Variation of monthly mean TEC (TECU) with average F_10.7 solar flux (SFU) during August. The top and middle panels show the variations for the descending phase of SC 22 (1980–1985) and the ascending phase of SC 23 (1986–1990), respectively. The linear regression fittings and correlation coefficients (r) between 〈TEC〉 and 〈F_10.7〉 are also shown in each panel. The bottom panel shows the monthly mean TEC variations for both the solar cycle phases together. The corresponding local times are indicated in the top panel. A prominent ionospheric hysteresis effect may be noted in the SC variation of TEC (see text).