This is an investigation of equinoctial asymmetry in Total electron content (TEC) variability at Niamey (Latitude: 13° 30' 49.18" N, Longitude: 2° 06' 35.28" E) using the Global Ionospheric Maps model constructed by the Center for Orbit Determination in Europe (CODG model) during solar cycle 23, that is, from year 1999 to year 2009. Niamey Center for Orbit Determination in Europe Total Electron Content (CODG TEC) from 1999 to 2009 show that ionization follows solar cycle and presents semi-annual variation with equinoctial asymmetry. In CODG TEC, generally, March/April maximum density is larger than that of September/October except during years 1999 and 2001. For all years (1999-2008), electronic density is higher between 1400 and 1700 UTC with the maximum at 1400 UTC. On one hand, Ap and aa index via pixel diagram and on the other hand, seasonal and sunspot cycle variation have been used to explain the exception of years 1999 and 2001. It was found that asymmetry of 1999 is due to solar wind particularly to fluctuating wind and asymmetry of 2001 results from CMEs.
Key words: Global positioning system (GPS), Center for Orbit Determination in Europe Total Electron Content (CODG TEC), ionization, asymmetry, equatorial ionosphere.
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