Magnetospheric Substorms and Auroral Particle Precipitation.

Low energy particle data gathered by the ESRO 1A satellite was analyzed. During geomagnetic storms, a gap was found to develop in the latitudinal profiles of electrons and protons precipitating in the early evening. An abrupt termination of particle activity on the poleward side of the precipitation region was observed on the night side during substorms. The main precipitation region on the night side also occasionally develops a low intensity poleward tail during quiet periods. These findings may indicate the formation of a neutral line close to the earth across the geomagnetic tail during substorms and the absence of such a barrier during quiet times. In addition, satellite observations of plasma injection into the inner magnetosphere as early as 18 hours indicate that direct plasma injection extends westward as far as dusk during intense substorms.