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The ionosphere is constantly changing. At low and middle latitudes, the changes result mainly from variations in the amount of extreme ultraviolet and x-ray radiation that the upper atmosphere receives from the Sun as the Earth spins on its axis and as the Sun becomes more active during the course of a solar cycle. At higher latitudes, they are caused by charged particle precipitation and electric fields that are imposed by the magnetosphere onto the auroral ionosphere. Changing ionospheric conditions are monitored by a world-wide network of radars and radio sounding devices ("ionosondes") to measure such quantities as electron density, plasma temperature, ion velocity, etc.
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