Physics‐based solar wind driver functions for the magnetosphere: Combining the reconnection‐coupled MHD generator with the viscous interaction

Abstract

Driver functions for the Earth's magnetosphere‐ionosphere system are derived from physical principles. Two processes act simultaneously: a reconnection‐coupled MHD generator 𝒢 and a viscous interaction. 𝒢 accounts for the dayside reconnection rate, the length of the reconnection X line, and current saturation limits for the solar wind generator. Two viscous drivers are derived: Bohm viscosity ℬ and the freestream‐turbulence effect ℱ. A problematic proxy effect is uncovered wherein the viscous driver functions also describe the strength of reconnection. Two magnetospheric‐driver functions written in terms of upstream solar wind parameters are constructed: 𝒢 + ℬ and 𝒢 + ℱ. The driver functions are tested against seven geomagnetic indices. The reaction of the geomagnetic indices to 𝒢 + ℬ and 𝒢 + ℱ is nonlinear: Nonlinear versions of the driver functions are supplied. Applying the driver functions at multiple time steps yields correlation coefficients of ~85% with the AE and Kp indices; it is argued that multiple time stepping removes high‐frequency uncorrelated signal from the drivers. Autocorrelation‐function analysis shows strong 1 day and 1 year periodicities in the AE index, which are not in the solar wind driver functions; correspondingly, high‐pass and low‐pass filtering finds uncorrelated signal at 1 day and 1 year timescales. Residuals (unpredicted variance) between the geomagnetic indices and the driver functions are analyzed: The residuals are anticorrelated with the solar wind velocity, the solar F10.7 radio flux, and the solar wind current saturation parameter. Removing diurnal, semiannual, and annual trends from the indices improves their correlation with the solar wind driver functions. Simplified versions of the driver functions are constructed: The simplified drivers perform approximately as well as the full drivers.