Hot Plasma Explosions Inflate Saturn’s Magnetic Field

MEDIA RELATIONS OFFICE
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Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

Feature: 2010-417 Dec. 14, 2010

Hot Plasma Explosions Inflate Saturn's Magnetic Field

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-417&cid=release_2010-417

A new analysis based on data from NASA's Cassini spacecraft finds a causal link
between mysterious, periodic signals from Saturn's magnetic field and explosions of hot
ionized gas, known as plasma, around the planet.

Scientists have found that enormous clouds of plasma periodically bloom around Saturn
and move around the planet like an unbalanced load of laundry on spin cycle. The
movement of this hot plasma produces a repeating signature "thump" in measurements of
Saturn's rotating magnetic environment and helps to illustrate why scientists have had
such a difficult time measuring the length of a day on Saturn.

"This is a breakthrough that may point us to the origin of the mysteriously changing
periodicities that cloud the true rotation period of Saturn," said Pontus Brandt, the lead
author on the paper and a Cassini team scientist based at the Johns Hopkins University
Applied Physics Laboratory in Laurel, Md. "The big question now is why these
explosions occur periodically."

The data show how plasma injections, electrical currents and Saturn's magnetic field --
phenomena that are invisible to the human eye -- are partners in an intricate choreography.
Periodic plasma explosions form islands of pressure that rotate around Saturn. The islands
of pressure "inflate" the magnetic field.

A new animation showing the linked behavior is available at http://www.nasa.gov/cassini
and http://saturn.jpl.nasa.gov .

The visualization shows how invisible hot plasma in Saturn's magnetosphere – the
magnetic bubble around the planet -- explodes and distorts magnetic field lines in
response to the pressure. Saturn's magnetosphere is not a perfect bubble because it is
blown back by the force of the solar wind, which contains charged particles streaming off
the sun.

The force of the solar wind stretches the magnetic field of the side of Saturn facing away
from the sun into a so-called magnetotail. The collapse of the magnetotail appears to kick
off a process that causes the hot plasma bursts, which in turn inflate the magnetic field in
the inner magnetosphere.

Scientists are still investigating what causes Saturn's magnetotail to collapse, but there are
strong indications that cold, dense plasma originally from Saturn's moon Enceladus
rotates with Saturn. Centrifugal forces stretch the magnetic field until part of the tail
snaps back.

The snapping back heats plasma around Saturn and the heated plasma becomes trapped in
the magnetic field. It rotates around the planet in islands at the speed of about 100
kilometers per second (200,000 mph). In the same way that high and low pressure
systems on Earth cause winds, the high pressures of space cause electrical currents.
Currents cause magnetic field distortions.

A radio signal known as Saturn Kilometric Radiation, which scientists have used to
estimate the length of a day on Saturn, is intimately linked to the behavior of Saturn's
magnetic field. Because Saturn has no surface or fixed point to clock its rotation rate,
scientists inferred the rotation rate from timing the peaks in this type of radio emission,
which is assumed to surge with each rotation of a planet. This method has worked for
Jupiter, but the Saturn signals have varied. Measurements from the early 1980s taken by
NASA's Voyager spacecraft, data obtained in 2000 by the ESA/NASA Ulysses mission,
and Cassini data from about 2003 to the present differ by a small, but significant degree.
As a result, scientists are not sure how long a Saturn day is.

"What's important about this new work is that scientists are beginning to describe the
global, causal relationships between some of the complex, invisible forces that shape the
Saturn environment," said Marcia Burton, the Cassini fields and particles investigation
scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The new results still
don't give us the length of a Saturn day, but they do give us important clues to begin
figuring it out. The Saturn day length, or Saturn's rotation rate, is important for
determining fundamental properties of Saturn, like the structure of its interior and the
speed of its winds."

Plasma is invisible to the human eye. But the ion and neutral camera on Cassini's
magnetospheric imaging instrument provides a three-dimensional view by detecting
energetic neutral atoms emitted from the plasma clouds around Saturn. Energetic neutral
atoms form when cold, neutral gas collides with electrically-charged particles in a cloud
of plasma. The resulting particles are neutrally charged, so they are able to escape
magnetic fields and zoom off into space. The emission of these particles often occurs in
the magnetic fields surrounding planets.

By stringing together images obtained every half hour, scientists produced movies of
plasma as it drifted around the planet. Scientists used these images to reconstruct the 3-D
pressure produced by the plasma clouds, and supplemented those results with plasma
pressures derived from the Cassini plasma spectrometer. Once scientists understood the
pressure and its evolution, they could calculate the associated magnetic field
perturbations along the Cassini flight path. The calculated field perturbation matched the
observed magnetic field "thumps" perfectly, confirming the source of the field
oscillations.

"We all know that changing rotation periods have been observed at pulsars, millions of
light years from our solar system, and now we find that a similar phenomenon is observed
right here at Saturn," said Tom Krimigis, principal investigator of the magnetospheric
imaging instrument, also based at the Applied Physics Laboratory and the Academy of
Athens, Greece. "With instruments right at the spot where it's happening, we can tell that
plasma flows and complex current systems can mask the real rotation period of the central
body. That's how observations in our solar system help us understand what is seen in
distant astrophysical objects."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of
the California Institute of Technology in Pasadena, Calif. manages the mission for
NASA's Science Mission Directorate, Washington, D.C. The magnetic imaging
instrument team is based at the Johns Hopkins University Applied Physics Laboratory,
Laurel, Md.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov
and http://www.nasa.gov/cassini .

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