Nighttime changes in the electron content of the ionosphere

The total electron content of the ionosphere was determined at Auckland (37°S) and at Invercargill (46°S) throughout each night from June 1965 to December 1966. Increases in total content were observed on nearly half the nights in winter but not in summer. Comparison with northern hemisphere results shows that this is a true seasonal change. A model assuming a constant source of ionization and a fixed loss rate was fitted to the measurements by a least-squares analysis. This showed that a nighttime influx of ionization also occurs in summer, although the increased density makes its effects less obvious. The influx is reduced at low latitudes and, possibly, at the equinoxes. Increases in magnetic activity have little effect on the nighttime source of ionization, but cause an increase in the recombination coefficient.