Ionosphere, according to Petterssen, lies beyond the ozonosphere at a height of about 60 km above the earth’s surface. At this level the ionization of atmosphere begins to occur. The first knowledge about the existence of this highly ionized layer at such great heights was acquired by means of radio-waves.
The credit for the discovery of this layer goes to Kennelly and Heaviside. Later on, more and more could be added to out knowledge about these ionized layers by means of aurora, sound waves and satellites etc.
The ionosphere, according to some other scientists, is supposed to start at a height of 80 km above the earth’s surface. The layer lying between 50 and 80 km is called the mesosphere. In this layer the temperature decreases with height. The upper boundary of this layer is called the mesopause.
In this layer the ionization of molecules and atoms occurs mainly as a result of ultraviolet, X-ray and gamma radiation. Ionization refers to the process by which atoms are changed to ions through the removal or addition of electrons, giving them an electrical charge.
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Above the ozonosphere, the temperature falls again reaching a minimum of about -100° C at a height of 80 km. above the earth’s surface.
Beyond this level the temperature increases again as a result of the absorption of short-wave solar radiation by the atoms of oxygen and nitrogen in the extremely rarefied air of the ionosphere.
It may be pointed out that when the air density is extremely low, very little energy is needed to produce substantial rise in temperature.
Even though the temperatures rise to very high values of more than 1000° C, such temperatures are altogether different from those experienced near the earth’s surface.
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Since gases at such great heights are very sparse, a very insignificant quantum of energy is produced by the fast moving air particles.