Don Savage
Headquarters, Washington, DC	October 17, 1996
(Phone: 202/358-1547)	EMBARGOED UNTIL: 4 P.M.(EDT)

Tammy Jones
Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, MD (Phone: 410/338-4514)

RELEASE: 96-210

Hubble Follows Rapid Changes In Jupiter's Aurora 

Hubble Space Telescope's sharp view of the rapid, spectacular dance of luminescent gasses high in Jupiter's 
atmosphere -- better known as aurora -- is allowing astronomers to map Jupiter's immense magnetic field 
and better understand how it generates such phenomena. 

"Now that we have pinpointed the general location of the auroral curtains and have mapped their daily 
changes, eventually we should be able to find out what causes the aurora on Jupiter," said John T. Clarke, 
an astronomer at the University of Michigan's College of Engineering. 

The new Hubble observations simultaneously show warped oval rings at the north and south poles (offset 
from Jupiter's spin axis by 10-15 degrees), as well as an auroral "footprint" created by a river of electrical 
current of about one million amperes flowing between Jupiter and the volcanic moon Io.

The Hubble images provide enough detail to allow Clarke and his colleagues to record daily changes in the 
auroras' intensity and motion. They find that changes in brightness occur over the course of a Jovian day, 
perhaps due to compression of Jupiter's magnetic field on the sun-facing side of the planet. They also find 
emission features that are fixed on the planet, co-rotating with it. 

This global view is complemented by in situ measurements of the magnetic field and charged particles by 
the Galileo spacecraft, now orbiting Jupiter. By comparing close-up and global views scientists expect to 
refine theories about how Jupiter creates and maintains its electrical, incandescent light shows.

The team of scientists, at the University of Michigan, Ann Arbor, NASA's Jet Propulsion Laboratory, 
Pasadena, CA, University of Wisconsin, Madison, and other institutions, studied Jupiter's auroras for two 
years with the telescope's Wide Field and Planetary Camera 2. Their results have led to two papers, one 
first authored by Clarke and the other by Gilda Ballester, also of the University of Michigan's College of 
Engineering. Both papers appear in the October 18 issue of Science. The images, taken in ultraviolet light, 
are the most sensitive and sharply-detailed views of the auroras to date. Previous observations of Jupiter's 
aurora have been recorded by Hubble's Faint Object Camera and by ground-based telescopes using near-
infrared filters. Hubble sees features as small as 186 miles across (300 kilometers). This allows Clarke and 
his colleagues to watch small-scale, rapid changes in the auroral pattern, map changes in both magnetic 
poles, and pinpoint the effects of emissions from Io. 

Auroras occur when charged particles (electrons, protons, and positive ions) are captured in the magnetic 
field surrounding a planet. Falling toward the magnetic north and south poles, they collide with molecules 
and atoms in a planet's upper atmosphere. The atoms become energized and release the extra energy in the 
form of light, just as gas in florescent and neon lights glows when an electric current is applied. 

By studying images of Jupiter's entire disk, the investigators found, surprisingly, the auroras mirror each 
other at the north and south poles. Though Earth's auroras at each pole also are carbon copies of each other, 
previous spatially-unresolved observations and theories for Jupiter suggested that some locations on the 
auroral ovals should be brighter. That's because, in Jupiter's case, the magnetic field has large asymmetries 
and more charged particles trapped in the field could, under specific mechanisms, eventually fall into the 
atmosphere at the weaker locations, and would thus create a brighter light show. 

A critical difference is that auroras on Earth are triggered by a barrage of charged particles from the Sun. 
This process is different on Jupiter, although not well understood. Fundamentally, the planet's immense 
magnetic field, coupled with its fast, 10-hour rotation, helps generate auroras that are 1,000 times more 
powerful than Earth's spectacular light shows.

The situation is complicated by material released by Jupiter's moon, Io. Scientists believe that volcanic 
eruptions on Io churn out particles that become ionized, expand radially, and are trapped by Jupiter's 
immense magnetic field. These charges are forced to co-rotate with the planet, creating an immense sheet 
of current that in turn modifies Jupiter's magnetic field. What has not been clear on Jupiter is the balance of 
the internal processes versus the Sun-driven processes, and how these processes produce the auroral lights.

On Earth, magnetic storms are triggered by large changes in the solar particles, producing very bright 
auroras. These storms can disrupt radio signals and communications systems, interfere with airplane 
navigation and cause power outages. One storm in 1989 knocked out a Quebec power station serving 9 
million people. The team has found that energetic auroral storms also occur on Jupiter, but that these 
storms may be triggered instead by internal processes.

Some of the material released by Io produces a fierce current of charged particles. The particles become 
ionized and are then drawn into Jupiter's intense magnetic field along an invisible "flux tube," which 
bridges both worlds. This creates small auroral spots just outside the ovals around both magnetic poles. By 
studying changes in the intensity of these spots, Clarke and his colleagues were able to map Jupiter's 
magnetic field as Io orbits through it. The scientists linked the spots to Io's "flux tube" because the auroral 
emissions rotate with Jupiter while the spots remain in a fixed location underneath Io.

"The size of the aurora at the magnetic footprint of Io is 600 to 1,200 miles (1,000 to 2,000 kilometers) 
across," Clarke said. "If you were at Jupiter's cloud tops, under Io's footprint, the aurora would fill the 
entire sky. You would see an explosion as the gasses 250 miles above you rapidly heated to more than 
10,000 degrees Fahrenheit. The aurora would speed overhead from east to west at more than 10,000 miles 
per hour (5 kilometers per second) because Jupiter's fast rotation moves it rapidly underneath Io, which 
orbits more slowly."

Clarke and his colleagues hope that future observations will yield more information about the auroras. The 
team is sharing data with the scientists operating the Galileo spacecraft, which moves through Jupiter's 
magnetic field repeatedly as it orbits the giant planet and surveys the Galilean satellites. Galileo can record 
the type of charged particles (ions, protons, electrons) in the field, their location and energy. Information 
from Hubble and Galileo is important because scientists can create a more accurate picture of the charged 
particles which produce the auroral lights, which eventually could lead them to its source on Io.

The Space Telescope Science Institute is operated by the Association of Universities for Research in 
Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The 
Hubble Space Telescope is a project of international cooperation between NASA and the European Space 
Agency.

- end -

EDITOR'S NOTE: An image accompanying this release is available to news media by calling the 
Headquarters Imaging Branch on 202/358-1900. Photo numbers are: 

Color:	96-HC-679
B&W:	96-H-679

Image files in GIF and JPEG format and captions may be accessed on Internet via anonymous ftp from 
ftp.stsci.edu in /pubinfo.

GIF	JPEG
PRC96-32	Jupiter Aurora	gif/jupaur.gif	jpeg/jupaur.jpg

Higher resolution digital versions (300 dpi JPEG) of the release photograph will be available temporarily in 
/pubinfo/hrtemp: 96-32.jpg (color) and 96-32bw.jpg (black/white).

GIF and JPEG images, captions and press release text are available via World Wide Web at:

http://www.stsci.edu/pubinfo/PR/96/32.html 

and via links at:

http://www.stsci.edu/pubinfo/Latest.html 
or
http://www.stsci.edu/pubinfo/Pictures.html