Shock drift acceleration at low energies

Abstract

Until now, studies of shock drift acceleration (SDA) at fast mode collisionless shocks have been confined to particles with initial energies greater than 10 keV. We present for the first time the results of test particle calculations of the interaction of low‐energy (1–8 keV) protons with both strong and weak oblique shocks. Since it is expected that the details of the shock structure will be important, we use a one‐dimensional hybrid, fully self‐consistent simulation to provide a realistic, time‐varying model of the shock fields. We follow the evolution, in these fields, of an isotropic, monoenergetic population of particles released just upstream of the shock. Our results indicate that SDA is a viable mechanism at oblique shocks for initial energies down to those normally classified as superthermal. We find that there is a minimum initial energy below which particles are not reflected; this threshold energy increases with both θBn (the shock normal angle) and MA (the Alfvén Mach number), and therefore implies that superthermal ions are more readily reflected as θBn and/or MA decreases. We suggest that the reflection of superthermal to very mildly energetic (around 1 keV) ions at the earth's bow shock could provide an explanation for the most energetic field‐aligned beams observed in the earth's foreshock. We have compared our results with those from a simplified model of a finite width shock transition, and we find that, statistically, it provides remarkably good agreement with the hybrid simulation results. The greatest differences between the two models appear for transmitted ions whose initial energy is low (1 keV).