Wave Acceleration of Energetic Ions in the Earth's Magnetosheath

Using field and particle observations from the THEMIS spacecraft we suggest a wave model for the local acceleration of energetic ions in the magnetosheath. An analysis of wave characteristics using interferometric techniques during intervals where energetic ions are observed reveals that on scales larger than a thermal ion gyro-radius the field fluctuations cannot clearly be categorized as a single MHD mode but have a mixed nature with fast mode fluctuations becoming increasingly dominant with increasing scale. From a simple consideration of ion acceleration through stochastic resonant interactions in the compressive fields in these waves (transit time acceleration) we derive a velocity space diffusion coefficient and show using Monte Carlo simulations that this mechanism may account for the observed energy scaling of the energetic magnetosheath ion distributions.