## 2 Materials and Methods  
The total electron content (TEC), amplitude scintillation index (S4) and phase scintillation index (\(\sigma_{\varphi}\)) data used in this study are obtained from the GNSS ionospheric scintillation and TEC monitoring (GISTM) multi-constellation, multi-frequency GNSS receiver (Septentrio make PolaRx5S) established at a low-latitude location, KL University, Guntur, India (Geographic 16\({}^{\circ}\)26\({}^{\prime}\)N 80\({}^{\circ}\)37\({}^{\prime}\)E, dip 22\({}^{\circ}\)32\({}^{\prime}\)). The raw data recorded by the receiver at a 50 Hz frequency is converted to ionospheric scintillation monitoring record (ISMR) format that contains the TEC values every 15 s along with many other parameters including scintillation indices, time of the week, elevation angle, azimuth angle, etc., for every minute. To validate the TEC values recorded by the GISTM receiver, we employ the recorded RINEX observation files in an alternative TEC calculation software, such as TayAbsTEC [72; 73; 74] and GPS-TEC analysis [75]. It is observed that the results are quite comparable to the GISTM recordings with GPS-TEC analysis plots apparently coinciding with the recorded TEC outputs. The signal combinations and satellite biases handled by the GISTM receiver for each constellation to determine TEC are presented in Table 1. The station biases corresponding to the antenna, connecting cable, and receiver are routinely calibrated against SBAS for better accuracy. In brief, the TEC is measured from P-code measurements, whereas the dTEC is computed from the carrier phase measurements only. On the contrary, in GPS-TECanalysis, the satellite differential code biases (DCBs) are taken from IGS CODE files and receiver biases are estimated by itself, whereas TayAbsTEC estimates both of the biases independently for determining TEC. It is worth mentioning that in the present case, the detrending method is used in the individual PRN signals for calculating TEC depletion assuming that the common receiver-specific errors are eliminated in the analysis and hence no external calibration is involved in this study. Table 1 shows the available signals from various GNSS constellations and their combinations used by the PolaRx5S receiver unit for estimating scintillation indices and TEC, respectively. At present, the station records data for GPS, Galileo, and BeiDou at three available frequencies, GLONASS and GAGAN at two frequencies, and Navigation with Indian Constellation (NavIC) at only one frequency. In the present study, we do not include NavIC and GAGAN constellations for the TEC variable as NavIC records data only in L5, whereas TEC recorded by the latter is uncorrected with biases. In the present study, we select four geomagnetically disturbed days (see Table 2) in the ascending phase of the 25 th solar cycle with a typical threshold of disturbance storm index (Dst) \(<-100\) nT for analyzing the triggering/suppression of EPBs through signatures of TEC depletion and scintillation indices in the GNSS signals. The associated interplanetary magnetic field (IMF Bz) as well as geomagnetic ap index (ap), and Kp indices downloaded from the Omni Web-NASA server ([[https://omniweb.gsfc.nasa.gov/form/dx1.html](https://omniweb.gsfc.nasa.gov/form/dx1.html)]([https://omniweb.gsfc.nasa.gov/form/dx1.html](https://omniweb.gsfc.nasa.gov/form/dx1.html)), accessed on 5 August 2023) are also explored for a clear understanding of the onset and evolution of the storms. The presence of bubbles is discovered by monitoring the sudden fall in TEC revealed in the detrended TEC curve. The detrended TEC curve is obtained by subtracting the observed TEC from its moving average with a specific window size which determines the number of observations required to calculate the moving average. Earlier studies used different window sizes like 10 min [13; 76; 77], 60 min [38], and 91 min [14], based on the location and resolution of the input data. We also try a 120 min window size to check for the fitting of the detrended TEC for possible plasma bubble detection.