Marks Aurora Season as
Northern Skies Glow with Light
This photograph of an aurora was taken in Wisconsin.
Jeffrey R. Hapeman of Lac du Flambeau, Wisconsin / NASA/JPL
What are the signs of spring? They are as familiar as a blooming
daffodil, a songbird at dawn, a surprising
of warmth from the afternoon sun. And, oh yes, don’t
forget the aurora borealis. Spring is aurora season. For reasons
fully understood by scientists, the weeks around the vernal
equinox are prone to Northern Lights. Canadians walking their
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.
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. So how could one season yield more auroras
“There’s a great deal we don’t understand
about auroras,” says UCLA space physicist Vassilis Angelopoulos.
For instance, “Auroras sometimes erupt with little warning
and surprising intensity. We call these events ‘sub-storms,’
and they are a big mystery.” What triggers the eruptions?
Where is sub-storm energy stored? (It has to gather somewhere
waiting to power the outburst.)
And, of course, why springtime?
To answer these
questions and others, NASA has deployed a fleet of five spacecraft
named THEMIS (short for “Time History of Events and Macroscale
Interactions during Substorms”) specially instrumented to
study auroras. Angelopoulos is the mission’s principal investigator.
This photograph of an aurora was taken in Alaska.
by Jan Curtis of the Geophysical Institute at the University
of Alaska / NASA/JPL
Auroras are much more than just pretty lights in
the sky. Underlying each display is a potent geomagnetic storm
with possible side-effects ranging from satellite malfunctions
in orbit to power outages on terra firma. Telecommunications,
air traffic, power grids and GPS systems are all vulnerable.
In a society that relies increasingly on space technology, understanding
these storms is vital.
Launched in February 2007, THEMIS has already
observed one geomagnetic storm with a total energy of five
hundred thousand billion (5
x 10^14) Joules. “That’s approximately equivalent to the
energy of a magnitude 5.5 earthquake,” says Angelopoulos. “This
storm moved twice as fast as anyone thought possible,” crossing
an entire polar time zone in 60 seconds flat!
THEMIS may have found the storm’s power
"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." Sibeck likens them to ropes because the magnetic
fields in question are organized much like the twisted hemp of
a mariner’s rope. Solar wind particles flow along the ropes
in whirligig trajectories leading from the sun to Earth.
Which brings us back to spring.
It turns out that magnetic connections between
the sun and Earth are favored in springtime. It’s a matter of geometry: As
Earth goes around in its orbit, Earth’s magnetic poles
wobble back and forth. (The poles don’t really wobble,
but the combination of Earth’s 23-degree polar tilt plus
orbital motion makes the poles seem wobble from the solar point
of view.) Around the time of the equinox, Earth’s magnetic
field is best oriented for “connecting-up” with the
sun, opening the door for solar wind energy to flow in and spark
But wait, there are two equinoxes, spring and fall, with similar
magnetic geometry. Indeed, autumn is aurora season, too. Geomagnetic
disturbances are almost twice as likely in spring-fall versus
winter-summer, according to historical records.
THEMIS is just getting started. The five spacecraft
are on a two-year mission to explore Earth’s magnetic field and
they are only now settling into their optimum science orbits. “With
five satellites, we can map the complex ebb and flow of energy
during geomagnetic storms better than any single satellite ever
could,” points out Angelopolous. “There’s no
telling what we might learn.”
One thing is certain, though. ‘Tis the season
for auroras — and lots of data for THEMIS. Says Sibeck, “We
welcome the spring!”