# Effects of Equatorial Plasma Bubbles on Multi-GNSS Signals: A Case Study over South China
## 3 Comparison of GNSS and Airglow Data  
All-sky 630 nm airglow images can visually display the morphology and motion state of EPBs [42]. In this section, the combined observations of airglow images and GNSS measurements are used to investigate the detailed influence of the spatial distribution and depletion degree of EPBs on GNSS signals. Figure 2 shows the comparison results of the airglow depletion, S4 index, and deviation VTEC (dVTEC) for the Galileo and GPS constellations during 12-20 UT on 3 April 2022. Considering that low elevation data may cause positional uncertainty between the GNSS signal path and airglow intensity, we select the observations with the highest elevation angle at each epoch in the airglow image coverage, as shown in Figure 2a,b. Relative airglow values less than 0, which have been discussed in the data processing section, are considered to indicate density depletion (i.e., an EPB) and are manually identified to ensure accuracy. The center of the EPB is with the strongest airglow depletion, while the edge of the EPB is the start and end of the airglow depletion. As shown in Figure 2c,d, when the airglows presented depletions, the S4 scintillation index increased obviously and the dVTEC showed prominent oscillation. Airglow depletions with larger magnitudes generally resulted in greater S4 values and dVTEC oscillations. In addition, when there was no airglow depletion, both the S4 index and dVTEC were relatively low. In Figure 2d, the airglow depletion at 13-14 UT was contaminated by an external light source near nightfall, which made it difficult to calculate the correct airglow depletion value; this value is not shown. The above results in Figure 2 reflect the relationship between EPBs and scintillation to some extent. However, uncertainties could be introduced from the movement of IPP trajectories of MEO satellites. Due to the orbital design of geostationary orbit (GEO) satellites, the IPP is considered to be in a fixed state, which provides advantageous conditions for further comparison with airglow images [43]. Figure 3 shows a comparison between the airglow depletion, S4 index, and dVTEC observed by GEO satellites. According to the longitude of the IPP, as shown in Figure 3a, the eight GEO satellites are arranged from east to west. The S4 and dVTEC results of the GEO satellites also show good agreement with the airglow depletion, as airglow depletion corresponds to a greater S4 index and stronger dVTEC oscillation. At \(\sim\)14.5 UT, as shown in Figure 3b-d, the airglow depletion seems to be interrupted, but scintillations still continue to appear. After confirmation from the original airglow image, it is found that this actually corresponds to two separate branch structures of the EPB tens of kilometers apart. The continuous unbroken scintillations may be due to the tilt of the signal path, which causes the signal to pass through the upper or lower part of the EPB, even though the IPP is located between the two branches. Therefore, the duration of airglow depletion is always less than the duration of scintillation because the signal path may pass through the EPB at various heights. The combined observations of airglow images and GEO satellite data clearly reveal the east-to-west zonal motion, depletion evolution, and lifetime of the EPB. Based on the spatial distribution and motion observed from airglow depletion, the core part of the EPB structure generally produced a greater S4 value, while the outer part of the EPB produced a smaller S4 value. For the EPB structure during 16-18 UT in Figure 3b-f and during 17.5-19 UT in Figure 3g,h, the depletion degree of the EPB gradually decreased. Correspondingly, the S4 index value and the dVTEC oscillation decreased. Near the end of the EPB lifetime at \(\sim\)18 UT in Figure 3e,f or \(\sim\)18.5 UT in Figure 3g,h, although the airglow still showed obvious depletion, the S4 index and dVTEC only presented slight changes.