## 3 Results and Discussion
Figure 9: ROTI variation at the present location (KLEF) and other geodetic GNSS stations under ICS network (IISC, HYDE, and LCK3), ranging from near equatorial latitude to outer edge of equatorial ionization anomaly (EIA) crest latitude in the Indian longitude sector on 23 April 2023, a geomagnetic storm day. Figure 10 depicts the TEC (black) retrieved from PRNs 2, 10, 18 and 32 belonging to the GPS constellation and the respective scintillation indices (S4 in blue and \(\sigma_{\varphi}\) in orange) for Signal 1 (refer to Table 1) along with the elevation angles (pink) on 23 April 2023. The possible association of EPBs can be inferred from the significant TEC depletions in the signals from selected PRNs. We note that the TEC depletion time and the scintillation activity are between 15:00 UT and 18:00 UT. In all the above PRNs, the minimum in the TEC depletion curve synchronizes with the maximum value of scintillation indices (S4 and \(\sigma_{\varphi}\)). All the depletions in the TEC detrended curve satisfy the plasma bubble criteria in terms of depletion depth (10 to 45 TECU), apparent duration (10 to 45 min), and maximum values (12.6 to 18.5 TECU) around the depletion. When an anomaly is observed in multiple station satellite pairs, the anomaly is due to the real ionospheric effect and not due to the receiver bias or data error. To further emphasize the association of EPBs, we analyze TEC depletions from other available GNSS constellations like GLONASS, Galileo, and BeilDou as multi-GNSS measurements could contribute to increasing the spatial coverage of the ionosphere and better understanding the local morphology of its fluctuations [86]. In Figure 11, we show the TEC (black) with elevation angle (pink), S4 (blue) and \(\sigma_{\varphi}\) (orange) of Signal 1 for PRNs 44, 58 and 59 under the GLONASS constellation. PRN 59 seems to be indicating the presence of EPB for a longer period than the other two PRNs as it is showing multiple depletion depths. All the depletions in the span of 15:00 UT to 18:00 UT satisfy the plasma bubble criteria and agree well with the S4 and \(\sigma_{\varphi}\) variations. Figure 12 depicts the TEC (black), elevation angle (pink), S4 (blue) and \(\sigma_{\varphi}\) (orange) of Signal 1 for the PRNs affected by the plasma bubble in the Galileo constellation. Only two PRNs (82 and 89) are affected by the plasma bubble. The depletion in PRN 89 is higher than in PRN 82 and the elevation angle is \(>\)40\({}^{\circ}\). Figure 13 projects the TEC (black) with elevation angle (pink), S4 (blue) and \(\sigma_{\varphi}\) (orange) of Signal 1 for the PRNs affected in the BeilDou constellation due to EPBs. A good number of satellites (PRN 142, 148, 149, 152, 153, and 156) from this constellation witness depletions fulfilling the criteria for possible association of EPBs. In the BeilDou constellation, more than two depletion depths are observed in all the PRNs showing the sensitivity of the constellation to the EPBs which are verified to match well with the collocated scintillation indices (S4 and \(\sigma_{\varphi}\)). Hence, from Figures 10-13, it is  
Figure 10: The sTEC (black), elevation angle (pink) and their respective scintillation indices S4 (blue) and \(\sigma_{\varphi}\) (orange) of the GPS PRNs 2, 10, 18 and 32, confirming depleted TEC between 15:00 UT and 18:00 UT on the 23 April 2023 geomagnetic storm day. evident that in all the constellations, the observed PRNs are encountered with depletions in TEC for a short period within the span of 15:00 UT to 18:00 UT (20:30 LT to 23:30 LT) that falls in the local post-sunset hours to midnight. The results are in good agreement with the earlier studies [38; 39; 87]. It is important to mention that an elevation angle threshold (>30\({}^{\circ}\)) is applied throughout the study irrespective of PRNs to avoid contaminations with possible multipath and ground interference errors.