Three solar cycles of day-time southern hemisphere Es activity

Abstract

The hourly values of the ionosonde parameters f o E s and f b E s for times near mid-day have been examined over three solar cycles 1947–82. The daily parameters were studied for two southern hemisphere stations: Christchurch (temperate zone) (−43°.6′ geographic, −48°.1′ geomagnetic) and Rarotonga (subtropical) (−21°.2′ geographic, −20°.7′ geomagnetic). The data represent the longest such analyzed sporadi- E record in the literature. The seasonal variations of per cent occurrence of f 0 E s ⩾ 5 MHz and f b E s ⩾ 4 MHz for both stations show no sunspot dependence, strong activity in local summer and, in the case of Christchurch, minor enhancements in local winter. Long-term f b E s occurrence is closely associated with Zurich sunspot number R z (correlation coefficients 0.93 for Christchurch and 0.76 for Rarotonga) with all seasons, showing the same in-phase R z control for both stations. Long-term f o E s occurrence for both stations and for all seasons exhibits no dependence on R z . There is evidence of a 6 year period in seasonal f o E s activity for Rarotonga. The data show a large and unexplained systematic decrease in f o E s occurrence for both stations and for all seasons by a factor of about 3 from 1947 to 1982.