Energy dissipation and ion heating at the heliospherictermination shock

摘要

The Los Alamos hybrid simulation code is used to examine heating and the partitionof dissipation energy at the perpendicular heliospheric termination shock in the presenceof pickup ions. The simulations are one-dimensional in space but three-dimensional infield and velocity components, and are carried out for a range of values of pickup ionrelative density. Results from the simulations show that because the solar wind ions arerelatively cold upstream, the temperature of these ions is raised by a relatively larger factorthan the temperature of the pickup ions. An analytic model for energy partition isdeveloped on the basis of the Rankine-Hugoniot relations and a polytropic energyequation. The polytropic index 1/ used in the Rankine-Hugoniot relations is varied toimprove agreement between the model and the simulations concerning the fraction ofdownstream heating in the pickup ions as well as the compression ratio at the shock.When the pickup ion density is less than 20%, the polytropic index is about 5/3, whereasfor pickup ion densities greater than 20%, the polytropic index tends toward 2.2,suggesting a fundamental change in the character of the shock, as seen in the simulations,when the pickup ion density is large. The model and the simulations both indicate for theupstream parameters chosen for Voyager 2 conditions that the pickup ion density isabout 25% and the pickup ions gain the larger share (approximately 90%) of thedownstream thermal pressure, consistent with Voyager 2 observations near the shock.