A search for local sources of the S-component at decameter wavelengths

Abstract

In an effort to find local sources of the Slowly Varying Component (SVC), an analysis is made of the episodical observations carried out since 1972 during periods of low solar activity at 20 and 25 MHz. In contrast to other writers who reported on successful observations of such sources (Kundu et al., 1977; Sastry et al., 1981, 1983), we have not found local sources, though we used the UTR-2 radio telescope to observe from several to several tens brightness distributions of the quiet Sun per day.

The multiple daily measurements allowed tracing the dynamics of the burst development. As has been found, bursts of high intensity can give rise to nonthermal radiation from the region of generation, thus producing a considerable increase in the maximum brightness temperature of scans across the Sun. The relaxation time of this process is 20 to 30 min. It is not improbable that this is the effect responsible for large variations in the maximum brightness temperatures of the scans that Sastry et al. (1981) connected with the SVC.

25 MHz radioheliograms of April 29 and 30, 1976, are presented which supplement the data of observations at Clark Lake (Kundu et al., 1977). It is shown that the sources observed there on April 27 and April 29, 1976, were most likely of nonthermal nature.

We conclude that at present, in spite of reports of some workers, there is no convincing evidence for the existence of local SVC sources at decameter wavelengths. Their reality could be confirmed or denied by more observations with better radio telescopes and a better account of the specific features of the decameter band.