Post-Sunset behaviour of the equatorial anomaly in the Indian sector

Abstract

A detailed Investigation is made of the relationship, In the post-sunset period, between the depth of the equatorial anomaly and the evening rise of F-region at the trough on a day-to-day basis, using pubhshed data of ionosonde stations In the Indian sector for a 2-yr period (1957-58) of high sunspot activity. The day-to-day changes In the post-sunset rise of F-region at the trough are found to show a significant positive correlation with those of the depth of the anomaly, and a significant negative correlation with those of fo F2 at the trough during all seasons and on quiet as well as disturbed days The day-to-day variability of fo F2 at the crest of the anomaly is also found to exhibit a significant positive correlation (with a delay of2 hr) with that of the evening rise of F-region at the trough during disturbed days of all the three seasons and on quiet days of summer and equinoxes. The results of the present study are In accordance with the prevailing View that the post-sunset enhancement of the anomaly during periods of high sunspot activity is due to a renewal of the 'fountain effect' It is suggested that the conspicuous absence of a Significant relationship of the day-to-day variability of fo F2 at the crest of the anomaly with that of the post-sunset rise of F- region at the trough during quiet days of Winter months is due to the effect of pole ward neutral winds in the post-sunset period.