Mechanisms of core‐mantle angular momentum exchange and the observed spectral properties of torsional oscillations

Abstract

Torsional oscillations of the fluid core produce decadal variations in the Earth's magnetic field and in the length of day. The magnetic field variations can be used to infer the fluid velocity of the oscillations. An additional constraint is imposed by the length‐of‐day variations because coupling mechanisms are required to transfer angular momentum between the core and mantle. Proposed mechanisms included electromagnetic, topographic, and gravitational couplings. We make use of a finite volume model of the core‐mantle system to evaluate each of these mechanisms with respect to observed spectral properties of torsional oscillations. We find that the different coupling mechanisms have different spectral characters which may be useful in distinguishing their importance within the Earth's core‐mantle system. For reasonable values of core and mantle properties, gravitational coupling between the mantle and inner core is the candidate mechanism most likely to be able to reproduce all spectral features of the observed transfer of angular momentum between the core and mantle. Possible spatial distributions of the torsional oscillation excitation source are also investigated.

Core–Mantle Interactions

“…One factor of h arises because the perturbation depends on the height of the topography; the second factor of h arises because the integral over the surface for the pressure torque depends on the presence of topography. For boundary topography of a few kilometers at large spatial scales, the resulting pressure torque is probably too small to explain the observed variations in the length of day (e.g., Mound and Buffett, 2005). Similar conclusions have been obtained using numerical geodynamo models that include the influence of boundary topography (Kuang and Chao, 2001).…”

“…Tests of this mechanism in numerical geodynamo models yield a torque on the mantle that is sufficient to explain the variations in the length of day (Aubert and Dumberry, 2010;Buffett and Glatzmaier, 2000). It is also possible to explain the length-ofday variations when the fluid motions in the outer core are restricted to the form of torsional oscillations (Dumberry and Mound, 2010;Mound and Buffett, 2005). These oscillations are of interest because the typical period is thought to be compatible with a nominal 60-year variation in the length of day (Braginsky, 1970(Braginsky, , 1984.…”

The Magnetic Field of Planet Earth

“…If such torques arise, the cylinders will start accelerating. This a mechanism by which the core and the mantle can exchange axial angular momentum on short time scales (Braginsky 1970(Braginsky , 1984Jault and Le Mouël 1991a), with the solid inner core possibly also playing an important role (Mound and Buffett 2005). Because the surface expression of such cylindrical accelerations must show up as part of the estimated core surface flows, those can be used to infer the amount of axial angular momentum the core is exchanging with the mantle (Jault et al 1988).…”

Earth Rotation Variations – Long Period

“…If the core-mantle boundary is not smooth but exhibits undulations or 'bumps,' then the flow of the core fluid can exert a torque on the mantle due to the fluid pressure acting on the boundary topography (Asari et al, 2006;Buffett, 1998;Hide, 1969Hide, , 1977Hide, , 1989Hide, , 1993Hide, , 1995aHide et al, 1993;Hinderer et al, 1990;Jault and Le Mouël, 1989Jault et al, 1996;Kuang and Bloxham, 1993;Kuang and Chao, 2001;Mound and Buffett, 2005). The strength of this topographic coupling, a mechanism first suggested by Hide (1969), depends on the amplitude of the topography at the coremantle boundary.…”

“…Electromagnetic torques arise from the interaction between the magnetic field within the core and the flow of electric currents in the weakly conducting mantle that are induced both by time variations of the magnetic field and by diffusion of electric currents from the core to the mantle (Bullard et al, 1950;Dumberry and Mound, 2008;Holme, 1998aHolme, ,b, 2000Jault and Le Mouël, 1991;Love and Bloxham, 1994;Mound and Buffett, 2005;Nakada, 2006Nakada, , 2009aNakada, , 2011Roberts, 1972;Rochester, 1960Rochester, , 1962Roden, 1963;Stewart et al, 1995;Stix and Roberts, 1984;Jault, 1999, 2000). The strength of this electromagnetic torque, a mechanism first suggested by Bullard et al (1950), depends both on the conductivity of the mantle and on the strength of the magnetic field crossing the core-mantle boundary.…”