High-resolution auroral acceleration signatures within a highly dynamic onset arc

On 16 March 2006, the Reimei spacecraft observed dynamic small-scale vortex-like structures along an east-west elongated onset arc, subsequently followed by the auroral expansion. Within a few seconds when Reimei crossed over the arc, detailed comparisons of the simultaneous conjugate auroral imaging and particle observations with high resolution were made to reveal the fine-scale acceleration processes acting within/around the flux tube of the bright auroral structures. In the surrounding dark region, Alfvenic accelerated electron fluxestime-dispersive, field-aligned electron bursts spanning a broad range of energiesbecame most noticeable. The dominant contributor to the central bright region, on the other hand, was monoenergetic electron fluxes in 3-7keV at larger pitch angles (>30 degrees) rather than the Alfvenic electron fluxes. The Reimei observations suggest that the Alfvenic acceleration process operates in the vicinity of small-scale auroral structures but plays a less significant role in powering the central bright emissions.