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

It is widely believed that astrophysical shocks can accelerate particles to ultra-relativistic energy, via the well-established diffusive shock acceleration mechanism. However, this mechanism requires seed particles with kinetic energy sufficiently high, whose origin is still an enigma. Here we show observational confirmation of an efficient electron pre-acceleration mechanism at 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 shock. The acceleration scenario is further confirmed by test-particle and numerical methods. The results demonstrate its ability to accelerate low-energy (approximately 17 eV) solar-wind electrons to >200k_BT_e. This study therefore provides important insights into the injection problem and generation of energetic particles in the universe.