Kelly Beatty, SkyandTelescope.com, New Scientist 19 Jun 09;
For the past couple of years, our sun has been at the minimum of its 11-year activity cycle. Its face has been virtually spotless for months on end, and there've been no dire alerts of titanic solar storms about to slam into Earth.
The problem is that this "quiet sun" has continued far too long – two years ago, a special task force predicted that the transition from the just-ended Cycle 23 to the upcoming Cycle 24 would come around March 2008. It didn't. (To be fair, there was sharp disagreement within the group at that time.)
Much fanfare accompanied the appearance of a tiny high-latitude sunspot in early 2008, supposedly heralding Cycle 24's arrival. Yet for months and months afterward the sun's face remained spotless.
Knowing when the upturn in solar activity begins and, more importantly, how strong it will get at maximum has grown in importance over the years. When the sun gets agitated, it buffets our planet with huge "storms" of high-speed plasma (ionised gas), punctuated by threatening flares of relativistic protons (see Space storm alert: 90 seconds from catastropheMovie Camera).
Rhythmic pulsations
But despite centuries-long records of sunspot counts and 50 years of mapping the sun's magnetic fields, scientists still don't understand what makes one cycle strong and another weak. The same task force that was bullish on Cycle 24 two years ago now believes the forthcoming activity could be the weakest in a century.
This week, two groups of researchers offered hope that we're finally understanding the sun's complex workings a little better.
The first comes from Rachel Howe and Frank Hill of the National Solar Observatory, who now believe that sunspots are linked to slow, eastward-moving "jet streams" about 7000 kilometres below the sun's visible surface, or photosphere. They've analysed 15 years of observations made using helioseismology, which monitors rhythmic pulsations at the surface created by pressure waves bouncing around the solar interior.
"Think of the sun as a musical instrument," Hill explains. "A piano has 88 keys, but the sun has five million 'notes' or modes of oscillation."
Deep-seated currents
Howe and Hill find that a pair of deep-seated currents migrate from the solar poles toward the equator during each cycle, and that the migration has been unusually sluggish of late.
They took three years to shift 10° towards the equator, and only now have they reached a solar latitude of 22°, the point at which activity perks up and sunspots start to appear. They can't yet tell whether the flow somehow causes sunspots, only that the two phenomena appear to be related.
"Had this analysis been available two years ago, we'd have seen the delayed onset of Cycle 24 coming," notes solar physicist Dean Pernell of NASA's Goddard Space Flight Center in Maryland.
The new findings were presented this week at a meeting of the American Astronomical Society's Solar Physics Division, which had a special session on Cycle 24.
Sunspot model
The second announcement concerns sunspots themselves and the arrangement of the intense magnetic tangles within them. Writing in Science Express, Matthias Rempel of the National Center for Atmospheric Research and three colleagues used a supercomputer grinding out 76 trillion calculations per second to create the first comprehensive, 3D model of these mysterious dark patches' inner workings.
The simulations reveal in detail how superheated gas streams along magnetic filaments from a spot's dark, central umbra to the lighter penumbra surrounding it.
Solar physicists first recognised this outward flow about 100 years ago. But, as Rempel's team notes, "The onset of these flows is closely related to the magnetic field inclination" and that outflows occur whenever the magnetic field is inclined more than 45° from vertical.
The hope is that a better understanding of sunspots will allow scientists to predict their behaviour more accurately and, in particular, to identify the ones most likely to trigger dangerous solar flares.
Courtesy of Sky and Telescope magazine
Sunspot Delay Due to Sluggish Solar "Jet Stream"?
Anne Minard, National Geographic News 19 Jun 09;
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.
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 slower 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.
