Variability of total electron content over an equatorial West African station during low solar activity



[1] Slant total electron content (STEC) data measured by the Global Positioning System receiver at Ilorin, Nigeria, with geographical coordinates 8.47°N, 4.68°E for the year 2009 (a low-activity year) was used to study the diurnal, monthly standard deviation and monthly median value of total electron content (TEC). The vertical total electron content (VTEC) values are estimated from the STEC data. The thin shell approximation with an ionospheric shell height of 350 km was used for the analysis. The diurnal variation of VTEC (DTEC) and its corresponding monthly median variation (MTEC) shows a minimum at presunrise between the hours of 05:00 and 06:00 LT. The DTEC values show a maximum variation range from ∼24 to ∼34 total electron content unit (TECU). The daytime maximum TEC values observed in all the months were broad with a slight daytime depression in May, June, July, and November. The maximum variation of MTEC after slight daytime depression is greater than its variation before the slight daytime depression in the months affected with the month of July as exception. The slight daytime depression was lowest in the month of May and has a value of 0.99 TECU. A postsunset decrease at 20:00 LT with corresponding enhancement 2 h later was observed in the month of March. This post sunset decrease and enhancement in the month of March could be a strong indicator of the abrupt onset of scintillations, plasma bubbles, and spread F phenomenon. The monthly standard deviation depicts summary behavior of all the diurnal variations in each month. Annual and seasonal variations were also investigated.