Near‐Field Acoustic and Gravity Wave Modes in the Ionosphere Triggered by Ground Motion and Tsunami During the 2024 Mw7.4 Hualien Earthquake

Abstract The interaction between the atmosphere and other components of the Earth system, such as the solid Earth and the oceans, remains an open and active area of research. In particular, the dynamic response of the ionosphere to forcing from below, such as earthquakes and tsunamis, is not yet fully understood. In this study, we investigate the ionospheric response to the Mw7.4 Hualien, Taiwan earthquake and trace each observed disturbance mode to its corresponding source mechanism. Using ground vertical velocity data from seismometer stations, Total Electron Content (TEC) measurements from 122 GNSS receivers, and tsunami observations from DART buoy data, we identify multi‐mode ionospheric disturbances propagating at speeds of ∼1.0, 0.60, and 0.22 km/s. The ∼1.0 km/s is the fastest mode directly driven by coseismic ground shaking. The slower mode of 0.60 km/s is most likely generated by the superposition of the primary acoustic waves (1.0 km/s) and ducted acoustic‐gravity waves; may therefore be interpreted as a secondary acoustic‐gravity wave. The slowest mode, propagating at 0.22 km/s, lies within the gravity wave range and is attributed to the regional‐scale tsunami triggered by the Mw7.4 earthquake. To the best of our knowledge, this represents the first observation of ionospheric disturbances generated by a moderate regional‐scale tsunami. Short‐Time Fourier Transform (STFT) analysis further reveals dominant center frequencies of ∼3.80, ∼2.60, and 1.30 mHz for the acoustic and gravity wave modes, respectively, consistent with known spectral characteristics of acoustic and tsunamigenic sources.