Fundamentals of collisionless shocks for astrophysical application, 1. Non-relativistic shocks


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 non-relativistic collisionless shocks is an indispensable step towards a general account of high-Mach 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 x-rays. Non-relativistic shocks have low Alfv´enic Mach numbers MA pmi=me(wpe=wce), with mi=me ion-to-electron 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 self-consistently produce sufficient dissipation for sustaining a stationary shock. As a consequence, they act as strong particle reflectors. All these shocks are microscopically thin, with shock-transition 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 quasi-perpendicular and quasi-parallel 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 non-stationary with completely different reformation cycles. From numerical full-particle simulations it becomes evident that, on ion-inertial scales close to the shock transition, quasi-parallel collisionless supercritical shocks behave locally quasi-perpendicular. This property is of vital importance for the particle dynamics near the quasi-parallel shock front. Considerable interest focusses on particle acceleration and the generation of radiation. Radiation from nonrelativistic shocks results mainly in wave-wave interactions among various plasma waves. Non-thermal charged particles can be further accelerated to high energies by a Fermi-like mechanism. The important question is, whether the shock can pre-accelerate shock-reflected particles to sufficiently high energies in order to create the seed-population of non-thermal particles which the Fermi mechanism requires. Based on preliminary full-particle numerical simulations this question is answered affirmatively. Such simulations provide evidence for the capability of high-Mach number collisionless shocks, even when being non-relativistic, of producing the seed population for the Fermi-process. Keywords: Collisionless shocks - Supercritical shocks - Shock kinetics - Shock reformation - Shock acceleration PACS Numbers: 95.30.Qd, 52.35.Tc, 52.72.+v

  author = {Treumann, R. A.},
  doi = {10.1007/s00159-009-0024-2},
  journal = {Astronomy \& Astrophysics Reviews},
  language = {en},
  number = {4},
  pages = {409-535},
  title = {Fundamentals of collisionless shocks for astrophysical application, 1. Non-relativistic shocks},
  volume = {17},
  year = {2009},
%O Journal Article
%A Treumann, R. A.
%R 10.1007/s00159-009-0024-2
%J Astronomy & Astrophysics Reviews
%G en
%N 4
%P 409-535
%T Fundamentals of collisionless shocks for astrophysical application, 1. Non-relativistic shocks
%V 17
%D 2009