Annual and Semiannual Variation in the Ionospheric F2-Layer Electron Density over the Indian Zone and Effect of Solar Activity on It

In situ measurement carried out by the Retarding potential Analyzer (RPA) on board SROSS C2 and ROCSAT during 1995 to 2003 covering ascending and descending periods of solar cycle 23 over Indian equatorial and low latitude were used to study the annual and semiannual variation of electron density at the topside F region. The ‘Semiannual anomaly’ which represent the electron density in equinox (March, April, September and October) is greater than that at solstice (May, June, July, August, November, December, January and February). The ‘annual anomaly’ which represents the electron density in winter (November, December, January, February) is higher than that in summer (May, June, July, August). The analysis has been carried out for the geomagnetic equator and ±10o magnetic latitudes. Observations reveal the existence of an equatorial asymmetry during daytime (10:00 – 14:00 hrs.) with higher electron density in spring (March, April) than in autumn (September, October) for both ascending and descending leg of the solar cycle. At the peak of the solar cycle, the density becomes equal for both equinoxes. Nighttime (22:00 – 00:00 hrs.) density in autumn is higher than that in spring for the ascending half of the solar cycle, becomes equal for both the equinoxes around the peak of the solar cycle. In the descending half the vernal density becomes higher than the autumnal density. The periodograms obtained from a Fourier analysis of the daytime average density shows that the annual variation is dominant over the semiannual variations for low to moderate solar activity whereas the semiannual peak becomes dominant over annual peak for high solar activity irrespective of the latitudes. At night, however, latitudinal differences have been observed. The annual variation is stronger than the semiannual variation at 10o N for low to moderate solar activity while for high solar activity the situation reverses. At 10o S and the magnetic equator, the annual variation is dominant for all levels of solar activity. Amplitude of the annual variation is higher in winter compared to that in summer. The physical and dynamical processes responsible for the observed annual and semi-annual trends in topside density will be identified and discussed.