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Space-age measurements of the total solar irradiance or "TSI". TSI is the sun's brightness summed across all the wavelengths of the electromagnetic spectrum--visible light and EUV included. TSI goes up and down with the 11 year solar cycle. Credit: C. Fröhlich.

To monitor these energetic photons, NASA is going to launch a sensor named "EVE," short for EUV Variability Experiment, onboard the Solar Dynamics Observatory as early as this winter.

"EVE gives us the highest time resolution (10 sec) and the highest spectral resolution (< 0.1 nm) that we've ever had for measuring the sun, and we'll have it 24/7," says Woods, the lead scientist for EVE. "This is a huge improvement over past missions."

Although EVE is designed to study solar activity, its first order of business is to study solar inactivity. SDO is going to launch during the deepest in almost 100 years. , flares and CMEs are at low ebb. That's okay with Woods. He considers solar minimum just as interesting as solar maximum.

"Solar minimum is a quiet time when we can establish a baseline for evaluating long-term trends," he explains. "All stars are variable at some level, and the sun is no exception. We want to compare the sun's brightness now to its brightness during previous minima and ask ourselves, is the sun getting brighter or dimmer?"

Lately, the answer seems to be dimmer. Measurements by a variety of spacecraft indicate a 12-year lessening of the sun's "irradiance" by about 0.02% at visible wavelengths and 6% at EUV wavelengths. These results, which compare the solar minimum of 2008-09 to the previous minimum of 1996, are still very preliminary. EVE will improve confidence in the trend by pinning down the EUV spectrum with unprecedented accuracy.

The sun's intrinsic variability and its potential for future changes are not fully understood—hence the need for EVE. "The EUV portion of the sun's spectrum is what changes most during a solar cycle," says Woods, "and that is the part of the spectrum we will be observing."

Woods gazes out his office window at the Colorado . It looks the same as usual. EVE, he knows, will have a different story to tell.

Source: Dr. Tony Phillips, Science@NASA
Category: Article series 1990

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