The electron content of the southern mid-latitude ionosphere, 1965–1971

Abstract

The electron content of the ionosphere ( I) was recorded continuously from June 1965 to August 1971, at latitudes of 34°S and 42°S. The diurnal ratio ( I max I min ) is about 3 in summer and 4 in winter, compared with 4–6 in the northern hemisphere and 20 at the dip equator. The electron content shows a peak near 19 hr in summer caused by upwards movements. The corresponding peak in N m is delayed to 22 hr, at solar maximum, by thermal contraction of the ionosphere. There is no seasonal anomaly at 34°S. The major anomaly is a large peak in April; at 42°S I increases by 70 per cent (and N m by 100 per cent) in 40 days at the March equinox. Changes with solar flux S show I ∝ S − 20 for summer day, summer night and winter day. Winter night values are almost constant because of a lowering of the O + H + transition height to 450 km near solar minimum. 27-day variations in I follow changes in S, with a lag of 1.0 ± 0.1 days, and give values of dI dS about 35 per cent less than for the solar cycle changes. Large day to day variations in I at periods of less than 10 days are attributed to changes in the loss rate. The per cent variation is approximately independent of season and solar activity, but increases at night in winter.