Comparison of equatorial GPS-TEC observations over an African station and an American station during the minimum and ascending phases of solar cycle 24

摘要

GPS-TEC data were observed at the same local time at two equatorial stationson both longitudes: Lagos (6.52° N, 3.4° E,3.04° S magnetic latitude), Nigeria; and Pucallpa(8.38° S, 74.57° W, 4.25° N magnetic latitude),Peru during the minimum (2009, 2010) and ascending (2011) phases of solarcycle 24. These data were grouped into daily, seasonal and solar activitysets. The day-to-day variations in vertical TEC (VTEC) recorded the maximumduring 14:00–16:00 LT and minimum during 04:00–06:00 LT at bothlongitudes. Seasonally, during solar minimum, maximum VTEC values wereobserved during March equinox and minimum during solstices. However, duringthe ascending phase of the solar activity, the maximum values were recordedduring the December solstice and minimum during the June solstice. VTEC alsoincreased with solar activity at both longitudes. On longitude by longitudecomparison, the African GPS station generally recorded higher VTEC valuesthan the American GPS station. Furthermore, harmonic analysis technique wasused to extract the annual and semi-annual components of the amplitudes ofthe TEC series at both stations. The semi-annual variations dominated the TECseries over the African equatorial station, while the annual variationsdominated those over the American equatorial station. The GPS-TEC-derivedaverages for non-storm days were compared with the corresponding valuesderived by the IRI-2007 with the NeQuick topside option. The NeQuick optionof IRI-2007 showed better performance at the American sector than the Africansector, but generally underestimating TEC during the early morning hours atboth longitudes.