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

Abstract

Abstract We use 214,410 hourly observations of transpolar voltage, Φ PC , 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 Φ PC responds to both dayside and nightside reconnection voltages (Φ D and Φ N ). We show that Φ PC 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 Φ D on Φ PC ) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of Φ N on Φ PC ). We also study the variation of Φ PC 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, n e , mean that the “map‐potential” reanalysis technique used to derive Φ 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 , Φ PC falls to about 15 kV for Δ t greater than about 15 h, indicating any viscous‐like voltage Φ V is considerably smaller than this. It is shown that both Φ PC and (− AL ) increase with increased solar wind dynamic pressure p SW , but not as much as the midlatitude geomagnetic index am . We conclude p SW increases both Φ D and Φ N 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.

Topics & Concepts