On the solar activity variations of nocturnal F region vertical drifts covering two solar cycles in the Indian longitude sector

摘要

A comprehensive analysis of the seasonal and solar cycle variabilities of nighttime vertical drift over the Indian longitude sector is accomplished using ionosonde data located at the magnetic equatorial location, Trivandrum (8.5°N, 76.5°E). The analysis extends over a span of two decades (1988–2008). The representative seasonal variations based on the extensive data of nocturnal vertical drift during three different solar activity epochs is arrived at, for the first time. Seasonally, it is seen thatmaximum post sunset V_d is obtained in vernal equinox (VE), followed by autumnal equinox (AE), winter solstice (WS), and summer solstice (SS) for high and moderate solar epochs, while for low solar epoch, maximum V_d occurs in WS followed by VE, AE, and SS. Further, the role of sunset times at themagnetic conjugate points inmodulating the time and magnitude of peak drifts during different solar epochs is ascertained. The equinoctial asymmetry in peak V_d during high andmoderate solar epochs is another significant outcome of this study. The solar activity dependence of vertical drift for a wide range of solar fluxes has been quantified for all the seasons. In the present era of GPS-based communication and navigation, these are important results that give a better handle in understanding essential factors that impact equatorial ionospheric phenomena.