The First Evidence for the Detection of CIDs Masked by Equatorial Plasma Bubbles From GPS‐TEC Data

The strong influence of equatorial plasma bubbles (EPBs) appears to mask co‐seismic ionospheric disturbances (CIDs) for the earthquakes occurring in the equatorial low‐latitudes during the post‐sunset time. Based on GPS‐TEC data, we attempted to unveil the CIDs for the two great seismic events that are the 2005 M 8.6 Nias and 2017 M 8.2 Mexico earthquakes. In the case of the Nias earthquake, the CIDs have been identified in the bandpass filtered vertical total electron content data obtained from selective GPS satellite‐receiver pairs. For the Mexico earthquake, the EPB anomalies are dominant, making the appearance of CIDs hazy. However, the N‐shaped vertical total electron content anomalies masked by the plasma bubble anomalies in the bandpass filtered data are clear after applying the frequency domain singular spectrum analysis (SSA). Furthermore, the frequency domain SSA indicates the acoustic and surface waves of CIDs for both earthquakes. It also shows the signature of propagating gravity waves for the Mexico earthquake that has generated a moderate tsunami. Thus, the present study illustrates that the CIDs concealed by the non‐stationary EPB anomalies for earthquakes occurring in low latitude equatorial regions during post‐sunset become visible. Plain Language Summary The studies carried out so far indicate that the equatorial plasma bubble (EPB) activity masks the co‐seismic ionospheric disturbances (CIDs) detected in GPS data in the low equatorial latitudes. However, EPB activity and generation of CIDs are linearly independent processes. Thus, EPBs may mask but cannot conceal the propagation of CID to the ionospheric heights. Furthermore, previous studies indicated that even a specially designed bandpass filter fails to separate the CIDs from background EPBs. But, the singular spectrum analysis (SSA) method is known for its robustness in separating such linearly independent components. In the present study, we reanalyze the GPS‐TEC data for the great 2005 Nias and 2017 Mexico earthquakes for plausible detection and separation of CIDs using the frequency domain SSA method. The CIDs emerged in the bandpass filtered VTEC data for some combinations of satellite and station pairs in the case of the Nias earthquake. But for the Mexico earthquake, EPB anomalies are dominant, and CIDs are not visible clearly. Interestingly, the CIDs corresponding to acoustic and surface waves for both earthquakes are visible after applying frequency domain SSA. Additionally