Using data from the Ulysses spacecraft, researchers have discovered that high-energy particles from the Sun sometimes go in unexpected directions.

by Dr Tony Phillips

Take a piece of paper. Make a little wad. If you're a kid, spit on it. Put it in a straw and blow hard.

If your teacher sends you to the principal's office, here's your excuse: you were making a model of relativistic protons accelerated in the shock front of a solar coronal mass ejection (CME). It was done in the name of science.

Really. Solar explosions and spitwads do have something in common. CME's hurl subatomic particles across the solar system at nearly light speed. Those particles are guided, much like a spitwad in a straw, by the Sun's magnetic field.

The Sun is a star-sized magnet; its magnetic field permeates the solar system all the way from Mercury to Pluto and beyond. We don't feel it on Earth only because our planet's own magnetic field is locally stronger - but in interplanetary space, the Sun's magnetic field rules.

Because the Sun rotates on its axis (once every 27 days) the Sun's magnetic field out among the planets has a spiral shape. Researchers call it "the Parker spiral" after the physicist who first described it. Using the Parker spiral, "space weather forecasters can predict where energetic solar particles will go," explains solar physicist Ming Zhang of the Florida Institute of Technology. That's a good thing, say, for spacewalking astronauts who want to know when a radiation storm is coming so they can duck inside their spaceship.

But Zhang and colleagues have recently discovered something troubling. Solar particles don't always follow the Parker spiral like they're supposed to.

Credit: Steve Suess, NASA. The Sun's spiraling magnetic field as viewed from ~100 AU away.

"We learned this," says Zhang, "using the Ulysses spacecraft."

Ulysses is a joint mission of the European Space Agency and NASA to study the Sun. The spacecraft left Earth in 1990 and swung by Jupiter two years later - an encounter that flung the craft high above the orbital plane of the nine planets. Ulysses' looping orbit carries it all around the Sun, even over the Sun's poles, which is just what scientists wanted.

"Here on Earth we see the Sun from one direction only - its equator," explains Zhang. "Ulysses lets us look at solar activity from many directions."

Such was the case on July 14, 2000 - Bastille Day in France - when a powerful explosion rocked the Sun. The source was a sunspot 20 times wider than Earth itself. For days, magnetic field lines above the spot had become increasingly tangled. The tension grew until, like a rubber band stretched overly taut, the lines of force snapped back - explosively.

A blast of electromagnetic radiation caused radio blackouts on Earth for hours. The explosion also hurled a massive cloud of gas (a CME) toward Earth, which, when it swept past our planet two days later, would trigger auroras as far south as Texas. At the leading edge of the CME, a shock wave accelerated subatomic particles to nearly light speed. Magnetic lines of force guided them to Earth where they temporarily disabled some satellites and knocked out one (the Japanese Advanced Satellite for Cosmology and Astrophysics) completely.

The high polar orbit of Ulysses carries it to a maximum solar latitude of 80.2 degrees south.

Ulysses watched these events from high (3 AU) above the Sun's southern hemisphere. "The spacecraft was at 60^o S heliographic latitude. The explosion was at 22^o N - almost directly in line with Earth," says Zhang. You can visualize it this way: Suppose the Sun had continents and countries like our planet does. The explosion happened in Saudi Arabia; Earth was overlooking equatorial Kenya; and Ulysses was hovering above the Antarctic Peninsula.

Ulysses' point of view, looking sideways at an explosion that blasted Earth nearly head-on, was key to Zhang's discovery.

Although the blast wasn't directed toward Ulysses, the craft was connected to the expanding CME by solar magnetic field lines. "At first, protons accelerated by the CME arrived from the direction we expected," says Zhang. "They followed the Parker spiral." But a few hours later Ulysses was blindsided by a blast of protons from 90^o away.

The spitwad had tunnelled through the straw!

"The technical name for this is cross-field diffusion," says Zhang. "It happens when magnetic fields get tangled." Particles find themselves able to drift or "diffuse" from one twisted line of force to another. "Pretty soon they're moving in unexpected directions," he says.

That's troubling, says Zhang, because subatomic particles accelerated by CME's can be "worse than radiation from nuclear bombs." Forecasting where those particles will go is crucial to the safety of astronauts and satellites. Nanosatellites, a new kind of miniature spacecraft on the drawing board, are particularly vulnerable, says Zhang. Their tiny electronics can be disabled by a single solar proton ("heavy ions are even more effective," he notes) - although nanosatellites can survive a radiation storm simply by turning off sensitive systems until the storm subsides.

more Spacewalking astronauts need good forecasts of solar radiation storms.

Researchers have developed computer models to predict the onset of radiation storms after solar flares and CME's, "but seldom do they include cross-field diffusion," says Zhang. "It is difficult to include," he explains, "because cross-field diffusion is a complicated process that happens in places where magnetic fields are tangled - in other words, where the Parker spiral is not quite right." Most of the solar system is unexplored territory, so researchers don't know where the tangles are.

Researchers have developed computer models to predict the onset of radiation storms after solar flares and CME's, "but seldom do they include cross-field diffusion," says Zhang. "It is difficult to include," he explains, "because cross-field diffusion is a complicated process that happens in places where magnetic fields are tangled - in other words, where the Parker spiral is not quite right." Most of the solar system is unexplored territory, so researchers don't know where the tangles are.

"We've still got a lot to learn," he concludes. How does cross-field diffusion work? Where is it most likely to happen? With some help from Ulysses, "we're finding the answers."