*Earths Magnetic FieldIntroductionEarths structureObservationsMagnetic observatoriesSatellitesDedicated field campaignsThe external fieldSource field for studies of the electrical conductivity at crustal and mantle levels The crustal fieldThe core fieldTime variationsPaleomagnetic observationsSecular variationsSatellite observations
*Earths Magnetic FieldThe GeodynamoGoverning equationsApproximationsSimulations
*Earths Magnetic FieldCrustal sources for the magnetic fieldRemanent magnetizationInduced magnetizationRelation to past and ongoing processes
*Excellent References Treatise on Geophysics Copyright 2007 Elsevier B.V. All rights reserved. Shortcut URL to this page: http://www.sciencedirect.com/science/referenceworks/9780444527486 Editor-in-Chief:Gerald Schubert
*Volume 5: Geomagnetism
5.01Geomagnetism in Perspective, Pages 1-31, M. KonoSummaryPlus | Chapter | PDF (2904 K) | View Related Articles 5.02The Present Field, Pages 33-75, N. Olsen, G. Hulot and T.J. SabakaSummaryPlus | Chapter | PDF (14345 K) | View Related Articles 5.03Magnetospheric Contributions to the Terrestrial Magnetic Field, Pages 77-92, W. Baumjohann and R. NakamuraSummaryPlus | Chapter | PDF (684 K) | View Related Articles 5.04Observation and Measurement Techniques, Pages 93-146, G.M. Turner, J.L. Rasson and C.V. ReevesSummaryPlus | Chapter | PDF (2319 K) | View Related Articles 5.05Geomagnetic Secular Variation and Its Applications to the Core, Pages 147-193, A. Jackson and C.C. FinlaySummaryPlus | Chapter | PDF (7793 K) | View Related Articles 5.06Crustal Magnetism, Pages 195-235, M.E. Purucker and K.A. WhalerSummaryPlus | Chapter | PDF (4208 K) | View Related Articles 5.07Geomagnetism, Pages 237-276, S. ConstableSummaryPlus | Chapter | PDF (1692 K) | View Related Articles 5.08Magnetizations in Rocks and Minerals, Pages 277-336, D.J. Dunlop and . zdemirSummaryPlus | Chapter | PDF (3105 K) | View Related Articles 5.09Centennial- to Millennial-Scale Geomagnetic Field Variations, Pages 337-372, C. ConstableSummaryPlus | Chapter | PDF (3744 K) | View Related Articles 5.10Geomagnetic Excursions, Pages 373-416, C. Laj and J.E.T. ChannellSummaryPlus | Chapter | PDF (1692 K) | View Related Articles 5.11Time-Averaged Field and Paleosecular Variation, Pages 417-453, C.L. Johnson and P. McFaddenSummaryPlus | Chapter | PDF (3723 K) | View Related Articles 5.12Source 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 5.13Paleointensities, Pages 509-563, L. Tauxe and T. YamazakiSummaryPlus | Chapter | PDF (2882 K) | View Related Articles 5.14True 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
*Volume 8: Core Dynamics
8.01Overview, Pages 1-30, P. OlsonSummaryPlus | Chapter | PDF (1025 K) | View Related Articles 8.02Energetics of the Core, Pages 31-65, F. NimmoSummaryPlus | Chapter | PDF (842 K) | View Related Articles 8.03Theory of the Geodynamo, Pages 67-105, P.H. RobertsSummaryPlus | Chapter | PDF (945 K) | View Related Articles 8.04Large-Scale Flow in the Core, Pages 107-130, R. HolmeSummaryPlus | Chapter | PDF (2755 K) | View Related Articles 8.05Thermal and Compositional Convection in the Outer Core, Pages 131-185, C.A. JonesSummaryPlus | Chapter | PDF (1763 K) | View Related Articles 8.06Turbulence and Small-Scale Dynamics in the Core, Pages 187-206, D.E. LoperSummaryPlus | Chapter | PDF (365 K) | View Related Articles 8.07Rotational Dynamics of the Core, Pages 207-243, A. TilgnerSummaryPlus | Chapter | PDF (2784 K) | View Related Articles 8.08Numerical Dynamo Simulations, Pages 245-282, U.R. Christensen and J. WichtSummaryPlus | Chapter | PDF (1868 K) | View Related Articles 8.09Magnetic Polarity Reversals in the Core, Pages 283-297, G.A. Glatzmaier and R.S. CoeSummaryPlus | Chapter | PDF (2692 K) | View Related Articles 8.10Inner-Core Dynamics, Pages 299-318, I. Sumita and M.I. BergmanSummaryPlus | Chapter | PDF (453 K) | View Related Articles 8.11Experiments on Core Dynamics, Pages 319-343, P. Cardin and P. OlsonSummaryPlus | Chapter | PDF (1731 K) | View Related Articles 8.12CoreMantle Interactions, Pages 345-358, B.A. Buffett
*Magnetic Pattern of the Oceans
*Magnetic Lineations. Mars
*Figure 23. Magnetic field map of one-third of the Southern Hemisphere of Mars. Note the eastwest trending bands of strong anomalies. Reprinted with permission from Connerney JEP, Acuna MH, Wasilewski PJ et al. (1999). Magnetic lineations in the ancient crust of Mars. Science 284: 794798. Copyright 1999 AAAS.
*P-wave Velocity PerturbationMid-Mantle
*Shear Wave Velocity Perturbation. Base of Mantle
*Representative large-scale mantle tomography for S-wave velocity structure (Grand, 2002) near the base of the mantle. Note the long-wavelength patterns of high velocities beneath the circum-Pacific and low velocities beneath the central Pacific and Africa. 1% contours are shown, with dotted lines highlighting the internal variations in the two large-low-shear-velocity provinces. Variations of 3% are imaged by this and other models with similar spatial patterns. Note the contrast in scale length of predominant heterogeneities with the mid-mantle pattern in Figure 3
*Importance of Earths Magnetic Field Earths 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.
The Geomagnetic Earth
*CLOSE Figure 3. A pictorial representation of the electromagnetic environment of Earth. From Constable S and Constable C (2004b) Observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity. Geochemistry Geophysics Geosystems 5: Q01006 (doi:10.1029/2003GC000634
*Sources of the Geomagnetic Field
*Bearing this definition in mind, the sources of the Earths magnetic field can then be understood asfalling into two main categories: magnetised media and electric currents. Obviously, magnetised sourcescan only be found inside the solid Earth. They occur in the form of rocks which have been magnetisedin the past (permanent magnetisation), but which also bear an additional magnetisation proportional tothe present ambient magnetic field (induced magnetisation). Clearly also, such rocks can only be foundin regions of the solid Earth, where the temperature is less than the Curie temperature of the mineralsultimately carrying the magnetisation. This restricts magnetised rocks to lie in the uppermost layers ofthe Earth. All other sources of the Earths magnetic field are electric currents. Those can be found inmost regions of the Earth: inside the metallic core, in the mantle and crust, in the oceans, and finallyabove the neutral atmosphere, in the ionosphere and magnetosphere (cf. Figure 1).
*MAGSAT (left) and Oersted (right) Satellites
*German CHAMP satellite
*rsted Satellite Orbit
*Left: Observatories providing data for the years 19952004 (red dots), and ground track of 24 hours of the rsted satellite on January 2, 2001 (blue curve). The satellite starts at 00UT at 57S and 72E, moves northward on the morning side of the Earth, and crosses the Equator at 58E (large black arrow). It continues crossing the polar cap, and moves southward on the evening side; 50min after the first equator crossing, the satellite crosses again the equator, at 226E (white arrow), on the dusk side of the orbit. The next equator crossing (after additional 50min) is at 33E (small black arrow), 24 westward of the first crossing 100min earlier, while moving again northward. Right: The path of a satellite at inclination i in orbit around the Earth.
* Magnetic Field Satellites
ScienceDirect - Treatise on Geophysics : The Present Field
Satellite Operation Inclination Altitude Data
OGO-2 Oct. 1965Sep. 1967 87 4101510 km Scalar only
OGO-4 Jul. 1967Jan. 1969 86 410910 km Scalar only
OGO-6 Jun. 1969Jun. 1971 82 4001100 km Scalar only
Magsat Nov. 1979May 1980 97 325550 km Scalar and vector
rsted Feb. 1999 97 650850 km Scalar and vector
CHAMP Jul. 2000 87 350450 km Scalar and vector
SAC-C/rsted-2 Jan. 2001Dec. 2004 97 698705 km Scalar only
Swarm 20102014 88 /87 530/
*Spherical Harmonic Representation of Magnetic Field
n = degree
*All models discussed so far are models of the Field of Internal Origin. But this field is the sum of the lowfrequency Core Field which reaches the Earths surface (and makes the Main Field) and of the CrustalField. Fortunately, we know that the crustal sources can only lie very near the Earths surface (above theCurie isotherm), in layers with reasonably well-known magnetic rock properties.
This makes it possible to predict the likely contribution of the Crustal Field to the observed Field of Internal Origin. Suchpredictions can be made either in a statistical way (e.g. Jackson ) or in a deterministic way (e.g.Purucker et al. ). In both cases the conclusion is the same: the Crustal Field can easily explain