Responses of the Equatorial Ionization Anomaly to the January 2026 Geomagnetic Storm

Abstract

Using ground‐based GNSS Vertical Total Electron Content (VTEC) and Swarm in situ electron density observations, this study investigates changes in the crest structure and intensity of the Equatorial Ionization Anomaly (EIA) over the American sector during the geomagnetic storm of 19–21 January 2026. The Kp index reached a maximum of 9− during this storm. On 19–20 January, the equatorial trough disappeared and the two VTEC crests evolved into a continuous equatorial enhancement that persisted for about two hours. Its subsequent rapid decay coincided with the Jicamarca downward drift reaching about −60 m/s, consistent with enhanced chemical loss at lower altitudes. On 20–21 January, a confined northern enhancement developed beside a broad southern depleted region, with a VTEC contrast exceeding 60 TECU. Swarm profiles distinguished localized depletions from the broad background trough and traced their extensions beyond ±30° magnetic latitude in both hemispheres. The magnitude of the preceding dayside eastward disturbance field was consistent with interplanetary electric field penetration, while the upward drift could have promoted initial plasma redistribution. Substorm‐related disturbances may have modulated shorter electric‐field variations. The later hemispheric contrast developed under asymmetric O/N ₂ changes and local meridional‐wind disturbances, which could have reinforced differences in plasma transport and loss. These observations connect the evolution of two different EIA morphologies with the corresponding electrodynamic and thermospheric disturbances. They provide an observational basis for understanding how storm‐time electric fields, thermospheric composition, and neutral winds reorganize the low‐latitude ionosphere.