Title: GEOMAGNETISM: a dynamo at the centre of the Earth
1GEOMAGNETISM a dynamo at the centre of the Earth
- Lecture 4
- Thermal Core-Mantle Interaction
2OVERVIEW
- 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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5The core-mantle boundary
6Thermal Core-Mantle Interaction
(hot)
(cold)
7- Downwelling below cold mantle concentrates flux
- Upwelling below hot mantle disperses flux
8Lateral 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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10Critical Rayleigh number for magnetoconvection
E10-9
11DYNAMO 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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14The Tangent Cylinder
15Z2 on core surface 1980
16GEOMAGNETIC FIELD AND LOWER MANTLE VS AS
FUNCTIONS OF LONGITUDE
17THE TIME-AVERAGED PALEOMAGNETIC FIELD LAST 5Ma
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19Reversal 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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21Transitions reversals and excursions(Love 1998)
22REVERSAL 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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25VGP PATHS
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27Reversal 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
28Matuyama-Brunhes, after Love Mazau (1997)
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31Matuyama-Brunhes VGPsmodified
32CONCLUSIONS
- 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