Quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during 2009–2015

Abstract

This study investigates the quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during 2009–2015. We obtained TEC observation data from four Global Navigation Satellite System (GNSS) stations over the African equatorial and low latitude, namely: Asab (ASAB, geographic coordinates 13.0°N, 42.65°E; geomagnetic coordinates 4.85°N, 114.34°E), Arba Minch University, (ARMI, 6.03°N, 37.56°E; 3.06°S, 109.29°E), Yamoussoukro (YKRO, 6.87°N, 5.24oW; 2.84°S, 67.41°E) and Mbarara (MBAR, 0.60°S, 30.74°E; 10.27°S, 102.36°E). Seven selected major Geomagnetic Storms (GSs) over solar cycle 24 were investigated and the responses of TEC and Rate of change of TEC Index (ROTI) as proxy for ionospheric irregularities to the storms were also studied. We recorded weaker ionospheric plasma density distributions around the magnetic equator (trough) than at the inner flank of the Equatorial Ionization Anomaly (EIA) crests: A clear evidence of EIA formation. Furthermore, hemispherical asymmetry was observed in the latitudinal distribution of TEC, with consistently higher distribution in the Northern Hemisphere (NH) than Southern Hemisphere (SH). TEC and ionospheric irregularities showed clear dependence on solar activity. Negative responses dominated GSs that occurred at night-time and positive effects for those that occurred during the daytime. The enhancement or inhibition of irregularities during GS is strongly dependent on the time of GS unfolding. As previously established, the daily maximum distribution of TEC at 1400–1600 LT and minimum at 0400–0600 LT were validated by this study, using long time (7 years) data. Seasonally, TEC showed semi-annual pattern of variations, with highest values in equinoxes, and lowest in solstices.