Ionospheric Response to the 25–26 August 2018 Geomagnetic Storm Using Detrended GPS-TEC and Wavelet Spectral Analysis at Low-MID Latitude Stations

This study investigates the ionospheric response to the moderate geomagnetic storm of 25–26 August 2018 (Dstmin ≈ –174 nT), using GPS-derived Total Electron Content (TEC) observations from six stations distributed across low and mid latitudes in the Indian and Asian sectors. A systematic detrending approach was applied to remove diurnal trends and long-period background variations, thereby isolating storm-driven disturbances. Continuous Wavelet Transform (CWT) analysis of the detrended TEC revealed statistically significant (95% confidence against first-order autoregressive (AR1) red-noise background) storm-associated oscillations in the 30–90-min range, corresponding to atmospheric gravity waves (AGWs) and medium-scale traveling ionospheric disturbances (MSTIDs). Low-latitude stations exhibited the strongest signatures, consistent with equatorial electrodynamic coupling and Equatorial Ionization Anomaly (EIA) modulation. A segmented CWT analysis effectively captured the pre-storm, main-phase, and recovery-phase evolution. These findings highlight coherent equatorward propagation of storm-induced AGWs/TIDs and demonstrate the utility of multi-station TEC-wavelet diagnostics for storm-ionosphere coupling studies.