Westward ionospheric electric field perturbations on the dayside associated with substorm processes

A controversy has risen in the direction of ionospheric electric field perturbation on the dayside caused by substorm expansion phase in recent years, i.e., eastward or westward. To exclude the effect of interplanetary magnetic field northward turning, the substorms without interplanetary field (IMF) trigger are required to investigate this issue. Previous works, such as that by Huang et al. (2004), showed that the eastward electric field perturbations caused by substorms can be observed. However, our case suggests that some substorms can produce strong westward electric field perturbations and drive westward equatorial electrojets on the dayside ionosphere. This westward electric field is created by an overshielding-like imbalance state of field-aligned currents (FACs), Region 2 (R2) FAC greater than Region 1 (R1) FAC, which is built up through R2 FAC enhancement rather than R1 FAC reduction due to IMF northward turning. The substorm processes should be responsible for the westward electric field especially through polar cap shrinkage and magnetic field dipolarization.