Thin shell dynamo models consistent with Mercury's weak observed magnetic field

Details Thin shell dynamo models consistent with Mercury's weak observed magnetic field Thin shell dynamo models consistent with Mercury's weak observed magnetic field

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p23a0247s&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #P23A-0247

Physics

5430 Interiors (8147), 5440 Magnetic Fields And Magnetism, 6235 Mercury, 1507 Core Processes (8115), 1510 Dynamo Theories

Scientific paper

Mariner 10 observed Mercury's magnetic field during 2 flybys of the planet between 1974 and 1975, revealing the presence of a magnetic field of internal origin with a dipole moment of ≈ 300nT-RM3 (1 RM = 2440 km). Such a field may be too large to be explained easily by remanent magnetization; however, energetic and magnetostrophic balance considerations suggest that it may be too small to be consistent with dynamo action, at least for an Earth-like dynamo. More specifically, energetic and magnetostrophic balance arguments provide an estimate of the toroidal magnetic field strength that the dynamo should generate: if the dynamo is Earth-like in the sense that it has similar toroidal-poloidal field strength scaling as the Earth, then the poloidal field should be much stronger than that observed. Before abandoning a dynamo explanation for Mercury's field, we question whether the same toroidal-poloidal scaling should hold for Mercury. Thermal evolution calculations estimate that Mercury's solid inner core may comprise between 0.5 and 0.8 of the total core radius, much larger than the Earth value of 0.35. As a result, the fluid convecting outer core where the dynamo is generated may be much thinner for Mercury than for Earth. Here we use 3-D numerical dynamo modeling to investigate dynamos operating in thin shell geometries. We examine the ratio of the dipole field at the core-mantle boundary to the toroidal field in the core for various shell thicknesses and Rayleigh numbers and find that some thin shell dynamos can produce magnetic fields with Mercury-like field partitioning. In these dynamos, the toroidal field is produced more efficiently through differential rotation than the poloidal field is produced through interactions of convective upwellings with the toroidal field. The poloidal field is also dominated by smaller-scale structure which was not observable by the Mariner 10 mission, compared to the dipole. This suggests that a hydromagnetic dynamo may be consistent with the weak surface field observed at Mercury and alternative explanations may not be required.

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