• The uppermost layer of the Earth’s atmosphere, where most of the atoms are ionised. Some 350km above sea level, the ionosphere has high temperatures of up to 1500° because high-energy solar photons are captured there, including those in X-ray wavelengths. This prevents radiation which would otherwise be fatal to human and other life from reaching sea level, so that without the ionosphere any life on Earth would have evolved very differently. The ionosphere is also useful for communications and radar, since it is possible to bounce radio signals off it for transmission beyond the visible horizon.

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