Observations of an intermediate shock in interplanetary space

Abstract

An interplanetary intermediate shock is identified from the bulk velocity, number density, and temperature of the solar wind protons and the three components of the interplanetary magnetic field observed by Voyager 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 discontinuity satisfy the Rankine‐Hugoniot relations for a magnetohydrodynamic (MHD) intermediate shock. This shock satisfies the following conditions. (1) The normal Alfvén‐Mach number (MA = Vn*/VA) is greater than unity in the preshock state and less than unity in the postshock state. (2) Both the fast‐mode Mach number (Mf = Vn*/Vf) in the preshock state and the slow‐mode Mach number (Msl = Vn*/Vsl) in the postshock state are less man unity, but the slow‐mode Mach number is greater than unity in the preshock state. (3) The projected components of the magnetic fields in the shock front for the pre‐ and postshock states have opposite signs. (4) The magnitudes of the magnetic fields decrease from the preshock to the postshock states. In the above expression, VA is the Alfvén speed based on the magnetic field component normal to the shock front, Vn* is the component of the bulk velocity normal to the shock front and measured in the shock frame of reference, and Vf and Vsl are the speeds of the fast‐ and slow‐mode magnetosonic waves in the direction of the shock normal, respectively. The discontinuity event in our discussion cannot be a rotational discontinuity because the Walen's relation is not satisfied. The identified intermediate, shock has MA = 1.04, θBn = 37°, and β = 0.56, where θBn is the angle between the preshock magnetic field and the shock normal direction and β is the ratio of thermal to magnetic energy densities. Using these parameters, a numerical solution of the 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 electron inertial force is neglected. The results of this simulation are compared with high‐resolution magnetic field data with a time resolution of 1.92‐s averages. The shock thickness of about 70 c/ωpi predicted from the hybrid code agrees with the observations. The general behavior of the magnetic field in the shock transition region is also very similar for the simulated and observed results. The macro‐ and microstructures of the intermediate shock obtained from the MHD and hybrid models resemble the observed structures.