Variations of GPS-TEC at an African Low Latitude Station during Geomagnetic Disturbances

Abstract

Global Positioning System (GPS) measurements have been used to investigate the storm-time seasonal variations of the total electron content (TEC). The selected storm events are all moderate (-100nT < Dst ? -50nT) and occurred at different seasons between the year 2002 and 2006 during the descending phase of solar cycle 23. The GPS measurements were obtained from the International GNSS Services (IGS) database for the receiver located at Lusaka (Geog. Lat. 15.43^oS, Long. 28.31^oE, -26.27^o dip), Zambia, a low latitude station in the African region. Analysis of the TEC data shows significant differences in the TEC variations during the storm events with noticeable local time and seasonal components. In most of the storms considered, positive storm effect was observed during their main phases with magnitudes and durations that differ seasonally. The positive phase is most prominent for storms that have their main phase during the daytime particularly in winter and equinoxes, while in summer, the positive phase is more noticeable between the pre-midnight and early morning period. The trend of storm-time TEC variations are the same for equinoxes and winter, with a consistent positive phase during the daytime and negative phase at nighttime. The TEC variations, however, differ in magnitude and duration. The response of TEC to the geomagnetic storms also presents a summer/winter and autumn/spring asymmetry. General observation shows a long lasting negative phase in daytime than at nighttime during summer; while it shows a fairly consistent behaviour with a positive and negative phase observed during the daytime and nighttime respectively, in winter. The average storm-time behavior observed at different seasons may suggest a way to improve the storm-time prediction of ionospheric models which have limited success in estimating short-time ionospheric dynamics.