Shock Physics in SNRs: an observational perspective
Cara Rakowski (Naval Research Lab (NRC fellow)) , J. Martin Laming (NRL), Parviz Ghavamian (Johns Hopkins)
The outer blast-waves of supernova remnants (SNRs) are an example of ``collisionless shocks'', i.e. the width of the shock transition is tiny compared to the Coulomb mean-free-path. In these shocks the particle heating to postshock temperatures and acceleration to cosmic ray energies must be mediated by plasma waves arising from instabilities, and not just from random Coulomb collisions. The 1/1835 mass ratio of electrons to protons makes the heating and acceleration of electrons particularly difficult. In this talk I will explain the spectroscopic techniques for determining the proton, ion and electron temperatures at a variety of supernova remnant shocks, and present the latest data on the electron to proton temperature ratio from this survey. The observed inverse square dependence of the electron to proton temperature ratio with shock velocity can be explained by a physical model for the electron heating, whereby lower hybrid waves excited in the shock cosmic-ray precursor damp by accelerating electrons along the local magnetic field, echoing recent suggestions in the literature that the cosmic rays are an integral part of the collisionless shock structure. New results including the growth rate for lower-hybrid waves in the cosmic-ray precursor will be discussed.