Geomagnetic control of ionospheric TEC and irregularities over the African sector using GNSS observations

Abstract
GNSS-derived total electron content (TEC) is a key observable for characterising ionospheric variability and assessing its impact on positioning, navigation, and timing (PNT) systems. However, most studies over the African sector focus on climatological means or event-based analyses, leaving the statistical distribution and extreme-value behaviour of TEC insufficiently quantified. This study addresses this gap using a comprehensive statistical analysis of GNSS TEC observations from five stations spanning equatorial, low-latitude, and mid-latitude African regions over the period 2015–2020. A probabilistic statistical framework is applied to characterise TEC distributions, latitudinal variability, geomagnetic modulation, and irregularity intensity. A Tail Amplification Factor (TAF) is introduced to quantify enhancements in upper-tail TEC behaviour under geomagnetically disturbed conditions. Results show that TEC exhibits strongly non-Gaussian behaviour, with persistent right-skewness and heavy tails across all stations. A clear latitudinal gradient is observed, with higher TEC values at low geomagnetic latitudes and systematic decreases toward mid-latitudes. Geomagnetic activity increases mean TEC by approximately 26% and enhances upper-tail percentiles by up to a factor of 1.29. Irregularity intensity increases under disturbed conditions, with a network-wide amplification of approximately 1.5. These results demonstrate that African ionospheric variability is governed by coupled latitudinal structure and geomagnetic forcing, with extreme-value behaviour playing a dominant role. The findings highlight the importance of distribution-aware approaches for improved ionospheric characterisation and GNSS performance assessment.