HR: 0800h
AN: G41C-0365    [Abstracts]
TI: A Six Year Oscillation in Length-of-Day: Signature of an Inner Core Normal Mode
AU: * Mound, J
EM: mound@geophysics.harvard.edu
AF: Harvard University, Dept. Earth & Planetary Sciences, Cambridge, MA 02138 United States
AU: Buffett, B
EM: buffett@geosci.uchicago.edu
AF: University of Chicago, Dept. of Geophysical Sciences, Chicago, IL 60637 United States
AB: An approximately 6 yr oscillation has been detected in high resolution time series of length-of-day (LOD) that span the last four decades. The LOD time series has been corrected for the effects of angular momentum exchange with the atmosphere and oceans; therefore, the remaining LOD variations can be attributed to interaction between the mantle and the core. Torsional oscillations of the fluid core are well known to be associated with LOD changes on decadal timescales; however, they can not explain the distinct interannual peak observed in the LOD spectrum. The inner core and mantle are gravitational coupled due to their aspherical density distributions. Misalignment of those density fields produces a restoring torque resulting in a normal of oscillation. The period of this mantle-inner core gravitational normal mode depends on the amplitude of density heterogeneity, which is characterised by a coupling constant. The gravitational coupling constant required to match the observed LOD oscillation falls within the range expected from models of mantle flow and can be used to further constrain such models. Detection of the mantle-inner core gravitational coupling normal mode places a lower bound on the viscosity of the inner core. A sufficiently soft inner core would respond by viscous deformation rather than rotation; the observed 6 yr LOD oscillation implies that the characteristic relaxation time of the inner core is on the order of years or longer, corresponding to a viscosity of at least 10¹⁷ N m.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 1239 Earth rotation variations
DE: 1507 Core processes (1213, 8115)
SC: Geodesy [G]
MN: Fall Meeting 2005