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Solar Probe Offers New and Closer Look at Everything Under the Sun

You might think that the weather forecast for the surface of the sun would be a pretty dreary affair. Hot and dry. High 10,000 Fahrenheit. Outlook: continued sunny with intermittent spots and flares for the next 4 billion years.

But recently scientists have learned that our local star is beset by its own kind of incendiary tempests, seasonal squalls and rogue currents of fire. And now new instruments that can peer beneath the outer luminous layers have revealed an utterly unexpected meteorological feature in the form of titanic "jet streams" of electrically charged gases that circle the poles, churning the atmosphere where it was thought to be most calm.

They also detected several mysterious "trade wind" bands of high-speed plasma 40,000 miles wide that ring the solar perimeter like the belts on Jupiter, as well as a global pattern of upper-level current flow north and south from the equator. Many of these effects, said Jesper Schou of Stanford University, exhibit "motion similar to weather patterns in the Earth's atmosphere."

The findings, reported last week by Schou, Craig DeForest and other Stanford researchers using the Solar and Heliospheric Observatory (SOHO) spacecraft, are "absolutely fascinating," said Juri Toomre, a University of Colorado astrophysicist not involved in the announcement. They "herald a new era of solar meteorology," said Douglas Gough of Cambridge University in England.

Researchers are hoping that the SOHO data will help explain such persistent enigmas as the origin and life cycle of sunspots, which influence the torrents of solar emissions that blast toward Earth, deranging radio communications.

"One of our ultimate goals," said John W. Harvey of the ground-based National Solar Observatory in Tucson, "is to learn enough about the sun to predict activity" accurately enough to warn Earthlings days in advance that a solar storm is on the way. "Any sort of insight," DeForest said, "could give us more lead time."

But in the short term, most solar scientists will settle for anything that makes clearer the nature of our familiar, but often inscrutable, star.

The sun, 98 percent of which is hydrogen and helium, is a perfectly ordinary middle-aged representative of the 100 billion stars in our galaxy. It can thus serve as "a Rosetta stone" for understanding others large and small, said George Withbroe of NASA's solar research program.

But first scientists need to understand its baffling quirks. For one, it swells and contracts at five-minute intervals, driven by sound waves that bounce around its innards. For another, it rotates at different speeds at different latitudes: At the equator, one revolution takes about 25 days; at the poles, about 33. The difference in speed between adjacent regions causes a sliding or shear force on the intervening gas. (Imagine two boats cruising side by side. If one is going 10 mph and the other 15, the water between them will be jerked into turbulence.)

Within that global pattern, the Stanford team observed half a dozen horizontal bands in both hemispheres of the sun that are traveling faster than adjoining areas (yellowish rings in the illustration). Solar surface activity is suspiciously high alongside them.

That makes sense: At the edges of those belts, where they rub against slower-moving gas, there should be considerable shear stress. That presumably causes kinks to form in the plasma -- gas that has been heated so much by fusion at the solar core that electrons have detached themselves from their atoms, creating a seething incandescent soup of negative electrons and positive nuclei.

The plasma rises and falls in the same patterns you see in a pot of boiling water. Hotter, less dense material rises until it reaches the surface, then cools and descends. This occurs in the outermost 30 percent of the sun, which is called the convection zone. Its top surface, the photosphere, generates the sunlight we see daily.

Because both the electrons and nuclei are electrically charged, any motion in the plasma will produce a magnetic field, just as a moving electric current in a wire does. When magnetic fields are induced by the rope-like kinks, they sometimes form weird loops that protrude all the way up through the outer surface. The magnetic field retards the upward convective flow of hot plasma, and so the photosphere looks darker where the fields poke through. These dark regions -- typically twice the Earth's diameter -- are sunspots.

Sunspots follow a strange but highly predictable pattern. Every 11 years, new batches start to form at high solar latitudes in both the northern and southern hemispheres, and gradually move toward the equator. Each spot in a cycle has the opposite magnetic orientation of the spots in the previous cycle, probably because the sun itself reverses its magnetic poles every 11 years: North becomes south and vice versa.

To better understand those and other solar idiosyncrasies, NASA and the European Space Agency launched SOHO in December 1995. The two-ton, 12-by-12-foot spacecraft, positioned 930,000 miles sunward from Earth, carries many instruments, some of which detect regional speed differences in sound-wave pulses that propagate through the solar gases. Such regions reveal their locations and relative velocities via the Doppler effect: Waves moving toward an observer appear closer together (that is, seem to have higher frequency and shorter wavelength); those moving away appear just the opposite.

That's how the Stanford group found the polar jet stream (invisible on the surface) and the fast-track horizontal bands. The bands' existence had been postulated in the 1980s, but the new data show they extend at least 12,000 miles deep.

Scientists had also long suspected that there was a general flow in both hemispheres from the equator to the poles. SOHO confirmed that the upper layer of the solar surface (down to a depth of at least 15,000 miles) drifts poleward at 50 mph. At that speed, it takes about a year to make the trip. Apparently when it reaches the higher latitudes, it descends and "we assume that the flows will return at some depth," Schou said, "though we haven't observed that yet."

That means surface currents are heading toward the poles at the same time that sunspot clusters are moving toward the equator. According to present theory, those seemingly contradictory events are possible because the big magnetic kinks that cause sunspots are thought to originate at the bottommost boundary of the convection layer, where currents are probably flowing toward the equator.

"Do we really understand" all the patterns, Gough said at a NASA briefing last week. "The answer is no. But we will. That is the faith of the scientist."

ANATOMY OF THE SUN

Using the Solar and Heliospheric Observatory (SOHO) spacecraft, scientists are assembling a much more detailed picture of the inner workings of the sun, including clues to such mysteries as the origin and life cycle of disruptive sunspots.

Miles outside

Outer Regions Temperature convection zone

Corona 3.24 millionF to 435,000

Transition zone 180,000F 1,240

Chromosphere 7,640F -- 50,000F 200 -- 1,240

Photosphere 11,240F -- 7,640F 0 -- 200

Sun's diameter: 840,000 miles

SOURCE: NASA photos