Ionisation is almost negligible at heights where gas molecules are few, and cannot occur near the ground either because the cosmic radiation has become insignificant by the time it has penetrated that far. Consequently, this kind of ionisation happens mainly at an intermediate elevation of 50-100 km.
The ionized region is called the ionosphere. It comprises several layers, including the D layer at around 80 km, the E layer around 110 km, and F layers at 170 and 270 km. The D layer disappears at night, because ionising radiation from the Sun is then absent, and there is sufficient air at 80 km for existing negative and positive ions to collide and neutralise each other. So the E layer becomes the base of the ionosphere. As a result, the space between the ground and the ionosphere becomes deeper at night, facilitating the bouncing of radio signals around the globe and consequently improving reception.
The electrical conductivity of the ionosphere also plays a part in the cycle of atmospheric electricity described in Chapter 9. Most strikingly, the ionosphere is responsible for the beautiful curtains, veils, arcs, rays and bands of greenish or dark-red light above 100 km in the sky at night, seen in the southern hemisphere within about 25 degrees of the south magnetic pole. James Cook named them Aurora Australis (or Southern Lights) in 1773. It has been observed from Hobart (43°S), as the south magnetic pole is currently located near the same longitude at 66°S. The light is due to the solar wind, atomic particles moving at around 500 km/s from the Sun.
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