Electron-foreshock instabilities in oblique high-Mach-number shocks

Motivated by simulations of oblique shocks in supernova remnants, we investigate which instabilities are excited by relativistic electron beams in the extended foreshock of non-relativistic high-Mach-number shocks. We analyze 2D PIC simulations to obtain phase-space distributions in various regions of the electron foreshock, where shock-reflected electrons interact with the incoming upstream flow and excite characteristic instabilities. Periodic PIC simulations and linear dispersion analyses are used to determine where and how these instabilities are generated.

Far from the shock front, the upstream plasma is dominated largely by the electron-acoustic instability. But in the near-upstream region of the foreshock, in which the beam of shock-reflected electrons is denser by an order of magnitude, oblique electromagnetic waves dominate the plasma. We show that, in contrast to a similar instability observed in the solar wind, this mode is not driven unstable by temperature anisotropy. Instead, this oblique-whistler instability is excited gyroresonantly by the isotropic beam of relativistic electrons and scatters most of them before they can reach the far upstream.