Spectral features of burst radio emission over the solar cycle

Abstract

We have investigated common burst spectral features for the 20th cycle of solar activity. The maximum daily radio fluxes in 8 frequency ranges are analysed. For every year the classification of these daily spectra is obtained by cluster analysis methods. There are two spectral minima for average spectra of clusters (in frequency ranges 4–3 and 0.5–0.25 GHz). As a rule their positions do not change during the solar cycle.

Every annual spectrum of weak bursts has three minima (in frequency ranges 4–3, 2–1, and 0.5–0.25 GHz). The positions of these minima remain invariable during the solar cycle. But anuual spectra of strong bursts depend essentially on the phase of solar activity.

The basic features of most burst spectra can be explained by gyrosynchrotron radiation of thermal and nonthermal electrons and plasma radiation at the plasma frequency and its second harmonic.