Chi‐Yen Lin

Global Ionospheric Specification (GIS) three‐dimensional electron density profiles constructed by assimilating the line‐of‐sight total electron content (TEC) from global navigation satellite system (GNSS) radio occultation (RO) measurements by FORMOSAT‐7/COSMIC‐2 (F7/C2) and ground‐based receiver network from international GNSS service, constitute a unique ionospheric database with 1‐hr cadence. Using the data spanning over half a solar cycle since the release of GIS as an operational Level 3 data product of F7/C2 mission, a comprehensive evaluation of the peak electron density (NmF ₂ ) is carried out by identifying data conjunctions with Global Ionosphere Radio Observatory (GIRO) Digisonde observations. The results reveal an overall correlation of 0.93, with GIS overestimating GIRO NmF ₂ by 6.4%. While the values do not show any specific dependence on geomagnetic activity, the correlation (bias) reduces (increases) when only high latitude (>35°) observations are included where no RO measurements are assimilated. A similar tendency is observed when only low solar activity years (F10.7 ≤ 100) are considered. The detailed local time dependence of the correlation and bias are examined in this study, with lowest correlation and large relative bias occurring during 0400 LT—0600 LT. The results show large local time variation of bias depending on latitude region and solar activity. Overall, the bias ranges from 10% to 30% during 1000 LT—2300 LT, whereas GIS may underestimate NmF ₂ by 5%–15% during 0700 LT—1000 LT. The results also demonstrate that GIS reveals identical day‐to‐day variations as seen in GIRO measurements and compares well with TECs from Global Ionosphere Map and ground GNSS receivers.

The 29 July 2025 Mw 8.8 Kamchatka earthquake generated notable seismic and tsunami waves. The total electron content (TEC) derived by more than 1,400 ground‐based global navigation satellite system (GNSS) receiving stations in Japan and Taiwan is employed to examine the spatiotemporal evolution of seismic‐ and tsunami‐traveling ionospheric disturbances (STIDs and TTIDs). A z‐score normalized process applied to GNSS TEC clearly shows STIDs, TTID ₁ and TTID ₂ traveling with horizontal speeds of 3.6 km/s, 273.2 m/s, and 215.4 m/s, respectively. TTID ₁ is consistent with the primary tsunami wavefront, while TTID ₂ is likely related to coastline‐reflected tsunami waves. The estimated TTID ₁ source is located within ∼90 km of the center of the main slip area, demonstrating a close spatial agreement between the tsunami ionospheric disturbance source and the tsunami source inverted from deep‐ocean pressure data.

This study evaluates 17 ionospheric models' performance over the Continental United States (CONUS) during the 2024 Mother's Day geomagnetic storm, using ground‐based GNSS Total Electron Content (TEC) and single‐frequency (SF) precise Single Point Positioning (SPP) technique. The storm produced significant positive storm effects during its main phase and notable TEC depletions during recovery phases. We evaluate each model's capability to (a) reproduce vertical TEC magnitudes and spatiotemporal patterns, (b) capture storm‐driven TEC anomalies, and (c) support practical SF GNSS positioning. Data assimilation (DA) models, specifically GloTEC and GIS, show the best performance in representing the TEC magnitude, morphology and variability, and they yield the lowest positioning errors. Physics‐based models exhibit mixed performance. They better capture negative storm effects but generally underestimate positive TEC enhancements, likely due to the underestimation or absence of penetration electric field. Empirical models struggle to represent the storm‐time dynamics, although IRI2020 partially captured the recovery‐phase TEC depletions. SF GNSS SPP results suggest that scientific models provide more realistic ionospheric representations than the Klobuchar model during storm conditions. However, large systematic biases during the main phase can still introduce significant positioning errors. This work establishes a performance baseline for ionospheric models under extreme conditions and demonstrates their readiness for practical application. The validation results are integrated into the CCMC Ionosphere‐Thermosphere Model Assessment and validation Platform (ITMAP), supporting community‐wide benchmarking and future model development.

The 29 July 2025 Mw 8.8 Kamchatka earthquake generated notable seismic and tsunami waves. The total electron content (TEC) derived by more than 1,400 ground-based GNSS receiving stations in Japan and Taiwan is employed to examine the spatiotemporal evolution of seismic- and tsunami-traveling ionospheric disturbances (STIDs and TTIDs). A normalized process applied to GNSS TEC clearly shows STIDs, TTID1 and TTID2 traveling with the horizontal speeds of 3.6 km/s, 273.2 m/s, and 215.4 m/s, respectively. TTID1 is consistent with the primary tsunami wavefront, while TTID2 is mainly attributed to coastline-reflected tsunami waves, with the Mw 6.9 aftershock likely providing a secondary contribution. The estimated TTID1 source is located within ~90 km of the center of the main slip area, which marks the first time an ionospheric source closely matches the tsunami source inverted from deep-ocean pressure data.

The ion density and velocity measured by the advanced ionospheric probe (AIP) onboard FORMOSAT‐5 (F5) and ion velocity meter (IVM) onboard FORMOSAT‐7/COSMIC‐2 (F7C2), and the electron density assimilated by the global ionospheric specification (GIS) are employed to study the ionospheric plasma structure and dynamics during the 10 May 2024 Mother's Day storm (Dst −412 nT). The F5/AIP and F7C2/IVM display a large‐scale hole over the magnetic equator in the Atlantic Ocean area (−10° to 25°N, −60° to 20°E) during the local midnight period, with the minimum ion density of 1.7 × 10 ⁴ #/cm ³ and 1.6 × 10 ³ #/cm ³ , respectively. In the hole area, F5/AIP and F7C2/IVM reveal upward ion velocities at 720 km altitude and downward ones at 550 km altitude, respectively, while GIS profiles show that the electron density yields the lower peak at ∼440 km and upper peak at ∼760 km altitude. This suggests that the downward and upward ion velocities result in the double‐peak feature.

The maiden launch of Gravity‐1, labeled as the most powerful all‐solid propellent rocket yet, on 11‐January‐2024 around 0530 UT from an offshore launch platform, Yellow‐Sea, China by OrienSpace was followed by the 48th launch of H‐IIA rocket from Tanegashima space center by Japanese Aerospace Exploration Agency at 0444 UT the next day. These rare launches on consecutive days over similar geographical regions and local‐times yielded rocket‐induced concentric traveling ionospheric disturbances (CTIDs) and depleted total electron content (TEC) by 50%–70% (20–60 TEC units (TECU) reduction). The CTIDs with periods 9–11 min and wavelengths 300–720 km propagated at speeds of 300–1,100 m/s, displaying acoustic dispersion characteristics. The TEC depletions covering a region of 3.5–4.5°, lasted for 60–90 min before eventually recovering. Three‐dimensional Global Ionospheric Specification (GIS) electron density profiles and TEC tomography inversions indicate ionospheric depletions over 250–320 km altitudes. The observed rate of TEC depletions during the two events are used to compare the water molecule (H₂O) content in the combustion exhausts of the two rockets by using SAMI2 (SAMI2 is Another Model of Ionosphere) simulations. The results show that Gravity‐1 emitted only 5%–7% of the H₂O contained in H‐IIA exhaust, while the rapid occurrence of TEC depletion covering a wider spatial area indicates its steeper vertical (closer to zenith) trajectory. The observations are further used to estimate the possible delay by the TEC gradients by solving the carrier‐phase ambiguity over selected stations. The results indicate a delay gradient of up to 60 mm/km, too small to threaten ground‐based augmentation systems (GBAS).

The receiver operating characteristic (ROC) and the area under the curve (AUC), initially developed for signal processing and psychology, are a test for assessing the performance of a binary classification problem at varying threshold values. The ion density (N_i) observed by FORMOSAT‐7/COSMIC‐2 is used to study traveling ionospheric disturbances (TIDs) triggered by the 15 January 2022 Tonga volcanic eruption. We examine parameters of N_i, differential N_i, and standard deviation N_i (STD_N_i) in January 2022, simulate TID wavefronts traveling with various speeds from 10 to 1,000 m/s, and apply the ROC curve to globally identify the significance of TIDs in STD_Ni triggered by the volcanic eruption. ROC and AUC results show that in addition to TIDs related to tsunami/tropospheric Lamb waves and a series of fast‐moving waves with propagation speeds of 180–350 and 450–600 m/s, respectively, those long‐lasting low‐speeds less than 70 m/s and high‐speeds about 690, 860, and 990 m/s meet 95% statistical significance, which confirms TIDs being detected. These show that the FORMOSAT‐7/COSMIC‐2 ion density can be used to globally detect various TIDs triggered by the Tonga volcanic eruption. The ROC test results also show a potential use case for detecting other geophysical signals in future applications.

Low Earth orbit (LEO) radio occultation|radio occultations (RO) constellations can provide global electron density profiles (EDPs) to better specify and forecast the ionosphere‐thermosphere (I‐T) system. To inform future RO constellation design, this study uses comprehensive Observing System Simulation Experiments (OSSEs) to assess the ionospheric specification impact of assimilating synthetic EDPs into a coupled I‐T model. These OSSEs use 10 different sets of RO constellation configurations containing 6 or 12 LEO satellites with base orbit parameter combinations of 520 or 800 km altitude, and 24° or 72° inclination. The OSSEs are performed using the Ensemble Adjustment Kalman Filter implemented in the data assimilation (DA) Research Testbed and the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). A different I‐T model is used for the nature run, the Whole Atmosphere Model‐Ionosphere Plasmasphere Electrodynamics (WAM‐IPE), to simulate the period of interest is the St. Patrick's Day storm on March 13–18, 2015. Errors from models and EDP retrieval are realistically accounted for in this study through distinct I‐T models and by retrieving synthetic EDPs through an extension Abel inversion algorithm. OSSE assessment, using multiple metrics, finds that greater EDP spatial coverage leading to improved specification at altitudes 300 km and above, with the 520 km altitude constellations performing best due to yielding the highest observation counts. A potential performance limit is suggested with two 6‐satellite constellations. Lastly, close examination of Abel inversion error impacts highlights major EDP limitations at altitudes below 200 km and dayside equatorial regions with large horizontal gradients and low electron density magnitudes.

The NASA global‐scale observations of the limb and disk (GOLD) mission is a measurement opportunity to scan the far ultraviolet airglow at ∼134–162 nm over the American Hemisphere since October 2018. The FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission has provided thousands of daily radio occultation soundings in the low‐ and mid‐latitude regions since July 2019. The nighttime OI–135.6 nm emission is mainly through radiative recombination, and the radiance is used to derive the peak electron density. Comparison with corresponding F7/C2 observations demonstrates good correlation in low‐latitudes, while is overestimated near mid‐latitudes in winter, induced by the photoelectrons emanating from magnetically conjugate Hemisphere. The machine learning technique Bagged Trees is implemented to develop an intensity to peak density model training from GOLD and F7/C2 observations. The validation demonstrates that Bagged Trees peak‐density has less influence from conjugate photoelectrons and indicates the power of machine learning techniques for geophysics data processing.