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

Abstract

AbstractOn 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, Alfvénic accelerated electron fluxes—time‐dispersive, field‐aligned electron bursts spanning a broad range of energies—became most noticeable. The dominant contributor to the central bright region, on the other hand, was monoenergetic electron fluxes in 3–7 keV at larger pitch angles (>30°) rather than the Alfvénic electron fluxes. The Reimei observations suggest that the Alfvénic acceleration process operates in the vicinity of small‐scale auroral structures but plays a less significant role in powering the central bright emissions.