FORMOSAT-3/COSMIC Research Articles

Early detection of Tonga volcanic-eruption from internal gravity wave effects on ionosphere, using satellite geodetic techniques

The occurrence of some natural hazards in the troposphere may lead to creation of Internal Gravity Waves (IGWs). These waves transfer energy from the lower troposphere to upper layers, and to the ionosphere. When these IGWs reach the ionosphere, they create significant variations in the ionospheric parameters. Therefore, they have considerable effects on performance of Global Navigation Satellite Systems (GNSS) receivers. In this study, we used double-frequency measurements of GNSS ground-based stations from GEONET network in New Zealand to detect the IGWs created by the tsunami induced from the 2022 Tonga volcanic eruption. In addition to GNSS measurements, FORMOSAT-7/COSMIC-2 (F7/C2) data, and SWARM data were also used to study these IGWs. It is known that many of the IGWs have horizontal phase speeds faster than that of the tsunami. As the volcanic-originated IGWs spread in cone-shape pattern, it is possible to detect these fast IGWs in the ionosphere earlier than the tsunami waves, reaching the tide gauges or DART buoys. In our study, we could detect the first IGWs at the New Zealand GNSS stations, 2 h earlier than the first registration of the tsunami waves at tide gauges and DART buoys near the New Zealand peninsula, which is located approximately 1.600 km from the Tonga Volcano. It can be concluded that IGWs can be used to warn tsunamis faster than the current early-warning systems, which make use of tide gauges and DART buoys. Furthermore, the spatial variations in ionospheric electron density (IED) were investigated using F7/C2 RO data. The results show that the volcanic-originated IGWs cause reduction in the IED peak value and altitude. The results of IED derived from F7/C2 and SWARM were in good agreement.

Application of the Bagged Trees Technique on Retrieving the Nighttime Ionospheric Peak Density From OI‐135.6 nm Airglow

AbstractThe 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.

Multi-Instrument Observation of the Ionospheric Irregularities and Disturbances during the 23–24 March 2023 Geomagnetic Storm

This work investigates the ionospheric response to the March 2023 geomagnetic storm over American and Asian sectors from total electron content (TEC), rate of TEC index, ionospheric heights, Swarm plasma density, radio occultation profiles of Formosat-7/Cosmic-2 (F7/C2), Fabry-Perot interferometer driven neutral winds, and E region electric field. During the storm’s main phase, post-sunset equatorial plasma bubbles (EPBs) extend to higher latitudes in the western American longitudes, showing significant longitudinal differences in the American sector. Over the Indian longitudes, suppression of post-sunset irregularities is observed, attributed to the westward prompt penetration electric field (PPEF). At the early recovery phase, the presence of post-midnight/near-sunrise EPBs till post-sunrise hours in the American sector is associated with the disturbance of dynamo-electric fields (DDEF). Additionally, a strong consistency between F7/C2 derived amplitude scintillation (S4) ≥ 0.5 and EPB occurrences is observed. Furthermore, a strong eastward electric field induced an increase in daytime TEC beyond the equatorial ionization anomaly crest in the American region, which occurred during the storm’s main phase. Both the Asian and American sectors exhibit negative ionospheric storms and inhibition of ionospheric irregularities at the recovery phase, which is dominated by the disturbance dynamo effect due to equatorward neutral winds. A slight increase in TEC in the Asian sector during the recovery phase could be explained by the combined effect of DDEF and thermospheric composition change. Overall, storm-time ionospheric variations are controlled by the combined effects of PPEF and DDEF. This study may further contribute to understanding the ionospheric responses under the influence of storm-phase and LT-dependent electric fields.

Ionospheric observations from formation flying spacecraft

The equatorial ionosphere exhibits remarkable variability on a variety of spatial and temporal scales, and therefore observations of this region remain critical. Remote sensing from the ground offers an opportunity to get high resolution observations, but only at a few locations. Remote sensing from space offers the possibility of characterizing the region on a global-scale but can miss some of the smaller-scale features. In situ observations from space capture the smallest observable scales, but only at the location of the spacecraft. Here we utilize a time period in which ICON and FORMOSAT-7/COSMIC-2 (F7/C2) FM4 were flying in formation - maintaining approximately the same relative position of less than a few hundred km of separation for approximately 2 weeks. Using in situ observations, small-scale feature can be seen by both spacecraft, which allows for any spatial and temporal ambiguity to be resolved and spatial gradients to be calculated. Results for observations equatorial plasma bubbles yield results similar to those in other studies, with the depth of depletion reducing with apex height. A small-scale uplift of the ionosphere and corresponding ion density enhancement that appear after the pre-reversal enhancement are seen. This small-scale feature spans only 8 min of local time. While the observations available cannot definitely determine the origin of this feature, it could be associated with a polarization field that precedes the formation of an equatorial plasma bubble. Quasi-periodic oscillations in the ion density and drifts are seen on more than one day, with spatial scales along-track of ∼ 500–1000 km. Using data from both spacecraft, it is shown that these are consistent with travelling ionospheric disturbances, in one case with a horizontal wavelength of 570 km and period of 24 min. The demonstration of such a variety of features that can be examined in this way during just a 2-week period of serendipitous conjunctions between these two spacecraft highlights the potential for future missions that are planned to maintain such a configuration over a more prolonged period of time.

Giant ionospheric density hole near the 2022 Hunga-Tonga volcanic eruption: multi-point satellite observations

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/N2 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

Response of the F2-peak electron density and height to plasma depletion bays in the equatorial nighttime ionosphere observed by FORMOSAT-7/COSMIC-2

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 F2-peak electron density (NmF2) and height (hmF2) are used to identify and examine the behavior of the plasma depletion bay (PDB) structures in the equatorial nighttime ionosphere. The NmF2 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 hmF2 over the PDB regions suggests that the electron density increases (decreases) are related to the NmF2 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.

Impact of the February 3–4, 2022 geomagnetic storm on ionospheric S4 amplitude scintillation index: Observations and implications

The coincidental loss of 38 out of 49 SpaceX Starlink satellites during their launch on February 3, 2022, concurrent with two moderate geomagnetic storms, opens a unique window into the study of ionospheric irregularities and their potential impacts on Low Earth Orbit assets. This research provides evidence for the first time on the influence of Prompt Penetration Electric (PPE) fields and Disturbance Dynamo (DD) fields on the GNSS S4 amplitude scintillation indices of this particular geomagnetic storm, using observed variations in the distance of Equatorial Ionospheric Anomaly (EIA) crests and the O/N2 density ratio. By examining observations from the F7/C2 (FORMOSAT-7/COSMIC-2) mission, the NASA/GSFC’s OMNI data set, the TIMED/GUVI (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager) O/N2 density ratio, and the GIM-TEC (Global Ionosphere Map of Total Electron Content) data, the study uncovers the complex dynamics of storm-induced irregularities and their correlation with suppressed S4 at low latitudes. It reveals the roles of PPE and DD in augmenting and mitigating S4 occurrences, respectively, during different storm phases. These findings contribute to enhancing the understanding of irregularity occurrence rates, scintillation effects, and geomagnetic storms across various longitudinal sectors, thereby providing a case study of changes to the scintillation environment during this moderate but high-profile geomagnetic event.

Investigation of the aberrant record from bus GPS receiver onboard FORMOSAT-7/COSMIC-2 satellite constellation in low Earth orbit

The FORMOSAT-7/COSMIC-2 (F7/C2) satellite constellation, a joint Taiwan-US collaboration mission, was launched on 25 June 2019. Following the FORMOSAT-3/COSMIC, the F7/C2 mission aims to provide global users the next-generation global navigation satellite system (GNSS) radio occultation data. The mission consists of six identical small satellites with an inclination angle of 24°, launched to the parking orbit of 720 km, and then transferred to the mission orbit of 550 km. Each satellite bus is equipped with two commercial off-the-shelf global positioning system receivers manufactured by Surrey Satellite Technology Ltd. for positioning, navigation, and timing functions. This receiver generates “GPS Receiver (bus GPSR) Warm Start Requested (WSR)” messages after commissioning, which could be considered as a kind of single event effect (SEE). More than 7000 WSR events have been recorded in 3 years after launch, providing another opportunity to investigate the SEE with the commercial off-the-shelf bus GPSR. In addition, the F7/C2 mission is the first satellite constellation that allows scientists to investigate the SEE using the same constellation at different altitudes of Low-Earth orbit. This study investigates and discusses the cumulative rate, distribution of WSRs, and their relationships between altitude and geomagnetic field. The statistical method is applied to define a warning area of the high probability occurring the SEE of this kind of component based on the geomagnetic field, whose corresponding strength is around 21,200/20,550 nT and close to the South Atlantic Anomaly area. Moreover, the relationships with space weather, including the disturbance of the Earth's magnetic field (Kp index) and the solar activity (F10.7 index), are also analyzed and reported.

Nighttime wavenumber-four and plasma depletion bays observed by FORMOSAT-5/AIP, ICON/IVM, and COSMIC-2/RO data

We examine nighttime longitudinal structures of ion density and ion velocity in wavenumber-four (WN4) and plasma depletion bays (PDBs) observed by FORMOSAT-5 (F5) and ICON, as well as electron density by FORMOSAT-7/COSMIC-2 (F7C2) in 2020. Three PDBs significantly occur in June and December solstices, while WN4 prominently appear in March and September equinox. F5/AIP (advanced ionospheric probe) ion velocities tend to be westward (eastward) at the PDB (WN4) longitudes, which shows that the ion density maximum and minimum are coincident with the eastward ion velocity maximum and minimum, respectively. ICON/IVM (ion drift meter) field-aligned velocities further reveal that the summer to winter trans-equatorial flow is essential to form PDBs, which further results in upward and downward ion drifts in the upstream and downstream areas (summer and winter hemisphere), respectively. Furthermore, F7C2 radio occultation electron density profiles reveal that three PDBs could prominently appear in June and December solstice in the upper ionosphere. F5/AIP and ICON/IVM ion velocities show that the zonal plasma drift is essential to form nighttime WN4 and PDBs, and the summer to winter trans-equatorial flow is important to PDBs.

Understanding the impact of assimilating FORMOSAT‐7/COSMIC‐2 radio occultation refractivity on tropical cyclone genesis: Observing system simulation experiments using Hurricane Gordon (2006) as a case study

AbstractStudies have shown that assimilating the radio occultation (RO) observations, including those from the FORMOSAT‐3/COSMIC (constellation observing systems for meteorology, ionosphere, and climate) (FS3‐C), provides positive impacts on tropical cyclone (TC) forecasts. The FS3‐C's successor, the FORMOSAT‐7/COSMIC‐2 (FS7‐C2), provides denser spatial data coverage over the Tropics and Subtropics, where severe weather systems often occur. This study investigates the impact of FS7‐C2 refractivity profiles on the prediction of TC genesis. A quick observing system simulation experiment is conducted for the period when Hurricanes Helene and Gordon (2006) occurred over the North Atlantic Ocean using a regional ensemble data assimilation system. Though assimilating FS3‐C or FS7‐C2 ROs successfully reproduces Helene's development, assimilating FS7‐C2 ROs better captures the genesis and development of Gordon with abundant moisture and vorticity in Gordon's core region, providing conditions favorable for the development of deep convection. A minimum area‐mean total precipitable water vapor of 54 mm, as well as the existence of mid‐level cyclonic vorticity (e.g., 500 hPa), at the storm core region in the initial condition is required for forecasting Gordon's genesis. Also, the assimilation of FS7‐C2 ROs in our experiments reduces the 500 hPa geopotential error by 22% and improves probabilistic quantitative precipitation forecast compared with assimilating FS3‐C ROs. Two sensitivity tests are conducted to evaluate the impact of low‐level negatively biased FS7‐C2 RO profiles and the removal of FS7‐C2 data below 3 km on Gordon's genesis. The former test does not degrade Gordon's genesis forecast skills due to a dipole error correlation between the background ROs and the moisture field over an observed RO profile near Gordon. The latter test does degrade Gordon's forecast skills but is still better than the assimilation of FS3‐C ROs since the features of low‐level moisture and mid‐level vorticity are preserved to some extent.

Comparisons of in situ ionospheric density using ion velocity meters onboard FORMOSAT-7/COSMIC-2 and ICON missions

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

Impact of Assimilating FORMOSAT-7/COSMIC-2 Radio Occultation Data on Typhoon Prediction Using a Regional Model

As the successor of FORMOSAT-3/COSMIC (FS3/C1), FORMOSAT-7/COSMIC-2 (FS7/C2) was successfully launched on 25 June 2019. FS3 radio occultation (RO) data has contributed greatly to Taiwan’s meteorological progress, improving model representations of marine boundary layer heights, cyclogenesis, tropical cyclones/typhoons, and Mei-Yu front systems. The operational CWBWRF numerical weather prediction model with the 3DEnVar data assimilating system in the Taiwan Central Weather Bureau (CWB) was adopted to evaluate the impact of assimilating FS7 RO data. The following two experiments were conducted: one assimilated the in-situ observations as in the CWB operational task (nRO), and the other additionally assimilated FS7 RO refractivity profiles (wRO). Both experiments utilized 6-h assimilating window and full cycle data assimilation strategy and made 120-h forecasts after each assimilation. Within over 70 synoptic verification cases, the biases of geopotential height, temperature, and wind were reduced in the upper model levels in wRO results, and the typhoon track and intensity prediction error reductions were statistically significant. In addition, the wRO experiment improved the typhoon structure in the initial conditions and led to a better typhoon structure forecast. These results showed that the FS7 RO refractivity assimilation could improve model forecast performance, leading to its operational use in the CWB.

Limb Sounders Tracking Tsunami-Induced Perturbations from the Stratosphere to the Ionosphere

In this study, we employ three types of satellite data from two different limb sounders: the FORMOSAT-3/COSMIC (F3/C) radio occultation (RO) technique and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument to study the vertical coupling of the 16-09-2015 Chile tsunami-induced perturbations from the stratosphere to the ionosphere. All three types of datasets, including temperature profiles from 10 to 55 km and 16 to 107 km, and electron density profiles from 120 to 550 km, recognized perturbations of different scales at different heights after the Chile tsunami. The vertical scales identified by the wavelet analysis are from 1–2 km, 5–9 km, and 25–50 km in the stratosphere, mesosphere, and ionosphere, respectively. Meanwhile, as a comparison and validation of the reliability, we also revisited the 11-03-2011 Tohoku earthquake/tsunami-related perturbations from the stratosphere to the ionosphere using the same data. It is believed that the two tsunamis both disturbed the whole atmosphere space, and the scale of these signals gradually increases with the increase in altitude but decreases with time. In addition, the tsunami-related ionospheric gravity wavefronts are examined by the F3/C observations. Another interesting point is that the temperature perturbations recorded by the SABER from 70–100 km altitude are found to arrive earlier than the 2015 tsunami wavefront. The findings in this study suggest that the limb-sounding technique is a useful instrument for detecting the tsunami-coupling gravity wave and benefits the tsunami warning system.

Detection of Vertical Changes in the Ionospheric Electron Density Structures by the Radio Occultation Technique Onboard the FORMOSAT-7/COSMIC2 Mission over the Eruption of the Tonga Underwater Volcano on 15 January 2022

A large near-surface perturbation such as the eruption of the Tonga underwater volcano on 15 January 2022 can generate disturbances in the Earth’s atmosphere and ionosphere. It is quite challenging to detect and investigate the disturbances in the vertical direction due to the lack of ground-based instruments, especially in the ocean area. To examine the vertical disturbances due to the Tonga eruption, this study utilizes the radio occultation (RO) technique onboard the satellites of the FORMOSAT-7/COSMIC2 (F7/C2) mission to sound the ionospheric electron density (Ne) profiles in the Central Pacific Ocean area around the eruption. The ionospheric Ne profiles show that the eruption almost annihilated the typical Chapman-layer structure over the eruption in the nighttime on 15 January. The Hilbert–Huang transform was applied to expose the vertical changes in the Ne structures as functions of wavelength and altitude. The analysis shows not only the occurrence of the small-scale disturbances with a wavelength of ~20 km from 100 km to 500 km altitudes, but also the significant attenuation of the structures with a wavelength >50 km, which has never been reported before. The time series of the total electron content from the ground-based Global Navigation Satellite System receiver near the eruption also verifies the significant long-lasting disturbances due to the eruption.

Extreme Poleward Expanding Super Plasma Bubbles Over Asia‐Pacific Region Triggered by Tonga Volcano Eruption During the Recovery‐Phase of Geomagnetic Storm

AbstractThe Tonga volcano eruption of 15 January 2022 unleashed a variety of atmospheric perturbations, coinciding with the recovery‐phase of a geomagnetic storm. The ensuing thermospheric variations created rare display of extreme poleward‐expanding conjugate plasma bubbles seen in the rate of total electron content index over 100–150°E, reaching ∼40°N geographic latitude. This is associated with fluctuations in FORMOSAT‐7/COSMIC‐2 (F7/C2) ion‐density measurements and spread‐F in ionograms. Preceding to this, an unusually strong pre‐reversal enhancement (PRE) occurred in the global ionospheric specification (GIS) electron density profiles derived from F7/C2 observations. The GIS also revealed a decrease of equatorial ionization anomaly (EIA) crest density due to the storm impact. Reduced E‐region conductivity by volcano‐induced waves and enhanced F‐region wind, further accelerated by reduced ion‐drag over the EIA, apparently intensified the PRE. Accompanied with the strong PRE, volcano‐induced seed perturbations triggered the super plasma bubble activity.

Introduction of TROPS ionospheric TEC products for FORMOSAT-7/COSMIC-2 mission

Six low Earth orbit (LEO) satellites were launched on June 25th, 2019 for a radio occultation (RO) mission for the FORMOSAT-7/COSMIC-2 (F7/C2) program. The GPS and GLONASS RO signals received by these F7/C2 satellites can be used to retrieve atmospheric and ionospheric parameter profiles for atmospheric and ionospheric research. In order to process the received RO signal, the processing system named Taiwan Radio Occultation Processing System (TROPS) is built. TROPS is developed by National Space Organization, Taiwan Analysis Center for COSMIC, and GPS Science and Application Research Center in Taiwan. The ionospheric products of TROPS are electron density profile, ionospheric scintillation index (S4 index), and absolute total electron content (TEC). S4 index has been calculated on board the satellites and other two products are retrieved by TROPS after the observation data downlink to ground. TEC is the linear integration of electron density along the signal propagation path. The electron density profile is retrieved from the relative TEC when the elevation angle of GNSS satellite is negative from F7/C2 satellite. The absolute TEC is the TEC from GNSS satellite to F7/C2 satellite. The difference between absolute and relative TEC is the TEC with/without differential code bias (DCB) correction. Currently, the data for the electron density profile and absolute TEC are provided by TROPS. Users can obtain the products freely from the internet. In this study, the retrieval method and the preliminary F7/C2 ionospheric TEC products retrieved by TROPS are presented in detail.

Retrospect and prospect of ionospheric weather observed by FORMOSAT-3/COSMIC and FORMOSAT-7/COSMIC-2

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.

Comparison of bottomside ionospheric profile parameters (B0 and B1) extracted from FORMOSAT-7/COSMIC-2 GNSS Radio occultations with Digisondes and IRI-2016 model

The bottomside thickness (B0) and shape (B1) are two important parameters considered in the bottomside electron density profile (EDP) specification of the International Reference Ionosphere (IRI) model that is the most widely accepted global empirical ionospheric model. In the present study, we investigated B0 and B1 as derived from least-square fitting of FORMOSAT-7/COSMIC-2 (F7/C2) radio occultation (RO) profiles by employing certain quality constraints in accordance to coincident Digisonde soundings. We performed a comparative analysis of the local time diurnal, seasonal and latitudinal behaviour of B0 and B1 parameters from a huge database of F7/C2 profiles accumulated during the last two years (2020 and 2021) with estimations from the IRI-2016 submodels: Bil-2000, Gul-1987, and ABT-2009, referring to the bottomside profile characteristics during the low solar activity period of 25th solar cycle. The results reveal daytime underestimation and nighttime overestimation features of IRI submodels. The typical pre-sunrise enhancement in B0 is clearly discernible in the F7/C2 observations which could not be reproduced in any of the IRI submodels. Moreover, the equatorial bottomside thickness anomaly (EBTA) presenting hemispheric asymmetry is not evidenced in the observed as well as modelled parameters. The sharp changeover in B0 magnitude from summer to winter solstice hemisphere and spring to fall equinox hemisphere in Gul-1987 indicates its inability to accurately represent the B0 variation at the equatorial and low latitude sector. Despite the fact that ABT-2009 performs relatively better than Gul-1987 sub-model based on F7/C2 and Digisonde datasets in different seasons, we demonstrate that there is room for possible improvement in terms of B0 and B1 representation.

Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations

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.

Instantaneous amplitude of low-latitude ionospheric irregularities probed by ROCSAT-1, DEMETER, and FORMOSAT-7/COSMIC-2

The occurrence probability of plasma irregularities has long been used to quantify that of ionospheric scintillations. We use the Hilbert-Huang transform to examine the occurrence probability and the instantaneous total amplitude of plasma irregularities in the low-latitude nighttime ionosphere observed by the low-inclination satellites, ROCSAT-1 during 1999–2004 and FORMOSAT-7/COSMIC-2 (F7C2) in 2020, as well as a high-inclination satellite, DEMETER, during 2006–2010. Generally, the longitudinal distribution of the occurrence probability is similar to that of the total amplitude, except that the probability of ion density irregularity observed by F7C2 and DEMETER yields nonphysical anomalous enhancements over the South American sector during May–August of 2007–2010 and 2020, respectively. S4 scintillations observed by F7C2 show that the anomalous enhancement results from the low ambient density in calculating the occurrence probability during the low solar activity year of 2020. We suggest the instantaneous total amplitude as a better indicator for describing ionospheric plasma irregularities.