INJECTION AND ACCELERATION OF THERMAL PROTONS AT QUASI-PARALLEL SHOCKS - A HYBRID SIMULATION PARAMETER SURVEY

Collisionless shocks that propagate along the mean magnetic field are known to accelerate some fi action of the incident charged particles d irectly from the thermal pool to energies that are considerably higher than the energy at which the plasma rams into the shock. Using hybrid simulations, we address two issues: (1) the dependence of the injecti on/acceleration of thermal protons to energies much higher than the pl asma ram energy on various shock parameters such as Mach number, plasm a beta, etc., and (2) the effect of the high-energy particles, acceler ated directly from the thermal population by the shock, on the macrosc opic Properties of the shock, most notably, on the density compression . We find that for supercritical Mach numbers the acceleration of the thermal plasma is efficient enough that the back pressure due to the e nergetic particles can significantly increase the density compression across the shock, above the value expected from the simple Rankine-Hug oniot prediction. Additionally, at low Alfven Mach number, where the a cceleration of the thermal plasma is inefficient, the density compress ion is smaller than the simple Rankine-Hugoniot prediction owing to th e nonresonant fire hose instability. The acceleration efficiency incre ases with Mach number except at very high Alfven Mach numbers, where i t begins to decrease for Mach numbers greater than similar to 10. This is due to the presence of a fixed, free-escape boundary that limits t he size of the foreshock region measured in units of the mean-free pat hs of the accelerated particles. Additionally we find that regardless of the upstream plasma parameters,the acceleration efficiency increase s with both the density compression ratio across the shock and the dis tance to the free-escape boundary measured in units of the mean-free p ath of the energetic particles. Both of these are consistent with anal ytic theory and numerical models that use a phenomenological scatterin g law.