Long term ionospheric electron content variations over Delhi

Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinat es: 28.63 degreesN, 77.22 degreesE; geomagnetic co-ordinates: 19.08 degrees N, 148.91 degreesE; dip Latitude 24.8 degreesN), India, for the period 1975 -80 and 1986-89 belonging to an ascending phase of solar activity during fi rst halves of solar cycles 21 and 22 respectively have been used to study t he diurnal, seasonal, solar and magnetic activity variations. The diurnal v ariation of seasonal mean of IEC on quiet days shows a secondary peak compa rable to the daytime peak in equinox and winter in high solar activity. IEC max (daytime maximum value of IEC, one per day) shows winter anomaly only d uring high solar activity at Delhi. Further, IECmax shows positive correlat ion with F-10.7 Up to about 200 flux units at equinox and 340 units both in winter and summer; for greater F10.7 values, IECmax is substantially const ant in all the seasons. IECmax and magnetic activity (A(p)) are found to be positively correlated in summer in high solar activity. Winter IECmax show s positive correlation with A(p) in low solar activity and negative correla tion in high solar activity in both the solar cycles. In equinox IECmax is independent of A(p) in both solar cycles in low solar activity. A study of day-to-day variations in IECmax shows single day and alternate day abnormal ities, semi-annual and annual variations controlled by the equatorial elect rojet strength, and 27-day periodicity attributable to the solar rotation.