In 2009, NASA's Interstellar Boundary Explorer (IBEX) mission science
team constructed the first-ever all-sky map of the interactions
occurring at the edge of the solar system, where the sun's influence
diminishes and interacts with the interstellar medium. A 2013 paper
provides a new explanation for a giant ribbon of energetic neutral atoms
– shown here in light green and blue -- streaming in from that
boundary. Credit: NASA/Goddard Space Flight Center Scientific
Visualization Studio. › View larger
The vast edges of our solar system – the boundary at the edge of our
heliosphere where material streaming out from the sun interacts with the
galactic material – is essentially invisible. It emits no light and no
conventional telescope can see it. However, particles from inside the
solar system bounce off this boundary and neutral atoms from that
collision stream inward. Those particles can be observed by instruments
on NASA’s Interstellar Boundary Explorer (IBEX). Since those atoms act
as fingerprints 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.
Scientists did not know what processes at the edge of the solar system
could cause this mysterious increase in neutral atoms, or why any part
of the boundary should be different from any other. In the years since,
scientists have devised models and theories to try to explain the
ribbon and now, building on earlier interpretations scientists have
added a new hypothesis to help solve this puzzle.
In a paper published in the Astrophysical Journal, researchers propose a
“retention theory” that for the first time explains the key observation
of the unexplained ribbon’s width. The paper appeared online on Feb. 4,
2013.
Indeed, since the discovery of the ribbon, over a dozen competing
theories seeking to explain the phenomenon have been put forth. The new
theory builds on one that was first published along with the discovery
of the ribbon in 2009 and then quantitatively simulated
in 2010. 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 their electrons get stripped away they become
charged ions and begin to gyrate rapidly around magnetic field lines.
This process frequently aims ions back toward the sun. So those ions
that pick up electrons at the right time might explain the extra boost
of neutral atoms that create the ribbon. The problems were that physical
processes might break down the distribution needed for it to work and
that models based on this process showed a ribbon narrower than IBEX
observed.
The new theory adds a key process: That rapid rotation creates 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 and produce the broader ribbon seen.
"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."
Models done with these waves taken into account agree with the available
observations, and the mathematical modeling results look remarkably
like what the ribbon actually looks like, says Schwadron.
“This is a perfect example of the scientific process," says David
McComas, who is the other author on the paper and the principal
investigator for the IBEX mission at the Southwest Research Institute in
San Antonio, Texas. "We observe something completely new and unexpected
with IBEX, develop various hypotheses to explain the observations, and
then develop mathematical models to try to validate the hypotheses.”
Although the retention theory may check all the boxes, the IBEX team is
still far from claiming that the ribbon is fully explained. A major test
for the retention theory will be watching how the ribbon changes in
step with observed changes in the solar wind.
"What we are learning with IBEX is that the interaction between the
sun's magnetic fields and the galactic magnetic field is much more
complicated than we previously thought," says Eric Christian, the
mission scientist for IBEX at NASA's Goddard Space Flight Center in
Greenbelt, Md. "By modifying an earlier model, this paper provides the
best explanation so far for the ribbon IBEX is seeing."
If the theory is correct, points out Schwadron, it will help us
understand more about 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,” he says.
IBEX is the latest in NASA’s series of low-cost, rapidly developed Small
Explorer space missions. Southwest Research Institute in San Antonio,
TX leads the IBEX mission with teams of national and international
partners. NASA’s Goddard Space Flight Center in Greenbelt, Md. manages
the Explorers Program for the Heliophysics Division of NASA’s Science
Mission Directorate in Washington, D.C.
For more information about the IBEX mission, go to: › http://www.nasa.gov/ibex
NASA's Goddard Space Flight Center, Greenbelt, MD
