Craft offers new views of Earth's auroras.

Like a shimmering curtain of light, bands of yellow-green and red dance across the sky, creating an otherworldly glow. More than just a riveting vision, Earth's auroras-the northern and southern lights-illustrate how energy from the sun travels to our planet.

Now, the recently launched POLAR satellite has taken the most detailed images of auroras ever made from space. In addition to visible-light pictures, the images include the first ultraviolet portrait of an aurora glimpsed simultaneously over the sunlit and nightside portions of the atmosphere and the first X-ray image of an entire aurora.

Researchers unveiled these pictures last week at a meeting of the American Geophysical Union in Baltimore.

The images shed light on the interaction between Earth's magnetic field and the solar wind, the breeze of charged particles emanating from the sun's outer atmosphere. As the wind approaches Earth, the planet's magnetic field steers the charged particles toward the North and South Poles. When the particles crash into the atmosphere high above the poles, they collide with local atoms, generating the eerie auroral glow.

In visible light, auroras reveal themselves only over the nightside part of Earth. On the dayside, the glare of sunlight overwhelms their faint glow.

POLAR, however, can view the dayside and nightside of an aurora simultaneously in both the ultraviolet and X rays.

The ultraviolet images demonstrate that the dayside and nightside of an aurora can vary from one another in both intensity and shape. "Each has a mind of its own," says George E. Parks, principal investigator of POLAR's ultraviolet imaging study at the University of Washington in Seattle. The differences, he notes, may reflect the fact that the magnetic field on the nightside has a cometlike tail that extends deep into space.

The X-ray images selectively track the motion of high-speed electrons in the solar wind, notes David Chenette of the Lockheed Martin Advanced Technology Center in Palo Alto, Calif., principal investigator of POLAR's ionospheric X-ray imaging experiment. "The X rays provide us with a very solid measure of the power that is [dumped] into the upper atmosphere from these electrons."

By examining Earth's auroras, POLAR tracks disturbances in the solar wind. The craft is part of a fleet of satellites, including the solar observatory SOHO (SN: 5/4/96, p. 277), that collectively studies the interaction between the sun and Earth. The goal of this fleet, explains Mario Acuna of NASA's Goddard Space Flight Center in Greenbelt, Md., "is to be able to predict when and where disturbances [originating from the sun] might occur in [Earth's] magnetosphere and ionosphere and how severe they might be."