Distinction and classification of the troughs and subtroughs in He + density from ISS-b satellite data at 1000– [formula omitted] altitudes

Abstract

The different types of light ion troughs (LIT) are distinguished and classified on the basis of systematic analysis of a large data set. Prominent features of the upper ionosphere such as the mid-latitude He + density trough, LIT, and the He + density subtrough, located equatorward of LIT, are mainly studied. We analyze ISS-b satellite observations of He + densities obtained at altitudes from 1000 to 1200 km during high solar activity from August 1978 to January 1980. The data show that the LIT position is located close and somewhat equatorward to the inner plasmapause for any local time, season and K p and show neither daytime and evening bulge. The probability of occurrence of LIT in nighttime winter conditions is ∼50% and tends to increase with K p, in daytime summer conditions it is ∼25% and tends to decrease with K p. Such behaviour is connected with corresponding variations in the LIT shape. We conclude that the He + density subtrough is not a unique and rare structure as supposed previously, since ∼570 cases of subtroughs were found in ∼4000 passes over the mid- and low-latitude ionosphere. They occur over a very broad interval of invariant latitudes (25–57°) with peaks at the latitudes of the outer plasmasphere (45–55°). Subtroughs are more often observed in summer afternoon/evening and in winter after midnight. They form more often under high magnetic activity than under quiet magnetic conditions.