Symmetry and Symmetry Breaking in Planetary Magnetic Fields

Friday, 19 December 2014: 4:15 PM
Hao Cao, University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States; California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, Christopher T Russell, Univ California, Los Angeles, CA, United States, Jonathan M Aurnou, University of California Los Angeles, Los Angeles, CA, United States, Krista M Soderlund, Univ TX Austin/Inst for Geophy, Austin, TX, United States and Michele Karen Dougherty, Imperial College London, Blackett Laboratory, London, United Kingdom
Six out of eight solar system planets currently possess global-scale intrinsic magnetic fields. Different symmetry and symmetry breaking with respect to the spin-axis and the equatorial plane of the host planet can be found for different planetary magnetic fields. With respect to the spin-axis, the magnetic fields of Mercury, Earth, Jupiter, and Saturn are dominated by the axisymmetric part while the magnetic fields of Uranus and Neptune show no such alignment. Moreover, non-axisymmetric components have not been determined unambiguously for the magnetic fields of Mercury and Saturn. With respect to the equatorial plane, the magnetic fields of Earth, Jupiter, and Saturn show small but non-negligible asymmetry while the magnetic field of Mercury shows a significant asymmetry. The magnetic fields of Uranus and Neptune likely possess similar strength in the two hemispheres divided by the equatorial plane, but this needs to be confirmed with future measurements.

Here we present our interpretation of the magnetic fields of Mercury and Saturn, both of which are often referred to as anomalous dipolar dynamos. For Mercury, we will show that volumetrically distributed buoyancy sources in its liquid iron core can naturally lead to equatorial symmetry breaking in the dynamo generated magnetic field as observed by MESSENGER. We will also show that the size of the solid inner core inside Mercury is likely smaller than 1000 km and could be detected indirectly with high-spatial-resolution magnetic field measurements near Mercury’s north pole. In addition, we will show that degree-2 longitudinal variations observed in the magnetic equator positions of Mercury could have an internal origin. For Saturn’s magnetic field, although its extreme axisymmetry could in principle be explained by a stably-stratified electrically-conducting layer on top of the dynamo region, more features such as equator-to-pole field contrasts cannot be explained by this same mechanism simultaneously. Towards this end, we will show the possible link between the features in Saturn’s magnetic field and dynamics in the semi-conducting region of Saturn.