SIMULATIONS OF ION ACCELERATION AT NON-RELATIVISTIC SHOCKS. I. ACCELERATION EFFICIENCY

Abstract

We use 2D and 3D hybrid (kinetic ions - fluid electrons) simulations to\ninvestigate particle acceleration and magnetic field amplification at\nnon-relativistic astrophysical shocks. We show that diffusive shock\nacceleration operates for quasi-parallel configurations (i.e., when the\nbackground magnetic field is almost aligned with the shock normal) and, for\nlarge sonic and Alfv\\'enic Mach numbers, produces universal power-law spectra\nproportional to p^(-4), where p is the particle momentum. The maximum energy of\naccelerated ions increases with time, and it is only limited by finite box size\nand run time. Acceleration is mainly efficient for parallel and quasi-parallel\nstrong shocks, where 10-20% of the bulk kinetic energy can be converted to\nenergetic particles, and becomes ineffective for quasi-perpendicular shocks.\nAlso, the generation of magnetic turbulence correlates with efficient ion\nacceleration, and vanishes for quasi-perpendicular configurations. At very\noblique shocks, ions can be accelerated via shock drift acceleration, but they\nonly gain a factor of a few in momentum, and their maximum energy does not\nincrease with time. These findings are consistent with the degree of\npolarization and the morphology of the radio and X-ray synchrotron emission\nobserved, for instance, in the remnant of SN 1006. We also discuss the\ntransition from thermal to non-thermal particles in the ion spectrum\n(supra-thermal region), and we identify two dynamical signatures peculiar of\nefficient particle acceleration, namely the formation of an upstream precursor\nand the alteration of standard shock jump conditions.\n