IBEX's ribbon in the sky: scientists unravel the mystery

A ribbon in the sky? Who, besides Stevie Wonder, knew?

Until last October, no one knew. Then NASA's Interstellar Boundary Explorer (IBEX) spacecraft revealed the unexpected feature as it mapped the frontier between the solar system and interstellar space.

Now, a team of space physicists posits that the ribbon seen in the IBEX map of the sky is likely being formed by atoms that originally came from the sun as part of the solar wind. The Milky Way's magnetic field is in effect reflecting them back in a kind of cosmic bank shot.

If the explanation holds up, IBEX's ribbon represents the first crude measurement of the local portion of the galaxy's magnetic field, says Jacob Heerikhuisen, a space physicist at the University of Alabama at Huntsville, who led the team.

"That's the most exciting thing for us," he says.

Researchers are trying to understand the boundary region between the solar system and interstellar space because that boundary shields the solar system from some 90 percent of the high-energy cosmic rays bombarding it, explains Nathan Schwadron, a space physicist at Boston University and member of the team reporting the results in the Jan. 10 issue of the Astrophysical Journal Letters.

Interstellar space represents "a harsh radiation environment," he says.

Unexpected finding

The ribbon's appearance on the IBEX map was a shock to mission scientists. They had anticipated that the map would reveal a fairly uniform distribution of fast-moving, or "energetic," neutral atoms across the sky.

These atoms form when electrically charged particles – largely protons – speed away from the sun at anywhere between 600,000 to 1.8 million miles an hour.

As this "solar wind" travels, some of these protons meet up with neutral atoms wandering into the solar system from interstellar space. Solar-wind particles can steal electrons from these incoming neutrals. This converts the outbound solar-wind particles into outbound neutral atoms. Because they are electrically neutral, they are oblivious to the sun's magnetic field and continue to hurtle beyond its influence into interstellar space.

IBEX revealed a fairly uniform distribution of these energetic neutral atoms across most of the sky, although they appeared in amounts two to three times higher than expected. But it also detected a "ribbon" of these atoms, distinguished by having three times the density of the surrounding ones.

A volley of ions

Mr. Heerikhuisen's team opted to test via a modeling exercise one explanation for the ribbon – the bank shot. The group realized that for any explanation of the ribbon's formation to work, it had to account for the ribbon's orientation across the sky. It also had to explain why the range of energies the neutral atoms display is virtually the same whether the atoms are part of the ribbon or not. And it had to include physics that weren't in typical computer simulations of the interface between the solar system and the material found in interstellar space.

As the team's model tells the story, when the outbound energetic neutral atoms leave the sun's sphere of influence – the so-called heliosphere – a fraction of them lose an electron and become ionized again. Now that they again carry an electrical charge, the ions respond to the galaxy's magnetic field. In a kind of cosmic backhand volley, the Milky Way's magnetic field sends some of these back at the solar system.

Along the way, the ions can once again grab electrons, reform as neutral atoms, and continue to flit back through the heliosphere to be picked up by IBEX's detectors.

One reality check on this model: What if IBEX records the ribbon changing position with time? Mr. Schwadron, one of Heerikhuisen's team, notes that their ribbon remains stable over long periods of time. But already, a second map unveiled at the American Geophysical Union's December meeting in San Francisco suggests that the ribbon has slightly changed position in the period between the maps.

Since the model is averaging the ribbon's position over those periods, it could accommodate some minor short-term shifts in position, Schwadron says.
"It will require lots of work to separate these out," he says.

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