Press Release

Saturn’s aurora defy scientists’ expectations

By SpaceRef Editor
February 17, 2005
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Saturn’s aurora defy scientists’ expectations

The dancing light of the auroras on
Saturn behaves in ways different from how scientists
have thought possible for the last 25 years. New
research by a team of US and European planetary
scientists led by John Clarke of Boston University,
USA, has overturned theories about how Saturn’s
magnetic field behaves and how its auroras are
generated. Their results will be published in the
February 17 issue of the journal Nature.

By choreographing the instruments aboard the Earth-
orbiting Hubble Space Telescope and the Cassini
spacecraft, while it was en route to Saturn, to look
at Saturn’s southern polar region, Clarke and his
team found that the planet’s auroras – 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
fill the sky over Saturn may, in fact, be a
phenomenon unique within our solar system.

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

The observations showed that Saturn’s auroras 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 auroras
last only about 10 minutes, Saturn’s auroras 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 auroras 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 auroras do
share characteristics with the other planets, they
are fundamentally unlike those on either Earth or
Jupiter. When Saturn’s auroras become brighter (and
thus more powerful), the ring of energy encircling
the pole shrinks in diameter. When Earth’s auroras
become brighter, the polar region for several minutes
is filled with light. Then the ring of light dims and
begins to expand. Jupiter’s auroras, 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 has entered orbit around Saturn,
Clarke and his team will be able to take a more
direct look at the how the planet’s auroras are
generated. The team is planning to probe how the
Sun’s magnetic field may fuel Saturn’s auroras and
what role the solar wind may play.

SpaceRef staff editor.