HR: 15:25h
AN: GP23B-08    [Abstracts]
TI: Symmetry and Stability of Earth's Magnetic Field
AU: * Coe, R S
EM: rcoe@pmc.ucsc.edu
AF: University of California, Earth Sciences Department, Santa Cruz, CA 95064 United States
AU: Glatzmaier, G A
EM: glatz@pmc.ucsc.edu
AF: University of California, Earth Sciences Department, Santa Cruz, CA 95064 United States
AB: Geophysicists have suggested various parameters that may correlate with time-averaged geomagnetic reversal rate, including axial dipole (AD), equatorial dipole (ED) and non-axial dipole (NAD) field strengths, their ratios, and their secular variation amplitudes. Merrill and McFadden (1988) proposed a more subtle relationship between reversal rate and symmetry of the geomagnetic field, and McFadden et al. (1991) used paleosecular variation data from lavas to demonstrate that this correlation exists. We show that geodynamo simulations also exhibit such a correlation between field symmetry and stability. The more stable simulation cases of Glatzmaier et al. (1999) have a ratio of time-averaged spectral energy of the equatorially antisymmetric (n+m odd, and excluding the AD) to equatorially symmetric (n+m even) spherical harmonic terms of the field at the core-mantle boundary that is significantly larger than the less stable cases. For instance, for the simulations with equatorially symmetric zonal heat flux at the core-mantle boundary (CMB), the ratio of odd to even spectral energy for the case that never reversed is more than ten times that for the case that reversed most frequently. Most intriguing is a simulation with uniform heat flux at the CMB and smaller inner core only one-quarter of its present radius, as was probably true long ago in Precambrian time. The odd/even spectral energy ratio for this case is about 7000 times larger than the uniform heat flux case with today's inner core and 40 times larger than the most stable case mentioned above, implying that the reversal rate may have been much lower early in the history of the geomagnetic field. Paleomagnetic evidence in favor of this hypothesis is suggestive, though not conclusive. (Glatzmaier, G. A., R. S. Coe, L. Hongre, and P. H. Roberts, 1999, Nature, 401, 885; McFadden, P.L., R.T. Merrill, M.W. McElhinny, and S. Lee, J. Geophy. Res., 96, 3923, 1991; Merrill, R.T. and P.L. McFadden, J. Geophy. Res., 93, 11589, 1988.)
DE: 1507 Core processes (1213, 8115)
DE: 1510 Dynamo: theories and simulations
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 1560 Time variations: secular and longer
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005