HR: 15:10h
AN: GP23A-06    [Abstracts]
TI: A Quadrupolar Magnetic Field at Mercury?
AU: * Christensen, U R
EM: christensen@mps.mpg.de
AF: Max Planck Institute for Solar System Research, Max-Planck Strasse 2, Katlenburg-Lindau, 37191, Germany
AU: Wicht, J
EM: wicht@mps.mpg.de
AF: Max Planck Institute for Solar System Research, Max-Planck Strasse 2, Katlenburg-Lindau, 37191, Germany
AB: During the Mariner 10 flybys Mercury's global magnetic field was detected. The available observations can be fitted by a nearly axial dipole, but the field geometry including the quadrupole-to-dipole ratio is poorly constrained. The surface field strength is only 1% of the geomagnetic field strength. For an Earth-like dynamo a thirty times stronger field is expected based on an assumed balance of Lorentz and Coriolis forces. We suggest that the dynamo operates only in a deep sublayer of the fluid part of the core, whereas its upper part is stably stratified because the temperature gradient is subadiabatic. Depending on the heat flow at the core mantle boundary, the sulphur concentration, and the size of the solid inner core, we estimate that the unstable layer has 20-60% of the outer core's thickness. Because Mercury rotates 60 times slower than Earth, the local Rossby number Ro_l, measuring the role of inertial forces relative to the Coriolis force, is estimated to be of order 10 compared to 0.1 in the geodynamo. Numerical dynamo models show that for high Ro_l the contribution of the dipole to the magnetic field is weak in the dynamo region, where higher multipole components dominate. They vary rapidly in time and are strongly attenuated in the stable conducting layer. The weak but slowly varying dipole and quadrupole components pass this layer to some degree and dominate the field geometry at the planetary surface. We have varied the basic control parameters, inner core size, and stable layer thickness in over 20 dynamo models. In some models the axial dipole and in others the axial quadrupole dominate the surface field whose strength is of the right order. While the dipole is more preferred for a larger size of the inner core, there is a trend towards quadrupolar solutions with an increasing value of Ro_l. Because our model values fall short of the estimated Mercury value for numerical reasons, we conclude that it is possible that Mercury's magnetic field is dominantly quadrupolar at the planet's surface. The Messenger flyby in January 2008 offers an opportunity to test this hypothesis as well as other model predictions, such as a high degree of axisymmetry of the field.
DE: 1510 Dynamo: theories and simulations
DE: 5430 Interiors (8147)
DE: 5440 Magnetic fields and magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting