Vertical-incidence ionograms display the virtual reflection height h' (f ) = ½ c τ (f ) of the HF wave with frequency f, where c is the free-space speed of the wave, and τ the measured pulse propagation time. The ordinary wave echoes are marked in red, the extraordinary echoes in green. Other colors indicate off vertical echoes. The calculated electron density profile shows the true (real) reflection heights of the ordinary wave. The topside profile is modeled assuming an α-Chapman profile.