Abstract
Abstract. 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, ROTIave) 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 ROTIave, 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 ROTIave 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 ROTIave 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.
Highlights
The ionosphere, which consists of free electrons and ions, frequently experiences irregular electron density
To show the performance of the electric field (EEF) model over the East African sector, we have presented the relationship between H and EEF for 5 international quiet days of each month of the year 2012
They reported that the vertical plasma drifts observed from Jicamarca incoherent scatter radar (ISR) has a positive and linear relation with H and H was widely taken as a proxy substitute for the EEF
Summary
The ionosphere, which consists of free electrons and ions, frequently experiences irregular electron density. The characteristics of ionospheric scintillation and ionospheric irregularities over the equatorial and low-latitude region in different longitudinal sectors during different solar and geomagnetic activities have been studied (e.g., Burke et al, 2004; Paznukhov et al, 2012; Oladipo and Schuler, 2013a; Seba and Tsegaye, 2015). The inhomogeneity of ionospheric electron distribution can cause sudden, rapid and irregular fluctuations of the amplitude and phase of the received signals, known as ionospheric scintillation (Wernik and Liu, 1974) This inhomogeneity, i.e, the spatial plasma density or TEC gradient, varies significantly in the low-latitude region because of geomagnetic storms, equatorial spread F (ESF) and the Appleton anomaly. The gradients of plasma density might be considered as an important parameter in the modeling of ionospheric irregularities and mitigating positioning errors on GNSS-based applications
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