|INTERLUDE 16-2 Solar—Terrestrial Relations|
|Our Sun has often been worshipped as a god with power over human destinies. Obviously, the steady stream of solar energy arriving at our planet every day is essential to our lives. But over the past century there have also been repeated claims of a correlation between the Sun's activity and Earth's weather. Only recently, however, has the subject become scientifically respectablethat is, more natural than supernatural.
In fact, there do seem to be some correlations between the 22-year solar cycle (two sunspot cycles, with oppositely directed magnetic fields) and periods of climatic dryness here on Earth. For example, near the start of the past eight cycles, there have been droughts in North Americaat least within the middle and western plains from South Dakota to New Mexico. The most recent of these droughts, which typically last 3 to 6 years, came in the late 1950s. The one expected in the 1980s, however, did not occur as clearly as anticipated.
Other possible Sun—Earth connections include a link between solar activity and increased atmospheric circulation on our planet. As circulation increases, terrestrial storm systems deepen, extend over wider ranges of latitude, and carry more moisture. The relationship is complex, and the subject controversial, because no one has yet shown any physical mechanism (other than the Sun's heat, which does not vary much during the solar cycle) that would allow solar activity to stir our terrestrial atmosphere. Without a better understanding of the physical mechanism involved, none of these effects can be incorporated into our weather forecasting models.
|Solar activity may also influence long-term climate on Earth. For example, the Maunder minimum seems to correspond fairly well to the coldest years of the so-called Little Ice Age that chilled northern Europe during the late 1600s. (Sec. 16.4) The accompanying winter scene actually captured the summer season in 17th-century Holland. How the active Sun, and its abundance of sunspots, may affect Earth's climate is a frontier problem in terrestrial climatology.
One correlation that is definitely established, and also better understood, is that between solar flares and geomagnetic activity at Earth. The extra radiation and particles thrown off by flares impinge on Earth's environment, overloading the Van Allen belts, causing brilliant auroras in our atmosphere and degrading our communication networks. These disturbances have been known for many years, but only recently have we associated them with the solar wind. We are only beginning to understand how the radiation and particles emitted by solar flares also interfere with terrestrial radars, power networks, and other technological equipment. Some power outages on Earth are actually caused not by increased customer demand or malfunctioning equipment but by flares on the Sun!
We cannot now predict when and where solar flares will occur. However, it would certainly be to our advantage to be able to do so, as this aspect of the active Sun affects our lives. This is a very fertile area of astronomical research and one for which there are clear terrestrial applications.