Variation of digisonde-derived scale height during quiet and disturbed geomagnetic conditions over an African equatorial station

摘要

One inherent parameter in the extrapolation of ionospheric topside profile is the scale height, which can be derived from an ionogram. For the first time in the African equatorial region, the variation of digisonde-derived scale height (H-m) is investigated at Ilorin (Geog. 8.50 degrees N; 4.50 degrees E; dip. -7.9 degrees) during quiet and disturbed conditions. Diurnal pattern of H-m revealed a consistent but insignificant hump before sunrise, followed by a marked increase, maximizing around local noon, and a decrease thereafter till nighttime. The daytime values, however, shows significant seasonal differences. A striking observation is the highest value in December solstice and lowest during June solstice. This is attributed to the influence of ion-neutral drag and topside thermal structure on the shape of the topside ionosphere. H-m also exhibits high correlation with the bottomside thickness parameter (B0) and the total electron content; however, it is generally moderate with F-2 layer critical frequency (foF2) and peak height (hmF2). These correlations do not show significant seasonal dependence. The large network of Global Positioning System (receivers may serve as a good data source for the reconstruction of the topside profile owing to the excellent relationship between H-m and total electron content. The linear model developed to describe the dependence of H-m on B0 generally gives a good representation of H-m with digisonde-B0 as input. Large discrepancies were observed with the three options of the IRI 2016-B0 used as input. The B0 models, if well established at the station, could be an alternative source to estimate H-m.