OBSERVATIONS OF AN INTERMEDIATE SHOCK IN INTERPLANETARY SPACE

An interplanetary intermediate shock is identified from the bulk veloc ity, number density, and temperature of the solar wind protons and the three components of the interplanetary magnetic field observed by Voy ager 1 on May 1 (day 122), 1980, when the spacecraft was at a distance of about 9 AU from the Sun. It is shown by a best fit procedure that the measured plasma and magnetic field on both sides of the discontinu ity satisfy the Rankine-Hugoniot relations for a magnetohydrodynamic ( MHD) intermediate shock. This shock satisfies the following conditions . (1) The normal Alfven-Mach number (M(A) = V(B)/V(A) is greater than unity in the preshock state and less than unity in the postshock stat e. (2) Both the fast-mode Mach number (M(f) = V(n)/V(f)) in the presh ock state and the slow-mode Mach number (M(sl) = V(n)/V(sl)) in the p ostshock state are less than unity, but the slow-mode Mach number is g reater than unity in the preshock state. (3) The projected components of the magnetic fields in the shock front for the pre- and postshock s tates have opposite signs. (4) The magnitudes of the magnetic fields d ecrease from the preshock to the postshock states. In the above expres sion, V(A) is the Alfven speed based on the magnetic field component n ormal to the shock front, V(n) is the component of the bulk velocity normal to the shock front and measured in the shock frame of reference , and V(f) and V(sl) are the speeds of the fast- and slow- mode magnet osonic waves in the direction of the shock normal, respectively. The d iscontinuity event in our discussion cannot be a rotational discontinu ity because the Walen's relation is not satisfied. The identified inte rmediate, shock has M(A) = 1.04, theta(Bn) =37-degrees, and beta =0.56 , where theta(Bn) is the angle between the preshock magnetic field and the shock normal direction and beta is the ratio of thermal to magnet ic energy densities. Using these parameters, a numerical solution of t he MHD equations for the shock is obtained. The simulated profiles of the bulk velocity, number density, temperature, and magnetic fields of the pre-and postshock states agree with those of the observed values. The same parameters are used to simulate an intermediate shock using a hybrid numerical code in which full ion dynamics is retained while e lectron inertial force is neglected. The results of this simulation ar e compared with high-resolution magnetic field data with a time resolu tion of 1.92-s averages. The shock thickness of about 70 c/omega(pi) p redicted from the hybrid code agrees with the observations. The genera l behavior of the magnetic field in the shock transition region is als o very similar for the simulated and observed results. The macro- and microstructures of the intermediate shock obtained from the MHD and hy brid models resemble the observed structures.