Since its October 2008 launch, NASA's Interstellar Boundary Explorer (IBEX) has provided images of the invisible interactions between our home in the galaxy and interstellar space. Particles emanating from this boundary produce a striking, narrow ribbon, which had yet to be explained despite more than a dozen possible theories. In a new "retention model," researchers from the University of New Hampshire and Southwest Research Institute suggest that charged particles trapped in this region create the ribbon as they escape as neutral atoms.
The Sun continually sends out a solar wind of charged particles or ions traveling in all directions at supersonic speeds. IBEX cameras measure energetic neutral atoms (ENAs) that form when charged particles become neutralized.
As solar wind ENAs leave the solar system, the majority move out in various directions, never to re-enter. However, some ENAs leave the solar system and impact other neutral atoms, becoming charges particles again. These newly formed pickup ions begin to gyrate around the local interstellar magnetic field just outside the solar system. In the regions where the magnetic field is perpendicular to their initial motion, they scatter rapidly and pile up. From those regions, some of those particles return to the solar system as secondary ENAs -- ENAs that leave the solar system and become charged and then re-neutralized, only to travel back into the solar system as ENAs a second time.
"The syrup you pour on a pancake piles up before slowly oozing out to the sides," says Dr. David McComas, IBEX principal investigator and assistant vice president of the SwRI Space Science and Engineering Division. "The secondary ENAs coming into the solar system after having been temporarily trapped in a region just outside the solar system do the same thing. As they pile up and get trapped or retained, they produce higher fluxes of ENAs from this region and form the bright ribbon seen by IBEX."
ENA energies observed in the ribbon correlate to the speed of the solar wind, which is slower (around 1 million miles per hour) at low latitudes and faster (up to 2 million miles per hour) at high latitudes.
"This was the clue that made us think the ribbon was caused by a secondary ENA source, because it so directly reflects the latitudinal structure of the solar wind," says McComas.
Simulations using a realistic solar wind structure showed remarkably good association with the IBEX data, closely reproducing the observed ribbon structure, location, and latitudinal ordering by energy. Thus far, the retention model appears best able to reproduce the IBEX observations. However, more studies are needed to confirm if variations in the solar wind affect the ribbon, as theorized.
Using information provided by this new model, future studies of the ribbon could help determine the properties of the nearby galactic magnetic field, opening a window into the physics of the nearby galactic medium. In addition, the IBEX ribbon could provide researchers with a means for measuring the strength of the interstellar magnetic field, as well as its direction.
The 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).
IBEX is the latest in NASA's series of low-cost, rapidly developed Small Explorer space missions. Southwest Research Institute in San Antonio leads the IBEX mission with a team of national and international partners. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the Explorers Program for NASA's Science Mission Directorate in Washington.
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