Ion alfvén velocity fluctuations and implications for the diffusion of streaming cosmic rays

The interstellar medium (ISM) of star-forming galaxies is magnetized and turbulent. Cosmic rays (CRs) propagate through it, those with energies from ? GeV ? TeV are likely subject to the streaming instability, whereby wave damping processes balances excitation resonant ionic Alfvén waves by CRs, reaching an equilibrium in which propagation speed CRs very close local ion velocity. transport therefore sensitive velocity fluctuations. In this paper we systematically study these fluctuations using a large ensemble compressible MHD turbulence simulations. We show that for sub-Alfvénic turbulence, as applies strongly ISM, probability density function (PDF) determined solely shocked gas forming parallel magnetic field, develop analytical models PDF up second moments. For super-Alfvénic correlated complex ways, correlations well contributions sets PDF. discuss implications findings underlying “macroscopic” diffusion mechanisms undergoing including modeling macroscopic coefficient plasmas. also describe how, highly-magnetized turbulent gas, PDF, hence column can be used access information about structure observations ISM.