Leakage from quasiparallel collisionless shocks: Implications for injection and shock dissipation

A simplified model of particle transport at a quasiparallel one-dimensional collisionless shock is suggested. In this model the magneto-hydrodynamics turbulence behind the shock is dominated by a circularly polarized, large amplitude Alfven wave originated upstream from the turbulence excited by particles leaking from the downstream medium. It is argued that such a wave, having significantly increased its magnetic field during the transmission through the shock interface, can effectively trap thermal ions, regulating their leakage upstream. Together with a background turbulence this wave also plays a fundamental role in thermalization of the incoming ion flow. The spectrum of leaking particles and the amplitude of the wave excited by these particles are self-consistently calculated. The injection rate into the first order Fermi acceleration based on this leakage mechanism is obtained and compared with computer simulations. The related problem of shock energy distribution between thermal and nonthermal components of the shocked plasma is discussed. The chemical composition of the leaking particles is studied. [S1063-651X(98)01610-9].