Effects of mirror reflection versus diffusion anisotropy on particle acceleration in obliqu.pdf

a r X i v : a s t r o - p h / 0 5 1 0 2 5 1 v 1 9 O c t 2 0 0 5 Mon. Not. R. Astron. Soc. 000, 1–6 (2005) Printed 5 February 2008 (MN LATEX style file v2.2) Effects of mirror reflection versus diffusion anisotropy on particle acceleration in oblique shocks Y. S. Honda 1? and M. Honda 2 1Kinki University Technical College, Kumano, Mie, 519-4395, Japan 2Plasma Astrophysics Laboratory, Institute for Global Science, Mie 519-5203, Japan Accepted 2005 Received 2005 3 June; in original form 2005 January ABSTRACT Cosmic ray particles are more rapidly accelerated in oblique shocks, with the magnetic field inclined with respect to the shock normal direction, than in parallel shocks, as a result of mirror reflection at the shock surface and slower diffusion in the shock nor- mal direction. We investigate quantitatively how these effects contribute to reducing the acceleration time over the whole range of magnetic field inclinations. It is shown that, for quasi-perpendicular inclination, the mirror effect plays a remarkable role in reducing the acceleration time; whereas, at relatively small inclination, the anisotropic diffusion effect is dominant in reducing that time. These results are important for a detailed understanding of the mechanism of particle acceleration by an oblique shock in space and heliospheric plasmas. Key words: acceleration of particles — diffusion — magnetic fields — shock waves — cosmic rays. 1 INTRODUCTION Diffusive shock acceleration (DSA) is one of the most favourable mechanisms to reproduce the energy spectrum of observed cosmic rays (for a review, see Drury 1983). Shock waves accompanied by the magnetic fields with small fluctu- ations (Alfve?n waves) are well established in space plasmas and operate as a powerful accelerator of charged particles (Blandford Eichler 1987). In earlier works, substantial ef- forts were devoted to studies of acceleration by the sim- ple ‘parallel shocks’ in which both the magnetic field and the direction of plasma