Boundary Layers & Magnetic Fields: Observations

Catherine L. JohnsonDepartment of Earth and Ocean Sciences

University of British Columbia, Vancouver

A Few Overview References - Treatise on Geophysics, 2007

1. Core energetics, Nimmo, vol 8, ch. 2 (sections 2,4,5)

2. Core-mantle interactions: Buffett, vol 8, ch. 123. Comparison w/ dynamo simulations: Christensen, vol

8, ch. 8, secn 44. Secular variation and historical field: Jackson &

Finlay, vol 5, ch. 55. Paleofield (kyr time scales): Constable, vol 5,

ch. 9 6. Paleofield (Myr time scales): Johnson & McFadden,

vol 5, ch. 11

Boundary layers & dynamos

1. Mantle convective style: relationship of Qcmb to Qad

2. Inner core solidification: latent heat and compositional buoyancy3. Radioactivity in the core

plate tectonicsEarth - dynamo

stagnant-lid convection Mars - no dynamo, remanent fieldVenus - no dynamo, no remanenceMercury - likely dynamo

conduction: Moon - no dynamoremanent field

from Buffet, 2007

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 kyrlake sediments, lavas: direction, |B| kyrlavas D, I, |B| Myrdeep 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

1. Since 3.5 Ga2. Intensity variations on both long and short time

scales

0 - 3.5 Ga

0 - 160 Ma

0 - 5 Ma

from Tauxe and Yamazaki, 2007

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, hl

m

Simulate distributions of observables: D, I, |B|, dispersion

Global Field Models

Persistent high latitude flux lobe => thermal coupling?

Low secular variation, Br, in Pacific => EM or thermal coupling?

Historical Geomagnetic Field Behavior (Centuries)

Br in T at CMB: 1590-1990 time average

Model gufm1Jackson et al., 2000

Persistent high latitude flux lobes?

Paleo-Field Behavior: I (Millenia)

Model CALS7K.2Korte & Constable, 2005

Br in T at CMB: 0 - 7 ka time average

DATA

lake sediments

archaeomag directions

archaeomag intensity

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Paleo-Field Behavior: I (Millions of Years)

Models: time-averaged b/c limited temporal information

Johnson & Constable, 1995Kelly & Gubbins, 1997

Johnson & Constable, 1997Historical Field: Jackson et al., 2000

Records of Reversals

Br at CMB: UFM1: 1840-1980

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 1. Time series of direction and relative intensity2. Large collection of new data from volcanics