The relation between the occurrence of ionospheric irregularities and the spatial gradient of total electron content (TEC) derived from two closely located stations (ASAB: 4.34^∘ N, 114.39^∘ E and DEBK: 3.71^∘ N, 109.34^∘ E, geomagnetic), located within the equatorial region, over Ethiopia, during the postsunset hours was investigated. In this study, the Global Positioning System (GPS)-derived TEC during the year 2014 obtained from the two stations were employed to investigate the relationship between the gradient of TEC and occurrence of ionospheric irregularities. The spatial gradient of TEC (ΔTEC∕Δlong) and its standard deviation over 15 min, σ(ΔTEC∕Δlong), were used in this study. The rate of change of TEC-derived indices (ROTI, ROTI_ave) were also utilized. Our results revealed that most of the maximum enhancement and reduction values in ΔTEC∕Δlong are noticeable during the time period between 19:00 and 24:00 LT. In some cases, the peak values in the spatial gradient of TEC are also observed during daytime and postmidnight hours. The intensity level of σ(ΔTEC∕Δlong) observed after postsunset show similar trends with ROTI_ave, and was stronger (weaker) during equinoctial (solstice) months. The observed enhancement of σ(ΔTEC∕Δlong) in the equinoctial season shows an equinoctial asymmetry where the March equinox was greater than the September equinox. During the postsunset period, the relation between the spatial gradient of TEC obtained from two closely located Global Navigation Satellite System (GNSS) receivers and the equatorial electric field (EEF) was observed. The variation in the gradient of TEC and ROTI_ave observed during the evening time period show similar trends with EEF with a delay of about 1–2 h between them. The relationship between σ(ΔTEC∕Δlong) and ROTI_ave correlate linearly with correlation coefficient of C=0.7975 and C=0.7915 over ASAB and DEBK, respectively. The majority of the maximum enhancement and reduction in the spatial gradient of TEC observed during the evening time period may be associated with ionospheric irregularities or equatorial plasma bubbles. In addition to latitudinal gradients, the longitudinal gradient of TEC has contributed significantly to the TEC fluctuations.

We employ multi-instrumental data to investigate the behavior of equatorial and low latitude ionosphere during the geomagnetic storm of November 3-6, 2021. We used TEC data obtained from GPS receiver stations located in the equatorial and low-latitudes of the Asian, African, and American sectors. It is found that the storm-time ionization level varies significantly in the trough and crest of EIA region over the three longitudes. ROTI is used to estimate the occurrence of ionospheric plasma irregularities during the storm. Usually, the main phase of the geomagnetic storm triggers the equatorial plasma irregularities and the recovery phase suppresses the occurrence of them. Here, we observed inhibition of the plasma irregularities over the three sectors during the main phase of the storm. We suspect this may be due to the injection of the PEFs which occur between local midnight and around noon during the main phase. The PEFs restrict the diffusion of plasma and therefore, suppress the occurrence of plasma irregularities during the main phase. During the recovery phase, moderate ionospheric irregularities occurred at local midnight in the American sector. In the African sector, the occurrence of weak irregularities can be seen before midnight on November 5 and 6. However, the Asian sector does not exhibit noticeable ionospheric irregularities during the storm. We conclude that the longitudinal variation in the development of ionospheric irregularities can be influenced by factors such as local time occurrence of maximum ring current, PPEF, disturbance wind dynamo electric field, and shielding electric field.