Diffusive Shock Acceleration in Oblique MHD Shocks: Comparison with Monte Carlo Methods and Observations

摘要

We report simulations of diffusive particle acceleration in obliquemagnetohydrodynamical (MHD) shocks. These calculations are based on extensionto oblique shocks of a numerical model for ``thermal leakage'' injection ofparticles at low energy into the cosmic-ray population. That technique,incorporated into a fully dynamical diffusion-convection formalism, wasrecently introduced for parallel shocks by Kang & Jones (1995). Here, we havecompared results of time dependent numerical simulations using our techniquewith Monte Carlo simulations by Ellison, Baring & Jones 1995 and with {it insitu} observations from the Ulysses spacecraft of oblique interplanetary shocksdiscussed by Baring etal (1995). Through the success of these comparisons wehave demonstrated that our {diffusion-convection} method and injectiontechniques provide a practical tool to capture essential physics of theinjection process and particle acceleration at oblique MHD shocks. In additionto the diffusion-convection simulations, we have included time dependenttwo-fluid simulations for a couple of the shocks to demonstrate the basicvalidity of that formalism in the oblique shock context. Using simple modelsfor the two-fluid closure parameters based on test-particle considerations, wefind good agreement with the dynamical properties of the more detaileddiffusion-convection results. We emphasize, however, that such two-fluidresults can be sensitive to the properties of these closure parameters when theflows are not truly steady. Furthermore, we emphasize through example how thevalidity of the two-fluid formalism does not necessarily mean that {itsteady-state} two-fluid models provide a reliable tool for predicting theefficiency of particle acceleration in real shocks.