Using vorticity to characterise meso-scale ionospheric flow variations

Using line-of-sight velocity measurements made by SuperDARN (The Super Dual Auroral Radar Network) radars with overlapping fields of view, it is possible to estimate the vorticity of the ionospheric convection flow over a wide range of scales. Here we exploit previous statistical analyses of 6 years of SuperDARN vorticity measurements to study the spatial variation of meso-scale flows in ionospheric convection. By making certain assumptions, we can statistically separate probability density functions (PDFs) of vorticity made at different locations in the ionosphere into two populations: (i) That due to the large-scale two-cell convection flow driven primarily by magnetic reconnection, and (ii) that due to meso-scale flow structures driven by processes such as turbulence. The resulting PDFs are fit by model functions using maximum likelihood estimation, and the spatial variation of the estimators is determined. The spatial variations of the large-scale vorticity estimators are ordered by the average convection flow, which is highly dependent on the IMF direction. The spatial variations of the meso-scale vorticity estimators appear independent of the senses of vorticity and IMF direction, but have a different character in the polar cap, the cusp, the auroral region, and the sub-auroral region.