綜合觀測與模擬結果顯示，2022年東加火山噴發對熱氣層–電離層系統造成由區域至全球尺度之顯著且長時間影響。近場結果指出，極端火山事件可透過大氣動力過程、電漿傳輸及電離層化學反應之共同作用，造成大尺度的電漿耗損與增強；遠場結果則顯示，火山產生之蘭姆波能有效將能量傳遞至高層大氣，並在遠離火山之區域引發顯著電離層抬升。本論文透過整合多種觀測資料與數值模擬，釐清極端火山事件引發之大氣–電離層耦合過程，進而理解低層大氣擾動對太空環境之影響。;The major eruption of the Hunga Tonga–Hunga Ha′apai volcano on 15 January 2022 generated atmospheric disturbances that propagated from the Earth′s surface into the upper atmosphere and ionosphere. Due to the enormous energy released by the eruption， various atmospheric waves， including Lamb waves， traveling atmospheric disturbances (TADs)， and traveling ionospheric disturbances (TIDs)， are observed globally. The eruption therefore provides a unique opportunity to study atmosphere–ionosphere coupling processes triggered by an extreme geophysical event.
This study examines the ionospheric responses to the 2022 Tonga volcanic eruption using a combination of ground-based and space-borne observations together with numerical simulations. Multiple observational datasets are employed， including ionosondes， HF Doppler sounding systems， barometers， tide gauges， Himawari-8 satellite observations， Global Ionosphere Maps (GIMTEC)， FORMOSAT-7/COSMIC-2 (F7C2) radio occultation and ion -velocity meter (IVM) measurements， FORMOSAT-5 (Fs-5) Advanced Ionospheric Probe (AIP) observations， and Global Ionospheric Specification (GIS) electron density reconstructions. In addition， the Global Ionosphere–Thermosphere Model (GITM) is conducted to investigate the physical mechanisms responsible for the observed ionospheric perturbations.
The first part of this dissertation investigates the near-field ionospheric response surrounding the eruption source region. GIMTEC reveals a long-lived umbrella-shaped plasma depletion during local nighttime and a subsequent daytime plasma enhancement extending approximately 1650 km from the volcano. During nighttime， TEC decreases by approximately 5–10 TECU， while F7C2 and Fs-5 observations indicate plasma density reductions of 30–97% and 72–95%， respectively. During daytime， the depleted region transitions into a broad plasma enhancement characterized by TEC increases of 2–18 TECU and ion density increases of 234–532%. Electron density reconstructions show that the disturbance extends from approximately 200 to 500 km altitude throughout much of the F-region ionosphere. Numerical simulations performed using the GITM reproduce the development of large-scale plasma depletion and reveal substantial plasma redistribution away from the eruption source region. Comparisons between observations and simulations suggest that atmospheric expansion， ionospheric uplift， enhanced molecular-ion recombination， and large-scale plasma transport collectively contribute to the formation of the observed umbrella-shaped ionospheric signatures.
The second part of this dissertation examines the far-field propagation of eruption-generated atmospheric and ionospheric disturbances. Surface pressure observations identify a tropospheric Lamb wave propagating at 300.8 ± 3.9 m/s across the western Pacific region. Corresponding ionospheric responses are detected by ionosondes and HF Doppler sounding systems in Taiwan and Japan. Two distinct TIDs are identified with propagation speeds of 539.0 ± 22.3 m/s and 338.3 ± 4.0 m/s， respectively， and are associated with thermospheric and tropospheric Lamb-wave modes. These disturbances produce ionospheric uplifts of approximately 86–155 km and 50–110 km and are observed 14–39 min prior to the arrival of the corresponding Lamb-wave fronts， indicating the presence of bow-wave structures. Numerical simulations successfully reproduce the large-scale propagation characteristics of the disturbances and provide further evidence of atmosphere–ionosphere coupling driven by eruption-generated Lamb waves.
The findings of this study demonstrate that the 2022 Tonga volcanic eruption produced profound and long-lasting modifications of the thermosphere–ionosphere system from regional to global scales. The near-field observations reveal that extreme volcanic forcing can generate large-scale plasma depletion and enhancement through the combined effects of atmospheric expansion， plasma transport， and ionospheric chemistry. The far-field observations further show that eruption-generated Lamb waves can couple efficiently into the ionosphere， producing substantial ionospheric uplift far from the source. By integrating multi-instrument observations with numerical simulations， this dissertation provides new insights into atmosphere–ionosphere coupling processes associated with extreme volcanic eruptions and improves our understanding of how disturbances originating in the lower atmosphere influence the space environment.
