Global Ionospheric TEC Variability: Long-Term Spectral–Wavelet Analysis and Geomagnetic Storm Event Case Studies

This study investigates global ionospheric variability using Global Total Electron Content (GTEC), defined as the spatial average of daily averaged Global Ionospheric Maps derived from the International GNSS Service. The analysis spans from 2010 to 2024, covering Solar Cycle 24 and the rising phase of Solar Cycle 25, and incorporates geomagnetic indices (Dst and Kp). Spectral, statistical, and wavelet-based techniques are applied to characterize long-term periodicities, storm-time perturbations, and phase relationships between ionospheric and geomagnetic parameters. Spectral analysis reveals persistent periodicities near 27 days, corresponding to the synodic solar rotation period, together with pronounced semiannual and annual modulations associated with seasonal solar–magnetospheric coupling. Time-resolved analysis shows that the quasi-27-day signal drifts slightly around the nominal solar rotation period depending on solar cycle phase but exhibits only weak correlation with geomagnetic storm occurrence. Three geomagnetic storms (25–27 October 2011; 25-30 May 2017; and 10–11 May 2024) are examined to investigate magnetosphere–ionosphere coupling under different solar conditions. Wavelet coherence analysis indicates that Dst generally leads GTEC variability, whereas Kp and GTEC exhibit partial in-phase coherence during storm main phases. Mean phase angles derived for these events confirm consistent magnetosphere-to-ionosphere causal ordering. During the 10–11 May 2024 superstorm, GTEC increased to about 8.2 TECU before undergoing rapid depletion despite continued geomagnetic disturbance. This transition is attributed to enhanced Joule heating and a reduced thermospheric O/ (Formula presented.) ratio. Overall, the results show that global TEC variability reflects both recurrent solar periodicities and transient storm-time processes.