Shelter from space storms: energy rebounds from Earth. (Science News This Week).

Spewing a continuous stream of ions and electrons at about 400 kilometers per second, the sun fills interplanetary space with its particles. The strength of this solar wind varies from a benign breeze to a fierce gale. Now, observations show that Earth's outer atmosphere interacts dramatically with the solar wind and shields the planet from it.

Scientists have long known that Earth's magnetic field diverts part of the solar wind and its inherent energy. The new data, generated by NASA's Imager for Magne-to-pause-to-Aurora Global Exploration (IMAGE) satellite, reveal that the outer atmosphere, too, diverts some of the dangerous energy. The finding was announced May 9 at a NASA press briefing in Washington, D.C., and is also the topic of two upcoming reports in the Journal of Geo-physical Research.

The newly recognized shield is the charged layer, known as the ionosphere, in the outer atmosphere. The ionosphere fills the interval between 300 and 1,000 km above Earth's surface. When a space storm hits this layer, it dumps up to a trillion watts of power into the atmosphere, says Stephen Fuselier of the Lockheed Martin Space Physics Lab in Palo Alto, Calif., a coauthor of the report. Low-intensity storms hit Earth weekly, while very-high-intensity storms happen once every 3 to 4 years.

The massive current from a space storm heats the ionosphere, which dissipates energy by immediately ejecting some of its own oxygen and hydrogen ions into space. A few hundred tons of gas--roughly equal to the volume of the Louisiana Superdome--are lost each storm, says Fuselier. "We knew Earth interacted [with space storms], but we had no idea it was so fast, so dramatic," says Fuselier.

Approximately half the energy from a storm is dissipated in this manner, adds coauthor Donald Mitchell of the John Hopkins Applied Physics Laboratory in Laurel, Md. Without the ionosphere's shielding effect, the lower atmosphere would heat up, he says.

"We knew quite a lot about Earth's interactions with space weather before," says Terry Onsager of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colo. However, the IMAGE satellite has enabled scientists to watch the global process unfold in its entirety, he says.

The satellite also revealed a downside to Earth's interaction with space storms, says Mitchell. Much of the charged gas ejected doesn't dissipate immediately and becomes trapped in Earth's enormous magnetic field. It follows the field lines like a train on a railroad track, accelerating to enormous speeds. The field concentrates the charged gas into a 1 billion [degrees] C, doughnut-shaped plasma cloud that encircles the planet during storms, says Mitchell.

The intense heat of the cloud emits highly charged particles, which disrupt the operation of communication and navigation satellites in space and power grids on Earth. "We pay a price for the protection," says Fuselier.