Figures 16.4, 16.6 and 16.7 illustrate the density structure in Earth's ionosphere.
Figure 16.6: Typical plasma density profile of Earth's ionosphere, showing the D, E, and F layers,
as functions of altitude [Brand, 1998].
Figure 16.7: Typical ionospheric density profiles for the day and night [Cravens, 1997].
These data can be
obtained by rocket and spacecraft measurement and by radar sounding from the ground and space. The D region
is where the ionosphere starts and becomes appreciable, covering the altitude range from about 50 to
90 km. Remembering that 
kHz for 
measured in cm 
, the Figures
show that the D region will reflect terrestrial signals with frequencies below about 2.5 MHz. The D layer is
associated with ionization by cosmic rays and X-rays and is not well understood [Luhmann, 1995].
The E layer lies between approximately 90 and 130 km is associated primarily with ionization by UV photons
and the ions 
and 
[Luhmann, 1995]. It is apparently well described by existing
theory. The F layers lie above 130 km and are associated mostly with 
ions. The lower, F1, layer
appears to be reasonably well understood, while the higher F2 layer does not. It is the E and F layers
which increase the maximum plasma frequency in Earth's ionosphere to values above about 10 MHz.
Note Figure 16.7's substantial differences between the ionospheric density profiles in the day and at night. These are due primarily to the much smaller ionization rate at night. However, variations in the recombination rate with altitude and vertical transport are also important. These effects lead to substantial changes in the propagation of radio waves produced on the ground.