The ionosphere is the layer of the Earth's atmosphere that is ionised by solar and cosmic radiation, located between 75 to 500 kms above the Earth.

The Sun emits a constant stream of plasma and UV and X-rays that flow out and ionise the Earth's ionosphere, during the daytime.

During the night, cosmic rays originating from sources throughout our own galaxy, ionise the ionosphere. Not being as strong as the Sun, the ionosphere is not charged as much at night time.

Radio waves that travel along the surface of the earth, are called "Surface Wave" or Ground Wave". Reception of surface radio waves fades out with the curvature of the earth, such as FM radio stations.

Radio waves also travel up into the sky or space above the earth, and are called "Space Waves' and "Sky Waves". Some of these radio signals can be received by aircraft, satellites and the international space station.

Other radio waves travel up to the ionosphere and are reflected back to earth, to be received hundreds or thousands of kms away. The distance between the radio transmitter and where the radio waves return to earth, is called the "Skip Zone" or skip distance. Some of these radio waves then bounce off the earth's surface, back up to the ionosphere and then reflected back to earth. This is called a double skip or double hop.

The ionosphere above the earth's surface consists of various layers, which change between daytime and night time. Also the layers change between summer and winter time.

The layers have a different effect on radio waves, depending on the radio frequency or band. The "D" layer absorbs lower frequencies (below 10 Mega Hertz) during the daytime.  At night time the "D" layers disappears allowing lower frequencies to be reflected back to earth by the other layers above. This is why AM broadcast stations are usually only received over long distances at night time.

Higher frequency radio waves (between 10 and 30 MHz) are reflected back to earth by one of the upper layers, depending on the actual transmitted frequency.

Amateur radio operators use different frequency bands to communicate over various distances around the world, depending on the time of day, season of the year and ionospheric conditions.

Very high frequencies (VHF) and ultra high frequency (UHF) radio transmissions pass straight through the ionospheric layers, into outer space.

 
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