Misc,
Particle acceleration and wave excitation in quasi-parallel high-Mach-number collisionless shocks: Particle-in-cell simulation
(Jul 8, 2014)
Abstract
We herein investigate shock formation and particle acceleration processes for
both protons and electrons in a quasi-parallel high-Mach-number collisionless
shock through a long-term, large-scale particle-in-cell simulation. We show
that both protons and electrons are accelerated in the shock and that these
accelerated particles generate large-amplitude Alfvénic waves in the
upstream region of the shock. After the upstream waves have grown sufficiently,
the local structure of the collisionless shock becomes substantially similar to
that of a quasi-perpendicular shock due to the large transverse magnetic field
of the waves. A fraction of protons are accelerated in the shock with a
power-law-like energy distribution. The rate of proton injection to the
acceleration process is approximately constant, and in the injection process,
the phase-trapping mechanism for the protons by the upstream waves can play an
important role. The dominant acceleration process is a Fermi-like process
through repeated shock crossings of the protons. This process is a `fast'
process in the sense that the time required for most of the accelerated protons
to complete one cycle of the acceleration process is much shorter than the
diffusion time. A fraction of the electrons is also accelerated by the same
mechanism, and have a power-law-like energy distribution. However, the
injection does not enter a steady state during the simulation, which may be
related to the intermittent activity of the upstream waves. Upstream of the
shock, a fraction of the electrons is pre-accelerated before reaching the
shock, which may contribute to steady electron injection at a later time.
