The Initial Assessment of Ionospheric Radio Occultation Data of MSS‐1 Satellite and Its Applications in Scintillation Exploration

Abstract

The Macau Science Satellites (MSS‐1) mission, consisting of twin low earth orbit satellites (Satellite A and B), is the first low‐inclination and high‐precision geomagnetic surveying satellite project in China. Among the multiple scientific payloads equipped with the scientific satellites, MSS‐1 carries a GNSS radio occultation (RO) receiver on Satellite A, and aims to observe the Earth's ionosphere and monitor the space weather especially in the South Atlantic Anomaly (SAA) area. This paper focuses on the ionospheric data of MSS‐1 obtained during the first 3 months after its launch, and assesses the RO products as well as initial scintillation measurements. Results show that MSS‐1 RO data have very good agreement with contemporary COSMIC‐2 and ionosonde observations. The scintillation amplitude indices S4 are comparable with that of COSMIC‐2 and well reflect the occurrence rates and distributions of equatorial plasma bubble and sporadic E (Es) in June solstice season. Special attentions are paid to the SAA longitude sector and inspire explorations on the seasonal variations and local‐time dependences of ionospheric irregularities.

Introduction

Macau Science Satellites (MSS-1) is the first satellite project of the planned series of geomagnetic survey satellites in China (). It consists of Satellite A and Satellite B and was launched successfully in 21 May 2023. MSS-1 is equipped with multiple scientific instruments including vector field magnetometer, flux gate magnetometer, coupled dark state magnetometer, energetic electron spectrometer, micro advanced stellar compass, GNSS receiver, satellite laser ranging, and solar X-ray detector. The main aims of the mission are to obtain the information of different layers of the Earth's system in the South Atlantic by detecting the geomagnetic anomaly area, study the origin and evolution of the geomagnetic field and the geomagnetic inversion mechanism, and attempt to depict a high-precision and high-resolution lithospheric geomagnetic map. The fundamental data are useful to analyze the interaction and interplay between the geomagnetic field with the Earth's external magnetic environment, atmospheric processes together with long-term change of climate. It focuses on the South Atlantic Anomaly (SAA) area and attempts to obtain high-energy electronic information of the inner radiation zone to serve Aerospace Safety and space weather forecast and research. MSS-1 is designed to be operated in circular orbits with an inclination of 41°. The orbit altitude is ∼450 km and the mass is around 550 kg. It is expected that the operation duration can be longer than 5 years.

Besides the high-precision geomagnetic field measuring instruments, MSS-1 carries a GNSS radio occultation (RO) receiver on the Satellite A platform to explore the Earth's ionosphere and space environment. Radio signals can be influenced by the ionosphere and atmosphere along the occultation ray path, in the form of time delay, excess phase, signal power variations, etc. Radio occultation observations have great advantages on vertical resolution and global coverage (Anthes et al., 2008). For past three decades, the development of RO missions has greatly enlarged the database of atmospheric and ionospheric observations and space weather monitoring. Scientific satellite missions such as Challenging Minisatellite Payload (CHAMP), COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) and its follow-on mission COSMIC-2, Communication/Navigation Outage Forecasting System (C/NOFS), MetOp series satellites and Compernicus projects have remarkable performances in facilitating the numerical weather prediction (Harnisch et al., 2013), space physics and space weather researches (Carrano et al., 2011; Zakharenkova et al., 2023). Besides the scientific missions, some commercial companies began to launch LEO satellites into space equipped with GNSS-RO receivers of tiny, low power, and cost-effective design. Several famous commercial companies like the Spire, GeoOptics, PlanetIQ and Yunyao have developed and launched hundreds of multi-payload LEO satellites in recent years (Angling et al., 2021; M. J. Wu et al., 2023). Furthermore, if the occultation signal passes through any inhomogeneity structures in the ionosphere, its wave front would be distorted and the received signal presents random fluctuations which is known as scintillation. The study of scintillation is crucial not only because it may cause signal fading, channel modeling and ranging resolution, but also can improve the interpretation of ionospheric geophysical parameters and the physics and dynamics of the upper atmosphere (Yeh & Liu, 1982). The GNSS RO receiver onboard MSS-1 provides high-rate amplitude and phase measurements that can be used to calculate the ionospheric scintillation index S4 and detect the occurrences and characteristics of ionospheric irregularities (Ludwig-Barbosa et al., 2023; Tsai et al., 2018). The radio signals traveling through the equatorial ionospheric irregularities in the longitude sector between 75^°W and 0^°E are promising in studying the geomagnetic filed especially at SAA region in corporation with other MSS-1 payloads.

This paper is organized as follow: in Section 2, basic introduction about the radio occultation payloads onboard MSS-1 is given, and data processing procedures are briefly summarized; Section 3 introduces the initial results of MSS-1 RO measurements and data validation with respect to COSMIC-2 and ionosonde data; Section 4 shows scintillation explorations based on 3 months' MSS-1 data including the amplitude scintillation index S4, spatial and temporal characteristics of E and F layer irregularities in the global scale and at specific SAA region; and the final section draws conclusions.

Materials and Methods

RO Payloads

The GNSS RO instrument onboard MSS-1 Satellite A consists of one pair of precise orbit determination (POD) antenna, one pair of ionospheric radio occultation (OCC) antenna, four high frequency cables, two preamplifiers and one GNSS receiver. The GNSS signals are received separately by the POD antenna and OCC antenna, filtered and amplified through the preamplifiers and delivered to the GNSS receiver. The signals are processed to generate in-phase and quadraphase samples, from which carrier phase can be measured onboard the satellite, and transmitted to the ground station network according to the planned transfer protocol. The schematics of MSS-1 GNSS RO system are summarized as in Figure 1. Detailed parameters of RO payload are summarized in Table 1. The antennas collect L1 and L2 signals from GPS and B1 and B3 signals from BEIDOU system. There are 12 signal channels allocated to the orbit determination measurements and four allocated to the ionospheric occultation observations. The positioning and ionospheric RO signals are of 1 Hz sampling rate. The high-rate (50 Hz) narrow band power (NBP) and wide band power (WBP) together with the satellite calculated carrier-to-noise ratio (CN0) are transmitted to the ground for further scintillation studies. Narrow band power and WBP are calculated by integral of I and Q values which are 1 kHz in-phase and quadraphase samples. The signal intensity can be obtained by the difference between NBP and WBP. The carrier-to-noise density ratio is also accessible through certain algorithms based on these band power. Details on the derivation of above signal parameters are introduced in Van-Dierendonck et al. (1993) and Wu et al. (2023). Limited by onboard space and resources, the antenna is faced forward and only receives rising occultations.

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Table 1 The Primary Parameters of RO Payload

Name	Parameters
Transmit system	GPS, BEIDOU
Signal frequencies	GPS L1/L2, BEIDOU B1/B3 (MEO, IGSO, GEO)
Antenna	Precise orbit determination antenna, occultation antenna
Measurements	Carrier phase, pseudo-range, signal power
Data sampling rate	1 Hz for ionospheric RO, 50 Hz for CN0, NBP and WBP
Straight line tangent height	Ionosphere 60–450 km
Signal tracking mode	Closed-loop

Data Processing

The total electron content (TECU) in the occultation signal link is related to the excess phase and can be obtained by combining the phase measurements at two frequencies of each system (Schreiner et al., 1999): $$TEC=\frac{1}{40.3}\frac{{f}_{1}^{2}{f}_{2}^{2}}{{f}_{1}^{2}{-f}_{2}^{2}}\left({L}_{1}-{L}_{2}\right),$$ where f_i represents the signal frequency and L_i is the corresponding phase measurement. The TEC is relative total electron content along the ray path from the RO receiver to the transmitter, and the L1/L2 carrier phase combination to compute it naturally eliminates the orbit and clock errors in the dual-frequency differencing. The phase ambiguity can be excluded by removing an initial value of the TEC series. The TEC consists of contribution from ionosphere below the altitude of LEO satellite (with negative elevation angle) and above the LEO (with positive elevation angle). To retrieve the electron density profile (EDP), the TEC contribution above LEO should be excluded at first. Since the antenna records TEC observations on both sides of the orbit with respect to the maximum impact parameter (as called occultation or non-occultation side), calibrated TEC is accessible by making difference between the symmetric points of the same impact parameter. It is based on the assumption that the occultation and the satellite planes are approximately coincident, and the ionospheric variations are relatively static during the data collection (Schreiner et al., 1999). Under the spherical symmetric assumption, calibrated TEC is regarded as the integration of the electron density of tangent point along the signal path. Therefore, the electron density at each tangent point can be retrieved by Abel inversion based on the calibrated TEC (TEC′): $$Ne\left({r}_{0}\right)=-\frac{1}{\pi }\int \nolimits_{{r}_{0}}^{{r}_{LEO}}\frac{d{TEC}^{\prime }/dr}{\sqrt{{r}^{2}-{r}_{0}^{2}}}dr.$$ In which r₀ and r_LEO is the distance from the Earth's center to the tangent point and to the LEO satellite respectively. Data with gaps larger than 5 s or truncated lower than 400 km are removed to guarantee the data continuity to the LEO satellite orbit.

The amplitude scintillation index S4 is the standard deviation of the signal intensities divided by the average signal intensity (SI): $${S}_{4}=\sqrt{\frac{\left\langle {SI}^{2}\right\rangle -{\langle SI\rangle }^{2}}{{\langle SI\rangle }^{2}}}$$, where ⟨⟩ represents the average of 1 s, and SI is obtained by the difference between NBP and WBP. The signal-to-noise ratio (SNR) are accessible by NBP and WBP (M. J. Wu et al., 2023). The downlink RO data are processed at the ground center and archived at the MSS-1 data archive (). In the future, all the MSS-1 Level 2 products (electron density profile and S4 index profile) will be accessible to all users via anonymous access. The EDPs and scintillation amplitude S4 indexes at 1 Hz are provided routinely with less than one day latency.

Figure 2 shows an occultation event observed at very early phase of MSS-1operation. The onboard CN0 series and ground processed NBP/WBP associated CN0 are demonstrated. It records the occurrence of sporadic E (Es) scintillation at around 100 km height with strong and sharp fluctuations in the CN0 series. The power spectrum density of signal is shown in the middle panel. “fp” is the Fresnel frequency; “p” is the slope of the roll-off frequency region fitted by least square estimation on the log-log power. On the right panel, the corresponding vertical electron density profile is retrieved by Abel inversion and confirms the existence of Es layer at similar altitude.

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Assessment of Data

Initial Results

The orbit inclination of MSS-1 is 41°, therefore the ionospheric observations generally focus on the equatorial and part of mid-altitudinal areas. Figure 3 displays the global distribution of GPS and Beidou RO measurements during the day 198 of year 2023. The lines show the track of tangent points mapped on the earth surface. Data are evenly distributed on both hemispheres and over lands and oceans. Local time distribution of the total numbers of occultation profiles obtained during the day of year (DOY) 152–243 (in June, July and August) in 2023 are shown in Figure 4. Benefiting from the low-inclination orbit, the occultation count in a day is relatively uniform and steady for both navigation systems. GPS RO observations are a bit more than that of Beidou. There are roughly 400 EDPs on average successfully retrieved every day during the first 3 months of MSS-1 mission.

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Comparison With COSMIC-2

The ionospheric occultation data of MSS-1 are validated by co-located COSMIC-2 products in June, July and August 2023. COSMIC-2 is a follow-on mission continuing the success of FORMOSAT-3/COSMIC mission and starts providing RO data and products since the end of 2019. The low-inclination orbit configuration (24^°) and operational altitude of ∼550 km determines its sounding coverage between ±40^° latitude. The COSMIC data analysis and archive center (CDAAC) is responsible for processing the downlink data and providing level 1 and 2 products in near real-time (). The ionospheric processing including the ionospheric excess phase and EDPs, absolute POD TEC for GPS and GLONASS, and the scintillation amplitude, phase indices and further geolocation, etc (Pedatella et al., 2021, 2023; Weiss et al., 2022). In previous validations between different data, the spatial collocation criterion varies from 2^° to 6^°, up to about 1,000–2,000 km (Garcia-Fernandez et al., 2005; Lei et al., 2007; Schreiner et al., 1999). The ionospheric meridional and zonal correlation lengths at middle latitudes are 7^° and 20^°, and reduce to 4^° and 11^° at low latitudes near the equator. The temporal criteria used varies from 30 min to 2 hr (Chang et al., 2022; Wang et al., 2019; M. J. Wu et al., 2023). According to Forsythe et al. (2020), the correlation time of ionosphere ranges from 1 to 3 hr, depending on the geomagnetic latitude region. It is reasonable to select temporal colocation within the correlation time. In this study, the co-located EDPs of MSS-1 and COSMIC-2 are matched by the window of 2^° latitude, 3^° longitude and 30 min. It is kind of strict criterion since we are trying to evaluate the RO profiles of different missions and azimuthal orientation. During June to August, there are 3,787 pairs of ionospheric RO observations are successfully matched and made statistics of the F2 layer peak density (NmF2) and peak height (hmF2). Figure 5 shows the scatter plot of MSS-1 and COSMIC-2 peak parameters. The correlation coefficient of hmF2 is about 0.95, and the mean difference is approaching 0 km, indicating very good agreement between the two missions. The standard deviation (STD) of hmF2 is 17.85 km. The relative mean difference of NmF2 is 3.27%, and the STD is 15.54%. The statistics between MSS-1 and COSMIC-2 are consistent with the comparison results of other RO missions (Forsythe et al., 2020; Schreiner et al., 1999; Y. Sun et al., 2018).

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Figure 6 shows the diurnal variations of NmF2 and hmF2 for MSS-1 and COSMIC-2 respectively at the geomagnetic latitude range of −35^° to 35^°. The F2 layer peak density obtained by both missions reflect the equatorial ionization anomaly (EIA) clearly in the northern summer solstice season, symbolized by the north and south crests and equatorial trough structures lying along the geomagnetic equator during the daytime. Driven by the plasma composition variations, the EIA presents a hemispheric asymmetry with more predominant enhanced crest than the winter hemisphere (Qian et al., 2013). The NmF2 derived by MSS-1 RO observations is slightly greater than that of COSMIC-2 especially at the northern crest area around the noon. The peak density heights generally have two peaks around 12 LT and 20 LT. The EIA is higher in post sunset hours than that of the noontime. The crests dismiss around the sunrise, and mostly locate north of the geomagnetic equator in the June solstice season (Yue et al., 2015).

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Comparison With Ionosonde

The ionosonde observations during DOY 152 and 243 of the year 2023 are downloaded from Lowell Global Ionosphere Radio Observatory data center at . Figure 7 shows the ionosonde stations with available ionograms during above period between −60^° and 60^° latitude. The co-located ionosonde and RO measurements are selected within 2^° latitude, 3^° longitude. Ionograms are recorded typically every 5–15 min. The closest time is selected within 30-min interval of the collocated RO occurrence. Ionosondes provide measurements of the critical frequency (foF2) and virtual height through ionograms and estimation of the true height of maximum electron density using special software like ARTIST, or the expert ionogram interpretation tool SAO Explorer (Reinisch & Galkin, 2011). The F2 peak density is transformed from foF2 by the relationship NmF2 (el/cm³) = 1.24 × 10⁴ · (foF2 (MHz))².

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There are 242 pairs during the daytime (LT 6–18) and 220 pairs at night successfully matched in the considered period. The statistics of hmF2 and NmF2 differences are shown in Figure 8. The hmF2 demonstrates good agreement between the MSS-1 results and ionosonde parameters. The mean deviation is about 14.94 and 10.85 km at daytime and nighttime respectively, and the STD is both around 18 km. The correlation coefficient for NmF2 is even higher, with relative mean deviation of 4%–5% and STD of 13%–15%. The statistic for all the co-located observations gets relative mean deviation of 4.85% and STD of 14.44%. The comparison with respect to ionosonde is comparable with the validation of other RO missions (Forsythe et al., 2020; Schreiner et al., 1999).

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Scintillation Explorations

S4 Indices

The main objective of MSS-1 GNSS payload is to monitor the space environment near the Earth. The high-rate signal amplitude data provided by RO receiver are promising in studying the ionospheric irregularities like the sporadic E (Es) and the equatorial plasma bubble (EPB) (or called as spread-F). With abundant capacity of data downlink, the 50 Hz CN0, NBP and WBP are downloaded to the ground all time and used to calculate the SNR and S4 indices. To verify the scintillation products, Figure 9 displays the histogram comparison between MSS-1 and COSMIC-2 S4 index during the same period. Only satellite “C2E1” of COSMIC-2 is used in the comparison. Therefore, the total number of observations of observations involved is comparable for MSS-1 and COSMIC-2. The scintillations are roughly separated to E region (80–120 km) and F region (150–450 km) by the altitude of maximum S4 occurrence. It is noted that COSMIC-2 transfers high-rate scintillation data when the onboard computed S4 exceeds a configurable threshold, therefore only the observations with S4 > 0.3 are taken into consideration. In general, the S4 indices of COSMIC-2 and MSS-1 have good agreement in the E and F regions. According to Figure 9 panel (a), COSMIC-2 records more Es scintillations with S4 less than 0.75, while the MSS-1 reveals more occurrences of strong scintillation cases with S4 higher than 0.75. The occurrence frequency of weak and medium scintillation (S4 < 1.0) slightly decreases at F-region (shown in panel (b)), while that of strong scintillation (S4 > 1.0) remains comparably high for both MSS-1 and COSMIC-2.

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Climatological Characteristics

To demonstrate climatological distribution of Es and EPB based on MSS-1 scintillation measurements, the occurrence rates are shown in Figure 10. The rate is represented by the ratio of the number of profiles with maximum S4 > 0.3 to the number of all the profiles in each 5^° × 10^° (geographic latitude × longitude) grid. Es and EPB scintillations are separated according to the vertical height of S4 maximum. During the June solstice, the Es occurrence is more frequent at the northern summer hemisphere, and demonstrates highly correlation with the geomagnetic field. The Es most likely occurs where the geographic latitudes move north and south following the dip angle changes. The occurrence rate for the majority areas of mid-latitude is over 60% except for the North America region. The longitudinal variations at middle latitude are attributed to the combination effects of the geomagnetic field, horizontal winds and ion sources. The increasing occurrences in the local summer is confirmed by both model and observational experiments (Yamazaki et al., 2022; Yu et al., 2019). The distribution of Es observed by MSS-1 agrees with former literature (Chu et al., 2014; Hodos et al., 2022). Panel (b) displays the geographical distribution of the global occurrence rates of EPBs or equatorial spread F which happens more frequently at night (as confirmed in Figure 11). High occurrences are found between −40^° and 40^° geomagnetic latitude band over the Pacific oceans and African areas. It is known that the F region scintillations generally follow the course of the geomagnetic field and are consistent with the appearance of crests of the EIA phenomenon (Kepkar et al., 2020) which is clearly shown in Figure 10, panel (b). Different with the mechanism of Es, EPBs are initiated by the Rayleigh-Tylor instability which is possibly seeding by atmospheric gravity waves, and vertical shear of plasma drift (Sultan, 1996).

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The vertical and temporal distributions of Es and EPB are further discussed in the June solstice season. To demonstrate the diurnal distribution, the occurrence rates in grid of 2.5 km × 1 hr and 20 km × 1 hr are calculated for Es and EPBs respectively. We assume that the scintillation source associated to the maximum S4 is located somewhere along the ray followed by the signal during its propagation. Therefore, the irregularity structure can be associated to the straight-line tangent height (distance of the line connecting the GNSS and the receiving satellites from the Earth's ellipsoid) of that particular ray. The corresponding tangent point (TP) is assumed as the equivalent location of the scintillation source. In Figure 11a, the most predominant feature of Es is the semidiurnal patterns during the northern summer. Two peaks of Es occurrences are found around LT 6–14 and LT 16–24. There is a downward drifting of Es height after sunset hours from 115 to 95 km, approximately at speed of 1.0 km/hr. Another descending trend begins at about LT 6 with slower rate. The prevalent semidiurnal variations are also observed by Chu et al. (2014) from COSMIC measurements and Wu et al. (2005) by CHAMP occultations. As for the EPB, the occurrence rate is much higher after local sunset till the early morning. The F-region scintillation is rarely observed during the daytime. The pre-reversal enhancement plays a significant role in influencing the plasma bubble growth and vertically lifting it after the sunset (Abadi et al., 2015). The height of EPB varies from 150 km to over 400 km, and it can be higher if not truncated by the orbit altitude of MSS-1 satellite.

SAA Scintillations

To focus on the scintillations happening especially over the SAA region, Figure 12 shows zoomed in maps between the geographic longitude 120^°W and 0^°E. The geomagnetic field configuration in the SAA sector is very different with those of other longitudes. In this area, the geographic latitudes of the dip equator shift vastly toward the poles and the geomagnetic field intensity is extremely decreased. Given the diurnal variation of EPB occurrence, we further discuss the possible reasons for its higher rate happening in the post-sunset period. In Figure 12, panel (a) demonstrates the mean nighttime hmF2 in June Solstice together with the occurrences of EPB irregularities respectively. The geolocation of irregularities is the same as mentioned in former section. The mean nighttime hmF2 is obtained by averaging the peak height from RO profiles observed during LT 0–4 and LT 18–24. The colormap represents the value of hmF2 and the black dots indicate the locations of scintillations respectively. Panel (a) shows that the EPBs most likely occur in the longitude sector where the electron density peak height increases and significant latitudinal gradients exist between geomagnetic latitudes ±40^° (Ko & Yeh, 2010). There is prominent coherence between the geomagnetic field lines and the EPB occurrences at this region. The 75^°W − 0^° longitude sector undergoes remarkable decrease in hmF2 at the southern winter hemisphere, as well as less occurrence of EPBs in the SAA area. The findings in this study agree well with previous literature.

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Besides increasing with enhancement of hmF2 along the geomagnetic latitudinal band 20^°–40^°, the occurrence of EPB is clustered noticeably at 20^°W − 0^° and 120^°W − 90^°W longitudinal sectors. Panel (b) illustrates another factor which may influence the detection of EPB. The ratio represented by the color map is calculated by the number of observations obtained during LT 0–4 and LT 18–24 divided by the total number of RO observations in each geographical grid. In this way, higher ratio means more observations recorded at post-sunset period. As a result of the low-inclination orbit design, the observations demonstrate slight differences in different regions during the first 3 months of operation. More observations are concentrated in the post-sunset period till the early morning in the North hemisphere, while the nighttime ratio in the southern hemisphere is kind of decreasing (especially south to the geomagnetic latitude −20^°). The distribution of EPBs happens to be coincident with the change of ratio for nighttime observations. More spots are found at the 20^°W − 0^° longitude sector where the ratio increases slightly. The local time difference of observations may influence the actual occurrence rates of EPB and possibly explain why the rates shown in Figure 10b are generally higher than previous studies in the June solstice season. It should be noted that the post-sunset equatorial F region irregularities are highly observed in the SAA longitude sector during the northern winter months while seldom occur in the June solstice at the same place (Su et al., 2008). Based on only 3-month measurements of MSS-1, it is difficult to perceive the characteristics of ionospheric irregularities depending on seasons and solar activities. The characteristics of EPB occurrence should be further analyzed when longer period of data are available.

Conclusions

This research focuses on the assessment of MSS-1 RO data in the first 3 months after launch. The occultation antenna is able to observe the ionospheric occultations from GPS and BEIDOU systems. There are over 400 EDPs successfully archived on average each day. The 50 Hz signal band power and CN0 are used to calculate the amplitude scintillation index S4 and conduct scintillation researches. We draw the following conclusions:

• In comparison with COSMIC-2 co-located profiles, the (relative) mean differences for NmF2 and hmF2 are 3.27% and −0.31 km respectively, and STDs are 15.54% and 17.85 km; the comparison with respect to ionosonde also shows high consistence between the MSS-1 results and ground-based ionospheric measurements.

• The S4 indices of COSMIC-2 and MSS-1 have good agreement in the 80–120 km and 150–450 km vertical range, except for more strong scintillations detected by MSS-1 with S4 >0.75.

• MSS-1 reveals that Es has predominant semidiurnal patterns in the occurrence rates at the northern mid-latitude area; the EPB occurrence rate is much higher after local sunset till the early morning during the June solstice.

• The correlations among the geomagnetic field lines, the electron density enhancement and the EPB occurrences in the SAA longitude sector are noticeable and correctly observed by MSS-1; more comprehensive seasonal dependences could be analyzed with long-term data in the future.

Acknowledgments

We are thankful for the data provided by COSMIC Data Analysis and Archive Center at and the GIRO data resources at . This research is funded by the National Key R&D Program of China (2020YFA0713501), the National Natural Science Foundation of China (12273094,12273093), and the Youth Innovation Promotion Association CAS. This work is supported by Macao Foundation.

Data Availability Statement

The COSMIC-2 ionospheric products used in this study are available at COSMIC Data Analysis and Archive Center (CDAAC) via (UCAR COSMIC Program, 2019); the ionosonde data of global ground stations are accessible at DIDBase FastChar tool via (Reinisch & Galkin, 2011). The MSS-1 data referred in this study is available at Zenodo (Wu, 2024) by .

References

Abadi, P., Otsuka, Y., & Tsugawa, T. (2015). Effects of pre‐reversal enhancement of E × B drift on the latitudinal extension of plasma bubble in Southeast Asia. Earth Planets and Space, 67(1), 74. [DOI: https://dx.doi.org/10.1186/s40623-015-0246-7]