Multi-instrumental observations of nighttime X-pattern of equatorial ionization anomaly during magnetically quiet time: case study

In this study, we provide a detailed case study of the X-pattern of equatorial ionization anomaly (EIA) observed on the night of September 12, 2021 by the Global-scale Observations of the Limb and Disk (GOLD) mission. Unlike most previous studies about the X-pattern observed under the severely disturbed background ionosphere, this event is observed under geomagnetically quiet and low solar activity conditions. GOLD’s continuous observations reveal that the X-pattern intensity evolves with local time, while its center’s longitude remains constant. The total electron content (TEC) data derived from the ground-based Global Navigation Satellite System (GNSS) network aligns well with GOLD observations in capturing the formation of the X-pattern, extending coverage to areas beyond GOLD’s observational reach. Additionally, the ESA’s Swarm mission show that both sides of the X-pattern can coincide with the occurrence of small-scale equatorial plasma bubbles (EPBs). To further analyze the possible drivers of the X-pattern, observations from the Ionospheric Connection Explorer (ICON) satellite were used. It shows that the latitudinal expansion (or width) between the EIA crests in two hemispheres is proportional (or inversely proportional) to the upward (or downward) plasma drift velocity, which suggests that the zonal electric field should have a notable influence on the formation of EIA X-pattern. Further simulations using the SAMI2 model support this mechanism, as the X-pattern of EIA is successfully reproduced by setting the vertical plasma drift to different values at different longitudes.