Abstract

Some features of diurnal variations of f_oF₂ are found to show a better response of solar activity impact than the hourly means of diurnal variation over a sunspot cycle. The ionization build-up-rate and the difference in pre- and post-noon peak are found to give 11- year periodicity for Ibadan (7.4ËšN, 3.9ËšE, 6ËšS dip) station while the hourly means are observed to give different periodicities. There is however, a phase lag between these features i.e. build-up rate of ionisation and the difference in post- and pre- noon peak of ionisation on one hand and the sunspot number on the other hand. The morning depression rate and evening decline do not however show solar cycle trend. For Singapore the solar cycle trend of ionisation build-up-rate and pre- and post noon peaks is slight while for Slough (51.5ËšN, 359.4ËšE, 66.5ËšS dip) another station investigated for similar results, the ionisation build-up-rate, difference in post-and pre-noon peaks, morning depression rate and evening decline do not show any solar cycle trend. No solar cycle trend is observed in morning depression rate and evening decline for Singapore (1.3ËšN, 103.8ËšE, 17.6ËšS dip).

Key words: Diurnal, Build-up-rate, Periodicity, and Phase-lag.