PARTICLE INJECTION AND THE STRUCTURE OF ENERGETIC-PARTICLE-MODIFIED SHOCKS

The energization of particles at shock waves is thought to be a fundamental process in the formation of shocks. In this paper, a new macroscopic ''self-consistent'' two-fluid model is presented for investigating the effects of particle injection from a thermal background plasma into an energetic-particle or cosmic-ray gas. By identifying only those particles with momenta greater than some momentum parameter p0, the injection term connecting the two fluids/gases can be identified for smoothly decelerating flows as well as for (sub)shocks. For smoothed flows the injection term is proportional to the flow gradient (and hence is increasingly important for steeper and steeper flow gradients), while at a subshock the injection term is proportional to the velocity jump. The results of including injection self-consistently are studied in the context of stationary shock structures; in particular, we address the following questions: (1) Can the nature and model of particle injection affect directly the nature and dynamics of the shock, and hence the efficiency of particle acceleration? (2) Can the shock itself regulate particle injection dynamically? It is shown that both questions can be answered in the affirmative. The class of admissible shock or injection fronts is found to be much richer in the case of the two-fluid injection model than in the noninjection model. A useful analogy between injection fronts and shocks in combustible media may be drawn. The general shock structure problem for injection fronts is studied, and the stability of such shocks is considered.