Magnetic structure of ionizing shock waves Part 2. Oblique shocks

Abstract

Oblique ionizing shock waves are considered as a subclass of the skew shocks discussed in part 1. It is shown explicitly that, for these cases, when the electrical conductivity is a scalar, the magnetic field and velocity vectors lie in a single plane throughout the shock structure. Details of the shock structure are given in the zero-magnetic-Prandtl-number limit for a range of values of the upstream magnetic pressure ratio as the transverse electric field is varied parametrically. Shocks possessing a magnetic structure are necessarily sub-Alfv énic downstream. Thus some structures are trans-Alfv énic (i.e. super-Alfv énic upstream), and others are completely sub-Alfv énic. Unlike the pre-ionized case, the former are stable to rotational Alfv én disturbances because of their ability to form skew shocks. Many features are qualitatively similar to those of skew shocks. For example, very high downstream temperatures may be obtained by Joule heating, and, in some cases, overall expansion shocks may occur. Although magnetic structures can always be found for a range of shock Alfvén numbers from zero up to a value greater than unity, there is an upper limit, above which only the gas shock exists, corresponding to a specific value of the electric field. There is no upper limit on the gas shook speeds. This contrasts with the skew shock case in which the corresponding (saddle-point) solution has an upper Alfvén number limit (slightly above two for the monatomic, infinite-Mach-number case), above which no skew shock solutions exist at all. Finally, the results of previous studies are reviewed in light of the structural requirements of ionizing shock waves.