Exact Expressions for the Critical Mach Numbers in the Two‐Fluid Model of Cosmic‐Ray–modified Shocks

Abstract

The acceleration of relativistic particles due to repeated scattering across a shock wave remains the most attractive model for the production of energetic cosmic rays. This process has been analyzed extensively during the past two decades using the model of diffusive shock acceleration. It is well known that one, two, or three distinct solutions for the flow structure can be found depending on the upstream parameters. Interestingly, in certain cases both smooth and discontinuous transitions exist for the same values of the upstream parameters. However, despite the fact that such multiple solutions to the shock structure were known to exist, the precise nature of the critical conditions delineating the number and character of shock transitions has remained unclear, mainly due to the inappropriate choice of parameters used in the determination of the upstream boundary conditions. In this paper we derive the exact critical conditions by reformulating the upstream boundary conditions in terms of two individual Mach numbers defined with respect to the cosmic-ray and gas sound speeds, respectively. The gas and cosmic-ray adiabatic indices are assumed to remain constant throughout the flow, although they may have arbitrary, independent values. Our results provide for the first time a complete, analytical classification of the parameter space of shock transitions in the two-fluid model. We use our formalism to analyze the possible shock structures for various values of the cosmic-ray and gas adiabatic indices. When multiple solutions are possible, we propose using the associated entropy distributions as a means for identifying the most stable configuration.