Supersonic solar wind ion flows downstream of the termination shock explained by a two-fluid shock model

Abstract

In the view of classic magnetohydrodynamics, it is generally expected that at a shock the supersonic upstream flow is converted into the subsonic downstream flow whereby the increased entropy appears in the increased downstream temperature. In the solar wind termination shock, it is expected that the supersonic upstream solar wind ion flow changes into a subsonic downstream ion flow. The Voyager-2 passage over this shock, however, demonstrated that the downstream solar wind ion flow still has a supersonic signature. In this paper, we present straightforward solution to this unexpected phenomenon by applying a two-fluid model to describe the passage of the solar wind over the termination shock. The two dynamically dependent, but thermodynamically independent fluids are the normal solar wind ions and the comoving suprathermal ions, or so-called pick-up ions. As we can show, the downstream solar wind ion flow can be either subsonic or supersonic depending on the upstream effective Mach number of the flow or the pick-up ion pressure. This is because most of the kinetic energy of the upstream flow is converted into thermal energy of the suprathermal ions, while the normal solar wind ions are heated only ineffectively, so that they can retain a supersonic signature even further downstream. With this two-fluid model, we can explain the main features of the shock structure observed by Voyager-2.