The Sciences

Here comes the Sun(spot)!

Bad AstronomyBy Phil PlaitJun 17, 2009 4:39 PM


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You'd think the return of blemishes on your face would be an unwelcome thing. But if the face is that of the Sun, and the blemish is actually a planet-sized knot of tangled magnetic fields, then it's actually most welcome indeed. Because now we're starting to understand why they're coming back.

After nearly two years of an acne-free surface, the first sunspots are starting to pop up on the Sun. Sunspots are regions on the Sun where the magnetic field lines of our nearest star erupt through its surface, and are an indicator of the amount of magnetic activity going on inside the Sun. Unlike a simple bar magnet, the solar magnetic field activity increases and decreases on a roughly 11-year cycle, and the number of sunspots follows in response. When the magnetic activity starts to rise after the cycle bottoms out, sunspots start to appear at a solar latitude of 22 degrees, and spread north and south from there. Right now we're at the bottom of the cycle, and sunspots are rare. But this two-year lack of spots has been the longest such period for nearly a century, and it's had solar astronomers scratching their heads. That's not too surprising, as the Sun is a fiendishly and vastly complex system of churning gas, and it's numbingly difficult to observe and model it. But astronomers have just made an important breakthrough in solar observations that links the way the gas under the Sun's surface behaves with the way sunspots form. This is very cool and exciting news! Imagine for a moment that I detonate a bomb under the ocean's surface. This would cause a rising bubble of steam as well as a big displacement of water. By observing the way the water moves on the surface of the ocean, I can figure out a lot about how deep the bomb was, how big the explosions was, and so on. I don't even need a bomb to do this, really; currents under the ocean's surface distort the water above them as well, generating waves and motions that can reveal what's going on at lower depths. Same goes for the Sun! Using the Global Oscillation Network Group (GONG, a series of telescopes designed to study sound waves moving across the Sun's surface) together with the space-borne Solar and Heliospheric Observatory's Michelson Doppler Imager, astronomers can measure the way the surface of the Sun reacts to streams and movements of gas below the surface, allowing them by proxy to determine just how the gas deep inside the Sun behaves. It's been known for a long time that the Sun doesn't rotate as a solid body; different parts of it spin at different rates. Underneath the surface of the Sun is a river of gas called the torsional oscillation, a jet-stream like movement located 1000 - 7000 km (600 - 4000 miles) down -- for comparison, the Earth is about 13,000 km (8000 miles) in diameter, while the Sun is 1.4 million km (860,000 miles) across. This stream is actually located just beneath the Sun's surface on this scale. The flow of this gas has been linked to the solar cycle, but the connection has been difficult to determine. It's known to form every 11 years near the Sun's poles, then starts to slowly migrate down to lower latitudes. But now, using GONG and MDI, astronomers have been tracking it, and found that recently the migration has been more sluggish than average. Instead of taking two years to move about ten degrees of solar latitude as it usually does, in this cycle it's taken three. This would explain why the sunspots have been delayed. Moreover, their observations have indicated the first sunspots to appear on the Sun this cycle did so right when the torsional oscillation reached the "magic" latitude of 22 degrees! Since this is where sunspots always start to appear once the solar cycle starts to rise again, it clinches the connection between this subsurface jet stream and the formation of the magnetic fields that cause sunspots. As Frank Hill, one of the astronomers who has been making these measurements, put it:

"It is exciting to see that just as this sluggish stream reaches the usual active latitude of 22 degrees, a year late, we finally begin to see new groups of sunspots emerging at the new active latitude." Since the current minimum is now one year longer than usual, [Drs. Rachel] Howe and Hill conclude that the extended solar minimum phase may have resulted from the slower migration of the flow.

The reason this is important is that it's extremely difficult to predict the sunspot cycle, and this measurement of the torsional oscillation gives us a handle on being able to do that -- no other methods so far have been able to predict this long period of low activity. And that in turn is important because the Sun's magnetic field is linked to solar activity: flares and giant coronal mass ejections, vast explosions of subatomic particles that can damage satellites and even cause blackouts on Earth (there may even be a connection between sunspots and climate and weather, although this is a very tenuous and difficult link to establish). Being able to predict sunspots means being able to predict how our tempestuous Sun behaves, and that means possibly having early warnings about dangerous outbursts from the Sun. This can save billions of dollars worth of satellites and power grid infrastructure on Earth, and may even save lives (blackouts tend to happen when the power grid is under stress during the periods of coldest winter and hottest summer, when you really don't want your power to go out). We depend on the Sun for light and heat which make life on Earth possible, but the flip side of that is that we're subject to its violent nature as well. But we're making great strides in understanding the Sun, and this knowledge has gigantic practical implications. Practical knowledge is useful (by definition), but I still reel at the idea that we can measure with confidence streams of gas that happen thousands of kilometers below the Sun's surface, forever hidden form direct view. Incredible. Actually, it's credible. It's science! Solar image from NASA/ESA/SOHO.

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