The effect of electromagnetic attenuation on the spatial spectrum of the geomagnetic field.

McQueen, Herbert; Hayes, L. M.; Conley, Daniel

doi:10.5636/jgg.35.103

Cited by 1 publication

(1 citation statement)

References 22 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…There is, however, some observational evidence on the time scales for change of the whole core field. McQueen et al (1983), using data from Barraclough et al (1975), have calculated the square roots of the ratios of the mean field energy to the mean secular variation energy as a function of harmonic degree, averaged over the period 1942.5 to 1980. The results are listed in table 6 .…”

Section: Free Decay Times and Observed Time Scalesmentioning

confidence: 99%

The dynamics of the Earth's inner and outer cores

Smylie¹,

Szeto²,

Rochester³

1984

Rep. Prog. Phys.

View full text Add to dashboard Cite

A brief historical introduction to the study of the Earth's core is followed by a review of recent advances in the dynamics of the liquid outer core and the rotation of the solid inner body. In particular, the scaling of the fluid motion equations is reviewed and a detailed derivation of the 'subseismic equation' which governs small-amplitude, low-frequency oscillations is given following methods first outlined by Rochester. Attention is given to the compressibility of the outer core and its role in dynamo theory as well as its effect on Proudman-Taylor flow, where it produces a fluid motion with helicity. The theorem of J B Taylor is also generalised to real core compressibility. Rotational motions of the inner core are examined under the strong gravity torque exerted on it by the rest of the Earth. Both the motion ignoring back reaction on the mantle's rotation and the full coupled problem are solved. The gravity torque is shown to be five orders of magnitude larger than inertial coupling which was previously considered dominant. The 'rapid' regular precessional mode which is nearly diurnal in space, largely ignored in the modern literature, may play a role in the time evolution of the dipole part of the main magnetic field. Inner-core behaviour during field reversals remains to be investigated. Energy considerations are shown to require nearly equilibrated electromagnetic torques on the inner core and mantle if they are to have differential rotations with respect to the outer core. For steady dynamos they must vanish identically. A variational principle for the induction equation is established which allows the constraints of vanishing torques to be introduced via Lagrange multipliers. It will also permit the abandonment of the traditional spherical vector harmonics as approximating functions, which are notoriously poorly convergent, in favour of finite elements. It is anticipated that torque constraints will limit the size of toroidal fields in the outer core but re-solution is required.

show abstract

Section: Free Decay Times and Observed Time Scalesmentioning

confidence: 99%