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Simulations and theory of ion injection at non-relativistic collisionless shocks

Author(s): Caprioli, D; Pop, AR; Spitkovsky, Anatoly

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Abstract: We use kinetic hybrid simulations (kinetic ions-fluid electrons) to characterize the fraction of ions that are accelerated to non-thermal energies at non-relativistic collisionless shocks. We investigate the properties of the shock discontinuity and show that shocks propagating almost along the background magnetic field (quasi-parallel shocks) reform quasi-periodically on ion cyclotron scales. Ions that impinge on the shock when the discontinuity is the steepest are specularly reflected. This is a necessary condition for being injected, but it is not sufficient. Also, by following the trajectories of reflected ions, we calculate the minimum energy needed for injection into diffusive shock acceleration, as a function of the shock inclination. We construct a minimal model that accounts for the ion reflection from quasi-periodic shock barrier, for the fraction of injected ions, and for the ion spectrum throughout the transition from thermal to non-thermal energies. This model captures the physics relevant for ion injection at non-relativistic astrophysical shocks with arbitrary strengths and magnetic inclinations, and represents a crucial ingredient for understanding the diffusive shock acceleration of cosmic rays.
Publication Date: 22-Dec-2014
Electronic Publication Date: 10-Jan-2015
Citation: Caprioli, D, Pop, AR, Spitkovsky, A. (2015). Simulations and theory of ion injection at non-relativistic collisionless shocks. Astrophysical Journal Letters, 798 (2), 10.1088/2041-8205/798/2/L28
DOI: doi:10.1088/2041-8205/798/2/L28
ISSN: 2041-8205
EISSN: 2041-8213
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Type of Material: Journal Article
Journal/Proceeding Title: Astrophysical Journal Letters
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.

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