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 of 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 climatology, and variability magnitude. The results reveal clear longitudinal day-to-day variability between East and West African sectors, with larger variability 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 0.678 to 0.848, whereas cross-sector correlations range from 0.476 to 0.689. 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.