GOLD Nighttime Limb Observations: A New Data Set for Investigating the Altitude‐Latitude Structure and Temporal Variability of the Equatorial Ionization Anomaly

Abstract Using NASA's Global‐scale Observations of the Limb and Disk (GOLD)'s Dark Limb Mode (DLM) observations, we provide a multi‐year characterization of nighttime Equatorial Ionization Anomaly (EIA) morphology and variability. From geostationary orbit, GOLD measures OI 135.6 nm limb radiance at ∼33°E and ∼128°W longitudes, yielding routine, time‐evolving altitude–latitude cross sections of the nighttime ionosphere (∼100–430 km) and enabling investigation of EIA morphology and temporal variability. Forward‐modeled line‐of‐sight integrations using WACCM‐X electron densities and GLOW‐derived volume emission rates indicate that GOLD limb radiance peaks at tangent altitudes ∼40 km lower than the contributing F‐region electron density peak at the limb longitude. Multi‐year (2019–2025) observations reveal limb‐to‐limb differences, pronounced day‐to‐day variability, and strong solar‐flux dependence in the EIA crest. This new DLM data set provides a distinct observational capability for ionosphere–thermosphere coupling studies and space‐weather applications.