Study of the D-region electron density ledge over the dip equator

Abstract

A large number of rocket measurements of electron density profiles in the D-region have been made over Thumba (8.5°N, 76.8°E) during the last two decades. An examination of these profiles reveals that the occurrence of a ledge in electron density depends on solar zenith angle, month and the level of solar activity. For example, in a flight conducted at 1230 hrs IST (Indian Standard Time) on 7 April 1972 no ledge was seen, whereas on the 21st day of the same month in 1975 at 1341 hrs IST, a ledge was visible. In winter, a ledge was not seen in any of the flights during which high solar activity occurred, whereas in one flight during low solar activity the ledge was visible. Also in all the flights during March a ledge was present. An ion chemical scheme has been used to explain this phenomenon theoretically. It is shown that the ledge in electron density can be explained by a simultaneous increase in electron production rate by a factor of about 100 and a decrease in electron loss coefficient by a factor of about 10 as the height increases from 85 to 90 km. It has also been found that a low value of NO concentration of about 10 5 cm −3 around the mesopause is conductive to the occurrence of a ledge.