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March
20, 2008: What are the signs of spring? They are
as familiar as a blooming Daffodil, a songbird at dawn, a
surprising shaft of warmth from the afternoon sun.
And,
oh yes, don't forget the aurora borealis.
Spring
is aurora season. For reasons not fully understood by scientists,
the weeks around the vernal equinox are prone to Northern
Lights. Canadians walking their dogs after dinner, Scandinavians
popping out to the sauna, Alaskan Huskies on the Iditarod
trail—all they have to do is look up and behold, green curtains
of light dancing across the night sky. Spring has arrived!
This
is a bit of a puzzle. Auroras are caused by solar activity,
but the Sun doesn't know what season it is on Earth. Yet it
seemed to know on March 1st when these lights erupted over
Tromso, Norway:
"It
was a very powerful outburst of Northern Lights," says
photographer Bjorn Jorgensen. "The ground actually turned
green!"
Such
outbursts are called auroral substorms and they have long
puzzled space physicists. Often sighted in springtime, "substorms
erupt with little warning and sometimes shocking intensity,"
says UCLA space physicist Vassilis Angelopoulos. "They're
a big mystery."
Angelopoulos
is the Principal Investigator of NASA's THEMIS mission--a
fleet of five spacecraft launched in Feb. 2007 to study the
substorm phenomenon. NASA's Polar spacecraft, which can detect
auroras in broad daylight using special UV filters, has also
joined the effort.
It
is a puzzle worthy of many spacecraft. Underlying each display
of pretty lights is a potent geomagnetic storm. THEMIS observed
one recently with a total energy of five hundred thousand
billion (5 x 1014) Joules. "That's approximately
equivalent to the energy of a magnitude 5 earthquake," says
Angelopoulos. Possible side-effects of
such storms range from satellite malfunctions to home power
outages; telecommunications, air traffic and GPS systems are
all vulnerable. "In a society that relies increasingly
on space technology, understanding substorms is vital."
THEMIS
may have found the substorm power supply--and a springtime
connection:
 "The
satellites have detected magnetic 'ropes' connecting Earth's
upper atmosphere directly to the Sun," says Dave Sibeck,
project scientist for the mission at the Goddard Space Flight
Center. "We believe that solar wind particles flow in
along these ropes, providing energy for geomagnetic storms
and auroras."
Right:
A magnetic map of a magnetospheric "rope" observed
in cross-section by the THEMIS satellites on May 20, 2007.
[Larger image]
It
turns out that rope-like magnetic connections between Sun
and Earth are favored in springtime. It's a matter of geometry:
As Earth goes around in its orbit, Earth's tilted magnetic
poles make different angles with respect to the Sun, tipping
back and forth with a one-year cadence. Around the time of
the equinox, Earth's magnetic field is best oriented for "connecting-up"
with the Sun.
But
wait, there are two equinoxes, spring and fall, with similar
magnetic geometry. Indeed, studies show that fall is aurora
season, too. Geomagnetic disturbances are almost twice as
likely in spring and fall vs. winter and summer, according
to 75 years of historical records analyzed by solar physicist
David Hathaway of the Marshall Space Flight Center: diagram.
So,
'tis the season for auroras--and lots of data for THEMIS.
Says Sibeck, "we welcome the spring!"
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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