4 Thermosphere

• From 80 km up to about 700 km above the surface.
• Nitrogen and oxygen absorb high-energy solar radiation, so the temperature rises dramatically, reaching over 2000 °C. (This temperature measures the very high kinetic energy of the very few particles present. Because the air is so thin, an astronaut or satellite there would not feel "hot": there are too few collisions to transfer much heat.)

The ionosphere

• is the lower part of the thermosphere, from 80 km to about 550 km.
• There, oxygen and nitrogen atoms absorb very high-frequency solar radiation (X-rays and gamma rays).
• This radiation ionises the gas atoms — they lose or gain electrons and become electrically charged — so this region has a high concentration of charged ions (hence "ionosphere").

• Radio communication: the ionosphere reflects electromagnetic (radio) waves, so signals can travel over very long distances around the curved Earth without satellites or cables (it works particularly for short-wave radio).
• The aurora: near the north and south poles, high-energy charged particles of the solar wind are guided by Earth's magnetic field toward the polar regions. There they collide with ions and atoms in the ionosphere and excite them (raise electrons to higher energy levels). When the atoms return to their ground state they release photons of light of various wavelengths — green and red from oxygen, blue and purple from nitrogen — producing the bright, colourful lights of the aurora (aurora borealis in the north, aurora australis in the south).