Radio Emissions from the Sun and the Interplanetary Medium

Abstract

The Sun is a radio emitter in a large spectral domain from submillimeter to kilometer wavelengths. Observations at different wavelengths sample different heights in the solar atmosphere, with longer wavelengths referring to higher heights above the photosphere. This review focuses on a few topics related to the physics of the Sun–Earth system. Section 2 summarizes the general context of radio wave propagation and radio emissions. The limits of the shortest and longest wavelengths that can be observed from ground are fixed, respectively, by the transparency of the Earth’s atmosphere and by the frequency cutoff of the ionosphere. Satellite observations extend the observable radio spectrum. Radio emissions result from thermal and nonthermal mechanisms. Section 3 summarizes the radio thermal observations of the Sun. The thermal emission is produced by thermal bremsstrahlung and also by gyro resonance emission of the electrons in the presence of a magnetic field (mainly in the microwave domain). Thermal emissions originate from regions with distinct physical parameters, and their detection will depend on the observing frequency. The different classes of radio bursts are presented in Sect. 4. They have distinct spectral characteristics and can last from a fraction of second to several hours; this section focuses more particularly on radio bursts that are produced by accelerated electron beams propagating along the magnetic field or by shocks. The impact on technologies of solar radio bursts is briefly discussed in Sect. 5. Observations of radio bursts reveal various forms of activity and acceleration processes that are associated sometimes to large-scale eruptive phenomena that include flares, filament eruptions, Coronal Mass Ejections (CMEs), and shocks. CMEs are the most spectacular largescale manifestations occurring at the Sun. Radio observations of CME events are presented in Sect. 5. It is shown that radio imaging and spectral observations provide signatures on the initial steps and development of CMEs. The last section concludes that radio observations offer significant insights to understand solar activity, its link with the interplanetary medium, and the consequence on the Sun–Earth system.