Nighttime VHF scintillations at 23°N magnetic latitude and their association with equatorial F region irregularities

A study of postsunset VHF scintillations observed during recent solar maximum period (1980) at 23°N magnetic latitude, which is substantially beyond the daytime crest of the equatorial anomaly along 84.4°E geographic meridian, is carried out in order to examine their association with the irregularities of the equatorial F region. This is an extension of the study reported earlier from a chain of stations along the same meridian but covering a magnetic latitude belt of 3°-21°N (Somayajulu et al, 1984). The characteristics and the occurrence pattern of scintillations observed at 23°N agree with the results reported upto 21°N. Also, the occurrence of scintillations at 23°N is found to be conditional to their prior occurrence at lower latitudes. This further confirms that the occurrence of nighttime scintillations up to 23°N magnetic latitude in the Indian zone are essentially controlled by the generation of equatorial F region irregularities. The study also provides evidence that the equatorial F region irregularities extend to altitudes in excess of 1300 km at least during equinoctial periods of high sunspot years and consequently cause scintillations in a wider latitudinal belt. The occurrence frequency of nighttime scientillations at 84°E, when compared with that of 93°E and 122°E longitudes for the same period of time, indicates strong longitudinal dependence, particularly in the Indian sector.