South Pole Flyby

Consider the following:

1. The sun's north magnetic north pole sticks out the south end of the sun. Magnetically, the sun is upside down!

"Most people don't know it, but we have the same situation here on Earth," notes Posner. "Our magnetic north pole sticks out of the geographic south pole." Magnetically, Earth and sun have a lot in common. "Both the sun's and Earth's magnetic poles are constantly on the move, and they occasionally do a complete flip, with N and S changing places." This flipping happens every 11 years on the sun in synch with the sunspot cycle. It happens every 300,000 years or so on Earth in synch with--what? No one knows. "Studying the polar magnetic field of the sun might give us some clues about the magnetic field of our own planet."

2. There are holes over the sun's poles --"coronal holes." These are places where the sun's magnetic field opens up and allows solar wind to escape. "Flying over the sun's poles, you get slapped in the face by a hot, million mph stream of protons and electrons," he says. Ulysses is experiencing and studying this polar wind right now. (Note: Earth has a polar hole, too--the ozone hole. The chemistry of the ozone hole is totally unrelated to the magneto-physics of coronal holes, but says Posner, "it is interesting that so many poles seem to have holes.")

3. Just as the sun's polar magnetic field allows solar wind out, it also allows galactic cosmic rays in. Could the space above the sun's poles be a place where we can sample interstellar matter without actually leaving the solar system? "That's what we thought before our first polar flyby in 1994," recalls Posner. "But we were wrong. Something is keeping cosmic rays out of the sun's polar regions. The current flyby gives us a chance to investigate this phenomenon."

Credit: SOHO

The sun's south polar coronal hole. Solar wind flows out of the dark region in this false-color UV image.

4. Another mystery: There is evidence from earlier flybys that the north pole and the south pole of the sun have different temperatures. "We're not sure why this should be," says Posner, "and we're anxious to learn if it is still the case." Today's south polar flyby will be followed by a north polar flyby in early 2008, allowing a direct north vs. south comparison.