Rocket measurements of the thermal and superthermal electron distributions in the prenoon topside auroral ionosphere and ionospheric cleft

Unprecedented measurements of thermal and superthermal electron velocity distributions in the northern winter prenoon topside auroral ionosphere and ionospheric cleft have been obtained using the Thermal Electron Capped Hemisphere Spectrometer (TECHS). The measurements encompass altitudes between 300--1450 km during a period of low solar and geomagnetic activity. The TECHS-derived thermal electron bulk plasma parameters show a broad cross-field, auroral ionospheric density depletion below 700 km in association with downward heat flux. The density depletion abruptly terminates at the equatorward thermal plasma density wall of the cleft ion fountain, which is characterized by factor of ∼5 density increase over a cross-field width of ∼30 km. In the deft ionosphere, TECHS measured a sequence of non-linearly increasing/decreasing dispersive low-energy electron flux signatures. Analysis of these dispersive signatures supports a time-of-flight model of low-energy electron plasma injection into an inverted-V potential structure located 0.35 RE above the payload.;The TECHS-measured field-aligned low-energy electron distributions associated with energy dispersive signatures show simultaneous beam-like features in the upwelling and precipitating distribution. Evidence of unstable distribution features are observed down to energies of ∼1 eV. Unstable thermal electron distributions are associated with ∼1 ms bursts of large amplitude (≥200 mV/m) Langmuir waves at the plasma frequency (fpe) and broadband high-frequency emission which cutoff at f pe. Cleft thermal electron plasmas are characterized by periods of reduced plasma density, enhanced thermal electron temperature, and heightened field-aligned velocities. One interval of transverse ion acceleration is identified by a deep density cavity, elevated thermal electron field-aligned drifts in excess of 100 km·s-1, enhanced (∼1 eV) thermal electron temperatures, and temperature anisotropies within the thermal electron velocity distribution.;The TECHS-derived thermal electron parameters have also been used to extend the ambient electron density-temperature relation to the development of electric potentials on spacecraft surfaces. This has led to a complete numerical modeling of the current balance equation for a sunlit spacecraft immersed in plasma, applicable to both positive and negative potential regimes. The results of this modeling suggest that electron temperature is a critical factor in driving spacecraft surfaces to negative potentials.