Saturn’s aurorae defy scientists' expectations

The instruments aboard the Earth-orbiting NASA/ESA Hubble Space Telescope and the NASA/ESA/ASI Cassini-Huygens spacecraft en route to Saturn were choreographed to look at Saturn's southern polar region.

The team, led by John Clarke of Boston University, USA, found that the planet's aurorae are fundamentally unlike those observed on either or Earth and Jupiter. The lights that occasionally fill the sky over Saturn may, in fact, be a phenomenon unique within the Solar System.

In Clarke's experiment, Hubble snapped ultraviolet pictures of Saturn's aurorae over several weeks and Cassini recorded radio emissions from the same regions while measuring the solar wind, a stream of charged particles that trigger aurorae. Those sets of measurements were combined to yield the most accurate glimpse yet of Saturn's aurorae.

The observations showed that Saturn's aurorae differ in character from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared with Earth, where aurorae last only about ten minutes, Saturn's aurorae can last for days.

The observations also indicated, surprisingly, that the solar wind may play a much larger role in Saturn's aurora than previously suspected. Hubble images, when combined with Cassini measurements of the solar wind, show that it is the pressure of the solar wind that appears to drive auroral storms on Saturn.

In Earth's case, it is mainly the Sun's magnetic field, carried in the solar wind that drives auroral storms. In Saturn’s case the orientation of the magnetic field plays no major role.

Seen from space, an aurora appears as a ring of light circling a planet's polar region, where magnetic poles typically reside. Auroral displays are initiated when charged particles in space collide with a planet's magnetic field and stream into the upper atmosphere. Collisions with gases in the planet's atmosphere produce flashes of glowing energy in the form of light and radio waves.

Scientists had long believed Saturn's aurorae possess properties akin to both Earth and Jupiter. Like Earth's, they were thought to be influenced by the solar wind. Like Jupiter's, they were assumed to be influenced by a ring of ions and charged particles encircling the planet.

The new results do show, however, a feature of Saturn's aurora that matches Earth's: Radio waves appear to be tied to the brightest auroral spots. This similarity suggests that the physical processes that generate these radio waves is just like those of Earth.

But, as the team observed, though Saturn's aurorae do share characteristics with the other planets, they are fundamentally unlike those on either Earth or Jupiter. When Saturn's aurorae become brighter (and thus more powerful), the ring of energy encircling the pole shrinks in diameter. When Earth's aurorae become brighter, the polar region for several minutes is filled with light. Then the ring of light dims and begins to expand. Jupiter's aurorae, however, are only weakly influenced by the solar wind, becoming brighter about once a month, at the most, in response to solar wind changes.

Recent model work has suggested that the key feature that make Saturn’s magnetic environment special, is Saturn’s strong magnetic field that works together with a dominating process where the magnetic field lines break and re-connect with other lines.

Saturn's 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. The new images also confirm that, at certain times, Saturn's auroral ring was more like a spiral, its ends not connected as the energy storm circled the pole.

Now that Cassini-Huygens has entered orbit around Saturn, Clarke and his team will be able to take a more direct look at the how the planet's aurorae are generated. The team is planning to probe how the Sun's magnetic field may fuel Saturn's aurorae and what role the solar wind may play.