Simultaneous equatorial plasma bubble observation using amplitude scintillations from GNSS and LEO satellites in low-latitude region

This study estimates the scale sizes of the plasma density irregularities and the longitudinal width associated with equatorial plasma bubbles (EPBs) in equatorial and low-latitude regions. By analyzing amplitude scintillation S4 indices and total electron content (TEC) measured from low earth orbit (LEO) satellite’s beacon signals with 400 MHz and Global Navigation Satellite System (GNSS) L1/E1 signals with 1575.42 MHz, recorded by receivers at the KMITL station in Bangkok, Thailand (geographic; 13.73° N, 100.77°E, magnetic: 7.26°N), we investigate the characteristics of these irregularities. We collected data of 154 LEO satellite pass events during nighttime on 21 disturbed days in four equinoctial months in 2021. Based on the presence or absence of the scintillation effects on GNSS and LEO beacon signals, the events are categorized into four classes to estimate the scale size of the plasma density irregularities. The analysis suggests that events with both GNSS and LEO scintillations, as well as events with GNSS scintillation alone, occur predominantly before midnight assuming the presence of the small-scale size of the irregularities within EPB. However, events with only LEO scintillation occur throughout the whole night and some events are observed before the events with both GNSS and LEO scintillations. Post-sunset LEO scintillation alone may be attributed to the onset of EPBs developing at low altitude, while post-midnight LEO scintillation events near the magnetic equator, observed during periods of low GNSS Rate of TEC Index (ROTI) values, are associated with bottom-side ionospheric irregularities but are not linked with EPB. The findings are consistent with previous researches on the generation and decay of electron density irregularities within plasma bubbles. However, this study provides new insights by using specific data sets and analysis techniques, offering a more comprehensive understanding of the association of LEO scintillations with bottom-side ionospheric irregularities near the magnetic equator, not observed in the ROTI map.


Introduction
Ionospheric scintillation refers to rapid fluctuations in the amplitude and phase of radio signals that pass through the ionospheric plasma density irregularities (Kung Chie Yeh and Chao-Han Liu 1982).Specifically, amplitude scintillations are frequently detected in the satellite signal near the magnetic equator and at low latitudes, where equatorial plasma bubbles (EPBs) are often observed (de Oliveira Moraes et al. 2012).EPBs have zonal widths ranging from tens to hundreds of kilometers in the magnetic east-west direction and plasma density irregularities associated with the EPB can have scale sizes ranging from tens of meters to a few kilometers (Yokoyama et al. 2014;Tsunoda 2015;Bhattacharyya 2022).Understanding the causes of scintillations related to EPBs is vital for space weather studies, and technologies related to satellite communication and navigation systems (Kintner 2009).
Within EPBs, plasma density irregularities with scale sizes close to the first Fresnel scale ( d F = √ 2 h where λ is the wavelength of the radio signal and h is the altitude of the ionospheric irregularity layer) induce amplitude scintillations in satellite signals (Bhattacharyya et al. 2000;Yang and Liu 2016).Particularly, small and intermediate-scale plasma irregularities (less than 1 km), can cause moderate-to-severe amplitude scintillations on radio signals within the very high frequency-ultra high frequency (VHF-UHF) band, including the Global Navigation Satellite System (GNSS) signals (Kintner 2009;Li et al. 2021;Salles et al. 2021).
The study of space weather study observes the spatial and temporal variations of ionospheric electron content (TEC) and scintillations using GNSS signals from medium-earth orbit (MEO) satellites and VHF beacon signals from low earth orbit (LEO) satellites.Yamamoto (2008) developed the "GNU Radio Beacon Receiver (GRBR)" system using LEO satellite beacon signals of 150 and 400 MHz to measure ionospheric TEC and scintillations.TEC data from multi-constellation GNSS satellite signals provide a better perceptional understanding of the ionospheric temporal variation, whereas scanning along an LEO satellite trace provides high spatial resolution for capturing ionospheric structures (Watthanasangmechai et al. 2016).
The intensity of scintillation in the satellite signals is proportional to the electron density irregularities having a scale size corresponding to the Fresnel scale.The S 4 index is commonly used to measure the amplitude scintillation intensity in ground-based received signal, defined as the standard deviation of the received signal power normalized by its mean value (Xiong et al. 2007).Analyzing scintillations from various frequency signals measured by the ground receivers provides valuable insights into prediction the scale sizes of the ionospheric irregularities associated with EPBs in the low-latitude region, while TEC variations from LEO satellites reveal the detailed structure of the EPB.However, due to the longitudinal and temporal variability of EPB characteristics and scintillation occurrences, further study is required to investigate the scintillation effects caused by EPB at low-latitude region, over Thailand.
In this study, we use amplitude scintillation S 4 indices from both the LEO F7/C2 satellite's beacon signals at 400 MHz through a GRBR receiver (Yamamoto 2008), and GNSS satellite's L1 or E1 signals at 1575.42 MHz recorded by a GNSS receiver at KMITL Bangkok station, Thailand, which is in the low-latitude region.We focused only on the scintillation events occurring during magnetically quiet conditions with Kp < 3.By observing the presence or absence of the scintillation effects on signals with different frequencies, we estimate the scale size of the plasma irregularities associated with EPBs because the scale size of the irregularities generating the scintillations corresponds to the Fresnel scale, which is determined by the wavelength or frequency of the signal that passes through irregularities.We analyzed the occurrence of the observed scintillation events in the four cases: scintillations on both GNSS L1/E1 signal and LEO beacon signal, scintillations on GNSS L1/E1 signals alone, and scintillations on LEO beacon signal only, and no scintillation.Investigating one sample event from each of the enhanced scintillation cases, we examined the internal structure of the electron density-depleted region in each case by observing the relative TEC and its standard deviation from the GRBR beacon receiver.Finally, we summarized the occurrence frequency distribution and duration of all scintillation events.

Theoretical background and data
When a radio signal passes through ionospheric plasma density irregularities with scale sizes below the first Fresnel zone, it experiences phase modulation, resulting in interference patterns or scintillations on the ground (Ghobadi et al. 2020).In low-latitude regions, these ionospheric irregularities are associated with EPBs or bottom-side ionospheric instabilities (Tsunoda 2015;Bhattacharyya 2022).Scintillation, even at moderate intensity, can degrade the accuracy and integrity of GNSS positioning (Kintner 2009;Salles et al. 2021).Therefore, there have been studies on the scintillation effects on multi-frequency GNSS signals in low-latitude regions (Rama Rao et al. 2006;Oliveira Moraes et al. 2017;Bumrungkit et al. 2022).
The characteristics of EPBs have been extensively studied using various data from multiple instruments, including GNSS receivers (Otsuka 2018;Joshi et al. 2019;Li et al. 2021).Buhari et al. (2014) and Manga et al. (2021) studied the two-dimensional structure of EBPs using the rate of total electron content change index (ROTI) data from a dense GNSS receiver network, as ROTI serves a reliable indicator of scintillation events.Additionally, VHF beacon signals from LEO satellites are employed to observe TEC variations due to their ability to scan ionospheric structures with a high spatial resolution.Using GRBR system developed by Yamamoto (2008), the latitudinal ionosphere structure was observed in (Watthanasangmechai et al. 2014) and the relationship between the large-scale wave structure (LSWS) and EPB occurrence was examined in (Liow et al. 2019).Moreover, Seechai et al. (2022) found that amplitude scintillations occurred more frequently on the beacon signal recorded on GRBR receiver than on GNSS signal because irregularities with smaller scale size have shorter lifetimes than the larger ones (Otsuka et al. 2009;Li et al. 2021).
In this study, we employed two types of receivers to measure S 4 indices of LEO satellite signals and GNSS satellite signals.The location of the receivers is at the KMITL Bangkok station (geographic; 13.73°N, 100.77°E, magnetic: 7.26°N), Thailand.The first receiver used is an open-source digital beacon receiver called GRBR, developed by Yamamoto (2008).The GRBR receiver information is publicly available at the URL https:// www.rish.kyoto-u.ac.jp/ ~yamam oto/ digit albea con/.The receiver stores data during the satellite passes, which are then analyzed by an off-line data analysis program.The GRBR receiver computes the S 4 index with a 1-second resolution, and relative TEC values and their standard deviation values with a 0.1-second resolution.Recently, the GRBR receiver was modified to receive two beacon signals (400 MHz, 965 MHz) from the FORMOSAT-7/ COSMIC-2 (F7/C2) constellation as explained in Hsiao et al. (2022).The GNSS receiver is a Septentrio PolaRx5 GNSS receiver, which computes S 4 index from the standard deviation of the 50-Hz raw signal power samples normalized to the average signal power over an 60-second interval.This study focused on the 400-MHz beacon signal from F7C2 LEO satellite and L1 frequency (1,575.42MHz) signal from two GNSS constellations, GPS and Galileo satellites.The F7/C2 satellites orbit the Earth at approximately 550 km altitude with a 24-degree inclination for about 97 min (Taiwan Space Agency 2023), while GNSS satellites orbit at an altitude of 22,000 km for 12 h.
During the study period from post-sunset up to postmidnight hours, the GRBR receiver detected up to nine LEO satellite passes at different times, each lasting about 5 min or less.We examined the S 4 indices of the beacon and GNSS signals recorded by the receivers during each LEO satellite passing event over 21 days with EPB events in the equinox months (March, April, September and October of 2021) with elevation angles above 30 degrees to remove multipath effects.Our study focuses scintillation effects on magnetically quiet days with Kp ≤ 3 because scintillation events can still occur during magnetically quiet periods in the low-latitude region (Groves et al. 1997).
The S 4 index, which measures scintillation in satellite signals, is normally computed as: where I is the intensity of the trans-ionosphere signal and 〈•〉 indicates the average over the interval of time of 1 min (Kung Chie Yeh and Chao-Han Liu 1982).However, the S 4 index in the GRBR receiver is estimated the ratio of the standard deviation and the average of the received power from the satellite's beacon signal at the receiver, i.e.: and the received power of the beacon signal at the receiver, P b is calculated as: where I b and Q b are the in-phase and quadrature-phase components of the received signal by the receiver.
In this study, we use a threshold of 0.8 for beacon signals based on the empirical study because the S 4 index from GRBR receiver during quiet time or daytime are (1) (2) S 4,GRBR = std(P b ) avg(P b ) , (3) mostly high as shown Fig. 1 where S 4 indices measured from two F7/C2 LEO satellites, F7 Flight Module 1 (F7 FM1) and F7 FM4 on 30 March 2021 (DOY 089), as recorded by the KMITL Bangkok station for approximately 3 min at different coordinated (UTC times), around 15:00 and 20:22, are illustrated.This study determined enhanced scintillation events when the S 4 value exceeds 0.8 continuously for a period of 5 s or longer.(2014).The standard deviation of the relative TEC in the GRBR receiver is computed using a window size of 10.For GNSS receiver, ROTI is the standard deviation of rate of TEC where the TEC is measured in unit of TECu (1 TECu = 10 16 electrons/m 2 ) and ROTI is in unit of TECu/min.To compute the ROTI from GNSS signals for 5 min interval, we use Eq. ( 2) from (Pi et al. 1997): where ROT is the rate of TEC which is the difference in TEC between two consecutive intervals of 60 s.In this study, we used the GNSS ROTI map developed by a GNSS receiver network under the KMITL's Excellence Center in GNSS and Space Weather, (http:// iono-gnss.kmitl.ac.th/) to check the locations and the area of the ionospheric irregularities over Thailand.

Results and discussion
In By observing the presence or absence of enhanced scintillations on these signals with different frequencies, we estimate the scale sizes of plasma irregularities in EPBs over Thailand.Measurements from the LEO satellite can the zonal width (east-west direction) of the EPBs along the satellite trajectory within a short time; therefore, we also analyzed the scintillation S 4 index and relative TEC values from the LEO signals to identify the longitudinal width of plasma-depleted regions inside EPBs.
The GRBR receiver detects multiple LEO satellite passes at different times.For each LEO satellite passing event, we examined the scintillation S 4 indices of both the beacon and GNSS signals recorded by the receivers during the nighttime.Based on the results, we classified into four cases as described in Table 1: Case 1) scintillations on both GNSS L1/E1 signal and LEO beacon signal; Case 2) scintillations on GNSS L1/E1 signals alone; (4 Case 3) scintillations on LEO beacon signal only; and Case 4) no scintillation.We examined one sample event form each scintillation case and then summarized all cases of 154 events.

Case 1: scintillations on both GNSS signal and beacon signals (DOY 089, 2021)
A Case 1 scintillation event was recorded from the LEO F7 FM4 satellite passing between 14:59:34 and 15:03:16 UTC (21:59:34-22:03:16 LT) on 30 March 2021 (DOY 089).Figure 3a shows scintillation levels at the IPPs (referenced to 350 km altitude) from the beacon signals (line), the GPS L1 signals (circle), and the Galileo E1 signals (star), recorded by the receivers at the KMITL Bangkok station.Enhanced scintillations were observed on both the L1 signal from a GPS satellite (G15) and the beacon signal from the LEO satellite, indicated by red color.We hypothesize that enhanced scintillation on the GNSS signal was caused by the small-scale irregularities (approximately 370 m) of EPBs. Figure 3b shows the ROIT map at 15:00 UTC (22:00 LT) over Thailand confirming that the LEO satellite passed through the area affected by EPBs, indicated by a red circle.
When comparing the TEC fluctuations during the enhanced scintillation period and low scintillation period, even small TEC fluctuations could still lead to the scintillation effect on the beacon signal with lower frequency.Thus, the enhanced S 4 index values in Figs. 3  and 4 enabled us to estimate the scale sizes of the plasma irregularities (approximately 370 m) associated EPB that can trigger GNSS scintillations, and the longitudinal   5a.We assume that this GNSS scintillation was triggered by small-scale irregularities (approximately 370 m) associated with EPB near magnetic equator.Although the GNSS E1 signal is affected, no scintillation was observed on the signal of the LEO satellite, which orbited at higher latitudes.The ROTI map in Fig. 5b shows a small area of ionospheric disturbances in close proximity to the location where the enhanced GNSS scintillation occurred.It is reasonable to assume that the EPBs were positioned higher than the LEO satellite's altitude (550 km) or far from the LEO satellite pass, as the beacon signal did not encounter any plasma irregularities.
Figure 6 shows the stable relative TEC without significant fluctuations along the pass of LEO satellite.TEC fluctuation from the beacon signal.Scintillation events occurred during periods of notable TEC perturbations.There are two abrupt density-depleted regions with size of 10 km and 2 km.

Summary of all cases
In this work, we examined the scintillation S 4 indices of the LEO F7/C2 satellite's beacon signals at 400 MHz and GNSS L1/E1 signals at 1575.42 MHz recorded at the KMITL Bangkok station in the low-latitude region.
The enhanced scintillations on the GNSS L1/E1 signal in Case 1 and Case 2 suggest that the estimated scale sizes of EPB irregularities were approximately 370 m, while Case 3 indicates that the LEO's beacon signal scintillations were caused by bottom-side ionospheric regularities with a scale size approximately 725 m.We interpret that the scintillation effects on the GNSS signal alone in Case 2 occurred when the EPBs were located higher than the LEO satellite's orbit altitude of 550 km or far from the LOE satellite pass.The enhanced scintillation period along the pass of the LEO satellite approximated the longitudinal width of the plasma-depleted region associated with EPBs or bottom-side ionospheric instabilities after midnight, estimated to be 270 km in Case 1, and 300 km in Case 3. We analyzed the statistics of Cases 1 to 4 at the KMITL Bangkok station in 2021.A total of 154 events of the LEO F7/C2 satellites passes were recorded on 21 days in March (4 days), April (4 days), September (1 days), and October (12 days) of 2021 (a low-solar activity year).These events include the 'No scintillation' case (Case 4), which is the largest number events recorded due to the solar minimum year.The bar chart in Fig. 9 shows the frequency distribution of Cases 1, 2, 3, and 4. Scintillations on the LEO satellite's beacon signal with lower frequency were more frequently detected each month compared to scintillations on the GNSS satellite's L1/E1 signals with a higher frequency, as expected.Case 2 exhibited the lowest frequency of scintillation events.
Finally, we examined the occurrence time of the scintillation events of Cases 1, 2, and 3, both of which involve scintillations of the beacon signals as depicted in Fig. 10.The shaded area indicates the post-midnight period.The findings indicate that scintillations on the beacon signal persist until post-midnight (20:00 UTC/ next day 3:00 LT) as discussed in (Otsuka 2018;Li et al. 2021).In contrast, scintillations on the GNSS L1/E1 signals triggered by small-scale irregularities of EPBs only occurred before midnight time in the lowlatitude region, consistent with previous studies in (Otsuka et al. 2009;Watthanasangmechai et al. 2016).These observations suggest that plasma bubbles with small-scale sizes close to the first Fresnel zone of the GNSS signal decay before midnight, and the large-scale plasma irregularities near pre-dawn do not generate scintillations on the GNSS signal.

Conclusion
This study estimated the scale sizes of equatorial ionospheric irregularities embedded in EPBs using amplitude scintillation S 4 indices of the LEO F7/C2 satellite's beacon signal at a frequency of 400 MHz and the GNSS satellite's L1 or E1 signals with a frequency of 1575.42MHz recorded by the GRBR and GNSS receivers at the KMITL Bangkok station, Thailand, in the low-latitude region during magnetically quiet condition (Kp < 3).For each LEO satellite pass during the nighttime period, we examined the scintillation S 4 indices of both the beacon and GNSS signals in 2021 and categorized the events into four cases depending on the presence and absence of the scintillations on signals.Enhanced scintillation example events reveal that the longitudinal widths of the EPB's depleted region are estimated to be ~ 260 km in the case of scintillations on both GNSS signal and beacon signal, and ~ 300 km in the case of scintillations on beacon signal only.
The findings are: • Scintillation effects on GNSS alone are the least frequent.developing equatorial plasma bubbles (EPBs) that are not fully developed yet.• LEO's beacon signal scintillations are observed post-midnight, while no GNSS signal scintillation is detected.These findings suggest a potential link between the observed LEO scintillation and the presence of bottom-side irregularities.However, further investigation with additional data is needed to substantiate this hypothesis.
Moreover, the work also presents the temporal characteristics of the scintillation events that occurred in the low-latitude region in Thailand.In future work, we will study the features of the equatorial ionospheric irregularities including post-sunset EPB and post-midnight bottom-side instability based on the scintillations on LEO and GNSS signals from more stations in Thailand.In addition, similar studies on the days of global disturbances (Kp > 4) during the solar maximum years should be investigated.
Fig. 1 S 4 index plots from a LEO F7 FM1 satellite during enhanced scintillation event and b F7 FM4 satellite during low scintillation event on 30 March 2021 (DOY 089) during quiet time this work, we analyzed the scintillation S 4 indices of the LEO F7/C2 satellite's beacon signals at a frequency of 400 MHz and the GNSS satellite's L1/E1 signals with a frequency of 1575.42MHz recorded by the receivers at KMITL Bangkok station, Thailand, in the low-latitude region.Theoretically, the amplitude scintillation is caused by irregularities with scale sizes close to the first Fresnel zone of the signal.With the peak of the ionospheric electron density irregularity was at 350 km altitude, the first Fresnel zones are approximately 370 m for L1, and 725 m for 400 MHz, respectively.

Fig. 3
Fig. 3 Maps of ionospheric pierce points (IPPs) of a S 4 index of GNSS [GPS (circle) + Galileo(star)] and LEO satellite signals received at the KMITL Bangkok station and b ROTI map of GNSS signals from the KMITL's receiver network over Thailand around 15:00 UTC (22:00 LT) on 30 March 2021 (DOY 089) and red color refers to enhanced scintillation (higher than the threshold) Fig. 5 Maps of ionospheric pierce points (IPPs) of a S 4 index of GNSS [GPS (circle)] and LEO satellite signals received at the KMITL Bangkok station and b ROTI map of GNSS signals from the KMITL's receiver network over Thailand around 14:35 UTC (21:35 LT) on 28 April 2021 (DOY 118) and red color refers to enhanced scintillation

Fig. 9
Fig.9The frequency of the scintillation events for Cases 1, 2, 3 and 4 for each month and all observation days

Table 1
Four cases of the scintillation events • Scale sizes of irregularities are approximately 370 m in most post-sunset EPBs.• Scintillation effects on both GNSS and LEO beacon signals occur only before midnight, while scintillation effects on LEO beacon signals can occur in both preand post-midnight.• Post-sunset scintillation on LEO satellite's beacon signal alone is possibly attributed to the onset of