The Ionospheric Responses During the 2020–2021 SSW From Multiple Simultaneous Observations

Through simultaneous observations with various instruments, the ionospheric responses under multiple modulations during the 2020–2021 SSW are explored. Unlike typical SSW events, the ionosphere exhibited a distinct decrease in NmF2 and TEC at daytime without regular semi‐diurnal variations. Analysis reveals this decrease was primarily driven by reductions in ΣO/N 2 and solar radiation, rather than conventional E × B drift or neutral wind transport. Crucially, thermospheric observations show enhanced SW2 and M2 tidal amplitudes after the SSW onset, suggesting that the enhanced tidal modulation (rather than minor FUV variations) served as the dominant mechanism for the observed ΣO/N 2 decrease. These findings highlight the importance of thermospheric composition changes in SSW‐ionosphere coupling.