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NASA's Interstellar Boundary Explorer (IBEX) spacecraft recently detected a mysterious ribbon of particles at the edge of the solar system. Scientists now say it may have been formed by atoms reflected back into the solar system by the Milky Way's magnetic field.

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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.

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.

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.

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.

In addition, we are not only seeing ENAs in an unexpected pattern, we are seeing many more ENAs from the ribbon as compared to the rest of the sky. The pattern of ENAs coming from other parts of the sky is essentially "dominated" by the ENA ribbon. There are so many more ribbon ENAs that the ribbon is the main feature seen in the first set of IBEX maps, even though ENAs from other parts of the sky are in that data, too.

In the time period for the second set of maps, IBEX saw lower numbers of ENAs coming various parts of the sky, including the region that is producing the ribbon. The scientists are not yet sure why, but it may have something to do with the lower amount of solar wind that has been flowing outward for the past several years.

Next, at higher energies in the first maps, we saw a bright "knot" of ENAs in the upper left (northern) portion of the ribbon. Below, the map cutout at the top shows the knot region from the first set of maps, and the middle one shows the same region in the second set of maps. Red, orange, and yellow colors indicate regions emitting higher numbers of ENAs; green and blue colors indicate lower numbers of ENAs.

Not only did IBEX see the ribbon, but we have seen it evolve over time. This is very exciting, and the scientists are very surprised! The team thought that we might detect small changes in the numbers of ENAs gradually over many years, but not over the course of only 6 months. "These observations show that the interaction of the Sun with the interstellar medium in our neighborhood of the Milky Way galaxy is far more dynamic and variable than anyone envisioned," says Dave McComas. 041b061a72