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.
One of the THEMIS probes exploring the space around
Earth, an artist's concept.
"The opening was huge—four 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.
Above: A computer model of solar wind flowing around
Earth's magnetic field on June 3, 2007.
Background colors represent solar wind density; red
is high density, blue is low. Solid black lines trace
the outer boundaries of Earth's magnetic field. Note
the layer of relatively dense material beneath the tips
of the white arrows; that is solar wind entering Earth's
magnetic field through the breach. Credit: Jimmy Raeder/UNH.
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."
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