Magnetic structure of ionizing shock waves Part 3. Normal shocks

Abstract

Normal ionizing shock waves are considered as a subclass of oblique shocks in which the upstream transverse magnetic field component is zero; i.e. the upstream field is normal to the plane of the shock. Non-trivial (switch-on) normal shocks involve a non-zero downstream transverse field component; magnetically trivial normal shocks are simply gas shocks with an imbedded constant normal magnetic field. As with oblique shocks, switch-on normal ionizing shock waves are plane- polarized, provided the conductivity is a scalar. Ohmic structures are discussed for several values of shock Alfv én number, treating the electric field as a free parameter, as usual. For Alfv én numbers extending from zero to two (for the infinite-Mach-number case), there is always a finite range of E field values. Above two, only the gas shock exists, and this requires a unique electric field value. Because the magnetic field magnitude increases through switch-on shocks, there is no mechanism available for converting magnetic energy into thermal energy, as is the case for oblique or skew shocks. Thus, there is no significant downstream heating above the viscous temperature; and, in some cases, slight downstream cooling may even occur. Expansion shocks are not possible in this geometry. Previous studies are reviewed in the light of structural requirements, and some erroneous results are clarified; in particular, it should be noted that MHD switchon solutions for the pre-ionized case are not imbedded in the family of ionizing switch-on solutions.