Interplanetary Electric Fields for Extreme Magnetic Storms

Using a list of sudden‐commencement storms, the ring‐current index, and 1‐h near‐Earth solar‐wind measurements from solar cycles 20–25, we develop extreme‐value statistical models relating storm intensity to the storm main‐phase maximum duskward interplanetary electric field . The conditional relationship is demonstrably sublinear—linear models are confidently rejected—indicating saturation of magnetospheric response under extreme solar‐wind forcing. An event like that of July 2012 ( mV/m), if Earth‐directed, would be associated with a median storm intensity of nT. Storms comparable to March 1989 ( nT) correspond to electric fields of mV/m, while Carrington‐class storms ( nT) correspond to mV/m—substantially lower than several previous estimates. These results indicate that solar‐wind conditions capable of driving extremely intense magnetic storms are less exceptional, and potentially more frequent, than previously thought.