Statistical Survey of Fine-Scale Auroral Structure at High Latitude: Evidence Consistent With Acceleration by Dissipative Non-Linear Inertial Alfvén Waves

Abstract

Relatively little is known about the causes of fine-scale aurora with scale sizes of $\sim 1$ km and less. We analyze images from the Auroral Structure and Kinetics (ASK) instrument, a multi-monochromatic spectral imager located in the Norwegian High Arctic. We analyze five categories of fine-scale aurora. The categories described in this paper are: “drift arcs”, “drift arcs (dynamic)”, “chocolate sauce”, “chocolate sauce (turbulent)”, and “psychedelic”; these are categories based on those in the citizen science project, Aurora Zoo. The analysis comprises: magnetic local time (MLT) dependence, characteristic precipitation energies, occurrence relative to the auroral oval, and adapted local intermittency measure (LIM) analysis of radially averaged image 2D power spectra. LIM analysis allows us to estimate the widths of the smallest structures typically associated with each aurora type. We find that chocolate sauce (turbulent) and psychedelic aurora are associated with nightside processes. This is based on their tendency to be higher energy, to occur at late dusk/midnight MLTs, and to occur on the poleward boundary of the auroral oval. For non-psychedelic aurora types, we find the smallest characteristic widths ($\sim 330-500$ m) and distribution of energies (typically less than 10 keV) to be consistent with the dissipative non-linear inertial Alfvén wave acceleration mechanism described by Wu and Chao (2004a), https://doi.org/10.1029/2003ja010126. For psychedelic aurora, the smallest characteristic widths ($\sim 200$ m) and high energies (median 10.9 keV) make it unlikely that it is produced by the aforementioned mechanism, which predicts arc widths of $\sim 0.3-4.6$ km and energies less than 10 keV.