The Sun and the Heliosphere

Solar radio emissions provide several unique diagnostics tools for the solar corona, which are otherwise inaccessible. However, imaging the highly dynamic coronal emissions spanning a large range of angular scales at radio wavelengths is extremely challenging. Due to its large number of antennas, MeerKAT radio telescope is possibly the globally best-suited instrument at GHz frequencies for providing high-fidelity spectroscopic snapshot solar images. At these frequencies, the Sun has a much higher flux density than any other astronomical source in the sky. Hence, observing the Sun with sensitive general-purpose radio telescopes like MeerKAT requires one to attenuate the solar signal very substantially for optimum instrument operation. Kansabanik et al. 2024 achieve this using an unconventional approach - rather than pointing straight at the Sun, the MeerKAT dishes are pointed 2.6 degrees away from the Sun. This effectively attenuates the solar signal by a factor of about 1000 and allows them to observe the Sun without saturating the telescope systems. The MeerKAT radio image of the Sun (top panel) shows extremely good morphological similarities with the EUV image as well as the simulated radio image (bottom panel) at corresponding frequency. The MeerKAT image comes from 15 minutes of observations. The observed and simulated images are remarkably similar, cyan circles mark corresponding features spanning a range of sizes and intensities. The observed spectra of these features are also consistent with the simulated spectra from synthetic MeerKAT images. This demonstrates the high fidelity of these images. The authors show that below ∼900 MHz MeerKAT images can recover essentially the entire flux density from the large angular-scale solar disk. Not surprisingly, at higher frequencies, the missing flux density can be as large as ∼50%. However, it can potentially be estimated and corrected. This work marks the first step towards commissioning solar observation with MeerKAT, which will enable a host of novel studies. This will not only make accessible a large unexplored phase space with significant discovery potential but also pave the way for solar science with the upcoming Square Kilometre Array-Mid telescope, for which MeerKAT is a precursor.

This work by Bawaji et al. continues the theme of building a detailed observational characterisation of the Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs). WINQSEs were discovered using the Murchison Widefield Array (MWA) by Mondal et al. (2020) and have thus far met all of the criterion for being the radio coutnerparts of nanoflares, hypothesised by Parker to explain coronal heating. Bawaji et al. investigate the morphological properties of WINQSEs, while also improving upon the earlier methodology used for detecting WINQSEs. They present a machine learning-based algorithm to detect WINQSEs, classify them based on their morphology, and model the isolated ones using 2D Gaussians. The figure shows the results from first using the t-SNE algorithm for condensing the information from the entire feature set for each of the detected WINQSEs into a two-dimensional space, and then using the DBSCAN algorithm to group similar features together. Interestingly, despite the expectations of their arising from intrinsically compact sources, Bawaji et al. find that WINQSEs tend to be resolved in their observations. They propose that this angular broadening arises due to coronal scattering and suggest that WINQSEs can provide ubiquitous and ever-present diagnostic of coronal scattering (and, in turn, coronal turbulence) in the quiet Sun regions, which has not been possible to date.

Coronal mass ejections (CMEs) are large-scale expulsions of magnetized plasma from the solar corona into the heliosphere. Measurements of the CME plasma parameters, particularly the magnetic field and the nonthermal electron population entrained in the CME, are crucial to understanding CME propagation, evolution, and geo-effectiveness. Spectral modeling of the gyrosynchrotron (GS) emission from CME plasma has long been regarded as one of the most promising remote-sensing techniques for estimating spatially-resolved CME magnetic fields and other plasma parameters. Imaging the very low flux density CME GS emission in the close proximity of the Sun, which has a flux density higher by many orders of magnitude, 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). The MWA allows us to detect faint GS radio emissions from the entire CME marked by the cyan box in the figure. The radio emission is shown by contours overlaid on white-light coronagraph difference images. Although routine detection of GS emission from CMEs is now within reach, the challenge has shifted to constraining the large number of free parameters in GS models, some 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 of MWA data. We show that spectropolarimetric imaging, even when only sensitive upper limits on the circular polarization flux density are available, not only is able to break the degeneracies in the model parameters but also yields tighter constraints on the key plasma parameters of interest than possible with total intensity spectroscopic imaging alone.

Since the discovery by Mondal et al. (2020) of Weak Impulsive Narrowband Quiet Sun Emissions (WIQNSEs), their observational characterisation has been a significant strand of research being pursued by this group. WINQSEs were originally interpreted to be the radio counterparts of Parker’s nanoflares and it is very exciting to see that all of the subsequent work has continued to respect and reinforce their interpretation. This work by Mondal et al. is the next step in this progression and focuses on the spectral nature of WINQSEs. Given that their strength is only a few percent of the background solar emission, they adopt an extremely conservative approach to reliably identify WINQSES. Only a handful of WINQSEs meet all of the stringent criteria. Their flux densities lie in the 20 − 50 Jy range and they have compact morphologies. For the first time, this work estimates their bandwidths and finds them to be less than 700 kHz, consistent with expectations based on earlier observations. These data come from a very quiet time and no sunspot was present on the visible disc of the Sun. The contours show a moment map for one of the five WINQSEs analysed, superposed on a map corresponding to its peak frequency, and the inset shows a zoomed in view. The contour levels are 0.8, 0.85, 0.9, 0.95 and 0.99 times the peak in the moment map.

The Square Kilometre Array Observatory (SKAO) is perhaps the most ambitious radio observatory 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. Additionally, its ability to make a vast previously unexplored phase space accessible promises a large discovery potential. The Indian solar and heliospheric physics community has been preparing for this science opportunity. Over the last many years, this effort has taken the form of leading the solar science enterprise with a SKA-Low precursor, the Murchison Widefield Array (MWA). This long term sustained effort has included the development of the necessary tools, including dedicated solar radio imaging pipelines, and has lead to many interesting discoveries, insights and realizations. These pipelines now represent the state-of-the-art and the science they have enabled spans the range from coronal heating to space weather, and coronal magnetography to discovering novel features in emissions from well known solar radio bursts. This article briefly summarises this journey, highlighting the major milestones on the way (till early 2022), and shares our future plans and long term objectives. The authors also discuss the novel heliospheric science which will be enabled by the future SKA-Low, primarily by the unprecedentedly detailed and sensitive studies of propagation effects suffered by the low frequency radio waves from distant cosmic sources as they traverse the magnetised heliospheric plasma.

Type II solar radio bursts are caused by magnetohydrodynamic shocks driven by solar eruptive events such as coronal mass ejections. This emission appears at the fundamental (F) and the harmonic (H) of the local plasma frequency and is seen to drift from higher to lower frequencies as the shock travels out through the corona. Often both F and H bands are further split into sub-bands. These split bands are generally believed to be coming from upstream and downstream regions of the shock. This hypothesis remains largely untested as locating the sites of emission for the split bands requires high quality spectroscopic snapshot imaging observations. Only recently, it has become possible to test this hypothesis using data from the Murchison Widefield Array (MWA) and the a robust interferometric imaging pipeline tuned for solar needs (Mondal et al., 2019). Bhunia et al. present combined results from imaging analysis of type II radio burst band-splitting and other fine structures, observed by the MWA and extreme ultraviolet observations from Solar Dynamics Observatory/Atmospheric Imaging Assembly. The symbols in the accompanying figure mark positions of the type II radio sources at four pairs of frequencies observed simultanoeusly in the higher (blue) and the lower (green) bands. The colour of the symbols gets darker with time. The blue and green arrows indicate the directions of motion of these sources. This study provides rare evidence that, at least in this particular instance, band-splitting is caused by emission coming from different parts of the shock (and not from regions upstream/downstream of the shock). They also notice small-scale motion in the location of the type II radio sources in MWA images, which are stongly correlated across neighouring times and frequencies. Bhunia et al. interpret these to be arising due to propagation effects incurred during passage of this emission through the turbulent coronal plasma and not because of the physical motion of the shock location. This also allows them to estimate the length scale of turbulent density perturbations, which is found to lie in the range 1-2 Mm.

Magnetic fields play a crucial role on the Sun, everywhere from the solar interior to the solar atmosphere. They provide the reservoir of energy for the heating of the solar atmosphere and the production of energetic particles, and drive solar activity, including eruptive events like flares and coronal mass ejections. However, measuring magnetic fields, particularly in the mid- and higher corona, is extremely challenging using observations at X-ray, ultraviolet, optical, and infrared wavelengths. The polarization of low-frequency radio emissions has long been recognized as one of the few effective observational probes of magnetic fields in this region. However, the extreme intrinsic variability of low-frequency solar radio emission makes it harder to extract this information from the radio data available from the standard instruments today. The ability to capture the detailed spectro-temporal and spatial variability of the polarized radio emissions is essential for radio coronal magnetography and this requires high dynamic-range spectropolarimetric snapshot imaging capability. This is now within reach with observations made using the Murchison Widefield Array (MWA), a precursor of the Square Kilometre Array Observatory (SKAO). Sophisticated and compute-intensive calibration and imaging processes are needed to make the requisite MWA solar images, each of which take several hours to complete. Designing and performing these analysis tasks requires a deep understanding of radio interferometry and the steep learning curve involved has been a deterrent limiting the use of solar data. Additionally, doing this manually is not only prone to human errors, but is simply infeasible, given the data volumes. To overcome these problems, Kansabanik et al. (2023) developed an 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), the implementation details of which are described in this work. A figure of merit for any parallel implementation of an algorithm is the improvement in run time with an increase in the availability of computational resources. The P-AIRCARS algorithm is embarrassingly parallel and as shown in the accompanying figure for a few different numbers of spectral channels and temporal samples, the run time for calibration scales very well with the increase in hardware resources. The highest dynamic range spectropolarimetric snapshot solar radio images now come from P-AIRCARS. Although the present implementation of P-AIRCARS is tuned for the MWA, the algorithm itself is quite general and will serve the needs of upcoming arrays like the SKAO-Low equally well. It is anticipated that P-AIRCARS will enable exciting new science with instruments like the MWA, encourage the wider use of radio imaging in the solar physics community, and hopefully form a stepping stone to the solar imaging pipeline for the SKAO.

In 2020, Mondal et al. reported the discovery of a new class of metrewave solar transient emission from quiet Sun regions, the strengths of which are only a few percent of the background emission. They have since been named Weak Impulsive Narrowband Quiet Sun Emission (WINQSEs). Their ubiquitous presence in quiet Sun regions, and narrow-band impulsive nature led Mondal et al. (2020) to suggest that these emissions might be the radio counterparts of the hypothesised nanoflares. Given the potential significance, this warrants detailed follow-up studies. In this work Mondal et al. have presented an analysis of data from an extremely quiet time. Not only do they detect numerous WINQSEs in these data, the improvements in methodology since the earlier work enable them to detect even weaker WINQSEs. The key properties of WINQSEs, namely, their impulsive nature and ubiquitous presence on the quiet Sun are observed in these data as well. The colour scale in the figure shows the fractional occupancy distribution of the detected WINQSEs for four different frequencies demonstrating their presence all over the Sun. The contours show the median map at respective frequencies with contour levels at 0.2, 0.4, 0.6, 0.8, 0.9, 0.95 times the peak in the median map. Interestingly, they find that the flux density distribution of the WINQSEs in this data set differs significantly from that found in the earlier work and demonstrate that these differences can justifiably be attributed to differences in methodology and the variations in the level of solar activity. In conjunction with another recent work (Sharma et al., 2022), which used an independent technique to detect WINQSEs, this work places the detection of WINQSEs on a firm pedestal.

Metre-wavelength solar emission spans angular scales from a few arcminutes to a few degrees. The brightness temperature of these emissions also varies by several orders of magnitude. Often, the faint radio emission from the quiet solar corona or coronal mass ejections is present simultaneously with the very bright radio emissions from solar radio bursts. To understand the global coronal properties, one has to detect both types of emissions simultaneously. At radio wavelengths, one cannot use a coronagraph to block the bright emission. Hence, one requires high-dynamic-range imaging to detect faint emission in the presence of very bright emission. The combination of 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, Mondal et al. 2019), produces the best spectroscopic snapshot solar images at low radio frequencies available to date. The present work demonstrates that even uncalibrated data from the MWA have a certain degree of coherency, which allows AIRCARS to make a reasonable starting point for boot-strapping a self-calibration algorithm even without a dedicated calibrator observation. The left panel of the figure shows an image after applying the calibration solutions from night-time calibrator observation, while the right panel shows the image made directly from the uncalibrated data which provides a reasonable starting point.  The strength of this algorithm makes AIRCARS 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 and other future radio interferometers for producing high-dynamic-range and high-fidelity images of the Sun.

A ubiquitous presence of weak energy releases is one of the most promising hypotheses to explain coronal heating, referred to as the nanoflare hypothesis. The accelerated electrons associated with such weak heating events are also expected to give rise to coherent impulsive emission via plasma instabilities in the metrewave radio band, making this a promising spectral window to look for their presence. Recently, Mondal et al. (2019) had reported the presence of weak and impulsive emissions from quiet Sun regions which seem to meet the requirements of being radio counterparts of the hypothesized nanoflares. Detection of such low-contrast weak emission signals from the quiet Sun is challenging and, given their implications, it is important to confirm their presence. In this work, using data from the Murchison Widefield Array, Sharma et al. use an independent robust approach for their detection by separating the dominant, slowly varying component of emission from the weak impulsive one in the visibility domain. By imaging these so-called ‘residual visibilities’, they detect milli-Solar Flux Unit-level bursts taking place all over the Sun and characterize their brightness temperatures, distributions, durations, and associations with features seen in extreme-UV images. The top panel of the figure shows the number of instances in a 30 min period where the residual flux density in a given pixel exceeded 6\sigma, where \sigma is the rms noise in the map far from the Sun for frequencies ranging from 108 MHz to 240 MHz. These features are seen to be present all over the Sun, though some clustering around active regions is seen at the highest frequencies. The lower panel shows the mean brightness temperature (Tb) of all the emission features identified in the upper panel which lies in the range of a few kK, order a percent of the solar thermal bremsstrahlung. These are among the weakest detections of non-thermal solar radio emissions. The black circle marks the optical disc of the Sun and the green contour the 5\sigma boundary of the radio Sun. Sharma et al. also constrain the energies of the nonthermal particles using inputs from the FORWARD coronal model along with some reasonable assumptions, and find them to lie in the subpico flare (~10^19-10^21 erg) range. They also report the discovery perhaps the weakest known type III radio burst yet and another emission feature showing the weakest known clear signature of the quasi-periodic pulsations.

The Sun is a magnetically active star. Its atmosphere, the solar corona, comprises hot magnetised plasma. Coronal magnetic fields are well known to be one of the crucial parameters determining the physics of the solar corona and are a key driver of space weather. Although the importance of the coronal magnetic field has long been appreciated, it is hard to measure the field strength. The polarisation properties of coronal emission at low radio frequencies can, in principle, be used for coronal magnetic field measurements. Precise polarimetry at these frequencies is intrinsically hard and it is made even more challenging by the very large range of brightness temperatures associated with different emission mechanisms (ranging from 10,000 K to 10,000 billion K), the variation in the fractional polarisation from close to 100% to less than 1%, and extreme temporal and spectral variability of the emission. Kansabanik et al. have developed a robust algorithm for accurate polarisation calibration of solar observations with the Murchison Widefield Array (MWA), a Square Kilometre Array (SKA) precursor. The algorithm delivers high dynamic range and high fidelity full Stokes solar radio images with residual leakages on par with the best images today; it is based on the Measurement Equation framework, which forms the basis of all modern radio interferometric calibration and imaging. The figure shows the total intensity and circular polarisation images of type-I, -II, and -III solar radio bursts, made using this algorithm. The red contours in the top panel show the Stokes I emission at 0.5% of the peak emission while the bottom panel shows percentage circular polarisation. The blue circles represent the optical disc of the Sun and the filled ellipses, the resolution of the observations. In all cases, the residual instrumental polarisation is less than 1%. The algorithm has been developed with the future SKA in mind. The high-fidelity spectropolarimetric snapshot solar radio imaging enables the exploration of previously inaccessible phase space and offers a considerable discovery potential.

The Sun is the highest flux-density source in the low-frequency radio sky. The flux density of even the quiet Sun exceeds many tens of thousands of Jy at metre wavelengths and can increase by multiple orders of magnitude during periods when active emission is present. Most astronomical sources, on the other hand, have flux densities below a few tens of Jy and only a handful of the brightest sources like Crab, Virgo-A, Cen-A, etc. reach a few thousand Jy. Sensitive radio instruments are optimized for observing faint astronomical sources. This leads to problems for solar flux density calibration as most telescopes require additional attenuation to be introduced in the signal path for solar observations, while most calibrators are too weak to be detected with this additional solar attenuation. In addition to dealing with the inclusion of another antenna-dependent element in the signal chain which needs to be calibrated, wide field-of-view (FoV) aperture array instruments like the Murchison Widefield Array (MWA) face another complication. To avoid contamination from the very strong solar emission, the flux density calibrators are usually observed before sunrise or after sunset. In this work, Kansabanik et al. present multiple independent approaches for absolute flux density calibration of solar MWA data and establish their consistency. Improving on the high-quality images delivered by the AIRCARS pipeline developed by Mondal et al. (2019), Kansabanik et al. present the first-ever detection of more than 80 background galactic and extra-galactic radio sources in the solar FoV, a bit like ``seeing stars in broad daylight''. The figure shows about a 3,600 square degrees FoV at 80 MHz, integrated over 2 minutes of time and over a bandwidth of 2 MHz, with sources down to a flux density of 4.6 Jy. The red circle marks the position of the Sun. The absence of imaging artefacts in the vicinity of the Sun is noteworthy and the RMS noise of the image is only about 1.5 times that of the GLEAM image of the same field. Kansabanik et al. use the GLEAM catalog flux density of these background radio sources to arrive at a robust flux density calibration method for solar observations. The other flux density calibration approaches demonstrated include using the presence of bright sources like Crab and Virgo-A in the solar FoV, and the use of a dedicated calibrator observation with and without the solar attenuators. These flux density calibration methods are a significant improvement over earlier approaches and are independent of the MWA array configuration. They deliver a flux density uncertainty of about 10% for solar observations even in the absence of dedicated calibrator observations and meet the requirements for obtaining accurate solar flux density calibrations for MWA data, needed for several solar scientific applications.

The NCRA solar physics group presented the first firm observational evidence for the presence of ubiquitous impulsive nonthermal radio emissions from the quiet solar corona (Mondal et al., 2020). These have since been christened WINQSEs – Weak Impulsive Narrow-band Quiet Sun Emissions - and are the smoking guns for the weak underlying magnetic reconnection processes or `nanoflares' which were hypothesised to explain the many decades-old coronal heating problem (Parker, 1988). The quantity of interest, from a coronal heating perspective, is the amount of energy deposited in the corona by the reconnection processes giving rise to these WINQSEs. It is, however, very hard to estimate this energy from radio emissions which arise from non-linear coherent emission processes. On the other hand, such energy estimates are routinely derived using well-established techniques from emissions in the extreme ultraviolet (EUV) and soft X-ray parts of the spectrum, which arise due to thermal processes. Mondal (2021) presents the first attempt to identify the EUV counterparts of these radio transients and use them to estimate the energy deposited into the corona during the events. By a careful comparison of the radio and EUV light curves, the author first identifies the EUV brightening associated with a closely spaced group of WINQSEs and then estimates the flare energy of this brightening to be ~10^25 ergs. The figure shows the radio contours at 132 MHz overlaid on an AIA 171 A map. The two possible EUV brightening candidates are shown in white boxes, with the likely one shown in the solid white box. This is the weakest EUV transient event for which a radio counterpart has been clearly identified. This cluster of WINQSEs had a peak flux density of ~2 mSFU, and the 10^25 ergs estimate is about an order of magnitude larger than nanoflare energies. This work demonstrates that even with current instrumentation, it is possible to identify the EUV counterparts of clusters of WINQSEs, and that the energies involved are consistent with the expectations based on the nanoflare hypothesis, making this an exciting line of exploration.

Using the data from the Murchison Widefield Array and their pipeline tailored for solar imaging (AIRCARS; Mondal 2019), the NCRA solar physics group has been focusing on studies of weak solar bursts. Here, Mondal and Oberoi present a high-fidelity snapshot spectroscopic radio imaging study of a weak type I solar noise storm that took place during an otherwise exceptionally quiet time. The flux density of the noise storm source varied between ∼0.6–24 SFU, about two orders of magnitude weaker than earlier studies along similar lines. The type I radio emission is believed to arise due to electron beams energized during magnetic reconnection activity. They track the observed morphology of the burst source for about 70 minutes to study the details of the reconnection and electron acceleration process during such quiet times. During this time interval, the authors identify multiple instances where the source's integrated flux density and area are strongly anticorrelated with each other. The authors also find that the presence of anticorrelation at one frequency does not necessarily imply its presence at other neighbouring frequencies in the same time window. The observed anticorrelation is interpreted as evidence for presence of MHD sausage wave modes in the magnetic loops and strands along which these electron beams are propagating. Their observations suggest that the sites of these small-scale reconnections are distributed along the magnetic flux tube. The authors hypothesize that small scale reconnections produce electron beams which quickly get collisionally damped. Hence, the plasma emission produced by them spans only a narrow bandwidth and the features seen even in neighbouring bands 12 to 28 MHz apart must arise from independent electron beams. These observations suggest a scenario where sausage MHD modes are stochastically excited in quiescent coronal loops. These sausage modes change the density of the non-thermal electrons responsible for the radio emission, thereby producing the observed anticorrelation between the area and the integrated flux density of the noise storm source. The work provides strong evidence that even during very quiescent times, there is discernible magnetic activity in the vicinity of active regions and in coronal loops. It also suggests that MHD oscillations in coronal magnetic loops and strands are likely quite ubiquitous. The radio emission from the weak electron beams propagating through these loops and strands serves to light them up, allowing their detection.

Metrewave emission from the quiet sun arises from thermal bremsstrahlung in the million-degree Kelvin (MK) corona, and can potentially be a rich source of coronal diagnostics. On its way to the observer, the radiation gets modified substantially due to the propagation effects – primarily refraction and scattering – as it traverses the magnetised and turbulent coronal medium, leading to a redistribution of the intensity in the image plane. By comparing high-fidelity full-disk metrewave solar maps during a quiet solar period and the corresponding modelled thermal bremsstrahlung emission, Sharma and Oberoi explore a novel approach to characterise and quantify these propagation effects. The solar radio maps between 100 and 240 MHz come from the Murchison Widefield Array (MWA). The FORWARD package, which does not include propagation effects, is used to simulate thermal bremsstrahlung images using the self-consistent Magnetohydrodynamic Algorithm outside a Sphere coronal model (Gibson et al., 2016). The authors attribute the differences between the observed and modelled maps to scattering and refraction. A good general correspondence between the modelled and observed brightness distributions is seen, though significant differences are also observed. The observed radio size of the Sun is found to be 25–30% larger in area. The emission peak corresponding to the only visible active region shifts by 8’–11’ and its size increases by 35–40%. Interestingly the direction of this shift is closer to the tangential direction than the radial direction, providing evidence for significant anisotropic propagation effects. Simple models suggest that the fraction of scattered flux density is always larger than a few tens of percent, and varies significantly between different regions (active and quiet regions, and coronal holes). Sharma and Oberoi estimate coronal density inhomogeneities to lie in the range 1–10%. In the figure, the top row shows the MWA maps and the bottom row those obtained using FORWARD. Only regions with brightness temperature > 0.2 MK are shown. Contour levels in all the maps are 70, 75, 80, 85, 90 and 95% of the peak. The authors also find that the flux densities estimated by the MWA and FORWARD are in excellent agreement at frequencies above 200 MHz, but, curiously, the MWA flux densities are systematically lower at lower frequencies. A likely reason is that the measurements used by FORWARD progressively lose accuracy with increasing height, where the emission at lower frequencies arises.

Explaining the presence of the million Kelvin corona sitting atop a 5800 Kelvin photosphere has been one of the longest-standing mysteries of solar physics. One of the hypotheses put forth for explaining this is the so called “nanoflare”-based coronal heating hypothesis (Parker, 1988). According to it, a large number magnetic reconnections keep taking place all of the over the Sun all the time; individually these small explosions involve only about a billionth of the energy of a large solar flare, but collectively they extract sufficient energy from the coronal magnetic fields to be able to heat and maintain the corona at a temperature of a million Kelvin. Considerable effort has been expended to look for observational evidence for the presence of these nanoflares in the X-ray and extreme ultraviolet bands, and has led to the conclusion that the observed distribution of even the weakest of the flares detected thus far is not consistent with the requirements for coronal heating. For the first time, Mondal et al. (2020) provide firm observational evidence for the presence of impulsive nonthermal radio emissions from the quiet solar corona, which form the smoking guns for the weak underlying magnetic reconnection processes. They meet all of the known criteria for coronal heating – they are found all through the quiet sun regions; and their radio flux density distribution has a power-law tail with a slope steeper than -2 at all frequencies (see figure). Mondal et al. estimate the energy that must be dumped in the corona to generate these impulsive emissions: this is consistent with the coronal heating requirements. These impulsive emissions have durations <1 second, their fractional time occupancy at a given region is <10%, and they show signs of clustering at small timescales. Additionally, the statistical properties of these impulsive emissions are very similar to those recently determined for magnetic switchbacks by the Parker Solar Probe. This study used data from the Murchison Widefield Array and was made possible by reliable detection of impulsive non-thermal solar emissions down to flux densities of a thousandth of a an SFU (1 SFU = 10,000 Jy), about two orders of magnitude fainter than earlier studies.

Measuring the physical parameters of Coronal Mass Ejections (CMEs), and particularly their entrained magnetic field, is crucial for understanding their physics and assessing their geo-effectiveness. At present, only remote sensing techniques can probe these quantities in the corona, the region where CMEs originate and acquire their defining characteristics. Radio observations offer a direct means for estimating the CME magnetic field by measuring the gyrosynchontron emission from CME plasma. Though simple in concept, this has proven to be challenging in practice, and there exist only a handful of successful examples in the literature. In this work, Mondal et al. measure various CME plasma parameters, including the magnetic field, by modeling the gyrosynchrotron emission from a CME. The radio imaging was done using the Murchison Widefield Array (MWA), and the high imaging dynamic range of these images allowed Mondal et al. to reliably detect these faint emissions. In fact, they were able to detect radio emission from a CME out to a larger distance (approximately 4.7 solar radii) than has been reported till date. The radio flux densities reported here are among the lowest measured in similar works. The MWA observations also provide much denser spectral sampling than has been available earlier, giving Mondal et al. the ability to more accurately constrain the model parameters. The present study is based on extensive flux density measurements of a slow, and otherwise unremarkable, CME. This suggests that new telescopes like the MWA should now be able to routinely detect the radio counterparts of CMEs and estimate their magnetic fields. The upper panel of the figure shows the average normalised radio contours (over the frequency range 108-145 MHz) superposed on a LASCO/C2 difference image. The green circles mark 3 and 4 solar radii. The contour levels start at 0.02% of the peak and increase in factors of two. The bottom panel shows the measured flux density from the region marked in yellow in the upper panel, along with the best-fit gyrosynchrotron spectral model.

Mulay et al. have used extreme UV imaging data from the AIA/SDO, radio imaging and spectra from the Murchison Widefield Array (MWA), and photospheric magnetic data from the HMI/SDO to carry out a detailed investigation of an active region jet and a nonthermal type III radio burst. The temperature of the jet spire and the footpoint regions were found to be similar to those reported in similar earlier studies, though the lower limits to the densities in these regions were estimated to be about an order of magnitude lower than the values reported earlier. A temporal and spatial correlation between the active region jet and the type III burst was established using high time resolution spectroscopic radio imaging. Observations of type III bursts are often used to establish the presence of open field lines. In the present instance, the MWA observations probe the regions ~0.13-0.52 R_Sun above the photosphere. A detailed examination of the data reveals that the nature of the observed radio emission is rather complex. A very interesting aspect of this work is that the observed and expected locations (based on the standard magnetic field extrapolation techniques) of radio burst sources do not match. The radio emission is very dynamic, changing across frequencies, and the differences between observed and expected emissions do not seem to follow a systematic pattern. A part of the observed differences can be explained by invoking the presence of significant propagation effects (refraction and scattering), which, in turn, provide evidence for large and dynamic density coronal inhomogeneities. The accompanying figure shows an AIA 171 Angstrom image at 03:51:47 UT with the red, green, and blue contours plotted over it (50, 55, 60, 65, 70, 75, 80, 85, 90, and 95% of the peak of the flux) representing observations at 101, 165, and 298 MHz, respectively. The yellow arrow indicates the location of the jet eruption. Orange and magenta lines indicate closed and open field lines, respectively, as given by extrapolation models. The red, green, and blue squares show the expected locations of radio sources at 101, 165, and 298 MHz, respectively. The different panels show the radio emission during each of the four episodes type III bursts, spanning a period of about 12 min.

Weak heating events are frequent and ubiquitous in the solar corona. They derive their energy from the local magnetic field and form a major source of local heating, signatures of which are seen in extreme UV (EUV) and X-ray bands. Associated radio emission arises from various plasma instabilities that lead to coherent radiation, making even a weak X-ray flare appear very bright in metrewave radio bands. Radio observations can hence probe non-equilibrium dynamics, providing complementary information about plasma evolution. However, a robust study of radio emission from one weak event among many simultaneous events, requires high dynamic range imaging at sub-second and sub-MHz resolutions due to the high spectro-temporal variability of these emissions. Such observations were not possible until recently. Mohan et al. present the first spectroscopic radio imaging study of a type-I noise storm, the data for which were obtained using the Murchison Widefield Array. This is also among the first spatially-resolved multi-waveband studies of active region loops hosting transient brightenings (ARTB), which are shown to be dynamically linked to metrewave type-I noise storms. Mohan et al. report the discovery of 30-second quasi-periodic oscillations (QPOs) in the radio light curve, riding on a baseline flux density. The strength of the QPOs and the baseline flux density are enhanced during a mircoflare associated with the ARTB. The interpretation suggested by Mohan et al. is that the sub-photospheric convective plasma flows lead to a build-up of magnetic stress across the braided magnetic field network. This stress gets released via numerous weak magnetic reconnection events. The observed relaxation time scale of 30 seconds corresponds to the Alfvén timescale for a the observed magnetic field braiding length scale. In the figure, the top panel shows the physical picture emerging from this study. The EUV bright loops are shown in red, and are co-located with the X-ray source. The observed radio emission comes from the marked region along the yellow loops at much larger coronal heights. The bottom panel shows the radio light curve after smoothing with a 10-second running mean filter. The vertical dashed lines are drawn at a separation of 30 seconds. The quasi-periodicity of episodes of emission is self evident.

McCauley et al. present the first circular polarisation (Stokes V) images of the Sun from the Murchison Widefield Array (MWA). These Stokes V images span the range form 80-240 MHz and were made using a heuristic polarisation calibration algorithm introduced here. They also present a survey of Stokes V features detected in over 100 observing runs near solar maximum during quiescent periods. These include detection of around 700 compact polarised sources with polarisation fractions ranging from less than 0.5% to nearly 100%. They are interpreted to be arising from a continuum of plasma emission noise storm (type-I bursts) sources associated with active regions. They also report a curious but characteristic “bullseye” structures observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. They also show that the large-scale polarimetric structure at their lowest frequencies is reasonably well-correlated with the line-of-sight magnetic field component inferred from a global potential field source surface model, while at higher frequencies this is not observed to be the case. The figure shows an example of Stokes I, V, and V/I at four frequencies across the MWA band for a coronal hole. Color bar units are in signal-to-noise [S/N] for I and V and percent for polarization fraction [V/I]. The green contours represent the 5 sigma level in Stokes I, the solid circles represent the optical disk, and the ellipses in the lower-left corners represent the synthesized beam sizes. The coronal hole is clearly visible in the 240 MHz Stokes I images, and transitions from being a dark to a bright structure as one proceeds to lower frequencies. The corresponding Stokes V bullseye structure is self evident.

Mohan et al. present results from carefully designed Giant Metrewave Radio Telescope (GMRT) low-frequency observations of Venus during its inferior conjunction. This ensured that the apparent angular size and flux density of Venus would be the largest observable from the Earth, making these the most detailed and sensitive observations of Venus that are possible with the GMRT. Mohan et al. used this opportunity to observe Venus at 234 MHz, 608 MHz and 1298 MHz. The figure shows the degree of polarisation maps for Venus at 607.67 MHz (top panel) and 1297.67 MHz (bottom panel), with the contour levels at 8, 12, 16, 20, 24, 28, 32, 36 and 40 percent; these are the lowest frequencies at which polarimetric maps have been made of Venus. Such polarimetric observations are essential for determining the sub-surface dielectric constant. As the penetration depth is substantially larger at low frequencies, metrewave observations allow us to probe the deeper sub-surface layers of Venus. This, in turn, is a very useful input for modeling the planetary surface dielectric properties. Using these observations, Mohan et al. determined the sub-surface dielectric constant to be ~4.5. At 234 MHz, they placed an upper limit of 321 K on the brightness temperature of Venus, firmly establishing that the brightness temperature of Venus begins to falls by about 1.4 GHz; the 234 MHz upper limit implies that the rate at which the temperature falls is even steeper than estimated earlier. This drop in the observed brightness temperature continues to pose a puzzle for present-day thermal emission models, which predict the brightness temperature to remain constant at low frequencies. However, the existing models do not take sub-surface properties into account, while emission at lower frequencies arises from deeper subsurface layers. These results suggest that sub-surface properties (dielectric properties through density and mineral content) can significantly impact the observed brightness temperature at low radio frequencies.

Although solar physics is one of the most mature branches of astrophysics, the Sun confronts us with many long standing problems that are fundamental in nature. Some of these problems, like the physics of shocks, are common across many astrophysical contexts and some others, like developing the ability to predict space weather, are of enormous societal relevance for the present technologically reliant society. Nindos et al. discuss how the Square Kilometre Array, the upcoming most ambitious radio telescope designed yet, can potentially lead to transformative advances in our understanding of the Sun and address some of these fundamental problems. In its first incarnation, SKA1 will comprise two instruments, the SKA1-Low aperture array (top panel) to be built in the Murchison region of Western Australia, and the SKA1-Mid dishes (bottom panel) to be build in the Karoo region of South Africa (image credit: SKA Organization). Nindos et al. summarise our current understanding of the key open problems in solar physics, based on work done across a large swathe of the electromagnetic spectrum. It then articulate the reasons why SKA observations can play an important role in answering some of these questions. These questions include: (1) the location and magnetic configuration of the electron acceleration site; (2) the mechanism(s) responsible for particle acceleration; (3) the flare-coronal mass ejection (CME) relationship; (4) the timing and evolution of CMEs from the early stages of development all the way to the outer corona; (5) the drivers of coronal shocks as well as the locations and efficiency of electron acceleration by shocks; and (6) the origin of solar energetic particles. This paper also showcases the recent work from the SKA precursors and pathfinders, namely the Murchison Widefield Array in Australia and the Low Frequency Array in Europe, which are already revealing previously unknown details of solar emissions and enabling more detailed and realistic modelling of solar phenomena. In addition, as is always the case with new instruments that outperform their predecessors in significant ways, it also emphasises the high probability of new discoveries that cannot yet be predicted.

At the site of their origin, solar metrewave 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 propagation effects. Effectively disentangling the intrinsic variations in emission from propagation effects has remained a challenge. Mohan et al. demonstrate a way to achieve this, using snapshot spectroscopic imaging study of weak type III bursts using data from the Murchison Widefield Array. All of the imaging for this work was done using AIRCARS, the automated solar radio imaging pipeline developed by the NCRA Solar Physics group. This study has lead to the discovery of second-scale Quasi-Periodic Oscillations (QPOs) in burst source sizes and orientation with simultaneous QPOs in intensity. Though the QPOs in intensity were previously known, they had never been imaged. In absence of any information about their size, these rapid oscillations were usually interpreted as a particular mode of magnetohydro-dynamic (MHD) oscillations in the coronal plasma. Their imaging lead to the realisation that the observed oscillations in source sizes are so large that the required speeds are two orders of magnitude larger than the typical Alfvén speeds expected at these coronal heights. This study thus rules out MHD oscillations and implies the presence of a quasi-periodic regulation mechanism operating much deeper in the corona. In addition, this study has also provided, for the first time, a way to quantify the density inhomogeneities in the low corona. The figure shows the variation in the area of the source of type III emission (red) and its intensity (green) measured in Solar Flux Units (1 SFU = 10,000 Jy) as a function of time for one of the six groups of type III bursts studied. The anti-correlation between the size and intensity time series is evident. QPOs in the orientation of the source of type III emission are also seen (blue).

Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral (< 100 kHz) and temporal (< 1 s) spans. With the new generation of instruments, it is now becoming possible to capture data with sufficient resolution (temporal, spectral and angular) that one can begin to characterize the solar morphology simultaneously along the axes of time and frequency. This, however, requires one to make around a million images per hour and renders the usual manual effort intensive approach impractical. The authors have hence developed an end-to-end imaging pipeline optimized for solar imaging - “Automated Imaging Routine for Compact Arrays for the Radio Sun (AIRCARS)”. They demonstrate AIRCARS on data from the Murchison Widefield Array (MWA). The dynamic range of the output images routinely varies from a few hundred to a few thousand. In the few cases, where they have pushed AIRCARS to its limits, the dynamic range can reach as high as ~100,000. These are now the highest dynamic range solar radio images at metre wavelengths, and are enabling exploration of pristine and interesting phase space. AIRCARS has the potential to transform the multi-petabyte MWA solar archive of raw data into science-ready images. AIRCARS can also be tuned to upcoming telescopes like the Square Kilometre Array, making it a very useful tool for the heliophysics community. The figure shows example AIRCARS images spanning the extremes of solar radio emissions. The bright compact emission seen in the top panel comes from a type II radio burst and the image has dynamic range of ~100,000. At a brightness temperature of roughly 1 billion K, it outshines the solar disc by about four orders of magnitude. The lower panel shows the Sun during a quite phase, when the brightness does not vary greatly across the disc. The dynamic range of this image is ~1,000.

At metre radio wavelengths, the thermal free-free emission from the million K coronal plasma forms the bulk of the solar emission. This broadband emission varies slowly in time and smoothly across frequency.  Superposed on this background emission are emissions from a variety of non-thermal mechanisms, which span a large range of strengths, and temporal and spectral scales. Studies with the Murchison Widefield Array (MWA) have recently shown that the weak short-lived narrow-band non-thermal emission features occur much more frequently than had been appreciated earlier. This is exciting because these weak non-thermal emission features may contain clues for solving the longstanding coronal heating problem. Sharma et al. attempt to quantify the weak non-thermal solar emissions using non-imaging techniques, taking advantage of the fine-grained data provided by the MWA to separate out the emission into a slowly varying component, which under moderately quiet solar conditions is expected to be dominated by thermal emission, and an impulsive component, expected to arise from non-thermal processes. They use a method based on a class of statistical data models called Gaussian mixture models (GMMs) to estimate both the strength of the emission components and their time-frequency occupancy. The top panel of the figure shows the observed distribution of the impulsive emission (black dots) superposed on the probability distribution function determined using the GMMs. The dashed and solid lines show, respectively, the individual Gaussian components and the sum of all the Gaussian components; the mean, width and weight of each component are listed on the top right. Surprisingly, Sharma et al. find that even during the moderately quiet solar conditions of the observations, the amount of energy radiated in impulsive non-thermal component is similar to that in the thermal component. Further, they detect evidence for the presence of non-thermal emission in as much as 20-45% of the frequency-time plane. Both of these aspects had not been realised till now. The bottom panel shows slowly varying and impulsive flux densities as a function of observing frequency. The non-thermal emissions studied here are about an order of magnitude weaker than the weakest similar emissions reported in the past. This work establishes the usefulness of the GMM technique for such studies, and gives some tantalising hints, though a lot more needs to be done to assess the role of the weak non-thermal features in coronal heating.

Low radio frequency solar emission spans a very large range in intensity, as well as temporal, spectral and spatial scales. Often multiple processes are going on simultaneously at different locations on the Sun, giving rise to different emissions. These emissions can differ greatly in their strengths and till recently one could usually only study the most intense of these sources. The significantly improved imaging dynamic range of the Mileura Widefield Array (MWA) is now making it possible to study comparatively weaker emissions in presence of more intense ones. In order to facilitate such studies, we have recently developed a new data product which will enable scientists to study the frequency and time variations of the emission coming from any specific patch on the Sun. Called SPREDS, an acronym for SPatially REsolved Dynamic Spectrum, it is named in analogy with the usual definition of a dynamic spectrum, which shows the variations of the emission in the time-frequency plane. We also presented the first flux calibrated solar images from the MWA. The accompanying figure shows an example: the top left panel shows a radio image of the Sun with some regions marked on it; the top right panel shows the dynamic spectrum for the entire Sun, which is the data product used most commonly at these frequencies; the remaining panels show the SPREDS from the corresponding regions marked in the solar disc. Note that the colour scale for these panels is in log scale, the differences between emissions from different regions on the Sun are self evident.

The surface of Venus has been studied by measuring radar reflections and thermal radio emission over the spectral range from several centimetres to metre wavelengths using Earth-based as well as orbiter platforms. Earlier non-imaging radio observations of Venus in the decimeter wavelength regime show a decreasing trend in the observed brightness temperature with increasing wavelength. The present-day thermal emission models however predict the brightness temperature to remain constant above wavelengths of about 10 cm. Mohan et al. report the first interferometric imaging observations of Venus below 620 MHz, which provide reliable brightness temperature measurements, and confirm this discrepancy. These observations were carried out at 606, 333 and 240 MHz using the GMRT. The brightness temperature values derived at the respective frequencies are 526 K, 409 K and <426 K, with errors of ∼7% which are generally consistent with the reported temperatures at 608 MHz and 430 MHz by previous investigators, but are much lower than those obtained by extrapolating from high-frequency observations at 1.38-22.46 GHz using the VLA. The circle and triangles show the measurements from this work, while the open boxes show the model prediction.

Magnetic reconnection has long been understood to be the primary mechanism responsible for the generation of non-thermal electron distributions, which in turn are responsible for the coherent non-thermal emissions at low radio frequencies. However a detailed understanding of the nature of particle acceleration due to reconnection is still lacking. Sharma et al. have carried out a first attempt to understand the details of this process using a 3D magnetohydrodynamics (MHD) framework. They investigate the role of turbulence on the reconnection rate and also study the distributions of energised particles using test-particles. They find that with increasing turbulent intensity the system enters what is usually termed the fast reconnection regime. The speeds of the energised particles are found to follow a Maxwellian distribution whose variance increases with the strength of the reconnecting field. The accompanying figure shows the joint normalised probability distribution functions of velocities of these energised particles along two perpendicular directions for (a) a low turbulence strength, (b) medium turbulence strength and (c) high turbulence strength cases.

Low radio frequency solar observations using the Murchison Widefield Array (MWA) have revealed the presence of numerous weak short-lived narrowband emission features, even during moderately quiet solar conditions. These non-thermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hence necessarily require an automated approach for their detection and characterization. Suresh et al. have developed an algorithm which employs continuous wavelet transform for feature detection in the dynamic spectrum. The green circles in the figure show the peaks of the features detected in an example MWA dynamic spectrum. The left and the right panels differ only in the colour bar range and show the efficacy of this implementation in detecting features across a range of intensities, and temporal and spectral spans. They represent the first statistically robust characterization of the properties of these features. This technique can reliably detect features weaker than 1 SFU (1 SFU = 10,000 Jy), the weakest non-thermal radio emissions so far reported in the literature. The features, which typically last for 1-2 seconds and span bandwidths of 4-5 MHz, can potentially provide an energetically significant contribution to coronal and chromospheric heating. They appear to ride on a broadband background continuum, hinting at the likelihood of their being weak, type-I solar bursts.

As the Sun is much brighter than the typical radio sources used for flux calibration, absolute flux calibration of solar observations is challenging. At low radio frequencies, this becomes even harder due to large fields of view of the instruments. Turning this large field-of-view into an advantage, Oberoi et al. have developed a technique suitable for a low resolution interferometric baseline to provide robust absolute solar flux calibration. Working with well-characterized antennas and receiver systems, this technique relies on using the available detailed full sky radio maps. It provides a reliable and computationally lean method for extracting parameters of physical interest using a small fraction of the voluminous interferometric data, which can be computationally prohibitively expensive to calibrate and image using conventional approaches. The figure shows an example application of this technique to data from the Murchison Widefield Array. It shows the computed values of solar flux in solar flux units (SFU; 1 SFU=10,000 Jy) as a function of time for ten spectral bands between 100 and 300 MHz.