Strong thermospheric response to the almost undetectable substorm on May 29, 2023

A ground based FPI (Fabry Perot Interferometer) at the Athabasca Observatory detected an unusual strong and storm-like equatorward meridional wind of up to 450 m/s on May 29, 2023, a geomagnetically quiet day (AE < 150 nT, Kp < 1). F18 DMSP SSUSI, a Far Ultra-Violet (FUV) spectrograph imager, observed a long lasting (∼7 h) auroral substorm on the same day. TIMED/GUVI data showed a O/N₂ depletion that extended to mid/low latitudes over a limited longitude range in the northern hemisphere. Concurrent SuperDARN measurements indicated strong plasma convection around the substorm location, suggesting a strong local heating (Joule and particle precipitation heating) near the substorm location. This strong and localized heating caused the storm-like response in the thermospheric meridional wind and composition. Furthermore, the FPI also observed a strong zonal wind (up to 180 m/s), which changed its direction from westward to eastward during the substorm. Such a change is due to the competition between the pressure gradient and Coriolis forces. In the Northern Hemisphere, the Coriolis force is westward with an equatorward meridional wind during the substorm; the direction of the pressure gradient force changed from westward to eastward due to changes in the relative locations of the observatory and the substorm. A strong IMF B_y and periodic variation in the IMF likely provide a favorable upstream condition for continuous energy input from the solar wind to the magnetosphere and/or the release of the stored magnetospheric energy into the thermosphere to drive the long duration substorm and the observed thermospheric changes.