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January
17, 2007
THEMIS
WILL JUDGE WHAT CAUSES HIGHLY
DYNAMIC AURORA
On a clear night over the far northern
areas of the world,
you may witness a hauntingly beautiful light display in the sky that
can
disrupt your satellite TV and leave you in the dark.
The eerie glow of the northern lights seems exquisite and quite
harmless. Most
times, it is harmless. The display, resembling a slow-moving ribbon
silently
undulating in the sky, is called the aurora. It is also visible in far
southern
regions around the South Pole.
Occasionally, however, the aurora
becomes much more
dynamic. The single auroral ribbon may split into several ribbons or
even break
into clusters that race north and south. This dynamic light show in the
polar
skies is associated with what scientists call a magnetospheric
substorm.
Substorms are very closely related to full-blown space storms that can
disable
spacecraft, radio communication, GPS navigation, and power systems
while
supplying killer electrons to the radiation belts surrounding Earth.
The
purpose of NASA's Time History of Events and Macroscale Interactions
during
Substorms (THEMIS) mission is to understand the physical instability
(trigger
mechanism) for magnetospheric substorms.
A clash of forces we can’t see with the human eye causes the
beauty and
destruction of space storms, though the aurora provides a dramatic
symptom.
Earth's molten iron core generates an invisible magnetic field that
surrounds our
planet. This magnetic field and the electrically charged matter under
its
control compose the Earth’s magnetosphere.
The sun constantly blows an invisible stream of electrically charged
gas,
called the solar wind, into space. The solar wind flows at very high
speed past
the Earth and its magnetosphere. In order to visualize what happens
when the
solar wind buffets the Earth’s magnetosphere, imagine a
windsock in a gale
force wind. The Earth's magnetosphere captures and stores small
fractions of
the colliding solar wind energy and particles on magnetic field lines
that
stretch like rubber bands.
During substorms, the solar wind overloads the magnetosphere with too
much
energy and the stretched magnetic field lines snap back like an
enormous
slingshot, energizing and flinging electrically charged particles
towards
Earth. Electrons, the particles that carry electric currents in
everything from
TVs to cell phones, stream down invisible lines of magnetic force into
the
upper atmosphere over the polar regions. This stream of electrons hits
atoms
and molecules in the upper atmosphere, energizing them and causing them
to glow
with the light we know as the aurora.
The same electrons sometimes charge spacecraft surfaces, resulting in
unexpected and unwanted electrical discharges. And those electrons that
enter
the radiation belts can ultimately find their energies boosted to
levels
millions of times more energetic than the photons that comprise the
light we
can see. Electrons with these energies can damage sensitive electronics
on
spacecraft and rip through molecules in living cells, potentially
causing
cancer in unshielded astronauts. Rapidly varying magnetic fields
associated
with magnetospheric substorms also induce electric currents in power
lines that
can cause blackouts by overloading equipment or causing short circuits.
Although the consequences of substorms are well-known, it is not clear
exactly
what finally snaps in the overloaded magnetosphere to trigger a
substorm.
Understanding what happens during substorms is important. "The worst
space
storms, the ones that knock-out spacecraft and endanger astronauts,
could be
just a series of substorms, one after the other," said David Sibeck of
NASA's Goddard Space Flight Center in Greenbelt, Md., project
scientist for the
THEMIS mission. "Substorms could be the building block of severe space
storms."
Just like meteorologists who study tornadoes to understand the most
severe
thunderstorms, space physicists study substorms for insight into the
most
severe space storms. “Substorm processes are fundamental to
our understanding
of space weather and how it affects satellites and humans in the
magnetosphere,” said Vassilis Angelopoulos, THEMIS principal
investigator at
the University of California's Berkeley Space Sciences Laboratory, in
Berkeley,
Calif. Scientists propose two possible triggers for substorms, but
until now,
there has been no way to distinguish between the two models.
Discerning between the two proposed substorm trigger mechanisms is
difficult
because the magnetosphere is so large. Over Earth's night (solar wind
down-stream) side, the solar wind stretches the magnetosphere far past
the
moon's orbit, to form the geomagnetic tail. Substorms start from a
small region
in space inside the geomagnetic tail, but within minutes cover a vast
region of
the magnetosphere. However, the two proposed trigger mechanisms predict
substorm onset in distinctly different locations within the geomagnetic
tail,
so the key to solving the mystery
lies in identifying the substorm point of origin.
Previous single-spacecraft studies of the Earth’s
magnetosphere have been
unable to pinpoint where and when substorms begin, leading to extensive
scientific debate on the topic. However, NASA's THEMIS mission will
solve this
mystery with coordinated measurements from a fleet of five identical
satellites, strategically placed in key positions in the magnetosphere,
in
order to isolate the point of substorm origin. The mission, named for
Themis,
the blindfolded Greek Goddess of Order and Justice, will resolve this
debate
like a fair, impartial judge.
THEMIS is scheduled for launch in February. When the five probes align
over the
North American continent, scientists will collect coordinated
measurements
down-stream of Earth, along the sun-Earth line, allowing the first
comprehensive look at the onset of substorms and how they trigger
auroral
eruptions. Over the mission's two-year lifetime, the probes should be
able to
observe some 30 substorms.
Down-stream alignments have been carefully planned to occur over North America once every four
days. For about 15 hours
surrounding the alignments, 20 ground stations in Canada
and Alaska
with automated all-sky cameras will document the aurora from Earth. The
combined spacecraft and ground observations will give scientists the
first
comprehensive look at the phenomena from Earth's upper atmosphere to
far into
space, enabling researchers to pinpoint where and when substorm
initiation
begins.
For
more information and images, visit:
http://www.nasa.gov/mission_pages/themis/news/Themis_intro.html
For more
information
about THEMIS, visit:
http://www.nasa.gov/mission_pages/themis/main/index.html
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Contact:
Cynthia O'Carroll
NASA Goddard Space Flight Center
301-286-4647
Cynthia.M.OCarroll@nasa.gov
This text is
derived from:
http://www.nasa.gov/mission_pages/themis/news/Themis_intro.html
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