Relation Between Actual and Virtual Ionospheric Height

Translation of virtual height of the ionosphere into actual height is achieved by fitting a parabolic maximum of electron density to observations of variation of virtual height with wave frequency. Methods for measuring the thickness of a region and the height of maximum electron density are described and applied. The simplest measure of the actual height of maximum electron density is the virtual height at five-sixths of the penetration frequency. This measure is found to be remarkably reliable for $F$ region at night. During the daytime it is often necessary to correct $F_1$ observations for presence of $E$ region, and $F_2$ observations for presence of $F_1$ region. A simple technique of correction is described and applied. The disadvantages of minimum virtual height for quantitative study of diurnal variation of ionospheric height are emphasized.