Nighttime Westward Equatorial Electrojet Observed During the January 2026 Geomagnetic Storm

Abstract

The equatorial electrojet (EEJ) is a narrow band of zonal electric current flowing in the E region ionosphere within approximately ±2° latitude of the magnetic equator. At night, the EEJ is generally considered very weak because of low ionospheric conductivity. However, evidence in the literature suggests that strong EEJ currents might occur on the night side during geomagnetic storms. This paper presents observations of the nighttime EEJ during the January 2026 geomagnetic storm. During the storm main phase, the EEJ strengths estimated using the conventional two‐station method indicate the presence of intense westward EEJ currents (150–200 mA/m) in the Thailand and Philippine sectors during pre‐sunrise hours. Observations from a chain of low‐latitude magnetometers distributed across the Thailand sector further reveal the latitudinal structure of the ground magnetic field signatures associated with the nighttime westward EEJ, providing additional evidence for the presence of nighttime westward EEJ currents. The nighttime westward EEJ in the Thailand and Philippine sectors coincides with strong eastward EEJ (∼250 mA/m) in the Peruvian sector on the dayside, consistent with the influence of prompt penetration electric fields from high latitudes. Intensifications of the nighttime westward and daytime eastward EEJ are accompanied by enhanced asymmetric geomagnetic field disturbances at mid latitudes and equatorward expansion of auroral electrojet signatures.