The Collaborative International Dictionary of English v.0.48:

ionosphere \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]

WordNet (r) 3.0 (2006):

ionosphere
    n 1: the outer region of the Earth's atmosphere; contains a high
         concentration of free electrons

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "ionosphere":
   Appleton layer, CAT, F layer, Heaviside-Kennelly layer,
   Van Allen belt, aerospace, aerosphere, air hole, air pocket,
   airspace, belt, bump, ceiling, chemosphere, crosswind, empty space,
   favorable wind, fog, front, head wind, high-pressure area, hole,
   isothermal region, jetstream, layer, low-pressure area,
   lower atmosphere, outer atmosphere, overcast, photosphere, pocket,
   roughness, soup, space, stratosphere, stratum, substratosphere,
   tail wind, tropopause, troposphere, trough, turbulence,
   upper atmosphere, visibility, visibility zero