A tiny satellite circling Earth is providing an unexpectedly complicated picture of the solar system's heliosphere, the invisible bubble that extends far beyond the planetary orbits to where the solar wind strikes the vast sea of particles and radiation that fill interstellar space, researchers said Thursday.
It turns out the heliosphere is changing much more rapidly than scientists ever expected, according to data published Thursday in the Journal of Geophysical Research.
Despite its great distance from Earth, the heliosphere is of great interest to astronomers because it shields the solar system from as much as 90% of the cosmic rays that would otherwise enter it.
As humans contemplate manned spaceflights of longer durations, "galactic cosmic radiation turns out to be the most important factor" for the safety of astronauts, astronomer David McComas of the Southwest Research Institute in San Antonio said at a news conference.
The sun emits a steady stream of particles traveling outward in all directions about 1 million mph. When they have traveled about 100 times farther than the distance between Earth and the sun — well beyond the outer planets — the particles collide with the interstellar medium. In the process, they deflect most cosmic rays back into space and produce uncharged particles that stream back into the inner solar system.
Researchers have long known about the existence of the heliosphere but had been unable to obtain images of it because it emits no light. That situation changed in 2008 with the launch of the Interstellar Boundary Explorer, or IBEX, which collects and analyzes the neutral particles. Because IBEX's orbit always maintains the same orientation with respect to the solar system, it is able to map the entire heliosphere as the Earth travels around the sun.
The first map from IBEX, released a year ago, "stunned many of us," said astronomer Nathan Schwadron of the University of New Hampshire, the IBEX science operations lead. Researchers were expecting to see a relatively uniform sphere. Instead, the satellite image depicted a ribbon of unusually high-energy particles that dipped downward from the North Pole across the solar system equator and back. There was also an unusually bright spot in the northern regions of the ribbon that researchers dubbed a knot.
"What amazed us was, this was never predicted," he said. And researchers still don't know what causes it. Everyone is "vying to understand what creates this ribbon," Schwadron said.
A follow-up map, with data compiled six months later and published Thursday, shows some changes in the structure of the ribbon. The most interesting finding is that the knot has dissipated.
"It's very surprising to see any variations at all," said McComas, who is the principal investigator on the mission. Because the solar cycle lasts 11 years, "we certainly thought we wouldn't see variations on this short a time scale."
Over the last two decades, the solar wind has weakened and the heliosphere has shrunk, allowing more cosmic radiation to enter. Any shrinkage in the heliosphere leads to increased cosmic radiation, which could be very dangerous to space travelers, McComas said.
"There is a lot of expectation that the sun is starting to pick up activity, and there may be more complicated interactions coming," he said. "With luck, we could observe them over the next solar cycle for a whole decade."
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