The ionosphere is a relatively thin shell around the earth between
about 70 and 1000 km above the earth's surface. It is the transition region
between the dense, electrically neutral atmosphere below and the very thin,
ionised plasmasphere and
magnetosphere above. When the
plasma up above is caused to move, magnetic field lines
communicate this motion down to the ionosphere as if they were pieces of string tying the different
plasma regions together. However, ions cannot move so easily in the
ionosphere because they must wade through a sea of neutral particles which
slow them down by frequent collisions. Yet the smaller electrons manage to
dodge the neutral particles and move freely. As a result of the different
electron and ion motions horizontal electrical currents flow in the ionosphere
which cause magnetic perturbations on the ground.
The motions can also help generate waves in the ionosphere that move with the
electrons and can be detected by radars.
The ionosphere acts as both a help and hindrance to long distance radio communication. The ionosphere helps by reflecting high frequency (HF) radio waves around the globe. It hinders by slowing down and absorbing all radio waves. The effects are very small on very- and ultra-high frequency (VHF and UHF) radio waves, but they can be important where radio waves from satellites are used for precise global positioning. The effects can be more severe on HF radio waves. Sometimes the HF ionospheric mirror won't reflect at all or it will reflect waves in the wrong direction.
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