Investigating Ionospheric TEC Variations in Solar and Geomagnetic Influences Across Solar Activity Phases
This study examines the variability of ionospheric total electron content (VTEC) in response to solar and geomagnetic drivers across solar cycles 23 to 25. While the dominant effect of solar radiation on VTEC is well-known, a comprehensive understanding of how these relationships and their time-lags vary across distinct solar cycle phases and across cycles of differing intensity has been lacking. Using global VTEC data from the Chinese Academy of Sciences Global Ionospheric Maps (CASG) and solar-geophysical indices from NASA's OMNI dataset spanning from 1998 to 2025, this study bridges that gap by quantifying correlation strengths and time-lag relationships between VTEC and parameters such as $F_{10.7}$ solar flux, R sunspot number, Kp, Ap, and Dst indices, and solar wind properties. Results show that solar proxies, particularly $F_{10.7}$ and R sunspot number, exhibit the strongest, most consistent correlations with VTEC, especially during the ascending and descending phases of the solar cycle, with a characteristic $\sim2$-day lag attributed to thermospheric oxygen dynamics and ionospheric recombination processes. In contrast, geomagnetic indices exhibit weaker and phase-dependent correlations, while direct correlations between solar wind parameters and global VTEC are weak, as their influence is primarily mediated by geomagnetic activity and exhibits strong regional and temporal heterogeneity. Phase-resolved analyses further reveal that geomagnetic activity plays a more prominent role during transitional phases, while maximum and minimum periods are dominated by EUV variability and non-solar drivers, respectively. These findings highlight the necessity of incorporating solar phase and time-lag dependencies in ionospheric modelling and forecasting efforts.