These images are taken from a computer animation illustrating the Earth's space storm shield in action. The solar wind is represented in Image 1 as a stream of yellow particles flowing from the Sun. The solar wind impacts the Earth's magnetic field, represented by the blue lines in Image 2. As the solar wind flows past the Earth's magnetic field, it generates enormous electric currents that heat Earth's space storm shield causing the shield to eject electrically charged oxygen atoms (oxygen ions) into space. The expelled oxygen ions are represented by the green particle streams in Image 3. The ejected oxygen ions 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 billion-degree plasma cloud around the planet, represented by the red cloud in Image 4. The red "ring of fire" around the Earth's polar regions represents the contribution of the particles to the aurora (the northern and southern lights). Photos: NASA

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 energy 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.