Lowdown on the sun

Every 11 years, the sun gets the doldrums. Solar storms are fewer and the strength of the solar wind, the stream of charged particles blown from the sun, declines. But new spacecraft observations have now gotten the true lowdown: The current solar minimum is the lowest — and one of the longest — recorded in the past 50 years, since modern measurements began

This period of low solar activity has already lasted six months longer than the last solar minimum, which was in 1994 and 1995.

The sun’s current state suggests that the heliosphere — the vast, protective magnetic cavity carved by the solar wind — has temporarily shriveled. This decline lets more harmful galactic cosmic rays into the solar system and makes it riskier for astronauts traveling beyond Earth’s own, much smaller magnetic bubble, or magnetosphere.

The findings may also shed new light on the origin of the solar wind. The low pressure of the solar wind, combined with other studies showing reduced solar magnetic activity, suggests that the wind is not only guided and shaped by the sun’s global magnetic field but is also powered by it, says David McComas of the Southwest Research Institute in San Antonio, Texas.

McComas and his colleagues describe the findings September 18 online in Geophysical Research Letters. Along with solar physicists, including Ed Smith of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., McComas also detailed the findings during a telephone press briefing on September 23

The pressure of the solar wind and the strength of the magnetic field in the current solar minimum “are at an all-time low, at least since the space age began four sunspot cycles ago,” says Smith. As when a tire is only partly inflated, the lower pressure suggests the heliosphere bubble isn’t quite as big as it used to be. That jibes with recent findings from the venerable Voyager 2 spacecraft, now journeying at the solar system’s outskirts.

“This extreme minimum has important implications for the solar dynamo for solar activity and perhaps even for terrestrial climate,” comments solar physicist Spiro Antiochos of NASA’s Goddard Space Flight Center in Greenbelt, Md. The dynamo is the roiling motions of gas inside the sun that generate the sun’s magnetic field.

Because the magnetic field at solar minimum acts as the seed field for the next minimum, “it may well be that the next cycle will be low, which could have a cooling effect on terrestrial weather,” Antiochos says. “There appears to be evidence for a correlation between lack of activity and terrestrial cooling.” Such cooling was observed during an extended period of unusually low solar activity about 300 years ago known as the Maunder Minimum.

Much of the new data come from NASA’s Ulysses spacecraft, launched in 1990 and the first craft to fly over the sun’s poles. Now just two months from its deathbed, when its hydrazine fuel is expected to freeze, Ulysses has found that the polar solar wind has only three-quarters the strength it had about a decade ago, during the last solar minimum.

Moreover, other spacecraft that measure the solar wind near the sun’s equator show a similar decline, demonstrating that the phenomenon involves the entire sun, McComas notes.

If the doldrums continue, the Voyager 1 and Voyager 2 spacecraft, already at the outskirts of the solar system, could reach the solar system’s very edge a year or two sooner than current estimates of 2010 to 2020, notes Smith. That’s because the heliosphere boundary is also the solar system boundary. However, Smith adds, “the pressure is expected to increase again as solar activity picks up, and cause the boundaries to move outward again.”

Voyager 2 recently encountered a region called the termination shock, where the solar wind slams into interstellar space, at a distance of about 83.7 AU from the sun. (1 AU is the average Earth-sun separation). That’s 10 AU closer to the sun than when the Voyager 1 craft encountered the termination shock in 2003, suggesting the heliosphere has indeed shrunk in the intervening years.

A mission called the Interstellar Boundary Explorer, scheduled for launch on October 19, may benefit most from the shrunken heliosphere. From near-Earth orbit, the mission will map the boundary of the solar system by detecting atoms that originated in the outer heliosphere.

The mission “should begin to see the heliosphere morph ‘in flight’” as the solar wind grows stronger and reinflates the bubble, comments Scott McIntosh of the National Center for Atmospheric Research in Boulder.

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Solar Low from Science News on Vimeo .