Injun 5 observations of low‐energy plasma in the high‐latitude topside ionosphere

Abstract

The two Air Force Geophysics Laboratory low‐energy spherical electrostatic analyzers aboard Injun 5 have provided observations which are used in conjunction with simultaneous Lepedea observations to study the energetic and ambient thermal plasmas in the evening sector topside ionosphere. Significant thermal electron flux enhancements are observed in the vicinity of the inverted V structures which could be due to either auroral return currents or ionospheric scale height changes. Enhanced hyperthermal (E > 28 eV) fluxes of positive ions, as well as vehicle potential modulations, are observed as the satellite passes through inverted V events. At 2500 km the polar cap electron density, temperature, and energy density were 89 ± 41 cm−3, 2234 ± 480°K, and 16.2 ± 5.8 eV/cm³, respectively. Higher energy densities were found during times of magnetic disturbance. Persistent fluxes of hyperthermal electrons are identified with the polar rain observed with Isis 2 experiments. Finally, evidence is cited for the existence of small‐scale (20 km) structures within the region of plasma sheet precipitation.