Chemistry_Grade_10-12 (1).pdf

2223 the mesosphere the mesosphere is located km

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22.2.3 The mesosphere The mesosphere is located about 50-80/85km above Earth’s surface. Within this layer, tem- perature decreases with increasing altitude. Temperatures in the upper mesosphere can fall as low as -100 C in some areas. Millions of meteors burn up daily in the mesosphere because of collisions with the gas particles that are present in this layer. This leads to a high concentration of iron and other metal atoms. 22.2.4 The thermosphere The thermosphere exists at altitudes above 80 km. In this part of the atmosphere, ultraviolet (UV) and shorter X-Ray radiation from the sun cause neutral gas atoms to be ionised . At these radiation frequencies, photons from the solar radiation are able to dislodge electrons from neu- tral atoms and molecules during a collision. A plasma is formed, which consists of negative free electrons and positive ions. This part of the atmosphere is called the ionosphere . At the same time that ionisation takes place however, an opposing process called recombination also begins. Some of the free electrons are drawn to the positive ions, and combine again with them if they are in close enough contact. Since the gas density increases at lower altitudes, the recombina- tion process occurs more often here because the gas molecules and ions are closer together. The 424
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CHAPTER 22. THE ATMOSPHERE - GRADE 11 22.2 ionisation process produces energy which means that the upper parts of the thermosphere, which are dominated by ionisation, have a higher temperature than the lower layers where recombina- tion takes place. Overall, temperature in the thermosphere increases with an increase in altitude. Extension: The ionosphere and radio waves The ionosphere is of practical importance because it allows radio waves to be trans- mitted. A radio wave is a type of electromagnetic radiation that humans use to transmit information without wires. When using high-frequency bands, the iono- sphere is used to reflect the transmitted radio beam. When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio wave energy is given up to this mechanical oscillation. The oscillating electron will then either recombine with a positive ion, or will re-radiate the original wave energy back downward again. The beam returns to the Earth’s surface, and may then be reflected back into the ionosphere for a second bounce. Interesting Fact erest Fact The ionosphere is also home to the auroras . Auroras are caused by the collision of charged particles (e.g. electrons) with atoms in the earth’s upper atmosphere. Charged particles are energised and so, when they collide with atoms, the atoms also become energised. Shortly afterwards, the atoms emit the energy they have gained, as light. Often these emissions are from oxygen atoms, resulting in a greenish glow (wavelength 557.7 nm) and, at lower energy levels or higher altitudes, a dark red glow (wavelength 630 nm). Many other colours can also be observed. For example, emissions from atomic nitrogen are blue, and emissions
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