Turbulent Reacceleration of Streaming Cosmic Rays and Damping of Compressive Turbulence

Subsonic, compressive turbulence transfers energy to cosmic rays (CRs), a process known as non-resonant reacceleration. It is often invoked to explain observed ratios of primary to secondary CRs at GeV energies, assuming wholly diffusive CR transport. However, such estimates ignore the impact of CR self-confinement and streaming. Furthermore, the back reaction of CRs, which damp compressive motions, on the turbulent flow has received scant attention. We study these issues in stirring box magnetohydrodynamic simulations using Athena++, with field-aligned diffusive and streaming CR transport. When streaming is included, reacceleration rates depend on plasma β. They are significantly slower than canonical reacceleration rates in low-β environments like the interstellar medium (ISM), but remain unchanged in high-β environments like the intracluster medium (ICM). Despite slow reacceleration rates, we show that CR "viscosity" can still significantly alter the turbulent cascade, steepening the turbulent power spectrum and damping small scale compressive turbulence. This "divergence-cleaning" renders small-scale turbulence largely solenoidal and could suppress fluctuations that are important for thermal instability and for resonant scattering of high-energy (E > 300 GeV) CRs.