This study examines the complex interrelationships among ionospheric NmF2 variations, solar activity F10.7 index, and crustal movements preceding the 2011 Tohoku-Oki earthquake. NmF2 from 5 ionospheric stations (Yakutsk, Khabarovsk, Wakkanai, Kokubunji, and Jeju) is studied together with vertical movement observed by Hi-net tilt meters in Japan. Results showed that while daytime NmF2 typically correlates with solar activity (F10.7 solar radio flux), significant deviations were observed during specific periods. During these intervals, NmF2 variations did not correlate with space weather parameters as expected but correlate with the vertical ground motion. We explain these phenomena as due to the enhanced dynamo electric field, which is produced originally by the vertical ground motion. Our findings suggest that the ionosphere before the large earthquake is influenced by the complex Lithosphere-Atmosphere-Iono sphere interaction such as crustal movements, acoustic-gravity waves, and dynamo electric field variations. (c) 2025 COSPAR. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Tsunamis can induce ionospheric disturbances that can be detected through analysis of global navigation satellite system signals collected by low Earth orbit (LEO) satellites. Consequently, tsunamis can be monitored by LEO satellites. For real‐time automatic tsunami monitoring, determining whether detected disturbances are attributed to a tsunami is crucial without prior information. This study proposes a disturbance detection threshold that investigates the statistical magnitude of monitoring values under normal conditions. The proposed method applies a bandpass filter with a bandwidth of 50–100 mHz to the total electron content normalized by the global ionospheric model to derive the monitoring value. Normal‐condition measurements were collected, and the standard deviation was analyzed based on the impact parameter altitude in a 3‐km window. The 3‐sigma peak value was then modeled to compute the threshold. Data from the FORMOSAT‐3/COSMIC satellite were collected during the 2011 Tohoku earthquake, focusing on two regions of interest: the Pacific Ocean in southeast Japan and the Pacific Ocean west of the United States. The threshold was determined using measurements from data collected 10 days before the earthquake. Applying the proposed threshold, disturbances were detected in one out of seven data points in the Pacific Ocean southeast of Japan and in one out of six data points in the Pacific Ocean west of the United States. A comparison of the detection results with the tsunami propagation model and Deep‐ocean Assessment and Reporting of Tsunamis data confirmed the detection of actual disturbances.

Abstract This study presents an innovative approach to improving the accuracy and reducing the error convergence time of static Precise Point Positioning (PPP) in Global Positioning System (GPS) navigation. The research focuses on the impact of the high spatial and temporal resolution of a regional ionospheric data assimilation model on PPP over Taiwan. The study further evaluates the performance of both static PPP with the ionospheric information using commonly used models such as Klobuchar and International Reference Ionosphere (IRI), as well as a global ionospheric data assimilation model. Compared to the default IRI, the data assimilated IRI model can improve the overall ionospheric total electron content by approximately 83%. Additionally, it can significantly reduce horizontal positioning errors and shorten the error convergence time more than 52% for static PPP, even during geomagnetic storm events. The study concludes that the high resolution of a regional ionospheric data assimilation model can enhance the accuracy and reduce the error convergence time of PPP navigation and positioning. This research provides valuable insights for future studies in this field, especially in the development of more precise and efficient models for correcting ionospheric delay in GPS navigation.

This paper discusses the relationship between the vertical ground motion and ionospheric disturbances before the Kumamoto earthquake on 16 April 2016, in Kyushu, Japan, using the vertical ground motion measured by slant gauges widely distributed in Kyushu, and the NmF2 observed by ionosondes in Japan and another region. We provide evidence that vertical ground motion excites internal gravity waves (IGWs) that disturb changes in the ionospheric plasma density. From the spectral analysis results of the vertical ground motion data, the summation of various period (frequency) components analyzed from the original data of the slant gauge shows a possible correlation with the change of NmF2 before the earthquake. On the other hand, the influence of the geomagnetic disturbance on vertical ground motion seems to exist. However, we cannot confirm that vertical ground motion is influenced by the geomagnetic disturbance (Kp index) and that the earthquake is triggered by the geomagnetic disturbance. There are two conditions for the vertical ground motion to disturb variations in the ionospheric plasma density: (1) The effective vertical ground motion period should be shorter than 5 h. In addition, (2) vertical ground motion should continue to exist so that wave energy can be continuously injected into the atmosphere. A possible mechanism with which to modify the ionosphere is discussed. The results of this study can provide a basis for the future ionospheric precursors of earthquakes by using the vertical ground motion.

FORMOSAT-3/COSMIC (F3/C) constellation of six micro-satellites was launched into the circular low-earth orbit at 800 km altitude with a 72-degree inclination angle on 15 April 2006, uniformly monitoring the ionosphere by the GPS (Global Positioning System) Radio Occultation (RO). Each F3/C satellite is equipped with a TIP (Tiny Ionospheric Photometer) observing 135.6 nm emissions and a TBB (Tri-Band Beacon) for conducting ionospheric tomography. More than 2000 RO profiles per day for the first time allows us globally studying three-dimensional ionospheric electron density structures and formation mechanisms of the equatorial ionization anomaly, middle-latitude trough, Weddell/Okhotsk Sea anomaly, etc. In addition, several new findings, such as plasma caves, plasma depletion bays, etc., have been reported. F3/C electron density profiles together with ground-based GPS total electron contents can be used to monitor, nowcast, and forecast ionospheric space weather. The S4 index of GPS signal scintillations recorded by F3/C is useful for ionospheric irregularities monitoring as well as for positioning, navigation, and communication applications. F3/C was officially decommissioned on 1 May 2020 and replaced by FORMOSAT-7/COSMIC-2 (F7/C2). F7/C2 constellation of six small satellites was launched into the circular low-Earth orbit at 550 km altitude with a 24-degree inclination angle on 25 June 2019. F7/C2 carries an advanced TGRS (Tri Gnss (global navigation satellite system) Radio occultation System) instrument, which tracks more than 4000 RO profiles per day. Each F7/C2 satellite also has a RFB (Radio Reference Beacon) on board for ionospheric tomography and an IVM (Ion Velocity Meter) for measuring ion temperature, velocity, and density. F7/C2 TGRS, IVM, and RFB shall continue to expand the F3/C success in the ionospheric space weather forecasting.

In this study, we have investigated the ionospheric responses on the August 2017 solar eclipse event by using a three-dimensional tomography algorithm with the ground-based GNSS (Global Navigation Satellite System) total electron content observations around Northern America. This three-dimensional ionospheric electron density structure from the tomography can provide us more information regarding the density variations and propagations of disturbances. Results show that the ionospheric electron density depletion triggered by the solar eclipse started from the higher ionosphere and then extended to lower altitudes. The maximum electron density depletion is around 40% compared with the previous day of solar eclipse. After around 30 min of the totality, the electron density continuously returned to the normal level. We further conduct a procedure of Fourier analyses to derive the vertical phase and group velocities of the electron density propagations. Results show that the opposite directions of the vertical phase and group velocities around 220–240 km altitude imply the energy/oscillation source by the solar eclipse. Graphical Abstract

This study examines the day-to-day variability of low-latitude ionosphere using global ionospheric specification (GIS) electron density profiles derived from FORMOSAT-7/COSMIC-2 radio occultation measurements during a deep solar minimum period of August 2019 to July 2020. The measurements reveal significant daily variations over dayside low latitudes, yielding about 10-20% standard deviation in equinoxes, 20-30% in solstices, reaching 40-50% in winter. The nighttime deviations could be 30-60%, being largest in solstices. Day-to-day variations are also observed in the longitudinal wave-4 structures. The period mostly remained geomagnetically quiet except for some moderate disturbances on a few days. Tidal decomposition of the GIS electron density shows that in-situ forced migrating diurnal (DW1) terdiurnal (TW3) oscillations and the background zonal mean yield only similar to 25% of the daily variations despite accounting for almost 75-90% of the observed electron density. Thus, forcing from lower atmosphere dominates the contribution (similar to 75%) to the observed daily variations. Only about one third of this lower atmospheric forcing comes from the migrating semidiurnal SW2 and the usually investigated non-migrating diurnal eastward DE2, DE3, stationary planetary wave SPW3, SPW4, and semidiurnal eastward SE1, and SE2 components. The residual tides other than those mentioned above, including secondary waves through non-linear interactions and other planetary waves, thus significantly influence the day-to-day variations in electron density and modify the longitudinal wave structures.

This study examines an unexpected and extreme positive ionospheric response to a minor magnetic storm on August 5, 2019 by using global ionosphere specification (GIS) 3D electron density profiles obtained by assimilating radio occultation total electron content (TEC) measurements of the recently launched FORMOSAT‐7/COSMIC‐2 satellites, and ground‐based global navigation satellite system (GNSS) TEC. The results reveal ∼300% enhancement of equatorial ionization anomaly (EIA) crests, appearing over 200–300 km altitudes, and a much intense localized density enhancement over the European sector. These are the most intense ionospheric response that has ever been detected for a small magnetic storm with Dst ∼ −53 nT (SYM‐H ∼ −64 nT). The enhancements are validated by using global ionosphere map (GIM) TEC and ground‐based GNSS TEC. The GIS vertical electron density structures during the storm are examined to understand the physical processes giving rise to such an intense ionosphere response during deep solar minimum conditions when the background electron density is very low. Altitude variations and poleward shifts of the locations of the EIA crests indicate that prompt penetration electric fields (PPEF) play an important role in producing the observed positive storm responses, with the storm‐induced equatorward circulation supporting the plasma accumulation against recombination losses. Additional physical mechanisms are required to fully explain the unexpected electron density enhancements for this minor storm event.

Abstract We investigate ionospheric disturbances using the total electron content (TEC) data obtained from the radio signals emitted from three satellites after the foreshock of the 2011 Tohoku Earthquake on March 9, 2011. Co‐seismic ionospheric disturbances (CIDs) appeared to extend from an onset point concentrically in all of the satellite data. We have found, however, that the geographic coordinates of the onset points did not coincide if the observed CIDs were assumed to occur at one altitude. Admitting that the altitudes of the onset points are different, we searched for coinciding geographic coordinates of the onset points by changing the altitudes and identified the altitude of the three onset points to be 107.8, 131.8, and 133.3 km and the onset time at these altitudes. As a result, the vertical velocity of an acoustic‐gravity wave is estimated to be 448 m/s from the travel time between the altitudes of 107.8 and 131.8 km and 370 m/s between the altitude of 107.8 and 133.3 km. This is comparable to the speed of sound calculated using the empirical model NRLMSISE‐00. The present study provides a method of determining the altitude and geographic coordinates of the onset point from the ionospheric TEC analysis independently of the seismic data. Furthermore, the source location of the acoustic‐gravity wave is simultaneously determined because its wavefront propagating vertically arrives earliest at an altitude that TEC disturbance is detectable.

A total solar eclipse passed over the continental United States from the west to the east coast on 21 August 2017. Measurements made by more than 2200 groundbased GNSS (Global Navigation Satellite System) receivers observed a significant decrease in ionospheric total electron content (TEC). Meanwhile, radio occultation soundings from the LEMUR2-LYNSEY-SYMO satellite record a double-peaked feature in the scanned TEC profile. A reproduction of the double-peaked feature on the TEC profile shows that the solar eclipse depresses the electron density and simultaneously tilts the ionosphere. This study, in turn, indicates the combination of the International Reference Ionosphere model and the GNSS TEC is a powerful tool for observing ionospheric space weather.

The ionospheric radio occultation (RO) inversion is a powerful tool in retrieving the global electron density profiles (EDPs) remotely by using the time delay of the signals received by Low Earth Orbit (LEO) satellites from the GPS and other GNSS satellites based on the spherical symmetry assumptions and the coplanar approximation. However, these assumptions may cause the inaccuracy in the electron density retrieval. In this study, for the first time, we present an ionospheric electron density comparison between the estimated topmost electron density profiles from the FORMOSAT-7/COSMIC-2 (F7/C2) RO and the co-located in-situ ion densities obtained from the Ion Velocity Meter (IVM) onboard the F7/C2 satellites and then further quantitatively evaluate the impacts of the abovementioned Abel inversion assumptions on the topside ionospheric electron density. Results showed the RO top-most electron density is overall in good agreement with the IVM in-situ ion density but is slightly underestimation. Furthermore, the dihedral angle of the LEO and the occultation plane is also highlighted the importance of the coplanar approximation in the Abel inversion.

This study investigates the medium-scale traveling ionospheric disturbances (MSTIDs) statistically at the low-latitude equatorial ionization anomaly (EIA) region in the northern hemisphere. We apply the automatic detection algorithm including the three-dimensional fast Fourier transform (3-D FFT) and support vector machine (SVM) on total electron content (TEC) observations, derived from a network of ground-based global navigation satellite system (GNSS) receivers in Taiwan (14.5° N geomagnetic latitude; 32.5° inclination), to identify MSTID from other waves or irregularity features. The obtained results are analyzed statistically to examine the behavior of low-latitude MSTIDs. Statistical results indicate the following characteristics. First, the southward (equatorward) MSTIDs are observed almost every day during 0800–2100 LT in Spring and Winter. At midnight, southward MSTIDs are more discernible in Summer and majority of them are propagating from Japan to Taiwan. Second, northward (poleward) MSTIDs are more frequently detected during 1200–2100 LT in Spring and Summer with the secondary peak of occurrence between day of year (DOY) 100–140 during 0000–0300 LT. The characteristics of the MSTIDs are interpreted with additional observations from radio occultation (RO) soundings of FORMOSAT-3/COSMIC as well as modeled atmospheric waves from the high-resolution Whole Atmosphere Community Climate Model (WACCM) suggesting that the nighttime MSTIDs in Summer is likely connected to the atmospheric gravity waves (AGWs).

A set of Observing System Simulation Experiments (OSSEs) are performed to assess the impact of assimilating Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron density profiles and ground-based Global Navigation Satellite System (GNSS) total electron content (TEC) observations in a whole atmosphere data assimilation system. The OSSEs are performed using the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) with data assimilation provided by the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Results from the OSSEs demonstrate that the assimilation of ionosphere observations improves the short-term (1hr) forecasts and analyses. The OSSEs show that the short-term forecasts and analyses are further improved when the ionosphere observations adjust the thermosphere neutral composition and temperature in addition to the ionosphere electron density. Based on an initialized forecast experiment, we find that adjusting the thermosphere neutral composition and temperature also leads to improved forecast skill in the ionosphere on longer time scales (i.e., beyond 1hr). Additionally, it is shown that using a 1 hr data assimilation cycle, and removal of second-order divergence damping in WACCMX+DART significantly improves tidal amplitudes, which were previously found to be too small. These initial results represent the first whole atmosphere data assimilation system with capabilities to assimilate observations from the troposphere to the ionosphere-thermosphere.

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.

Here, we introduce a low-cost airglow imaging system developed for observing plasma bubble signatures in 630.0-nm airglow emission from the F region of the ionosphere. The system is composed of a small camera, optical filter, and fish-eye lens, and is operated using free software that automatically records video from the camera. A pilot system was deployed in Ishigaki Island in the southern part of Japan (Lat 24.4, Lon 124.4, Mlat 19.6) and was operated for ~ 1.5 years from 2014 to 2016 corresponding to the recent solar maximum period. The pilot observations demonstrated that it was difficult to identify the plasma bubble signature in the raw image captured every 4 s. However, the quality of the image could be improved by reducing the random noise of instrumental origin through an integration of 30 consecutive raw images obtained in 2 min and further by subtracting the 1-h averaged background image. We compared the deviation images to those from a co-existing airglow imager of OMTIs, which is equipped with a back-illuminated cooled CCD camera with a high quantum efficiency of ~ 90%. It was confirmed that the low-cost airglow imager is capable of imaging the spatial structure of plasma bubbles, including their bifurcating traces. The results of these pilot observations in Ishigaki Island will allow us to distribute the low-cost imager in a wide area and construct a network for monitoring plasma bubbles and their space weather impacts on satellite navigation systems.