Abstract:
Total electron content records from four southern hemisphere stations show 43 SITECs in the period 1966-77. Relating these to solar flare data shows that the probability of a given flare producing a SITEC increases with the peak 10.7 cm solar flux; with the value of cos x> where x is the solar zenith angle; and with decreasing central distance of the flare. The sizes of SITECs also increase significantly with the solar flux. SITECs are generally larger at lower latitudes in summer, but show little latitudinal variation in winter. This pattern occurs at both solar maximum and solar minimum, and is explained by seasonal changes in cos x and in ionospheric loss rates. SITEC size vs. latitude, however, is more variable during magnetically disturbed periods. Relative loss rates at different stations are obtained from the observed rise rates of corresponding SITECs. Results show that rapid ionospheric recombination (due to molecular ions) is more important at higher latitudes in summer and at lower latitudes in winter. This reflects primarily seasonal and latitudinal changes in the ratio N2/O, at heights above approximately 150 km. Similar variations are revealed in an analysis of 5 SITECs recorded at two northern hemisphere stations. SITEC rise rate measurements therefore provide a useful tool for studying differences in atmospheric composition at different sites. A comparison is made between total electron content at two pairs of stations with nearly the same geographic and geomagnetic latitudes in the northern and southern hemispheres. Differences in the shapes of the monthly mean diurnal curves and the larger diurnal ratio TECmax/TECmjn in the northern hemisphere are attributed to some combination of (1) the opposite effects in the opposite hemispheres of the eastern magnetic declination on the neutral wind induced vertical ionisation drift, and (2) the effect of differing electron temperatures at the conjugate points on the local loss rate. The nighttime TEC shows an annual maximum in summer at all stations reflecting the longer hours of production and a seasonal difference in neutral wind direction. The daytime TEC shows an annual variation with generally larger TEC in December - January and at the equinoxes than in June - July at all stations. These annual variations in TEC are reproduced from MSIS model values of 0, O2 and N2 densities and cos x at the four ionospheric points. It is suggested that the annual variation in O/O2 is related to the effect of the changing earth-sun distance on the rate of 0 photodissociation and on thermospheric circulation. The TEC storm behaviour is analysed at the four stations by calculating mean TEC during storms vs. local time, mean percentage deviations from quiet conditions during storms vs. local time, and mean percentage deviations vs. time from storm commencement. Calculations are carried out separately for summer and winter and for two ranges of storm commencement time. Average behaviour of slab thickness at Auckland during storms is also calculated. Results from two individual storms at the four stations show the large variability of ionospheric storm behaviour. New results from this study are (1) after the day of the storm commencements the mean deviations from the quiet behaviour are more positive or less negative at the southern hemisphere stations, and (2) TEC storm variations may persist for ten days in winter at 34°N and 34°S. The occurrence of traveling ionospheric disturbances in TEC at 34°N and 34°S is compared for two summer months and two winter months for day and night. Results discussed suggest that medium scale gravity waves propagate from high to low latitudes, and more are removed by day than by night by dissipative processes.