Electron acceleration mechanisms at Earth’s quasi-perpendicular bow shock

We use the Magnetospheric Multiscale mission to observe electron acceleration events at Earth’s quasi-perpendicular bow shock. Acceleration mechanisms up to mildly relativistic electron energies are investigated in order to provide more insight into the long-standing injection problem. The events are chosen for their diversity in observed high energy electron flux and shock angle, θ_Bn, enabling the Stochastic Shock Drift Acceleration theory to be further tested for different shock parameters. An alternative acceleration mechanism is also presented. The electron acceleration region of this unusual event is associated with a decrease in wave activity, inconsistent with common electron acceleration mechanisms such as Diffusive Shock Acceleration and Stochastic Shock Drift Acceleration. The energetic electron population is shown to have a bi-directional pitch-angle distribution, indicating parallel heating along the magnetic field lines.
We propose a two-step acceleration process where energetic field-aligned electrons originating from the electron foreshock act as a seed population further accelerated by a shrinking magnetic bottle process. Furthermore, we present evidence for electron pitch-angle scattering at the shocks and discuss its importance and different roles for the different electron acceleration mechanisms mentioned above.