N. Nishitani

Solar cycles 24–25 were quiet until a geomagnetic storm with a Sym‐H index of −170 nT occurred in late March 2023. On March 23–24, a Fabry‐Perot interferometer (FPI; 630 nm) in Tromsø, Norway, recorded the highest thermospheric wind speed of over 500 m/s since 2009. Comparisons with magnetometer readings in Scandinavia showed that a large amount of electromagnetic energy was transferred to the ionosphere‐thermosphere system. Total electron content maps suggested an enlarged auroral oval and revealed that the FPI observed winds near the polar cap instead of inside the oval for a long period during the storm main phase. The FPI wind had a strong equatorward component during the storm, likely because of the powerful anti‐sunward ionospheric plasma flow in the polar cap. The positive Y‐component of the IMF for 6 days before the storm caused a successive westward component of the FPI‐measured wind during the storm main phase. On March 24, the first day of the storm recovery phase, thermospheric wind disturbed and the ionospheric density decreased significantly at high latitudes. This density depression lasted for several days, and a large amount of electromagnetic energy during the storm modified the thermospheric dynamics and ionospheric plasma density.