Ion acceleration at the equatorward edge of the cusp: Low altitude observations of patchy merging

Controversy continues unabated concerning the detection of signatures of merging at low altitudes. One phenomenon required by nearly all models of merging is the acceleration of ions in the merging region by about the Alfvén velocity. We report the observation of spatially discrete patches of precipitating accelerated ions at the equatorward edge of the cusp. These regions of accelerated bulk flow velocity, which occur at or near the transition between boundary layer and cusp proper, are spatially distinct from the overall energy‐latitude dispersion in the main cusp and are best fitted by Maxwellian distributions with convection velocities of 450‐640 km/s; such velocities are 200–300 km/s faster than in the main cusp ion distribution. The acceleration cannot be attributed to electrostatic potentials, for the electron distributions in the patchy ion regions are also accelerated. Based on an initial survey of 13 such events, occurrence of distinctly separate accelerated ion patches appears to be primarily or exclusively a southward B z phenomenon.