Continuous penetration of the interplanetary electric field to the equatorial ionosphere over eight hours during intense geomagnetic storms

The interplanetary electric field penetrates to the low‐latitude ionosphere and has significant influence on the ionospheric electrodynamics during intense geomagnetic storms. It is critical to understand how much of the interplanetary electric field can penetrate to the ionosphere and how long penetration electric fields can exist. In this study, we present the Defense Meteorological Satellite Program (DMSP) satellite measurements of the equatorial ionospheric ion vertical velocity in the dusk sector during two intense geomagnetic storms. We find that the equatorial ionospheric electric field at dusk is approximately proportional to the interplanetary electric field for 8–10 hours during the storm main phase. The penetration efficiency is relatively constant over the entire interval and is 5% at dusk in one storm case. The long‐lasting storm‐time penetration electric field causes significant redistribution of the ionospheric plasma. This study reveals that penetration electric fields can last for 8–10 hours without effective shielding when the magnetic storm activity is strengthening during continuous southward interplanetary magnetic field. The result implies that the interplanetary electric field is linearly coupled with the dusk equatorial ionosphere during the entire main phase of intense magnetic storms, which is very important for understanding the storm‐time ionospheric electric field and electrodynamics.