Abstract/Summary

We use 214410 hourly observations of transpolar voltage, {\Phi}_{PC}, from 25 years’ observations by the SuperDARN radars to confirm the central tenet of the Expanding-Contracting Polar Cap model of ionospheric convection that {\Phi}_{PC} responds to both dayside and nightside reconnection voltages ({\Phi}_{D} and {\Phi}_{N}). We show {\Phi}_{PC} increases at a fixed level of the nightside auroral electrojet AL index with increasingly southward IMF (identifying the well-known effect of {\Phi}_{D} on {\Phi}_{PC}) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of {\Phi}_{N} on {\Phi}_{PC}). We also study the variation of {\Phi}_{PC} with time elapsed {\Delta}t since the IMF last pointed southward and show low/large values occur when (-AL) is small/large. Lower numbers of radar echoes, n_{e}, mean that the “map-potential” re-analysis technique used to derive {\Phi}_{PC} is influenced by the model used: we present a sensitivity study of the effect of the threshold of n_{e} required to avoid this. We show that for any threshold n_{e}, {\Phi}_{PC} falls to about 15 kV for {\Delta}t greater than about 15 hours, indicating any viscous-like voltage {\Phi}_{V} is considerably smaller than this. It is shown that both {\Phi}_{PC} and (-AL) increase with increased solar wind dynamic pressure p_{SW}, but not as much as the mid-latitude geomagnetic index am. We conclude p_{SW} increases both {\Phi}_{D} and {\Phi}_{N} through increasing the magnetic shear across the relevant current sheet but has a larger effect on mid-latitude geomagnetic indices because of the effect of additional energy stored in the tail lobes.