THE EFFECTS OF LARGE-SCALE AND SMALL-SCALE DENSITY STRUCTURES ON THE RADIO-EMISSION FROM CORONAL STREAMERS

The radio observations of the coronal streamers obtained using Clark L ake radioheliograph at 73.8, 50.0, and 38.5 MHz during a period of min imum activity in September 1986 are presented. Streamers appear to cor relate with two prominent disk sources whose intensities fluctuated ra ndomly. The variations in half-power diameter of the radio Sun are fou nd to correspond with the variations in the white-light extents of the coronal streamers. It appears that the shape of the radio Sun is not a function of the phase of the solar cycle; instead it depends on the relative positions of the streamers in the corona. The observed peak b rightness temperatures, T(B), of the streamers are found to be very lo w, being congruent-to 6 x 10(4) K. We compute the brightness temperatu re distribution along the equator by tracing the rays in the coronal p lasma. The rays are deflected away by the streamers before reaching th e critical density level, whereas they penetrate deeper into the coron al hole for small angles between the line of sight and the streamer ax is. As a consequence, it is found that the streamers and coronal holes appear in the calculated equatorial brightness distribution as irregu lar brightness depressions and enhancements, respectively. The fine st ructures are found to disappear when the scattering due to small-scale density inhomogeneities is included in the ray-tracing calculations. The required relative level of density fluctuations, epsilon1 = DELTAN /N, is found to be greater than 12% to reduce the peak brightness temp erature from 10(6) K to 6 x 10(4) K for all the three frequencies.