Dissipation in oblique slow shocks

摘要

Ion and electron dynamics and dissipation in collisionless slow mode shocks are examined using one-dimensional hybrid (kinetic ions, massless fluid electrons) and full particle (kinetic ions and electrons) simulations. In the hybrid code, two types of electron fluid models are used to determine the role of electrons: an adiabatic electron fluid model, which uses a scalar electron pressure, and the electron pressure tensor model, in which effects of the downstream electron temperature anisotropy with respect to the local magnetic field direction can be modeled. Comparing results from full particle and hybrid simulations that use an adiabatic electron fluid, the dynamics of slow shocks with the shock normal angle Bn ranging from moderate to very oblique (60°–84°) are explored for the upstream ion and electron β value of 0.1. At moderate Bn, the two types of simulations give comparable results, and the shock dissipation is provided primarily by the ions. However, as Bn increases, more significant differences are found between the two types of simulations, which indicates that the ion dissipation alone is inadequate to set up the shock and that additional electron physics is needed. At highly oblique Bn, the downstream electron temperature becomes anisotropic (T e > T e ) in full particle simulations, similar to that observed for the ion plasma in both type of simulations, while the electron inertial effects are negligible. The electron anisotropy results from both the large mirror effects and the electron acceleration/heating by the parallel electric field of very obliquely propagating kinetic Alfvén waves excited by ion-ion streaming in the shock. The additional electron dynamics in the wave fields lead to spiky structures in the shock ramp in the density and the ion and electron parallel temperature/pressure. We present hybrid simulations of very oblique slow shocks ( Bn = 84°) that retain the electron pressure tensor calculations. Results show that the inclusion of the downstream electron temperature anisotropy and of the resulting quasi-viscous effects in hybrid methods allows slow shocks to be set up at very oblique angles, as observed in the distant tail, when dissipation from ion-ion streaming becomes much weaker.