Keeping a close eye on the sun's irradiance

Although sunspots cause a decrease in irradiance they're accompanied by bright white blotches called faculae that cause an overall increase in solar irradiance.

Like sunspots, solar prominences are more likely to occur during the most active part of the solar cycle. Yet, despite their striking appearance, they have little impact on the sun's total solar irradiance.

For more than two centuries, scientists have wondered how much heat and light the sun expels, and whether this energy varies enough to change Earth's climate. In the absence of a good method for measuring the sun's output, the scientific conversation was often heavy with speculation.

By 1976, that began to change when Jack Eddy, a solar astronomer from Colorado examined historical records of sunspots and published a seminal paper that showed some century-long variations in solar activity are connected with major climatic shifts. Eddy helped show that an extended lull in solar activity during the 17th Century - called the Maunder Minimum - was likely connected to a decades-long cold period on Earth called the "Little Ice Age."

Two years after Eddy published his paper, Nasa launched the first in a series of satellite instruments called radiometers, which measure the amount of sunlight striking the top of Earth's atmosphere, or total solar irradiance. Radiometers have provided unparalleled details about how the sun's irradiance has varied in the decades since. Such measurements have helped validate and expand upon Eddy's findings. And they've led to a number of other discoveries, and questions, about the sun.

Without radiometers, scientists would probably still wonder how much energy the sun emits and whether it varies with the sunspot cycle. They wouldn't know of the competition between dark sunspots and bright spots called faculae that drives irradiance variations.

It's well known today that the sun's irradiance fluctuates constantly in conjunction with sunspots, which become more and less abundant every 11 years due to turbulent magnetic fields that course through the sun's interior and erupt onto its surface.

But as recently as the 1970s, scientists assumed that the sun's irradiance was unchanging; the amount of energy it expels was even called the "solar constant".

It was data from radiometers aboard Nimbus 7, launched in 1978, and the Solar Maximum Mission, launched two years later, that were the death knell to the solar constant.

Likewise, in 2003, a radiometer aboard Nasa's Solar Radiation and Climate Experiment satellite observed large sunspot patches that caused irradiance to drop by as much 0.34 per cent, the largest short-term decrease ever recorded.

Measurements collected during the 1980s and 1990s gave scientists the evidence they needed to prove that irradiance is actually a balance between darkening from sunspots and brightening from accompanying hot regions called faculae, a word meaning "bright torch" in Latin.

When solar activity increases, as it does every 11 years or so, both sunspots and faculae become more numerous. But during the peak of a cycle, the faculae brighten the sun more than sunspots dim it.

Though most scientists believe the 0.1 per cent variation is too subtle to explain all of the recent warming, it's not impossible that long-term patterns - proceeding over hundreds or thousands of years - could cause more severe swings that could have profound impacts on climate.

Source: Nasa