Astronomersprobing the vast northern lights that ring Jupiter's north pole have found amysterious X-ray ``hot spot'' that flares up like a beacon every 45 minutes.

Scientistssaid it could take years to explain this pulsating region, which itsdiscoverers speculate may be related to bursts of radio waves that emanate fromthe giant planet at a similar interval.

``Itcame as a complete surprise, but scientists live for that. Sometimes the thingsthat are most unexpected are the most important,'' said Christopher Russell, aUniversity of California, Los Angeles professor of geophysics. Russell was notinvolved in the research.

Thediscovery of the hot spot is one of several surprises that have emerged from aunique opportunity scientists had last year to study Jupiter and the enormousmagnetic cocoon that surrounds it.

Theoccasion was a space science first _ the first time two spacecraft had visitedJupiter, or any outer planet, at the same time.

Inearly January 2001, the Galileo orbiter that has been circling Jupiter since1995 and the Cassini probe, which swung past Jupiter on its way to Saturn,passed through Jupiter's magnetosphere _ a zone of magnetically chargedparticles trapped within its magnetic field.

Imagestaken during the same period by the Hubble space telescope and the ChandraX-Ray Observatory, both in orbit around the Earth, complemented thespacecraft's observations.

Sevenpapers that arose from that data were highlighted in the Feb. 28 issue of thejournal Nature, describing various aspects of Jupiter's magnetosphere and itsinteraction with the planet's upper atmosphere.

Thedata transmitted back showed, as scientists had predicted, that Jupiter'smagnetosphere changes shape as it is buffeted by interplanetary shock wavescreated by the solar wind _ the stream of particles thrown off by the sun.Earth's magnetosphere acts similarly.

ButJupiter's is more complex and far larger. At about 100 times as wide as theplanet, it's so large that if it were visible to the naked eye, it would appearlarger than the full moon to an observer on Earth. And its cometlike tailextends past the orbit of Saturn.

RandyGladstone, a planetary scientist at the Southwest Research Institute in SanAntonio who was lead author on the X-ray hot spot paper, said his team is stillin the early stages of trying to figure out what is causing the pulsating spot.

Hesaid the disturbance accounts for most of the X-ray emissions that are seen inJupiter's northern auroras. Scientists had thought those emissions wereproduced by sulfur and oxygen ions blasted into space by Jupiter's volcanicmoon, Io, and then energized by circulating through the Jovian magnetic field.

Now,it's not clear what is behind those emissions, Gladstone said. The evidencepoints to an origin much farther away from Jupiter, near the edge of itsmagnetosphere, where it meets the solar wind.

``Somethingis causing these emissions _ from X-rays to ultraviolet to radio (waves).There's something connecting all of these emissions to have them happen overall these wavelengths,'' Gladstone said.

JohnClarke, a professor of astronomy at Boston University, and colleagues authoredanother paper that reported the discovery of a ghostly glowing trail etched byIo into Jupiter's ionosphere, the region high in its atmosphere where aurorasform.

Histeam also found that two of Jupiter's three other large moons _ Ganymede andEuropa _ etch smaller, oval ``magnetic footprints.''

Scientistsalready knew Io, the most volcanically active body known in the solar system,was producing a similar footprint.

ButClarke said that since Europa and Ganymede do not have volcanoes but stillproduce footprints in Jupiter's ionosphere, it appears some unknown mechanismis causing all three moons to leave their mark there.

Jupiter'sfourth large moon, Callisto, may cast such a spot, too. But whether it does mayremain unanswered for years to come because no new missions to Jupiter areplanned, and the Galileo spacecraft will end with a fiery plunge into Jupiternext year.

Aproposed spacecraft that would be the first to visit Pluto, the outermostplanet, would swing by Jupiter on its decade-long journey, giving it a chanceto study the planet. But that mission's funding remains in question.