Kinetic Alfven Waves and Electron Physics in Oblique Slow Shocks

One-dimensional particle-in-cell (PIC; kinetic ions and electrons) and hybrid (kinetic ions; adiabatic and massless fluid electron) simulations of highly oblique slow shocks (θB n=84° and β=0.1) [Yin et al., J. Geophys. Res., 110, A09218, 2005] have shown that the dissipation from the ions is too weak to form a shock and that kinetic electron physics is required. The study also showed that the downstream electron temperature becomes anisotropic ({T^e}_∥ > {T^e}_\perp), as observed in slow shocks in space. The electron anisotropy results, in part, from the electron acceleration/heating by parallel electric fields of obliquely propagating kinetic Alfvén waves (KAW) excited by ion-ion streaming, which cannot be modeled accurately in hybrid simulations. In the shock ramp, spiky structures occur in density and electron parallel temperature, where the ion parallel temperature decreases due to the reduction of the ion backstreaming speed. In this work, KAW and electron physics in oblique slow shocks are further examined under lower electron beta conditions. It is found that as the electron beta is reduced, the resonant interaction of electrons and the wave parallel electric fields shifts to the tail of the electron velocity distribution, providing more efficient parallel heating. As a consequence, for β_e=0.02, the electron physics is shown to influence the formation of a θB n=75° shock. Electron effects are more enhanced at a highly oblique shock angle (θBn=84°) when both the growth rate and the range of unstable modes on the KAW branch increase. Detailed electron and ion phase-space vortices in the shock ramp formed by electron and KAW interactions and the reduction of the ion back-streaming speed, respectively, are observed in the simulations and confirmed in homogenerous geometries in one and two-spatial dimensions. Results from this study conclude that the inability of the hybrid method to model the Landau resonance of KAW with electrons and the resulting time-evolving electron parallel heating leads to differences between the PIC and hybrid simulations.