FORMOSAT-3/COSMIC Research Articles

Temperature response to the June 2020 solar eclipse observed by FORMOSAT-7/COSMIC2 in the Tibet sector

This study explores the response of atmospheric temperature to the annular solar eclipse at the summer solstice on 21 June 2020. The radio occultation (RO) technique of the FORMOSAT-7/COSMIC2 (F7/C2) mission observes the temperature in the troposphere and stratosphere. The RO observations show that the temperature decreases significantly (near 4 to 8 °C) between 5 and 8 km altitudes over the Tibetan Plateau area within the 80% obscuration during the eclipse. The tropopause temperature increases by ~ 2 to 5 °C over the same area. By contrast, the tropopause temperature decreases by ~ 4° to 5 °C over the Indian Ocean. The F7/C2 RO technique captured not only the sudden tropospheric cooling and stratospheric warming over Tibet during the eclipse but also the possible response over the Indian Ocean away from the greatest eclipse.

Impact of Radio Occultation Data on the Prediction of Typhoon Haishen (2020) with WRFDA Hybrid Assimilation

FORMOSAT-7/COSMIC-2 (FS7/C2) satellite was successfully launched in June 2019. The satellite provides about 5000 radio occultation (RO) soundings daily over the tropical and partial subtropical regions. Such plentiful RO soundings with high accuracy and vertical resolution could be used to improve model initial analysis for typhoon prediction. In this study, assimilation experiments with and without the RO data were conducted with the WRFDA hybrid system for the prediction of Typhoon Haishen (2020). The experimental results show a remarkable improvement in typhoon track prediction with RO data assimilation, especially when using a nonlocal refractivity operator. Results in cycling DA and forecast are analyzed and verified for the RO data impact. Diagnostics of potential vorticity (PV) tendency budget helps explain the typhoon translation induced by different physical processes in the budget. The typhoon translation is essentially dominated by horizontal PV advection, but the track deviation can increase due to the vertical PV advection with opposite effects in the absence of RO data. Sensitivity experiments for different model initial times, physics schemes, and RO observation amounts show positive RO data impacts on typhoon prediction, mainly contributed from FS7. Complementary, an improved forecast of Typhoon Hagupit (2020) is also illustrated for the RO data impact.

Near Real‐Time Global Plasma Irregularity Monitoring by FORMOSAT‐7/COSMIC‐2

AbstractThis study presents initial results of the ionospheric scintillation in the F layer using the S4 index derived from the radio occultation experiment (RO‐S4) on FORMOSAT‐7/COSMIC‐2 (F7/C2). With the sufficiently dense RO‐S4 observations at low latitudes, it is possible to construct hourly, global scintillation maps to monitor equatorial plasma bubbles (EPBs). The preliminary F7/C2 RO‐S4 during August 2019 to April 2020 show clear scintillation distributions around American and the Atlantic Ocean longitudes. The RO‐S4 near Jicamarca are compared with range‐time‐intensity (RTI) maps of the 50 MHz radar, and the results show that the occurrence of intense RO‐S4 in the range 0.125–0.5 are co‐located with the bottomside of the spread‐F patterns. Increases in RO‐S4 at the upward phase of bottom‐side oscillations is theoretically consistent with large‐scale wave seeding of the EPBs. The locations and occurrences of the RO‐S4 greater than 0.5 are consistent with airglows depletions from the NASA GOLD mission. Climatology analyses show that monthly occurrences of RO‐S4 > 0.5 agree well with the monthly EPB occurrences in GOLD 135.6 nm image, and show a similar longitudinal distribution to that of DMSP and C/NOFS in‐situ measurements. The results suggest that the RO‐S4 intensities can be utilized to identify EPBs of specific scales.

Ionospheric total electron content response to annular solar eclipse on June 21, 2020

The effects of physical events on the ionosphere structure is an important field of study, especially for navigation and radio communication. The paper presents the spatio-temporal ionospheric TEC response to the recent annular solar eclipse on June 21, 2020, which spans across two continents, Africa and Asia, and 14 countries. This eclipse took place on the same day as the June Solstice. The Global Navigation Satellite System (GNSS) based TEC data of the Global Ionosphere Maps (GIMs), 9 International GNSS Service (IGS) stations and FORMOSAT-7/COSMIC-2 (F7/C2) were utilized to analyze TEC response during the eclipse. The phases of the TEC time series were determined by taking the difference of the observed TEC values on eclipse day from the previous 5-day median TEC values. The results showed clear depletions in the TEC time series on June 21. These decreases were between 1 and 9 TECU (15–60%) depending on the location of IGS stations. The depletions are relatively higher at the stations close to the path of annular eclipse than those farther away. Furthermore, a reduction of about −10 TECU in the form of an equatorial plasma bubble (EPB) was observed in GIMs at ~20° away from the equator towards northpole, between 08:00–11:00 UT where its maximum phase is located in southeast Japan. Additionally, an overall depletion of ~10% was observed in F7/C2 derived TEC at an altitude of 240 km (hmF2) in all regions affected by the solar eclipse, whereas, significant TEC fluctuations between the altitudes of 100 km − 140 km were analyzed using the Savitzky-Golay smoothing filter. To prove TEC depletions are not caused by space weather, the variation of the sunspot number (SSN), solar wind (VSW), disturbance storm-time (Dst), and Kp indices were investigated from 16th to 22nd June. The quiet space weather before and during the solar eclipse proved that the observed depletions in the TEC time series and profiles were caused by the annular solar eclipse.

Observation and Simulation of the Development of Equatorial Plasma Bubbles: Post‐Sunset Rise or Upwelling Growth?

AbstractThis study investigates the underlying physics of equatorial plasma bubbles (EPBs) on 11 December 2019, under solar minimum conditions. The Global‐scale Observations of the Limb and Disk (GOLD) ultraviolet nightglow images exhibit a periodic distribution of reduced emissions related to EPBs. Remarkably, FORMOSAT‐7/COSMIC‐2 (F7/C2) observes a significant altitudinal difference of ~45 km in the bottomside ionosphere between two nearly collocated electron density profiles before the onset of EPBs, indicating the presence of an upwelling. Distinct ionospheric perturbations are also observed in F7/C2 and ground‐based Global Positioning System observations, suggesting that gravity waves may contribute to the upwelling. Simulations with SAMI3/ESF are further carried out to evaluate the upwelling growth and pre‐reversal enhancement (PRE) effect on EPB development. Simulations reveal that the crests of upwellings show a localized uplift of ~50 km, and EPBs only develop from the crest of upwellings. The uplift altitude of upwellings is comparable to the F7/C2 observations and the post‐sunset rise in moderate solar conditions. The polarization electric field (Ep) developed within the upwellings can drive vertical Ep × B drifts of ~32–35 m/s, which are comparable to the PRE vertical E × B drifts. We find that the PRE alone cannot drive EPBs without upwelling growth, but it can facilitate the upwelling growth. Observations and simulations allow us to conclude that upwelling growth could play a vital role in the formation of EPBs.

The Early Results and Validation of FORMOSAT‐7/COSMIC‐2 Space Weather Products: Global Ionospheric Specification and Ne‐Aided Abel Electron Density Profile

AbstractThe 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.

Plasma Depletion Bays in the Equatorial Ionosphere Observed by FORMOSAT‐3/COSMIC During 2007–2014

AbstractA new feature of plasma depletion bay (PDB) on the longitudinal structure over the equatorial and low latitudes is observed by the FORMOSAT‐3/COSMIC (F3/C) electron density profiles. The existence of the PDB feature is confirmed by the OI 135.6 nm radiance from TIMED/GUVI, which together with F3/C electron density shows that one North PDB extending to the Southern Hemisphere prominently appears over Southwest America while three South PDBs extending to the Northern Hemisphere occur over North Atlantic, India Ocean, and Southeast Asia. Three‐dimensional F3/C ionospheric electron densities are further used to examine PDB structures at various local times, seasons, solar activities, and altitudes during 2007−2014. It is found that the north PDB is observed during October−March, while the south PDBs mostly exist during April−September. These PDBs can be observed within 250–350 km altitudes in the nighttime, appearing pronounced over 275–300 km altitudes around 2300–0100 LT, in the low solar activity year of 2007. Global effective wind patterns computed using the horizontal wind model HWM93 in solstice months suggest that summer‐to‐winter field‐aligned winds projected from zonal neutral winds play an important role.

Climatology of the Equatorial Plasma Bubbles Captured by FORMOSAT‐3/COSMIC

AbstractThe FORMOSAT‐3/COSMIC (F3/C) satellites are used to study the climatology of equatorial plasma bubbles (EPBs) during the low to moderate solar flux years (2008–2013). We use the F3/C total electron content to identify the presence of EPBs and investigate the background conditions for the initiation of EPBs. The results reveal that the EPB activities have strong solar dependence. The longitudinal and seasonal trends of EPBs are highly correlated to the angle between the dusk solar terminator and magnetic field lines near the magnetic equator. Asymmetries of EPBs between solstices and equinoxes exist and could be due partly to the asymmetry of equatorial ionization anomaly structures, which result in longitudinal differences as well. EPBs extend to higher altitudes and latitudes during the ascending phase of Solar Cycle 24 (2011–2013) due mainly to the increase of background electron density. However, an altitudinal asymmetry of EPBs occurs in moderate solar flux years, which is likely due to the suppression or lower growth and occurrence rates of EPBs. In addition to vertical drift, tidal forcing also contributes to the longitudinal and seasonal distributions of EPBs. Upwellings and precursor waves preceding the EPBs are observed climatologically, which likely play a vital role in initiating the EPBs. This study also reveals a vertical connection between the equatorial ionospheric irregularities and atmospheric forcing on a climatological basis.

Fluctuations on vertical profiles of the ionospheric electron density perturbed by the March 11, 2011 M9.0 Tohoku earthquake and tsunami

The GPS radio occultation (RO) of FORMOSAT-3/COSMIC (F3/C) with 1–2.5-km-altitude resolution is used to scan fluctuations on vertical profiles of the ionospheric electron density induced by seismic and tsunami waves of the March 11, 2011 M9.0 Tohoku earthquake. The Hilbert–Huang transform is applied to derive the instantaneous wave number and amplitude of the fluctuations in 22 (7 before and 15 after) F3/C profiles within 3000 km in radius from the epicenter during the earthquake. Fluctuations of the vertical electron density profile induced by underneath Rayleigh and tsunami waves and their post-waves are examined. It is found that the right underneath Rayleigh wave can induce significant long-wavelength fluctuations of 50–105 km on the vertical profile of the electron density, while tsunami and their post-waves activate the prominent short-wavelength fluctuations less than 16 km on those profiles in the poleward side of the epicenter. Results show that RO is a powerful tool which is suitable to probe vertical fluctuations in the ionospheric electron density.

Vertical Structure of the Ionospheric Response Following the Mw 7.9 Wenchuan Earthquake on 12 May 2008

The response of the vertical ionospheric electron density structure to the Mw 7.9 Wenchuan earthquake on 12 May 2008 was recorded by the FORMOSAT-3/COSMIC (F3/C) radio occultation (RO) precise orbit determination (POD) total electron content (TEC). The horizontal propagation characteristics of the seismo-traveling ionospheric disturbances were identified by combining ground-based Global Navigation Satellite System (GNSS) TEC data and RO TEC data. The postseismic ionospheric infrasound wavefront with wavelength of around 80 km at altitude of 150–350 km was scanned vertically by an individual RO TEC profile. A large amount of TEC profiles around the epicenter from 0000 to 1200 UT provide evidence of continental earthquake-induced vertical oscillations that lasted in the ionosphere for several hours. RO-observed vertical perturbations appear on both the southward and northward sides of the epicenter. The vertical structure of the ionospheric oscillations with wavelength ranging from 10 to 80 km increased with altitude between 230 and 650 km.

Global ionospheric modeling based on multi-GNSS, satellite altimetry, and Formosat-3/COSMIC data

Ionosphere total electron content (TEC) from global ionospheric maps (GIM) is widely applied in both ionospheric delay correction and research on space weather monitoring. Global ionospheric modeling based on multisource data is an effective method to improve conventional GIM accuracy and reliability. In this study, a global ionospheric model is constructed from multi-GNSS (here, GPS/GLONASS/BDS), satellite altimetry and Formosat-3/COSMIC (F3/C) observations using a spherical harmonic (SH) function. The results show that compared to the conventional GIM derived from GPS/GLONASS data, the combined GIM performance from multisource data improves significantly; the RMS versus external data decreases from [2, 5] to [2, 3] TECU, and the BIAS decreases from [− 3, 1] to [− 1, 1] TECU. Specifically, BDS observations improve the IPP distributions, especially over the region of Australia; compared with GPS-based ionospheric TEC. Our calculated GIM with BDS data has better performance than that without BDS data. By combining JASON 2 and GPS/GLONASS data, the residual distribution is more concentrated, and the RMS is improved effectively in mid-high latitudes of the southern hemisphere and in the equatorial region. F3/C TEC also exhibits relatively minor improvements on GIM; the standard deviation reduces from 2.89 to 1.92 TECU, and the BIAS regarding extra F3/C data decreases from − 2.02 to − 1.71 TECU.

Determination of near real-time GNSS satellite clocks for the FORMOSAT-7/COSMIC-2 satellite mission

In this study, we determine the near real-time (NRT) clocks of the Global Positioning System (GPS) and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) satellites in the Taiwan RO Process System (TROPS), which is mainly designed for the data processing in both the FORMOSAT-3/COSMIC (F3C) and FORMOSAT-7/COSMIC-2 (F7C2) satellite missions. The accuracy of GNSS clocks defines the quality of the atmospheric excess phase, which is used for the retrieval of bending angle profiles in GNSS radio occultation (RO) observations. The accuracy of the NRT GNSS clocks is assessed by comparing the clock rate, clock stability and clock-induced positioning error on receivers with the final solutions given by the European Space Agency (ESA). Overall, the standard deviations of the clock rates from TROPS agree with those from ESA within 0.05 mm/s over 2304 clock solutions. Additionally, we find that the clock stability of the GPS Block IIF type (3 × 10−13) is an order of magnitude better than that of IIR Block types (3 × 10−12) over a time interval of 30 s. In comparison, the stabilities of GLONASS clocks are approximately 3 × 10−12. We quantify the NRT clock error on the receiver positioning by using the precise point positioning technique obtained from the Bernese GNSS software. The 3-dimensional clock-induced positioning error is approximately 3.3, 3.2 and 0.9 cm for station AUCK and 6.9, 6.3 and 3.1 cm for station NRC1 for the GPS-only, GLONASS-only and GPS + GLONASS cases, respectively. For GNSS-RO applications, the bending angle profiles derived using TROPS GPS clocks agree with the COSMIC Data Analysis and Archive Center products to within 0.01–1.00 μrad. However, this is not the case for the GLONASS clock, because the GLONASS clock-induced errors on the RO profile are 10–100 times greater than those induced by the GPS clock. This suggests that different weightings should be used for RO applications, such as data assimilation, when different satellite clocks are involved in GNSS-RO retrievals. This study serves as a reference for assessing the impact of GNSS clocks on both GNSS-POD (precise orbit determination) and GNSS-RO in preparation for the F7C2 satellite mission.

Data Assimilation of Ground‐Based GPS and Radio Occultation Total Electron Content for Global Ionospheric Specification

AbstractThis study presents an approach based on the Gauss‐Markov Kalman filter to assimilate the total electron content observed from ground‐based GPS receivers and space‐based radio occultation instrumentations (such as FORMOSAT‐3/COSMIC (F3/C) and FORMOSAT‐7/COSMIC‐2 (F7/C2)) into the International Reference Ionosphere. Observing System Simulation Experiments (OSSEs) show that the data assimilation procedure consisting of the forecast and the measurement update steps can better improve the accuracy of the data assimilation analysis than the assimilation procedure using the measurement update alone. Compared with F3/C, the denser F7/C2 occultation observations can improve the analysis accuracy significantly as suggested by OSSEs. The real data assimilation results are further validated with global ionosphere maps, the global ground‐based GPS measurements, and the ionospheric F2 peak height and electron density sounded by ionosondes. Both the OSSEs and validation results confirm that a number of improvements to the data assimilation procedure presented in this paper can indeed be used to reconstruct the three‐dimensional ionospheric electron density adequately.

Climate Change Detection with the Global GPS Information System

The Intergovernmental Panel on Climate Change (IPCC) has predicted a persistent and wide-spread warming of temperatures close to the surface over the entire globe in the 21st century, due to the continuously increases in the concentrations of anthropogenic greenhouse gases in the atmosphere. How realistic are the predictions made by the IPCC models, especially over the polar region where the highest increases in temperatures were predicted? Due to the lack of accurate temperature measurements over polar region, the estimate of temperature trends over the polar region remains to be determined. The polar region is also one of the regions where temperature trends remain uncertain. In this work we use profiles of temperature measurements from the FORMOSAT-3/COSMIC (F3C) to determine global temperature trends during the period 2006-2016. The radio occultation (RO) technique gives the F3C a unique and accurate temperature measurement when verified against radiosonde measurements, and when compared with other passive-based temperature measurements using the thermal infra-red emission method. The RO methods use known sources from the radio waves provided by the Global Positioning System (GPS). With a good global coverage by the F3C, we can work out temperature trends on a global scale. Our analysis shows following main results. The distinctive warming over the northern hemisphere polar regions. This warming in the polar region, identified by the F3C observations, confirms the predictions made by the IPCC models. As warming trends in the polar region are the most worried concerns for the global warming, which is likely to lead to progressive reduction in sea ice content, melting glaciers, etc. Our work with the F3C data proves this is terrifying picture is actually happening now. Our analysis also shows distinctive warming over the southern hemisphere polar regions. Again, this proves significant warming during the cold season in the polar region which is consistent with what we have found in the northern hemisphere polar regions. In conclusion, the F3C data demonstrates that significant warming has occurred during the period 2007-2013, and most pronounced warming areas have occurred over the polar region. The pronounced warming over the polar region likely to have worsen effects on sea ice lost, and melting glaciers.

An empirical model of L-band scintillation S4 index constructed by using FORMOSAT-3/COSMIC data

Modern society relies heavily on the Global Navigation Satellite System (GNSS) technology for applications such as satellite communication, navigation, and positioning on the ground and/or aviation in the troposphere/stratosphere. However, ionospheric scintillations can severely impact GNSS systems and their related applications. In this study, a global empirical ionospheric scintillation model is constructed with S4-index data obtained by the FORMOSAT-3/COSMIC (F3/C) satellites during 2007–2014 (hereafter referred to as the F3CGS4 model). This model describes the S4-index as a function of local time, day of year, dip-latitude, and solar activity using the index PF10.7. The model reproduces the F3/C S4-index observations well, and yields good agreement with ground-based reception of satellite signals. This confirms that the constructed model can be used to forecast global L-band scintillations on the ground and in the near surface atmosphere.

Global ionosphere map constructed by using total electron content from ground-based GNSS receiver and FORMOSAT-3/COSMIC GPS occultation experiment

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.

Ionospheric F2 region perturbed by the 25 April 2015 Nepal earthquake

AbstractSeismic waves can be detected in the Earth's atmosphere and ionosphere; however, their impacts on ionospheric electron density (Ne) structures near the altitude of peak Ne (hmF2) have not yet been fully determined due to the lack of sufficient observations sampled in the vertical direction. Here we apply a ground‐based Global Positioning System (GPS) receiving network in Asia as well as the space‐based GPS occultation experiment on board the FORMOSAT‐3/COSMIC (F3/C) satellite to vertically scan the ionospheric Ne structures, which were perturbed by the magnitude Mw7.8 Nepal earthquake that occurred on 25 April 2015. The F3/C altitudinal Ne profiles show that the Nepal earthquake‐induced air perturbations penetrate into the ionosphere at supersonic speeds of approximately 800 m/s and change the Ne structure by 10% near hmF2. The vertical scale of the Ne perturbation is 150 km, while the hmF2 is uplifted by more than 30 km within 1 min. Those results reveal that the earthquake‐induced ground displacement should be considered as a significant force that perturbs the vertical Ne structure of the ionosphere.

Spread F occurrence and drift under the crest of the equatorial ionization anomaly from continuous Doppler sounding and FORMOSAT-3/COSMIC scintillation data

A relatively new method based on measurements by multipoint continuous Doppler sounding is applied to study the occurrence rate, propagation velocities, and directions of spread F structures over Tucumán, Northern Argentina, and Taiwan, both of which were under the crest of the equatorial ionization anomaly in 2014. In addition, spread F is studied globally over the same time period from the S4 scintillation index measured onboard FORMOSAT-3/COSMIC (F3/C) satellite. It is shown that the continuous Doppler sounding gives results that are consistent with S4 data and with previous optical, global positioning system (GPS), and satellite measurements. Most of the spread F events were observed from September to March, i.e., during the local summer half of the year in Tucumán, whereas in Taiwan, the highest occurrence rate was observed around equinoxes. The occurrence rate in Tucumán was about four times higher than that in Taiwan. The propagation velocities and directions were estimated from the Doppler shift spectrograms. The spread structures related to spread F propagated roughly eastward at velocities from ~70 to ~200 m s−1 during nighttime hours. The mean observed horizontal velocity was 140 m s−1 over Tucumán and 107 m s−1 over Taiwan. The local times at which the highest velocities were observed roughly correspond to local times with highest values of scintillation index S4, at ~20 to 23 LT. In addition, a comparison of measured drift velocities with neutral wind velocities predicted by models is provided. The observed velocities usually exceeded the horizontal neutral wind velocities predicted by the HWM14 model for the locations and times of observations.

Worst-Case GPS Scintillations on the Ground Estimated from Radio Occultation Observations of FORMOSAT-3/COSMIC During 2007–2014

The FORMOSAT-3/COSMIC (F3/C) satellite probes the S4 scintillation index profile of GPS signals by using the radio occultation (RO) technique. In this study, for practical use on the Earth’s surface, a method is developed to convert and integrate the probed RO S4 index, so obtaining the scintillation on the ground. To estimate the worst case, the maximum value on each profile probed by F3/C, which is termed S4max, is isolated. The isolated data are further used to construct the global three-dimensional distributions of S4max for various local times, seasons, solar activities, and locations. The converted S4max for the first time estimates the global distribution of ionospheric scintillations in the GPS L1 band C/A code signal on the ground. The results show that the worst-case scintillations appear within the low-latitude region of ±30°N, peaking around ±20°N magnetic latitude; they begin at 1900 MLT, reach their maximum at 2100 MLT, and vanish by about 0200–0300 MLT. The most pronounced low-latitude scintillation occurs over the South American and African sectors.

A self-contained GIM-aided Abel retrieval method to improve GNSS-Radio Occultation retrieved electron density profiles

A self-contained global ionospheric maps (GIM)-aided Abel retrieval method is adopted that can account for spherical non-uniformity along the radio wave path in the vicinity of the radio occultation (RO) region using the asymmetry factors derived from global background NmF2 maps. This procedure does not require any supplementary data sources to augment the Abel retrieval in order to represent the horizontal gradients in the ionosphere. This simulation study demonstrates that when sufficiently dense RO observations are available, the NmF2 values initially derived from classical Abel retrieval (with the assumption of spherical symmetry) are sufficiently good and useful to represent the spherical non-uniformities in electron density distribution. The Ne(h) profiles retrieved from this Abel retrieval procedure aided with asymmetry factors provide significant improvement compared to classical Abel retrieval at both F- and E-regions. The artificial large-scale structures, such as wave number-3 latitudinal structure and plasma caves underneath the equatorial ionization anomaly crests, were largely eliminated. This improved retrieval procedure also reduces the computational complexity and is suitable when the number of occultations increased by many times in the future when FORMOSAT-7/COSMIC-2 (F7/C2) and other RO missions become operational. The computation simplicity in this self-contained GIM-aided Abel retrieval procedure enables more accurate retrieval of Ne(h) profiles in near real time.