San Francisco -- It was the first of three balloons being sent aloft by scientists into the sky over Kiruna, Sweden, last year. Aboard were several instruments, including two X-ray detectors. The 48-hour flight seemed uneventful -- that is, until researchers started examining the data in a laboratory.

What was found by three graduate students -- Kirsten Lorentzen of the University of Washington and Robin Millan and Jason Foat of the University of California at Berkeley -- has scientists scrambling for an explanation: an intense stream of X-rays, occurring in seven bursts, each separated by only a few minutes and lasting for a total of half an hour. The evidence was clear that the high energy bursts came not from outer space, but from the Earth's upper atmosphere.

"The source is simply not known," says Lorentzen, who, with Millan, is presenting posters on the mystery at the fall meeting of the American Geophysical Union here (Dec. 10 at 8:30 a.m.). There are high-energy X-rays located in the magnetosphere, the intense magnetic field that surrounds the Earth, Lorentzen says. "But they don't usually enter the Earth's atmosphere, and certainly not in big bursts like this."

Lorentzen, Millan and Foat made their discovery when they were taking part in an international campaign organized by the Universite Paul Sabatier, Toulouse, France, to study the aurora from stratospheric balloons in conjuction with scientific satellites passing overhead.

The aurora, also known as the northern lights, is a phenomenon visible more than 60 miles above the Earth in a region where electrons collide with atmospheric particles. The electrons come from near space around the Earth, and travel along the planet's magnetic field lines. In addition to creating the aurora's heavenly lights, the electrons also create a type of radiation known as bremsstrahlung, or X-rays. This radiation cannot penetrate the thickest layers of the Earth's lower atmosphere, but can be observed from balloons at a height of about 20 miles.

Last year's balloon observing mission carried several types of X-ray instruments, including an X-ray imaging camera built by Lorentzen at the UW and a germanium X-ray spectrometer built at UC Berkeley by physics professor Robert Lin and his colleagues. The spectrometer measured the energy of the X-ray bursts, and the camera took X-ray images of the mysterious event.

What is new about this discovery, says Lorentzen, is that the X-rays were recorded during the day and there was no auroral activity overhead. Although high energy X-rays have been observed in astrophysics before, she says, this is the first time that X-rays of such intense energy -- mega-electron-volts -- have been detected emerging from around the Earth.

Lorentzen notes that it is known from satellite observations that high energy electrons become trapped in the Van Allen radiation belt surrounding the Earth, but it is not known how they could penetrate the planet's atmosphere to produce the type of energy bursts recorded.

"This is a scientific mystery, a very difficult problem," she says. "We don't understand the mechanism that causes this type of event."

Foat is now at the University of California, San Diego.