Press and Public Relations Department
Max Planck Society for the Advancement of Science
Prof. Joachim Saur
University of Cologne, Institute of Geophysics and Meteorology
Tel.: +49 221 470-2310
Fax: +49 221 470-5161
Dr. Norbert Krupp
Max Planck Institute for Solar System Research, Katlenburg-Lindau
Tel.: +49 5556 979154
Fax: +49 5556 979-6154
March 6th, 2006
News SP / 2006 (23)
Electrons Flying "Backwards" in Saturn's Sky
Scientists at the Max Planck Institute for Solar System Research
discover unusual particle rays in Saturn's polar region
Polar lights are fascinating to look at on Earth. On other planets, they
can also be spectacular. Scientists from the Max Planck Institute for
Solar System Research in Katlenberg, Lindau, Germany, have now observed
Saturn's polar region using the particle spectrometer MIMI, on the
Cassini Space Probe. They discovered electrons not only being
accelerated toward the planet, but also away from it (Nature, February
We can see polar lights on Earth when electrons above the atmosphere are
accelerated downwards. They light up when they hit the upper atmosphere.
Some years ago, researchers discovered that electrons inside the polar
region can also be accelerated away from the Earth -- that is,
"backwards". These anti-planetary electrons do not cause the sky to
light up, and scientists have been puzzled about how they originate.
Until now it has also been unclear whether anti-planetary electrons only
occur on Earth. An international team led by Joachim Saur at the
University of Cologne have now found electrons on Saturn that are
accelerated "backwards" -- that is, in an anti-planetary direction.
These particles were measured using "Magnetospheric Imaging Instruments"
(MIMI) on NASA's Cassini Space Probe. One of these instruments' sensors,
the "Low Energy Magnetospheric Measurement System" (LEMMS), was
developed and built by scientists at the Max Planck Institute for Solar
The rotation of the space probe helped the researchers to determine the
direction, number, and strength of the electron rays. They compared
these results with recordings of the polar region and a global model of
Saturn's magnetic field. It turned out that the region of polar light
matched up very well with the lowest point of the magnetic field lines
in which electron rays were measured.
Because the electron ray is strongly focussed (with an angle of beam
spread less than 10 degrees), the scientists were able to determine
where its source lies: somewhere above the polar region, but inside a
distance of maximum five radii of Saturn. Because the electron rays
measured on the Earth, Jupiter, and Saturn are so similar, it appears
that there must be some fundamental process underlying the creation of
Doing these measurements, Norbert Krupp and his colleagues Andreas Lagg
and Elias Roussos from the Max Planck Institute for Solar System
Research worked closely with scientists from the Institute for
Geophysics and Meteorology at the University of Cologne and the Applied
Physics Laboratory of Johns Hopkins University in Baltimore. US
scientists led by Tom Krimigis are responsible for service and
coordination of the instrument on the Cassini Space Probe.
J. Saur, B.H. Mauk, D. G. Mitchell, N. Krupp, K.K. Khurana, S. Livi, S.
M. Krimigis, P. T. Newell, D. J. Williams, P.C. Brandt, A. Lagg, E.
Roussos, M. K. Dougherty
Electron beams at Saturn indicate universality of anti-planetward
Nature, 439, 699-702 (09 Feb 2006)
Beware -- particle rays! Researchers have measured electron rays, using
the instrument MIMI on the Cassini Space Probe. These rays fly
"backwards" -- that is, away from, rather than toward, the planet
Saturn, in its polar region.
Image: University of Cologne / Joachim Saur