Low Latitude Tec Variations for Three Consecutive Solar Cycles

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Hourly values of the total electron content (TEC) for the low latitude station of Hawaii (21.2°N, 157.7°W) during the last three consecutive solar cycles 20, 21 and 22 are used to study the long term and short term variations of TEC for this low latitude station. It is found that for perfectly quiet days, a linear variation for the daytime peak values of TEC (TECmax) with S10.7 solar flux exists even for high solar flux values of S10.7 during the winter and equinox seasons of the three solar cycles. However, during summer a ‘levelling off’ effect in TECmax is observed beyond about 180 units of S10.7. The long term dependence of TECmax on solar activity is found to be less sensitive during summer compared to winter and equinox for the three solar cycles. In contrast to the behaviour of TECmax, the solar activity dependence of TECmin (morning minimum values of TEC) for the three solar cycles is found to be strongest in summer and weakest in winter. The time difference between TECmax and TECmin remains more or less constant for the entire solar flux range during winter and equinox in the three solar cycles. However, during summer this time difference shows a decreasing trend with increasing solar flux indicating a ‘levelling off’ effect of TECmax for higher solar flux values in summer. There is good correspondence in the diurnal variation of the day to day variations in TEC for the respective seasons of the solar maximum and minimum phases during the three solar cyles.