DAY-TO-DAY VARIABILITY IN IONOSPHERIC ELECTRON-CONTENT

Published values of ionospheric electron content (IEC) are used to stu dy its day-to-day variability at a number of stations extending from e quatorial to mid-latitudes in Indian and American sectors for high and low solar activity years. It is seen that the variability is larger a t night than by day, highest in February and November (D months) and l owest in equinox (E) months. The solar activity dependence is such tha t the variability is higher during high sunspot years than during low sunspot years for mid- and subauroral latitudes while for low latitude s this trend is reversed from 08 to 23 h local time. From a correlativ e study of the variability in IEC vis-a-vis that in possible causative factors such as solar 10.7 cm flux, magnetic activity, equatorial and auroral electrojet, the relative contributions of the above factors t o the variability in IEC at different locations are brought out. The s patial correlation of TEC is also studied.