Accelerated ions ahead of earthward propagating dipolarization fronts

We report on the evolving ion distributions associated with the arrival of an earthward propagating dipolarization front in the near-Earth magnetotail using Time History of Events and Macroscale Interactions during Substorms (THEMIS). Ion distributions exhibit steady duskward anisotropy well before the front arrival, suggesting thin current sheet formation at similar to 11 R-E, during the growth phase of a moderate geomagnetic substorm. As the dipolarization front moves closer, an additional, earthward streaming ion population appears, resulting in an earthward velocity moment. This population eventually overwhelms the preexisting duskward anisotropy and merges with the earthward convecting bulk flow once the dipolarization front arrives. Test-particle simulations show that the observed ion evolution is consistent with a picture of ions reflected and accelerated by the approaching front and moving ahead of it.