The efficient acceleration of thermal protons by perpendicular shocks

The physics of charged-particle acceleration from near-thermal to much higher energies by collisionless shocks is investigated using large-scale self-consistent plasma simulations. The focus here is on acceleration at shocks that move normal to the average magnetic field. It is shown that a fraction of thermal protons incident on a perpendicular shock are readily accelerated to energies much higher than the ram energy of the incident plasma. This indicates that there is not an injection problem at perpendicular shocks. It is found that some (initially) thermal protons are reflected by the shock and move upstream along magnetic field lines that are multiply connected to other locations on the shock. This leads to efficient acceleration and results in a distribution function, averaged over a large spatial region downstream of the shock, having a high-energy tail that originates directly from the thermal population. It can be concluded from our results that perpendicular shocks are important sites of particle acceleration in a wide variety of astrophysical plasmas.