Observations of the Spectrum of High-Intensity Solar Radiation at Metre Wavelengths. III. Isolated Bursts

Abstract

Observations are described of the spectrum of " isolated bursts" of solar radio-frequency radiation in the frequency range 70-130 Mc/s. These bursts last for a few seconds and have a bandwidth of the order of tens of megacycles per second. Prior observations indicate that they are not circularly polarized. They occur sporadically, often in small groups ; many hours sometimes elapse between successive bursts or groups. Although, in general, their spectra show diverse features, some of them (referred to as " type III " bursts) are of a distinct type characterized by a rapid drift, with time, of the frequency of maximum intensity towards the lower frequencies, at a rate of the order of 20 Mc/s. per second. Characteristics of the spectra of type III bursts are described in detail. The results are discussed and hypotheses of origin examined. It is shown in particular that the frequency drift of type III bursts cannot be attributed to the selective group retardation of waves in the solar atmosphere emanating from a fixed source. The frequency drift may, however, be associated with the rapid motion of a source travelling outwards through the solar atmosphere.