Detection of Vertical Changes in the Ionospheric Electron Density Structures by the Radio Occultation Technique Onboard the FORMOSAT-7/COSMIC2 Mission over the Eruption of the Tonga Underwater Volcano on 15 January 2022

A large near-surface perturbation such as the eruption of the Tonga underwater volcano on 15 January 2022 can generate disturbances in the Earth’s atmosphere and ionosphere. It is quite challenging to detect and investigate the disturbances in the vertical direction due to the lack of ground-based instruments, especially in the ocean area. To examine the vertical disturbances due to the Tonga eruption, this study utilizes the radio occultation (RO) technique onboard the satellites of the FORMOSAT-7/COSMIC2 (F7/C2) mission to sound the ionospheric electron density (Ne) profiles in the Central Pacific Ocean area around the eruption. The ionospheric Ne profiles show that the eruption almost annihilated the typical Chapman-layer structure over the eruption in the nighttime on 15 January. The Hilbert–Huang transform was applied to expose the vertical changes in the Ne structures as functions of wavelength and altitude. The analysis shows not only the occurrence of the small-scale disturbances with a wavelength of ~20 km from 100 km to 500 km altitudes, but also the significant attenuation of the structures with a wavelength >50 km, which has never been reported before. The time series of the total electron content from the ground-based Global Navigation Satellite System receiver near the eruption also verifies the significant long-lasting disturbances due to the eruption.