Saturation of the auroral electrojet current and the polar cap potential

It is shown that the auroral electrojet index AE tends to saturate, or approach a limiting value, for large southward interplanetary magnetic field, namely, BZ < −15 nT. The saturation also occurs during individual major geomagnetic storms. The saturation of AE is the result of a saturation in the polar cap potential drop. The polar cap potential has a limiting value due to the nonlinear nature of the magnetosphere‐ionosphere (M‐I) coupling. Results of a M‐I coupling model show that the saturation level of the polar cap potential is lowered as the diffuse auroral conductance increases. The data presented also show an increase in AE for large BZ values, indicating increased auroral activity for BZ > 20 nT.