i·on·o·sphere 
(ī-ŏnə-sfîr′)
(ī-ŏnə-sfîr′)n.
A region of the earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of radio waves. It extends from a height of 70 kilometers (43 miles) to 400 kilometers (250 miles) above the surface.
i·on′o·spheric (-sfîrĭk, -sfĕr-) adj.