Solar energetic particles and their association with radio emissions

Abstract

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.

Figures (6)

• Figure 1: Frequency coverage and operational periods of major solar radio imaging instruments worldwide. The plot illustrates the long-term continuity and complementary frequency ranges of facilities from metric to microwave wavelengths Together, these observatories provide multi-frequency imaging coverage of the solar atmosphere over several decades, enabling studies of radio bursts, active regions, and coronal dynamics across multiple solar cycles.
• Figure 2: Typical daily observation windows and frequency coverage of solar radio imaging instruments. Each shaded region represents the frequency band and local observing time (in Universal Time) for facilities across different longitudes. The plot highlights the near-continuous global coverage achieved through coordinated observations from Asia (GRAPH, DART, CSRH), Europe (NRH, RATAN-600, SunDish, MRO, SRH), and the Americas (EOVSA), facilitating 24-hour monitoring of solar radio emissions from 0.05 GHz to 37 GHz. The figure shows the daily maximum observation window for each instrument which would correspond to summer time observations, however, these observation windows can vary.
• Figure 3: SolarMACH plot showing the current constellation of spacecraft monitoring the Sun gieseler2023. Multiple spacecraft are also located close to Earth. The solid spirals show the magnetic connections of each spacecraft and the Earth to the Sun along the interplanetary magnetic field.
• Figure 4: Connecting remote radio and X-ray observations to solar energetic electrons. a. Three-dimensional representation of the electron acceleration signatures (hard X-rays and Type II radio bursts) that occurred during a CME eruption of 3 October 2023. The purple mesh represents the CME shock outline and the purple and blue magnetic field lines show the magnetic connections to two spacecraft. b. Solar-MACH plot of the spacecraft configuration on 3 October 2023 where the black arrow represents the coordinates of the flare. c. X-rays, radio bursts and energetic electrons observed by Solar Orbiter/STIX, PSP/RFS and LOFAR, and Solar Orbiter/EPT/HET, respectively. The type II bursts shows a good temporal association with the in situ energetic electrons and it is also the remote feature that is best magnetically connected to Solar Orbiter. Figure adapted from morosan2025.
• Figure 5: Coronal Mass Ejections (CME) and signatures of accelerated electrons. (a) SDO/AIA $171$ Å image at 11:20:57 UT, superimposed with a potential field source surface (PFSS) extrapolation at noon showing both open (blue) and closed (white) field lines within region surrounding the active region and northern sunspot, and a LASCO C2 image showing the streamer-puff (above) and narrow (below) CME fronts at 11:36:05 UT, as shown in 2020ApJ...893..115C. (b-d) Dynamic spectra showing samples of the spike emission. (e) Dynamic spectra of a Type IIIb J-burst. All dynamic spectra are background subtracted where the background is defined using a region at the start of each dynamic spectra containing no bursts at all frequencies. Image taken from 2023ApJ...946...33C.