SPREAD-F at anomaly crest regions in the indian and american longitudes

Occurrence features of spread-F over Ahmedabad, situated near the anomaly crest region in Indian zone are studied from the quarter hourly ionograms for the years 1983-95. The occurrences of range and frequency types of spread-F over Ahmedabad are seasonal and solar cycle dependent. Frequency type of spread-F is most frequent during June-solstices of low sunspot years. The range type of spread-F is frequent during equinoxes and December-solstices (with lower occurrence) of high sunspot years. ne occurrence features of range spread at Ahmedabad are compared with the occurrence features of range spread over Cachoeira Paulista, a station near the anomaly crest region in Brazil in the American sector. The occurrence of range spread-F over Cachoeira Paulista shows strong seasonal dependence with a distinct maximum during local summer (December solstices) for both low and high sunspot years. The occurrence of range spread over Cachoeira Paulista is much higher than over Ahmedabad. (C) 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.