Investigation of the relationship between the spatial gradient of total electron content (TEC) and the occurrence of ionospheric irregularities Teshome Dugassa 1,2 , John Bosco Habarulema 3,4 , and Melessew Nigussie 5 1 Ethiopian Space Science and Technology Institute, Department of Space Science and Application, Addis Ababa, Ethiopia 2 Bule Hora University, College of Natural and Computational Science, Department of Physics, Bule Hora, Ethiopia 3 South Africa National Space Agency, Space Science, Hermanus, South Africa 4 Department of Physics and Electronics, Rhodes University, Grahamstown, South Africa 5 Washera Geospace and Radar Science Laboratory, Physics Department, Bahir Dar University, Bahir Dar, Ethiopia Correspondence: Teshome Dugassa (tdugassa2016@gmail.com), John Bosco Habarulema (jhabarulema@sansa.org.za), Melessew Nigussie (melessewnigussie@yahoo.com) Abstract. The relation between the occurrence of ionospheric irregularity and the spatial gradient of total electron content (TEC) during the post-sunset hours over the equatorial region is investigated. Different instruments and techniques have been applied to study the behavior of these ionospheric irregularities. In this study, the Global Positioning System (GPS) based derived TEC was employed to investigate the relation between the spatial gradient of TEC between two nearby located stations and the occurrence of ionospheric irregularity over the East Africa longitudinal sector. The gradient of TEC between the two 5 stations (ASAB: 4.34 ◦ N, 114.39 ◦ E and DEBK: 3.71 ◦ N, 109.34 ◦ E, geomagnetic) located within the equatorial region of Africa were considered in this study during the year 2014. The rate of change of TEC based derived index (ROTI , ROTI ave ) is also used to observe the relation between the spatial gradient of TEC and the occurrence of ionospheric irregularities over the stations. The result obtained shows that most of the maximum enhancement/reduction in the spatial gradient of TEC observed in March and September equinoxes are noticeable between 19:00 LT - 24:00 LT as the large-scale ionospheric irregularities 10 do. Moreover, the observed spatial gradient of TEC shows two peaks (in March and September) and they exhibit equinoctial asymmetry where the March equinox is greater than September equinox. The maximum enhancement/reduction in the gradient of TEC and ROTI ave during the evening time period also show similar trends but after 1-2 hrs from the equatorial electric field (EEF). The relationship between the spatial gradient of TEC and ROTI observed during the nighttime hours correlate moderately with correlation coefficient of C = 0.58 and C = 0.53 over ASAB and DEBK, respectively. The vast majority of 15 the spatial gradient of TEC are associated with ionospheric irregularities that occur during the evening period. In addition to latitudinal gradients, the spatial gradient of TEC has a significant contribution on the computation of TEC fluctuation. The spatial gradient of electron density (TEC) near solar-terminator obtained from two nearby located Global Navigation Satellite System (GNSS) receivers may be used as an alternative method to estimate the strength of the zonal electric field. 20 Key words: Spatial gradient of TEC, ROTI ave , ionospheric irregularities 1