Electron heating and acceleration in the shock transition region: Background plasma parameter dependence

Abstract

In addition to the well-known plasma parameters of Mach number (MA) and plasma β, the other plasma parameter of the frequency ratio of the electron plasma to gyration, ωpe/Ωce (∝n/B), is emphasized as another important parameter for the plasma heating/acceleration process in the shock transition region. The process of energy conversion from the ion bulk flow into energetic electrons through field activity due to plasma instabilities cannot be described only by such macro averaged parameters as MA and β. The evolution of plasma instabilities largely depends on the background plasma properties, i.e., magnetic field B and density n, which is represented by ωpe/Ωce. Recent studies have shown that electron phase space hole generation due to the nonlinear evolution of Buneman instability between reflected ions and inflow electrons plays an important role in particle energization in the shock transition region. However, even under the same Mach number and plasma β, the formation of electron phase space holes depends on the frequency ratio ωpe/Ωce. Moreover, depending on ωpe/Ωce, different heating and acceleration properties can be obtained. The effect of the frequency ratio ωpe/Ωce on the electron energization is investigated in the shock transition region by using periodic simulations with realistic mass ratio. When ωpe/Ωce⩽1 no electron phase space hole is generated and mild heating of the electrons with a nonthermal energy spectrum is observed. In the region of ωpe/Ωce ⩾ 10 a clear series of electron phase space holes is generated and, after that, ion phase space holes are generated in the resultant hot electrons. These ion phase space holes play an important role in the subsequent strong electron energization.