GEOMAGNETISM: a dynamo at the centre of the Earth

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GEOMAGNETISM a dynamo at the centre of the Earth

• Lecture 4
• Thermal Core-Mantle Interaction

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OVERVIEW

• Lateral variations in temperature in the lower
  mantle determine the heat flux across the
  core-mantle boundary
• this affects core convection
• e.g. by promoting downwelling beneath cold
  mantle, upwelling beneath hot regions...
• ...this affects the magnetic field
• e.g. by downwelling concentrating magnetic flux,
  upwelling dispersing it

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The core-mantle boundary
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Thermal Core-Mantle Interaction
(hot)
(cold)
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• Downwelling below cold mantle concentrates flux
• Upwelling below hot mantle disperses flux

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Lateral variations in heat flux across the CMB
can

• Drive thermal winds
• lock core convection
• Force a lateral scale on the core convection
• none of the above!

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Critical Rayleigh number for magnetoconvection
E10-9
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DYNAMO CATASTROPHE

• The Rayleigh number is fixed
• The critical Rayleigh number depends on field
  strength
• Vigour of convection varies with supercritical
  Ra
• So does the dynamo action
• If the magnetic field drops, so does the vigour
  of convection, so does the dynamo action
• The dynamo dies

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The Tangent Cylinder
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Z2 on core surface 1980
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GEOMAGNETIC FIELD AND LOWER MANTLE VS AS
FUNCTIONS OF LONGITUDE
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THE TIME-AVERAGED PALEOMAGNETIC FIELD LAST 5Ma
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Reversal transition paths (1)

• Many reversal transition fields have now been
  captured in lava flows and sediments
• These are plotted for each site by mapping into a
  virtual geomagnetic pole
• The VGPs trace a path from north to south or
  south to north that depends on the site and the
  transition field
• Many paths tend to lie in the Americas or in
  Asia, the other half of a great circle

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Transitions reversals and excursions(Love 1998)
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REVERSAL SCHEME OF GUBBINS SARSON (1994)

• Small changes in fluid flow change a steady field
  to an oscillatory one, a dynamo wave
• Flows generating oscillatory solutions are rare
• Fluctuations in fluid flow can move the dynamo
  between steady and oscillatory regimes
• Both oscillatory and steady fields have flux
  concentrated by downwellings induced by cold
  mantle
• The oscillatory fields give persistent VGP paths
  lying close to the longitudes of downwelling

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VGP PATHS
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Reversal transition paths (2)

• Persistent path selection requires departures
  from axial symmetry, as in the boundary
  conditions
• The path is site-dependent unless the transition
  field is a perfect axial dipole
• If some sites record an American path and some an
  Asian path for the same transition, other sites
  could be expected to record neither path
• A site recording an Americas path for a
  normal-reverse transition must record an Asian
  path for a reverse-normal transition

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Matuyama-Brunhes, after Love Mazau (1997)
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Matuyama-Brunhes VGPsmodified
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CONCLUSIONS

• Thermal interactions can influence core
  convection and the magnetic field.
• The same mechanism of flux concentration can
• explain the present-day morphology
• produce a non-axisymmetric time average
• explain persistent VGP reversal transition paths
• explain persistent low secular variation in the
  Pacific