A Complete Chain of the Generating Processes of Ionospheric Negative Storm Over the North America.

A negative ionospheric storm occurred over the North American sector on 4 November 2021. By the integration of hemispheric power (HP), Ionospheric Connection Explorer (ICON), Global‐scale Observations of the Limb and Disk and global navigation satellite system total electron content (TEC) observations, a complete observation chain is obtained for the first time. At 07:11 UT, prominent energy was input into high latitudes. From 11:51 to 15:49 UT, strong southwestward neutral wind was observed by ICON over the south of North America, followed by significant ∑O/N2 depletion (60%) and temperature enhancement (∼300 K at ∼150 km). TEC depletion started at 12:00 UT in the northeastern North America and expanded to most of the United States. The TEC reduction showed the latitudinally tilted structure similar to that of ∑O/N2 depletion and temperature enhancement, which was shaped by the southwestward neutral wind. The thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) reproduced the observations qualitatively and was utilized to investigate the details of the negative storm generation and evolution processes. Strong equatorward wind and ∑O/N2 depletion reached the North America at midnight, much earlier than the TEC depletion, which began at sunrise. At 90°W, the time delay between TEC and ∑O/N2 depletions increased from ∼3.3 hr at 30°N to ∼11 hr at 60°N. Several factors contributed to the time delay including the time difference between midnight and sunrise, ∑O/N2 transport direction, westward wind and the variation of solar elevation angle with latitude. Plain Language Summary: During geomagnetic storms, the increase/decrease of ionospheric plasma density during geomagnetic storms relative to geomagnetic calm period is called positive/negative ionospheric storms. Due to the lack of observations of thermospheric properties, there are very few studies on negative storms over middle latitudes using direct neutral wind and temperature observations. This paper investigates the negative storm over the North American sector on 4 November 2021. A complete observation chain of the negative storm generating processes is presented by using hemispherical power index, neutral wind, neutral temperature and composition, and total electron content (TEC) measurements. Strong southward and westward neutral wind were observed by Ionospheric Connection Explorer satellite over middle and low latitudes. Global‐scale Observations of the Limb and Disk satellite also recorded prominent thermospheric composition changes and temperature enhancement. Subsequently, TEC depletion occurred was consistent with that of neutral temperature and composition variations. A numerical model is employed to uncover the details of the processes. Thermospheric composition changes occurred between 40° and 60°N firstly at 06:00 UT in the North America sector while TEC depletion appeared at 12:00 UT. There were four factors contributed to the time lag between TEC and composition changes including the time difference between midnight and sunrise, composition transport, westward wind and solar elevation angle. Key Points: During a negative storm, strong southwestward neutral winds and prominent ∑O/N2 depletion were observed by Ionospheric Connection Explorer and Global‐scale Observations of the Limb and Disk, respectivelyA complete observation chain of the negative ionospheric storm generation at middle latitudes is first established for the first timeThe time delay between TEC and ∑O/N2 depletions increased from 3.3 hr at 30°N to ∼11 hr at 60°N [ABSTRACT FROM AUTHOR]