Liquid Metal MHD and the Geodynamo

  • H. K. Moffatt
Part of the Mechanics of Fluids and Transport Processes book series (MFTP, volume 10)


The magnetic field of the Earth is generated by dynamo action associated with the upwelling of buoyant material in the liquid outer core. It is argued that this upwelling occurs in the form of mushroom-shaped blobs of material released from the mushy zone at the inner core boundary (ICB), and having a very small density defect δρ/ρ. The rise of buoyant material with velocity w is compensated by the slow rate of growth of the solid inner core. The resulting mass balance, combined with approximate geostrophic force balance in the core leads to estimates
$$\delta \rho /\rho \sim 3 \times {10^{ - 9}},w \sim 2 \times {10^{ - 4}}m/s.$$
Each rising blob drives a Taylor column, and the helicity and α-effect associated with this flow is estimated. A mean-field dynamo driven by this α-effect in conjunction with differential rotation generates a magnetic field whose strength is determined in order of magnitude by the plausible assumption of magnetostrophic equilibrium.


Inner Core Mushy Zone Differential Rotation Liquid Core Magnetic Reynolds Number 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. [1]
    BRAGINSKII S.I., 1963 ‘Structure of the F-layer and reasons for convection in the Earth’s core’, Dokl. Akad. Nank SSSR 149 8.Google Scholar
  2. [2]
    LOPER D.E. and ROBERTS P.H., 1978 ‘On the motion of an iron-alloy core, containing a slurry I General theory’, Geophys. Astrophys. Fluid Dyn. 9 289.ADSzbMATHCrossRefGoogle Scholar
  3. [3]
    GUBBINS D., 1976 ‘Observational constraints on the generation process of the Earth’s magnetic field’, Geophys.J. 47 19.ADSCrossRefGoogle Scholar
  4. [4]
    GUBBINS D., 1987 ‘Mechanisms for geomagnetic polarity reversals’, Nature 326, 167.ADSCrossRefGoogle Scholar
  5. [5]
    MOFFATT H.K., 1978 Magnetic Field Generation in Electrically Conducting Fluids,Cambridge University Press, 47 et seq.Google Scholar
  6. [6]
    STEENBECK, M., KRAUSE, F., and RADLER K-H, 1966 ‘A calculation of the mean electromotive force in an electrically conducting fluid in turbulent motion, under the influence of Coriolis forces’. Z. Naturforsch. 21a, 369 [in German].ADSGoogle Scholar
  7. [7]
    MOFFATT H.K., 1970 ‘Turbulent dynamo action at low magnetic Reynolds number’, J.Fluid Mech. 41, 435.ADSzbMATHCrossRefGoogle Scholar
  8. [8]
    KRAUSE F. and RADLER K-H, 1979 Mean-field Magnetohydrodynamics and Dynamo Theory. Pergamon Press.Google Scholar
  9. [9]
    LOPER D.E. and ROBERTS P.H., 1981 ‘A study of conditions at the inner core boundary of the earth’. Phys. Earth Planet. Inter. 24, 302.ADSCrossRefGoogle Scholar
  10. [10]
    HOWARD L.N., 1966 ‘Convection at high Rayleigh number’ in Applied Mechanics (Proc. XIth Int. Cong. Appl. Mech. Munich 1964, Ed. H. G6rtler ) 1109.Google Scholar
  11. [11]
    GREENSPAN H.P., 1968 The Theory of Rotating Fluids,Cambridge University Press.Google Scholar
  12. [12]
    HUNT J.C.R. and HUSSAIN A.K.M.F., 1988 ‘A note on velocity, vorticity and helicity of fluid elements and fluid volumes’. Unpublished.Google Scholar
  13. [13]
    FEARN D.R., LOPER D.E. and ROBERTS P.H., 1981 ‘Structure of the Earth’s inner core’. Nature 292–232.Google Scholar
  14. [14]
    LOPER D.E. 1983 ‘Structure of the inner core boundary’, Geoph. Astrophys. Fluid Dyn. 25, 139.ADSzbMATHCrossRefGoogle Scholar

Copyright information

© Kluwer Academic Publishers 1989

Authors and Affiliations

  • H. K. Moffatt
    • 1
  1. 1.Department of Applied Mathematics and Theoretical PhysicsCambridgeUK

Personalised recommendations