Extreme Geomagnetic Storms may have a Larger Impact than we realize

The Earth’s magnetosphere is driven by its external medium. This is the shocked solar wind plasma and fields within the magnetosheath, and not the solar wind we measure ~230 RE upstream at the L1 Lagrange point. As most space physics studies use the solar wind driver at L1, the random uncertainty in this measurement relative to the true shocked solar wind driver that couples with the magnetosphere leads to a systematic statistical bias due to the regression-to-the-mean effect. This effect creates an appearance of saturation of the geomagnetic response, such as the cross-polar cap potential or the westward auroral electrojet, at extreme values of the solar wind driving. Once we account for the systematic bias due to random error, we can see that the geomagnetic response to the correct solar wind driving is linear. Hence, we are currently underestimating the geomagnetic response to extreme geomagnetic storms. The real effect of extreme geomagnetic storms might be larger than twice what was previously thought.