NASA - National Aeronautics and Space Administration

Monitoring even the slightest changes in the Sun's output is crucial for understanding our delicate climate system. So when Venus' transit past the Sun in June had the power to decrease that energy by even a slight amount, it was news for scientists.

Right: The Solar X-ray Imager instrument on the GOES spacecraft caught this view of the Venus Transit. The entire event lasted about six hours and various movies have been combined to show this complete shot. Click on image for movie. Credit: NASA/NOAA

In fact, NASA's Solar Radiation and Climate Experiment (SORCE) satellite recently confirmed that a vast number of objects in our solar system like sunspots and planets reduce some of the Sun's energy reaching the Earth. Sunlight hitting the Earth during the Venus Transit actually fell by one-tenth of one percent, according to SORCE. Although that might not seem significant, it's "astronomical" in the world of astronomy!

Left: When Venus came between the Earth and the Sun for 6 hours on June 8th, it reduced sunlight to Earth by one-tenth of a percent. SORCE measured a drop in the Sun's energy by about 1.36 Watts for each square meter of Earth's cross-section. Click on image for larger version. CREDIT: NASA/LASP

And during the October and November solar storms (the most intense ones ever observed), very large sunspots moving across the face of the Sun dimmed Earth-bound sunlight by about three-tenths of one percent. SORCE became the first satellite to make measurement of the total energy output from sunspots and their associated solar flares - violent eruptions of the Sun. SORCE also recorded the energy from all wavelengths, something no other satellite has ever done.

A sunspot is simply a region on the Sun's surface that is marked by lower temperature and intense magnetic activity. Although they are blindingly bright, they are roughly 2700 degrees Fahrenheit (1500 degrees Celsius) cooler than their surroundings. Such a temperature contrast appears visibly as dark spots on the Sun's surface. Some sunspots can last for months, while others fade within a few hours. They tend to expand and contract as they wobble around the Sun, and can be as large as 80,000 km in diameter, fitting nearly six planet Earths within one.

Left: In October 2003, large sunspots moving across the Sun blocked three-tenths of one percent of sunlight to Earth for about 4 days. Click on image for larger version. Credit: NASA/LASP

Three tenths of one percent is probably about the same amount of solar energy reduction that triggered the chaotic period known as the "Little Ice Age." That decrease lasted some 50 years, while this decrease lasted about a week, though. The point is that the slightest changes can make a world of difference.

During the Little Ice Age, exceptionally cold temperatures raged throughout Europe and the rest of the world, but there were virtually no sunspots observed. Some scientists hypothesize that during that period, low solar activity led to a dramatic decrease in Sun's ultraviolet radiation levels, impacting ozone formation in the stratosphere. This affected circulation in the upper atmosphere, ultimately reducing westerly winds and cooling the continents during the wintertime. Same difference in output but a different cause, all of which means big mysteries need to be solved.

Today SORCE measurements provide data of extraordinary detail and accuracy, equipping today's climate scientists with essential information on the Sun's energy output. This allows researchers to better understand how the Sun's energy level can affect weather on short timescales, like during the Venus transit, or over longer periods, such as the Little Ice Age. SORCE also gives insight into how increases in greenhouse gases might ultimately affect Earth's climate.

NASA Goddard Space Flight Center/Additional Multimedia
Related Climate "Image of the Week"
SORCE Web Site
SORCE News Page
More on Sunspots
Related story on the Little Ice Age
October Solar One-Two Punch
More on Venus Transit

Mike Bettwy
NASA Goddard Space Flight Center