Magnetosheath control of the cross polar cap potential: Correcting for measurement uncertainty in space weather

This study quantifies the variation and apparent saturation of the cross polar cap potential (phiPC) with respect to the motional electric field (EM) in the solar wind and magnetosheath. The electric potential across the polar cap (phiPC) is often observed to respond linearly to solar wind driving during weak driving and nonlinearly during strong driving, with phiPC eventually responding less and less to increased driving. This effect is called “polar cap potential saturation” and has been observed in many studies that correlate some measure of solar wind driving (typically the motional electric field EM) with phiPC. In this study, it is shown that measurement error in the solar wind driver creates a nonlinear bias in its variation relative to phiPC. This is accomplished by associating a decade of radar measurements of phiPC with simultaneous probabilistic predictions of EM in the solar wind just upstream of the bow shock and in the magnetosheath. After correcting for the bias in the solar wind and magnetosheath EM caused by measurement uncertainty, the extent of saturation between phiPC and the corrected solar wind EM is reduced. More importantly, phiPC saturates with respect to the solar wind EM but not the magnetosheath EM. This effect is caused by the magnetosheath EM being reduced relative to the solar wind EM when the solar wind Alfven Mach number is low. These findings both support the theory that the phiPC saturation is due to the magnetosheath flow being magnetically dominated for large solar wind magnetic field magnitudes.