SOLAR-ACTIVITY DEPENDENCE OF IONOSPHERIC ELECTRON-CONTENT AND SLAB THICKNESS USING DIFFERENT SOLAR INDEXES

Ionospheric electron content (IEC) and slab thickness (tau) data for t he period 1977 to 1980 from Lunping (23.03 degrees N; 121.90 degrees E subionospheric) have been examined for their solar activity dependenc e. Local noontime monthly means as well as values for the 5 QQ days in a month have been examined separately with different solar indices, n amely: solar EUV flux (170-190 Angstrom), S-10.7cm flux and sun spot n umber (SSN) on a seasonal basis. Both IEC and tau parameters exhibit b etter correlation with solar EUV and S-10.7cm fluxes than with SSN for all seasons. IEC increases linearly with both EUV and S-10.7cm flux w hereas with SSN it shows a distinct nonlinear relationship during all seasons in both monthly mean and 5 QQ days' values. This study indicat es that for correlating and predicting the variations (especially the medium term) in the ionospheric parameters, both EUV and S-10.7cm flux es have an advantage over SSN.