Boundary Layers & Magnetic Fields: Observations Catherine L. Johnson Department of Earth and Ocean Sciences University of British Columbia, Vancouver A Few Overview References - Treatise on Geophysics, 2007 Core energetics, Nimmo, vol 8, ch. 2 (sections 2,4,5) Core-mantle interactions: Buffett, vol 8, ch. 12 Comparison w/ dynamo simulations: Christensen, vol 8, ch. 8, secn 4 Secular variation and historical field: Jackson & Finlay, vol 5, ch. 5 Paleofield (kyr time scales): Constable, vol 5, ch. 9 Paleofield (Myr time scales): Johnson & McFadden, vol 5, ch. 11

Boundary layers & dynamos Mantle convective style: relationship of Qcmb to Qad Inner core solidification: latent heat and compositional buoyancy 3. Radioactivity in the core from Buffet, 2007 conduction: Moon - no dynamo remanent field stagnant-lid convection Mars - no dynamo, remanent field Venus - no dynamo, no remanence Mercury - likely dynamo plate tectonics Earth - dynamo

Boundary Layers and the Geodynamo Are there observable geomagnetic diagnostics of boundary conditions? qcmb (mean and spatial variations), IC growth => time-averaged morphology and intensity, TAF => (paleo) secular variation, (P)SV => reversal rates & field structure during a reversal Are there observational constraints useful for building models of the geodynamo?

Boundary Layers and the Geodynamo Are there observable geomagnetic diagnostics of boundary conditions? qcmb (mean and spatial variations), IC growth DATA: satellites: Bx, By, Bz since 1980 observatories, surveys: Bx, By, Bz centuries archaeomagnetic artefacts: |B|, direction kyr lake sediments, lavas: direction, |B| kyr lavas D, I, |B| Myr deep sea sediments I, relative |B|, D Myr Issues: temporal, spatial distribution, not full vector measurement, paleo-site position

Longevity and Mean Intensity of Earth’s Field 0 - 5 Ma 0 - 3.5 Ga 0 - 160 Ma from Tauxe and Yamazaki, 2007 Since 3.5 Ga Intensity variations on both long and short time scales

Global Field Models TAF: Regularized inversions for SH coefficients (Guy - next week) Linear - Bx, By, Bz; non-linear - D, I, |B| PSV: Forward model statistical distributions of glm, hlm Simulate distributions of observables: D, I, |B|, dispersion

Historical Geomagnetic Field Behavior (Centuries) Br in T at CMB: 1590-1990 time average Model gufm1 Jackson et al., 2000 Persistent high latitude flux lobe => thermal coupling? Low secular variation, sBr, in Pacific => EM or thermal coupling?

Paleo-Field Behavior: I (Millenia) DATA lake sediments archaeomag directions archaeomag intensity Model CALS7K.2 Korte & Constable, 2005 Br in T at CMB: 0 - 7 ka time average Persistent high latitude flux lobes?

Paleo-Field Behavior: I (Millions of Years) Models: time-averaged b/c limited temporal information Johnson & Constable, 1995 Kelly & Gubbins, 1997 Johnson & Constable, 1997 Historical Field: Jackson et al., 2000

Records of Reversals Br at CMB: UFM1: 1840-1980 If ULVZ patches are metallic or metal-rich then during reversals, as dipole field decays currents are generated in patches resulting in a secondary field. Costin and Buffett (2003, submitted): Geometry of secondary field, generated in metallic or metal-rich patches during reversals and superposed onto transitional field due to dynamo, can govern VGP paths as observed. Confined VGP paths due to thermal or electromagnetic coupling? e.g., Costin & Buffett, 2004

Paleofield Models: Next Generation…. Continuous global models for 0 - 2 Ma: temporal evolution, spectrum, statistics Reconcile / Merge different data types Time series of direction and relative intensity Large collection of new data from volcanics Calibration, spectrum