Earth’s Magnetic Field
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Transcript of Earth’s Magnetic Field
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Earth’s Magnetic Field• Introduction
– Earth’s structure• Observations
– Magnetic observatories– Satellites– Dedicated field campaigns
• The external field– Source field for studies of the electrical
conductivity at crustal and mantle levels • The crustal field• The core field• Time variations
• Paleomagnetic observations• Secular variations• Satellite observations
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Earth’s Magnetic Field
• The Geodynamo– Governing equations– Approximations– Simulations
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Earth’s Magnetic Field
• Crustal sources for the magnetic field– Remanent magnetization– Induced magnetization– Relation to past and ongoing processes
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Volume 5: Geomagnetism
5.01 Geomagnetism in Perspective, Pages 1-31, M. KonoSummaryPlus | Chapter | PDF (2904 K) | View Related Articles 5.02 The Present Field, Pages 33-75, N. Olsen, G. Hulot and T.J. SabakaSummaryPlus | Chapter | PDF (14345 K) | View Related Articles 5.03 Magnetospheric Contributions to the Terrestrial Magnetic Field, Pages 77-92, W. Baumjohann and R. NakamuraSummaryPlus | Chapter | PDF (684 K) | View Related Articles 5.04 Observation and Measurement Techniques, Pages 93-146, G.M. Turner, J.L. Rasson and C.V. ReevesSummaryPlus | Chapter | PDF (2319 K) | View Related Articles 5.05 Geomagnetic Secular Variation and Its Applications to the Core, Pages 147-193, A. Jackson and C.C. FinlaySummaryPlus | Chapter | PDF (7793 K) | View Related Articles 5.06 Crustal Magnetism, Pages 195-235, M.E. Purucker and K.A. WhalerSummaryPlus | Chapter | PDF (4208 K) | View Related Articles 5.07 Geomagnetism, Pages 237-276, S. ConstableSummaryPlus | Chapter | PDF (1692 K) | View Related Articles 5.08 Magnetizations in Rocks and Minerals, Pages 277-336, D.J. Dunlop and Ö. ÖzdemirSummaryPlus | Chapter | PDF (3105 K) | View Related Articles 5.09 Centennial- to Millennial-Scale Geomagnetic Field Variations, Pages 337-372, C. ConstableSummaryPlus | Chapter | PDF (3744 K) | View Related Articles 5.10 Geomagnetic Excursions, Pages 373-416, C. Laj and J.E.T. ChannellSummaryPlus | Chapter | PDF (1692 K) | View Related Articles 5.11 Time-Averaged Field and Paleosecular Variation, Pages 417-453, C.L. Johnson and P. McFaddenSummaryPlus | Chapter | PDF (3723 K) | View Related Articles 5.12 Source of Oceanic Magnetic Anomalies and the Geomagnetic Polarity Timescale, Pages 455-507, J.S. Gee and D.V. KentSummaryPlus | Chapter | PDF (3684 K) | View Related Articles 5.13 Paleointensities, Pages 509-563, L. Tauxe and T. YamazakiSummaryPlus | Chapter | PDF (2882 K) | View Related Articles 5.14 True Polar Wander: Linking Deep and Shallow Geodynamics to Hydro- and Bio-Spheric Hypotheses, Pages 565-589, T.D. Raub, J.L. Kirschvink and D.A.D. EvansSummaryPlus | Chapter | PDF (1558 K) | View Related Articles
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Volume 8: Core Dynamics
8.01 Overview, Pages 1-30, P. OlsonSummaryPlus | Chapter | PDF (1025 K) | View Related Articles 8.02 Energetics of the Core, Pages 31-65, F. NimmoSummaryPlus | Chapter | PDF (842 K) | View Related Articles 8.03 Theory of the Geodynamo, Pages 67-105, P.H. RobertsSummaryPlus | Chapter | PDF (945 K) | View Related Articles 8.04 Large-Scale Flow in the Core, Pages 107-130, R. HolmeSummaryPlus | Chapter | PDF (2755 K) | View Related Articles 8.05 Thermal and Compositional Convection in the Outer Core, Pages 131-185, C.A. JonesSummaryPlus | Chapter | PDF (1763 K) | View Related Articles 8.06 Turbulence and Small-Scale Dynamics in the Core, Pages 187-206, D.E. LoperSummaryPlus | Chapter | PDF (365 K) | View Related Articles 8.07 Rotational Dynamics of the Core, Pages 207-243, A. TilgnerSummaryPlus | Chapter | PDF (2784 K) | View Related Articles 8.08 Numerical Dynamo Simulations, Pages 245-282, U.R. Christensen and J. WichtSummaryPlus | Chapter | PDF (1868 K) | View Related Articles 8.09 Magnetic Polarity Reversals in the Core, Pages 283-297, G.A. Glatzmaier and R.S. CoeSummaryPlus | Chapter | PDF (2692 K) | View Related Articles 8.10 Inner-Core Dynamics, Pages 299-318, I. Sumita and M.I. BergmanSummaryPlus | Chapter | PDF (453 K) | View Related Articles 8.11 Experiments on Core Dynamics, Pages 319-343, P. Cardin and P. OlsonSummaryPlus | Chapter | PDF (1731 K) | View Related Articles 8.12 Core–Mantle Interactions, Pages 345-358, B.A. Buffett
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Magnetic Pattern of the Oceans
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Magnetic Lineations. Mars
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P-wave Velocity PerturbationMid-Mantle
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Shear Wave Velocity Perturbation. Base of Mantle
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Importance of Earth’s Magnetic Field
Earth’s magnetic field is necessary for life on Earth.The magnetic field protects us against the flow of charged particles from the sun and acts a kind of shield. Some researchers believe that evolution of life is accelerated during periods of weak magnetic fields, because this would enhance genetic changes – mutations.
The magnetic field on the continents and their shelves is used for prospecting after oil, gas and mineral deposits.
The interpretation of the magnetic field on the oceans had a major impact on the development of plate tectonics.
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The Geomagnetic Earth
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Sources of the Geomagnetic Field
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MAGSAT (left) and Oersted (right) Satellites
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German CHAMP satellite
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Örsted Satellite Orbit
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Magnetic Field Satellites
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Spherical Harmonic Representation of Magnetic Field
Internal source region
External source region
Source free region”Region of observations”
),(),( 1 nn
nn SrSrVVVn = degree
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Geomagnetic Spectrum
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Crustal Magnetic Fields
From Maus (2007)n = 100
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Curie Depth Antarctica
Crustal MagField
Crustal Thickness
Curie DepthHeat Flow
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Geomagnetic Jargong
Frontiers
Is Earth's magnetic field reversing?
Catherine Constable & Monika Korte
Earth and Planetary Science Letters 246 (2006) 1–16
Page 2
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Reversals are documented in the oceanic crust 170 My back.
Reversals have taken place on the average everty 250000 year during the past 20 My.
On the average the rotation poles and the magnetic poles coincide.
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Normal polarity
Reverse polarity
Age[My]
4 3 2 1 0 1 2 3 4
Variations in the magnetic field over a mid-ocean ridge
Lithosphere
Calculated magnetic field from the model of sea-floor spreading
Measured magnetic field across a mid-ocean ridge
Molten magma fills the gap, solidifies, cools below the Curie temperature (560oC) and becomes magnetized in the direction of the prevalent magnetic field
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The magnetization along a 42 m long core from the Pacific at 4415 m water depth
Inklination close to zero at the equator
The sedimentation varies between 1-5 cm/1000 år. The sediments contain small amounts of magnetite which constitute small magnets that direct themselves into the direction of the Earth magnetic field on their way through the water column
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The Gauss–Matuyama (2.58 Ma) reversal record of VGPs recorded in sediments deposited in Searles Lake, California (Glen et al., 1999b). Note the highly complex VGP path, with initial and final excursions in orange, multiple rapid oscillations in black, and main reversing phase including two large swings from high to equatorial latitudes in red.
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Difference between Örsted (2000) och Magsat (1980) measurements
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Tangential flow pattern in the outer core at the CM transition
Anticyclonic patches transporting oppositely directed magnetic flux, i.e. negative feedback.
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The Earth’s magnetic field is generated by electric currents in the outer liquid core, which mainly consists of iron
The iron in the core moves turbulently at speeds of about 20 km/y (i.e. 1 million times faster than the movements in the Earth’s mantle)
When the electrically conductive metal moves in the magnetic field, a new magnetic field is generated which may amplify the existing field
This self-amplifying effect is called the Geo-dynamo
GEO-DYNAMO
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Important Constraints on Models of the Geodynamo
•Westward drift of non-dipolar field•Excursions
– Frequency– Strength distribution
•Reversals– Frequency– Duration
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Aborted Reversal Simulation
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Spectra of characteristic length and timescales in core dynamics. MAC – Magnetic, Archimedean, Coriolis waves
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Snapshot of Magnetic Field
The field is sheared around the ‘tangent
cylinder to the inner-core equator
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Snapshots of a reversal
Three snapshots of a simulated magnetic field at 500 years before the mid-point in the dipole reversal, at the mid-point and at 500 years after the mid-point.
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Dynamo Simulations. Varying Heat Flow at CMB
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More Snapshots
Surface
CMB
PoloidalToroidalFields