Mission Features

Total Solar Irradiance: The Sun Also Changes
 
SOHO images of the sun showing most of an entire solar cycle During periods of peak activity (front three images) sunspots, solar flares and coronal mass ejections are more common, and the sun emits slightly more energy than during periods of low activity (back images). The amount of energy that strikes Earth’s atmosphere -- called total solar irradiance (TSI) -- fluctuates by about 0.1 percent over the course of the sun's 11-year cycle, even though the soft X-ray wavelengths shown in this image vary by much greater amounts. Credit: Steele Hill, SOHO, NASA/ESA
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Researchers once erroneously referred to the amount of energy arriving from the sun as the "solar constant". In reality, they have discovered that the sun's luminosity can change over long time scales.

Since 1978, satellite instruments have allowed solar scientists to make extraordinarily precise TSI measurements to check just how "constant" the solar constant actually is. The instruments have measured with such accuracy that scientists have been able to detect tiny variations in TSI that occur with the solar cycle.

During periods of intense solar activity—characterized by peaks in sunspots, flares, and hotspots called faculae—TSI increases by approximately a tenth of a percent. Overall, TSI varies by approximately 0.1 percent—or about 2 watts per square meter between the most and least active part of an 11-year solar cycle.

Today, scientists who study the links between solar activity and climate are confident that the small variations in TSI associated with the eleven-year solar cycle cannot explain the intensity and speed of warming trends seen on Earth during the last century. The 0.1 percent shift in TSI simply isn't enough to have a strong influence, and there's no convincing evidence that suggests TSI has trended upward enough over the last century to affect climate.

However, it's possible—probable, in fact—that the sun experiences sizable shifts in TSI over much longer time scales that could impact climate. For example, a 70-year period called the Maunder Minimum, which featured exceptionally low numbers of sunspots, is thought to be connected to a period of especially low TSI that helped drive Europe’s Little Ice Age. On a much longer time scale, it's also known that the sun has increased its luminosity significantly—by about 30 percent—over its 4.55 billion year lifespan.

Glory carries an instrument—the Total Irradiance Monitor (TIM) that will help maintain the long-term TSI record, which is critical for scientists who are trying to determine definitively if longer-term solar cycles exist and how they might behave.