Sun storm squeezes Earth's ionosphere.

A spacecraft has found the first direct evidence that storms generated on the sun squeeze Earth's upper atmosphere, ejecting gases into space. A satellite called Polar found that the flow of ionized gas from Earth's poles increased dramatically just as a solar storm plowed into Earth on Sept. 24 and 25.

The storm originated on the sun as a magnetized cloud of ionized gas. A shock wave generated by the storm rammed into the magnetic shell that surrounds Earth, giving enough of a kick to gas trapped in the ionosphere, a layer of the upper atmosphere, to expel several hundred tons of gas, mainly oxygen.

Researchers already knew that hydrogen, helium, and oxygen ions from the ionosphere leak into space, but they had never correlated the flow with a solar storm. Trapped in the wake of the solar wind, most of the ejected gas eventually returns to Earth. The returning gas is accelerated and heated by the same processes that create Earth's radiation belts and spectacular auroras and helps supply the raw material for these displays.

Thomas E. Moore of NASA's Goddard Space Flight Center in Greenbelt, Md., reported the findings last month at a meeting of the American Geophysical Union in San Francisco.

Before Polar, astronomers had found it difficult to detect charged particles migrating from the ionosphere. The electrical charge that naturally builds up on the surface of spacecraft, due to ionizing radiation from the sun, interfered with observations. Polar's detectors use a plume of xenon ions and electrons to dispose of any charge that builds up, enabling the craft to track the flow of ions from the upper atmosphere.