Suprathermal Ions Upstream of the Earth's Bow Shock

摘要

Two principal mechanisms have been proposed for achieving acceleration to MeV range energies: (1) Fermi acceleration through multiple scattering off of irregularities up- and downstream of the shock; and (2) shock-drift acceleration, in which particles gain energy by drifting in the inhomogeneous magnetic field of the shock in a direction parallel to the convection electric field and the solar wind. Both of these energization mechanisms require an initial seed population of suprathermal particles. Although it has been suggested that the ordinary ion heating that occurs at the shock might provide enough hot particles for the Fermi process. The effectiveness of the shock-acceleration mechanisms is enhanced if there exists an initial suprathermal ion population created by additional shock-related processes. Such suprathermal particles are indeed observed in association with collisionless shocks, and an extensive body of observational and theoretical work has been directed toward characterizing them and understanding their origin. It is the purpose of this report to review the nature of the various populations of shock associated suprathermal ions, with particular emphasis on the ion foreshock region of the Earth's bow shock. We also assess the current understanding of the possible origins of those populations. (ERA citation 09:031870)