9. Our Changing Atmosphere
The varying Sun and its affect on the Earth's atmosphere
The Sun is a magnetic variable star
Our lives depend on the Sunís continued presence and steady output. It illuminates our days, warms our world, and makes life on Earth possible.The total amount of the Sunís life-sustaining energy is called the ďsolar constantĒ, perhaps because no variations could be detected in it for a very long time. Yet, as reliable as the Sun appears, it is an inconstant companion. Its luminous output varies in tandem with the Sunís 11-year magnetic activity cycle.
Stable detectors placed aboard satellites above the Earthís atmosphere have been precisely monitoring the Sunís total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant (Fig. 9.1). It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.
The Earthís varying Sun-layered atmosphere
Not only does the atmospheric pressure decrease as we go upward, the temperature of the air also changes, but it is not a simple fall-off. It falls and rises in two full cycles as we move off into space (Fig. 9.2).
The temperature decreases steadily with increasing height in the lowest region of our atmosphere, called the troposphere from the Greek tropo for turning. Visible sunlight passes harmlessly through this region to warm the ground below. The temperature above the ground tends to fall at higher altitudes where the air expands in the lower pressure and becomes cooler. The temperature increases at greater heights within the next atmospheric layer, named the stratosphere. The Sunís invisible ultraviolet radiation is largely absorbed in the stratosphere, where it warms the gas and helps make ozone.
The threat of dangerous and even lethal ultraviolet rays caused world-wide concern when it was discovered that everyday, man-made chemicals are punching a hole in the ozone layer (Fig. 9.3). The chemicals, called chlorofluorocarbons or CFCs for short, were therefore completely banned by international agreement in 1990. Still, the ozone layer is not expected to regain full strength until well into the latter half of the twenty-first century.
The mesosphere, from the Greek meso for intermediate, lies just above the stratosphere. The temperature declines rapidly with increasing height in the mesosphere, reaching the lowest levels in the entire atmosphere. The main reason for the decreasing temperatures is the falling ozone concentration and decreased absorption of solar ultraviolet.
The temperature then begins to rise again with altitude in the ionosphere, a permanent, spherical shell of electrons and ions, reaching temperatures that are hotter than the ground. The ionosphere is created and heated by absorbing the extreme ultraviolet and X-ray portions of the Sunís energy. This radiation tears electrons off the atoms and molecules in the upper atmosphere, thereby creating ions and free electrons that are not attached to atoms.
Solar X-rays and extreme ultraviolet radiation both produce and significantly alter the Earthís ionosphere. Their greater intensity near the maximum of the 11-year magnetic activity cycle produces increased ionization, greater heat, and expansion of our upper atmosphere. At a given height, the temperature, the density of free electrons, and the density of neutral, unionized atoms all rise and fall in synchronism with solar activity over its 11-year cycle (Fig. 9.4). This Sun-induced change in the content and structure of the ionosphere affects its ability to mirror radio waves.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University