The ionosphere is formed in the Earth's upper atmosphere by incident solar radiation interacting with, and removing electrons, from different gases. Different wavelengths of solar electromagnetic energy, in the ultraviolet and X-ray parts of the spectrum, ionise different component gases in the atmosphere. A single ionospheric layer may be regarded as being formed by a specific wavelength interacting with a single species of air molecule. We can visualise this in diagrammatic form:
As the solar radiation travels down through the atmosphere it encounters a greater density of air and the rate of ionisation becomes greater. However, in doing so, it loses intensity and eventually the ionisation rate decreases. This results in an ionospheric layer which shows a steep increase as altitude increases (the bottomside of the layer) and a more gradual decrease after the peak (the topside of the layer). Such a layer is called a Chapman layer after the scientist who first derived the form of this ionospheric profile, which is given by the formula:
where h is the height and α is the secant of the solar zenith angle. Ne is the electron density.
Australian Space Academy