8.1.2 Ionosphere

The ionosphere is the ionized plasma in the upper atmosphere, produced by the dissociation of atmospheric atoms. It is generally divided into layers D, E and F1 at low altitudes and F2 at higher altitude. The low altitude layers are significant only during daylight hours. F2 is permanent and the densest, peaking at around 300 km altitude. For most space applications, it is the electron density which is the most important characteristic. Immediately above the F2 peak, density falls off nearly exponentially with height. At mid to low latitudes, the density falloff slows down at higher altitudes as the magnetic field traps plasma to form the plasmasphere.

Intense auroral charging environments are associated with discrete aurorae which are generally found between 60 and 77 degrees magnetic latitude. However, at times of enhanced geomagnetic activity, the auroral zone is moved southwards. In this region, ionospheric density can become irregular on the scale of metres to kilometres vertically and metres to hundreds of kilometres horizontally. The energetic electrons have energies of 10s of keV. Plasma density in this region can suddenly increase by a factor of up to 100 during magnetically active periods.

In the Polar Cap, typically above 70° latitude, there is a strong wintersummer asymmetry because of the tilt of the Earth,. Strong drift motion occurs due to electric fields. In winter, density is maintained by this drift and by “polar rain”, a weak electron flux from the solar wind with energy around 100eV. When both these processes are depressed, ionospheric density in this region can become very low.

Ionospheric plasma characteristics are expressed in the International Reference Ionosphere (IRI) [RN.9]. IRI is an empirical model based on a large volume of ground and space data. IRI describes monthly average conditions but can be updated to time-specific conditions using measured characteristic parameters, e.g., F-peak density and height, if these are available. This model calculates densities, temperatures and composition in the altitude range 50 to 1 500 km in the non-auroral ionosphere. The model describes the densities of O⁺, H⁺, He⁺, NO⁺, O₂⁺, N⁺, and Cluster ions. External drivers for the IRI model are the sunspot number and the ionospheric index IG; in both cases the 12-month running mean of the index is used.