Boundary Layers & Magnetic Fields: Observations Catherine L. Johnson Department of Earth and Ocean...
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Transcript of Boundary Layers & Magnetic Fields: Observations Catherine L. Johnson Department of Earth and Ocean...
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
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
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