Starting at about 80 km above the Earth’s surface, the atmosphere contains an ionised particle component called the ionosphere. The upper ionosphere extends to a height of about 600 km. It is hard electromagnetic radiation from the Sun in the UV and extreme UV range that provides the energy needed to ionise gaseous molecules and atoms present within this zone of the atmosphere.
The degree of ionisation increases with altitude. For example, at a height of 100 km, it is estimated that only one in 10 million atoms and molecules are ionised, whereas all particles are ionised high in the ionosphere.
Several distinct layers of the ionosphere have been identified based upon their ion and electron densities. The outermost region – the F region – has the highest concentration of free electrons and ions. During daylight hours, this region splits in two – F2 being the outer one and F1 the inner one. The F2 region is the principal reflecting layer for high-frequency (HF) radio communications during both day and night.
At a height of between 90–150 km lies the E region. Here, the degree of ionisation is lower than in the F region, with about one electron present for every 108 neutral particles.
Below E is D at 80–100 km with an even smaller electron content. During the night-time, regions E and D disappear, leaving only the F region as a fully ionised layer.
Diagram © University of Waikato, 2014