Anne Minard, National Geographic News 24 Sep 08;
Even the sun appears headed for a recession.
The Ulysses space probe has detected fewer sunspots, decreased solar winds, and a weakening magnetic field—the lowest solar activity observed in 50 years, NASA scientists said yesterday.
That translates into a shrinking of the heliosphere, the invisible buffer that extends beyond Pluto and guards the planets—ours included—from bombardment by cosmic rays.
Speaking yesterday at a NASA teleconference, scientists refused to draw conclusions from their observations, especially with respect to whether the changes are influencing Earth's climate.
"That area of science is in the realm of speculation at this point," said Nancy Crooker, a researcher at Boston University.
But David J. McComas of the Southwest Research Institute, who leads one of the experiments onboard Ulysses, called the changes "significant."
"This is a whole-sun phenomenon. The entire sun is blowing significantly less hard than it was 10 to 15 years ago," he said.
"Over the entire record of sun observations, this is the longest prolonged low pressure that we've observed."
Variable Star
Some variance in solar activity is normal for the sun, which has a 22-year magnetic cycle and an 11-year sunspot cycle.
But McComas said in a statement that researchers have been "surprised to find that the solar wind is much less powerful than it had been in the previous solar minimum."
Ed Smith, a NASA Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena, California, also added that the drop in solar winds has lasted longer than predicted.
Solar Influence
Scientists noted that while solar activity is low compared to the past 50 years of data, the sun's output has dipped before.
In the early 1600s Galileo and other astronomers observed only about 50 sunspots over a 30-year period. Normally, the early scientists would have witnessed closer to 50,000.
Scientists have also speculated for centuries about an intuitive link between the sun's intensity and Earth's climate.
There is evidence of the sun causing short-term impacts on Earth's weather.
The so-called Maunder Minimum, a time of low solar activity, lasted from about 1645 to 1715. During this time, access to Greenland was largely cut off by ice, and canals in Holland routinely froze solid, according to NASA.
Glaciers advanced in the Alps, and sea ice increased so much that no open water flowed around Iceland in the year 1695.
The latest observations show that the sun is even more mercurial than previous research could have found. "The sun is a variable star after all," Crooker said.
Less protection from the sun's heliosphere may also make space exploration more dangerous, according to Crooker.
Astronauts could encounter more lethal cosmic rays without the sun's protection, for example.
Most of the effects of a shrinking heliosphere, however, will be felt billions of miles beyond Pluto, at the edges of the sun's influence.
If the solar wind stays weak, NASA's Voyager 1—launched in 1977 and now headed beyond our solar system—should reach the edge of the heliosphere earlier than expected, becoming the first craft to enter interstellar space.
Mission Sunset
Launched in 1990, the joint NASA and European Space Agency Ulysses mission has lasted four times longer than expected.
The probe, which is slowly freezing to death and is expected to shut down within months, observed a dramatic slowdown in solar activity during its third and final orbit around the sun last year.
While the demise of Ulysses is imminent, NASA will soon develop the Solar Probe mission, which will fly close to the sun to determine what heats its corona—the outer layer—and accelerates solar wind.
