solar cycle effects on gnss-derived ionospheric total electron content ...

Chapter 5MAPPING GNSS-DERIVED TEC OVER SOUTH AFRICADURING DIFFERENT EPOCHS OF SOLAR CYCLE 235.1 IntroductionThe current trend in ionospheric physics research has proven that the dual frequency (L1 = 1575.42MHz and L2 = 1227.60 MHz) signals transmitted by the GNSS, and received by the network of GPSreceivers distributed worldwide provide a unique opportunity to determine the higher resolutionspatial and temporal ionospheric TEC at regional and global level (e.g. Jakowski and Paasch, 1984;Klobuchar, 1991; Komjathy and Langley, 1996; Jakowski, 1996; Komjathy, 1997; Mannucci et al.1998). The magnitude of TEC is highly variable and depends on several factors such as local time,geographical location, season and <strong>solar</strong> activity <strong>cycle</strong> (e.g. Jakowski, 1996; Jakowski et al. 1999;Jakowski, et al. 2002; Immel et al. 2003; Tsurutani, et al. 2004, Skoug et al. 2004; Jee, et al. 2005;Mannucci et al. 2005; Fedrizzi, et al. 2001). Recent studies (Jakowski et al. 2001; Jakowski et al.2002) illustrate that TEC monitoring using the GNSS network can contribute to space weathermonitoring.Based on the validation of UNB-IMT results with Ionosonde TEC measurements over South Africapresented in Chapter 4, it is possible to adopt and use this model as a tool to characterise variationsand dynamics ionospheric TEC over southern Africa. In this Chapter, UNB-IMT is applied to studythe <strong>solar</strong> <strong>cycle</strong> variations of TEC maps over the South African region using data from the SouthernAfrican GNSS networks described in Chapter 4. For this task, the monthly averaged sunspot numberwas used as a proxy for <strong>solar</strong> activity <strong>cycle</strong> 23 as shown in Figure 5.1. From these monthly averagedvalues, it is well established that the Sun has a quasi-periodic ~11 year activity <strong>cycle</strong> (e.g. Smith and76