SEPARATRIX CHARACTERISTICS IN STEADY MHD FLOWS

The appearance of separatrix surfaces in the mixed elliptic/hyperbolic equations describing steady MHD hows is discussed and some examples a re presented. It is shown that in the domains where the MHD equations are hyperbolic, there exist some special characteristics which corresp ond to stationary shocks of zeroth strength. These separatrix characte ristics split the hyperbolic domains into subdomains wherein communica tion via the corresponding signal speed is not always possible. The ap propriate boundary conditions required for the determination of a uniq ue solution are also discussed.