A quantitative deconstruction of the morphology of high-latitude ionospheric convection

We present an analysis of ionospheric convection data derived from velocity measurements made by the Super Dual Auroral Radar Network (SuperDARN). Our analysis uses an established technique for combining the network data to produce maps of large-scale convection by fitting a spherical harmonic expansion of the ionospheric electric potential to the radar measurements. We discuss how the basis functions of the spherical harmonic expansion describe different characteristic elements of the ionospheric convection pattern and show how their associated coefficients can be used to quantify the morphology of the convection, much like the total transpolar voltage is used to quantify its strength, in relation to upstream interplanetary magnetic field conditions and associated magnetospheric activity. We find that similar to 2/3 of the voltage associated with the typical convection pattern is described by a simple twin vortex basis function. The magnitude of the twin vortex is strongly dependent on IMF B-Z and the degree of its (typically westward) rotation is weakly dependent on IMF B-Y. The remaining similar to 1/3 of the total voltage is associated with deviations from the basic twin vortex pattern, introduced by the addition of other basis functions, such as IMF B-Y associated dusk-dawn asymmetries, nightside convection features associated with tail activity, and "reverse" high-latitude convection cells associated with intervals of IMF B-Z > 0.