December 11, 2007

Our Solar System is Squashed

FRANCISCO--New observations from the edge of our solar system show what
scientists have suspected for several years: The solar system is squashed.

The edge of
the solar system is roughly where the solar wind runs up against thin gas found
between the stars in the rest of the Milky Way. This "wind," actually a thin
gas of electrically charged particles, blows outward in all directions from the
sun at between 1 million and 2 million mph, forming a bubble called the
heliosphere with boundaries far beyond the orbit of Pluto.

boundary between the heliosphere and interstellar space is an abrupt shock
wave, called the solar wind termination shock. In December 2004, NASA's Voyager 1
spacecraft crossed this boundary and hinted that the shockwave was dented, or
uneven in places.

Its sister
spacecraft, Voyager 2, was also launched in 1977 on a mission to the outer
planets. Voyager 2 crossed the shock boundary Aug. 30 this year, about 10
billion miles away from where its twin crossed. The probe confirmed that the
shock wave was squashed–it was pushed in closer to the sun by the local
interstellar magnetic field where Voyager 2 crossed compared to where Voyager 1
did, by about 1 billion miles.

"So there's
something outside pushing in on the southern hemisphere of the heliosphere,"
said Voyager mission scientist Ed Stone, that is "otherwise distorting a more
or less spherical surface."

Stone says
this "dent" in the heliosphere is evidence of how strong the
interstellar magnetic field is.

Stone spoke
during a presentation of the research here today at a meeting of the American
Geophysical Union.

Voyager 1 only crossed the shock
once, Voyager 2 had multiple crossings, because the shock wave
sloshes back and forth like surf on a beach. This allowed for many
measurements of the velocity, density and temperature of the solar wind.

The data
from Voyager
shows that the shock wave is unusual.

In a normal
shock wave, fast-moving material slows down and forms a denser, hotter region
as it encounters an obstacle. However, Voyager 2 found a much lower
temperature beyond the shock than was predicted. This probably indicates that
the energy is being transferred to cosmic ray particles that were accelerated
to high speeds at the shock, scientists say.

So the shock
barriers acts like what Stone calls a "cosmic ray accelerator"–sort of like a
cosmic ping-pong game, some of the ions are reflected back across the shock.
Over time, the ions gain a large amount of energy from the solar wind before
they rocket out into space.

are still unsure where exactly these cosmic rays originate in the shock
wave–they do know that they don't originate where either Voyager craft crossed
the shock.

Voyagers are currently in the heliosheath and will continue outward past the
last reaches of the sun's wind, "and we will be then, for the first time, in
interstellar space," Stone said, with scientists hoping to use the Voyager
craft to explore space beyond our solar system.

With NASA's
upcoming mission, the Interstellar Boundary Explorer (IBEX), Stone and his
colleagues hope more variations in the shock wave will be revealed, with
possibly some effects in the east and west directions.

will tell us what it looks like from all directions," he told