A Giant Breach in Earth's Magnetic Field
- From: baalke@xxxxxxxxxxxxx
- Date: Wed, 17 Dec 2008 10:06:27 -0800 (PST)
http://science.nasa.gov/headlines/y2008/16dec_giantbreach.htm
A Giant Breach in Earth's Magnetic Field
NASA Science News
Dec. 16, 2008: NASA's five THEMIS spacecraft have discovered a breach
in
Earth's magnetic field ten times larger than anything previously
thought
to exist. Solar wind can flow in through the opening to "load up" the
magnetosphere for powerful geomagnetic storms. But the breach itself
is
not the biggest surprise. Researchers are even more amazed at the
strange and unexpected way it forms, overturning long-held ideas of
space physics.
"At first I didn't believe it," says THEMIS project scientist David
Sibeck of the Goddard Space Flight Center. "This finding fundamentally
alters our understanding of the solar wind-magnetosphere interaction."
The magnetosphere is a bubble of
magnetism that surrounds Earth and protects us from solar wind.
Exploring the bubble is a key goal of the THEMIS mission, launched in
February 2007. The big discovery came on June 3, 2007, when the five
probes serendipitously flew through the breach just as it was opening.
Onboard sensors recorded a torrent of solar wind particles streaming
into the magnetosphere, signaling an event of unexpected size and
importance.
"The opening was hugeâfour times wider than Earth itself," says Wenhui
Li, a space physicist at the University of New Hampshire who has been
analyzing the data. Li's colleague Jimmy Raeder, also of New
Hampshire,
says "1027 particles per second were flowing into the
magnetosphere - that's a 1 followed by 27 zeros. This kind of influx
is an
order of magnitude greater than what we thought was possible."
The event began with little warning when a gentle gust of solar wind
delivered a bundle of magnetic fields from the Sun to Earth. Like an
octopus wrapping its tentacles around a big clam, solar magnetic
fields
draped themselves around the magnetosphere and cracked it open. The
cracking was accomplished by means of a process called "magnetic
reconnection." High above Earth's poles, solar and terrestrial
magnetic
fields linked up (reconnected) to form conduits for solar wind.
Conduits
over the Arctic and Antarctic quickly expanded; within minutes they
overlapped over Earth's equator to create the biggest magnetic breach
ever recorded by Earth-orbiting spacecraft.
The size of the breach took researchers by surprise. "We've seen
things
like this before," says Raeder, "but never on such a large scale. The
entire day-side of the magnetosphere was open to the solar wind."
The circumstances were even more surprising. Space physicists have
long
believed that holes in Earth's magnetosphere open only in response to
solar magnetic fields that point south. The great breach of June 2007,
however, opened in response to a solar magnetic field that pointed
north.
"To the lay person, this may sound like a quibble, but to a space
physicist, it is almost seismic," says Sibeck. "When I tell my
colleagues, most react with skepticism, as if I'm trying to convince
them that the sun rises in the west."
Here is why they can't believe their ears: The solar wind presses
against Earth's magnetosphere almost directly above the equator where
our planet's magnetic field points north. Suppose a bundle of solar
magnetism comes along, and it points north, too. The two fields should
reinforce one another, strengthening Earth's magnetic defenses and
slamming the door shut on the solar wind. In the language of space
physics, a north-pointing solar magnetic field is called a "northern
IMF" and it is synonymous with shields up!
"So, you can imagine our surprise when a northern IMF came along and
shields went down instead," says Sibeck. "This completely overturns
our
understanding of things."
Northern IMF events don't actually trigger geomagnetic storms, notes
Raeder, but they do set the stage for storms by loading the
magnetosphere with plasma. A loaded magnetosphere is primed for
auroras,
power outages, and other disturbances that can result when, say, a CME
(coronal mass ejection) hits.
The years ahead could be especially lively. Raeder explains: "We're
entering Solar Cycle 24. For reasons not fully understood, CMEs in
even-numbered solar cycles (like 24) tend to hit Earth with a leading
edge that is magnetized north. Such a CME should open a breach and
load
the magnetosphere with plasma just before the storm gets underway.
It's
the perfect sequence for a really big event."
Sibeck agrees. "This could result in stronger geomagnetic storms than
we
have seen in many years."
A video version of this story may be found here
<http://www.nasa.gov/mpg/297403main_THEMIS_svsLG.mpg>. For more
information about the THEMIS mission, visit http://nasa.gov/themis
.
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