Ionospheric Electron Content Measurements at a Northern Low Midlatitude Station through Half a Solar Cycle

Abstract

Ionospheric electron content (IEC) measurements with the help of geostationary satellite radio beacon have been made at Delhi during the period 1975-80. Results for three 1-yr periods representing low medium and high solar activity conditions are presented. Detailed study of the diurnal seasonal and solar cycle variations has revealed a number of new features which were not observed earlier from observations made with low orbiting satellites. The IEC, its early morning build-up rate and nighttime decay rate of ionization show seasonal as well as semiannual variations with peak in equinoctial months which indicates that response of the ionosphere to solar radiations is greatest in this season. A secondary enhancement was observed in the evening hours frequently during winter and equinox of medium and high solar activity period. The seasonal anomaly, i.e. when IEC in winter exceeds that in summer appears during medium and high solar activity period only. Another interesting observation is that IEC shows nonlinear dependence with 10.7 cm solar flux. After increasing linearly up to about 200 units, it decreases with varying rates in all the three seasons with the result that at about 230 units, winter IEC becomes lower than the summer values. Study of the behaviour of IEC during different hours of the day with season and solar activity has revealed that IEC behaves differently in daytime and nighttime. Also the harmonic components beyond the third one do not show any significant values.