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Ulysses Spacecraft Observes Surprising Reach of Solar Activity

Steve Koppes
(Phone: (773) 702-8366)
Email: s-koppes@uchicago.edu

Once every 11 years, the sun's magnetic field flips over. During the commotion that accompanies these field reversals, large solar flares send great geysers of hot gas and huge quantities of charged particles erupting from the surface and streaming into space during a period called "solar maximum."

New data collected by University of Chicago scientists via the Ulysses space probe shows that there is no escaping the pervasive influence of these potentially harmful charged particles, not even at the seldom-visited high solar latitudes.

"What we found, which I think nobody really expected, is the widespread distribution of particles accelerated following a single solar flare basically all the way through the heliosphere," the region of space dominated by the sun, said Bruce McKibben, a physicist at the University's Enrico Fermi Institute.

Solar flares - sudden local brightenings on the sun's surface - usually produce X-rays and radio emissions as well as charged particles and gas eruptions. "The energetic particles following a flare can sometimes increase the radiation intensity in space to harmful levels, and increased radiation can linger in the vicinity of Earth in the inner heliosophere for days, weeks, or even as much as a month after a large flare," McKibben explained.

Scientists had expected to find that the sun's magnetic field lines, which are stretched out into space by the solar wind, would keep charged particles originating in solar flares near the equator confined to a fairly narrow band near their origination point. Instead, these particles have mysteriously migrated to high solar latitudes in a matter of just three or four days, making radiation levels almost the same over the poles as at the equator.

It thus appears that even at polar latitudes, where only Ulysses has gone before, spacecraft can be exposed to almost the same solar energetic particle environment as spacecraft on the plane of the ecliptic, where the Earth and the planets follow their orbits around the sun. McKibben will present a paper that describes the rapid and pervasive dissemination of solar particles Aug. 9 at the 27th International Cosmic Ray Conference in Hamburg, Germany. The paper will draw on observations from all of the instruments that are part of the Cosmic and Solar Particles Investigation (COSPIN) consortium.

McKibben leads the high-energy charged-particle experiment on Ulysses provided by the COSPIN consortium. The consortium consists of scientific groups from five nations, each of which provided a detector system covering specific aspects of the high-energy particle radiation in space. The Pioneer 6 and 7 spacecraft had previously detected evidence of the widespread distribution of charged particles from the sun in the late 1960s and early 1970s. But the data then were limited to spacecraft orbiting the sun in the ecliptic plane near the sun's equator.

"We've now extended that into the third dimension," McKibben said.

Exactly how solar particles travel so rapidly across the magnetic fields of interplanetary space remains a mystery, McKibben said. "There's clearly some mechanism that moves them across magnetic fields much faster than we have any reason to believe they should. There are ideas but none of them seem completely adequate, at least to me, and I'm puzzled."

Last year the sun entered a solar maximum, which scientists expect will last for up to three or four years. Charged particles released by the sun are the chief danger during periods of high solar activity, said Cliff Lopate, another Fermi Institute physicist.

"When the sun becomes active, it can release enormous numbers of charged particles in extremely brief periods of time," Lopate said. "Because there are so many of them, the particles can damage both electronic and biological systems in space."

The Earth's magnetic field shields the International Space Station fairly well against the sun's radiation, but "solar maximum would be a bad time to go to Mars," McKibben said. "It appears that you can get significant enhancements in radiation exposure anywhere in the solar system following an event anywhere on the sun."

The sun's severe radiation zone extends as far out as the orbit of Jupiter, "so there's not very much protection in going away from the sun, either," McKibben said.