Puzzling X-ray flashes in the upper atmosphere
During a routine survey of the atmosphere some 30km up, scientific equipment recorded a series of extremely high-energy X-ray flashes. The origin of these bursts is a mystery to scientists. It was the first of three balloons sent into the skies over Kiruna, Sweden, by scientists last year. Various instruments were on board, including two X-ray detectors. The 48-hour flight appeared uneventful - that is, until the researchers began analyzing the data in the lab.
What the three graduate students - Kirsten Lorentzen from the University of Washington and Robin Millan and Jason Foat from the University of California at Berkeley - discovered had scientists desperate for an explanation: an intense stream of X-rays, split up seven eruptions in quick succession, lasting a total of half an hour. The source of the flares was undoubtedly the Earth's upper atmosphere, not space.
"The source is just not known yet," says Lorentzen, who co-presented several posters about these mysteries with Millan at the American Geophysical Union fall meeting. He explains that high-energy X-rays occur in the magnetosphere, the magnetic field that surrounds the earth. "However, they don't normally enter the Earth's atmosphere and certainly not in large outbursts like this one."
Lorentzen, Millan and Foat made their discovery while taking part in an international study organized by the Université Paul Sabatier in Toulouse. In the course of this, the northern lights were studied simultaneously with stratospheric balloons and with scientific satellites.
Northern Lights are caused when electrons collide with atmospheric particles at an altitude of more than 100 kilometers. The electrons come from nearer space and move along the earth's magnetic field lines. In addition to the Northern Lights, the electrons also produce a type of radiation known as reflex radiation. This radiation cannot penetrate the thickest layers of the Earth's lower atmosphere, but it can be measured from balloons at an altitude of about 30 km.
Last year the balloons carried various types of X-ray measuring equipment, including an X-ray camera developed by Lorentzen and a germanium X-ray spectrometer. The spectrometer measured the energy of the X-ray bursts while the camera captured images of the mysterious event.
What's new about this discovery, says Lorentzen, is that the X-rays were taken during the day and that there was no activity like the Northern Lights at higher levels. Although high-energy X-rays have been observed before in astrophysics, she explains, this is the first time X-rays of such high energy -- on the order of mega-electronvolts -- have been detected originating near Earth.
According to Lorentzen, it is known from satellite observations that high-energy electrons are trapped in the Van Allen radiation belt surrounding the Earth. However, no one knows how they could enter the planet's atmosphere to create these types of bursts of energy. "This is a scientific puzzle and an extremely difficult problem," she says. "We don't yet understand the mechanism that causes this event."
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