Anshu Kumari

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 ∼750 MHz and frequency drift rate of ∼0.5 MHz s ⁻¹ ) associated with the X2.3-class solar flare that occurred on 2024 November 6. A propagating extreme-ultraviolet (EUV) disturbance was observed shortly after the flare onset in the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) field of view, while radio spectrographs recorded the type II burst between 13:46 and 13:56 UT over a frequency range from ∼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 on board Aditya-L1 and imaging observations from STIX on board Solar Orbiter reveal signatures of efficient nonthermal electron acceleration associated with the flare. A nonlinear force-free field 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.

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 the evolution of PCH. Using daily Nobeyama Radioheliograph observations spanning 1992–2018, we quantify microwave PB peak temperature variations and compare them with the temporal evolution of PCH area extracted from Solar Dynamics Observatory/Atmospheric Imaging Assembly–based Spatial Possibilistic Clustering Algorithm coronal hole catalogs 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, which are often associated with the eruption of jets. Overall, this study aims to investigate the impact of PCHs, polar magnetic fields, and small-scale polar activity on PB observed at 17 GHz and its long-term evolution.

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 yr 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 ± 500 km, an altitude where plasma magnetohydrodynamics expects magnetic dominance ( β < 1). The derived latitudinal velocity profile (5–15 m s ^–1 ) 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.

Context. Shocks in the solar corona are capable of accelerating electrons that in turn generate radio emission known as type II radio bursts. The characteristics and morphology of these radio bursts in the dynamic spectrum reflect the evolution of the shock itself, together with the properties of the local corona where the shock propagates. Aims. In this work we study the evolution of a complex type II radio burst with a multilane structure to find the locations where the radio emission is produced and relate them to the properties of the local environment where the shock propagates. Methods. Using radio imaging, we were able to separately track each lane composing the type II burst and relate the position of the emission to the properties of the ambient medium, such as density, Alfvén speed, and magnetic field. Results. We show that the radio burst morphology in the dynamic spectrum changes with time and is related to the complexity of the local environment. The initial stage of the radio emission is characterized by a single broad lane in the spectrum, while the later stages of the radio signature evolve in a multilane scenario. The radio imaging reveals how the initial stage of the radio emission separates with time into different locations along the shock front as the density and orientation of the magnetic field change along the shock propagation. At the time when the spectrum shows a multilane shape, we find a clear separation of the imaged radio sources propagating in regions with different densities. Conclusions. By combining radio imaging with the properties of the local corona, we describe the evolution of a type II radio burst and, for the first time, identify three distinct radio emission regions above the coronal mass ejection front. Two regions were located at the flanks, producing earlier radio emission than the central position, in accordance with the complexity of density and Alfvén speed values in the regions where radio emission is generated. This unprecedented observation of a triple-source structure provides new insights into the nature of multilane type II bursts.

Context. Solar radio bursts can provide important insights into the underlying physical mechanisms that drive the small and large-scale eruptions on the Sun. Since metric radio observations can give us direct observational access to the inner and middle corona, they are often used as an important tool to monitor and understand the coronal dynamics. Aims. While the sizes of the radio sources that can be observed in the solar corona are essential for understanding the nature of density turbulence within the solar corona and its subsequent influence on the angular broadening observed in radio source measurements, the smallest radio sources associated with solar radio bursts have so far been limited by observational techniques and the radio instrument’s baselines. Methods. We selected three type II bursts that were observed with the LOFAR core and remote stations in the Solar Cycle 24. We estimated the sizes and shapes (ellipticity) of the radio sources from 20–200 MHz using a two-dimensional (2D) Gaussian approximation. Results. Our analysis shows that the smallest radio source size for type II bursts in the solar corona that can be observed in the solar atmosphere at low frequencies is 1.5′±0.5′ at 150 MHz. However, even though the observations were taken with remote baselines (with a maximum distance of ∼85 km), the effective baselines were much shorter (∼15 km), likely due to snapshot imaging of the Sun. Conclusions. Our results show that the radio source sizes are less affected by scattering than suggested in previous studies. Our measurements indicate smaller source sizes at frequencies below 95 MHz compared to previous reports, though some overlap exists with measurements at higher frequencies when using smaller baselines.

Context. Although the differential rotation rate on the solar surface has long been studied using optical and extreme ultraviolet (EUV) observations, associating these measurements with 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. Aims. We aim to characterise the differential rotation profile of the upper chromosphere using cleaned solar full-disc 17 GHz radio imaging from the Nobeyama Radioheliograph spanning a little over two solar cycles (1992–2020). Methods. A tracer-independent method based on automated image correlation was employed on daily full-disc 17 GHz radio maps. This method determines the angular velocities in 16 latitudinal bins of 15° each by maximising the 2D cross-correlation of overlapping image segments. Results. The best-fit parameters for the differential rotation profile are A = 14.520 ± 0.006°/day, B = –1.443 ± 0.099°/day, and C =–0.433 ± 0.267°/day. These results suggest that the upper chromosphere rotates significantly faster than the photosphere at all latitudes, with a relatively flatter latitudinal profile. We also observed a very weak anti-correlation, ρ_s = −0.383 (94.73%), between the equatorial rotation rate and solar activity. Conclusions. 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 (A) 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.