Ionospheric photoelectrons observed in the magnetosphere at distances up to 7 earth radii

Photoelectrons of ionospheric origin have been observed for the first time at high altitudes (up to 7R_E geocentric distance) using the suprathermal plasma analysers (SPA) on the GEOS satellites. At such high altitudes the photoelectron flux is confined within a few degrees of the magnetic field direction. We show how this flux may be identified and extracted from the background which is a combination of locally produced photoelectrons and ambient plasma. GEOS-2 results are presented to illustrate the “turn-on” of the photoelectron flux at dawn in the ionosphere. Data from GEOS-1 are used to study the behaviour of the photoelectron flux with equatorial geocentric distance from 3 to 7R_E. The results compare favourably with theoretical models and with ionospheric observations at mid latitudes.