Trapping of cosmic rays in MHD turbulence

Abstract

Astrophysical plasmas are turbulent and magnetized. The interaction between cosmic rays (CRs) and magnetohydrodynamic (MHD) turbulence is a fundamental astrophysical process. Based on the current understanding of MHD turbulence, we revisit the trapping of CRs by magnetic mirrors in the context of MHD turbulence. In compressible MHD turbulence, isotropic fast modes dominate both trapping and gyroresonant scattering of CRs. The presence of trapping significantly suppresses the pitch-angle scattering and the spatial diffusion of CRs along the magnetic field. The resulting parallel diffusion coefficient has a weaker dependence on CR energy at higher energies. In incompressible MHD turbulence, the trapping by pseudo-Alfvén modes dominates over the gyroresonant scattering by anisotropic Alfvén and pseudo-Alfvén modes at all pitch angles and prevents CRs from diffusion.

American Musicals: Stage and screen / L’écran et la scène. Anne Martina & Julie Vatain-Corfdir (dir.). Paris : Sorbonne Université Presses. Paris, 2019, 254 p.

“…The influence of large-scale modes can be followed in a guiding-center approximation, whereas the resonant modes that provoke non-adiabatic excursions of the pitch angle act sporadically along the particle trajectory. Such a description lies close in spirit to that of Xu & Lazarian (2020) and Lazarian & Xu (2021), who study the spatial diffusion of particles trapped in large-scale mirrors yet subject to a pitch-angle scattering process. A key difference however, is that in the present picture, the interactions that lead to pitch-angle scattering are intense, localized in space and associated with sharp bends in the magnetic lines, not to QLT-like resonances.…”

“…Those forces influence µ in an adiabatic manner, decreasing it in regions of increasing magnetic field strength and vice-versa. The overall process can lead to spatial diffusion, but it should not lead by itself to a scaling of λ with r g , given that the dominant effect is tied to the largest length scales on which most of the turbulent power is concentrated (Malyshkin & Kulsrud 2001;Chandran 2000a;Xu & Lazarian 2020;Lazarian & Xu 2021).…”