Saturns aurora defy scientists expectations
Secrets of Saturns polar light show unveiled by BU-led team of astronomers
Aurora on Saturn behave in ways different from how scientists have thought possible for the last 25 years, according to new research by a team of astronomers led by John Clarke, a professor in BUs Department of Astronomy and in the departments Center for Space Physics. The teams findings have overturned theories about how Saturns magnetosphere behaves and how its aurora are generated. Their results will be published in the February 17 issue of Nature.
In an unusual coordination of two spacecraft, the team was able to gather what proved to be startling data on Saturns aurora. By choreographing the instruments aboard the Saturn-bound Cassini spacecraft and the Hubble Space Telescope circling Earth to look at Saturns southern polar region, Clarke and his team found that the planets aurora, long thought of as a cross between those of Earth and Jupiter, are fundamentally unlike those observed on either of the other two planets. The lights that occasionally paint the sky over Saturn may, in fact, be a phenomenon unique within our solar system.
In Clarkes experiment, Hubble snapped ultraviolet pictures of Saturns aurora over several weeks and Cassini recorded radio emissions from the same regions while measuring the solar wind. Those measurements sets were combined to yield the most accurate glimpse yet of Saturns aurora.
The observations showed that Saturns aurora differ in character from day to day, as they do on Earth, moving around on some days and stationary on others. But compared to Earths auroral displays, which last only about 10 minutes, Saturns aurora can last for days.
The observations also indicated, surprisingly, that the suns magnetic field and solar wind may play a much larger role in Saturns aurora than previously suspected. Hubble images show that some displays remain stationary as the planet rotates beneath, as happens on Earth, but also show that, as on Jupiter, the aurora sometime move along with Saturn as it spins on its axis. This difference suggests that Saturns aurora are driven in an unexpected manner by the suns magnetic field and the solar wind and that the planets aurora possibly have different physical states at different times.
Seen from space, an aurora appears as a ring of energy circling a planets polar region. Auroral displays are spurred when charged particles in space interact with a planets magnetosphere and stream into the upper atmosphere. Collisions with atoms and molecules produce flashes of radiant energy in the form of visible, ultraviolet, and infrared light.
Scientists had long believed Saturns aurora possess properties akin to auroras on Earth and Jupiter--like Earths, they were thought to be influenced by the solar wind; and like Jupiters, they were assumed to be influenced by a ring of ions and charged particles encircling the planet.
But, as the team observed, although Saturns aurora do share characteristics with the other planets, they are fundamentally unlike the auroral displays on either Earth or Jupiter. When Saturns aurora become brighter (and thus more powerful), the ring of energy encircling the pole shrinks in diameter. By contrast, when Earths aurora become brighter, the polar region fills with light for several minutes, then dims, and the ring of light expands. Jupiters aurora, in comparison, are only weakly influenced by the solar wind, becoming brighter about once a month.
Saturns auroral displays also become brighter on the sector of the planet where night turns to day as the storms increase in intensity, unlike either of the other two planets. At certain times, Saturns auroral ring was more like a spiral, its ends not connected as the energy storm circled the pole.
Now that Cassini has entered orbit around Saturn, Clarke and his team will be able to take a more direct look at the how the planets aurora are generated. According to Clarke, the teams next effort will be to study in greater detail how Saturns auroral emissions are influenced by the planets magnetic field.
The Cassini/Huygens mission was launched in 1997 and is jointly operated by the European Space Agency and NASAs Jet Propulsion Laboratory in Pasadena, California. Cassini arrived in orbit around Saturn in July 2004 and will spend four years exploring the sixth planet, its moons and mysterious rings. NASAs Hubble Space Telescope is a cooperative program with the European Space Agency and is operated by the Space Telescope Science Institute on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. Hubble has spent the last 14 years orbiting Earth snapping pictures of the cosmos.
Faculty research in BUs Department of Astronomy is coordinated through its Institute for Astrophysical Research and its Center for Space Physics. Boston University, with an enrollment of more than 29,000 in its 17 schools and colleges, is the fourth-largest independent university in the United States.
Ann Marie Menting | EurekAlert!