Two-Step Electron Acceleration During a Flux Rope Crossing of Earth's Bow Shock

The generation of cosmic ray particles with ultra-relativistic energies is a crucial issue in astrophysical and space physics. Shocks have been acknowledged as efficient accelerators in the universe which can produce energetic particles through a variety of mechanisms, such as diffusive shock acceleration (DSA), shock surfing acceleration (SSA) and shock drift acceleration (SDA). Using the data set obtained by the Magnetospheric Multiscale mission, we report an unusually sharp enhancement of energetic electron flux in the upstream region of Earth's bow shock, associated with a bipolar variation of the interplanetary magnetic field. To explain these observations, we propose a two-step acceleration scenario where electrons trapped in a flux rope (FR) first undergo head-on collisions and are further accelerated by FR contraction as the FR crosses the shock. Such a scenario can well explain the spacecraft observations. This work can improve our understanding of particle acceleration processes at shocks.