Solar Constant Cycle

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Solar constant cycle

The solar constant is a measure of the amount of radiant energy reaching the earth's outer atmosphere from the sun. More precisely, it is equal to the rate at which solar radiation falls on a unit area of a plane surface at the top of the atmosphere and oriented at a perpendicular distance of 9.277 x 10⁷ mi (1.496 X 10⁸ km) from the sun. That distance is the average (mean) distance between earth and sun during the course of a single year.

Measuring the solar constant has historically been a difficult challenge since there were few good methods for measuring energy levels at the top of the atmosphere. A solution to this problem was made possible in 1980 with the launching of the Solar Maximum (Solar Max) Mission spacecraft. After repair by the Challenger astronauts in 1984, Solar Max remained in orbit, taking measurements of solar phenomena until 1989. As a result of the data provided by Solar Max, the solar constant has now been determined to be 1.96 calories per 0.3 in² (2 cm²) per minute, or 1,367 watts per 3.3 ft (1 m). This result confirms the value of 2.0 calories per cm² per minute long used by scientists.

In addition to obtaining a good value for the solar constant, however, Solar Max made another interesting discovery. The solar constant is not, after all, really constant. It varies on a daily, weekly, monthly, and yearly basis, and probably much longer. The variations in the solar constant are not large, averaging at a few tenths of a percent.

Scientists have determined the cause of some variability in the solar constant and are hypothesizing others. For example, the presence of sunspots results in a decrease in the solar constant of a few tenths of a percent. Sunspots are regions on the sun's surface where temperatures are significantly lower than surrounding areas. Since the sun presents a somewhat cooler face to the earth when sunspots are present, a decrease in the solar constant is not surprising.

Other surface features on the sun also affect the solar constant. Solar flares, for example, are outbursts or explosions of energy on the surface that may last for many weeks. Correlations have been made between the presence of solar flares and the solar constant over the twenty-seven-day period of the sun's rotation.

A longer-lasting effect is caused by the eleven-year sunspot cycle. As the number of sunspots increase during the cycle, so does the solar constant. For some unknown reason, this long-term effect is just the opposite of that observed for single sunspots.

Scientists believe that historical studies of the solar constant may provide some clues about changes in the earth's climate . The nineteenth-century British astronomer E. Walter Maunder pointed out that sunspots were essentially absent during the period of 1645 to 1715, a period now called the Maunder minimum. The earth's climate experienced a dramatic change at about the same time, with temperatures dropping to record lows. These changes are now known as the Little Ice Age . It seems possible that further study of the solar constant cycle may explain the connection between these two phenomena.

[David E. Newton ]