The Effects of a Small Geomagnetic Storm on Earth's Thermosphere and Ionosphere: ICON Observations of the 25 January 2021 Disturbance.

Isolated geomagnetic storms during solar minimum offer the opportunity to observe the effects of solar wind on Earth's atmosphere without interference from other background changes in the ionosphere‐thermosphere system. Here, we observe the effects of a minor storm on the ionosphere at middle and low latitudes, where disturbances in thermospheric winds and composition can produce storm‐time changes in ionospheric densities. Using data from the NASA Ionospheric Connection Explorer (ICON), we investigate the competing effects of composition changes and neutral wind disturbances in modifying the ionosphere during the 25 January 2021 storm. Large enhancements in plasma densities are seen across the dayside late on 25 January, associated with large changes in meridional winds and significant high‐latitude electric current enhancements as measured by the auroral electrojet index. At the same time, we find evidence of rapid transport of heated air from the southern auroral zone to the equator in the East‐Asian sector with the appearance of deeply reduced O/N2 in the morning sector and storm‐time northward winds prior to its appearance. We observe significant, rapid erosion of the high plasma densities in the sector where thermospheric disturbance appears over 5 hr of observation. The noon‐time plasma density enhancements are short‐lived and we observe no westward perturbation in the low‐middle latitude, dayside zonal winds over a 5‐day period including the storm. Thus, we attribute the ionospheric changes to prompt penetration and meridional wind forcing, while ruling out the presence of a storm‐time disturbance dynamo and its related ionospheric effects. Key Points: A minor geomagnetic storm on 25 January 2021 produced remarkably large changes in the equatorial thermosphere and ionosphereAuroral heating effects were transported to the equator by storm‐time winds, with heating effects and winds affecting ionospheric densityIonospheric densities drop rapidly in region of O/N2 disturbance over 5 hr of observation [ABSTRACT FROM AUTHOR]