Periodic fadings in VHF radio-satellite transmissions during the solar eclipse on 23 October 1976

Abstract

THE localised time-dependent cooling action of the lunar shadow during an eclipse may generate gravity waves at ionospheric heights1,2. The bow wave thus formed would be detected at great distances from the eclipse path. Experimental data in the northern hemisphere from the eclipse on 7 March 1970, indicated the presence of gravity waves in the F-region, deduced from the incoherent backscatter technique3, vertical incidence soundings4, and recordings of the Faraday rotation angle of VHF transmissions from geostationary satellites5. The detected travelling ionospheric disturbance (TID) had the horizontal phase velocity of 400m s−1 (ref. 4) and 620±120 m s−1 (ref. 5). TID was reported to travel north to south4, or 279±25° east of north5. We report here our observations of large-magnitude periodic fadings during the eclipse of 23 October 1976, which indicate that the eclipse is associated with generation of wave disturbance affecting radio-satellite transmissions in the upper VHF. The waves had a 40-km wavelength, and propagated from north to south with a velocity of 556 m s−1.