geomagnetism: a dynamo at the centre of the earth

GEOMAGNETISM: a dynamo at the centre of the Earth

Lecture 4

Thermal Core-Mantle Interaction

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

The core-mantle boundary

Thermal Core-Mantle Interaction

(hot)

(cold)

•Downwelling below cold mantle concentrates flux

•Upwelling below hot mantle disperses flux

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!

Critical Rayleigh number for magnetoconvection

E=10-9

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

The Cylinder

Z2 on core surface 1980

GEOMAGNETIC FIELD AND LOWER MANTLE VS AS FUNCTIONS OF LONGITUDE

THE TIME-AVERAGED PALEOMAGNETIC FIELD LAST 5Ma

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

Transitions: reversals and excursions(Love 1998)

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

VGP PATHS

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

Matuyama-Brunhes, after Love & Mazau (1997)

Matuyama-Brunhes VGPsmodified

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