Solar Constant and Variable Sun
Variations in the Sun's Luminosity and the Sunspot Cycle
Aug 4, 2008
The Sun's energy output each second, its luminosity, cannot vary as much as many variable stars do or the consequences to Earth's climate would be disastrous. The fact that Earth's climate is fairly stable tells us that the Sun's luminosity must also be fairly stable, but there may be small variations.
Early Measurements
Charles Greeley Abbot of the Smithsonian Institute spent the first half of the twentieth century trying to accurately measure variations in the Sun's luminosity. Abbot and his coworkers measured the solar constant, also called the solar irradiance, which is the amount of solar energy striking a square meter above Earth's atmosphere every second. Abbot's data span 55 years. He claimed to have detected changes in the Sun's luminosity, but other scientists of the day thought that the solar variations were too small to detect with the limits to the accuracy of the data.
Satellite Measurements
Abbot's work was hampered by the need to accurately correct for the amount of the Sun's energy absorbed by Earth's atmosphere. Extremely accurate measurements of the solar constant and the Sun's luminosity require going into space, above Earth's atmosphere.
Working before the space age, Abbot tried to minimize the atmospheric corrections, by making his measurements from remote mountaintops. The dawning of the space age however allowed scientists to measure the Sun's energy output from above the atmosphere. In 1978 the Nimbus 7 satellite was equipped with an accurate radiometer to accurately measure the solar constant. The Solar Maximum Mission, which also had a very accurate radiometer followed shortly thereafter in 1980. These two satellites, as well as subsequent missions, have very accurately measured the solar constant over the entire 11 year sunspot cycle.
Sunspots are dark spots on the Sun's visible surface, the photosphere, that are caused by the Sun's turbulent magnetic field. There is an 11 year cycle in the number of sunspots and in the amount of other related magnetic activity.
The satellite data show that the Sun's luminosity is roughly 0.15% higher during sunspot maximum than during sunspot minimum. It seems strange that the Sun is actually brighter when dark spots cover more of its surface. However the Sun also has less well known bright spots, called faculae, that also increase in number when sunspots do. The effect of faculae is slightly stronger than the effect of sunspots. The satellite data confirm Abbot's claims about solar variability over the course of the solar activity cycle.
Changes in the Sun's luminosity over the 11 year cycle do not likely affect Earth's climate because they are small compared to year to year variations. There are however less well understood longer cycles in the amount of solar activity. For example, the Maunder minimum was a period in the late 17th century when very few sunspots were observed. This also corresponds to the time of the Little Ice Age. If the Sun is less luminous during sunspot minimum, then the Little Ice Age may have been caused by variations in the Sun's luminosity.
Further Reading
Frazier, Kendrick. Our Turbulent Sun. Prentice-Hall, 1980.
Golub, Leon and Pasachoff, Jay M. Nearest Star The Surprising Science of Our Sun. Harvard, 2001.
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