The Structure and Emission of a Non-Radiative Shock

Abstract

Faint filaments are observed a few arcmin outside the bright optical filaments of the Cygnus Loop. They show nearly pure Balmer line emission spectra, and they are interpreted as emission from non-radiative shocks (1). Each neutral H atom passing through the shock front emits on average about 0.1 Hα photon before it is ionized. Since this radiation arises very close to the shock front, rather than in an extended post-shock cooling zone (2), it can be used to study the physics of the shock front itself. The structure of a shock poses several important questions (3). There may be an electron thermal conduction precursor ahead of the shock and there may be plasma turbulence. The shock thermalizes 3/4 of the bulk velocity of the incoming particles, so the ions initially have nearly all of the thermal energy. The electron and ion temperatures can reach equilibrium on the Coulomb collision time scale, but plasma turbulence may bring them into equilibrium much more rapidly. The Coulomb equilibration time scale is similar to the hydrogen ionization time, so that the hydrogen line emission will depend on the nature of the equilibration. The interpretation of the Hα line profile in terms of the shock velocity also depends on this equilibration, so this question is important for comparison of shock models with X-ray spectra.