The Thermosphere, the second-highest layer of Earth's atmosphere, starts from the Mesopause, the top most part of the Mesosphere, at an altitude of about 85 km (260,000 ft) and extends up to at an altitude range of 500–1000 km (1,600,000 - 3,300,000 ft). However, the height of the Thermopause varies considerably due to changes in solar activity. In this region temperatures increase with height. This increase in temperature is due to the process of absorption of the ultraviolet and X-Ray radiation from the Sun. The temperature of this layer can range from about 500° C (932° F) to 2,000° C (3,632° F) or even higher, though the gas molecules are so far apart that its temperature in the usual sense is not practically very meaningful. It would not feel hot to a human in direct contact, because the density of its air is too low to conduct a significant amount of energy to or from the skin. This layer is completely cloudless and free of water vapor.

The Thermosphere is the layer, where the space shuttles and the International Space Station orbits the Earth. This is also the layer where the auroras, the Northern Lights and Southern Lights occur. Actually, the charged particles from space collide with atoms and molecules in the Thermosphere and the process excite them into higher states of energy. As a result, the atoms shed this excess energy by emitting photons of light, which we see as the colourful Aurora Borealies and Aurora Australis.

As the Thermopause lies at the lower boundary of the Exosphere, it is also referred to as the ‘Exobase’. The lower part of the Thermosphere contains the ‘Ionosphere’, which extends from 80 to 550 km above Earth's surface. The Scientists consider it as an extension of the Thermosphere. The importance of the Ionosphere is great, though it represents even less than 0.1% of the total mass of the Earth's atmosphere.

Actually, the upper atmosphere is ionized by solar radiation. The Sun's energy is so strong at this level, that it breaks apart the molecules. In other words, the energetic solar radiation knocks electrons off molecules and atoms, turning them into "ions" with a positive charge.

The ionosphere reflects and absorbs radio waves, allowing us to receive shortwave radio broadcasts in New Zealand from other parts of the world.