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 in Ann Arbor, NASA's Jet Propulsion Laboratory, Pasadena, CA, University of Wisconsin, Madison, Goddard Space Flight Center, Greenbelt, MD 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 even 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 communication 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 cloudtops, 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 also 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.