DAY-TO-DAY VARIABILITY OF IONOSPHERIC ELECTRON-CONTENT OVER WALTAIR

Polarization rotation data (136 MHz) recorded from the geostationary s atellites ETS-II and SIRIO at a low latitude station, Waltair (17.7-de grees-N, 83.3-degrees-E) during high (1978-79) and low(1984-85) sunspo t number years have been used to study the day-to-day variability of t he ionospheric electron content (IEC). The ratio of the standard devia tion of daily IEC values to the mean IEC values has been used as a mea sure of the variability. The daytime ratios are reasonably constant an d independent of season during both the sunspot number years. The nigh ttime ratios are higher (20-80%) during high solar activity period com pared to those (15-30%) of low solar activity period. A correlative st udy is made on the day-to-day variability in IEC(max) with possible ca usative factors, such as solar 10.7 cm flux, electrojet strength (DELT AH) and magnetic index (A(p)) during the low solar activity period. Th e IEC at Waltair shows a linearly increasing trend with 10.7 cm flux d uring three different seasons and with the electrojet strength only in summer, while it does not seem to depend on the magnetic index. Howev er, the percentage variations in the diurnal IEC, when considered sepa rately for magnetically quiet and disturbed days during three differen t seasons of low solar activity period, show a clear 'mirror-image' be haviour only during summer.