Incoherent Solar Radio Emission

TL;DR

This paper reviews incoherent solar radio emission mechanisms—free-free, gyroresonance, and gyrosynchrotron—and discusses their diagnostic applications for probing the solar atmosphere and magnetic fields.

Contribution

It provides a detailed analysis of the emission mechanisms and highlights their potential for solar atmospheric diagnostics, especially in non-flaring and flaring conditions.

Findings

Free-free emission dominates in the non-flaring Sun.

Gyroresonance emission reveals magnetic field strength and orientation.

Gyrosynchrotron emission is key during solar flares at >1-2 GHz.

Abstract

Incoherent solar radio radiation comes from the free-free, gyroresonance, and gyrosynchrotron emission mechanisms. Free-free is primarily produced from Coulomb collisions between thermal electrons and ions. Gyroresonance and gyrosynchrotron result from the acceleration of low-energy electrons and mildly relativistic electrons, respectively, in the presence of a magnetic field. In the non-flaring Sun, free-free is the dominant emission mechanism with the exception of regions of strong magnetic fields which emit gyroresonance at microwaves. Due to its ubiquitous presence, free-free emission can be used to probe the non-flaring solar atmosphere above temperature minimum. Gyroresonance opacity depends strongly on the magnetic field strength and orientation; hence it provides a unique tool for the estimation of coronal magnetic fields. Gyrosynchrotron is the primary emission mechanism in flares at frequencies higher than 1-2 GHz and depends on the properties of both the magnetic field and the accelerated electrons, as well as the properties of the ambient plasma. In this paper we discuss in detail the above mechanisms and their diagnostic potential.