Quantifying Spectral Breaks in the Optical Flow Vorticity Spectrum of a Discrete Auroral Arc

Using ASI observations during the overnight flight of the NASA GREECE mission over Venetie, AK, we investigate multi-scale plasma dynamics in the auroral acceleration region. 2-dimensional wavelet cross correlation is applied to successive image frames to obtain optical flow velocity fields at varying spatial scales. Vorticity fields are derived from finite differencing, and vorticity spectra are computed and corrected optical smearing from the auroral green line. Using predicted vorticity scaling relations for quasi-static and Alfvenic arcs, we estimate the electron inertial length at the break between quasi-static and Alfvenic regimes, occurring when the perpendicular length scale is comparable to the electron inertial length. This calculation yields electron densities consistent with typical values in the AAR. Additionally, we observe a left-handed bias in vorticity near the transition from quasi-static to Alfvenic behavior for 48% of events surveyed, too low to draw statistically significant conclusions regarding its physical origin. Some events display a broadened transition between the two regimes, suggesting the transition isn’t tied to a single length scale. Most importantly, this study demonstrates the feasibility to perform multi-scale optical flow vorticity analysis from ground-based imagery.