Abstract
Ionospheric Total Electron Content (TEC) variability during geomagnetic storms is a major space weather concern because it can degrade the accuracy and reliability of Global Navigation Satellite System (GNSS) applications. Despite growing dependence on GNSS technologies across Africa, observations of storm-time TEC responses remain limited due to the sparse distribution of ionospheric monitoring stations. This study investigates the temporal evolution and magnitude of storm-induced TEC disturbances over the African sector and provides observational constraints for regional ionospheric modelling and space weather forecasting. The study assesses temporal variations and magnitude of ionospheric TEC perturbations (ΔTEC) relative to interplanetary magnetic field Bz (IMF Bz), solar wind speed (Vx), Disturbance storm time (Dst) index, and Auroral Electrojet (AE) index during geomagnetic storm recovery phases. GNSS-derived ΔTEC data from five African-sector stations NKLG (0.33°N), DJIG (11.52°N), RBAY (−28.79°), RBAT (33.99°N), and HER (−34.88°) were analyzed for the 12–14 October 2016 (Dst min = −86 nT) and 23–25 April 2023 (Dst min = −213 nT) geomagnetic storms. Both events exhibited well-defined storm sudden commencement (SSC), main phase (MP), and recovery phase (RP). Positive TEC enhancements during the main phase were associated with prompt penetration electric fields (PPEFs), followed by negative TEC depletions during recovery attributed to disturbance dynamo (DD) effects. TEC variability increased with storm intensity, with the April 2023 storm producing depletions reaching −35.6 TECU at HER compared with −2.7 TECU during October 2016. Temporal delays between Dst minimum and maximum TEC depletion ranged from 6 to 12 h, with equatorial stations responding earlier than mid-latitude stations. Southward IMF Bz turnings and elevated solar wind speeds were closely associated with TEC perturbations. The results demonstrate that storm intensity and latitude strongly influence TEC variability across the African sector and highlight the importance of regional observations for understanding ionospheric storm dynamics and improving TEC prediction and modelling capabilities.

Key words: Geomagnetic storm, Ionospheric storm, Ionospheric Disturbance dynamo, Total Electron Content (TEC), Prompt Penetration Electric field (PPEF)