Ionospheric response to minor geomagnetic storms in deep solar minimum revealed by Global Ionosphere Specification electron density based on FORMOSAT-7/COSMIC-2 observations

Global Ionosphere Specification (GIS) 3D electron density constructed by using the radio occultation slant total electron content (TEC) measurements of the FORMOSAT-7/COSMIC-2 (F7/C2) constellation and ground-based global navigation satellite system TEC, revealed extreme positive ionospheric response during a minor G1 geomagnetic storm on 5 August 2019. The results revealed over 300% enhancement of the equatorial ionization anomaly (EIA) crest electron density and localized enhancements over the European sector. Enhancements of such magnitude are usually reported during severe geomagnetic storms in solar maximum period. This study examines the ionospheric response observed in GIS electron density during 13 similar events occurred in 2019-2021. A superposed epoch analysis is carried out, with the time of maximum interplanetary electric field (IEFy) as the zero-epoch time. Local-time and latitude variations of the average GIS electron density corresponding to the zero-epoch are examined by using previous 5-day average electron density as the quiet-time reference. The results reveal similar positive storm response during other events, with an average electron density enhancement of 100-200% over low latitudes in the afternoon period. Maximum electron density response occurs within about 3.5-4 hours after the zero-epoch. In the night sector, the enhancement occurs after 5-12 hours of the zero-epoch. The latitude variation reveals classic storm-time behavior in solar maximum, with stronger response occurring earlier over mid- and low-latitudes and propagating to equatorial region. The storm effect lasts for about 3-days over the mid- and low-latitudes. The results are further compared with the corresponding variations of IMF parameters and O/N2 ratio, and the possible factors that contribute to the observed extreme positive response are examined.