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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 HemisphereThe machine learning technique bagged tree is used to develop an intensity to NmF2 model based on GOLD and F7/C2 observationsThe machine learning model can reduce the overestimation of NmF2 influenced by the photoelectrons emanating from the opposite Hemisphere [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The conversion of airglow intensity to volume emission rate (VER) is a common method for studying the ionosphere, but the contribution of the intensity conversion process to the uncertainty in estimated electron or ion density is significant. The Abel inversion is a commonly used method for retrieving VERs from vertical profiles of airglow intensities accumulated along the rays horizontally at the tangent point, but it requires that the intensities converge to zero at their uppermost height, which is often not the case due to observational limitations. In this study, we present a method for optimizing the retrieval of VER from satellite-measured airglow intensities using the techniques of deep learning and Abel inversion. This method can be applied to fill in unobserved or discontinuous observations in airglow intensity profiles with the Chapman function, allowing them to be used with the Abel inversion to determine VERs. We validate the method using limb 135.6 nm airglow emission intensity data from the NASA Global-scale Observations of the Limb and Disk (GOLD) mission. Our training process involves using three hidden layers with varying numbers of neurons, and we compare the performance of the best-performing deep learning models to Abel-transformed results from real-time observations. The combination of Abel inversion and deep learning has the potential to optimize the process of converting intensity to VER and improve the capacity for analyzing ionospheric observations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 variationsDigisonde validations demonstrate that the GIS NmF2 has excellent performance when there are RO observations available for assimilation [ABSTRACT FROM AUTHOR]

• Plasma depletion bays (PDBs) at the F 2 -peak are for the first time observed by FORMOSAT-7/COSMIC-2. • The F 2 -peak height ascends and descends in the summer and winter hemisphere along the PDB longitude, respectively. • The summer-to-winter field-aligned plasma flow essential for the PDB formation. FORMOSAT-7/COSMIC-2 (F7/C2) satellite cluster, launched in June 2019, performs radio occultation experiments observing the vertical electron density profiles in the low-latitude ionosphere. The F7/C2 F 2 -peak electron density (N m F 2) and height (h m F 2) are used to identify and examine the behavior of the plasma depletion bay (PDB) structures in the equatorial nighttime ionosphere. The N m F 2 reveals three distinct PDBs within 60°W–15°E, 30−90°E, and 110−160°E during June solstice, while a single PDB appears within 150−60°W during December solstice. All the four PDBs' features simultaneously appear during March and September equinoxes. Asymmetries in the h m F 2 over the PDB regions suggests that the electron density increases (decreases) are related to the N m F 2 ascending (descending) in the summer (winter) hemispheres. Agreements between F7/C2 observations and HWM93 results show that the asymmetry of vertical motions driven by the neutral wind is the main causal mechanism for the longitudinal variation of PDBs. [ABSTRACT FROM AUTHOR]