Efficient acceleration of energetic electrons upstream of Earth’s bow shock

Astrophysical collisionless shocks exist widely in the universe on almost all scales, from planet-sized shocks in the heliosphere to extra-galactic radio jets thousands of light-years across ^1,2 . It is widely believed that strong shocks can accelerate particles to ultra-relativistic energy ^3-5 , via the well-established diffusive shock acceleration mechanism where particles gain kinetic energy by crossing a shock repeatedly ^6-8 . However, this mechanism requires a seed particle population with kinetic energy sufficiently high to accomplish the multiple shock crossings ^9-11 , whose origin is still an enigma. This is known as the injection problem—one of the most pivotal puzzles in the high-energy astrophysics which has hitherto not been well resolved ^10,11 . Here we report a novel and efficient electron acceleration mechanism observed upstream of the Earth’s bow shock. This mechanism relies on a special V-shaped magnetic field configuration in the upstream solar wind, which channels the shock-reflected electrons back and thus enables them to be reflected by the shock many times. This special field configuration arises when a solar-wind discontinuity—an ubiquitous and inherent structure in space plasmas—approaches and intersects the bow shock. We propose a numerical model that can reproduce the acceleration and explain the spacecraft observations well. The observations and numerical model together provide compelling evidence for the ability of this mechanism to accelerate low-energy (~17 eV) solar-wind electrons to >200 k _B T _e . This study therefore provides new and important insights into the injection problem of shock acceleration theories and can help to understand the generation of energetic particles in the cosmos.