Effects of shock parameters on upstream energetic electron burst events

Recent simulation results have revealed that energetic electron bursts are produced cyclically at the shock reformation period upstream of reforming shocks and are qualitatively very different from the continuous beam expected from time‐stationary shocks (Yuan et al., 2007a). This paper extends our previous studies by numerically investigating the dependence of electron burst events on shock parameters (the upstream plasma β and Mach number M A ). The test particle approximation is made for electrons, and the electron trajectories are traced exactly in the time‐dependent electromagnetic field profiles, generated by one‐dimensional hybrid simulation code. Simulation results indicate that the upstream incoming electrons can be reflected nonuniformly or continuously depending on the shock parameters. Bursty energetic electron events take place when the plasma beta is low ( β ≤ 0.4) and the shock Mach number is high ( M A ≥ 6). Time‐varying loss cone, beam, and ring beam features are observed in the upstream electron distribution functions. The beam density, speed, average kinetic energy, and speed spread cyclically change with time by factor of ∼2–4. In contrast, continuously reflected electrons are observed for low beta ( β ≤ 0.4), low Mach number ( M A ≤ 4) shocks, even when the shock is reforming because the changes in shock fields are relative small. The electron burst events disappear and the observed upstream electron distribution function contours are steady state. A continuous electron beam is formed, which is qualitative the same as the beam from steady‐state shocks. Increasing the plasma beta (providing the shock is still reforming) has minor effects on the upstream electron beam features.