An intense wave of X-rays and gamma radiation, bursting from a huge magnetic flare on a mysterious star 20,000 light-years away, hit Earth's upper atmosphere last month and knocked sensitive detectors askew on at least seven spacecraft as the wave sped through the solar system, scientists reported Tuesday.

The radiation posed no health risks to humans or animals, but for five minutes, as it reached the night side of the Earth, it caused a powerful increase in the activity of the electrically charged ionosphere that is normally quiescent at night, and it sharply decreased the range of many radio stations.Radiation detectors aboard satellites orbiting the Earth and others exploring deep space mea-sured radiation pulses so strong that their instruments could no longer record them, ground con-trol-lers said.

Little of the tremendous radiation surge reached the Earth's surface, however, because X-rays and gamma rays can barely penetrate the ionosphere.

The wave was first detected on the night of August 27 by a Stanford University team that operates a string of very low frequency radio transmitters across North America. The group is headed by Umran Inan, a Stanford electrical engineer, and since the discovery, scientists all over the world have been analyzing the phenomenon.

"It is extremely rare for an event occurring outside the solar system to have any measurable effect on the Earth," Inan said. "It was as if night was briefly turned into day in the ionosphere."

The radiation came from a newly discovered and truly extraordinary type of star that astronomers have termed a "magnetar." These stars are more massive than the sun, but their dense superheavy matter is barely a dozen miles in diameter. Their magnetic fields, more powerful than any thing else in the known universe, is so intense that it powers a steady glow of X-rays from the star's surface, according to National Aeronautics and Space Administration astronomers at the Marshall Space Flight Center in Huntsville, Ala.

Last month's dramatic pulse of radiation originated in a sudden rippling surge of energy that created a gigantic starquake on the magnetar's surface, the astronomers said.