Statistical behavior of large-scale ionospheric disturbances during geomagnetic storms using 20-year GNSS-TEC data

To establish the statistical behavior of global electron density variations in the ionosphere during the main and recovery phases of geomagnetic storms, we conducted a superposed epoch analysis of interplanetary magnetic field (IMF), solar wind, geomagnetic indices (AE and SYM-H), and global navigation satellite system (GNSS) total electron content (TEC) data for 20 years (20002019). In this study, we analyzed the ratio of the TEC difference (rTEC) to the quiet-day TEC for 663 geomagnetic storms with the minimum SYM-H value of less than -40 nT. During the main phase, the rTEC enhancement related to the SED base appeared at the daytime (918 h, geomagnetic local time: GMLT) mid-latitudes in both hemispheres. Around the noon or afternoon, the rTEC enhancement related to the SED plume was observed at high latitudes. In the low-latitude and equatorial regions, the clear rTEC enhancement occurred in the nighttime after -6.0 h epoch time. After the onset of the recovery phase, the magnitude of the mid-latitude rTEC enhancement decreased rapidly within 4 h, and the rTEC values at high latitudes became negative in all the GMLTs. The equatorial rTEC value increased significantly in the nighttime while that in the off equatorial region decreased slightly in a wide GMLT range (824 h, GMLT). These situations persisted at least for 16 h. The equatorial rTEC signatures suggest that an over-shielding or disturbance dynamo field suppresses the equatorial fountain effect.