Comparative Analysis of Middle Atmospheric Ozone Response to Four Large Geomagnetic Storms of Solar Cycle 25

Abstract. Solar Cycle 25 (SC25) produced several major geomagnetic storms with distinctly different combinations of energetic electron precipitation (EEP) and solar proton event (SPE) forcing, providing a unique opportunity to investigate short-term ozone responses in the mesosphere-thermosphere system. We analyse four major storms occurring in May 2024, October 2024, November 2025, and January 2026 using Microwave Limb Sounder (MLS) observations and specified-dynamics WACCM-D simulations. Significant mesospheric ozone depletion was observed during all four events, with maximum ozone losses ranging from approximately 30 % to 80 %. Model-observation comparisons show that the dominant particle forcing varied substantially among the storms. The May 2024 superstorm was predominantly EEP-driven, whereas the October 2024 event was largely SPE-driven. The November 2025 and January 2026 storms represent mixed SPE-EEP cases, with a stronger proton contribution during January 2026. Negative ozone anomalies were consistently observed in the lower thermosphere during all four events. Although not reproduced by WACCM-D, their repeated occurrence suggests a lower-thermospheric response to major geomagnetic storms. Model simulations further reveal the indirect energetic particle precipitation (EPP) effect in the stratosphere, characterised by descending ozone depletion features dominated by EEP.

The four SC25 storms demonstrate the value of combining satellite observations with chemistry-climate models to separate the contributions of SPEs, EEP, and atmospheric dynamics, while also revealing several areas where current representations of particle forcing and upper-atmospheric coupling can be further improved. Maintaining long-term observations of both EPP and atmospheric composition remains crucial for improving our understanding of atmosphere-space coupling during geomagnetic storms.