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GEOMAGNETISM: a dynamo at the centre of the Earth

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OVERVIEW Lateral variations in temperature in the lower mantle determine the heat flux across the ... Persistent path selection requires departures from axial ... – PowerPoint PPT presentation

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Title: GEOMAGNETISM: a dynamo at the centre of the Earth


1
GEOMAGNETISM a dynamo at the centre of the Earth
  • Lecture 4
  • Thermal Core-Mantle Interaction

2
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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5
The core-mantle boundary
6
Thermal Core-Mantle Interaction
(hot)
(cold)
7
  • Downwelling below cold mantle concentrates flux
  • Upwelling below hot mantle disperses flux

8
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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10
Critical Rayleigh number for magnetoconvection
E10-9
11
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
15
Z2 on core surface 1980
16
GEOMAGNETIC FIELD AND LOWER MANTLE VS AS
FUNCTIONS OF LONGITUDE
17
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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21
Transitions reversals and excursions(Love 1998)
22
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

28
Matuyama-Brunhes, after Love Mazau (1997)
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Matuyama-Brunhes VGPsmodified
32
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
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