But it looks so constant...
That's only a limitation of the human eye. Modern telescopes and spacecraft have penetrated the sun's blinding glare and found a maelstrom of unpredictable turmoil. Solar flares explode with the power of a billion atomic bombs. Clouds of magnetized gas (CMEs) big enough to swallow planets break away from the stellar surface. Holes in the sun's atmosphere spew million mile-per-hour gusts of solar wind. And those are the things that can happen in just one day.
'Solar Constant' is an Oxymoron
Astronomers were once so convinced of the sun's constancy, they called the irradiance of the sun "the solar constant," and they set out to measure it as they would any constant of Nature. By definition, the solar constant is the amount of solar energy deposited at the top of Earth's atmosphere in units of watts per meter-squared. All wavelengths of radiation are included—radio, infrared, visible light, ultraviolet, x-rays and so on. The approximate value of the solar constant is 1361 W/m2.
Clouds, atmospheric absorption and other factors complicate measurements from Earth's surface, so NASA has taken the measuring devices to space. Today, VIRGO, ACRIM and SORCE are making measurements with precisions approaching 10 parts per million per year. Future instruments scheduled for flight on NASA's Glory and NOAA's NPOESS spacecraft aim for even higher precisions.
To the amazement of many researchers, the solar constant has turned out to be not constant.
"'Solar constant' is an oxymoron," says Judith Lean of the Naval Research Lab. "Satellite data show that the sun's total irradiance rises and falls with the sunspot cycle by a significant amount."
At solar maximum, the sun is about 0.1% brighter than it is at solar minimum. That may not sound like much, but consider the following: A 0.1% change in 1361 W/m2 equals 1.4 Watts/m2. Averaging this number over the spherical Earth and correcting for Earth's reflectivity yields 0.24 Watts for every square meter of our planet.
"Add it all up and you get a lot of energy," says Lean. "How this might affect weather and climate is a matter of—at times passionate—debate."
Because SDO specializes in extreme ultraviolet wavelengths, it won't be making direct measurements of the total solar irradiance, which requires sensitivity across the entire electromagnetic spectrum. Nevertheless, a combination of data from SDO and other spacecraft could shed new light on this important topic—and perhaps reveal other oxymorons as well.