A review of the climatology features and mid- and high-latitude forcing of the equatorial electrojet

The equatorial electrojet (EEJ) is a narrow eastward current that flows in the daytime equatorial E region. It is an important part of low-latitude ionospheric electrodynamics. This review presents a synthesis of recent advances in EEJ research. The main topics include historical observations, observation and inversion methods, local time and longitudinal variations, mid- and high-latitude forcing, solar activity forcing, and EEJ models. Ground-based and satellite observations show clear local time and longitudinal variations of the EEJ. These variations are mainly controlled by E-region conductivity, atmospheric tides, and geomagnetic field geometry. Among these factors, nonmigrating tides play a dominant role in the longitudinal pattern. Under disturbed conditions, mid- and high-latitude processes, such as geomagnetic storms, substorms, sudden changes in solar wind dynamic pressure, sudden stratospheric warming events, and subauroral polarization streams, can substantially modify the EEJ intensity and even reverse its direction through coupling processes. In addition, solar activity, including solar flares and solar eclipses, can modulate the EEJ primarily through rapid changes in ionospheric conductivity and associated electrodynamic adjustments. Although physics-based electrodynamic models, data-driven empirical models, and global numerical simulations have provided valuable insights across different scales, their quantitative consistency and capability of reproducing EEJ disturbances remain limited. This review points out several open issues. These include the combined effects of different driving factors, the nonlinear response of the EEJ, and the lack of reliable EEJ models during disturbed conditions. Finally, future studies should focus on combined observations, improved coupling models, and prediction of EEJ variability.