Skywave
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Skywave is the propagation of radio waves by the refraction caused by the Earth's highly charged ionosphere. A skywave is a radio wave from a distant radio station (or rarely, a TV station) that is refracted in the ionosphere. It brings in distant stations at night as clear as local stations.
Low to mid frequencies below approximately 10 MHz (longer than 30 meters) including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), travel most efficiently this way at night, acting more like the Earth's natural electrical field than other higher frequencies above 30 MHz (shorter than 30 Meters) which have properties more akin to light). Frequencies Lower than 30 kHz have a wavelength longer than the distance between the Earth and the Ionosphere.
The Ionosphere, though refracting, can usually bend radio waves enough so they come back to the surface of the Earth.The Ionosphere has been called a natural radio "mirror". Because the solar wind "blows" the ionosphere toward the earth on the day side, and away from it on the night side, the refractive layer of the ionosphere is much higher above the surface at night. The amount of ionisation and the distance of the higher charge in the ionosphere effect the amount of refraction.