The Multi-fluid Nature of the Termination Shock

Abstract:

After the crossing of the termination shock by the Voyager spacecraft, it became clear that pickup ions (PUIs) dominate the thermodynamics of the heliosheath. Particle-in-cell simulations by Wu et al. [2010] have shown that the sum of the thermal solar wind and non-thermal PUI distributions downstream of the termination shock can be approximated with a 2-Maxwellian distribution. Therefore the heliosheath can be described as multi-fluid plasma comprising of cold thermal solar wind ions, hot pickup ions (PUI) and electrons. The abundance of the hot pickup ion population has remained unknown, since the plasma instrument on board Voyager 2 can only detect the colder thermal ion component. Upstream of the termination shock, where the solar wind bulk flow is quasi-perpendicular to the Parker spiral-like heliospheric magnetic field, the two ion fluids are fully coupled. However, in the heliosheath, where the ion flows start to divert from the radial direction, PUIs and thermal solar wind ions become decoupled in the parallel direction, resulting in differential ion flow velocities. This multi-fluid nature of the heliosheath cannot be captured in current single-fluid MHD models of the heliosphere. Here we present our new multi-ion Hall MHD model of the termination shock, which is able to resolve finite gyroradius effects [Zieger et al., 2014]. The addition of hot PUIs to the mixture of thermal solar wind protons and cold electrons results in the mode splitting of fast magnetosonic waves into a high-frequency fast mode (or PUI mode) and a low-frequency fast mode (or thermal proton mode). We show that the multi-fluid nature of the solar wind predicts two termination shocks, one in the thermal and the other in the pickup ion component. We demonstrate that the thermal ion shock is a dispersive shock wave, with a trailing wave train, which is a quasi-stationary nonlinear wave mode, also known as oscilliton. We constrain the previously unknown PUI abundance and the PUI temperature by fitting simulated multi-fluid termination shock profiles to Voyager 2 observations. Our model provides self-consistent energy partitioning between the ion species across the termination shock and predicts the preferential heating of the thermal ion component. The nonlinear oscilliton mode can be a source of compressional turbulence in the heliosheath.