## 1 Introduction
It shows that a relatively denser coverage of global ROTI datasets from a huge infrastructure of conventional geodetic GNSS receivers than any other ground-based monitoring units across the globe appeals exploitation of TEC depletion strengths and ROTI for understanding long-term global morphology of EPBs [38; 39; 40; 41]. TEC or slant total electron content (sTEC) is the number of electrons present in the ray path between the satellite and the receiver measured in TECU where 1 TECU = 10\({}^{16}\) electrons/m\({}^{2}\)[42]. [PERSON] et al. [13] used the sTEC data obtained from the GPS satellites at six stations all over the world to find the plasma bubble occurrence. The model response at different seasons and temporal variations was observed. The in-depth analysis showed a TEC depletion ranging from 6 TECU to 18 TECU from all the stations. [PERSON] et al. [14] collected the sTEC values every 10 min to find out the plasma bubble formation in the equatorial Africa region for the early 6 months of 2004. The results show that the average plasma bubble depletion depth is 9 TECU while the average apparent depletion time is 90 min. [PERSON] et al. [38] used the sTEC data from high-rate GNSS receivers at African GNSS stations in 2014 and 2015 to verify the possible EPB occurrence. The preliminary observations in the TEC detrended curve showed the characterization depth of EPB ranging from 10 TECU to 40 TECU, and the depletion time ranging from 15 to 68 min. [PERSON] et al. [39] used data from 23 GPS receivers along the dip equator to verify the plasma bubble occurrences from 2000 to 2006. Their observations from the monthly occurrence rates of the EPB in all the longitudinal stations emphasize that sunset time lag played a crucial role in the evolution of EPBs. With the introduction of a greater number of low earth orbiting (LEO) satellites, a great opportunity was provided for studying the global spatiotemporal features of EPBs and ionospheric irregularities from using onboard high resolution magnetic field measurements, GNSS radio occultation (RO) or Langmuir probe (LP) measurements which can address the limitations of sparse coverage from ground-based observations up to some extent. Unlike other available constellations, the special configuration in Swarm constellation provides an opportunity for investigating the plasma irregularities in both meridional and zonal directions for understanding their relationship with plasma depletion and transient loss of lock in GNSS signals [43]. [PERSON] et al. [44] conducted a multi-instrumental analysis with F3/C1 RO electron density profiles and scintillation indices along with in situ plasma probe observations from C/NOFS, Swarm, and DMSP satellites to characterize development of the storm induced EPBs and scintillation effects. Global DMSP profiles of plasma densities during the high solar activity periods detects the post-sunset variation of electric fields in the equatorial ionosphere, which is the source of increased occurrence of EPBs as closely aligned magnetic field lines with the terminator favors the formation of bubbles [45]. Unlike other ground-based or LEO observations, with the two-dimensional images of the equatorial and low-latitude ionosphere taken by Global-scale Observations of the Limb and Disk (GOLD) imager from geostationary orbit provide scopes for unambiguous measurement of the formation, drifting direction, and evolution of EPBs on a global scale morphology [46]. [PERSON] et al. [47] investigated the longitudinal discrepancies in the onset time and occurrence rate of plasma depletions using GOLD data, demonstrating that the depletions can occur an hour or more after the onset of EPBs depending on the season and longitude of observation. Results of [PERSON] et al. [48] reported that EPBs are clearly discernable in the GOLD nighttime O1 135.6 radiance and may serve as reference for identifying its link with TEC depletion as observed by GNSS-based measurements. Recent work of [PERSON] et al.