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Measurements of the plasma parameters of coronal mass ejections (CMEs), particularly the magnetic field and nonthermal electron population entrained in the CME plasma, are crucial to understand their propagation, evolution, and geo-effectiveness. Spectral modeling of gyrosynchrotron (GS) emission from CME plasma has been regarded as one of the most promising remote-sensing techniques for estimating spatially resolved CME plasma parameters. Imaging the very low flux density CME GS emission in close proximity to the Sun with orders of magnitude higher flux density has, however, proven to be rather challenging. This challenge has only recently been met using the high dynamic range imaging capability of the Murchison Widefield Array (MWA). Although routine detection of GS is now within reach, the challenge has shifted to constraining the large number of free parameters in GS models, a few of which are degenerate, using the limited number of spectral points at which the observations are typically available. These degeneracies can be broken using polarimetric imaging. For the first time, we demonstrate this using our recently developed capability of high-fidelity polarimetric imaging on the data from the MWA. We show that spectropolarimetric imaging, even when only sensitive upper limits on circularly polarization flux density are available, is not only able to break the degeneracies but also yields tighter constraints on the plasma parameters of key interest than possible with total intensity spectroscopic imaging alone., Published at the Astrophysical Journal (23 pages, 15 figures, 3 tables)

Results from 11 years of radio timing for eclipsing black widow millisecond pulsar (MSP) binary, J1544+4937, is presented in this paper. We report a phase-connected timing model for this MSP, using observations with the Giant Metrewave Radio Telescope (GMRT) at multiple frequencies and with Green Bank Telescope (GBT). This is the longest-duration timing study of any galactic field MSP with the GMRT. While extending the timing baseline from the existing 1.5 years to about a decade we report the first detection for a significant value of proper motion ($\mathrm{\mu_{T}} \sim$ 10.14(5) $\mathrm{mas/year}$) for this pulsar. Temporal variations of dispersion measure ($\mathrm{\Delta DM~ \sim 10^{-3}}$ pc $\mathrm{cm^{-3}}$) manifested by significant determination of 1st, 2nd, and 3rd order DM derivatives are observed along the line of sight to the pulsar. We also noticed frequency-dependent DM variations of the order of $\mathrm{10^{-3}~ pc~ cm^{-3}}$, which could arise due to spatial electron density variations in the interstellar medium. This study has revealed a secular variation of the orbital period for this MSP for the first time. We investigated possible causes and propose that variation in the gravitational quadrupole moment of the companion could be responsible for the observed temporal changes in the orbital period., Comment: 12 pages, 5 Figures, 2 Table, Accepted in the Astrophysical Journal

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 $, Published in: 2022 3rd URSI Atlantic and Asia Pacific Radio Science Meeting (AT-AP-RASC)

Imaging the low-frequency radio Sun is an intrinsically challenging problem. Meter-wavelength solar emission spans angular scales from a few arcminutes to a few degrees. These emissions show temporal and spectral variability on a sub-second and sub-MHz scales. The brightness temperature of these emissions also varies by many orders of magnitude, which requires high-dynamic-range spectroscopic snapshot imaging. With the unique array configuration of the Murchison Widefield Array (MWA), and the robust calibration and imaging pipeline, Automated Imaging Routine for the Compact Arrays for the Radio Sun (AIRCARS) produces the best spectroscopic snapshot solar images available to date. The working principle and the strength of this algorithm are demonstrated using statistical analysis and simulation. AIRCARS uses the partial phase stability of the MWA, which has a compact core with a large number of antenna elements distributed over a small array footprint. The strength of this algorithm makes it a state-of-the-art calibration and imaging pipeline for low-frequency solar imaging, which is expected to be highly suitable for the upcoming Square Kilometre Array (SKA) and other future radio interferometers for producing high-dynamic-range and high-fidelity images of the Sun., 22 pages, 11 figures. Published at Solar Physics

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 $, 27 pages, 9 figures, Published at the Astrophysical Journal

Imaging the low-frequency radio Sun at high fidelity is an intrinsically challenging problem. Solar emissions span angular scales of a few degrees to the instrumental resolution and are composed of highly variable emissions over both temporal and spectral axis and, also covering a few orders of magnitude in brightness temperature. Unique array design of the Murchison Widefield Array(MWA), which is a Square Kilometre Array (SKA) precursor, the state-of-the-art calibration and imaging pipeline, called AIRCARS, produces the high dynamic range spectroscopic snapshot solar images than ever made. The working principle and the strength of AIRCARS are discussed based on statistical analysis and simulation. AIRCARS can perform calibration of the solar observations and other bright astronomical sources even without any dedicated calibrator observations. AIRCARS uses the partial phase stability of the arrays with a compact core having a large number of antennas distributed over a small array footprint. It has been shown that AIRCARS can also perform calibration of radio interferometers with a much longer array footprint, but having a compact core. The strength of the novel algorithm of the AIRCARS makes it a state-of-the-art calibration pipeline, which is highly suitable for the upcoming SKA and other future radio interferometers for performing calibration and producing high dynamic range and high fidelity solar images of the Sun.

Type III solar radio bursts form a well known class of active solar emissions and are associated with energetic electron beams propagation outwards through the coronal plasma, and in the process giving rise to their characteristic rapid spectral drifts. Though they have been the subject of a large number of studies since their discovery in the 1950s, the high fidelity and dynamic range spectroscopic snapshot images from the new technology instruments, like the Murchison Widefield Array (MWA) are enabling the exploration of a previously inaccessible part of phase space and leading to the discovery of previously unknown aspects of these well known bursts even in recent times (e.g. Mohan et al., 2019, ApJ, 875). We have now developed a robust and general full Stokes polarization calibration and imaging algorithm optimized for MWA solar observations.. Referred to as “Polarimetry using Automated Imaging Routine for Compact Arrays for the Radio Sun'' (P-AIRCARS; Kansabanik et al., 2022, in prep.), it can deliver full Stokes solar images with leakages on par with usual astronomical radio maps. Here we use this novel capability to carry out a detailed polarimetric study of a type III solar radio burst observed with the MWA. This is, once again, enabling an exploration of new phase space with an exciting discovery potential. Preliminary results show that these type III bursts show presence of linearly polarized emission. While conventional wisdom says that all traces of linear polarization should get washed out due to differential Faraday rotation in the corona, we have convincing reasons to believe that this emission is solar in origin. Here we present the current status of our first detailed polarimetric imaging study oa this type III radio source.

Magnetic field measurements at middle and higher coronal heights are challenging using conventional techniques with observations at visible or extreme ultra-violet wavelengths. Low radio frequencies are ideal for probing magnetic fields at higher coronal heights. Polarization properties of solar radio emissions are known to be a rich source of information about the emission mechanisms and magnetic fields of the corona. Nonetheless, largely due to technical challenges, precise polarimetric solar observations at low radio frequencies have remained challenging. The degree of polarization of solar radio emission varies dramatically over time, frequency, and also in morphology, depending on the emission mechanism. The radio bursts show a moderate to a high degree of circular polarization, while the quiet sun thermal emissions show a very low degree of circular polarization (

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., 17 pages, 8 figures, 3 tables, Published in the Astrophysical Journal

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., Comment: 34 pages, 18 figures, accepted for publication in Journal of Astronomy and Astrophysics

The dynamics and the structure of the solar corona are determined by its magnetic field. Measuring coronal magnetic fields is, however, extremely hard. The polarization of low-frequency radio emissions has long been recognized as one of the few effective observational probes of magnetic fields in the mid and high corona. However, the extreme intrinsic variability of this emission, the limited ability of most of the available existing instrumentation (until recently) to capture it, and the technical challenges involved have all contributed to its use being severely limited. The high dynamic-range spectropolarimetric snapshot imaging capability that is needed for radio coronal magnetography is now within reach. This has been enabled by the confluence of data from the Murchison Widefield Array (MWA), a Square Kilometre Array (SKA) precursor, and our unsupervised and robust polarization calibration and imaging software pipeline dedicated to the Sun-Polarimetry using the Automated Imaging Routine for Compact Arrays of the Radio Sun (P-AIRCARS). Here, we present the architecture and implementation details of P-AIRCARS. Although the present implementation of P-AIRCARS is tuned to the MWA, the algorithm itself can easily be adapted for future arrays, such as SKA1-Low. We hope and expect that P-AIRCARS will enable exciting new science with instruments like the MWA, and that it will encourage the wider use of radio imaging in the larger solar physics community., Comment: 22 pages, 11 figures, 1 table. Published at the Astrophysical Journal Supplement Series

Coronal Mass Ejections (CMEs) are large-scale explosive eruptions of magnetised plasma from the Sun into the Heliosphere. Measuring the physical parameters of CMEs is crucial for understanding their physics and for assessing their geo-effectiveness. Radio observations offer the most direct means for estimating these plasma parameters when gyrosynchrotron (GS) emission is detected from the CME plasma. However, since the first detection by Bastian et al.2001, only a handful of studies have successfully detected GS emission from CME plasma. This is usually attributed to the challenges involved in obtaining the high dynamic range imaging required for observing this faint gyrosynchrotron emission in the vicinity of active solar emissions.The newly developed imaging pipeline (Mondal et al., 2019) designed for the data from Murchison Widefield Array (MWA) marks a significant improvement in metrewave solar radio imaging. Our work suggests that we should now be able to routinely detect GS emission from CME plasma. We present an example where we have successfully detected radio emission from CME plasma and modelled it as GS emission, leading to reliable estimates of CME magnetic field as well as the distribution of energetic electrons (Mondal et al. 2020). In a different example we are able to detect the radio emission from the CME plasma out to as far as 8.3 solar radii. We find that the observed spectra are not always consistent with simple GS models. This highlights that more complicated physics might be at play and points to the need for building more detailed models for interpreting these emissions. We hope that with the availability of polarimetric imaging capability, which we are in the process of developing, this technique will provide a robust way to routinely measure CME magnetic fields along with its other physical parameters. We note that these are the weakest detections of GS emissions from CME plasma reported yet.

It has been a long standing problem as to how the solar corona can maintain its million K temperature, while the photosphere, which is the lowest layer of the solar atmosphere, is only at a temperature of 5800 K. A very promising theory to explain this is the “nanoflare” hypothesis, which suggests that numerous flares of energies ~1024 ergs are always happening in the solar corona, and maintain its million K temperature. However, detecting these nanoflares directly is challenging with the current instrumentation as they are hypothesised to occur at very small spatial, temporal and energy scales. These nanoflares are expected to produce nonthermal electrons, which are expected to emit in the radio band. These nonthermal emissions are often brighter than their thermal counterparts and might be detectable with current radio instruments. Due to their importance multiple searches for these nonthermal emissions have been done, but thus far they have been limited to active regions. The quiet corona is also hot, and often comprises the bulk of the coronal region, so it is equally important to understand the physical processes which maintain this medium at MK temperatures. We describe the results from our effort to use the data from the Murchison Widefield Array (MWA) to search for impulsive radio emissions in the quiet solar corona. By pushing the detection threshold of nonthermal emission by about two orders of magnitude lower than previous studies, we have uncovered ubiquitous very impulsive nonthermal emissions from the quiet sun. We refer to these emissions as Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs). Using independent observations spanning very different solar conditions we show that WINQSEs are present throughout the quiet corona at all times. Their occurrence rate lies in the range of many hundreds to about a thousand per minute, implying that on average order 10 or so WINQSEs are present in every 0.5 s MWA image. Preliminary estimates suggest that WINQSEs have a bandwidth of ~2 MHz. Buoyed by their possible connection to the hypothesised “nanoflares”, we are pursuing several projects to characterise and understand them. These include developing machine learning algorithms to identify WINQSEs in radio images and characterise their morphologies; exploring the ability of the present generation EUV and X-ray instruments to estimate the energy corresponding to the brightest of WINQSEs; and attempting very high time resolution imaging to explore their temporal structure. In this talk, I will present the results from the past and ongoing projects about WINQSEs and argue that these might be a key step towards detecting “nanoflares” and the resolution of the coronal heating problem.

The frequency dependent eclipses of the radio emission from millisecond pulsars (MSPs) in compact binary systems provide an opportunity to understand the eclipse mechanism and to determine the nature of the eclipsing medium. We combine multifrequency observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) and model the broadband radio spectrum in the optically thick to thin transition regime to constrain the eclipse mechanism. The best-fit model to the eclipse phase spectra favors synchrotron absorption by relativistic electrons. We are able to strongly constrain the frequency of onset of the eclipse to 345$\pm$5 $\mathrm{MHz}$, which is an order of magnitude more precise than previous estimates. The dependence on the magnetic field strength of synchrotron absorption allowed us to estimate the magnetic field strength of the eclipse medium to be $\sim$13 $\mathrm{G}$, which is very similar to the values obtained by considering a pressure balance between the incident pulsar wind and the stellar wind of the companion. Applying this method to other millisecond binary pulsars will enable us to determine if the eclipse mechanisms are all the same and also estimate the wind and magnetic field properties of the companion stars. The method could also be applied to other systems where pulsars interact with companion winds in binary systems and in all cases it will lead to a better understanding of the evolutionary processes., Comment: 13 pages, 5 Figures, 1 Table, Published in the Astrophysical Journal