We used dense global navigation satellite system data from China to track the propagation of traveling ionospheric disturbances (TIDs) triggered by the 2022 January 15 Tonga volcanic eruption. We identified two TIDs originating from the eruption. One, which has been reported widely by a number of recent investigations, had a velocity of ∼361 m/s. However, another long‐distance propagating TID with a velocity of ∼264 m/s has not been widely discussed. The velocities of these TIDs coincide with previous simulation results of gravity‐wave L0 and L1 modes. We propose that these TIDs were caused by the L0 and L1 ducted modes of gravity waves excited by the volcanic eruption. However, the L1 mode is usually negligible due to its weak amplitude in comparison with that of the L0 mode. The enormous energy release resulted in a stronger amplitude of the L1 mode, which induced a detectable TID in the ionosphere.

This article presents observational results of the vertical structural evolution of ionospheric holes during two rocket launches. A combination of observations from the newly built Sanya incoherent scatter radar (SYISR) and Global Navigation Satellite System networks was employed. The vertical structural evolution of two rocket‐induced ionospheric holes over the SYISR could be divided into three stages. Firstly, 10–11 min after launch, the holes initially appeared in the topside ionosphere. Then, the holes expanded rapidly toward higher and lower altitudes. At ∼30 min after launch, the holes extended to an altitudinal range of ∼200 km to over 700 km. The maximum depletion altitude was 425 km for the afternoon event and 283 km for the midnight event. Finally, the holes recovered slowly, with faster recovery at higher altitudes. Our work contributes to further understanding the vertical structural evolutionary mechanisms of the ionospheric holes and relevant ionospheric processes.

The Multiband Ultraviolet Spectrum Imager (MUSI) is an optical remote sensing instrument scheduled to launch on the Fengyun‐4C meteorological satellite in 2024. MUSI is designed to measure the airglow emissions between ∼120 and 160 nm above the East Asia and Pacific region from a geostationary orbit to infer ionospheric parameters. Here we develop a lookup‐table algorithm for retrieving the peak electron density (n_mF₂) and the Total Electron Content (TEC) from nighttime observations of the OI 135.6 nm emission. The algorithm takes into account the north‐south asymmetry of the equatorial ionization anomaly and the atmospheric variations with solar activity and local time. Analysis of synthetic observations shows that the assumptions made in the algorithm only lead to ∼2%–4% uncertainties in the retrievals under the modeled ionospheric conditions. The algorithm is verified by retrieving n_mF₂ and TEC from the Global‐Scale Observations of the Limb and Disk (GOLD) mission and comparing with coincided radio measurements. The comparison indicates almost negligible systematic differences (typically less than ∼3%) between the GOLD and the ionosonde n_mF₂, which are within the error limit of the GOLD retrievals. The accuracy of the GOLD TEC retrievals is shown to be comparable to (or in some sense even better than) that of the measurements from the Global Navigation Satellite System. Several simplified yet robust retrieval methods are also developed, which can be readily implemented for both existing and upcoming missions. Our results promote far ultraviolet (FUV) remote sensing to be a key technology for accurate determination of the ionospheric state.