摘要：This paper presents a statistical analysis of ionospheric response over ionosonde stations Grahamstown (33.3°S, 26.5°E, geographic) and Madimbo (22.4°S, 30.9°E, geographic), South Africa, during geomagnetic storm conditions which occurred during the period 1996–2011. Such a climatological study is important in establishing local ionospheric behavior trend which later forms a basis for accurate modeling and forecasting electron density and critical frequency of the F2 layer (foF2) useful for high-frequency communication. The analysis was done using foF2 and total electron content (TEC), and to identify the geomagnetically disturbed conditions, the Dst index with a storm criterion of Dst ≤−50 nT was used. Results show a strong solar cycle dependence with negative ionospheric storm effects following the solar cycle and positive ionospheric storm effects occurring most frequently during solar minimum. Seasonally, negative and positive ionospheric storm effects occurred most in summer (63.24%) and in winter (53.62%), respectively. An important finding is that only negative ionospheric storms were observed during great geomagnetic storm activity (Dst ≤−350 nT). For periods when both foF2 and TEC data (from colocated ionosonde and GPS receiver stations) were available, a similar response in terms of variational trend was observed. Hence, GPS data can be used to effectively identify the ionospheric response in the absence of ionosonde data.
