Abstract

Abstract. We investigated total electron content (TEC) at Ilorin (8.50∘ N 4.65∘ E, dip lat. 2.95) for the year 2010, a year of low solar activity in 2010 with Rz=15.8. The investigation involved the use of TEC derived from GPS, estimated TEC from digisonde portable sounder data (DPS), and the International Reference Ionosphere (IRI) and NeQuick 2 (NeQ) models. During the sunrise period, we found that the rate of increase in DPS TEC, IRI TEC, and NeQ TEC was higher compared with GPS TEC. One reason for this can be attributed to an overestimation of plasmaspheric electron content (PEC) contribution in modeled TEC and DPS TEC. A correction factor around the sunrise, where our finding showed a significant percentage deviation between the modeled TEC and GPS TEC, will correct the differences. Our finding revealed that during the daytime when PEC contribution is known to be absent or insignificant, GPS TEC and DPS TEC in April, September, and December predict TEC very well. The lowest discrepancies were observed in May, June, and July (June solstice) between the observed values and all the model values at all hours. There is an overestimation in DPS TEC that could be due to extrapolation error while integrating from the peak electron density of F2 (NmF2) to around ∼1000 km in the Ne profile. The underestimation observed in NeQ TEC must have come from the inadequate representation of contribution from PEC on the topside of the NeQ model profile, whereas the exaggeration of PEC contribution in IRI TEC amounts to overestimation in GPS TEC. The excess bite-out observed in DPS TEC and modeled TEC indicates over-prediction of the fountain effect in these models. Therefore, the daytime bite-out observed in these models requires a modifier that could moderate the perceived fountain effect morphology in the models accordingly. The daytime DPS TEC performs better than the daytime IRI TEC and NeQ TEC in all the months. However, the dusk period requires attention due to the highest percentage deviation recorded, especially for the models, in March, November, and December. Seasonally, we found that all the TECs maximize and minimize during the March equinox and June solstice, respectively. Therefore, GPS TEC and modeled TEC reveal the semiannual variations in TEC.

Highlights

  • Total electron content (TEC) is the total number of free electrons in a columnar of 1 m2 along the radio path from the satellite to the receiver on the Earth

  • We found around the sunrise period, the model TEC rises faster than the GPS TEC, but International Reference Ionosphere (IRI) TEC rises faster compared to digisonde portable sounder (DPS) TEC and NeQuick 2 (NeQ) TEC

  • We found that NeQ TEC underestimated TECNeQ−GPS by between ∼ 0.11 (May) and ∼ 9.72 (November)

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Summary

Introduction

Total electron content (TEC) is the total number of free electrons in a columnar of 1 m2 along the radio path from the satellite to the receiver on the Earth. TEC exhibits diurnal, seasonal, solar cycle, and geographical variations. The physical and dynamical morphology of the TEC over a given location is of great importance in transionospheric communications during both quiet and disturbed geomagnetic conditions (Aravindan and Iyer, 1990; Akala et al, 2012; Olawepo et al, 2015; Tariku, 2015 and de Jesus et al, 2016). GPS TEC is quantified from the GPS orbiting satellites to the GPS receiver station on the Earth, with an approximate distance of 20 200 km (Liu et al, 2006). A typical GPS TEC measurement incorporates the complete plasmaspheric electron content (PEC). The digisonde portable sounder (DPS) estimates the bottomside and topside TEC to obtain the total TEC from the electron density (Ne) profile

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