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Sept.
23, 2008: In a briefing today at NASA headquarters,
solar physicists announced that the solar wind is losing power.
"The
average pressure of the solar wind has dropped more than 20%
since the mid-1990s," says Dave McComas of the Southwest
Research Institute in San Antonio, Texas. "This is the
weakest it's been since we began monitoring solar wind almost
50 years ago."
McComas
is principal investigator for the SWOOPS solar wind sensor
onboard the Ulysses spacecraft, which measured the decrease.
Ulysses, launched in 1990, circles the sun in a unique orbit
that carries it over both the sun's poles and equator, giving
Ulysses a global view of solar wind activity:
Above:
Global measurements of solar wind pressure by Ulysses. Green
curves trace the solar wind in 1992-1998, while blue curves
denote lower pressure winds in 2004-2008. [Larger
image]
Curiously,
the speed of the million mph solar wind hasn't decreased
much—only 3%. The change in pressure comes mainly from reductions
in temperature and density. The solar wind is 13% cooler and
20% less dense.
"What
we're seeing is a long term trend, a steady decrease in pressure
that began sometime in the mid-1990s," explains Arik
Posner, NASA's Ulysses Program Scientist in Washington DC.
How
unusual is this event?
"It's
hard to say. We've only been monitoring solar wind since the
early years of the Space Age—from the early 60s to the present,"
says Posner. "Over that period of time, it's unique.
How the event stands out over centuries or millennia, however,
is anybody's guess. We don't have data going back that far."
Flagging
solar wind has repercussions across the entire solar system—beginning
with the heliosphere.
The
heliosphere is a bubble of magnetism springing from the sun
and inflated to colossal proportions by the solar wind. Every
planet from Mercury to Pluto and beyond is inside it. The
heliosphere is our solar system's first line of defense against
galactic cosmic rays. High-energy particles from black holes
and supernovas try to enter the solar system, but most are
deflected by the heliosphere's magnetic fields.
Right:
The heliosphere. Click
to view a larger image showing the rest of the bubble.
"The
solar wind isn't inflating the heliosphere as much as it used
to," says McComas. "That means less shielding against
cosmic rays."
In
addition to weakened solar wind, "Ulysses also finds
that the sun's underlying magnetic field has weakened by more
than 30% since the mid-1990s," says Posner. "This
reduces natural shielding even more."
Unpublished
Ulysses cosmic ray data show that, indeed, high energy (GeV)
electrons, a minor but telltale component of cosmic rays around
Earth, have jumped in number by about 20%.
These
extra particles pose no threat to people on Earth's surface.
Our thick atmosphere and planetary magnetic field provide
additional layers of protection that keep us safe.
But
any extra cosmic rays can have consequences. If the trend
continues, astronauts on the Moon or en route to Mars would
get a higher dose of space radiation. Robotic space probes
and satellites in high Earth orbit face an increased risk
of instrument malfunctions and reboots due to cosmic ray strikes.
Also, there are controversial studies linking cosmic ray fluxes
to cloudiness and climate change on Earth. That link may be
tested in the years ahead.
Above:
The temperature and density of electrons in the solar wind
have dropped since the mid-1990s, according to measurements
made by the Ulysses spacecraft. [Larger
image]
Some
of most dramatic effects of the phenomenon may be felt by
NASA's two Voyager spacecraft. After traveling outward for
30+ years, the two probes are now at the edge of the heliosphere.
With the heliosphere shrinking, the Voyagers may soon find
themselves on the outside looking in, thrust into interstellar
space long before anyone expected. No spacecraft has ever
been outside the heliosphere before and no one knows what
the Voyagers may find there.
NASA
is about to launch a new spacecraft named IBEX (short for
Interstellar Boundary Explorer) that can monitor the dimensions
of the heliosphere without actually traveling to the edge
of the solar system. IBEX may actually be able to "see"
the heliosphere shrinking and anticipate the Voyager's exit.
Moreover, IBEX will reveal how our solar system's cosmic ray
shield reacts to changes in solar wind.
"The
potential for discovery," says McComas, "is breathtaking."
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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