Equatorial ionospheric response to the 10.7 cm radio flux over two sunspot cycles (1969-1991)

It is evident that fluctuations in a standard ionospheric parameter, the minimum (virtual) height (h^′F) of the equatorial F-region in the African (Ouagadougou), Asian (Manila) and American (Huancayo) longitudinal sectors, closely resemble changes in solar activity as deduced from the 10.7 cm solar flux index (S), over two solar cycles (1969-91). The monthly median hourly value of h^′F, particularly in the post-sunset period (18-20 LT), are positively correlated with the monthly average S. The value of h^′F can be deduced from an empirical formula: h^′F=0.68S+218.3, with the correlation coefficient (r) between h^′F and S being 0.78. The diurnal distribution of r during daytime (06-14 LT) was radically different for the African and Asian longitudinal sectors during 1980-1991, with the most pronounced difference in the post-noon period (12-14 LT) when the correlation coefficients r for the Asian and African sectors are 0.8 and 0.2, respectively. Thus, the daytime F-region in the African sector responded far less to changes in solar activity than the Asian F-region during this cycle. This longitudinal anomaly was however absent in the preceding cycle (1969-1979) when the African and Asian sectors were both characterised by low daytime and pronounced post-sunset correlation coefficient r. The American sector appears to have a high correlation coefficient r in daytime increasing to a small maximum in the post-sunset interval. The post-sunset enhancement in r is a characteristic feature for equatorial stations only (corrected geomagnetic latitude <10^°).