## 3 Results and Discussion
It is important to mention that GAGAN does not have any frequency matching with Signal 2, whereas the NavIC system does not carry legacy Signals 1 and 2 as per the categories of signals listed in Table 1. The high intensities of the S4 and \(\sigma_{\varphi}\) coincide with the depletion depth. While the GPS constellation has the maximum S4 values compared to all the other constellations, BeiDou shows the maximum level of \(\sigma_{\varphi}\) among all constellations. In Figure 16a,b, we present S4 and \(\sigma_{\varphi}\) variations for Signal 3 under global (GPS, Galileo, and BeiDou) and Indian (GAGAN and NavIC) constellations. However, we cannot present the corresponding scintillation indices for GLONASS as it does not include Signal 3 at present in the ISMR recordings. Figures 15b and 16a show frequent enhancement of scintillation indices corresponding to signals under the BeiDou constellation, demonstrating relatively longer effects of EPBs. The scintillation values in all the PRNs in Figures 13-16 lie within moderate to strong scintillation activity as per the classification scheme for S4 and \(\sigma_{\varphi}\) followed in our study. A quick overview of the maximum levels of S4 and \(\sigma_{\varphi}\) in all the GNSS constellations and their respective frequencies can be obtained from Table 4. It is also clear from S4 and \(\sigma_{\varphi}\) variability that the level of the scintillation effect is inversely proportional to the signal frequency demonstrating relative dependance of scintillation effects on the penetrating power of the signals. The results reported are consistent with the previous studies showing the scintillations associated with EPBs during the post-sunet hours and their decaying after the midnight period [14; 39; 76]. Moreover, the impact of the depleted TEC during scintillation activity is realized in terms of ionospheric delays that are supported by enhanced S4, \(\sigma_{\varphi}\), and ROTI indices in our study. Concerning the association of TEC depletions and scintillation activity with the occurrence of EPBs at the present low-latitude location, the EPBs are considered to be the depleted plasma density regions typically seen under the influence of the equatorial electrodynamics. In the present work, we determine the TEC depletion characteristics by detrending the high-resolution (15 s) sTEC observables from signals under different GNSS constellations at a low-latitude location with a 60 min window size during the geomagnetically disturbed days. Threshold constraints like depletion depth (10 TECU) and depletion duration (10 to 180 min) are applied to selectively choose the sTEC depletions that highly correspond to the EPB occurrences over the region. [PERSON] et al. [90] employed a moving average window of 10 min to extract the depletions associated with EPBs by a depletion depth threshold of 5 TECU. Similarly, [PERSON] et al. [38] used a 60 min window size to calculate the moving average and obtained the detrended TEC to find out the possible occurrence of the plasma bubble with a minimum depletion depth of 10 TECU. A similar method was applied by [PERSON] et al. [14] with a 91 min window size and 10 TECU depletion depth threshold for a 10 min resolution sTEC data to find the plasma bubble activity in the African region. However, the earlier literature and the observations from the present work demonstrate that a 60 min window size with a depletion depth of 10 TECU is the best combination of thresholds to suitably identify the depletions highly associated with EPBs. Thus, the present method of indirectly detecting the presence of EPBs could disregard TEC  
Figure 15: (**a**) The S4 (blue) and \(\sigma_{\varphi}\) (orange) of Signal 2 for all the available PRNs, showing the TEC depletions in the GPS, GLONASS and Galileo constellations between 15:00 UT and 18:00 UT on the 23 April 2023 geomagnetic storm day. (**b**) The S4 (blue) and \(\sigma_{\varphi}\) (orange) of Signal 2 for all the available PRNs showing the TEC depletions in the BeiDou constellation.