Spectral distributions of microwave bursts

Abstract

The lack of open literature publication of the distributional properties of the cm-λ spectra of solar microwave bursts has lead to some erroneous concepts of the typical characteristics of these spectra. To provide more accurate information, this paper sets forth various distributions of the peak flux density spectra of large numbers of bursts, based on observations of the Sagamore Hill Radio Observatory at nine discrete frequencies between 245 and 35000 MHz over the years 1968–1971. As a foundation for the distribution studies, the basic spectral classification system is outlined. The majority of burst spectra were found to contain a cm-λ component having a single spectral maximum in the 1400 to 35000 MHz range; such spectra are designated C type. A study of the correlation of the spectral maximum frequency fmax of the cm component and the photospheric magnetic field strength of the associated region shows a tendency for greater correlation at higher fmax for stronger magnetic sssfields. A study of the correlation for C type spectra between fmax and the quasi-cutoff frequency fqc on the low-frequency side shows that for bursts of moderate peak flux density (50–500 sfu) fqc is well correlated with fmax; a good fit to the relation fmax=Afqc is found with A =3.4. The possible attenuating mechanisms responsible for the spectral shaping of the cm component are discussed.