Total electron content records from four southern hemisphere stations show 43 SITECs (sudden increases of the total electron content) in the period 1966--1977. Relating these to solar flare data shows that the probability that a given flare will produce a SITEC increases with the peak 10.7-cm solar flux; with the value of cos &khgr;, where &khgr; 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 &khgr; and in ionospheric loss rates. SITEC size versus latitude, howecer, 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 five 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.