This paper provides an overview of the contributions of the space-based global navigation satellite system (GNSS) radio occultation (RO) measurements from the FORMOSAT-7/COSMIC2 (F7/C2) mission in advancing our understanding of ionospheric plasma physics in the purview of space weather. The global positioning system (GPS) occultation experiment (GOX) onboard FORMOSAT-3/COSMIC (F3/C), with more than four and half million ionospheric RO soundings during April 2006–May 2020, offered a unique three-dimensional (3D) perspective to examine the global electron density distribution and unravel the underlying physical processes. The current F7/C2 carries TGRS (Tri-GNSS radio occultation system) has tracked more than 4000 RO profiles within ±35° latitudes per day since 25 June 2019. Taking advantage of the larger number of low-latitude soundings, the F7/C2 TGRS observations were used here to examine the 3D electron density structures and electrodynamics of the equatorial ionization anomaly, plasma depletion bays, and four-peaked patterns, as well as the S4 index of GNSS signal scintillations in the equatorial and low-latitude ionosphere, which have been previously investigated by using F3/C measurements. The results demonstrated that the denser low-latitude soundings enable the construction of monthly global electron density maps as well the altitude-latitude profiles with higher spatial and temporal resolution windows, and revealed longitudinal and seasonal characteristics in greater detail. The enhanced F7/C2 RO observations were further applied by the Central Weather Bureau/Space Weather Operation Office (CWB/SWOO) in Taiwan and the National Oceanic and Atmospheric Administration/Space Weather Prediction Center (NOAA/SWPC) in the United States to specify the ionospheric conditions for issuing alerts and warnings for positioning, navigation, and communication customers. A brief description of the two models is also provided.

The ionospheric total electron content (TEC) derived from dense ground‐based Global Navigation Satellite System receivers over the continental United States and those from global ionosphere maps are utilized to find the ionosphere response to the August 21, 2017 total solar eclipse. Maximum obscurations and their associated TEC major depressions appear simultaneously at midlatitudes, while major depressions elongate toward the magnetic equator with some delays in the equatorial ionization anomaly (EIA) region. The former is due to the photochemical loss process, while the latter is caused by the plasma transport of E×B drifts and lunar gravitation forces. TECs of predawn reductions, morning enhancements, afternoon reductions, and nighttime enhancements reveal that the semidiurnal lunar tide are essential. Since a solar eclipse always occurs on a new moon day, the lunar tide results in the early EIA appearance and major depressions being underestimated/diminished before and overestimated/enhanced after about 14:00 local time.

This paper concurrently investigates the midlatitude trough and the plasmapause positions in the ionosphere by colocated measurements of the electron density, electron temperature, and whistler count probed by DEMETER (Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions) satellite in the nighttime at 2230 LT (local time; 1900–0100 MLT, magnetic local time, mainly in the premidnight period) during the 4‐year period of 2006–2009. More than 13,000 Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions orbits of the electron density and the electron temperature are used to search the trough position, while the same amount of the whistler count is employed to determine the plasmapause position at the satellite altitude. The plasmapause is more sensitive to solar activity, which moves equatorward 1.0–1.2° form the low to high solar activity of the study period. On the other hand, the midlatitude trough is more sensitive to seasonal variation, which shifts poleward 1.7–2.5° from the winter to summer month in the study period. The midlatitude trough usually appears in the poleward side of the plasmapause during the study period. Both of the midlatitude trough and the plasmapause move equatorward during the magnetic disturbed condition. For the magnetic disturbed Kp ≥ 6₋, the midlatitude trough can appear in the equatorward side of the plasmapause.

This study conducts a comprehensive analysis of observations related to the M_L6.2 Hualien earthquake that struck eastern Taiwan on 6 February 2018, focusing particularly on gamma-ray emissions and total electron content (TEC) as earthquake precursors. Prior research has shown that significant gamma-ray enhancements are frequently detected at the YMSG (Yangmingshan gamma-ray) station prior to major earthquakes in eastern and northeastern Taiwan, suggesting that gamma-ray anomalies may serve as reliable indicators for identifying seismic precursors in this area. Our findings reveal a significant rise in gamma-ray emissions at the YMSG station from 19 January to 4 February 2018, which corresponds to a precursor period of approximately 18 days before the Hualien earthquake. Positive and negative TEC anomalies were observed in Taiwan on 20–21 January and 5 February, respectively, and may be considered as ionospheric precursors to the earthquake. Additionally, deep-learning techniques applied to TEC data facilitate the detection of ionospheric precursors associated with the Hualien earthquake, enabling forecasts of an approaching seismic event. Collectively, these observations indicate that all identified anomalies are regarded as short-term precursors, explicable through the theoretical framework of lithosphere–ionosphere coupling (LIC).

Intense eruptions of the Tonga volcano activated prominent traveling atmospheric disturbances (TADs) at 04:05UT on 15 January 2022. Himawari‐8 satellite images depict that TADs of the tropospheric Lamb wavefront propagate with a speed of 315 m/s and arrive in Taiwan at 11:30UT. Networks of 98 barometers, 28 tide gauges, an ionosonde, and 10 magnetometers are used to study the responses of magnetic fields to TADs in Taiwan. The horizontal components in magnetic field changes of the Taiwan magnetometers all point toward and away from the Tonga volcano at 11:00–12:00UT upon the tropospheric Lamb wavefront arrival and at 22:00–23:00UT when the thermospheric Lamb wavefront with speeds of 487 m/s coming, respectively. Analyses of the raytracing and beamforming techniques on the horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves efficiently activate traveling ionospheric disturbances and modify ionospheric currents of the globe.