Characteristics of storm-enhanced density as seen in GNSS total electron content at midlatitudes during geomagnetic storms

Abstract:

Characteristics of global electron density variations in the ionosphere during two geomagnetic storms on 7 and 8 November 2004 and 27 and 28 May 2017 were investigated using total electron content (TEC) obtained from the global navigation satellite system (GNSS). In both cases, enhanced TEC regions first appeared in the midlatitude regions in the afternoon sector approximately 2 h after southward turning of the interplanetary magnetic field (IMF). At this time, the enhanced TEC regions related to the equatorial ionization anomaly (EIA) were not observed in the low-latitude regions. The TEC enhancements in the midlatitude regions appeared with a magnetic conjugacy in the opposite hemisphere, and this feature correspond to one of the storm‐enhanced density (SED) features. The amplitude of the TEC enhancement was lager in the winter hemisphere than that in the summer hemisphere. These observational facts suggest that the origin of SED is not an extension of EIA to the midlatitude regions and the SED structures are directly formed in the midlatitude regions. An enhanced eastward electric field due to the southward turning of the IMF, which is called a prompt penetration electric field, carries ionospheric plasma in the sunlit region to a higher altitude, where recombination rate is low, and then solar EUV radiation generates new plasma at the lower altitude. Thus, the TEC values increase in the dayside ionosphere within 2 hours from the uplift of the ionosphere due to the eastward electric field. From the above consideration, we conclude that the enhanced eastward electric field due to the prompt penetration electric field mainly contributes to the SED generation at a lower latitude of the midlatitude trough.