First Synoptic Observations of Geomagnetic Storm Effects on the Global‐Scale OI 135.6‐nm Dayglow in the Thermosphere by the GOLD Mission

The Global‐scale Observations of the Limb and Disk (GOLD) mission provides, for the first time, a synoptic view of the thermospheric response to a moderate geomagnetic storm at a 30‐min cadence. The observations showed that the OI 135.6‐nm emission substantially decreased at middle and high latitudes, but it increased at low latitudes. The depletion pattern between 4 and 6 November 2018 was likely due to the strong upwelling of the high‐latitude thermosphere. The deepest depletion in brightness that occurred near local noon was of ~300–400 R (~35% of the quiet time background level) and displayed a striking westward movement. First‐principles modeling reasonably reproduced the overall morphology of brightness response to the geomagnetic storm despite the depletion magnitude in brightness being underestimated. The postprocessing of the simulation results revealed that the westward movement of the deepest depletion was attributable to the horizontal wind changes driven by the geomagnetic storm. Plain Language Summary Geomagnetic storms are triggered by enhanced solar wind emitted from the Sun, and a large portion of the absorbed solar energy dissipates into the high latitudes of the Earth's atmosphere through Joule heating and ion drag. Geomagnetic storms usually bring tremendous disturbances into the thermosphere‐ionosphere (e.g., neutral and plasma density), which significantly impact human activities (e.g., satellite orbit determination, navigation, and telecommunications). Accurate prediction of the response to geomagnetic storms in the thermosphere‐ionosphere is one of the core scientific objectives of space weather. The newly launched Global‐scale Observations of the Limb and Disk mission provides unprecedented synoptic measurements of ionosphere‐thermosphere properties. This study reports a large response of the OI 135.6‐nm emission to a moderate geomagnetic storm that occurred on 8–9 November 2018 observed by Global‐scale Observations of the Limb and Disk, which demonstrates a huge potential to advance our knowledge of the thermosphere‐ionosphere system and ability to simulate and forecast space weather. Key Points GOLD provided, for the first time, synoptic observations of the thermospheric response to a geomagnetic storm with a 30‐min cadence Dayglow brightness depletions occurred at high latitudes and were extended into midlatitudes with evident interhemispheric asymmetry The deepest depletion occurred near local noon and moved westward during and