Attenuation of galactic radio noise at 25 Mhz and 21.3 Mhz in the ionosphere over Ahmedabad during 1957-1964

Abstract

Measurement of the intensity of galactic radio noise at Ahmedabad from 1957 to 1964 has shown that its attenuation has decreased steadily with the decline of solar activity. Calculation of the absorption in the D region and in the F region up to the F2 peak using N(h) profiles and effective collision frequencies of electrons has shown that a large part of the absorption of galactic radio noise takes place in the F region over Ahmedabad, both below and above the F2 peak. It is also found that the nighttime attenuation in low solar activity period (1964–1965) is much larger than can be explained on the basis of electron-ion collision processes alone. It is suggested that scattering of galactic radio noise by inhomogeneities in the F region is a source of extra attenuation during night in a low solar activity period, since it is known that the occurrence of spread F shows a considerable increase during night at Ahmedabad in low sunspot years. The absorption measurements also suggest that electron temperatures are appreciably higher than neutral gas temperatures in the early morning hours and that the excess temperature of electrons is higher in low sunspot years than in high sunspot years.