Simulation of Energetic Electron Bursts Upstream of Re-Forming Shocks

Energetic electrons are widely observed in vicinity of planetary bow shocks, interplanetary shocks, and coronal mass ejection driven shocks. In this paper the electron energization is numerically studied by tracing exact test particle electron trajectories in the time-dependent electromagnetic fields typical for the Earth's bow shock. The time-dependent re-forming shock profiles are self-consistently generated by one-dimensional multiscale hybrid simulations. It is shown that energetic electron bursts occur cyclically at the shock re-formation period. The upstream electron distributions show time-varying loss-cone, beam, and ring-beam features in velocity space. The beam density, beam speed, average beam kinetic energy, velocity spread, and associated wave growth rate in the upstream region of the shock cyclically change with time by a factor of ~2-4. These characteristics are qualitatively very different from the continuous beam expected for a time-stationary shock and are important for the plasma instabilities/radio emission phenomena near the shocks.