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Extending the Content
Unit : The Atmosphere and the Oceans
Chapter 11: Atmosphere  

Earth's atmosphere extends high above the tallest mountains into the dark reaches of outer space. Some portions of the upper atmosphere, far beyond where the human eye can see, profoundly affect life on Earth. For example, you've already learned about the ozone layer, a sub-layer of the stratosphere that absorbs ultraviolet radiation from the Sun. Another important atmospheric sub-layer is the ionosphere, which is found in the lower portion of the thermosphere.

The Ionosphere
The ionosphere is located about 100 to 400 km above Earth's surface. There, ionized particles are produced by molecular interactions with solar radiation. In the ionosphere, atoms of nitrogen and oxygen absorb short wavelengths of sunlight, such as those associated with gamma rays, x-rays, and some ultraviolet radiation. This absorption of solar radiation causes the atoms to lose electrons and to change to positively charged atoms, or ions. Because these ions are electrically charged, they can be used to transmit and receive communication signals on Earth. For instance, long radio waves in the high-frequency band (HF) are reflected by particles in the ionosphere. This allows the radio waves to be "bounced" off the ionosphere and picked up by receivers around the world. Radio waves in the very-high-frequency band (VHF) and the ultra-high-frequency band (UHF), however, are too short to be reflected by the ionosphere, and are thus transmitted by other means. The electrically charged nature of the ionosphere does have a downside-during periods of intense solar activity, radio communications are often disrupted by high-energy particles from the Sun that enter the ionosphere and produce magnetic storms.

Sky Lights
The electrical nature of the ionosphere also causes phenomena known as the aurora borealis, or northern lights, and the aurora australis, or southern lights. The former is found in the northern hemisphere and the latter is found in the southern hemisphere. In both cases, bands of dramatically colored lights appear in the night sky. For the duration of their appearance, these lights constantly shift and undergo changes in intensity. The lower portions of the bands are usually about 65 km above Earth's surface. The upper portions may be as high as 900 km. A band of light can stretch for thousands of kilometers across the horizon.

Auroral lights are caused by high-energy, radiation particles from the Sun, which interact with the ionosphere. These solar particles are deflected by Earth's magnetic field toward the poles. For this reason, auroral lights are usually spotted above 65 north and south latitudes. As solar particles collide with gas molecules in the atmosphere, light is emitted. The most common auroral light is green, but most colors in the visible spectrum, including red, blue, orange, and violet, have been spotted.

Auroral lights do not appear to affect weather in the lower atmosphere. However, they are linked to changes in solar activity-as solar activity increases, incidents of auroral lights increase. Solar activity, in turn, has been linked to changes in climate. Thus, studies of auroral lights may hold some key to understanding climatic changes.

Auroral lights

LINK-UP: Find out more information about auroral lights.

Unlike auroral lights, airglow is an ionospheric phenomenon that can be observed at all latitudes. Best described as a faint light that spreads across the night sky, airglow is caused by chemical reactions that take place between molecules of air and solar radiation. Sparked by incoming solar rays, molecules of oxygen, hydrogen, nitrogen, and sodium recombine and react to produce hazy red, green, and yellow lights high in the atmosphere. Airglow is generally found at altitudes of 95 to 160 km. On a moonless night, it sends more visible light to Earth than all the stars in the sky.

VHF and UHF waves are used to transmit television signals. These waves, however, cannot be reflected by the ionosphere and thus travel into space. How have scientists overcome this problem? Research and write about how VHF and UHF radio waves are transmitted and received on Earth. Share your results with the class.


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