18.7 Ionosphere
Above the ozone layer, there's another useful zone in the thermosphere, beginning around 75 km (47 mi) and extending to 500 km (310 mi) above the ground. Here, the sun's rays, as well as rays from space, break some of the atmospheric gas molecules into ions. This layer of ionized atoms is the ionosphere. The ionosphere actually contains up to four layers that combine and change between day and night and with the seasons. The ionosphere reflects relatively long radio communication waves. Until the second half of the twentieth century, radio users depended almost entirely on this effect to communicate over long distances. Even today, some radio stations and amateur radio broadcasters send their messages around the curve of the earth by bouncing their radio signals off the ionosphere, especially at night. Some shortwave radio signals can return to Earth thousands of miles away from their transmitting antennae. For example, a Christian missionary radio station in Carriacou, an island just off the coast of Venezuela, has listeners in Finland and Russia without using communication satellites!