Solar Radio Burst in the metric to kilometric range

Abstract

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.

Figures (16)

• Figure 1: Daily observing windows and operational frequency ranges of non-imaging solar radio flux and light-curve instruments. Each horizontal bar represents the combined time–frequency coverage for a specific instrument, plotted in UT (horizontal axis) and frequency in MHz (vertical axis, logarithmic scale). These instruments measure integrated solar radio flux at one or multiple discrete frequencies, providing high-cadence time series used to study flare energetics, emission mechanisms, event timing, and long-term solar radio variability. Instruments with observing sessions crossing midnight (e.g., NoRP) are represented with two split rectangles. Together, these facilities offer continuous global monitoring across 1–400 GHz, which is crucial for characterizing impulsive and slowly varying solar radio emissions.
• Figure 2: Single frequency data, i.e. time series of the flux density in sfu, recorded by the solar multichannel radiopolarimeter of the TSRS. The flux density, i.e., the L-hand and R-hand circular polarization measurements (top and bottom panel, respectively), were routinely taken in the metric range. In the first 80s of recordings, the structured continuum is strongly L-polarized (about 90 %), and the continuum in the following 160s is weakly L-polarized. (Modified from Magdalenic2008Thesis).
• Figure 3: Operating time windows and frequency coverage for major solar dynamic spectrometers and spectropolarimeters that record full-Sun radio spectra with fine spectral and temporal resolution. These instruments produce dynamic spectra essential for identifying and classifying solar radio bursts (Types I–V), diagnosing electron acceleration, tracing the propagation of CMEs/shocks, and probing coronal/heliospheric plasma. Frequency ranges span from long-wavelength decametric systems (e.g., URAN-2 at 8–33 MHz) to microwave spectrometers (e.g., SSRT 4–8 GHz) and high-frequency broadband systems such as Phoenix-3 (100–5000 MHz). Many facilities operate only during local daytime (e.g., Humain, Trieste, Gauribidanur), while others maintain seasonal schedules (e.g., SSRT spectrograph). Instruments are shown excluding the distributed CALLISTO network, focusing instead on dedicated high-performance spectrometers with long-term archives. This global ensemble provides near-continuous spectral coverage from 10 MHz to several gigahertz, enabling comprehensive studies of solar radio bursts.
• Figure 4: Dynamic spectra observed by ground-based and space-based instruments, panels a) and b) respectively. The slowly drifting type II radio bursts, i.e. signatures of shock waves and fast drifting type III radio bursts, i.e. signatures of fast electron beams, are marked in two spectra. a) Dynamic radio spectrum observed by Green Bank Solar Radio Burst Spectrometer in the frequency range 70–18 MHz, observed on May 31, 2005. b) The radio emission in DH to km wavelength range observed by WIND/Waves instrument bougeret1995 on December 7, 2020.
• Figure 5: Time–frequency coverage for solar-dedicated radio interferometers and imaging arrays, plotted across UT and operating frequencies. These instruments are designed specifically for high-resolution imaging of the solar corona, mapping active regions, flares, CMEs, and coronal magnetic field topology. Shown are facilities such as GRAPH (40–120 MHz), CSRH / MUSER (0.4–15 GHz), SunDish (18–26 GHz), Siberian Radioheliograph (4–8 GHz), Metsähovi 37 GHz solar system, NoRH (17 & 34 GHz), EOVSA (1–18 GHz), OVRO-LWA, NRH (150–450 MHz), RATAN-600, ARUN (1–12 GHz solar imager) and others. Instruments crossing midnight (e.g., NoRH, EOVSA) are represented with wrap-around blocks. Collectively, these systems provide imaging across 0.04–37 GHz, offering unprecedented diagnostics of coronal magnetic fields, source heights, electron populations, and plasma dynamics from metric to millimeter wavelengths.