Electrodynamics and Field-Aligned Wind Drivers of the Ionospheric Lunar Tidal Response to the 2020–2021 Sudden Stratospheric Warming: Insights from COSMIC-2, ICON, SD-WACCM-X, and TIE-GCM

Sudden stratospheric warming (SSW) events provide a pathway for strong coupling between the lower atmosphere and the ionosphere. This study investigates how the breakdown of the stratospheric polar vortex during the 2020–2021 SSW influenced the lunar semidiurnal (M2) tide in F-region electron density. We analyze Global Ionospheric Specification (GIS) electron density data derived from COSMIC-2 observations and vertical plasma drifts measured by the Ion Velocity Meter on board ICON. During the SSW, when the polar vortex was strongly disrupted, the M2 lunar tide increased by up to ~16% at low latitudes near the Equatorial Ionization Anomaly (EIA) around 300 km altitude relative to daily mean zonal mean. The vertical plasma drift response exhibits similar variability. Simulations using SD-WACCM-X reproduce consistent M2 signatures in E-region zonal winds, F-region vertical drifts, and electron density. To isolate the physical mechanisms, we analyze the ion continuity equation using TIE-GCM driven by SD-WACCM-X lower boundary conditions. The results show that M2 tides primarily affect F-region electron density through electrodynamically driven vertical plasma drifts (E-region dynamo), which increase by ~114% during the SSW, while transport by neutral winds (field-aligned transport) increases by ~43%. These findings indicate that the E-region dynamo provides the dominant pathway for lunar tidal modulation of the ionosphere during SSW events, with neutral wind transport playing a secondary role.