Solar cycle effects on large scale equatorial F-region plasma depletions

In recent years, scientists have used wide-angle imaging observations of F-region nightglow emissions (e.g., OI630 nm and OI777.4 nn) to map the F-region equatorial ionospheric irregularity structures, characterized by large scale plasma depletions, generally known as transequatorial plasma bubbles. In Brazil, an OI630 nm all-sky imaging system has been in operation at Cachoeira Paulista (22.7 degrees S, 45.0 degrees W) since March 1987. This has provided an extensive data-base of the OI630 nm all-sky imaging observations in the equatorial ionospheric anomaly region. In this paper, the morphology of large scale equatorial F-region plasma depletions under sunspot maximum and minimum conditions is presented and discussed. It has been observed that the occurrences of plasma depleted regions are relatively lesser during low solar activity than during high solar activity. The seasonal occurrence pattern is fairly similar for high and low solar activities. (C) 1999 COSPAR. Published by Elsevier Science Ltd.