The "injection problem" for quasiparallel shocks

For a particle to be accelerated diffusively at a shock by the first-order Fermi acceleration mechanism, the particle must be sufficiently energetic t hat it can scatter across all the micro- and macrostructure of the shock, e xperiencing compression between the converging upstream and downstream stat es. This is the well-known "injection problem." Here the interaction of ion s with the ramp of a quasiparallel shock is investigated. Some ions inciden t on the shock experience specular reflection, caused either by the cross-s hock electrostatic potential or by mirroring as the magnetic field is bent and compressed through the ramp. Scattering of reflected ions by self-gener ated and pre-existing turbulence in the region upstream of the shock then a cts to trap backstreaming ions and return them to the ramp, where some expe rience further reflections. Such repeated reflections and scattering energi ze a subpopulation of ions up to energies sufficiently large that they can be diffusively shock accelerated. Two ion distributions are considered: pic kup ions which are assumed to be described by a shell distribution, are the rmal solar wind ions which may be described by a kappa distribution. Inject ion efficiencies are found analytically to be very high for pickup ions and much lower for thermal solar wind ions, suggesting that this injection mec hanism, stochastic reflected ion or SRI acceleration, is a natural precurso r for the acceleration of the anomalous cosmic ray component at a quasipara llel shock. While significantly less efficient, SRI acceleration is also vi able for thermal solar wind ions described by a kappa distribution. (C) 200 1 American Institute of Physics.