A High-Flying X-Ray Mystery

SAN FRANCISCO--X-rays powerful enough to penetrate an inch of aluminum pulse through Earth's upper atmosphere, geophysicists reported today at the fall meeting of the American Geophysical Union. The bursts strike some 35 kilometers above the planet's surface--the lowest altitude at which scientists have observed x-rays that powerful. Although the x-rays are probably emitted by particles spiraling inward from the magnetic bubble that envelops Earth, their exact source is a mystery.

The observations are lucky offshoots of an experiment designed to study aurora--the "northern lights." Electrons plunging toward the poles from space trigger the glow of the aurora when they collide with air particles, and they also emit x-rays as they slow down high in the atmosphere. Two teams from the University of Washington in Seattle and the University of California, Berkeley, set out to study the x-rays with instruments lofted by balloon. The outcome of the 1996 flight above Sweden, the researchers say, was totally unexpected.

For a total of 30 minutes during the 2-day flight, the instruments detected x-ray bursts at energies ranging above a million electron-volts--at least 10 times higher than usual for the upper atmosphere. What's more, the bursts were measured at times when Earth's magnetic field--which accelerates the electrons that spark auroral displays--was quiet, says University of Washington geophysicist Kirsten Lorentzen. "It's a real mystery," Lorentzen says. "We don't know of a mechanism that can boost electrons to such high energies close to Earth."

"This is impressive and exciting work," says Lawrence Zanetti, a physicist at NASA Headquarters in Washington, D.C., who studies Earth's magnetosphere. Learning the origins of the new high-energy bursts could help researchers predict when particles accelerated by Earth's magnetic field might wreak havoc on satellites. "But right now, we have no idea where they come from," Zanetti says. NASA has funded a weeks-long circumpolar balloon launch, set for June 1998, to study the events in more detail.