Observation of Fresnel-type Fadings at Delhi, a Low Midlatitude Station, over Half a Solar Cycle

Abstract

Fresnel-type fading patterns have been observed on recordings of orbiting as well as geostationary satellites made at Delhi for over half a solar cycle. Results show that the occurrence of Fresnel-type fading patterns is most frequent during low solar activity but less during high solar activity. These patterns were most frequently recorded in the time interval between local sunset and midnight mainly in summer months. Patterns occur mostly in groups with average periodicity ranging from 2-10 min. The orbiting satellite observations indicate the tendency of patterns to occur at zenith angles greater than 45° but no azimuth preference is observed in the occurrence of these irregularities. The height of the ionospheric irregularities responsible for such patterns, corresponds to the E-region height. The occurrences of these patterns are associated with the occurrence of a long-lived or a short-lived blanketing type sporadic-E at Delhi with <i>f</i>₀Es values normally greater than 5 MHz whereas no correlation with the occurrence of spread-F was observed. It was also observed that in addition to high <i>f</i>_oEs value, a diffused nature of sporadic-E layer, indicated by range spread on ionograms, is also one of the important factors associated with the production of these diffraction patterns.