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

Abstract

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, 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.

A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

“…Note that we have no low-altitude evidence of the type of electron acceleration mechanism that causes this type of aurora. Motoba and Hirahara 48 showed evidence from low-altitude satellite measurements where electrons at a range of pitch angles are observed at the same time as the onset arc. Previous evidence from FAST spacecraft measurements and kinetic simulations of Alfvenic acceleration 49 indicates that acceleration due to large Alfven waves can result in mono-energetic populations of electrons with a wide range of pitch angles (Fig.…”

The tails of the satellite auroral footprints at Jupiter

“…If the footprint tails are generated by such a quasi‐static process while the spots are related to dispersive Alfvén waves, then the energy distribution of the precipitating electron population would be expected to be quasi‐monoenergetic in the tails and broader in the spots. Such a discrepancy between narrow distributions associated to inverted‐Vs and broader distributions related to Alfvén waves are frequently observed at Earth [e.g., Paschmann et al , ; Motoba and Hirahara , ]. However, in the IFP case, observations of the vertical brightness profile show a similar and broad distribution for both the main spot and the tail [ Bonfond et al , ; Bonfond , ].…”

Physical Processes of Meso-Scale, Dynamic Auroral Forms

“…(g) Zoomed-in MAC 427.8 nm images. Reproduced from Motoba and Hirahara (2016) rates and propagation speeds for reasonable plasma conditions in the source region, it is unlikely that ground-based observations will be able to unequivocally identify the source mechanism. However, coordinated auroral and in-situ observations of the instability region will be challenging, given its predicted small size and the necessity to measure, for example, plasma gradients in three dimensions on short time-scales.…”