Long-Lasting Penetration Electric Fields during Geomagnetic Storms: Observations and Mechanisms

Abstract

Penetration electric fields originating from the solar wind/magnetosphere, shielding electric fields produced by the Region 2 field-aligned currents, and disturbance dynamo electric fields produced by disturbance neutral winds are the major disturbance electric fields in the middle- and low-latitude ionosphere during geomagnetic storms. Understanding the interplay of electric fields from different sources is critical for understanding storm time ionospheric electrodynamics. In this study, we analyze ionospheric data measured by five Defense Meteorological Satellite Program satellites in the dusk-evening sector during a magnetic storm. A peculiar feature in this case is that the vertical ion drift was enhanced in the upward direction by up to 180 m/s, corresponding to the occurrence of an eastward penetration electric field in the equatorial ionosphere, for 14 hr, including 3 hr of the storm main phase and the first 11 hr of the recovery phase. The variations of other ionospheric parameters (the ion density, ion composition, electron temperature, and zonal drift) were well correlated with the variation of the vertical ion drifts, providing further evidence of prolonged penetration electric fields. Observations of short-lived penetration electric fields are also presented, and the causes for short-lived penetration electric fields are discussed. Important issues on the shielding effects are addressed. Scenarios for occurrence of long-lasting penetration electric fields are proposed. Some outstanding problems associated with penetration electric fields are listed for future studies.