17-20The diurnal variations of the annual and semi-annual periodicities in the F-region of the ionosphere over Lunping are investigated by analyzing the monthly mean values of ionospheric electron content (IEC) for a period of three consecutive years representing low, medium and high sunspot activity. Significant solar cycle variations in both amplitude and phase have been observed. The diurnal maximum in IEC occurs progressively later with increase in solar activity. The phase of the annual component shifts from winter in solar minimum to late autumn in solar maximum, whereas the phase of the semi-annual maxima is only in equinox conforming to the global behaviour. The variations are mostly explained in terms of the E × 8 drifts and changes in the atmospheric composition
