Low latitude features of annual and semiannual periodicities in NmF2 in American and Indian zones

Abstract

The monthly median NmF2 values during the high sunspot period 1958 from a number of low latitude stations in the American zone are subjected to harmonic analysis. The results are presented as a function of local time and are compared with those obtained earlier from the Indian zone. The gross features in the two zones are found to be identical. The semiannual periodicity is shown to have maxima in April and October at all stations for all hours of the day and this is attributed to similar changes in thermospheric atomic oxygen concentration controlling the production of ionization. The annual maximum exhibits systematic changes in the amplitude and phase as a function of latitude and local time. One of the main features is a north-south asymmetry in the amplitude of the annual variation which can also be attributed to larger concentration of atomic oxygen in January than that in July. Further, a geomagnetic control is seen to exist on the annual variation at both the eastern and western longitude zones. This is explainable in terms of plasma exchange between magnetically conjugate latitudes.