GNU Collaborative International Dictionary of English

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1.       
   ionosphere n. the outer region of the earth's atmosphere, beyond which is the exosphere; it contains a high concentration of free electrons and ions, and extends from about 50 miles to about 250 miles above the earth's surface, but the height shows daily and seasonal variation, and the composition of the particles in the ionosphere is also affected by events such as solar flares.
   
   [WordNet 1.5 +PJC]