Page 24 - Curriculum Visions Dynamic Book

The upper atmosphere
At heights of thousands of kilometres the atmosphere gradually merges with the solar wind.
Most of the gas molecUles in the atmosphere are concentrated in the lowest 10–12 km—a region called the troposphere. That is where the clouds form, and the
water cycle operates (see page 29). Above it lies the quieter stratosphere to an altitude of about 50 kilometres. It is in the stratosphere that ozone molecules absorb ultraviolet rays.
Most of the atmosphere consists of electrically neutral atoms and molecules, but at high altitudes more and more particles are electrically charged. This region is called the ionosphere. It extends throughout the mesosphere and thermosphere but becomes most important above 100 kilometres. Here, because of the greater proportion of charged, or ionized, particles, the auroras are generated (see pages 20–21). (Note: The radiation belts and magnetic field of the Earth are not related to the belts of the atmosphere, with the Van Allen belts being doughnut-shaped radiation zones that begin 1,000 km above the Earth’s surface. The magnetosphere goes through the entire atmosphere and beyond.)
 This picture shows the Earth’s limb (the edge of the atmosphere).
It is highlighted by this view where the Moon is seen through the outer atmosphere. Notice
how the thinner upper atmosphere is shown by the clarity of the Moon.
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