THEORY OF ION INJECTION AT SHOCKS

We have extended the theory of diffusive particle acceleration at para llel shocks to the low energy end where the spectrum matches the therm al plasma. The standard theory ceases to work in this region due to th e high anisotropy of the distribution function, and replaces the kinet ics of the particles there by a crude parameterization which is common ly referred to as ''injection''. We develop an analytical formalism, w hich treats injection in terms of thermal particles leaking from downs tream and recrossing the shock front until they have gained energies a cceptable for the standard description. This allows one to relate the injection rate to the hydrodynamic parameters of the subshock. In a qu asilinear description the upstream pitch angle scattering of ions, not of electrons, is provided by the unstable growth of MHD waves due to the presence of the ''warm beam'' of ions, leaking from the downstream plasma.