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.
