Sunspot Delay Due to Sluggish Solar "Jet Stream"?

June 19, 2009

A sluggish, jet stream-like flow deep inside the sun could be to blame for the delay in increased solar activity that has been stumping astronomers.

(Read "Sun Oddly Quiet—Hints at Next 'Little Ice Age'?")

The jet stream, which is actually a plasma current called a torsional oscillation, has been migrating more slowly than usual through the star's interior, according to a team led by Frank Hill of the National Solar Observatory in Tucson, Arizona.

Every 11 years the sun generates new jet streams near its poles. These streams slowly shift from east to west toward the solar equator over a period of 17 years.

When the stream reaches a certain latitude, the sun starts producing new sunspots—relatively cool, dark regions on the sun that mark areas of magnetic disturbance.

But the stream associated with the current cycle of solar activity has been moving even more slowly than normal, Hill said.

Obvious in Hindsight?

Based on new data from sun-tracking instruments known as the Global Oscillation Network Group (GONG) and the Solar and Heliospheric Observatory (SOHO), Hill and colleagues saw that it took an extra year for the stream to cross a distance of 10 degrees latitude, compared with previous solar cycles.

The new measurements also show that the stream has finally reached the critical latitude linked to sunspot production, which could explain why solar activity finally seems to be picking up.

"It's not clear whether this [slower jet stream] is a cause or a consequence" of the mysterious solar quiet, Hill said. "But the fact that we see it a couple of years in advance [of the sun's extended quiet] makes me think it's a cause."

Jesper Schou, an astrophysicist at Stanford University who works on SOHO, said that, since the stream's sluggish motion had shown up in previous data, in hindsight the solar quiet might have been predicted.

But both GONG and SOHO, right now the best instruments for monitoring the sun's interior, have been in operation for only 14 years. That's a relatively short period for scientists to get comfortable with the type of data being returned. By contrast, sunspots have been tracked as a measure of solar activity for hundreds of years.

"You need some amount of confidence" with the data before recognizing any discrepancies, Schou said. "After a while it's like, Oh, it looks very obvious."

Findings presented this week at a meeting of the American Astronomical Society Solar Physics Division in Boulder, Colorado.

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