The region of the Earth's upper atmosphere extending from 31 miles to 250 miles above sea level.

It contains a small percentage of free electrons and ions produced by photo ionization of the constituents of the atmosphere by solar ultraviolet radiation at very short wavelengths (less than 1000 angstroms).

The ionosphere significantly influences radio wave propagation of frequencies less than about 30 MHz.

The existence of the ionosphere was first proposed by Balfour Stewart in 1882 to account for differences in the earth′s magnetic field.

Critical Frequency

In propagation work, the highest frequency at which radio energy will be reflected by an ionospheric layer rather than penetrate through it.

Maximum Usable Frequency

The frequency that has a 50% probability of attaining ionospheric reflection support over a specified skywave path throughout the month at the same time each day.

See also: Earth, Earth Current, Ionospheric Storms, Sky Waves, Stratosphere, Thermosphere, Troposphere.

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Subjects: Physics

HAARP A scientific endeavor aimed at studying the properties and behavior of the ionosphere, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and defense purposes.