The Relationship Between Solar Radio and Hard X-ray Emission

TL;DR

This review explores the relationship between solar radio and hard X-ray emissions, highlighting how combined observations improve understanding of electron acceleration, magnetic fields, and emission mechanisms during solar flares.

Contribution

It provides a comprehensive synthesis of joint radio and X-ray observations, including methods for comparing electron energy distributions and insights into emission mechanisms and physical conditions in solar flares.

Findings

Radio and X-ray emissions show similar temporal behavior during flares.

Differences exist between electron populations at different energies.

Millimeter/submillimeter observations reveal puzzling results compared to X-ray data.

Abstract

This review discusses the complementary relationship between radio and hard X-ray observations of the Sun using primarily results from the era of the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite. A primary focus of joint radio and hard X-ray studies of solar flares uses observations of nonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlung hard X-rays to study the properties of electrons accelerated in the main flare site, since it is well established that these two emissions show very similar temporal behavior. A quantitative prescription is given for comparing the electron energy distributions derived separately from the two wavelength ranges: this is an important application with the potential for measuring the magnetic field strength in the flaring region, and reveals significant differences between the electrons in different energy ranges. Examples of the use of simultaneous data from the two wavelength ranges to derive physical conditions are then discussed, including the case of microflares, and the comparison of images at radio and hard X-ray wavelengths is presented. There have been puzzling results obtained from observations of solar flares at millimeter and submillimeter wavelengths, and the comparison of these results with corresponding hard X-ray data is presented. Finally, the review discusses the association of hard X-ray releases with radio emission at decimeter and meter wavelengths, which is dominated by plasma emission (at lower frequencies) and electron cyclotron maser emission (at higher frequencies), both coherent emission mechanisms that require small numbers of energetic electrons. These comparisons show broad general associations but detailed correspondence remains more elusive.