What Lies inside the Sun?

Artist's concept image of the SDO satellite orbiting the Sun.When a pond's surface is calm, you can see more clearly into its depths. As it turns out, the same is true for the sun.

Researchers are excited that the sun's surface is calmer now than it's been in almost a century. It's a rare opportunity — the first since the Space Age began — to peer more clearly into the sun's mysterious interior.

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To take advantage of the opportunity, NASA is about to launch the Solar Dynamics Observatory (SDO). SDO is expected to launch this year, as early as November 2009.

"That's perfect timing," says Dean Pesnell, a solar physicist at the Goddard Space Flight Center in Greenbelt, Md. "The sun is experiencing a century-class solar minimum, offering the clearest possible views."

"SDO will actually see through the sun's surface," added Pesnell. "The process is a little like taking an ultrasound of a pregnant mother. You can see the baby right through the skin."

The sensor that performs this trick is called the Helioseismic and Magnetic Imager (HMI). It can sense acoustic waves moving through the sun, and turn those waves into a fairly clear image of the interior.

"There's a lot going on inside the sun that we don't understand," notes Todd Hoeksema, a solar physicist at Stanford University, Palo Alto, Calif. where the HMI was built. "The Solar Dynamics Observatory is bound to deliver some big discoveries."

The biggest discovery of all would be the inner workings of the solar dynamo. Deep beneath the sun's visible surface, massive currents of electrically-charged gas (plasma) circulate in patterns that give rise to the sun's powerful magnetic field. Almost all solar activity from sunspots to solar flares is regulated by this inner dynamo.

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"Understanding how the dynamo works is a holy grail for stellar physics," says Pesnell. "It is the key to forecasting solar activity and space weather."

The problem is these deep flows are hidden from view. The sun's surface is bright and opaque, so it is impossible to look through it. Instead, solar physicists study the sun's interior the same way that geologists look deep into the Earth—via seismology. Just as earthquakes trigger seismic waves that travel through the Earth, shifting mass in the sun sends pressure waves rippling through its interior. These p modes (p for pressure) bounce around inside the sun, causing the star to ring like an enormous bell. HMI detects the surface vibrations, which in turn can be analyzed to reveal the depths.

Sunspots can get in the way of the waves, distorting their times and frequencies, making it tricky to figure out what's really happening inside the complicated solar interior. That's why the current lack of sunspots is good for helioseismology.

"You have more sensitivity to what's happening deep in the sun when there's not as much interference from the surface," Hoeksema explains. At the peak of the solar cycle, sunspots are numerous. So far in 2009, the sun's surface has been free of sunspots about 80 percent of the time — the most tranquil it's been since 1913.

From a geosynchronous orbit 22,000 miles above Earth's surface, SDO will observe helioseismic waves more precisely than ever before. The current gold standard for observing the sun is a satellite called the Solar and Heliospheric Observatory, or SOHO, which maps helioseismic activity with mega-pixel resolution once every minute or so. HMI will up the ante to 16 megapixels every 45 seconds, resulting in a far more detailed view of the solar interior.

SDO will also improve on SOHO by beaming its torrent of raw data down to Earth unprocessed. Because of limited download bandwidth, SOHO performs some calculations while the data are still onboard the spacecraft. Only the results are beamed to scientists. Since SOHO was launched in 1995, scientists have since devised better ways to process the data and correct for errors. But there's no way they can apply these new techniques to SOHO imagery because the data have already been "crunched" by the time they leave the spacecraft.

To give scientists full access to its original data, SDO will have a continuous, 150 megabit per second download link. In comparison, most home high-speed internet connections are only 1 to 10 megabits per second.

Somewhere in all those p modes will be the telltale signs of solar jet streams, subsurface winds, proto-sunspots, and the solar dynamo itself — all "seen" with unprecedented clarity.

With the sun so calm, now is a great time to look.

Related links:

> Helioseismology (Stanford University)

> Goddard SDO page

> HMI on Twitter

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