arXiv subjects

Surajit Mondal

Publications and source records attributed to Surajit Mondal.

At least 55 records · Page 3Linked to original sources

arXiv2023

Space Weather Research using Spectropolarimetric Radio Imaging Combined With Aditya-L1 and PUNCH Missions

Low-frequency radio observations have been expected to serve as a powerful tool for Space Weather (SW) observations for decades. Radio observations are sensitive to a wide range of SW-related observations ranging from emissions from coronal mass ejections (CMEs) to the solar wind. Ground-based radio observatories allow one gathering of high-sensitivity data at high time and spectral resolution for an extended period, which remains a challenge for most space-based observatories. While radio techniques like Interplanetary Scintillation (IPS) are well established, radio imaging studies have remained technically challenging. This is now changing with the confluence of data from instruments, like the Murchison Widefield Array (MWA), and robust unsupervised analysis pipelines. This pipeline delivers full Stokes radio images with unprecedented fidelity and dynamic range. This will serve as a powerful tool for coronal and heliospheric studies. We present the recent developments and achievements to measure the magnetic fields of the CME plasma and shock front at coronal heights and also share the current status of the objective to measure the heliospheric Faraday rotation towards numerous background linearly polarised radio sources with the Sun in the field of view. We envision that in the coming years, the availability of new-generation radio instruments combined with the Aditya-L1 and PUNCH mission will mark the start of a new era in Space Weather modeling and prediction.

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arXiv2023

Radio Studies of the Middle Corona: Current State and New Prospects in the Next Decade

The "middle corona," defined by West et al. (2022) as the region between ~1.5-6 solar radii, is a critical transition region that connects the highly structured lower corona to the outer corona where the magnetic field becomes predominantly radial. At radio wavelengths, remote-sensing of the middle corona falls in the meter-decameter wavelength range where a critical transition of radio emission mechanisms occurs. In addition, plasma properties of the middle corona can be probed by trans-coronal radio propagation methods including radio scintillation and Faraday rotation techniques. Together they offer a wealth of diagnostic tools for the middle corona, complementing current and planned missions at other wavelengths. These diagnostics include unique means for detecting and measuring the magnetic field and energetic electrons associated with coronal mass ejections, mapping coronal shocks and electron beam trajectories, as well as constraining the plasma density, magnetic field, and turbulence of the "young" solar wind. Following a brief overview of pertinent radio diagnostic methods, this white paper will discuss the current state of radio studies on the middle corona, challenges to obtaining a more comprehensive picture, and recommend an outlook in the next decade. Our specific recommendations for advancing the middle coronal sciences from the radio perspective are: (1) Prioritizing solar-dedicated radio facilities in the ~0.1-1 GHz range with broadband, high-dynamic-range imaging spectropolarimetry capabilities. (2) Developing facilities and techniques to perform multi-perspective, multiple lines-of-sight trans-coronal radio Faraday Rotation measurements.

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arXiv2023

Radio Imaging Spectropolarimetry of CMEs and CME Progenitors

Coronal mass ejections (CMEs) are the most important drivers of space weather. Central to most CMEs is thought to be the eruption of a bundle of highly twisted magnetic field lines known as magnetic flux ropes. A comprehensive understanding of CMEs and their impacts hence requires detailed observations of physical parameters that lead to the formation, destabilization, and eventual eruption of the magnetic flux ropes. Recent advances in remote-sensing observations of coronal cavities, filament channels, sigmoids, EUV "hot channels," white light CMEs, and in situ observations of magnetic clouds points to the possibility of significant progress in understanding CMEs. In this white paper, we provide a brief overview of the potential of radio diagnostics for CMEs and CME progenitors, with a particular focus on the unique means for constraining their magnetic field and energetic electron population. Using synthetic observations based on realistic 3D MHD models, we also demonstrate the transformative potential of advancing such diagnostics by using broadband radio imaging spectropolarimetry with a high image dynamic range and high image fidelity. To achieve this goal, a solar-dedicated radio facility with such capabilities is recommended for implementation in the coming decade.

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arXiv2023

Quantifying Energy Release in Solar Flares and Solar Eruptive Events: New Frontiers with a Next-Generation Solar Radio Facility

Solar flares and the often associated solar eruptive events serve as an outstanding laboratory to study the magnetic reconnection and the associated energy release and conversion processes under plasma conditions difficult to reproduce in the laboratory, and with considerable spatiotemporal details not possible elsewhere in the universe. In the past decade, thanks to advances in multi-wavelength imaging spectroscopy, as well as developments in theories and numerical modeling, significant progress has been made in improving our understanding of solar flare/eruption energy release. In particular, broadband imaging spectroscopy at microwave wavelengths offered by the Expanded Owens Valley Solar Array (EOVSA) has enabled the revolutionary capability of measuring the time-evolving coronal magnetic fields at or near the flare reconnection region. However, owing to EOVSA's limited dynamic range, imaging fidelity, and angular resolution, such measurements can only be done in a region around the brightest source(s) where the signal-to-noise is sufficiently large. In this white paper, after a brief introduction to the outstanding questions and challenges pertinent to magnetic energy release in solar flares and eruptions, we will demonstrate how a next-generation radio facility with many (~100-200) antenna elements can bring the next revolution by enabling high dynamic range, high fidelity broadband imaging spectropolarimetry along with a sub-second time resolution and arcsecond-level angular resolution. We recommend to prioritize the implementation of such a ground-based instrument within this decade. We also call for facilitating multi-wavelength, multi-messenger observations and advanced numerical modeling in order to achieve a comprehensive understanding of the "system science" of solar flares and eruptions.

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arXiv2022

Study of radio transients from the quiet Sun during an extremely quiet time

In this work we study a class of recently discovered metrewave solar transients referred to as Weak Impulsive Narrowband Quiet Sun Emission \citep[WINQSEs,][]{mondal2020}. Their strength is a few percent of the quiet Sun background and are characterised by their very impulsive, narrow-band and ubiquitous presence in quiet Sun regions. \citet{mondal2020} hypothesised that these emissions might be the radio counterparts of the nanoflares and their potential significance warrants detailed studies. Here we present an analysis of data from an extremely quiet time and with an improved methodology over the previous work. As before, we detect numerous WINQSEs, which we have used for their further characterisation. Their key properties, namely, their impulsive nature and ubiquitous presence on the quiet Sun are observed in these data as well. Interestingly, we also find some of the observed properties to differ significantly from the earlier work. With this demonstration of routine detection of WINQSEs, we hope to engender interest in the larger community to build a deeper understanding of WINQSEs.

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arXiv2022

Preparing for Solar and Heliospheric Science with the SKAO: An Indian Perspective

The Square Kilometre Array Observatory (SKAO) is perhaps the most ambitious radio telescope envisaged yet. It will enable unprecedented studies of the Sun, the corona and the heliosphere and help to answer many of the outstanding questions in these areas. Its ability to make a vast previously unexplored phase space accessible, also promises a large discovery potential. The Indian solar and heliospheric physics community have been preparing for this science opportunity. A significant part of this effort has been towards playing a leading role in pursuing science with SKAO precursor instruments. This article briefly summarises the current status of the various aspects of work done as a part of this enterprise and our future goals.

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arXiv2022

A novel algorithm for high fidelity spectro-polarimetric snapshot imaging of the low-frequency radio Sun using SKA-low precursor

Magnetic field couples the solar interior to the solar atmosphere, known as the corona. The coronal magnetic field is one of the crucial parameters which determines the coronal structures and regulates the space weather phenomena like flares, coronal mass ejections, energetic particle events, and solar winds. Measuring the magnetic field at middle and higher coronal heights are extremely difficult problem and to date there is no single measurement technique available to measure the higher coronal magnetic fields routinely. polarization measurements of the low-frequency radio emissions are an ideal tool to probe the coronal magnetic fields at higher coronal heights ($>1R_\odot$). To date, most of the low-frequency polarization observations of the Sun were limited to bright solar radio bursts. Here we developed a novel algorithm for performing precise polarization calibration of the solar observations done with the Murchison Widefield Array, a future Square Kilometer Array (SKA) precursor. We have brought down the instrumental polarization $<1\%$. We anticipate this method will allow us to detect very low-level polarised emissions from coronal thermal emissions, which will become a tool for routine measurements of the global coronal magnetic fields at higher coronal heights. This method can be easily adapted for future SKA and open a window of new discoveries using high fidelity spectro-polarimetric snapshot imaging of the Sun at low radio frequencies.

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arXiv2022

Tackling the Unique Challenges of Low-frequency Solar Polarimetry with the Square Kilometre Array Low Precursor: The Algorithm

Coronal magnetic fields are well known to be one of the crucial parameters defining coronal physics and space weather. However, measuring the global coronal magnetic fields remains challenging. The polarization properties of coronal radio emissions are sensitive to coronal magnetic fields. While they can prove to be useful probes of coronal and heliospheric magnetic fields, their usage has been limited by technical and algorithmic challenges. We present a robust algorithm for precise polarization calibration and imaging of low-radio frequency solar observations and demonstrate it on data from the Murchison Widefield Array, a Square Kilometer Array (SKA) precursor. This algorithm is based on the Measurement Equation framework, which forms the basis of all modern radio interferometric calibration and imaging. It delivers high dynamic range and fidelity full Stokes solar radio images with instrumental polarization leakages $<1\%$, on par with general astronomical radio imaging, and represents the state-of-the-art. Opening up this rewarding, yet unexplored, phase space will enable multiple novel science investigations and offer considerable discovery potential. Examples include detection of low-level of circularly polarization from thermal coronal emission to estimate large-scale quiescent coronal fields; polarization of faint gyrosynchrotron emissions from coronal mass ejections for robust estimation of plasma parameters; and detection of the first-ever linear polarization at these frequencies. This method has been developed with the SKA in mind and will enable a new era of high fidelity spectro-polarimetric snapshot solar imaging at low-radio frequencies.

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arXiv2022

Robust absolute solar flux density calibration for the Murchison Widefield Array

Sensitive radio instruments are optimized for observing faint astronomical sources, and usually need to attenuate the received signal when observing the Sun. There are only a handful of flux density calibrators which can comfortably be observed with the same attenuation setup as the Sun. Additionally, for wide field-of-view (FoV) instruments like the Murchison Widefield Array (MWA) calibrator observations are generally done when the Sun is below the horizon to avoid the contamination from solar emissions. These considerations imply that the usual radio interferometric approach to flux density calibration is not applicable for solar imaging. A novel technique, relying on a good sky model and detailed characterization of the MWA hardware, was developed for solar flux density calibration for MWA. Though successful, this technique is not general enough to be extended to the data from the extended configuration of the MWA Phase II. Here, we present a robust flux density calibration method for solar observations with MWA independent of the array configuration. We use different approaches -- the serendipitous presence of strong sources; detection of numerous background sources using high dynamic range images in the FoV along with the Sun and observations of strong flux density calibrators with and without the additional attenuation used for solar observations; to obtain the flux scaling parameters required for the flux density calibration. Using the present method, we have achieved an absolute flux density uncertainty $\sim10\%$ for solar observations even in the absence of dedicated calibrator observations.

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arXiv2021

A search for counterparts of quiet sun radio transients in extreme ultraviolet data

Recently nonthermal radio transients from the quiet sun have been discovered and it has been hypothesised using rough calculations that they might be important for coronal heating. It is well realized that energy calculations using coherent emissions are often subject to poorly constrained parameters and hence have large uncertainties associated with them. However energy estimates using observations in the extreme ultraviolet (EUV) and soft X-ray bands are routinely done and the techniques are pretty well established. This work presents our first attempt to identify the EUV counterparts of these radio transients and then use the counterpart to estimate the energy deposited into the corona during the event. I show that the group of radio transients studied here was associated with an brightening observed in the extreme ultraviolet waveband and was produced due to a flare of energy $\sim 10^{25}$ ergs. The fact that the flux density of the radio transient is only $\sim 2\,$mSFU suggests that it might be possible to do large statistical studies in future for understanding the relationship between these radio transients and other EUV and X-ray counterparts and also for understanding their importance in coronal heating.

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arXiv2021

Insights from snapshot spectroscopic radio observations of a weak Type I noise storm

We present a high fidelity snapshot spectroscopic radio imaging study of a weak type I solar noise storm which took place during an otherwise exceptionally quiet time. Using high fidelity images from the Murchison Widefield Array, we track the observed morphology of the burst source for 70 minutes and identify multiple instances where its integrated flux density and area are strongly anti-correlated with each other. The type I radio emission is believed to arise due to electron beams energized during magnetic reconnection activity. The observed anti-correlation is interpreted as evidence for presence of MHD sausage wave modes in the magnetic loops and strands along which these electron beams are propagating. Our observations suggest that the sites of these small scale reconnections are distributed along the magnetic flux tube. We hypothesise that small scale reconnections produces electron beams which quickly get collisionally damped. Hence, the plasma emission produced by them span only a narrow bandwidth and the features seen even a few MHz apart must arise from independent electron beams.

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arXiv2021

Exploring Coronal Heating Using Unsupervised Machine-Learning

The perplexing mystery of what maintains the solar coronal temperature at about a million K, while the visible disc of the Sun is only at 5800 K, has been a long standing problem in solar physics. A recent study by Mondal(2020) has provided the first evidence for the presence of numerous ubiquitous impulsive emissions at low radio frequencies from the quiet sun regions, which could hold the key to solving this mystery. These features occur at rates of about five hundred events per minute, and their strength is only a few percent of the background steady emission. One of the next steps for exploring the feasibility of this resolution to the coronal heating problem is to understand the morphology of these emissions. To meet this objective we have developed a technique based on an unsupervised machine learning approach for characterising the morphology of these impulsive emissions. Here we present the results of application of this technique to over 8000 images spanning 70 minutes of data in which about 34,500 features could robustly be characterised as 2D elliptical Gaussians.

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arXiv2021

First radio evidence for impulsive heating contribution to the quiet solar corona

This letter explores the relevance of nanoflare based models for heating the quiet sun corona. Using metrewave data from the Murchison Widefield Array, we present the first successful detection of impulsive emissions down to flux densities of $\sim$mSFU, about two orders of magnitude weaker than earlier attempts. These impulsive emissions have durations $\lesssim 1$s and are present throughout the quiet solar corona. The fractional time occupancy of these impulsive emissions at a given region is $\lesssim 10\%$. The histograms of these impulsive emissions follow a powerlaw distribution and show signs of clustering at small timescales. Our estimate of the energy which must be dumped in the corona to generate these impulsive emissions is consistent with the coronal heating requirements. Additionally, the statistical properties of these impulsive emissions are very similar to those recently determined for magnetic switchbacks by the Parker Solar Probe (PSP). We hope that this work will lead to a renewed interest in relating these weak impulsive emissions to the energy deposited in the corona, the quantity of physical interesting from a coronal heating perspective, and explore their relationship with the magnetic switchbacks observed by the PSP.

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arXiv2020

Radio emission from a decade old Type I superluminous supernova, PTF10hgi: Comparison with FRB121102

We perform a comparative study between the only radio detected Type I superluminous supernova (SLSN) PTF10hgi, and the most active repeating fast radio burst FRB121102. This study has its root in the hypothesized FRB-SLSN connection that states that magnetars born in SLSN can power FRBs. The wideband spectrum (0.6--15 GHz) of PTF10hgi presented here, provides strong evidence for the magnetar wind nebular origin of the radio emission. The same spectrum also enables us to make robust estimates of the radius and the magnetic field of the radio emitting region and demonstrates that the nebula is powered by the rotational energy of the magnetar. This spectrum is then compared with that of FRB121102 which we extend down to 400 MHz using archival data. The newly added measurements put very tight constraint on the emission models of the compact persistent source associated with FRB121102. We find that while both sources can be powered by the rotational energy of the underlying magnetar, the average energy injection rate is much higher in FRB121102. Hence, we hypothesise that, if PTF10hgi is indeed emitting fast radio bursts, those will be much weaker energetically than those from FRB121102.

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arXiv2020

A 3D framework to explore the propagation effects in stars exhibiting electron cyclotron maser emission

Recently, coherent radio emission has been discovered from a number of hot magnetic stars, via the process of electron cyclotron maser emission (ECME). This emission, observed in the form of highly circularly polarized pulses, have interesting properties which contain information about the host star. One of the important properties of ECME is the frequency dependence of the pulse arrival time. This has been attributed to propagation effect by Trigilio et al. (2011), and could explain the sequence observed for CU Vir qualitatively (Lo et al. 2012). However no quantitative treatment exists for this phenomenon which is a promising tool to estimate the density in the stellar magnetosphere. Besides, the effect of propagation through the magnetosphere on ECME has been thought to be limited to giving rise to a particular sequence of arrival of pulses, and in some cases producing the upper cut-off frequency for ECME (Leto et al. 2019). Here, we present a framework to deal with the propagation effect by considering continuous refraction in the inner magnetosphere of the star. This framework is capable of incorporating any type of density distribution, and in principle any type of magnetic field, though we limit ourselves to a dipolar magnetic field for this work. We show by simulation that for stars with high obliquity, the propagation effect can influence not only the sequence of arrival of pulses drastically, but also the pulse shapes, and the observability of a pulse from a particular magnetosphere.

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arXiv2020

Estimation of the physical parameters of a CME at high coronal heights using low frequency radio observations

Measuring the physical parameters of Coronal Mass Ejections (CMEs), particularly their entrained magnetic field, is crucial for understanding their physics and for assessing their geo-effectiveness. At the moment, only remote sensing techniques can probe these quantities in the corona, the region where CMEs form and acquire their defining characteristics. Radio observations offer the most direct means for estimating the magnetic field when gyrosynchontron emission is detected. In this work we measure various CME plasma parameters, including its magnetic field, by modelling the gyrosynchrotron emission from a CME. The dense spectral coverage over a wide frequency range provided by the Murchison Widefield Array (MWA) affords a much better spectral sampling than possible before. The MWA images also provide much higher imaging dynamic range, enabling us to image these weak emissions. Hence we are able to detect radio emission from a CME at larger distances ($4.73 R_\odot$) than have been reported before. The flux densities reported here are amongst the lowest measured in similar works. Our ability to make extensive measurements on a slow and otherwise unremarkable CME suggest that with the availability of data from the new generation instruments like the MWA, it should now be possible to make routine direct detections of radio counterparts of CMEs.

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arXiv2019

The Low-Frequency Solar Corona in Circular Polarization

We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 - 240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5% to nearly 100%. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.

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arXiv2019

Evidence for Super-Alfvenic oscillations in sources of Solar type III radio bursts

At the site of their origin, solar meterwave radio bursts contain pristine information about the local coronal magnetic field and plasma parameters. On its way through the turbulent corona, this radiation gets substantially modified due to the propagation effects. Effectively disentangling the intrinsic variations in emission from propagation effects has remained a challenge. We demonstrate a way to achieve this, using snapshot spectroscopic imaging study of weak type III bursts using data from the Murchison Widefield Array (MWA). We use this study to present the first observational evidence for second-scale Quasi-Periodic Oscillations (QPOs) in burst source sizes and orientation with simultaneous QPOs in intensity. The observed oscillations in source sizes are so fast and so large that, they would require two orders of magnitude larger Alfven speed than the typical 0.5 Mm/s, if interpreted within a MHD framework. These observations imply the presence of a quasi-periodic regulation mechanism operating at the particle injection site, modulating the geometry of energetic electron beams that generate type III bursts. In addition, we introduce a method to characterize plasma turbulence in mid coronal ranges, using such frequent weak bursts. We also detect evidence for a systematic drift in the location of the burst sources superposed on the random jitter induced by scattering. We interpret this as the motion of the open flux tube within which the energetic electron beams travel.

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