Diurnal variations in the ionosphere deduced from satellite radio signals

Abstract

Recordings of signals from satellite 1958δ2 (Sputnik III) have been used to derive the mean diurnal variation in total ion content in the ionosphere above Sydney, Australia, in summer and winter. There are marked seasonal differences suggesting that the smooth variation expected from ion production and loss is greatly modified by transport of ionization. The main effect is to exaggerate the diurnal variation in winter and minimize it in summer. In addition to the midday maximum a secondary maximum occurs at 1800 hours local time in both seasons and a smaller one at 0500 hours in winter. The satellite observations have been compared with the maximum ionization of the F region as deduced from ionosonde records and the ratio of the total ionization to that at the maximum used as a measure of the effective thickness of the ionosphere. This ‘thickness index’ also has a greater diurnal variation in winter than in summer. It is found that the average ionization over 24 hours is greater in summer but it is distributed more evenly over the 24 hours giving higher values at night and lower by day than in winter. Moreover, the daytime thickness is greater in summer than in winter, thereby increasing the discrepancy in observed midday foF2 values which are appreciably higher in winter. The afternoon peak of ionization is shown to be associated with an increase in thickness in both seasons, whereas the early morning maximum appears only in winter and also appears to be associated with thickening. These variations are in agreement with results from satellite observations in the northern hemisphere and also with the limited appropriate observations from rockets. The possible relationship of the variations to known ionospheric movements is discussed briefly and suggestions made for further analyses and observations.