The valley approximation

The virtual height of a layer will be affected by the amount of ionisation below it. The ionisation in each layer is known from the critical frequency, but information about the ionisation between the layers cannot be measured by an ionosonde, since the electron concentration there is less than the critical frequency of the lower layer, and any wave which has a frequency sufficient to penetrate the first layer will not be reflected back to the ionosonde by these lower density electrons.

In order to re-create the true-height-profile, the ionisation in the valley must be estimated. A maximum estimate for the true-height of the upper peak can be made by assuming no electrons between the layers. Likewise, a minimum estimate for the true-height of the upper layer can be made by assuming that the ionisation between the layers is the same as the peak density of the lower layer (ie, that there is no valley, merely a cusp). In true-height profile analysis therefore, the valley approximation is very important. Work has been done to model the variation in height and depth of the valley with time of day, date and latitude. The true-height analysis program POLAN approximates the valley in this way.

Some work has been done on estimating the size of the valley from the MSIS86 atmospheric model. This work is however, limited to the mid-latitude ionosphere. For more information consult the reference Aeronomical calculations of valley size in the ionosphere by J. E. Titheridge, Adv Spac. Res. 10, pp 821-824, 1990