Inner core tilt and polar motion

Dumberry, Mathieu; Bloxham, Jeremy

doi:10.1046/j.1365-246x.2002.01756.x

Cited by 49 publications

(85 citation statements)

References 64 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…5). A wide range of values for the viscosity of the inner core have been estimated, ranging from 10 11 to 10 22 Pa s (Dumberry & Bloxham 2002;Van Orman 2004;Reaman et al 2011). The most recent estimates of 10 17 Pa s and 10 15 -10 18 Pa s come from length of day variations and from mineral physics experiments (Gleason & Mao 2013), respectively.…”

Section: Discussionmentioning

confidence: 99%

The feasibility of thermal and compositional convection in Earth's inner core

Lythgoe¹,

Rudge²,

Neufeld

et al. 2015

Geophysical Journal International

View full text Add to dashboard Cite

S U M M A R YInner core convection, and the corresponding variations in grain size and alignment, has been proposed to explain the complex seismic structure of the inner core, including its anisotropy, lateral variations and the F-layer at the base of the outer core. We develop a parametrized convection model to investigate the possibility of convection in the inner core, focusing on the dominance of the plume mode of convection versus the translation mode. We investigate thermal and compositional convection separately so as to study the end-members of the system. In the thermal case the dominant mode of convection is strongly dependent on the viscosity of the inner core, the magnitude of which is poorly constrained. Furthermore recent estimates of a large core thermal conductivity result in stable thermal stratification, hindering convection. However, an unstable density stratification may arise due to the pressure dependant partition coefficient of certain light elements. We show that this unstable stratification leads to compositionally driven convection, and that inner core translation is likely to be the dominant convective mode due to the low compositional diffusivity. The style of convection resulting from a combination of both thermal and compositional effects is not easy to understand. For reasonable parameter estimates, the stabilizing thermal buoyancy is greater than the destabilizing compositional buoyancy. However we anticipate complex double diffusive processes to occur given the very different thermal and compositional diffusivities.

show abstract

Section: Discussionmentioning

confidence: 99%

The feasibility of thermal and compositional convection in Earth's inner core

Lythgoe¹,

Rudge²,

Neufeld

et al. 2015

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…The values for the magnetic field components at the ICB are chosen to match Dumberry & Bloxham (2002); both the ICB and CMB dipole field strengths are in agreement with constraints obtained from analyses of tidally forced nutation series (Mathews et al 2002). Relaxation of the inner-core shape is also included in the model with characteristic relaxation times, ϒ, of either 1 or 100 yr.…”

Section: Resu Lt Smentioning

confidence: 97%

Electromagnetic torques in the core and resonant excitation of decadal polar motion

Mound¹

2017

Preprint

View full text Add to dashboard Cite

S U M M A R YMotion of the rotation axis of the Earth contains decadal variations with amplitudes on the order of 10 mas. The origin of these decadal polar motions is unknown. A class of rotational normal modes of the core-mantle system termed torsional oscillations are known to affect the length of day (LOD) at decadal periods and have also been suggested as a possible excitation source for the observed decadal polar motion. Torsional oscillations involve relative motion between the outer core and the surrounding solid bodies, producing electromagnetic torques at the inner-core boundary (ICB) and core-mantle boundary (CMB). It has been proposed that the ICB torque can explain the excitation of the approximately 30-yr-period polar motion termed the Markowitz wobble. This paper uses the results of a torsional oscillation model to calculate the torques generated at Markowitz and other decadal periods and finds, in contrast to previous results, that electromagnetic torques at the ICB can not explain the observed polar motion.

show abstract

“…These free precessions may participate in the polar motion if they are excited by time-dependent torques on the inner core. For a forcing at decade timescale, resonant excitation of the ICW may increase the amplitude of polar motion (Dumberry & Bloxham 2002). We allow viscoelastic inner core deformations in our formulation by including a linear decay term to eq.…”

Section: Angular Momentum Dynamicsmentioning

confidence: 99%

“…17 and fig. 2(c) of Dumberry & Bloxham (2002)]. This difference arises as a consequence of the role of the fluid core in the generation of the torque.…”

Section: Static Angular Momentum Balancementioning

confidence: 99%

“…Electromagnetic stress at the ICB results predominantly from the shear of the radial magnetic field by horizontal flows. Assuming a purely axisymmetric zonal flow in the core (representing torsional oscillations), a non-rotating inner core and a non-axisymmetric magnetic field at the ICB, Dumberry & Bloxham (2002) concluded that an electromagnetic torque of the correct magnitude (10 20 N m) is possible. However, strong electromagnetic coupling in the axial direction tend to prevent differential velocity at the ICB: the free modes of torsional oscillations inside the tangent cylinder are restricted to a rigid body rotation and the inner core is axially rotating at the same angular velocity.…”

Section: O N C L U S I O Nmentioning

confidence: 99%

See 1 more Smart Citation

Gravitational torque on the inner core and decadal polar motion

Dumberry¹

2008

Geophysical Journal International

Self Cite

View full text Add to dashboard Cite

S U M M A R YA decadal polar motion with an amplitude of approximately 25 milliarcsecs (mas) is observed over the last century, a motion known as the Markowitz wobble. The origin of this motion remains unknown. In this paper, we investigate the possibility that a time-dependent axial misalignment between the density structures of the inner core and mantle can explain this signal. The longitudinal displacement of the inner core density structure leads to a change in the global moment of inertia of the Earth. In addition, as a result of the density misalignment, a gravitational equatorial torque leads to a tilt of the oblate geometric figure of the inner core, causing a further change in the global moment of inertia. To conserve angular momentum, an adjustment of the rotation vector must occur, leading to a polar motion. We develop theoretical expressions for the change in the moment of inertia and the gravitational torque in terms of the angle of longitudinal misalignment and the density structure of the mantle. A model to compute the polar motion in response to time-dependent axial inner core rotations is also presented. We show that the polar motion produced by this mechanism can be polarized about a longitudinal axis and is expected to have decadal periodicities, two general characteristics of the Markowitz wobble. The amplitude of the polar motion depends primarily on the Y 1 2 spherical harmonic component of mantle density, on the longitudinal misalignment between the inner core and mantle, and on the bulk viscosity of the inner core. We establish constraints on the first two of these quantities from considerations of the axial component of this gravitational torque and from observed changes in length of day. These constraints suggest that the maximum polar motion from this mechanism is smaller than 1 mas, and too small to explain the Markowitz wobble.

show abstract

Inner core tilt and polar motion

Cited by 49 publications

References 64 publications

The feasibility of thermal and compositional convection in Earth's inner core

The feasibility of thermal and compositional convection in Earth's inner core

Electromagnetic torques in the core and resonant excitation of decadal polar motion

Gravitational torque on the inner core and decadal polar motion

Contact Info

Product

Resources

About