arXiv subjects

Anshu Kumari

Publications and source records attributed to Anshu Kumari.

At least 19 recordsLinked to original sources

arXiv2026

A high-frequency type II radio burst associated with an X2.3 class flare

Radio observations provide a powerful diagnostic of the solar corona, enabling investigations of dynamic phenomena associated with solar flares, coronal mass ejections (CMEs), and shock waves. We present a multiwavelength analysis of a rare high-frequency type II radio burst (starting frequency of $\sim$750 MHz and frequency drift rate of $\sim$0.5 MHz s$^{-1}$) associated with the X2.3-class solar flare that occurred on 6 November 2024. A propagating EUV disturbance was observed shortly after the flare onset in the SDO/AIA field of view, while radio spectrographs recorded the type II burst between 13:46 and 13:56 UT over a frequency range of $\sim$750 to 45 MHz. Radio imaging observations from the Nançay Radioheliograph (NRH) show that the radio sources propagate southward during the event. X-ray spectroscopy from HEL1OS onboard Aditya-L1 and imaging observations from STIX onboard Solar Orbiter reveal signatures of efficient non-thermal electron acceleration associated with the flare. An NLFFF extrapolation identifies a pre-eruptive magnetic flux rope in the source region, while white-light coronagraph observations obtained during the event show no detectable large-scale CME. The speed of the erupting flux rope, derived from stereoscopic EUV observations, is consistent, within measurement uncertainties, with the shock speed inferred from the radio dynamic spectrum. Together, these observations suggest that compact flux rope eruptions, even in the absence of a detectable white-light CME, can generate low-coronal shocks capable of producing high-frequency type II radio emission.

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arXiv2026

Multi-lane type II radio bursts: Insights into shock propagation in the corona

Type II solar radio bursts are considered as the signatures of the coronal shocks. These bursts are generated from plasma waves excited by magnetohydrodynamic (MHD) shocks, and then converted into radio waves at the local plasma frequency and/or its harmonics. Hence, these bursts often have fundamental-harmonic (FH) and band-splitting (SB) structures, which provide insights into shock generation and propagation in the corona, hence, in turn, the corresponding coronal conditions. In the present study, we analysed an unusual multi-lane type II burst observed with ground-based solar radio spectrographs on May 29, 2024, between 14:24 and 14:43 UT. The start and end frequencies of the type II burst were 450 MHz and 25 MHz, respectively. By combining spectral information with radio imaging data, we found that radio waves were escaping from the corona via emissions from distinct shock regions. In addition, along with the traditional FH and SB, there were multi-lane structures in the type II bursts. Our analysis suggests complex, inhomogeneous shock dynamics near the leading edge (LE) of the coronal mass ejection (CME). This indicates that the plasma material compresses more strongly in these forefront regions. This was confirmed by radio imaging observations, which showed that the higher-frequency emission occurred at a higher altitude than the lower-frequency emission. Our results suggest that the shock geometry and plasma inhomogeneity play an important role in the generation of type II bursts, leading to traditional fundamental-harmonic split-band (FH-SB) pairs with additional splitting in the type II bands.

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arXiv2026

RAISE: A Low-Frequency Space-Based Payload for Solar Radio and RFI Measurements on the SMiLE Mission

We present the Radio-wave Apparatus for Investigating Solar & Earth interference (RAISE), a compact low-frequency radio payload proposed for hosting on the SMiLE mission. Operating from Low Earth Orbit, RAISE targets a spectral regime that is largely inaccessible from the ground due to ionospheric effects. The payload is designed to enable space-based observations of low-frequency solar radio emissions while simultaneously characterizing terrestrial and ionospheric radio frequency interference in the near-Earth environment. RAISE employs mode-dependent Earth-pointing and Sun-pointing observations to generate dynamic spectral measurements relevant to space weather studies and low-frequency radio mission planning. As an experimental and technology demonstration payload, RAISE provides essential heritage for future space-based low-frequency radio astronomy and space weather missions.

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arXiv2026

Solar energetic particles and their association with radio emissions

Energetic particle populations are ubiquitous throughout the Universe. In our solar system, the most prominent sources of energetic particles are solar flares or collisionless shocks often driven by huge eruptions of magnetised plasma called coronal mass ejections (CMEs). Remotely, low energy electrons from the Sun can be observed as solar radio bursts that are produced by accelerated electron beams undergoing beam-plasma interactions. There are still many open questions on the generation of solar energetic particles (SEP): how and where are SEPs accelerated during solar flares and CMEs and how they escape the solar atmosphere? Another important question is: what is the link between the solar radio bursts and the observed SEPs at spacecraft? SKA can provide high-resolution radio images combined with spectroscopic observations to determine the acceleration time, trajectory and escape of low energy electrons from the solar corona. The synergy between SKA and current space missions will help investigate solar activity and energetic particles across a wide range of wavelengths and particle energies. Particle data from spacecraft can be used to make a connection between radio bursts and SEPs by comparing SEP inferred injection times and energies to those of electrons generating radio bursts at the Sun. Radio observations in turn can be used to distinguish between flare and shock acceleration since different radio bursts pinpoint towards different energetic processes. Since the acceleration region and origin of SEPs of various properties is still largely debated, radio observations have the potential to be an invaluable tool in unraveling these processes.

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arXiv2026

Probing the Solar Corona and the Solar Wind Using Angular Broadening Observations with the SKA

Angular broadening observations of compact radio sources provide a powerful method for probing the solar corona and solar wind. Such observations enable studies of the phase structure function, turbulence amplitude, intermediate-scale density fluctuations, solar-wind heating rates, and dissipation scales. When a compact radio source is observed through coronal or solar-wind plasma, several observable effects can arise: (1) the apparent source size increases because of scattering by turbulent plasma, producing angular broadening; (2) the source flux density decreases because of scattering and absorption; (3) the observed angular broadening can be anisotropic, reflecting anisotropic turbulence in the corona and solar wind; and (4) the position angle of the anisotropy, measured from north through east, can help infer the orientation of the coronal magnetic field. These effects provide insights into the physical processes governing the solar wind and its interaction with electromagnetic waves, and they offer constraints on coronal turbulence and magnetic-field structure. At present, angular broadening studies remain limited and have mostly focused on very bright radio sources such as Tau A. The unprecedented sensitivity and angular resolution of the Square Kilometre Array are expected to greatly expand the number of suitable background sources, opening a new window on the solar corona, solar wind, and heliosphere.

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arXiv2026

Role of SKA in Advancing Remote Measurements of Magnetic Fields of Solar Coronal Mass Ejections

Coronal Mass Ejections (CMEs) are large expulsions of magnetized plasma from the Sun into interplanetary space and are the primary drivers of extreme space weather variations. The strength and topology of CME magnetic fields largely determine their impact on Earth. Although visible-light coronagraphs routinely observe CMEs and provide their geometric and kinematic properties, they cannot directly measure CME vector magnetic fields. These fields evolve from initiation through the inner heliosphere due to interactions with other CMEs, coronal structures, and the ambient solar wind, leading to significant structural deformation. Such evolution complicates predictions of the CME magnetic field at Earth. Accurate measurements of CME magnetic fields in the corona and heliosphere are therefore essential for advancing space weather forecasting. Radio observations spanning MHz to GHz frequencies provide a powerful remote-sensing approach for measuring CME magnetic fields from the ground. Recent observations with Square Kilometre Array (SKA) precursors and pathfinder instruments, as well as other new-generation facilities, have demonstrated the potential of these radio techniques for CME magnetic-field diagnostics. At the same time, these studies have highlighted several limitations of current instruments. The higher sensitivity, wider instantaneous bandwidth, and broader frequency coverage of the SKA will open a new observational window, enabling these techniques to be fully exploited for constraining SpWx models and improving predictive accuracy. However, such observations are non-standard and require special consideration in scheduling, calibration, and imaging. Developments achieved with SKA precursors and pathfinders are paving the way for robust CME magnetic-field measurements with the SKA.

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arXiv2026

Coronal Magnetography using Spectropolarimetry with SKA Telescopes

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

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arXiv2026

Radio Wave Propagation as a Probe of the Solar Corona and Solar Wind

Radio waves propagating through an inhomogeneous, turbulent medium such as the solar corona and solar wind become distorted, causing the initially plane wavefronts becomes corrugated and acquire an RMS phase deviation across the wavefront. This leads to observable effects such as angular broadening of radio sources or intensity scintillation. Such waves can be used to probe the solar wind through various techniques, including angular broadening and interplanetary scintillation observations. Such observations enable the study of several key properties, such as the phase structure function, amplitude of turbulence, density modulation index, solar wind heating rates, magnetic field topology, and dissipation scales. These phenomena provide critical insights into the physical processes governing the solar corona and solar wind and its interaction with radio waves, offering valuable constraints on both coronal and solar wind turbulence and coronal magnetic field configurations. Currently, the limited number of radio sources near the ecliptic restricts our observations. However, the SKA-Low and SKA-Mid are expected to detect a significantly larger number of radio sources, thereby providing deeper insights into the solar corona, solar wind, and heliosphere. Long-term observations will be crucial to understanding how the above-mentioned parameters vary with heliocentric distance and over the solar cycle.

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arXiv2026

Solar Radio Bursts in the metric to kilometric range

Solar radio bursts (SRBs) are intense emissions observed in radio wavelengths most frequently during solar transients, such as coronal mass ejections (CMEs) and flares. SRBs are direct signatures of accelerated electrons in the solar atmosphere. These solar transients have a direct impact on the near-Earth atmosphere. SRBs serve as key diagnostic tools for plasma processes, particle accelerations, magnetic field dynamics in the solar corona and the heliosphere, which are the root cause of these solar transients. There are several key science question which solar radio observations can answer, such as: When $\&$ where is the bulk of the energy released in flares?, what are the physical properties of the energy release site?, what are the properties of heated plasma $\&$ accelerated particles?, how does the transport of heated plasma $\&$ accelerated particles?, what bearing do flares have on the question of coronal heating? The Square Kilometre Array (SKA), with its unprecedented sensitivity, temporal, spectral, and spatial resolution, as well as dynamic range, is expected to provide an enhanced understanding of the physics behind solar transients with unprecedented detail.

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arXiv2026

Microwave Polar Brightening and Its Connection to Polar Coronal Holes

Polar brightening (PB) observed at microwave frequencies serves as an important probe to study the thermal and magnetic properties in the Sun's polar regions. Building on earlier studies that linked microwave PB to polar faculae, small-scale loops, and the polar coronal holes (PCHs), we present a comprehensive analysis of the long-term behaviour of 17 GHz microwave PB and its relation to polar magnetic field and coronal hole evolution. Using daily Nobeyama Radioheliograph observations spanning 1992 to 2018, we quantify microwave PB peak temperature variations and compare them with the temporal evolution of PCH area extracted from SDO/AIA-based SPoCA coronal hole catalogues during the period 2010-2018. We also examine the correspondence between microwave PB and the polar magnetic field to assess the nature of their association. Our results show a strong correlation between microwave PB peak temperature and PCH area, as well as with the polar magnetic-field strength. In addition, we found that regions of enhanced microwave emission are frequently associated with small-scale loop structures, consistent with Coronal Bright Points (CBPs), which are often associated with the eruption of jets. Overall, this study aims to investigate the impact of coronal holes, polar magnetic fields, and small-scale polar activity on polar brightening observed at 17 GHz and its long-term evolution.

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arXiv2026

Radio Signature of Higher Atmospheric Meridional Flow and Implications for Magnetic Trees in the Sun

The coupling between plasma flows and magnetic fields in the solar atmosphere governs the transport of angular momentum and the redistribution of magnetic flux, yet its manifestation in the magnetically dominated upper chromosphere remains uncertain. Using 27 years of 17 GHz full-disk solar radio imaging observations from the Nobeyama Radioheliograph, we report the first detection of a poleward flow signature at heights of $3000\pm500$ km, an altitude where plasma magnetohydrodynamics expects magnetic dominance ($β<1$). The derived latitudinal velocity profile ($5-15$ m/s) mirrors the established photospheric meridional circulation, displaying modulation with solar cycle parameters. Comparison with long-term synoptic magnetograms reveals that the motion of 17 GHz brightness features closely tracks poleward magnetic flux transport, implying a deep magnetic anchoring of these structures. This finding provides the first observational evidence that chromospheric flows at radio wavelengths reflect subsurface meridional dynamics, consistent with the "magnetic tree" hypothesis, which links high-altitude motion to deep-seated magnetic connectivity.

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arXiv2025

UK White Paper on Magnetic Reconnection

Magnetic reconnection powers explosive releases of magnetic energy, heating and particle acceleration throughout the plasma universe. Knowledge of this universal process is vital to understanding the Heliosphere, as it plays a key role in solar flares, coronal mass ejections, coronal heating, solar wind acceleration, geomagnetic storms, and interactions between the solar wind and planetary magnetospheres. As such, reconnection underpins multiple science objectives of multiple future space missions. The UK plays a leading role in this international field, through a combination of in situ measurements from Earth's magnetosphere and the solar wind, observations of the solar corona and chromosphere, and world-class numerical simulations and theory. This white paper identifies: Nine priority science objectives for reconnection research in the next decade; Recommendations to guide investment in theory, simulations and infrastructure; Mission priorities and required measurements to ensure the UK maintains and improves its world-class credentials in reconnection science.

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arXiv2025

A type II solar radio burst without a coronal mass ejection association

Type II solar radio bursts are commonly associated with shocks generated by coronal mass ejections (CMEs), where plasma waves are excited by magnetohydrodynamic (MHD) processes and converted into radio waves at the local plasma frequency or its harmonics. However, there are instances where type II bursts occur in the absence of whitelight CMEs. We analysed one such metric type II radio burst observed on November 2, 2023, characterized by split band features and fundamental-harmonic lanes. Notably, no CME was detected with space-based coronagraphs during this event. However, an intense M1.6 class flare was observed just before the type II burst and an extreme ultraviolet (EUV) disturbance was observed expanding into surrounding regions. The absence of any whitelight CME seen in any coronagraph field of view even though the EUV shock had a moderate speed of $\approx500~km/s$, which was close to the shock speed derived from radio observations, %indicates that the shock in the inner corona was most-likely produced by the very intense solar flare and the type II was associated with the EUV disturbance seen in the lower corona. These observations indicate that the shock in the inner corona was most-likely driven by the EUV ejecta seen in the lower corona, but the ejecta did not survive as a CME in the coronagraph field of view.

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arXiv2025

Insights into Chromospheric Large-Scale Flows using Nobeyama 17 GHz Radio Observations I. The Differential Rotation Profile

Although the differential rotation rate on the solar surface has long been studied using optical and extreme ultraviolet (EUV) observations, associating these measurements to specific atmospheric heights remains challenging due to the temperature-dependent emission of tracers observed in EUV wavelengths. Radio observations, being primarily influenced by coherent plasma processes and/or thermal bremsstrahlung, offer a more height-stable diagnostic and thus provide an independent means to test and validate rotational trends observed at other EUV wavelengths. We aim to characterize the differential rotation profile of the upper chromosphere using cleaned solar full-disc 17 GHz radio imaging from the Nobeyama Radioheliograph (NoRH), spanning a little over two solar cycles (1992 - 2020). A tracer-independent method based on automated image correlation was employed on daily full-disc 17 GHz radio maps. Our results suggest that the upper chromosphere rotates significantly faster than the photosphere at all latitudes, with a relatively flatter latitudinal profile. A very weak anti-correlation between the equatorial rotation rate and solar activity is also observed. Our findings reaffirm the potential of radio observations to probe the dynamics of the solar chromosphere with reduced height ambiguity. The overlap of the equatorial rotation rate found in this study with that for $304$ Å in the EUV regime lends additional support to the view that the equatorial rotation rates increase with height above the photosphere. Future coordinated studies at wavelengths with better-constrained height formation will be crucial for further understanding the complex dynamics of the solar atmosphere.

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arXiv2025

Can metric radio bursts be used as a diagnostics tool for interplanetary coronal mass ejections?

Metric radio bursts are often said to be valuable diagnostic tools for studying the near-sun kinematics and energetics of the Interplanetary Coronal Mass Ejections (ICMEs). Radio observations also serve as an indirect tool to estimate the coronal magnetic fields. However, how these estimated coronal magnetic fields are related to the magnetic field strength in the ICME at 1 AU has rarely been explored. We aim to establish a relation between the coronal magnetic fields obtained from the radio observations very close to the Sun and the magnetic field measured at 1 AU when the ICME arrives at the Earth. We performed statistical analysis of all metric type II radio bursts in solar cycles 23 and 24, which were found to be associated with ICMEs. We estimated the coronal magnetic field associated with the corresponding CME near the Sun (middle corona) using a split-band radio technique and compared those with the magnetic fields recorded at 1 AU with in-situ observations. We found that the estimated magnetic fields near the Sun using radio techniques are not well correlated with the magnetic fields measured at 1 AU using in-situ observations. This could be due to the complex evolution of the magnetic field as it propagates through the heliosphere. Our results suggest that while metric radio observations can serve as effective proxies for estimating magnetic fields near the Sun, they may not be as effective close to the Earth. At least, no linear relation could be established using metric radio emissions to estimate the magnetic fields at 1 AU with acceptable error margins.

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arXiv2024

The effect of data-driving and relaxation model on magnetic flux rope evolution and stability

We investigate the effect of data-driving on flux rope eruptivity in magnetic field simulations by analysing fully data-driven modelling results of active region (AR) 12473 and AR11176, as well as preforming relaxation runs for AR12473 (found to be eruptive). Here, the driving is switched off systematically at different time steps. We analyse the behaviour of fundamental quantities, essential for understanding the eruptivity of magnetic flux ropes (MFRs). The data-driven simulations are carried out with the time-dependent magnetofrictional model (TMFM) for AR12473 and AR11176. For the relaxation runs, we employ the magnetofrictional method (MFM) and a zero-beta magnetohydrodynamic (MHD) model to investigate how significant the differences between the two relaxation procedures are when started from the same initial conditions. To determine the eruptivity of the MFRs, we calculate characteristic geometric properties, such as the cross-section, MFR height along with stability parameters, such as MFR twist and the decay index. For eruptive cases, we investigate the effect of sustained driving beyond the point of eruptivity on the MFR properties. We find that the fully-driven AR12473 MFR is eruptive while the AR11176 MFR is not. For the relaxation runs, we find that the MFM MFRs are eruptive when the driving is stopped around the flare time or later, while the MHD MFRs show eruptive behaviour even if the driving is switched off one and a half days before the flare occurs. We find that characteristic MFR properties can vary greatly even for the eruptive cases of different relaxation simulations. The results suggest that data driving can significantly influence the evolution of the eruption, with differences appearing even when the relaxation time is set to later stages of the simulation when the MFRs have already entered an eruptive phase.

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arXiv2024

CME-associated type-IV radio bursts: The solar paradigm and the unique case of AD Leo

The type-IV bursts, associated with coronal mass ejections (CMEs), occasionally extend to the decameter-hectrometric (DH) range. We present a comprehensive catalog of simultaneous multi-vantage point observations of DH type-IV bursts by Wind and STEREO spacecraft since 2006. 73% of the bursts are associated with fast ($> 900\,km\,s^{-1}$) and wide ($>60^0$) CMEs, which are mostly geoeffective halo CMEs. Also, we find that the bursts are best observed by the spacecraft located within $|60^0|$ line of sight (LOS), highlighting the importance of LOS towards active latitudes while choosing target stars for a type-IV search campaign. In young active M dwarfs, CME-associated bursts have remained elusive despite many monitoring campaigns. We present the first detection of long-duration type-III, type-IV, and type-V bursts during an active event in AD Leo (M3.5V; $0.4M_\odot$). The observed burst characteristics support a multipole model over a solar-like active region magnetic field profile on the star.

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arXiv2024

Inter-planetary type-IV solar radio bursts: A comprehensive catalog and statistical results

Decameter hectometric (DH; 1-14 MHz) type-IV radio bursts are produced by flare-accelerated electrons trapped in post-flare loops or the moving magnetic structures associated with the CMEs. From a space weather perspective, it is important to systematically compile these bursts, explore their spectro-temporal characteristics, and study the associated CMEs. We present a comprehensive catalog of DH type-IV bursts observed by the Radio and Plasma Wave Investigation (WAVES) instruments onboard Wind and STEREO spacecraft, covering the period of white-light CME observations by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the SOHO mission between November 1996 and May 2023. The catalog has 139 bursts, of which 73% are associated with a fast (>900 km/s) and wide (>60$^o$) CME, with a mean CME speed of 1301 km/s. All DH type-IV bursts are white-light CME-associated, with 78% of the events associated with halo CMEs. The CME source latitudes are within $\pm$45$^o$. 77 events had multi-vantage point observations from different spacecraft, letting us explore the impact of line of sight on the dynamic spectra. For 48 of the 77 events, there was good data from at least two spacecraft. We find that, unless occulted by nearby plasma structures, a type-IV burst is best viewed when observed within $\pm$60$^o$ line of sight. Also, the bursts with a duration above 120 min, have source longitudes within $\pm$60$^o$. Our inferences confirm the inherent directivity in the type-IV emission. Additionally, the catalog forms a sun-as-a-star DH type-IV burst database.

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