Published online by Cambridge University Press: 13 March 2009
Ionizing shock waves may involve a non-zero transverse electric field in the non-conducting upstream region. The possibility of this field means that two extra parameters are involved in ionizing shock transitions, in addition to the usual three MHD shock parameters. The present paper investigates the question of existence or absence of structurally determined relationships between the five parameters. The results show that ionizing shock waves can be categorized into three types, depending on the nature of their downstream singular points in a three-dimensional phase space. Thus, if shock Mach number, Alfven number, and upstream magnetic field angle are given, type 2 shooks are characterized by a unique transverse electric field in both magnitude and orientation, type 3 shocks allow one degree of freedom to the electric field but specify a relationship between the magnitude and orientation, and in type 4 shocks both electric field parameters are free to range between certain limits. These structural conditions correspond exactly to previously investigated evolutionarity conditions based on wave perturbations, the interpretation of which has been incomplete due to a lack of knowledge of the possibility of skew shocks. In light of the author's subsequent work on skew shocks and the new general three-dimensional results, a complete map of the behaviour of ionizing shock waves is now possible, and the present article attempts to construct its essential features. Earlier works are reviewed and clarified in reference to this general framework.