Transition of slow shocks to fast shocks in the interplanetary space

Abstract

As a CME pushes its way through preceding slower solar wind, large disturbances in the interaction region may evolve to form transient MHD shocks. The shocks can be slow shocks in the coronal space, but they appear as fast shocks near 1 AU. We use a polytropic MHD model to study the transition of slow shocks to fast shocks in an interaction region sandwiched between a faster solar wind and a slower solar wind near the equatorial plane. The polytropic index controls the radial variation of the solar wind conditions outside the shock. The simulation results of two examples show smooth transition of the shock system from slow shocks near 0.15 AU to fast shocks near 1 AU and beyond. A parametric study of shock solutions in the dimensionless parameter space shows that the transition process is attributed chiefly to the increase of β with the heliocentric distance and to a lesser degree to the increase of the shock angle θ. Under the initial condition of small β and θ near 0.1 AU, the interaction region evolves to form a pair of slow shocks inside 0.15 AU. As the interaction region convects outward, the increases of β and θ cause a transition of the shock system between 0.15 and 0.3 AU from a slow shock pair to a double shock pair consisting of both slow and fast shocks. As the system moves outward, β and θ continue to increase, the fast shock grows stronger, and the slow shock becomes weaker. Eventually, the slow shocks fade away, and the shock system finally evolves to a pair of fast shocks.