Spatial structure of ionospheric convection velocities in regions of open and closed magnetic field topology

We present a spatial structure function analysis of ionospheric velocity, measured by the Halley Super Dual Auroral Radar Network (SuperDARN) radar over five years. We show evidence for scale-free velocity fluctuations for separations from 45 km to ∼1000 km. For larger separations a deviation from scale-free structure is seen that we interpret as the influence of the global convection pattern. We find evidence for scale-free structure in regions of both open and closed field line topology, though with different power-law exponents. The measured power-law exponents poleward and equatorward of the open-closed field line boundary are 0.31 and 0.39 respectively. We propose that the scale-free spatial structure of ionospheric velocity fluctuations on open magnetic field lines is a reflection of the spatial structure of the solar wind and the scale-free spatial structure of ionospheric velocity fluctuations on closed magnetic field lines is dictated by the internal dynamics of the magnetosphere-ionosphere system.