THE ACCELERATION OF LOW-ENERGY PROTONS BY QUASI-PERPENDICULAR INTERPLANETARY SHOCKS

New aspects of the acceleration of low energy (35-1000 keV) protons by quasi-perpendicular interplanetary shocks are presented, using observations and numerical simulations. Time reversed trajectory calculations of particles are used to derive the behaviour of the angular distribution and spectrum through the shock. These calculations show that for simple planar geometries of the magnetic field and for a powerlaw spectrum of pre-accelerated particles the expected omnidirectional enhancements are smaller than observed. Pitch angle distributions in the vicinity of six interplanetary shocks have been determined from the measurements carried out onboard the ISEE-3 spacecraft. Reflection of particles was clearly identifiable by the loss cone type angular distribution observed upstream of the shock. Downstream of the shock, the shape and the energy dependence of the pitch angle distributions provide support for the scatter-free model in some cases. However, the observed spikes at the shock passage and bidirectional upstream distributions measured at the nearest to perpendicular shocks, together with other features of particle spectra and angular distributions which cannot be readily explained by model calculations suggest that fluctuations in the magnetic field might also seriously affect the acceleration process. © 1990.