The ionosphere is that part of the upper atmosphere where free electrons occur in sufficient density to have an appreciable influence on the propagation of radio frequency electromagnetic waves. This ionization depends primarily on the Sun and its activity. The major part of the ionization is produced by solar X-ray andultraviolet radiation and by corpuscular radiation from the Sun. Although the Sun is the largest contributor toward the ionization, cosmic rays make a small contribution.
Any atmospheric disturbance effects the distribution of the ionization.
The ionosphere is a dynamic system controlled by many parameters including acoustic motions of the atmosphere, electromagnetic emissions, and variations in the geomagnetic field . Because of its extreme sensitivity to atmospheric changes, the ionosphere is a very sensitive monitor of atmospheric events. In some circles it is thought that there is persuasive evidence of an ionospheric precursor to large earthquakes that can be used a predictor. These EMEs have been detected in the ionosphere up to six days prior to a large earthquake, such as with the May 1960, Chilean 8.3 earthquake. The most accurate way of measuring the ionosphere is with a ground-based ionosonde, which records data as ionograms.