Ionospheric positive storm phases at the magnetic equator close to sunset

Abstract

During geomagnetic activities, perturbed electric fields at middle and low latitudes of the ionosphere may result from the effect of prompt penetration from high latitudes and the disturbance dynamo mechanism, respectively. The polarity of penetration electric fields depends on the orientation of the interplanetary magnetic field (IMF) Bz, and that of electric fields associated with the disturbance dynamo is almost opposite to that in the quiet time condition. Although a few hours are required to build up the perturbed electric fields through the disturbance dynamo mechanism, the dynamo electric fields can persist for several hours after geomagnetic activities cease. It turns out that the low‐latitude electric field disturbances associated with the disturbance dynamo mechanism should be more persistent than that associated with the direct penetration from the polar cap, especially for the period in the recovery phase. Model results show a significant, westward disturbance dynamo electric field at period close to sunset, but it seems not important in the empirical model. This perturbed electric field at sunset will weaken the intensity of the prereversal enhancement and of the following fountain effect in the storm time. Relative to the quiet time condition, it produces an enhancement in the total electron content. This phenomenon is seasonal. It only occurs at period close to equinox, when the hemispheric wind (summer to winter) is minimal. These theoretical results have been substantiated by model results of the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model and by the global integration and modeling in this study. Both results show that the positive phase of the ionospheric storm at the magnetic equator close to sunset is produced by the westward disturbance dynamo electric field.