PARTICLE INJECTION AND ACCELERATION AT EARTHS BOW SHOCK - COMPARISON OF UPSTREAM AND DOWNSTREAM EVENTS

We investigate the injection and acceleration of thermal solar wind ions at the quasi-parallel Earth's bow shock during radial interplanetary magnetic field conditions. Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) satellite observations of complete proton spectra, and of heavy ion spectra above 10 keV Q-1, made on 1984 September 12 near the nose of the shock, are presented and compared to the predictions of a Monte Carlo shock simulation which includes diffusive shock acceleration. These diffuse ion observations are of particular importance because they include three shock crossings which were observed within a short time span (less than 20 minutes), allowing the comparison of upstream and downstream events as the observation point moved through the shock under very similar solar wind conditions. We find that the spectral observations are in good agreement with the predictions of the simulation when it is assumed that all accelerated ions originate in the solar wind and are injected into the acceleration mechanism by thermal leakage from the downstream plasma. The efficiency, which is determined directly from the downsteam observations, is high, with at least 15% of the solar wind energy flux going into accelerated particles. Our comparisons allow constraints to be placed on the rigidity dependence of the scattering mean free path and suggest that the upstream solar wind must be slowed substantially by backstreaming accelerated ions prior to undergoing a sharp transition in the viscous subshock.