1. A layer that overlaps many of the other layers from 30 to 250 miles (48 to 402 kilometers), as part of the upper mesosphere and the lower thermosphere.

The ionosphere is the part of the atmosphere in which the air is ionized by such factors as the sun's ultraviolet radiation.

Layers within the ionosphere are also responsible for reflecting radio waves and are important to long-distance communication.

2. A region above a planet where the breakup of atmospheric gases by sunlight leads to large concentrations of free electrons and ions.

These are capable of seriously disrupting radio communications on earth.