20, Michael Charlton6, Shane Maloney21, Nicky Chorley22, Himanshu6, Sanskar Modi6, James Paul Mason6, Naman96396, Jose Ivan Campos Rozo23, Larry Manley6, Agneet Chatterjee24, John Evans6, Michael Malocha6, Monica G. Bobra25, Sourav Ghosh24, Airmansmith976, Dominik Stańczak26, Ruben De Visscher6, Shresth Verma27, Ankit Agrawal6, Dumindu Buddhika6, Swapnil Sharma6, Jongyeob Park28, Matt Bates6, Dhruv Goel6, Garrison Taylor29, Goran Cetusic6, Jacob6, Mateo Inchaurrandieta6, Sally Dacie30, Sanjeev Dubey6, Deepankar Sharma6, Erik M. Bray6, Jai Ram Rideout31, Serge Zahniy5, Tomas Meszaros6, Abhigyan Bose6, André Chicrala32, Ankit6, Chloé Guennou6, Daniel D’Avella6, Daniel Williams33, Jordan Ballew6, Nick Murphy34, Priyank Lodha6, Thomas Robitaille6, Yash Krishan6, Andrew Hill6, Arthur Eigenbrot35, Benjamin Mampaey36, Bernhard M. Wiedemann6, Carlos Molina6, Duygu Keşkek6, Ishtyaq Habib6, Joseph Letts6, Juanjo Bazán37, Quinn Arbolante38, Reid Gomillion6, Yash Kothari6, Yash Sharma6, Abigail L. Stevens39, 40, Adrian Price-Whelan41, Ambar Mehrotra6, Arseniy Kustov6, Brandon Stone6, Trung Kien Dang42, Emmanuel Arias6, Fionnlagh Mackenzie Dover1, Freek Verstringe36, Gulshan Kumar43, Harsh Mathur44, Igor Babuschkin6, Jaylen Wimbish6, Juan Camilo Buitrago-Casas6, Kalpesh Krishna45, Kaustubh Hiware46, Manas Mangaonkar6, Matthew Mendero6, Mickaël Schoentgen6, Norbert G Gyenge47, Ole Streicher48, Rajasekhar Reddy Mekala6, Rishabh Mishra6, Shashank Srikanth43, Sarthak Jain6, Tannmay Yadav49, Tessa D. Wilkinson6, Tiago M. D. Pereira50, 51, Yudhik Agrawal12, jamescalixto6, yasintoda6, and Sophie A. Murray52