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. Plain Language Summary The cosmic ray comprises ions and electrons with energies up to 1,020 eV and thus its generation mechanism is an important issue. A conventional theory known as diffusive shock acceleration is a promising candidate but it needs seed electrons with sufficient initial energies. How these electrons are generated is still unclear. In this study, we report an unusually sharp enhancement of energetic electron flux with a large variation of magnetic field direction in the upstream region of Earth's bow shock, and propose a two‐step acceleration scenario to explain the spacecraft measurements. Electrons first gain energies from the head‐on collisions with the bow shock, and then bounce between the two ends of a circular magnetic field of shrinkage, analogous to a ball reflecting between two converging walls. This scenario has been mathematically confirmed and shows good agreement with the spacecraft measurements. Key Points Efficient electron acceleration is observed when a flux rope collides with the Earth's bow shock Electrons trapped in the flux rope are successively accelerated by the head‐on reflection at shock front and the flux rope contraction This two‐step acceleration scenario is mathematically reproduced and fit the spacecraft observations well