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EARTH'S
SPACE STORM SHIELD OFFERS PROTECTION AT A PRICE New
observations from a NASA spacecraft reveal that a layer in the Earth's outer atmosphere
acts like a heat shield by absorbing energy from space storms, which reduces their
ability to heat the lower atmosphere. However, it imposes a heavy toll for its
services by creating a billion-degree cloud of electrified gas, or plasma, that
surrounds our planet. The plasma cloud is so ferociously hot, its particles act
like radiation, occasionally disrupting satellites in mid to high orbits. This
discovery from NASA's Imager for Magnetopause to Aurora Global Exploration (IMAGE)
spacecraft confirms that the Earth actively participates in space storms. Although
past space missions gave provisional evidence for this behavior, IMAGE provides
the first global picture of the active role Earth's ionosphere plays in space
storms, which is very different from the earlier view that the solar wind itself
supplied the energetic particles responsible for these storms. The
Earth's space storm shield is a tenuous layer of the outer atmosphere (outer ionosphere)
between 180-620 miles (300-1,000 kilometers) high that includes electrically charged
atoms. "Just as a heat shield sacrifices itself by allowing its outer layers
to slough off during the fiery reentry of a spacecraft, Earth's shield absorbs
space storm energy by throwing some of its charged particles into space,"
said Stephen Fuselier of the Lockheed-Martin Advanced Technology Center, Palo
Alto, Calif., who is lead author of the first of two papers on this discovery
to be published in the Journal of Geophysical Research. "But
this protection comes with a high price, because the expelled particles gain tremendous
speed as they leave the atmosphere, become trapped by the Earth's magnetic field
and ultimately encircle the Earth, where they form a hot plasma cloud around the
planet," said Donald Mitchell of the Johns Hopkins Applied Physics Laboratory,
Laurel, Md., who is lead author of the second paper. Approximately half of the
energy deposited by space storms in our atmosphere is absorbed this way, according
to the researchers. The
solar wind, a thin, high-velocity plasma, blows constantly from the Sun at an
average speed of 250 miles per second (400 kilometers/sec). If the Earth had no
global magnetic field, or magnetosphere, the solar wind would impact our atmosphere
directly and gradually erode it away. Instead, the solar wind slams into the Earth's
magnetosphere and is diverted around our planet. Buffeting of the magnetosphere
is more intense during space storms, when explosive events on the Sun give the
solar wind an unusually high velocity or density, or a particularly potent magnetic
field configuration. Although
the magnetosphere does a good job staving off the solar wind, Earth is not home
free. Since the solar wind plasma is comprised of electrically charged particles
that are moving rapidly past the Earth's magnetic field, a multimillion amp electric
current is generated, which flows down the Earth's invisible magnetic field lines
and pumps up to a trillion watts of power into the magnetosphere, especially above
the polar regions, where the aurora (northern and southern lights) form. Without
the space storm shield, heat from these enormous electric currents would cause
our lower atmosphere (lower ionosphere) to expand and increase orbit-disrupting
drag on spacecraft. The
first result from IMAGE shows the Earth's shield in action as it absorbs a space
storm's electric current and is ejected into space. Fuselier used the Low Energy
Neutral Atom imager (LENA) instrument on IMAGE to discover that electrically charged
oxygen atoms are ejected into space immediately in response to the bursts of heating
of the ionosphere by the massive electric currents. The amount of ionosphere lost
during a typical space storm is around a few hundred tons, about equal to the
mass of the air in the Louisiana Superdome, according to the team. The
second IMAGE observation shows the price paid for the shield's protection. Because
of their electric charge, the expelled oxygen ions feel magnetic forces and are
trapped within the Earth's vast magnetosphere, where they follow magnetic field
lines like cars on a highway. Scientists know that the magnetosphere distorts
under the impact of the solar wind, like an umbrella in a windstorm. In particular,
the region of the magnetosphere facing away from the Sun is stretched into a long,
tail-like shape as the solar wind blows by. Because magnetic fields have tension,
they resist stretching and behave like rubber bands. When the stretching becomes
too great, the night-side magnetosphere snaps back towards Earth, carrying the
ejected ions from the ionosphere with it like an enormous slingshot. Mitchell
used the High Energy Neutral Atom imager (HENA) instrument on IMAGE to observe
that these ions, now accelerated to enormous velocities (about 2,500 miles per
second or 4,000 km/sec), appear immediately in the aurora and in the cloud of
hot plasma that encircles the Earth during space storms. Earth contributes material
and the solar wind supplies the energy which transforms this cool atmospheric
material into a dangerously hot plasma cloud. If it were not for the Earth's own
ionosphere supplying material, the hot plasma cloud would be very much diminished.
This
new view is helping scientists to better understand the effects of space storms,
which create moving plasma clouds that interfere with navigation using Global
Positioning System satellites. Back
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