= Ionosphere
{wiki}
The ionosphere is a region of the <upper atmosphere> of <Earth> with substantial partial <ionization>. Its mobile <Electrons> and <ions> constitute a <plasma> mixed with neutral gas. For <radio communication>, the crucial question is whether a chosen <radio wave> returns toward the ground, is absorbed, or passes through. A <skywave> can connect stations beyond their direct view across the curved surface of <Earth>, while <wave interference> between a <ground wave> and a <skywave> can make reception fade even when the <radio transmitter> remains steady.
The <ionosphere> is useful both as a route for <radio communication> and as a remote laboratory: varying the probing <frequency> changes how deeply the <radio wave> penetrates the <plasma>. The <Appleton-Barnett radio interference experiment> turned an unwanted fluctuation in reception into a measurement of the overhead route. <Cold-plasma radio dispersion> explains why this is a tunable medium, while <ionospheric virtual height> explains why a measured delay is not automatically an altitude.
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