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An earthquake of magnitude 9.0 occurred near the east coast of Honshu (Tohoku area), Japan, producing overwhelming Earth surface motions and inducing devastating tsunamis, which then traveled into the ionosphere and significantly disturbed the electron density within it (hereafter referred to as seismotraveling ionospheric disturbances (STIDs)). The total electron content (TEC) derived from nationwide GPS receiving networks in Japan and Taiwan is employed to monitor STIDs triggered by seismic and tsunami waves of the Tohoku earthquake. The STIDs first appear as a disk‐shaped TEC increase about 7 min after the earthquake occurrence centered at about 200 km east of the epicenter, near the west edge of the Japan Trench. Fast propagating disturbances related to Rayleigh waves quickly travel away from the epicenter along the main island of Japan with a speed of 2.3–3.3 km/s, accompanied by sequences of concentric circular TEC wavefronts and followed by circular ripples (close to a tsunami speed of about 720–800 km/h) that travel away from the STID center. These are the most remarkable STIDs ever observed where signatures of Rayleigh waves, tsunami waves, etc., simultaneously appear in the ionosphere. Key Points Ionospheric disturbances generated by earthquake and tsunami Greatest disturbances ever seen containing signatures of following waves Rayleigh, acoustic, and tsunami‐generated waves

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. Key Points FORMOSAT-3/COSMIC and FORMOSAT-7/COSMIC-2 uniformly observe 3D electron density. FORMOSAT-3 and FORMOSAT-7 enable ionospheric weather forecasting. FORMOSAT-7/COSMIC-2 TGRS and IVM have a better understanding of the electrodynamics of ionospheric plasma.

Effects of rapidly changing ionospheric weather are critical in high accuracy positioning, navigation, and communication applications. A system used to construct the global total electron content (TEC) distribution for monitoring the ionospheric weather in near-real time is needed in the modern society. Here we build the TEC map named Taiwan Ionosphere Group for Education and Research (TIGER) Global Ionospheric Map (GIM) from observations of ground-based GNSS receivers and space-based FORMOSAT-3/COSMIC (F3/C) GPS radio occultation observations using the spherical harmonic expansion and Kalman filter update formula. The TIGER GIM (TGIM) will be published in near-real time of 4-h delay with a spatial resolution of 2.5° in latitude and 5° in longitude and a high temporal resolution of every 5 min. The F3/C TEC results in an improvement on the GIM of about 15.5%, especially over the ocean areas. The TGIM highly correlates with the GIMs published by other international organizations. Therefore, the routinely published TGIM in near-real time is not only for communication, positioning, and navigation applications but also for monitoring and scientific study of ionospheric weathers, such as magnetic storms and seismo-ionospheric anomalies.

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 topmost 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 paper uses the tropospheric pressure, temperature, and water vapor pressure observed by six micro-satellites of the FORMOSAT-3/COSMIC (F3/C) constellation to study El Niño-Southern Oscillation (ENSO) events from July 2006 to January 2012. The temperature and pressure profiles are used to derive the height, pressure, temperature, and potential temperature of the lapse rate tropopause. Following the calculation of the standard normalized Southern Oscillation Index (SOI) and the Niño 3.4 index, the corresponding indices of the four tropopause parameters are derived. Good agreements between the standard and F3/C-derived indices show that the derived indices are significant for monitoring ENSO signatures. With the uniform coverage, the global F3/C profiles are used to construct three-dimensional structures of temperature, pressure, and water vapor pressure to gain a better understanding of tropospheric structures and dynamics during the ENSO events.

A network of 6 ground-based GPS receivers in East Asia was employed to study seismo-traveling ionospheric disturbances (STIDs) triggered by an M 8.0 earthquake which occurred at Wenchuan on 12 May 2008. The network detected 5 STIDs on the south side of the epicenter area. A study on the distances of the detected STIDs to the epicenter versus their associated traveling times shows that the horizontal speed is about 600 m s-1. Applying the circle method, we find that the 5 circles intercept at a point right above the epicenter when the horizontal speed of 600 m s-1 is given. Global searches of the ray-tracing and the beam-forming techniques confirm that the STIDs are induced by vertical motions in the Earth's surface during the Wenchuan Earthquake.

The ionospheric radiance and electron density observed by the tiny ionospheric photometer (TIP) and GPS occultation experiment (GOX) payloads on FORMOSAT-3/COSMIC satellites are applied to determine the boundaries of the auroral oval and its width in the winter nighttime ionosphere for both hemispheres. The TIP collects ionospheric emission at 135.6 nm due to electron impact excitation, while the GOX offers ionospheric electron density profiles with radio occultation (RO) technique. Comparison between them shows similar patterns of the plasma structure in the polar caps. The mean width of the auroral bands ranges between about 2 and 11° latitude in the winter nighttime and it varies with longitudes. The comparison by month suggests that the mean radius of the auroral ovals varies with the intensity of the auroral radiance.

The tiny ionospheric photometer (TIP) and GPS occultation experiment (GOX) onboard FORMOSAT-3/COSMIC (F3/C) are employed to measure the OI 135.6 nm intensities in the nadir direction and the total electron content (TEC) between the F3/C and GPS satellite in the ionosphere, respectively. Due to its very high sensitivity ~600 counts/Rayleigh and rather narrow nadir pointing 3.8° ncircular field-of-view, the TIP provides accurate characterization of ionospheric electron density gradients in the horizontal direction. Meanwhile, a technique of the low earth orbit (LEO) tomography is applied to analyze the GOX data obtaining the 3D distribution of ionosphere electron density. Here, we combine the two observations to carry out the LEO-TIP tomographic inversions, and demonstrate that the peak electron density (NmF2) retrieved from the TIP combined together with the peak altitude (hmF2) information from the LEO tomography profiles provides more realistic electron density.

A new era of studying the ionospheric space weather effects has come after launch of the innovative satellite constellation, named as Formosa Satellite 3 or Constellation Observing System for Meteorology, Ionosphere, and Climate (abbreviated as FORMOSAT-3/COSMIC or F3/C in short), performing a radio occultation experiment capable of observing the global ionosphere three-dimensionally. This is the first time that a satellite constellation provides instantaneously both the lower and upper parts of the ionospheric electron density up to the satellite altitude. With more than 2500 soundings of the ionospheric vertical electron density profiles every day, ionospheric plasma structures over many continents and most of oceans, where ground-based observation is limited, are now observed continuously. Important ionospheric research topics, such as space weather effects to the ionosphere, variations of ionospheric plasma structure and dynamics produced by solar outputs, and atmosphere-ionosphere coupling processes, can be widely studied and modeled based on the three-dimensional ionospheric images constructed by the F3/C observations. After one year in orbit, a great amount of radio occultation soundings allow us to construct global ionospheric maps to study the ionospheric seasonal effects and atmosphere-ionosphere interactions. Taking advantage of the uniqueness of dense global coverage, the major physical mechanisms of the two studies are given. For study of the seasonal variation during solstice, electron density images of the mid- and low-latitude ionosphere show a clear north-to-south asymmetry which may be affected by the summer-to-winter neutral wind. Meanwhile a significant longitudinal variation at midnight is also seen in the solstitial season. Another interesting result is the four stronger equatorial ionization anomaly (EIA) regions located at different longitudes. This four-peaked EIA structure may result from upward propagating nonmigrating tides originated from troposphere. F3/C's observation of the daytime four-peaked structure provides an important evidence to support the proposed forming mechanism.

The global positioning system (GPS) can be used to monitor the seismic perturbation induced by the 2011 off the Pacific coast of Tohoku Earthquake (magnitude 9.0), Japan, on March 11, 2011, and to trace the tsunami across the Pacific Ocean by measuring the changes in the ionospheric total electron content (TEC). We estimate the vertical and horizontal mean speeds of the seismic and tsunami waves using the time and distance of the TEC perturbation, and then, taking into account those determined speeds, trace back to the epicenter and the tsunami origin by applying a 3-dimensional spherical model. The results show that both the tracked epicenter and the tsunami origin are quite close to the epicenter reported by the USGS, with a mean horizontal propagation speed of 2.3 km/s after the earthquake and about 210 m/s after the tsunami. This consistency confirms that the perturbation sources in the ionosphere are due to the earthquake. This implies that the GPS-TEC measurements have the potential to be part of a lower cost, ground-based, tsunami monitoring system.