The FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission was launched on 25 June 2019 with six low‐Earth‐orbit satellites and can provide thousands of daily radio occultation (RO) soundings in the ...low‐latitude and midlatitude regions. This study shows the preliminary results of space weather data products based on F7/C2 RO sounding: global ionospheric specification (GIS) electron density and Ne‐aided Abel and Abel electron density profiles. GIS is the ionospheric data assimilation product based on the Gauss‐Markov Kalman filter, assimilating the ground‐based Global Positioning System and space‐based F7/C2 RO slant total electron content, providing continuous global three‐dimensional electron density distribution. The Ne‐aided Abel inversion implements four‐dimensional climatological electron density constructed from previous RO observations, which has the advantage of providing altitudinal information on the horizontal gradient to reduce the retrieval error due to the spherical symmetry assumption of the Abel inversion. The comparisons show that climatological structures are consistent with each other above 300 km altitude. Both the Abel electron density profiles and GIS detect electron density variations during a minor geomagnetic storm that occurred within the study period. Moreover, GIS is further capable of reconstructing the variation of equatorial ionization anomaly crests. Detailed validations of all the three products are carried out using manually scaled digisonde NmF2 (hmF2), yielding correlation coefficients of 0.885 (0.885) for both Abel inversions and 0.903 (0.862) for GIS. The results show that both GIS and Ne‐aided Abel are reliable products in studying ionosphere climatology, with the additional advantage of GIS for space weather research and day‐to‐day variations.
Plain Language Summary
This study presents two ionosphere products from the innovative satellite constellation mission launched recently. Global ionospheric specification is an ionospheric data product that assimilates ground‐based Global Positioning System and FORMOSAT‐7/COSMIC‐2 radio occultation observation of total electron content, to generate hourly global three‐dimensional electron density for monitoring space weather condition. Ne‐aided Abel electron density profile is an improved retrieval product of FORMOSAT‐7/COSMIC‐2 radio occultation observations by imposing asymmetry information of ionosphere to mitigate the error introduced by the assumption of spherical symmetry in the Abel inversion. The comparisons and validations confirm that these two data products are reliable for the study of ionosphere climatology and weather. They are operationally produced and released at Taiwan Analysis Center for COSMIC.
Key Points
All the three F7/C2 products capture similar climatological structure of ionosphere in longitudes (Wave 4) and latitudes (EIA crests)
Abel electron density profiles detect responses to geomagnetic storm, but GIS performs better in reconstructing the EIA crests variations
Digisonde validations demonstrate that the GIS NmF2 has excellent performance when there are RO observations available for assimilation
A moon shadow of the total solar eclipse swept through the continent of United States (CONUS) from west to east on 21 August 2017. Massive total electron content (integration of electron density from ...0 km to 20,200 km altitude) observations from 2,255 ground‐based Global Navigation Satellite System receivers show that the moon shadow ship generates a great ionospheric bow wave front which extends ~1,500 km away from the totality path covering the entire CONUS. The bow wave front consists of the acoustic shock wave due to the supersonic/near‐supersonic moon shadow ship and the significant plasma recombination due to the reduction in solar irradiation within the shadow area. The deep bow wave trough (−0.02 total electron content unit (1 TECU = 1016 el m−2) area) nearly coincides with the 100% obscuration moving along the totality path over the CONUS through the entire eclipse period. The supersonic moon shadow ship induces a bow wave crest in front of the ship (~80% obscuration). It is the first time to find the acoustic shock wave‐formed bow wave trough and crest near the totality.
Key Points
A 3,000 km wide ionospheric bow wave front is induced by the moon shadow ship of the 21 August 2017 total solar eclipse over CONUS
The TEC depression contributes to the wide bow wave front
The acoustic shock wave induces the bow wave crest and trough near the totality
This paper presents two empirical models, the low wind bagged trees (LWBT) and high wind bagged trees (HWBT) ensemble models to estimate ocean surface wind speed using spaceborne Global Navigation ...Satellite System Reflectometry (GNSS-R) data. The models are empirically trained using NASA's Cyclone GNSS (CYGNSS) mission level 1 data (version 2.1). The truth label for the LWBT model is the wind speed product derived from European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-5 and Global Data Assimilation System (GDAS), while the label for the HWBT model is wind speed measurements from Stepped Frequency Microwave Radiometer (SFMR). Testing results show that the LWBT and HWBT models achieved global wind speed retrieval root-mean-square-error (RMSE) of ~1.5 m/s and ~1.4 m/s, respectively, corresponding to an improvement of 29% and 65% with respect to the CYGNSS Level 2 standard wind speed product. The maximum bias is reduced by 65% and 60% for LWBT and HWBT over the Level 2 wind speeds, respectively. Two typhoon case studies are presented to corroborate the model performances and their retrieved wind speeds are consistent with reports from World Meteorological Organization (WMO) and with the measurement provided by the Huangmao Zhou (HMZ) weather station.
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.
We report the preliminary inter-satellite comparisons of the in situ ion density measurements by the ion velocity meter (IVM) onboard FORMOSAT-7/COSMIC-2 (F7/C2) and Ionospheric Connection Explorer ...(ICON) missions, during the solar minimum period of December 2019 to November 2020. The initial comparisons reveal identical diurnal, seasonal, and latitude/longitude variations in the two ion-density measurements, with F7/C2 consistently yielding stronger values than ICON, which could partly result from the difference in their orbit altitudes. The diurnal variation in the equatorial region did not show any effect of pre-reversal enhancement (PRE) during 2019–2020. The daytime plasma distributions show larger ion densities over a narrow latitudinal belt around the geomagnetic equator in all seasons, and the low-latitude densities reveal signatures of hemispherical asymmetry, with larger values occurring in the summer hemisphere. The observations also reveal distinct wavenumber-4 longitudinal modulation, which is most prominent in equinox and becomes less distinguishable during December solstice months. The simultaneous observations from F7/C2 IVM and ICON IVM also provide opportunities to study the spatial configuration and time evolution of ionospheric irregularities in the equatorial and low latitude regions. The F7/C2 and ICON simultaneously observed the equatorial plasma bubbles (EPBs) occurring around Taiwan on 18 October 2020, and the observations are consistent with each other. The EPBs were also observed by an all-sky imager located in Taiwan, comparing the satellite observations.
Graphical Abstract
Key points
F7/C2 IVM shows similar patterns of diurnal, seasonal and latitude/longitude variations of ion density to ICON IVM but with stronger magnitudes.
Distinct latitudinal and longitudinal variations of plasma distributions along seasons were observed during 2019-2020.
Simultaneous observations by the multi-satellite constellation of F7/C2 and ICON and all-sky imager provide opportunity to monitor evolutions of EPBs.
Global Ionospheric Specification (GIS) is based on the Gauss–Markov Kalman filter to assimilate the slant total electron content (TEC) observed from ground-based GPS receivers and space-based radio ...occultation instrumentations in order to reconstruct three-dimensional (3D) ionospheric electron density structure, and it can remotely sense and monitor the weather condition in space. In this study, five minutes of high temporal resolution GIS is implemented in order to reconstruct the 3D electron density structure on the 21 August 2017 total solar eclipse and analyze the variations induced by the moon’s shadow. To obtain more information of the ionosphere, from the extend 2200 GPS stations on the continental United States, are added for assimilation. The results show the ionosphere peak height (hmF2) uplift was 30–50 km altitude in latitude 25–40°N, and that the electron density depletion at higher altitudes (400 km) has a more noticeable time delay than at low altitudes (200 km), especially in low-latitude regions.
A giant ionospheric hole was simultaneously detected in the in situ measurements of FORMOSAT-7/COSMIC-2 (F7/C2), Ionospheric Connection Explorer (ICON), Swarm missions, and ground-based total ...electron content (TEC) by global navigation satellite system receivers, and F7/C2 Global Ionosphere Specification (GIS) data near Tonga, following the explosive volcano eruption on 15 January 2022. The TEC maps displayed the huge depletions that developed near Tonga after the eruption and gradually evolved. The ICON IVM, F7/C2 IVM and Swarm-LP detected large depletions not only near Tonga, but also in the EIA trough region. The GIS observations clearly show the ionospheric hole that extends spatially near Tonga, especially strongly south/southward. The simultaneous observations showed that the ionosphere hole near Tonga combined with the EIA trough and finally evolved into a giant ionosphere hole around 07 UT. The ionospheric hole, which occurred at 05 UT near Tonga, extended over a wide area of 160°-200°E and 25°S-20°N and lasted for about 11 h. The F7/C2 and ICON satellites overpasses showed large ion density depletions by the hole at orbit altitudes, accompanied by enhancements in ion temperature and field-aligned and perpendicular ion drift. Such a long-lasting giant ionospheric hole by a seismic event has not been reported earlier, creating a unique ionospheric environment near Tonga after the eruption. The strong successive impulses by multiple volcano eruptions, together with O/N
2
decrease in the summer hemisphere, interhemispheric wind, and water vapor injection into high altitudes apparently yielded such a giant ionospheric hole, 4–6 times larger than that observed during the Tohoku earthquake.
Graphical Abstract
Key points
The simultaneous measurements reveal the evolution of the ionospheric hole after the Tonga volcanic eruption.
The ionospheric hole that had occurred near Tonga merged with the EIA trough depletion in the northern hemisphere, forming a giant ionospheric hole.
The long-lasting giant ionospheric hole is caused by impulsive pressure, trans-equatorial wind, O/N2 depletion, and water vapor injection.
Using the physics-based thermosphere-ionosphere model (NCAR-TIEGCM) with an ensemble Kalman filter, this study reports the first data assimilative analysis of the ionosphere responses to the solar ...eclipse on 21 August 2017. The system, using a 2-min assimilation cycle of data from ground-based GNSS observations, show dynamic variations of the equatorial ionization anomaly (EIA) due to the electrodynamic effects of the solar eclipse. Two major ionospheric responses are captured: (1) an early appearance of EIA at the westward boundary of moon shadow and (2) an enhanced EIA at lower latitudes and suppressed EIA at the higher latitudes. These eclipse-induced conjugate EIA variations are produced by an eastward electric field perturbation around the magnetic equator and a westward electric field perturbation at the higher latitudes.
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.
Plain Language Summary
The global‐scale observations of the limb and disk (GOLD) mission can scan wide ranges of far‐ultraviolet airglow at ∼134–162 nm over the American Hemisphere, providing the intensity of the airglow radiance and related maximum electron density of the ionosphere. Meanwhile, the FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission receives GNSS radio signal to do radio occultation soundings and provides more than 4,000 of vertical electron density profiles daily. This study examines the two observations from different sounding techniques and algorithms, which generally yield good agreements, except that in the winter Hemisphere, the GOLD images experience the twilight airglow excited by conjugate photoelectrons emanating during May, June, and July. Moreover, the machine learning technique Bagged Trees is implemented to develop an intensity to peak density model training from two satellite observations, and the validation shows that the machine learning technique can detect and reduce the influence of the conjugate photoelectrons. The results indicate that the model can improve and increase the accuracy of the application of GOLD intensity for retrieving the peak electron density of the ionosphere.
Key Points
The F7/C2 NmF2 and global‐scale observations of the limb and disk (GOLD) 135.6 nm airglow peak density are consistent except in the mid‐latitude region in the winter Hemisphere
The machine learning technique bagged tree is used to develop an intensity to NmF2 model based on GOLD and F7/C2 observations
The machine learning model can reduce the overestimation of NmF2 influenced by the photoelectrons emanating from the opposite Hemisphere
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.
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