University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Friday, September 20th 2019
2:30 pm:
Speaker: Colby Haggerty, University of Chicago
Subject: Hybrid Plasma Simulations of the Acceleration and Back-Reaction of Cosmic Rays on Astrophysical Shocks

High energy low density cosmic rays (CR) are a ubiquitous feature of astrophysical systems and make up a sizable fraction of the energy budget in the interstellar medium. The shock waves associated with supernova remnants are believed to be the principal accelerators of galactic CRs, through a collisionless acceleration process referred to as diffusive shock acceleration (DSA). We present simulations of collisionless plasma shocks performed with the first hybrid code to include relativistic ion dynamics (dHybridR). In these simulations, we show evidence of modifications to the fluid shock jump conditions caused by the CR pressure. The rapid transition to CR modified shocks occurs soon after the onset of the shock. CR modified shocks are expected to have a harder power law slope, however we find a significantly steeper spectral index of nearly p^-5 for early times and hardening as the simulation progresses to approximately p^-4.3 as the compression ratio saturates. To understand these simulation results we present a non-linear theory of DSA which includes considerations for both the magnetic field and the CR modified jump conditions. The steep spectra is shown to be caused by the enhanced magnetic field downstream of the shock. This magnetic field was originally generated from the CR streaming instability upstream of the shock, and thus the compressed/enhanced magnetic field acts to regulate the energetic run away problem that CR modified shocks presents for non-linear DSA.

Faculty Host: Thomas W. Jones

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