Total electron content of the equatorial ionosphere

Abstract

Total electron content (N_t) variations in the ionosphere above the magnetic equator (Thumba dip 0.6°S) obtained by the Faraday rotation measurements of beacon signals from S66 satellites are described for the period December 1965-August 1968. The N_t value reaches a minimum around 05 hr and a broad maximum between 14-18 hr, the diurnal ratio being more than 20. During no-echo condition at pre-sunrise hours, N_t is found to be abnormally low. The equivalent slab-thickness at Thumba is between 150 and 250 km except around 14 hr when it reaches a high value around 500 km. The electrons are almost equally distributed above and below the peak for the daytime hours, but in the latter part of the night the ratio of top-side to the bottom-side electron content exceeds the value of 5. This high ratio is suggested as being due to very low value of maximum electron density which during the pre-sunrise period becomes comparable to the electron density at great heights where there is no diurnal change of electron density. Combining the data of Thumba and Ahmedabad, the diurnal development of the equatorial anomaly in N_t is described.