Electrons in the ionospheric source cone - Evidence for runaway electrons as carriers of downward Birkeland currents

Extremely intense field-aligned fluxes of low energy electrons have been observed with ISIS-2 streaming out of the ionosphere at auroral latitudes. Fluxes in excess of 10 billion per sq cm sec ster at energies below 500 eV with peak fluxes from 10 to 100 eV were detected at 1400 km. The electrons are very strongly field-aligned, having pitch angles confined within 10 deg of the magnetic field. Since they are so intense and so highly collimated they cannot be produced by atmospheric backscattering of a primary auroral beam. These electrons are sometimes associated with ionospheric ions that have been accelerated transverse to the magnetic field. They occur in regions of downward field-aligned current, and may thus be carriers of the current, thus far unidentified. It is suggested that they are runaway electrons from the ionosphere produced by a downward field-aligned component of the electric field.