On anomalous departures from a linear relation between the polar cap index and its controlling factors in solar wind and magnetotail

Abstract

The polar cap (PC) index has been found to respond linearly to both the solar wind direct driving and the magnetotail energy unloading. The influence of the solar wind direct driving is quantified by the electric field (EK‐R) proposed by Kivelson and Ridley (2008). Here, EK‐R is representative of models in which the electric field imposed on the ionosphere by magnetopause reconnection saturates for extreme solar wind driving. The influence of the magnetotail energy unloading is described by the modified AL index (ALU). The model generally describes the variation of the PC index quite well at times of strong geomagnetic activity, but, occasionally, there are large discrepancies. In this study, I investigate 18 events in which the model predictions deviate substantially from the measured PC indices and, in the process, identify limitations on the use of the PC index. For southward oriented interplanetary magnetic field (IMF), sources other than EK‐R and ALU may contribute to the PC index significantly. The effect is equivalent to altering the baseline values of magnetic field H and D components when deriving the PC index. For northward oriented IMF, the high‐latitude reconnection can drive antisunward convection and excursions of the auroral oval that can dramatically influence the PC index but are not captured by the linear model.