Solar Radio Astronomy

Abstract

This chapter deals with solar activity, describing the radio emission related to sunspots and flares in the different frequency ranges. In the laboratory, many mechanisms are available for the generation of radio waves, however only a few of them seem to be efficient enough in the solar atmosphere to give intensity detectable at the earth. They are thermal emission by free-free transitions, plasma oscillations, probably generated by the Cerenkov effect, and the gyro and synchrotron radiations emitted by energetic particles rotating in the solar magnetic field. As in the visible range, the highly ionized gas of the solar atmosphere emits radiation by free-free transitions of the hydrogen atoms. The effect of the magnetic field must be taken into account to study the emission and the propagation of radio waves. The classical magnetoionic theory can be used where the wave is split into two components by the magnetic field, the ordinary and extraordinary waves, with different refractive indices, and different polarizations. The presence of the polarized component of the slowly varying component can be explained by the effect of the magnetic field of the sunspot, giving an increase of the optical depth different for the different modes at the gyromagnetic frequency and its harmonics.