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 12 September 2021 by the Global-scale Observations of the Limb and Disk (GOLD). Unlike most previous studies about X-pattern observed under the severely disturbed background ionosphere, this event is under geomagnetically quiet and low solar activity conditions. The GOLD’s continuous observations reveal that the X-pattern intensity evolves with local time, while the center 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 satellites show that the both sides of the X-pattern can be accompanied by the occurrence of small-scale EPBs. To further analyze the possible driver for 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 (downward) plasma drift velocity, which suggests that the zonal electric field should have a notable influence on the formation of X-pattern of EIA. 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.