Longitudinal Day-to-Day Variability of the Rate of Change of the TEC Index (ROTI) over the African Equatorial Ionization Anomaly Region

The rate of change in the total electron content index (ROTI) is a widely used proxy for phase fluctuations associated with ionospheric irregularities derived from GNSS total electron content measurements. This study investigates the longitudinal day-to-day variability of ionospheric irregularities over the African Equatorial Ionization Anomaly (EIA) region during 2014 using ROTI from six GNSS stations at low latitudes. Day-to-day variability is quantified through differences between consecutive daily ROTI values (ΔROTI), relative ROTI (RROTI), correlation coefficients, daily deviations from climatological trend, and variability magnitude. The results demonstrate notable longitudinal variations in day-to-day variability between the East and West African sectors, with predominantly higher variability observed in West Africa. Diurnal ROTI peaks reach about 2 TECU/min in East Africa and about 3 TECU/min in West Africa. Correlation coefficients within the same longitude sector range from 0.68 to 0.85, whereas cross-sector correlations range from 0.48 to 0.69. ΔROTI and RROTI indicate that day-to-day variability is driven mainly by short-period lower-atmospheric forcing rather than slow solar forcing. Relative variations range from about −70% to +150%, with predominantly negative values during solstices and positive values during equinoxes.