The main electron trough during the rising solar cycle

Results of an analysis of Explorer 22 Langmuir probe measurements are presented for the winter periods between November 1, 1964, and February 15, 1967. The atmospheric phenomenon characterized by the data is the nightside main electron trough at 1000 km. The electron-density patterns imply that as the solar phase advanced, the mean quiet-time trough position was negligibly affected but the density increased as much as threefold. An increased variability among individual latitude profiles of electron density also accompanied the increase in mean sunspot number. The latitudinal distribution of electron temperature exhibited a nightside maximum at the trough that showed negligible changes in amplitude, width, and location over the period of analysis. The nightside temperature maximum can be related to the plasmapause phenomenon by invoking field-aligned heat conduction from the protonosphere. Potentially, downward heat conduction from hot protonospheric plasma within the plasmasphere can produce electron temperatures that increase with latitude at 1000 km, whereas outside the plasmasphere, where densities are greatly reduced, the temperatures can decrease with latitude because the low densities above 1000 km retard the downward heat conduction process.