# ICRC 2021

Jul 12 – 23, 2021
Online
Europe/Berlin timezone

## Turbulent Reacceleration of Streaming Cosmic Rays: Fluid Simulations

Jul 15, 2021, 12:00 PM
1h 30m
06

#### 06

Poster CRD | Cosmic Ray Direct

### Speaker

Chad Bustard (Kavli Institute for Theoretical Physics)

### Description

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).

This 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.

### Keywords

Confinement, reacceleration, turbulence, particle acceleration

Subcategory Theoretical Results

### Primary author

Chad Bustard (Kavli Institute for Theoretical Physics)

### Co-author

Prof. S. Peng Oh (University of California - Santa Barbara)