Magnetopause reconnection layer bounded by switch-off shocks: Part 2. Pressure anisotropy

摘要

The jump conditions are analyzed in detail for two slow shocks bounding a reconnection plasma jet, observed on 3 August 2008 by the spacecraft THEMIS D (Time History of Events and Macroscale Interactions during Substorms) on the dayside, low-latitude magnetopause. Both shocks are near the switch-off limit. They have been previously examined by Sonnerup et al. (2016), on the basis of the simplest MHD version of the jump conditions. In the present paper, those conditions now include the pressure anisotropy, normal heat fluxes, and a finite normal magnetic field component, the effects of all of which are found to be small. We also present and discuss the, mostly field-aligned, measured total heat fluxes, which are found to be substantial and directed away from the reconnection site. We show that the double-adiabatic (Chew-Goldberger-Low) invariants are far from invariant. Their combination indicates a large entropy increase across the shock on the magnetospheric side with a much smaller increase across the shock on the magnetosheath side. The detailed cause of the entropy changes remains unclear but appears to involve irreversible transfer of energy between thermal motion parallel and perpendicular to the magnetic field. The new results confirm the previously found presence of heavy ions and the values of the effective ion mass on both sides of the event. They also confirm the need for an ion pressure correction in the shock on the magnetospheric side.