Field-aligned fluxes in the geomagnetic equator

During 12 electron injection events observed by Explorer 45 during the December, 1971, and February, 1972, in magnetic storms it was found that the lowest observable pitch angle particles (down to 7 degrees in one case) arrived first. Pitch angle dispersion and shell-splitting effects are shown to be inadequate explanations for most of these pitch angle distributions. It is suggested that parallel electric fields may be supplying 1- to 5-keV electrons to the magnetosphere from the ionosphere. Various models of parallel electric fields are discussed.