The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions in the ionosphere. read more
The Thermosphere has no real upper limit, rather, it extends out into space to the magnetosphere, which is not part of the atmosphere. The Thermosphere is so big that it is divided into two additional parts: the Ionosphere and the Exosphere. read more