Scattering of Cosmic Rays by Magnetohydrodynamic Interstellar Turbulence

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Abstract
Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for substantial revisions in our understanding of cosmic ray transport. In this paper we use recently obtained scaling laws for MHD modes to calculate the scattering frequency for cosmic rays in ISM. We consider gyroresonance with MHD modes (Alfvenic, slow and fast) and transit-time damping (TTD) by fast modes. We conclude that the gyroresonance with fast modes is the dominant contribution to cosmic ray scattering for the typical interstellar conditions. We also find that TTD is not an efficient scattering mechanism owing to the substantial Landau damping of modes with wave vector at small angle to magnetic field. Our model of CR scattering by fast modes reproduces the turnoff of CR escape length around 1 GeV, which matches the observed peak of the secondary/primary ratio Boron/Carbon. More importantly, the scattering by fast modes produces a steeper spectrum of the escape lengths than Kolmogorov model, and thus agrees better with observations.

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