A mysterious phenomenon was discovered at the outer most border of our solar system when a special instrument was launched which could observe this region for the first time. This border is defined as the heliopause, where material streaming out from the sun interacts with the galactic material - the whole bubble around the sun is the heliosphere and shields the solar system against galactic radiation, but is also molded by it, similar to the Earth's magnetosphere. It was invisible until recently, since it emits no light, but neutral atoms are actually bouncing back to the core of our system after particle collisions within the boundary region. Those particles can now be observed by instruments on NASA's Interstellar Boundary Explorer (IBEX). Since the bounced back atoms act as identifier for the boundary from which they came, IBEX can map that boundary in a way never before done. In 2009, IBEX saw "something in that map that no one could explain: a vast ribbon dancing across this boundary that produced many more energetic neutral atoms than the surrounding areas."

But yesterday a new hypothesis was published in a paper in the Astrophysical Journal, in which researchers propose a "retention theory" that for the first time explains the key observation of the unexplained ribbon's width. In a truly scientific quest to solve the puzzle, models and theories have been devised since the discovery of the phenomenon, eventually piling up to a dozen competing theories.

What kind of processes at the edge of the solar system could cause the puzzling increase in neutral atoms, and why are some parts of the boundary different from others? The newest theory actually returns to the roots and builds on the hypothesis that was published first, along with the original discovery of the ribbon in 2009. This theory posited that the ribbon exists in a special location where neutral hydrogen atoms from the solar wind cross the local galactic magnetic field. Neutral atoms are not affected by magnetic fields, but when they get ionized by having their electrons stripped away they begin to gyrate rapidly around magnetic field lines, which can frequently aim ions back toward the sun, where IBEX records them. So those ions that pick up electrons at the right time might explain the extra boost of neutral atoms that create the ribbon. But what is the right time, and which elements does it affect? A quantitative computer simulation of the theory in 2010 resulted in a ribbon that was narrower than IBEX observed.

The new theory offers a solution that leads to the broader ribbon that is observed by adding a key process: Rapid rotation could create waves or vibrations in the magnetic field, and the charged ions then become physically trapped in a region by these waves, which in turn would amplify the ion density. Simulating this in new mathematical models yields a picture that comes very close to the original.

"Think of the ribbon as a harbor and the solar wind particles it contains as boats," says Nathan Schwadron, the first author on the paper and scientist at The University of New Hampshire, Durham. "The boats can be trapped in the harbor if the ocean waves outside it are powerful enough. This is the nature of the new ribbon model. The ribbon is a region where particles, originally from the solar wind, become trapped or retained due to intense waves and vibrations in the magnetic field."

While the theory looks good so far, it is still work in progress, and more modelling and testing will be done to see if there are errors, and accordingly, adjustments to be added. The IBEX instrument will continue to be useful in this process, since ongoing observations can be compared with the dynamic changes in the strength of the solar wind, which needs to fit the current model and predictions. The results are relevant to understand better how our heliosphere interacts with the rest of the universe. "The ribbon can be used to tell us how we're moving through the magnetic fields of the interstellar medium and how those magnetic fields then influence our space environment," Schwadron says.

Paper: "Spatial Retention of Ions Producing the IBEX Ribbon," by N.A. Schwadron and D.J. McComas was published Feb. 4 in the Astrophysical Journal. The IBEX team's papers on the first six models about the ribbon's origin were published in Science (2009).