Ionospheric Variability Across Solar Cycles 23 and 24 in the Indian Longitude Sector: GPS Observations and IRI Model Validation

Abstract This study investigates Total Electron Content (TEC) variations across the Indian longitude sector (75°–85°E) from 2004 to 2014, spanning descending phase solar cycle 23 and ascending/peak phases of solar cycle 24, using GPS data from four stations: Bangalore (IISC), Varanasi (BHUP), Bishkek (POL), and Novosibirsk (NOVM). We analyze latitudinal, seasonal, and solar activity‐driven TEC variability and evaluate IRI‐2016 and IRI‐2020 model performance. Low‐latitude stations (IISC, BHUP) exhibit strong equinoctial peaks (∼40–70 TECU), semi‐annual anomalies, noontime bite‐outs driven by the equatorial ionization anomaly and drifts, and nighttime enhancements during high solar activity. Mid‐latitude stations (POL, NOVM) show lower TEC (∼20–35 TECU) with seasonal variations influenced by thermospheric winds. Equinoctial asymmetry is evident, with October TEC surpassing March–April in 2011–2013 due to rising solar EUV flux. TEC strongly correlates with F10.7/SSN ( R 2 ∼0.65–0.78, higher at low latitudes), reflecting strong solar control at low‐latitudes. At low latitudes, IRI‐2020 outperforms IRI‐2016 with lower RMSE, while at mid‐latitudes IRI‐2016 shows slightly better performance. Differential TEC (DTEC) reveals larger deviations at low latitudes ranging from −26.8 to +30.81 TECU, compared to smaller deviations at mid‐latitudes ranging from −8.71 to +18.50 TECU. Both models capture equinoctial enhancements but overestimate winter anomaly occurrence and show large errors during equinoxes, particularly at low latitudes. Unlike prior studies, this work examines TEC across an entire solar cycle, revealing regional model limitations. These findings enhance ionospheric model validation, critical for improving satellite navigation and space weather forecasting in the Indian sector.