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

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!

10. Critical Rayleigh number for magnetoconvection E=10-9

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

14. 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

19. 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

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

25. VGP PATHS

27. 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)

31. 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