Spatial Scale of Ionospheric Equatorial Electrojet Using Longitude Gradient of Magnetic Data from Swarm Satellites

The equatorial electrojet (EEJ) is an eastward electric current system in the ionosphere located along the dip equator. Investigating the small-scale spatial gradients of the EEJ is essential to elucidate its fine structure and associated controlling sources. In this study, magnetic gradients were estimated to investigate the small-scale spatial structures of EEJ. These data were simultaneously measured by the side-by-side, parallel-flying Swarm-A and Swarm-C satellites, which maintain a fixed separation of approximately 1.4° in longitude. Several features emerge from our statistical results: (1) Approximately 44% of cases exhibit absolute longitudinal gradients less than 0.2 nT/100 km, which corresponds to a difference of about 10 nT over a typical distance of 50° longitude. This is not a necessary relationship; it merely illustrates that 0.2 nT/100 km is a relatively larger value. From this rough estimation, it is considered that the EEJ’s small-scale variations could be more pronounced than we expected as shown by its large-scale variations. Over 90% of the cases have gradient values below 0.6 nT/100 km. (2) Cases with a gradient exceeding 0.9 nT/100 km are primarily located over the South Atlantic Anomaly, northern Africa, and Pacific regions. (3) Since positive gradient refers to eastward increasing of EEJ’s magnitude while negative value corresponds to eastward decrease, special distributions of both positive and negative gradients with higher values suggest longitudinal variations in EEJ have a typical and consistent pattern. This pattern is largely consistent but not the same as those found in previous studies. The results in our study can be used to understand the local structure of the EEJ.