Persistent Eastward EEJ Enhancement During the Geomagnetic Storm Recovery Phases

Recent case studies disclosed that sometimes the dayside ionospheric equatorial electric field underwent persistent long-duration (>4 hr) eastward enhancements during geomagnetic storm recovery phases, rather than anticipated decrease or reversal. However, it is still unknown the occurrence and morphology of the persistent enhancement as well as its impact on the low-latitude ionospheric plasma distribution. For the first time, we examined the issue with superposed epoch analyses of equatorial electrojet (EEJ) and GNSS total electron content (TEC) responses in the American (49 degrees W) and Indian (78 degrees E) sectors. The occurrence rates of the persistent enhancement during storm recovery phases are more than 30%, with higher occurrence rates in second to fourth days of the recovery phase, equinoxes, and American sector. The corresponding averaged EEJ and TEC responses were disclosed and compared with those storm samples without the persistent enhancement. On the first day of recovery phase, the dayside equatorial ionosphere was dominated by the prevalent westward electric field and causative inhibited equatorial fountain. In contrast, after the first day, the dayside morphologies showed different patterns: (a) for the persistent cases, the large eastward EEJ and TEC enhancement indicated the enhanced eastward electric field at forenoon and causative intensified equatorial fountain. (b) For the storms without the persistent cases, the general westward EEJ and equatorial TEC enhancements were dominated. Our study indicates that dayside eastward enhancement is an important disturbed pattern of equatorial ionospheric electric field during the late recovery phases, and the enhancement generally causes the intensification of equatorial ionization anomaly.