Fundamentals of collisionless shocks for astrophysical application, 1. Nonrelativistic shocks
Treumann, R. A. (2009),
Fundamentals of collisionless shocks for astrophysical application, 1. Nonrelativistic shocks,
Astronomy & Astrophysics Reviews, 17(4), 409535, doi:10.1007/s0015900900242.
 Abstract
 Abstract A comprehensive review is given of the theory and properties of nonrelativistic
shocks in hot collisionless plasmas in view of their possible application in astrophysics. Understanding
nonrelativistic collisionless shocks is an indispensable step towards a general
account of highMach number collisionless astrophysical shocks and their effects in dissipating
flow energy, heating matter, accelerating particles to high – presumably cosmic ray –
energies, and generation of detectable radiation from radio to xrays. Nonrelativistic shocks
have low Alfv´enic Mach numbers MA pmi=me(wpe=wce), with mi=me iontoelectron
mass ratio, and wpe;wce the electron plasma and cyclotron frequencies, respectively. Though
of high temperature, their temperatures (in energy units) are limited to T < mec2, such
that particle creation is inhibited, classical theory is applicable, and reaction of radiation
on the dynamics of the shock can be neglected. The majority of such shocks is supercritical,
i.e. they are unable to selfconsistently produce sufficient dissipation for sustaining a
stationary shock. As a consequence, they act as strong particle reflectors. All these shocks
are microscopically thin, with shocktransition width of the order of the ion inertial length
li = c=wpi. The full theory of these shocks is developed, and the different possible types of
shocks are defined. Since all collisionless shocks are magnetised, the most important distinction
is between quasiperpendicular and quasiparallel shocks. The former propagate about
perpendicular, the latter about parallel to the upstream magnetic field. Their manifestly different
behaviour is described in detail. In particular, both types of shocks are nonstationary
with completely different reformation cycles. From numerical fullparticle simulations it
becomes evident that, on ioninertial scales close to the shock transition, quasiparallel collisionless
supercritical shocks behave locally quasiperpendicular. This property is of vital
importance for the particle dynamics near the quasiparallel shock front. Considerable interest
focusses on particle acceleration and the generation of radiation. Radiation from nonrelativistic
shocks results mainly in wavewave interactions among various plasma waves.
Nonthermal charged particles can be further accelerated to high energies by a Fermilike
mechanism. The important question is, whether the shock can preaccelerate shockreflected
particles to sufficiently high energies in order to create the seedpopulation of nonthermal
particles which the Fermi mechanism requires. Based on preliminary fullparticle numerical
simulations this question is answered affirmatively. Such simulations provide evidence for
the capability of highMach number collisionless shocks, even when being nonrelativistic,
of producing the seed population for the Fermiprocess.
Keywords: Collisionless shocks  Supercritical shocks  Shock kinetics  Shock reformation  Shock acceleration
PACS Numbers: 95.30.Qd, 52.35.Tc, 52.72.+v
 BibTeX

@article{id1407,
author = {R. A. Treumann},
journal = {Astronomy {\&} Astrophysics Reviews},
month = {aug},
number = {4},
pages = {409535},
title = {{Fundamentals of collisionless shocks for astrophysical application, 1. Nonrelativistic shocks}},
volume = {17},
year = {2009},
doi = {10.1007/s0015900900242},
}
 EndNote

%0 Journal Article
%A Treumann, R. A.
%D 2009
%N 4
%V 17
%J Astronomy & Astrophysics Reviews
%P 409535
%T Fundamentals of collisionless shocks for astrophysical application, 1. Nonrelativistic shocks
%8 aug