Turbulent Reacceleration of Streaming Cosmic Rays: Fluid Simulations

Discussion timeslot (ZOOM-Meeting): 15. July 2021 - 12:00
ZOOM-Meeting URL: https://desy.zoom.us/j/91896950007
ZOOM-Meeting ID: 91896950007
ZOOM-Meeting Passcode: ICRC2021
Corresponding Session: https://icrc2021-venue.desy.de/channel/14-CRs-and-ISM-CRD/45
Live-Stream URL: https://icrc2021-venue.desy.de/livestream/Discussion-06/7

Abstract:
We present MHD+CR simulations probing reacceleration of pre-existing cosmic rays by long-wavelength, subsonic, compressive turbulence. With purely diffusive transport, we recover the scaling relations of Ptuskin 1988, where the reacceleration time reaches a minimum at the ``sweet spot” diffusion coefficient of the sound speed times the outer turbulence scale, $D_{rm crit} sim c_{s}L$. For GeV energy cosmic rays, however, where self-confinement and streaming transport likely dominate, reacceleration rates are highly suppressed at low plasma $beta$, collisionless energy loss $propto v_{A} cdot nabla P_{CR}$ largely offsets energy gain, even when additional diffusion at the "sweet spot” value of $D_{rm crit}$ is included. At higher plasma $beta$ (when diffusive transport dominates), which may be appropriate in galaxy halo environments, the energy gain time is again quite short (as low as a few eddy turnover times). rnrnThis in-situ cosmic ray production, especially if coupled with reacceleration by large-scale shocks, can increase non-thermal pressure support in the circumgalactic medium, as required to explain COS-Halos absorption line measurements, and could leave an imprint in diffuse gamma-ray emission. In low-$beta$ environments like the interstellar medium, reacceleration of GeV-energy cosmic rays can likely be ignored. This may alleviate tension between current cosmic ray reacceleration models and recent observations by Voyager 1 and AMS-02 that favor pure diffusion / convection models.'

Authors: Chad Bustard
Co-Authors: S. Peng Oh
Indico-ID: 1056
Proceeding URL: https://pos.sissa.it/395/171