A Survey of 25 Years' Transpolar Voltage Data From the SuperDARN Radar Network and the Expanding-Contracting Polar Cap Model

Abstract

We use 214,410 hourly observations of transpolar voltage, ΦPC, from 25 years' observations by the Super Dual Auroral Radar Network radars to confirm the central tenet of the expanding-contracting polar cap model of ionospheric convection that ΦPC responds to both dayside and nightside reconnection voltages (ΦD and ΦN). We show that ΦPC not only increases at a fixed level of the nightside auroral electrojet AL index with increasingly southward interplanetary magnetic field (IMF) (identifying the well-known effect of ΦD on ΦPC) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of ΦN on ΦPC). We also study the variation of ΦPC with time elapsed since the IMF last pointed southward, Δt, and show that low/large values occur when (−AL) is small/large. Lower numbers of radar echoes, ne, mean that the “map-potential” reanalysis technique used to derive ΦPC is influenced by the model used: we present a sensitivity study of the effect of the threshold of ne required to avoid this. We show that for any threshold ne, ΦPC falls to about 15 kV for Δt greater than about 15 h, indicating any viscous-like voltage ΦV is considerably smaller than this. It is shown that both ΦPC and (−AL) increase with increased solar wind dynamic pressure pSW, but not as much as the midlatitude geomagnetic index am. We conclude pSW increases both ΦD and ΦN through increasing the magnetic shear across the relevant current sheet but has a larger effect on midlatitude geomagnetic indices because of the effect of additional energy stored in the tail lobes.