Radio wave reflections from large horizontal gradients in the topside ionosphere

Radio wave echoes due to total reflection from the edge of large-scale depressions of electron density are observed by topside sounding satellites. The resulting traces on swept-frequency ionograms have a minimum frequency which is the nose of a loop-shaped trace. The nose frequency and the shape of the traces depend upon the distribution of electron density in the depression and the location of the sounder relative to the sides of the depression.