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 and reducing 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 the
planet.

The plasma cloud is so ferociously hot, its particles act
like radiation, occasionally disrupting satellites in mid- to
high-altitude orbits. This discovery from NASA’s Imager for
Magnetopause to Aurora Global Exploration (IMAGE) spacecraft
confirms 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 and 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
6bbrgy by throwing some of its charged particles into space,”
said Stephen Fuselier of the Lockheed-Martin Advanced
Technology Center, Palo Alto, Calif., 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., lead author
of the second paper. Approximately half the energy deposited
by space storms in the 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
the atmosphere directly and gradually erode it. Instead, the
solar wind slams into the Earth’s magnetosphere and is
diverted around the 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 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 Earth’s lower
atmosphere (lower ionosphere) to expand and increase orbit-
disrupting drag on spacecraft.

The first IMAGE result 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
instrument on IMAGE to discover electrically charged oxygen
atoms are ejected into space immediately in response to the
bursts of ionosphere-heating 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
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 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 that 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. Images and more information are available
at:
http://www.gsfc.nasa.gov/topstory/20020509imagessu.html