Investigation of the relationship between the spatial gradient of total electron content (TEC) between two nearby stations and the occurrence of ionospheric irregularities

Abstract

Abstract. The relation between the occurrence of ionospheric irregularities and the spatial gradient of total electron content (TEC) derived from two closely located\nstations (ASAB: 4.34 ∘ \u2009N, 114.39 ∘ \u2009E and DEBK: 3.71 ∘ \u2009N, 109.34 ∘ \u2009E, geomagnetic), located within the equatorial region,\nover Ethiopia, during the postsunset hours was investigated. In this study, the Global Positioning System (GPS)-derived TEC during the year 2014\nobtained from the two stations were employed to investigate the relationship between the gradient of TEC and occurrence of ionospheric irregularities. The\nspatial gradient of TEC ( ΔTEC∕Δlong ) and its standard deviation over 15\u2009min, σ(ΔTEC∕Δlong ), were used in this study. The\nrate 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\n ΔTEC∕Δlong are noticeable during the time period between 19:00 and 24:00\u2009LT. In some cases, the peak values in the spatial gradient\nof TEC are also observed during daytime and postmidnight hours. The intensity level of σ(ΔTEC∕Δlong) observed after postsunset show\nsimilar trends with ROTI ave , and was stronger (weaker) during equinoctial (solstice) months. The observed enhancement of σ(ΔTEC∕Δlong) \nin the equinoctial season shows an equinoctial asymmetry where the March equinox was greater than the September equinox. During the postsunset period, the\nrelation between the spatial gradient of TEC obtained from two closely located Global Navigation Satellite System (GNSS) receivers and the equatorial electric\nfield (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\ndelay of about 1–2\u2009h between them. The relationship between σ(ΔTEC∕Δlong) and ROTI ave correlate linearly with correlation\ncoefficient 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\nobserved during the evening time period may be associated with ionospheric irregularities or equatorial plasma bubbles. In addition to latitudinal gradients,\nthe longitudinal gradient of TEC has contributed significantly to the TEC fluctuations.