The response of the dayside equatorial electrojet to step-like changes of IMF B_Z

The equatorial electrojet (EEJ) is driven by zonal electric fields, which are known to be well correlated with the interplanetary electric field and therefore with the interplanetary magnetic field (IMF) B_Z component. In the present study, we investigate how the equatorial H magnetic component, and therefore the EEJ, responds to step-like changes of IMF B_Z. The reduction of southward IMF B_Z (northward turning) and that of northward IMF B_Z (southward turning) are examined separately. The result shows that for the northward turnings, the EEJ immediately starts to weaken with the accuracy of the estimates of the travel times of the IMF changes. The time constant of the response is much longer, and the equatorial H component decreases continuously by 40 nT for 30 min after the northward turnings. In contrast, the response of the EEJ to the southward turnings is far less clear in both magnitude and timing, and it does not depend on whether or not IMF B_Z actually becomes southward. The difference in the EEJ response to the northward and southward turnings reflects at least partially the fact that the magnetosphere-ionosphere system is more sensitive to IMF B _Z when IMF is southward than northward. It is suggested that the electric field penetrates from the polar region to the dip equator through a global current system that connects the auroral electrojets and the EEJ, and the ionospheric conductance in the polar region may play an important role in the formation of such a current system. Key Points EEJ starts to weaken within 1 min from the arrival of northward IMF turnings. The time constant of EEJ response to northward turnings is ~30 minutes. The response of EEJ to southward turnings is not clear in magnitude or timing.