Characteristic energy spectra of 1- to 500-eV electrons observed in the high-latitude ionosphere from Atmosphere Explorer C

Energy spectra of electrons between 1 and 500 eV have been obtained at altitudes mainly below 300 km over both north and south high-latitude regions with the photoelectron spectrometer experiment on the Atmosphere Explorer C satellite. Although the high-latitude fluxes of 1- to 500-eV electrons are highly variable, it has been possible to organize the observed events into four major classes: (1) fluxes of electrons having a peak in energy below 100 eV, approximately isotropic over the upper 2π hemisphere, and located at high invariant latitudes on the dayside which apparently gain access to the ionosphere through the dayside magnetospheric cusps; (2) structured fluxes of electrons showing one or more discrete but variable peaks in energy, reminiscent of ‘inverted V’ phenomena at higher energies, observed at all magnetic local times; (3) fluxes which show a monotonically decreasing differential flux versus energy dependence proportional to E−α, where E > 30 eV and 1 < α < 2, characteristic of auroral secondary electrons; (4) intense fluxes of electrons with rapidly changing and random energy spectra, characteristic of passage through auroral arcs or breakup regions. Fluxes of types 3 and 4 depend on precipitation of high-energy auroral primary particles, while those of types 1 and 2 appear to be a consequence of fundamental processes which couple the magnetosphere and the ionosphere. Detailed data are presented on classes 1 and 2, including occurrence and location as a function of magnetic activity.