Cosmic radio noise absorption on 25 mc/s andFscatter

Abstract

In a previous communication [Bhonsle and Ramanathan, 1958], the authors have reported on the measurements of cosmic radio noise on 25 Mc/s at Ahmedabad over a period of 1 year from March 1957 to February 1958 and have given estimates of the values of the attenuation of cosmic radio noise in the atmosphere. On the assumption that the attenuation occurs mainly in the D region and F regions of the ionosphere, it was shown that the total attenuation could be divided into two components, a D-region and an F2-region component. On examination of the mean diurnal and seasonal variations, the curves of total attenuation (Fig. 1) showed the following features: (1) minimum attenuation occurs before sunrise; (2) a day-time maximum in the attenuation occurs near or shortly after noon; and (3) a second maximum in attenuation occurs after sunset and before midnight, usually between 20 and 22 hours local time, in the winter equinoctial and months. Adopting a method previously used by Mitra and Shain [1953], D-region absorption and F attenuation were separately estimated. The D-region absorption was found to be directly dependent on solar zenith distance with maximum amplitude in summer and minimum in winter. The deviative F-region attenuation was not a simple function of the solar zenith distance; above a certain minimum value of the critical frequency foF2, it increased rapidly with increase in foF2. The F2 attenuation in the first half of the night often exceeded its maximum daytime value by about 2 db. It was evident that the increase in the value of foF2 in the early part of the night and its nearer approach to 25 Mc/s were inadequate to account for the premidnight peak in F2 attenuation during winter and equinoctial months. There was some evidence to show that the premidnight peak was connected with F scatter. But the occurrence of a rather sharp rise in attenuation shortly after sunset and its reaching a maximum at 21 to 22 hours were puzzling features.