Saturn's variable radio period

Details Saturn's variable radio period Saturn's variable radio period

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm22b..02l&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM22B-02

Astronomy and Astrophysics

Astronomy

2736 Magnetosphere/Ionosphere Interactions (2431), 2740 Magnetospheric Configuration And Dynamics, 2756 Planetary Magnetospheres (5443, 5737, 6033)

Scientific paper

Since Voyager's era, it is commonly assumed that rotation rate of the interiors of outer planets can be derived from the periodic variation of their radio emission. Indeed, Voyager analysis of auroral radio emissions originating from Jupiter, Saturn, Uranus and Neptune could indeed provide estimates of the planetary sidereal periods, within an accuracy depending on the observing time span and in agreement with an uniform rotation rate. However, later measurements of Saturn kilometric radiation, - first, since 1994, by the sensitive radio astronomy instrument aboard Ulysses spacecraft and then, since 2003, by the one aboard Cassini in orbit around the planet -, rather demonstrate that the apparent radio period of Saturn is slowly changing, within about 1% of the mean value. Since true variations of the rotation period of Saturn's deep interior are certainly at much lower level, for obvious conservation of energy and angular momentum reasons, the observed period change is rather to be interpreted as a slow drift in phase of the radio source and/or associated magnetospheric features, within the planetary corotating frame. Quite interestingly, similar drift was recently found in time series of the magnetic field azimuthal component, as measured by Cassini's magnetometer. Since the field azimuthal component is not of internal origin, and auroral radio emission links inner and outer parts of the magnetophere, one get compiling evidence for the existence of some long-term, global change of Saturn' magnetosphere at the time scale of years. Some possibilities are briefly reviewed and discussed.

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