Long‐duration penetration of the interplanetary electric field to the low‐latitude ionosphere during the main phase of magnetic storms

It is well known that the interplanetary electric field can penetrate to the low‐latitude ionosphere. It is generally believed that the penetration of electric fields can last only for ∼30 min because of the shielding effect in the ring current. In this paper we present the observations of the dayside ionospheric electric field enhancements at middle and low latitudes in association with reorientations of the interplanetary magnetic field (IMF). In six cases, the eastward electric field in the dayside equatorial ionosphere, measured by the Jicamarca incoherent scatter radar, was enhanced for 2–3 hours after the IMF turned southward and remained continuously southward. In one case the eastward electric field in the dayside midlatitude ionosphere, measured by the Millstone Hill incoherent scatter radar, was continuously enhanced for ∼10 hours during southward IMF. Since Millstone Hill is close to the equatorward boundary of the auroral zone during magnetic storms, the penetration electric field there may be different from that at the equatorial ionosphere. The most striking feature of the measurements is that the enhancements of the ionospheric electric field can last for many hours without significant decay. The electric field enhancements in the middle‐ and low‐latitude ionosphere are closely related to magnetic activity and occur during the main phase of magnetic storms. The observations show that the interplanetary electric field can continuously penetrate to the low‐latitude ionosphere without shielding for many hours as long as the strengthening of the magnetic activity is going on under storm conditions.