The Sun and the Heliosphere

(Divya Oberoi, Atul Mohan, Surajit Mondal, K. Hariharan, Former members: P. K. Manoharan, Rohit Sharma)

Radio waves provide a view of the Sun that is very different from that at other wavelengths. Low-frequency solar radio emission varies rapidly in time, frequency and spatial location; this variability has long posed a challenge to solar studies. Besides using the GMRT, NCRA-TIFR researchers are involved in mapping the Sun with the Murchison Widefield Array (MWA), a new radio telescope in Australia. The unique high fidelity imaging capability of the MWA over short time intervals and narrow frequency widths allows it to track changes in the solar emission across time, frequency and morphology.

A constant stream of charged and magnetized plasma flows out from the upper solar atmosphere. As radio waves from distant sources traverse this inhomogeneous and turbulent “solar wind”, their wave fronts get distorted. For compact sources, this leads to the phenomenon of Interplanetary Scintillation (IPS), analogous to the optical twinkling of stars. IPS provides an excellent remote sensing probe for the heliosphere, and the ORT has played a pivotal role in the development and application of IPS techniques. IPS monitoring with the ORT is being used to provide insight into solar activity, including solar bursts, coronal mass ejections, and solar-wind driven magnetic storms that affect the near-Earth environment.

Recent Results
GMRT polarisation and brightness temperature observations of Venus
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.
Solar physics with the Square Kilometre Array
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.
Discovery of super-Alfvénic oscillations in solar type-III radio bursts
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).
An unsupervised imaging pipeline for generation of high dynamic range solar radio images
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.
Quantifying the weakest non-thermal solar emissions via non-imaging studies
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.
Introducing SPatially REsolved Dynamic Spectra for the Sun
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.
Radio observation of Venus using the GMRT
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.
Energisation of Charged Particles by Fast Magnetic Reconnection
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.
Wavelet-based Characterization of Small-scale Solar Emission Features
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.
Estimating Solar Flux Density at Low Radio Frequencies
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.

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