Acceleration of diffuse ions upstream from parallel shocks

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Abstract
We review the preliminary results of diffuse ion generation at parallel shocks using a combination of hybrid simulation and particle splitting techniques. Our basic motivation is to obtain improved statistics relative earlier studies, and to examine in detail the spatial and velocity space distribution of the diffuse ion population. We find that for a parallel shock with Alfvén Mach number of 5 and plasma β of 2, approximately 2% of the incoming ion flux is reflected and scattered back upstream. This diffuse population has a shallow density maximum at a velocity corresponding to specular reflection and has a distribution that is approximately exponential in energy. Further, the velocity space distribution of these ions is limited to shock normal velocities less than the speed of the plasma core. We interpret these results as being due to the finite length of the simulation system, and the low level of upstream waves excited by the backstreaming ions. Implications for particle acceleration and cosmic ray generation are discussed.

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