The lower part of the Thermosphere; a layer of ionised air extending from the Mesopause to approximately 600km above the surface.
The ionosphere contains a high proportion of free electrons which influence radio propagation. High Frequency (HF) radio waves hitting the free electrons in the ionosphere cause them to vibrate and re-radiate the energy back down at the same frequency, effectively bouncing the radio wave back towards the Earth. This characteristic of the ionosphere has long been exploited to enable long range radio communications. The distribution and concentration of ions within the ionosphere is not uniform, varies between day and night, and is influenced by a number of factors including solar radiation, geomagnetic storms, and Lightning, all of which affect radio propagation. Movement of ions across the earth’s magnetic field generates electrical currents within the ionosphere. Aurora, also known as the Northern Lights or Southern Lights occur in the Ionosphere, close to the Mesopause during geomagnetic storms.