The Ionosphere

The Ionosphere is an electrified region in the upper levels of the atmosphere consisting of ions and free electrons.   As high-energy radiation from the sun strikes this region, air molecules absorb it, breaking down into ions and free electrons.  This region extends from about 60 km in altitude up to the top of the thermosphere.  A maximum density of free electrons occurs at about 300 km (Wallace and Hobbs 17).

AM radio wave communication relies on the ionosphere.  Radio waves are reflected off the lowest region of the ionosphere, the "D Layer".  Periods of high solar activity affect radio reception.  The increased electron density that results from active solar activity causes greater absorbtion of radio waves.  This then hinders reception quality.

It is important to understand that the ionosphere is not its own separate region of the atmosphere.  Rather, it begins in the upper regions of the mesosphere and then extends up through the depth of the thermosphere.  This electrified region shows yet another way layers in the atmosphere can be defined.

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