Scientists have gotten their best "look" ever at the
invisible ring of energetic ions trapped in
Saturn's giant magnetic field, finding that it is
asymmetric and dynamic, unlike similar rings that
appear around Earth.
Using the Magnetospheric Imaging Instrument on NASA's
Cassini spacecraft, a team led by
Stamatios Krimigis of Johns Hopkins' Applied Physics
Laboratory discovered that Saturn's ring of
energetic ions — called a "ring current" — is a
warped disc that is deflected by the solar wind out of the
equatorial plane on the planet's night side and thickens
dramatically on the day side. The images were
obtained by a unique camera that Krimigis says "visualizes
the invisible" and show the plasma and
radiation belts in Saturn's environment.
In the Dec. 13 issue of the journal Nature, Krimigis'
team describes how Saturn's ring current
changes over time; it's a dynamic system, doughnut-shaped
but sometimes appearing like someone took
a bite out of it. The scientists also found that Saturn's
ring current is persistently asymmetric — unlike
Earth's — and rotates closely in step with Saturn
itself. Ring currents form when hot ionized gas, known
as plasma, becomes trapped on a planet's magnetic field
lines. The main source of the plasma that
forms Saturn's ring current is material from the gas vented
by geysers on the moon Enceladus.
At Earth, ring currents form during large solar
wind-driven magnetic storms, although they
fade quickly as the driving solar wind disturbance recedes
into deep space. At Saturn, the
Magnetospheric Imaging Instrument observed that the ring
current's intensity seemed only weakly
related to solar activity.
"We might get a more intense reading when a solar wind
pressure spike passes by," said APL's
Donald Mitchell, a MIMI co-investigator, "but the surprise
is that Saturn's ring current didn't become
symmetric or dissipate as it does at Earth. It stayed lumpy
and rotated around the planet several
times. We don't know exactly why that happens, but we have
seen it exhibit this behavior repeatedly."
The presence of a ring current around Saturn was first
suggested in the early 1980s from
magnetic anomalies observed by NASA's Pioneer 11 and
Voyager 1 and 2 spacecraft. But Saturn's ring
current had never been mapped on a global scale; only small
areas were mapped previously, and not in
this detail. MIMI was designed for just this purpose;
developed by an APL-led international team,
MIMI has three distinct sensors, one of which contributed
the images for this work.
False-color images accompanying the Nature article
were taken by MIMI's ion and neutral
camera and show the intensity of the energetic neutral
atoms emitted from the ring current through
a process called charge exchange. This happens when a
trapped energetic ion steals an electron from a
cold gas atom, becomes neutral and escapes the magnetic
field. Scientists are using these images to
create a map of the invisible ring current, which is
roughly five times farther from Saturn than its
famous icy rings.
MIMI gathered the images for the Nature paper in March
2007 as Cassini looped nearly
920,000 miles over Saturn's poles, giving the instrument a
bird's-eye view of the magnetic activity
swirling around the planet.
The Cassini-Huygens mission is a cooperative project
of NASA, the European Space Agency and
the Italian Space Agency. The Jet Propulsion Laboratory, a
division of the California Institute of
Technology, manages the mission for NASA's Science Mission
Directorate. Krimigis is the principal
investigator for MIMI, which was designed and built and is
operated by an APL-led team.
For more information about the Cassini-Huygens
mission, go to
http://saturn.jpl.nasa.gov and
the MIMI team's home page,
sd-www.jhuapl.edu/CASSINI/index.html.
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