arXiv subjects

Soham Dey

Publications and source records attributed to Soham Dey.

7 recordsLinked to original sources

arXiv2026

Coronal Magnetography using Spectropolarimetry with SKA Telescopes

The solar coronal magnetic field drives nearly every aspect of solar phenomena and activity -- from flares, coronal mass ejections, and solar wind that governs space weather to the much weaker nanoflares. These magnetic fields are routinely measured at the visible surface of the Sun, the photosphere. However, detailed and direct measurements of the magnetic fields in the solar atmosphere, particularly in the coronal layer, have remained rather limited. Mostly, these are estimated from vector magnetic field measurements at photospheric heights through different extrapolation models. In the case of the corona, these extrapolations lack observational constraints from the corona, especially during periods of intense activity when magnetic structures evolve rapidly. Measurements of coronal magnetic fields from observations, therefore, remain one of the most crucial and unresolved challenges in solar and space-weather research. Radio observations of the Sun hold considerable potential in this regard. Observations of diverse emission mechanisms, ranging from plasma emissions at lower frequencies to thermal Bremsstrahlung and gyro-resonance at higher frequencies, provide multiple avenues to probe the coronal magnetic fields, unique at radio wavelengths. SKAO, with its broad frequency coverage (0.05 to 15 GHz), will allow us to probe wide range of coronal layers through unprecedented high-fidelity polarimetric imaging at high temporal, spectral, and spatial resolutions. This chapter details how the coronal magnetic field measurements can be achieved through spectro-polarimetric imaging of the Sun with the SKAO.

astro-ph.SR↗

arXiv2026

State-of-the-art Observation, Calibration, and Imaging Framework for Solar and Heliospheric Sciences with SKA

The Sun is a surprisingly difficult radio source to observe and image, even with the SKA. It is multiple orders brighter than the typical radio sources, which sensitive radio telescopes like SKA are optimized for. So, configuring the signal chain to enable solar observations while maintaining linearity is the very first non-standard requirement to be met. Next, solar radio emission spans an impressive range along every single phase-space parameter that can be used to describe it -- time scales from solar cycles to millisecond; spectral scales from smooth thermal emission to $\sim$100 kHz coherent emission; brightness temperatures from $10^4$ K for gyrosynchrotron emissions to $10^{13}$ K for bright type-III bursts; fractional polarizations from less than 1\% to nearly 100\%; and angular scales extending beyond a degree. Capturing the dynamics in solar radio emission in their full glory requires, on the one hand, that all the data that goes into making an image be acquired over very short temporal and spectral spans and, on the other, also imposes requirements for very high imaging dynamic range with high polarization purity. Extracting the information at the requisite temporal and spectral scales from SKA data will require a spectropolarimetric snapshot capability with high dynamic range and fidelity. Additionally, some of the most interesting insights into solar physics and space weather come from studying solar activity, which remains inherently unpredictable. This chapter discusses the various considerations that need to be addressed to help realize the promise of solar and heliospheric science from SKA.

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arXiv2025

STORMY : A Real-time Triggering Framework using Yamagawa Solar Spectrograph for Active Solar Emission Observations with the MWA

Some of the most interesting insights into solar physics and space weather come from studying radio emissions associated with solar activity, which remain inherently unpredictable. Hence, a real-time triggering system is needed for solar observations with the versatile new-generation radio telescopes to efficiently capture these episodes of solar activity with the precious and limited solar observing time. We have developed such a system, Solar Triggered Observations of Radio bursts using MWA and Yamagawa (STROMY) for the Murchison Widefield Array (MWA), the precursor for the low frequency telescope of upcoming Square Kilometre Array Observatory (SKAO). It is based on near-real-time data from the Yamagawa solar spectrograph, located at a similar longitude to the MWA. We have devised, implemented, and tested algorithms to perform an effective denoising of the data to identify signatures of solar activity in the Yamagawa data in near real-time. End-to-end tests of triggered observations have been successfully carried out at the MWA. STORMY is operational at the MWA for the routine solar observations, a timely development in the view of the ongoing solar maximum. We present this new observing framework and discuss how it can enable efficient capturing of event-rich solar data with existing instruments, like the LOw Frequency ARray (LOFAR), Owens Valley Radio Observatory - Long Wavelength Array (OVRO-LWA) etc., and pave the way for triggered observing with the SKAO, especially the SKA-Low.

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arXiv2025

An automated, self-calibration based pipeline for high-fidelity solar imaging with LOFAR: SIMPL

The LOw Frequency ARray (LOFAR) is capable of imaging spectroscopy of the Sun in the 10-240 MHz frequency range, with high spectral, temporal, and spatial resolution. However, the complex and rapidly varying nature of solar radio emission - spanning several orders of magnitude in brightness further exacerbated by the strong ionospheric phase distortions during daytime observations, poses major challenges for calibration, imaging, and automation. We aim to develop a fully automated, high-fidelity imaging pipeline optimized for LOFAR solar observations, capable of handling the intrinsic variability of solar emission and producing science-ready images with minimal human intervention. We have built the Solar Imaging Pipeline for LOFAR (SIMPL), which integrates excision of radio-frequency interference (RFI) for the solar-specific scenarios, calibration strategies, and self-calibration. The pipeline is designed to enable scalable and uniform processing of large archival datasets. SIMPL achieves more than an order-of-magnitude improvement in imaging dynamic range compared to previous efforts and reliably produces high-quality spectroscopic snapshot images. It has been tested across a wide range of solar conditions. It is currently being employed to process a decade of LOFAR solar observations, providing science-ready FITS images for the community and enabling both comprehensive and novel studies of solar radio phenomena -- ranging from quiet Sun emission and faint non-thermal features to active regions and their associated dynamic events, such as transient bursts.

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arXiv2025

First Robust Detection of Linear Polarization from Metric Solar Emissions: Challenging Established Paradigms

Polarimetric radio observations of the Sun can provide rich information about emission mechanisms and the propagation medium. For the past five decades, solar polarimetric studies at low radio frequencies have almost always assumed the absence of linear polarization. This has been based on the expectations from coronal propagation effects. Here we present the first robust evidence of linear polarization from solar emissions at meter wavelengths using simultaneous measurements with two telescopes of very different designs separated by thousands of kilometers - the Murchison Widefield Array and the upgraded Giant Metrewave Radio Telescope. Both datasets show consistent linear polarization fractions, confirming this detection. Rapid changes in morphology, as well as the fractional linear polarization at small time and frequency spans, further rule out any possibilities of an instrumental origin. Assuming the absence of linear polarization in solar radio emissions can result in incorrect interpretation of solar observations as well as those of other flare stars, which are often guided by learnings from solar studies. This discovery highlights the need for relaxing this assumption, and is essential for precise estimation of polarization signatures, ultimately leading to a better understanding of the plasma conditions in the Sun and other stars.

astro-ph.SR↗

arXiv2025

A Formalism for Calibrating the Instrumental Polarization of Radio Interferometric Arrays at Meter Wavelengths using Unpolarized Sky: A Demonstration using the MWA Observations

Calibration of instrumental polarization is critical for measuring polarized radio emissions from astrophysical sources to extract the magnetic field information in astrophysical, heliospheric, and terrestrial plasmas. At meter wavelengths, calibration of radio polarimetric observations is particularly challenging because of the scarcity of bright polarized sources due to significant Faraday depolarization. Here, we present a novel formalism for polarization calibration using an unpolarized sky model. The formalism is specifically designed for wide-field, low-frequency instruments like the Murchison Widefield Array (MWA), the LOw Frequency ARray (LOFAR), New Extension in Nan\c{c}ay Upgrading LoFAR (NenuFAR), Owens Valley Radio Observatory - Long Wavelength Array (OVRO-LWA), low-frequency telescope of the Square Kilometre Array Observatory (SKAO-low), etc. By leveraging the apparent polarization of the unpolarized sky induced by the polarized primary beam of the radio telescope, this method avoids dependence on bright polarized calibrators. It is also immune to ionospheric Faraday rotation. The validation of the approach via MWA observations confirms the accuracy of the method. This formalism provides a robust framework for low-frequency polarization calibration. It addresses the longstanding calibration challenges and advances the field of low-frequency polarimetry by enabling polarization studies of astrophysical radio sources.

astro-ph.IM↗

arXiv2024

New Insights into Type-I Solar Noise Storms from High Angular Resolution Spectroscopic Imaging with the Upgraded Giant Metrewave Radio Telescope

Type-I solar noise storms are perhaps the most commonly observed active radio emissions from the Sun at meter-wavelengths. Noise storms have a long-lived and wideband continuum background with superposed islands of much brighter narrowband and short-lived emissions, known as type-I bursts. There is a serious paucity of studies focusing on the morphology of these two types of emissions, primarily because of the belief that coronal scattering will always wash out any features at small angular scales. However, it is important to investigate their spatial structures in detail to make a spatio-temporal connection with observations at extreme-ultraviolet/ X-ray bands to understand the detailed nature of these emissions. In this work, we use high angular resolution observations from the upgraded Giant Metrewave Radio Telescope to demonstrate that it is possible to detect structures with angular scales as small as $\sim 9\arcsec$, about three times smaller than the smallest structure reported to date from noise storms. Our observations also suggest while the individual type-I bursts are narrowband in nature, the bursts are probably caused by traveling disturbance(s) inducing magnetic reconnections at different coronal heights, and thus leading to correlated change in the morphology of the type-I bursts observed at a wide range of frequencies.

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