19 June 2009

Mystery of missing sunspots solved

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The Sun is in the pits of a century-class solar minimum, and sunspots have been puzzlingly scarce for several years. Now, solar physicists think they understand why.
Helioseismic map of the solar interior

A helioseismic map of the solar interior. Tilted red-yellow bands trace solar jet streams. Black contours denote sunspot activity. When the jet streams reach a critical latitude around 22º, sunspot activity intensifies. Credit: NASA/NSO

A jet stream deep inside the Sun may be migrating slower than usual through the star’s interior, giving rise to the current lack of sunspots.

A team at the U.S. National Solar Observatory in Tucson, Arizona, used a technique called helioseismology to detect and track the jet stream down to depths of 7,000 km below the surface of the star.

The physicists, Rachel Howe and Frank Hill, announced the results this week at an American Astronomical Society press conference in Boulder, Colorado.

The Sun generates new jet streams near its poles every 11 years. The streams migrate slowly from the poles to the equator and when a jet stream reaches the critical latitude of 22º, new-cycle sunspots begin to appear.

Howe and Hill found that the stream associated with the next solar cycle has moved sluggishly, taking three years to cover a 10º range in latitude compared to only two years for the previous solar cycle.

The jet stream is now, finally, reaching the critical latitude, heralding a return of solar activity in the months and years ahead.

“It is exciting to see”, says Hill, “that just as this sluggish stream reaches the usual active latitude of 22º, a year late, we finally begin to see new groups of sunspots emerging.”

The current solar minimum has been so long and deep, it prompted some scientists to speculate that the Sun might enter a long period with no sunspot activity at all, akin to the Maunder Minimum of the 17th century.

This new result dispels those concerns. The sun’s internal magnetic dynamo is still operating, and the sunspot cycle is not broken.

Because it flows beneath the surface of the Sun, the jet stream is not directly visible. Hill and Howe tracked its hidden motions via helioseismology – the study of pressure waves within the Sun. Shifting masses inside the sun send these waves rippling through the stellar interior.

So-called ‘p modes’ (p for pressure) bounce around the interior and cause the Sun to ring like an enormous bell. By studying the vibrations of the Sun’s surface, it is possible to figure out what is happening inside. Similar techniques are used by geologists to map the interior of our planet.

In this case, researchers combined data from a network of telescopes (the Global Oscillation Network Group or GONG) that measures solar vibrations from various locations around Earth and NASA’s Solar and Heliospheric Observatory (SOHO) in space.

“This is an important discovery,” commented Dean Pesnell of NASA’s Goddard Space Flight Centre in Maryland, USA. “It shows how flows inside the Sun are tied to the creation of sunspots and how jet streams can affect the timing of the solar cycle.”

There is, however, much more to learn. “We still don’t understand exactly how jet streams trigger sunspot production,” said Pesnell. “Nor do we fully understand how the jet streams themselves are generated.”

To solve these mysteries, and others, NASA plans to launch the Solar Dynamics Observatory (SDO) later this year. SDO is equipped with sophisticated helioseismology sensors that will allow it to probe the solar interior better than ever before.

“The Helioseismic and Magnetic Imager (HMI) on SDO will improve our understanding of these jet streams and other internal flows by providing full disk images at ever-increasing depths in the sun,” said Pesnell.

Continued tracking and study of solar jet streams could help researchers do something unprecedented – accurately predict the unfolding of future solar cycles.

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Dr Tony Phillips is a science writer and editor at NASA.
This is an edited version of a feature first published on the Science@NASA web site.
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