| Abstract 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 D=max{−Dst} $D=max left{-Dstright}$ to the storm main‐phase maximum duskward interplanetary electric field E $E$. The conditional relationship D | E $Dvert E$ 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 (E=69.6 $E=69.6$ mV/m), if Earth‐directed, would be associated with a median storm intensity of D=495378648 $D=49{5}{378}^{648}$ nT. Storms comparable to March 1989 (D=594 $D=594$ nT) correspond to electric fields of E=543976 $E=5{4}{39}^{76}$ mV/m, while Carrington‐class storms (D=964 $D=964$ nT) correspond to E=9568133 $E=9{5}_{68}^{133}$ 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. |