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Astronomers made an unprecedented dual observation of Jupiter Thursday, using two different space telescopes to study Jupiter's upper atmosphere, particularly the planet's aurora, a colorful display of charged particles similar to the Earth's aurora borealis.

The observations were especially important because of a volcanic eruption on Jupiter's moon Io last week, and because of last summer's collision of Comet Shoemaker Levy 9 with Jupiter.

"There is an enormous amount of energy being put into the atmosphere," said Johns Hopkins University astrophysicist Paul Feldman. "Clearly, these energetic particles are controlling, to a large extent, the fate of the upper atmosphere of Jupiter. We are trying to get a better idea of how it responds to changes."

Astronomers used the Hopkins Ultraviolet Telescope (HUT), now orbiting Earth in the space shuttle Endeavour, to collect precise spectrographic data of Jupiter's upper atmosphere. At the same time, scientists used the Hubble Space Telescope to observe the same region of Jupiter's atmosphere, producing ultraviolet images to compare against the ultraviolet spectral data, Dr. Feldman said during a news conference at the Marshall Space Flight Center in Huntsville, Ala.

HUT is one of three ultraviolet telescopes in the Astro-2 observatory, now being operated from the payload bay of Endeavour.

"We are very excited that we were able to obtain ... exactly simultaneous observations of the same event, one giving us an image of the event, the other giving us a very high quality spectrum at the same time," said Dr. Feldman, a Johns Hopkins professor who is heading up HUT's observations of the Jovian atmosphere.

Scientists will use the high-quality spectrographic data from HUT and the images from Hubble to learn more about the nature and amount of energy produced by particles deposited into Jupiter's upper atmosphere by volcanic eruptions on Io and the cometary impacts.

Auroras on Jupiter's north and south poles grew dramatically brighter within two weeks of the cometary impacts. Auroras also are influenced by Io's volcanic particles and the solar winds, a constant stream of charged particles from the Sun.

Scientists not only want to learn more about how particles from Io's volcanism and solar winds affect the aurora. They also hope to learn more about how heat from the auroras is affecting Jupiter's atmosphere.

Since the Voyager spacecraft flew by Jupiter in 1979, scientists have been using spectrographs in space to study the characteristics of Jovian auroras. Scientists believe that most of the particles in Jupiter's magnetosphere are produced in volcanic eruptions on Io. In space the particles are first ionized -- powerful radiation removes electrons from their atoms -- giving them an electric charge. Then they are caught up in the Jovian magnetosphere -- a region of magnetic field lines around planets that traps charged particles.

Astronomers are debating whether the solar winds or Io's volcanic discharges exert the most influence on Jupiter's magnetosphere, Dr. Feldman said.

"I hope our observations will go some way towards resolving that problem," he said.

The space telescopes have provided scientists with a global view of Jupiter's atmosphere.

"We are measuring the total amount of energy deposited into the atmosphere at any given time by these magnetospheric particles." Dr. Feldman said, noting that researchers are trying to establish how the particles effect atmospheric characteristics like its composition and density.

For example, researchers are studying whether the recent eruption on Io would have produced more ions in the magnetosphere, ultimately leading to brighter auroras.

"We will be able to tell that from the analyses of both the intensity of the emissions observed by HUT and by looking at the distribution and brightness of the HST images," Dr. Feldman said.

Another point of interest is that the observations are taking place during a low point in the 11-year solar cycle, called the solar minimum. Astronomers made similar observations when the solar cycle was peaking, during the solar maximum, with the Astro-1 observatory on the space shuttle Columbia in December 1990.

"We are going to look for changes in the affects of the Sun on the Jovian atmosphere by comparing data from the two missions," Dr. Feldman said.

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