General Features of the Ionosphere at Thumba

Abstract

The paper describes the characteristics of the ionosphere over the magnetic equator as revealed from the ionograms obtained by an automatic ionospheric sounder at Thumba. The well-known midday biteout of the f₀F₂ at the equatorial stations is almost absent during the winter months and is most pronounced during the equinoxes. The equinoctial maxima of f₀F₂ seem to be more pronounced for the midnight than for midday hours. The solar cycle variation of f₀F₂ for any particular solar hour are inversely proportional to the changes in the F₂-layer height during the same period. Another interesting feature of the F₂-layer at Thumba has been the development of extremely pronounced and sharp peak of h_pF₂ around 20 hr during the equinoxes. The absence of ionospheric echoes in the early morning hours is very predominant during the low sunspot years. The spread-F at Thumba is of range splitting type during early part of the night and frequency spread type during early morning hours. The midday f₀F₂ at Thumba are less than the corresponding value at Kodaikanal during the winter months while during the summer months the f₀F₂ are less at Kodaikanal than at Thumba indicating the movements of ionospheric equator with season. The values of f₀E_s are very low compared to those at Kodaikanal and this seems to be instrumental rather than genuine differences.