Kinetic structure of slow shocks: Effects of the electromagnetic ion/ion cyclotron instability

Abstract

Using a one‐dimensional electromagnetic hybrid code (particle ions, fluid electrons), the structure of and ion heating at slow magnetosonic shocks are investigated in the low beta regime. In particular, the effects of the waves generated at or upstream of the shock by the electromagnetic ion/ion cyclotron (EMIIC) instability are studied. To understand the role of shock initialization and boundary conditions on the resulting solutions, three methods of shock formation are utilized. One is a relaxation method, where the shock is started as a structureless discontinuity and allowed to evolve to a possible steady state solution. It is shown that this method of shock formation can lead to spurious results and therefore is not reliable. The other two investigated methods are similar, in that the shock is formed via flow interactions in a dynamic fashion. One of these is the usual piston method, where the plasma is injected from one side of the simulation box and reflected from the other side. In a new, third method (flow‐flow) the plasma is injected from both boundaries, which results in the formation of a pair of slow shocks. Both of these shock formation methods result in reliable solutions. Using the piston or flow‐flow interactions, it is found that slow magnetosonic shocks have a number of different structures depending on the plasma parameters and the shock normal angle. One set of solutions corresponds to the classical structure where a coherent Alfvén wave train is formed downstream of the shock. However, Alfvén waves generated by the backstreaming ions via the electromagnetic ion/ion cyclotron instability are also observed upstream of the shock. The second class of solutions corresponds to structures with Alfvén turbulence upstream and downstream of the shock. These shocks are found to have a nonsteady behavior, due to transport of upstream waves generated by the EMIIC instability into the downstream region. The nonsteady behavior of the shock prevents the formation of a coherent trailing wave train. A third class of structure is associated with much weaker laminar shocks, where little or no wave activity is present both upstream and downstream of the shock. Using the full electromagnetic dispersion equation, it is shown that large linear damping of Alfvén or slow waves prevents the formation of a wave train downstream of such shocks. A fourth class of solutions is associated with no wave activity in the downstream but has Alfvén waves at and upstream of the shock. The excitation of these waves at the shock leads to a nonsteady behavior, even though their Poynting vector is toward the upstream. By using the average magnetic moment and deviations from its upstream value, the role of the EMIIC instability in ion dissipation has also been investigated. It is shown that ion orbits are adiabatic at weak laminar shocks, leading to anisotropic heating with T∥ > T⊥. On the other hand, when upstream Alfvén waves are present, they lead to incident ion scattering; before the shock transition region. This scattering, however, does not always lead to complete thermalization immediately downstream of the shock. As such, the length scales associated with the magnetic structure and ion dissipation may vary considerably at slow shocks.