A statistical model of vorticity in the polar ionosphere and implications for extreme values

Measurements of vorticity in the Earth’s ionosphere enable the characterisation of turbulent structure in the ionospheric plasma flow and how it varies spatially in relation to large-scale magnetic field-aligned current (FAC) systems. We have determined the spatial variation of the probability density function (PDF) of ionospheric vorticity measurements made by the Super Dual Auroral Radar Network (SuperDARN) across the northern polar ionosphere for the 6-year interval 2000-2005 inclusive. These PDFs are highly leptokurtic, with heavy tails, and are well-modelled by Tsallis q-exponential probability distributions. The parameters of the model q-exponential distributions have been determined using maximum likelihood estimation (MLE), resulting in a statistical model of ionospheric vorticity that covers the polar ionosphere. The spatial variation of the model parameters is highly variable, with the shape and scale of the model distributions varying systematically in relation to the well-known FAC regions, showing that FACs have a major influence on the character of ionospheric plasma vorticity. From the model distributions we estimate the probability of observing extreme vorticity values with the SuperDARN radars (beyond thresholds of 5, 10, 20, and 40 mHz) across the northern polar ionosphere.