Localized ΣO/N ₂ Enhancements During Non‐Superstorms: Tongue, Vortex, and the Tongue‐to‐Vortex Transition

Abstract

We unify the neutral tongue and vortex structures under a common framework of localized column density ratio of O to N ₂ (ΣO/N ₂ ) enhancements to investigate their similarities and differences during non‐superstorms based on ΣO/N ₂ data sets from GUVI and GOLD disk observations, as well as TIEGCM simulations. Three intense‐to‐moderate geomagnetic storms occurring in October 2002 April 2020, and May 2021 are examined. The results demonstrate that the vortex and tongue structures are different spatial configurations of the same underlying process: the advection and redistribution of thermospheric composition anomalies by storm‐time neutral winds with varying flow directions, velocities, and degrees of vorticity. The simulations further indicate that the tongue is usually the predecessor of the vortex, and persistent storm‐time energy input is identified as a critical factor controlling this transition during intense and moderate geomagnetic storms. These findings confirm that vortex structures can occur during non‐superstorm conditions and tend to develop at relatively higher geomagnetic latitudes than those reported during superstorms. Additionally, the downward vertical winds and the spatial distribution of composition perturbations are shown to play important roles in determining the morphology and intensity of localized ΣO/N ₂ enhancement structures. It is also noteworthy that these structures frequently originate and evolve on the nightside, making them difficult to detect using dayside‐only observations. Therefore, the combined use of global observations and numerical simulations is essential for improving our understanding of these complex thermospheric structures and their underlying physical mechanisms during space weather events.