Confinement and Isotropization of Galactic Cosmic Rays by Molecular‐Cloud Magnetic Mirrors When Turbulent Scattering Is Weak

Abstract

Theoretical studies of magnetohydrodynamic (MHD) turbulence and observations of solar wind —uc- tuations suggest that MHD turbulence in the interstellar medium is anisotropic at small scales, with smooth variations along the background magnetic —eld and sharp variations perpendicular to the back- ground —eld. Turbulence with this anisotropy is inefficient at scattering cosmic rays, and thus the scat- tering rate l may be smaller than has been traditionally assumed in diUusion models of Galactic cosmic-ray propagation, at least for cosmic-ray energies E above 1011¨1012 eV at which self-con—nement is not possible. In this paper, it is shown that Galactic cosmic rays can be eUectively con—ned through magnetic re—ection by molecular clouds, even when turbulent scattering is weak. Elmegreens quasi- fractal model of molecular-cloud structure is used to argue that a typical magnetic —eld line passes through a molecular cloud complex once every D300 pc. Once inside the complex, the —eld line will in most cases be focused into one or more dense clumps in which the magnetic —eld can be much stronger than the average —eld in the intercloud medium (ICM). Cosmic rays following —eld lines into cloud com- plexes are most often magnetically re—ected back into the ICM, since strong-—eld regions act as magnetic mirrors. For a broad range of cosmic-ray energies, a cosmic ray initially following some particular —eld line separates from that —eld line sufficiently slowly that the cosmic ray can be trapped between neigh- boring cloud complexes for long periods of time. The suppression of cosmic-ray diUusion due to mag- netic trapping is calculated in this paper with the use of phenomenological arguments, asymptotic analysis, and Monte Carlo particle simulations. Formulas for the coefficient of diUusion perpendicular to the Galactic disk are derived for several diUerent parameter regimes within the E-l plane. In one of these parameter regimes in which scattering is weak, it is shown that molecular-cloud magnetic mirrors strongly reduce cosmic-ray anisotropy in the ICM, and analytic formulas for the angular harmonics are derived. Subject headings: acceleration of particlescosmic raysISM: cloudsISM: magnetic —elds ¨ turbulence