Energy burst from X-ray star briefly turns ionospheric ‘night into day’
BY DAVID F. SALISBURY
On the night of August 27th Earth's upper atmosphere was bathed briefly by an invisible burst of gamma- and X-ray radiation. This pulse the most powerful to strike Earth from beyond the solar system ever detected had a significant effect on Earth's upper atmosphere, report Stanford researchers. It is the first time that a significant change in Earth's environment has been traced to energy from a distant star.
"It was as if night was briefly turned into day in the ionosphere," says Umran Inan, professor of electrical engineering at Stanford and head of the research group that observed the atmospheric disturbance. The ionosphere is the portion of the atmosphere between 60 to 80 kilometers in altitude that plays an important role in radio communications. So it is quite likely that the range of high-powered radio stations, which usually reach much farther at night due to lower ionospheric activity levels, was suddenly reduced to their lower, daytime ranges during the burst's five-minute duration, he says.
The characteristics of the unusual gamma-ray burst, the nature of the unusual X-ray star that appears to be its source, and the dramatic effect that it had on the ionosphere are the subjects of a National Aeronautics and Space Agency space science update being held on Tuesday, Sept. 29, at 2 p.m. ET at NASA Headquarters in Washington, D.C.
Stanford's Very Low Frequency Research Group operates a string of stations in North America that use VLF radio transmissions to monitor changes in the ionosphere, a region that is above the range of weather balloons and below that of satellites. Normally, the researchers use the network, called the Holographic Array for Ionospheric Lightning research (HAIL), to study the effects that localized phenomena like lightning have on the ionosphere. But they were perfectly positioned to measure the impact of the extra-solar energy pulse.
During the five minutes that the gamma- and X-ray radiation impinged on Earth's upper atmosphere, the researchers found that the level of ionization or electrical activity in the ionosphere, which is normally quiescent at night, suddenly flared to daytime levels, Inan says.
The Stanford scientists saw the effect clearly in stations monitoring the ionosphere over the western United States, which were included in the area illuminated by the burst, but not over the eastern part of the country, which was in shadow. Inan's group also saw evidence of a 5.16-second pulsation that corresponds with the X-ray star's previously established rotation rate, indicating that the ionization in Earth's atmosphere varied in accordance with the pulsations of the gamma-ray burst.
In 1988, Inan and Gerald Fishman from the NASA/Marshall Space Flight Center reported observing a similar, but much smaller, ionospheric disturbance associated with a gamma-ray burst that occurred in 1983. At the time, the source of gamma-ray bursts was unknown. So the most the scientists could claim was that "this may be the first time that a transient extra-solar phenomenon has measurably affected a part of the Earth's environment." Inan's group detected another small disturbance of this type in 1996, but did not report it.
Kevin Hurley at University of California-Berkeley has used satellite information to calculate the total energy that an X-ray star, called SGR 1900+14, must have released to produce the August 27 burst. He calculates that, if the energy could be harnessed, it would be enough to power all of human civilization on Earth for a billion billion years. Of course, only a tiny fraction of this energy reached Earth.
According to space agency scientists, the radiation reaching Earth had an intensity slightly less than that of a dental X-ray. Both gamma rays and X-rays cause ionization directly, by stripping electrons from atoms and leaving them electrically charged. Nevertheless, the burst had a remarkable impact on the ionosphere.
"It is amazing that such a burst could produce ionization levels similar to those produced by all the radiation coming from the sun," Inan says. "When we put this information into our mathematical models it may provide us with important new insights about how the ionosphere behaves in response to sudden ionization changes."
The nature and intensity of the Aug. 27 gamma ray burst supports the theory that SGR 1900+14 is an exotic object called a magnetar predicted by Robert Duncan of the University of Texas-Austin and Christopher Thompson of the University of North Carolina-Chapell Hill.
A magnetar is a special kind of neutron star. A neutron star is the collapsed core that is left behind when a massive star explodes. It is extremely dense, weighing more than the sun but squeezed into a ball less than 12 miles in diameter. A magnetar differs from an ordinary neutron star by possessing a magnetic field far greater than any other object known. Its field would be so intense that a steady X-ray glow would emanate from its surface and periodic starquakes would produce bursts of gamma rays and occasional cataclysmic flares like the one observed on August 27.
"All this goes to show that the
Earth does not exist in splendid isolation," says Inan. "We now
know that the Earth's physical environment is affected not only by
our own sun but by energy originating from distant parts of our