Particle precipitaton into the evening auroral oval in 1--2 hours preceding substorm onset has been studied using measurements from the EISCAT incoherent scatter radar, complemented by measurements from the Viking satellite. The ionization between 100 and 160 km altitude is found to correspond to precipitation of approximately Maxwellian populations of ions with average energies of 10--20 keV and of electrons with average energies of 1--3 keV. A high-energy tail in the electron precipitaton is found to account for the ionization below 100 km altitude in the equatorward and poleward parts of the precipitation zone. A distinct population of high-energy electrons, with energies of several tens of keV or more, is, however, required to explain the ionization profiles in the center of the zone. The ions and separate population of high-energy electrons are precipitated from close to the respective stable trapping boundaries (where the particle pitch angle distribution changes from isotropic to trapped).

It is suggested that the latter electrons are scattered into the loss cone as the magnetic field becomes more taillike during the substorm growth phase and the radius of field line curvature in the equatorial plane becomes unusually small. The southward drifting band of high radio wave absorption, which has previously been identified by many authors as a characteristic feature of the substorm growth phase in riometer records, is found in the cases presented here to be explained mainly by the separate, high-energy electron population. The lower-energy electron population has the spectral characteristics expected for a plasma sheet origin but is found both equatorward and poleward of the trapping boundary for >100-kev electrons. Southward drifting auroral arcs which also characterize the substorm growth phase are found to be located immediately poleward of the zone of higher-energy electron precipitation, i.e., immediately poleward of the stable trapping boundary for >10-keV electrons.

This suggests that the latitude at which a substorm is initiated, which is often that of a previously southward drifting arc, may be controlled by the presence of the trapping boundary. ¿ Copyright 1990 by the American Geophysical Union