Long-lasting goodshielding at the equatorial ionosphere

During magnetic storm times, a dawnward electric field may build up at the Alfven layer in the inner magnetosphere to cancel an enhanced duskward magnetospheric convection electric field. This effect is known as “shielding” because the dawnward electric field prevents the duskward electric field from penetrating into the inner magnetosphere. The term “goodshielding” refers to the perfect shielding status, in which the penetration electric field is almost completely canceled by shielding electric field. Observational evidence of goodshielding has rarely been reported before. Here we present two long‐lasting (16 and 23 min) goodshielding cases as unambiguous evidence. Both cases show similar historical patterns of shielding status: long‐duration electric field penetration (undershielding), goodshielding, and then overshielding. The goodshielding time periods, featuring quiet‐level ionospheric electric field at the dayside equator, are not directly related to the changes of interplanetary magnetic field (IMF) BZ, but the transitions from goodshielding to overshielding coincide with reduction of convection electric field. Both cases show that the equatorial electric field does not significantly respond to convection electric field enhancements during goodshielding, indicating that the shielding electric field may quickly adjust to shield newly enhanced convection electric fields. The quick shielding is possibly attributed to the fact that the ring current has become stronger and has been brought closer to the Earth. These new features may provide some clues to understand the magnetosphere‐ionosphere coupling processes during storm times.