Microwave imaging spectroscopy of the solar corona

Abstract

The study of the solar corona plays a key role in understanding solar activity. Microwave imaging spectroscopy -measuring microwave spectra at every point on the solar disk (or image) -is the most promising method for studying the lower corona and the chromosphere-corona transition region. Solar microwave emission originates in the low corona and in the upper chromosphere and is produced both by thermal and non-thermal electrons. The main types of emission are bremsstrahlung, gyroresonance and gyrosynchrotron emission. The quiet Sun emission is dominated by the bremsstrahlung while the active region emission is due to the gyroresonance mechanism. The gyrosynchrotron emission is generated by non-thermal electrons during solar flares. Microwave emission spectra allow us to estimate the temperature and density of the plasma in active regions. Microwave spectroscopy of the solar corona is perhaps the only method for measuring the coronal magnetic field both for the quiet Sun and for transient events. The Siberian Radioheliograph (SRH), a new generation solar radio telescope, is described, and the first observations of solar activity in the broadband frequency range 3-24 GHz are presented. The estimation of the magnetic field scale length is presented. The scale length was estimated by using the SRH data -spectra of an active region emission in the frequency range 3-12 GHz.