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Solar system's edge surprises astronomers

New observations reveal a dense ribbon structure that current models don’t explain

3:16pm, October 15, 2009
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The edge of the solar system is tied up with a ribbon, astronomers have discovered. The first global map of the solar system reveals that its edge is nothing like what had been predicted. Neutral atoms, which are the only way to image the fringes of the solar system, are densely packed into a narrow ribbon rather than evenly distributed.

“Our maps show structure and energy spectra that are completely different from what any model has predicted,” says study coauthor Herbert Funsten of the Los Alamos National Laboratory in New Mexico.

NASA’s Interstellar Boundary Explorer satellite, or IBEX, discovered the narrow ribbon, which completes nearly a full circle across the sky. The density of neutral atoms in the band is two to three times that in adjacent regions.

These and related findings, reported in six papers posted online October 15 in Science, will not only send theorists back to the drawing board, researchers say, but may ultimately provide new insight on the interaction between the heliosphere — the vast bubble in which the solar system resides — and surrounding space.

The bubble is inflated by solar wind, the high-speed stream of charged particles blowing out from the sun to the solar system’s very edge. For 48 years, researchers have assumed that the solar wind sculpted the structure at the heliosphere’s boundary with interstellar space, says Tom Krimigis of Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md.. But the newly found ribbon’s orientation suggests that the galaxy’s magnetic field, just outside the heliosphere, seems to be the chief organizer of structure in this region, says theorist Nathan Schwadron of Boston University, a lead author of one of the studies.

It’s not known whether the ribbon lasts for just a few years or is a permanent feature.

Equally puzzling are observations of the same boundary region with an instrument on the Cassini spacecraft, which recorded the density of atoms at higher energies, above 6,000 electron volts. From its vantage point at Saturn, Cassini sees a belt rather than a ribbonlike structure, a team led by Krimigis also reports in Science. The belt is substantially broader than the ribbon seen by IBEX but is in the same general area.

The heliosphere shields the solar system from 90 percent of energetic cosmic rays — high-speed charged particles that would otherwise bombard the planets and harm life. Understanding more about the heliosphere and its ability to filter out galactic cosmic rays could be critical for assessing the safety of human space travel, Schwadron notes. The new findings may also help predict how the heliosphere varies in shape and size as it moves through the galaxy and encounters regions of space having different densities and magnetic field strengths.

The ribbon found by IBEX, recorded at energies between 200 and 6,000 electron volts, is brightest at about 1,000 electron volts and lies between about 100 and 125 astronomical units from the sun, notes David McComas of the Southwest Research Institute in San Antonio. One astronomical unit is the distance between the Earth and the sun. The atoms recorded by IBEX, which orbits Earth, took a year or two, depending on their energies, to reach the craft from the outer edge of the heliosphere.

The IBEX ribbon runs perpendicular to the direction of the galaxy’s magnetic field at the interstellar boundary, an indication that the field has a much stronger than expected influence on the sun’s environs, report Schwadron and his colleagues. One possibility is that pressure from this external magnetic field has forced particles just inside the heliosphere to bunch together into a ribbon.

“First and foremost, this is a big surprise because we thought we know a lot about this region, the edge of the heliosphere,” McComas says. The Voyager 1 craft in 2004 (SN: 1/3/04, p. 7) and the Voyager 2 craft in 2007 (SN: 8/2/08, p. 7) journeyed to opposite sides of this fringe region of the solar system and crossed the termination shock — where the solar wind encounters a shock that precedes the influx of particles drifting into the solar system from interstellar space. Both craft recorded the density of particles and the strength of the magnetic fields.

Both Voyager 1 and 2 missed seeing the newly found ribbon because it spans a region between their flight paths, says McComas. No existing model can explain the ribbon, he adds, which was found independently by two instruments on IBEX.

Researchers had assumed that the pressure from the solar wind would compress in the heliosphere in the direction that the solar system was moving through space and create a cometlike tail in the opposite direction, notes Krimigis. “Now we know that’s wrong,” he says.

IBEX has also generated the first maps of neutral hydrogen and oxygen atoms entering the solar system from interstellar space. Previous observations had traced only incoming helium atoms. The sensitivity of the IBEX instruments allowed researchers to record the relatively small number of oxygen atoms that travel from beyond the termination shock, about 16 billion kilometers from Earth, to the spacecraft, notes study coauthor Stephen Fuselier of the Lockheed Martin Advanced Technology Center in Palo Alto, Calif..
Hydrogen atoms are more abundant than either helium or oxygen but their low mass means they are easily swept aside by the high-speed solar wind and can’t readily be detected. The sun’s unusually low activity during the current minimum in the solar cycle allowed more of the hydrogen atoms from the outer heliosphere to travel unimpeded to the inner solar system, enabling IBEX to record those atoms, Fuselier says.

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