Observations of Thermospheric Wind and Composition Changes and their Impact on Ionospheric Disturbances in the European Sector during Nov 4, 2021 Geomagnetic Storms

On November 1st and 2nd, 2021, four Halo coronal mass ejections were ejected from the Sun, releasing enormous amounts of high-energy particles into interplanetary space. These were directed towards the Earth and reached our planet on November 3rd and 4th, 2021, generating the first G3-level extreme geomagnetic storm since the beginning of the 25th solar cycle. In this study, we analyzed the thermospheric and ionospheric responses in the European sector to a G3-level storm using various observational data. In particular, we found positive ionospheric storms in the middle and low latitudes of Europe probably caused by the propagation of equatorward and westward neutral winds from polar region which were observed by the Fabry-Perot interferometer and Ionospheric Connection Explorer/Michelson Interferometer for Global High-resolution Thermospheric Imaging (ICON/MIGHTI) data. In contrast, we detected negative storms at high latitudes probably due to an increase in thermospheric density (upwelling), as confirmed by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) data. Fortunately, we were able to confirm these two antithetical responses using European ionosonde and total electron contents (TEC) observation chains distributed over a wide range of latitudes, and for the first time, to define the imaginary boundary line between the two responses.