S. K. Chakraborty* and R. Hajra
Department of Physics, Raja Peary Mohan Col lege, Uttarpara, Hooghly 712
258, India
Abstract. Long term (1979-90) total electron content (TEC) data have been analyzed to show its dependence on EUV radiation from the sun. TEC obser- vation has been made from Calcutta (22.58o N, 88.38 o E geographic, dip: 32 N), situated virtually below the northern crest of equatorial ionization anomaly. Day to day changes in TEC at different local times do not show any significant correlation with F10.7 solar flux. A good correlation is, however, observed be- tween F10.7 solar flux and monthly mean TEC when both are considered on long term basis, i.e., either in the ascending (1986-90) or in the descending (1979-85) phases. In the early morning hours correlation coefficient maximizes around 0800-1000 IST interval. The flux independent nature of diurnal TEC is prominent around the noon time hours of few months with F10.7 values greater than 150. Variation of TEC for the whole time period (1979-90) also exhibits a prominent hysteresis effect. A remarkable feature of hysteresis effect is its local time dependent nature. Solar flux normalized TEC exhibit clear seasonal dependence with asymmetrical variations in the two equinoxes. A further nor- malization leads to prominent local time dependent feature. Based on solar flux, seasonal and local time dependent features of TEC an empirical formula has been developed to represent the TEC variation in the early morning hours. It yields a quantitative estimate of solar flux dependent nature of TEC variation.
Keywords: solar-terrestrial relations - Sun: UV radiation - solar activity