The puzzle of the variable radio period of Saturn (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Philippe Zarka
Observatoire de Paris, LESIA and CNRS
Tél: 33 1 45 07 76 63 Fax: 33 1 45 07 28 06

Laurent Lamy
Observatoire de Paris, LESIA and CNRS
Tél: 33 1 45 07 76 68 Fax: 33 1 45 07 28 06

Baptiste Cecconi
Observatoire de Paris, LESIA and CNRS
Tél: 33 1 45 07 77 59 Fax: 33 1 45 07 28 06

Renée Prangé
Observatoire de Paris, LESIA and CNRS
Tél: 33 1 45 07 71 65 Fax: 33 1 45 07 28 06

7 November 2007

The puzzle of the variable radio period of Saturn

The auroral radio emission of giant planets are usually used to estimate
their rate of internal rotation. But in the case of Saturn, these
emissions present important variations at the month scale, which cannot be
due to the rotation. A team of astronomers, led by the LESIA, from Paris
Observatory, just showed that these variations are controlled by external
mechanism, certainly related to the solar wind.

Measurements of the rotation rate of giant planets using visible
wavelengths are relatively inaccurate because they only give the
combination of the internal rotation and the wind speeds, all unknown a
priori. As planetary auroral radio emissions are produced by electrons
moving along planetary magnetic field lines, they should be tied -- via
the magnetic field -- to the planetary interior, and this is why they are
used to measure the internal rotation of giant planets. For Jupiter, one
obtains thus a stable period with an accuracy better than one part in one
million. In the case of Saturn, Voyager radio measurements provided a
period of 10h 39m 24s +/- 7s, adopted as Saturn's rotation period. But
subsequent observations by Ulysses and Cassini showed that this period
actually fluctuates by +/- 6 min (thus +/- 1%) at a timescale of a few
months to years. Similar variations seem to exist for the magnetic field
measured in-situ in Saturn's magnetosphere.

The origin of this huge variation (which would represent +/- 15 min. if
compared to the duration of the terrestrial day) obviously cannot be due
to a change of the internal planetary rotation: contrary to an ice-skater,
Saturn has no arms to fold in order to change its rotation rate while
keeping its angular momentum constant, nor has it any efficient and rapid
source or sink of angular momentum. These period variations are one of the
main enigmas studied by the Cassini scientific community. Its solution is
the key of the determination of atmospheric wind speeds, would constrain
Saturn's internal structure and shape (polar flattening), and is required
to be able to define a longitude system on Saturn allowing to organize the
observations over long time intervals.

The astronomers have concentrated their search on short-term period
variations (a few days), in order to look for correlations with other
phenomena more easily than for slow variations over several months. For
that purpose, they have developed a method allowing us to measure these
short-term variations with an accuracy better than 1% on a timescale of
about 8 days. We have obtained the following two results [ref. 1]:

(i) Saturn's radio period varies at a timescale of 20-30 days, with an
amplitude larger than that of long-term variations (which could therefore
simply be the residual of the time averaging of short-term ones);

(ii) these variations at 20-30 days are correlated with those of the solar
wind speed around Saturn (especially the speed, and not the solar wind
pressure or density). This proves that the origin of the variations of
Saturn's "radio clock" is -- at least in part -- external to the Saturn
system, and that the solar wind speed is the key parameter of this
external control.

One of the theories previously proposed for explaining the long-term
variations of the radio period already suggested such an external control
and a specific role for the solar wind speed [ref. 2]. This theory, which
also applies to short-term variations, is strongly supported by the above
recent results. It offers a frame in which we should eventually be able to
subtract solar-wind induced variations in order to obtain the true
internal rotation period of Saturn (if such one period does exist), thanks
to the goniometric (i.e. "radio astrometry") capabilities of the
radioastronomy experiment (RPWS) onboard Cassini spacecraft.

[1] P. Zarka, L. Lamy, B. Cecconi, R. Prangé & H. O. Rucker, Modulation of
Saturn's radio clock by solar wind speed, Nature, 8 Nov. 2007.
[2] B. Cecconi & P. Zarka, Model of a variable radio period for Saturn,,
J. Geophys. Res. 110, A12203, 2005.

Reference

Modulation of Saturn's radio clock by solar wind speed
Philippe Zarka(1), Laurent Lamy(1), Baptiste Cecconi(1), Renée Prangé(1) &
Helmut O. Rucker(2)
(1) LESIA, Laboratoire d'Etudes Spatiales et d'Instrumentation en
Astrophysique, Observatoire de Paris, Centre National de la Recherche
Scientifique, Université Pierre et Marie Curie, Université Paris Diderot,
92190 Meudon, France
(2)Space Research Institute, Austrian Academy of Sciences, A-8042 Graz,
Austria
Nature, 8 November 2007,
http://www.obspm.fr/actual/nouvelle/...ature_2007.pdf

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/nov07/saturn-f1.jpg (61KB)]
Artist view of the solar wind interacting with Saturn's magnetosphere and
causing variations of its "rotational clock" measured by the periodicity
of its radio emissions.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/nov07/saturn-f2.gif (131KB)]
Comparison of short-term variations of Saturn's radio emissions (boldface
lines on the 2 panels), with solar wind speed (thin line in (a)) and solar
wind pressure (thin line in (b)). The data shown concern the year before
Cassini's insertion in orbit around Saturn. Solar wind variations around
Saturn are obtained by ballistic projection of measurements made in the
vicinity of the Earth. The correlation with the speed is excellent, while
that with the pressure is very poor.