IMF Control of Substorm Intensity

The auroral electrojet index, AL, is commonly considered as a proxy of substorm intensity. The development of the auroral electrojets is known to be mainly controlled by changes in the north-south component (Bz) of the interplanetary magnetic field (IMF). Using events in which only the IMF Bz varied significantly, it is found that the absolute value of AL is reduced by a southward turning of IMF for some events. The reduction of the westward auroral electrojet by southward IMF is classified as a new type of auroral electrojet response to the IMF Bz in addition to previously documented responses of the auroral electrojets to the directly driven process, the internally triggered process, and a northward turning of the IMF.