NASA’s Interstellar Boundary Explorer (IBEX) spacecraft has helped explain the striking narrow ribbon of charged particles emanating from the boundary of the solar system.
There have been a number of theories – but now, in a new ‘retention model’, researchers from the University of New Hampshire and Southwest Research are suggesting that charged particles trapped in this region create the ribbon as they escape as neutral atoms.
The sun sends out a continual solar wind of charged particles, or ions, traveling in all directions at supersonic speeds. IBEX cameras measure energetic neutral atoms (ENAs) that form when these charged particles become neutralized.
After solar wind ENAs leave the solar system, most never re-enter. However, some impact other neutral atoms, becoming charged particles once again. These newly formed pickup ions begin to gyrate around the local interstellar magnetic field just outside the solar system; and, inregions where the magnetic field is perpendicular to their initial motion, they scatter rapidly and pile up.
From those regions, some particles return to the solar system as secondary ENAs.
“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.”
Simulations showed ‘remarkably good association with the IBEX data’, says the team, closely reproducing the observed ribbon structure, location, and latitudinal ordering by energy.
And ENA energies observed in the ribbon correlate to the speed of the solar wind, which is slower at low latitudes and faster at high ones.
“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.