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

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 that maximum post sunset Vd 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 Vd occurs in WS followed by VE, AE, and SS. Further, the role of sunset times at the magnetic conjugate points in modulating the time and magnitude of peak drifts during different solar epochs is ascertained. The equinoctial asymmetry in peak Vd during high and moderate 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. Key Points Control of conjugate point sunset times on magnitude and time of peak Vd Equinoctial asymmetry in PRE during both solar epochs Seasonal and solar activity variations of long term nocturnal vertical drift