Ionisation ledges in the equatorial ionosphere - A tool to detect cross-equatorial winds

The spatial distribution of the intensity of the ionisation ledges in the equatorial topside ionosphere is delineated from the ionograms obtained from the topside sounder experiment onboard ISIS satellites. They are magnetic field aligned and thus arch-shaped over the magnetic equator. Their spatial extents in the two hemispheres on either side of the magnetic equator are equal in the equinoctial and unequal in the solsticial months. The ledge length in the winter hemisphere is found to be longer than that in summer hemisphere. It is interpreted as due to the presence of cross-equatorial winds from summer to winter hemisphere and the calculated wind speeds agree with the values theoretically shown to be required for explaining the asymmetry in the equatorial anomaly of the F2 layer during solstice periods.