High Degree Moments and Secular Variation of Saturn's Intrinsic Magnetic Field: Implications for the Planet Interior

Details High Degree Moments and Secular Variation of Saturn's Intrinsic Magnetic Field: Implications for the Planet Interior High Degree Moments and Secular Variation of Saturn's Intrinsic Magnetic Field: Implications for the Planet Interior

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13c1688c&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P13C-1688

Physics

[6030] Planetary Sciences: Comets And Small Bodies / Magnetic Fields And Magnetism

Scientific paper

The Cassini mission is providing an excellent opportunity to characterize the intrinsic magnetic field of Saturn. In this study, we first conduct a parameter space search based on the Cassini SOI measurements which approached to within 1.33 Rs. This analysis reveals that the high degree moments of the field are unexpectedly small and the total field spectrum shows a zig-zag pattern at the planet surface up to degree 5. When downward continued, the power spectrum of the odd terms becomes flat near 0.4 Rs which suggests a much deeper dynamo than previously estimated based on pressure ionization arguments. Such a deeply seated dynamo is consistent with the energy flux scaling law for the dipole moment. When downward continued to the estimated dynamo surface the field is strongly concentrated at the two rotation poles. Dynamo-generated intrinsic magnetic fields of planets are characterized by their secular variation. To extract the secular variation rate for Saturn's field, we first model the contributions to the measured magnetic field from various sources in the magnetosphere: the ring current, the mysterious ~11h periodic perturbations, the magnetopause current and tail current, etc. Modeling the external contributions is based on measurements outside the L-shell = 3.8 Rs orbit by orbit. The parameters extracted are then used to calculate the external field inside the L-shell = 3.8 Rs for the corresponding orbit. After removing the external contributions, the secular variation rate of the field is investigated. No clear secular variation trend is found, and upper limits on the secular variation rate are extracted. We search for the non-axisymmetric moments as well. Our more careful analysis compared to previous ones only results in tighter upper bounds on the amplitude of non-axisymmetric moments. We discuss the implications of the zig-zag power spectrum of the magnetic field, and the lack of detectable non-axisymmetric magnetic moments and secular variation for the interior structure of the planet and compare with similar properties of the Jovian magnetic field.

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