New theory resolves mystery of anomalous cosmic rays (Forwarded)

Southwest Research Institute
PO Drawer 28510
San Antonio, TX 78228-0510

For more information, contact:

Maria Martinez, Southwest Research Institute
(210) 522-3305

Kira Edler, Boston University
(617) 358-1240

Harvey Leifert, American Geophysical Union
(202) 777-7507

February 17, 2006

New theory resolves mystery of anomalous cosmic rays

San Antonio -- When Voyager 1 finally crossed the "termination shock" at
the edge of interstellar space in December 2004, space physicists
anticipated the long-sought discovery of the source of anomalous cosmic
rays. These cosmic rays, among the most energetic particle radiation in
the solar system, are thought to be produced at the termination shock --
the boundary at the edge of the solar system where the
million-mile-per-hour solar wind abruptly slows. A mystery unfolded
instead when Voyager data showed 20 years of predictions to be wrong.

A new theory published in the February 17 issue of the Geophysical
Research Letters by Dr. David McComas of Southwest Research Institute and
Dr. Nathan Schwadron of Boston University explains why the energization of
anomalous cosmic rays is almost entirely absent where Voyager passed
through the blunt nose of the termination shock. While the shape of the
shock was formerly thought to be unimportant, the new theory explains how
this shape is the major factor in particle energization.

McComas and Schwadron say that understanding the role of the termination
shock's shape in the energization of anomalous cosmic rays may be a
stepping stone to understanding the influence of shock shapes for
energization of particle radiation throughout the cosmos. Shocks energize
many forms of this dangerous particle radiation, which pose significant
hazards to astronauts on space missions, such as future manned missions to
the Moon and Mars. "Models showed we should see the source energy spectrum
of anomalous cosmic rays at the termination shock," says McComas, senior
executive director of the SwRI Space Science and Engineering Division. "We
were pretty sure we knew what we'd see, but when we got there it wasn't
what we expected and it clearly was not the source of the anomalous cosmic
rays."

Researchers were uncertain where the termination shock would even be
found, but they knew there would be a jump in magnetic fields, a
deceleration of plasma and other signs. "It's like walking across a field
when you don't know where the edge of the property is," says McComas. "You
know you're at the boundary when you finally see the fence."

The shape of the termination shock wasn't thought to be important, so most
researchers treated it as being circular, with the magnetic field from the
solar wind spiraling out and piercing through it at a single point.
McComas and Schwadron showed that acceleration of anomalous cosmic rays
can be easily explained by including a more realistic termination shock
shape. "In fact, the termination shock couldn't be circular because the
solar system is moving through the galaxy, which would create more of a
flattened egg shape," says Schwadron. "A flattening of the nose of the
termination shock leads to a time dependant acceleration process."

The production of anomalous cosmic rays requires a connection to the
termination shock (the point where it's pierced by the magnetic field
line) and the ability for energetic particles to reside near that
connection for up to about a year. Using the new model, simple
calculations showed particles could remain at a connection point for about
300 days, further evidence of a valid model.

Voyager 1 didn't see the energetic anomalous cosmic rays when it crossed
the termination shock. "The 20-million-electron-volts-per-particle helium
that we saw was less than 10 percent of what was predicted. Similarly, we
saw only 5 percent of what was predicted for
4-million-electron-volts-per-particle oxygen," says McComas. "We weren't
off by 5 or 10 percent, we were off by factors of 10 and 20."

The new model shows that particles can indeed be accelerated at the
termination shock, but not at the nose where Voyager crossed it. "The
particles don't get accelerated up to the highest energies until the field
line has moved a long way out and its 'feet' have moved back along the
sides of the termination shock," says McComas. "This means the source of
the energetic anomalous cosmic rays must be on the flanks."

The Voyager 2 spacecraft is also moving out of the solar system, making
single-point measurements as it travels. It is expected to pass the
termination shock, farther back from the nose, within the next 2­3 years.
"The explanation given here provides predictions that Voyager 2 should
observe a larger jump in energetic particle fluxes and a more unfolded
anomalous cosmic ray spectrum as it crosses the termination shock," says
Schwadron.

The Interstellar Boundary Explorer (IBEX) spacecraft, scheduled to launch
in the summer of 2008, will be the first to make global images of the
interactions around the termination shock. At that time, researchers will
be able to view global interactions at the termination shock's nose,
flanks and tail. Combined with data from Voyagers 1 and 2, these
observations will enable researchers to understand the global interaction
of the solar system with the galaxy for the first time. "Even without
IBEX, this is a big step in understanding what's going on at the
termination shock," says McComas. "We really feel that our answer to this
mystery is just too simple to be wrong."

SwRI leads the IBEX science mission for NASA. The Goddard Space Flight
Center manages the Explorer Program for the Science Mission Directorate.

The paper, "An Explanation of the Voyager Paradox: Particle Acceleration
at a Blunt Termination Shock," is available in the February 17 issue of
the Geophysical Research Letters.

SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 3,000 employees
and an annual research volume of more than $435 million.

Founded in 1839, Boston University is an internationally recognized
institution of higher education and research. With more than 30,000
students, it is the fourth largest independent university in the United
States. BU contains 17 colleges and schools along with a number of
multi-disciplinary centers and institutes which are central to the
school's research and teaching mission.

The American Geophysical Union is a nonprofit scientific organization
focused on the dissemination of scientific information in the
interdisciplinary and international field of geophysics. Its membership
includes more than 41,000 scientists from 130 countries.

Editors: An image that illustrates the theorized shape of the termination
shock and the spiraling magnetic field line is available at
http://www.swri.org/press/2006/schematic.htm

For more information about the IBEX mission, visit
http://www.ibex.swri.edu


.