geomagnetic field

*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

*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

215.pdf

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

*Geomagnetic Spectrum

*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 [1994]) or in a deterministic way (e.g.Purucker et al. [2002]). In both cases the conclusion is the same: the Crustal Field can easily explain thesmall scales of the Field of Internal Origin but cannot explain its largest scales. The transition occursaround the degree 14 of the Spherical Harmonic representation of the field, and shows up nicely as astrong inflection point in the spatial Mauersberger-Lowes spectrum (recall eq. 26) of the Field of InternalOrigin at the Earths surface (see Figure 14a in section 4.1, where this spectrum is further discussed; seealso Chapter 6 of the present volume, where predictions of the Crustal Field contributions are illustrated).This then shows that the Crustal Field is dominating the small scales (say degree N = 15 and above) ofthe Field of Internal Origin, to which it can be identified, while the Core Field is dominating the largescales (say degree N = 13 and below), to which it can also be identified. This of course also meansthat the smallest scales of the Core Field are permanently screened by a constant unknown Crustal Fieldand that the largest scales of the Crustal Field cannot be observed. Altogether it thus appears that atany given epoch, only the low temporal frequencies (because of the slightly conducting Mantle) and thelarge spatial scales (because of the magnetised Crust) of the Core Field can potentially be identified atthe Earths surface where it makes the Main Field. However, the screening of the Core Field by thequasi-permanent Crustal Field will not affect the first (and higher) time derivative of the small scalesof the Core Field, which can unambiguously be identified to the observed Secular Variation up to thehighest recoverable degrees, even beyond degree 13 (which can now be achieved, as will be shown insection 4.1).

*Crustal Magnetic FieldsFrom Maus (2007)n = 100

*Curie Depth AntarcticaCrustal MagFieldCrustal ThicknessCurie DepthHeat Flow

*Geomagnetic Jargong

FrontiersIs Earth's magnetic field reversing?Catherine Constable & Monika KorteEarth and Planetary Science Letters 246 (2006) 116Page 2

*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.

*Normal polarityReverse polarityAge[My]

4 3 2 1 0 1 2 3 4 Variations in the magnetic field over a mid-ocean ridgeLithosphereCalculated magnetic field from the model of sea-floor spreadingMeasured magnetic field across a mid-ocean ridgeMolten magma fills the gap, solidifies, cools below the Curie temperature (560oC) and becomes magnetized in the direction of the prevalent magnetic field

*The magnetization along a 42 m long core from the Pacific at 4415 m water depthInklination close to zero at the equatorThe sedimentation varies between 1-5 cm/1000 r. 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

*

*June 2005) | doi:10.1038/nature03674; Received 23 July 2004; Accepted 13 April 2005Geomagnetic dipole strength and reversal rate over the past two million yearsJean-Pierre Valet1, Laure Meynadier2 and Yohan Guyodo3 Gomagntisme et Palomagntisme (UMR CNRS 7577), Gochimie et Cosmochimie (UMR CNRS 7579), Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France Laboratoire des Sciences du Climat et de l'Environnement, Avenue de la Terrasse, 91190 Gif-sur-Yvette, FranceCorrespondence to: Jean-Pierre Valet1 Correspondence and requests for materials should be addressed to J.-P.V. (Email:[email protected]).Top of page AbstractIndependent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1, 2 and thus provide information regarding the processes governing the geodynamo. So far, this procedure has been limited to the past 800,000years (800kyr; ref. 3), which does not include any geomagnetic reversals. Here we present a composite curve that shows the evolution of the dipole moment during the past two million years. This reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas, which were used for calibration. We show that, at least during this period, the time-averaged field was higher during periods without reversals but the amplitude of the short-term oscillations remained the same. As a consequence, few intervals of very low intensity, and thus fewer instabilities, are expected during periods with a strong average dipole moment, whereas more excursions and reversals are expected during periods of weak field intensity.

We also observe that the axial dipole begins to decay 6080kyr before reversals, but rebuilds itself in the opposite direction in only a few thousand years.

*

The GaussMatuyama (2.58Ma) 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.

*Difference between rsted (2000) och Magsat (1980) measurements

*Tangential flow pattern in the outer core at the CM transition

Anticyclonic patches transporting oppositely directed magnetic flux, i.e. negative feedback.

*

The Earths 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 Earths 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

*Important Constraints on Models of the GeodynamoWestward drift of non-dipolar fieldExcursionsFrequencyStrength distributionReversalsFrequencyDuration

*Aborted Reversal Simulation

*An aborted reversal during the case c simulation of Glatzmaier et al. (1999) that occurs around 88000 years in the record of Figure 14(c). Left: True dipole path and VGP paths for five locations around the globe. Right: Longitudinally averaged poloidal flux in the outer core at the midpoint of the directional excursion shows that while the field reversed in most of the outer core and above, it retained the original normal polarity close to the inner core and within it. Note that each plotted Time Step represents about 100 years and 3500 numerical time steps.

*Models of the Core Field

*Spectra of characteristic length and timescales in core dynamics. MAC Magnetic, Archimedean, Coriolis waves

*Snapshot of Magnetic Field

The field is sheared around the tangent cylinder to the inner-core equator

*A snapshot of the simulated geomagnetic field produced by Glatzmaier and Roberts (1995). A set of magnetic lines of force illustrated the 3D structure of the field, which is intense and complicated inside the fluid core and smooth and dipole-dominated outside the core. The rotation axis of the model Earth is vertical in the illustration and yellow lines represent outward directed field and blue line represent inward directed field. The field is sheared around the tangent cylinder to the inner-core equator.

*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.

*Dynamo Simulations. Varying Heat Flow at CMB

*Eight dynamo simulations with different imposed patterns of radial heat flux at the CMB. The top row shows the patterns of CMB heat flux. Solid contours represent greater heat flux out of the core relative to the mean; broken contours represent less. Case g has a uniform CMB heat flux and case h has a pattern based on seismic tomography, assuming lower sound speed corresponds to warmer mantle and therefore smaller heat flux out of the core. The second row shows the trajectory of the south magnetic pole of the dipole part of the field outside the core, spanning the times indicated in the plots below; the marker dots are about 100 years apart. The plots in the third and fourth rows show the south magnetic pole latitude and the magnitude of the dipole moment (in units of 1022 A m2) vs time (in units of 1000 years). Reproduced from Glatzmaier GA, Coe RS, Hongre L, and Roberts PH (1999) The role of the Earths mantle in controlling the frequency of geomagnetic reversals. Nature 401: 885890 with permission from Nature.

*More Snapshots

SurfaceCMBPoloidalToroidalFields

*A sequence of snapshots of the longitudinally averaged magnetic field through the interior of the core and of the radial component of the field at the CMB and at what would be the surface of the Earth, displayed at roughly 3000-year intervals, spanning the first dipole reversal of case h in Figure 14. In the plots of the average field, the small circle represents the inner-core boundary and the large circle is the CMB. The poloidal field is shown as magnetic field lines on the left-hand sides of these plots (blue is clockwise and red is counter-clockwise). The toroidal field direction and intensity are represented as contours (not magnetic field lines) on the right-hand sides (red is eastward and blue is westward). Hammer (equal area) projections of the entire CMB and surface are used to display the radial field (with the two different surfaces displayed as the same size). Reds represent outward-directed field and blues inward field. The surface field, which is typically an order of magnitude weaker, was multiplied by 10 to enhance the color contrast. Adapted from Glatzmaier GA, Coe RS, Hongre L, and Roberts PH (1999) The role of the Earths mantle in controlling the frequency of geomagnetic reversals. Nature 401: 885890.

*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. *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 *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*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).*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.

*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 [1994]) or in a deterministic way (e.g.Purucker et al. [2002]). In both cases the conclusion is the same: the Crustal Field can easily explain thesmall scales of the Field of Internal Origin but cannot explain its largest scales. The transition occursaround the degree 14 of the Spherical Harmonic representation of the field, and shows up nicely as astrong inflection point in the spatial Mauersberger-Lowes spectrum (recall eq. 26) of the Field of InternalOrigin at the Earths surface (see Figure 14a in section 4.1, where this spectrum is further discussed; seealso Chapter 6 of the present volume, where predictions of the Crustal Field contributions are illustrated).This then shows that the Crustal Field is dominating the small scales (say degree N = 15 and above) ofthe Field of Internal Origin, to which it can be identified, while the Core Field is dominating the largescales (say degree N = 13 and below), to which it can also be identified. This of course also meansthat the smallest scales of the Core Field are permanently screened by a constant unknown Crustal Fieldand that the largest scales of the Crustal Field cannot be observed. Altogether it thus appears that atany given epoch, only the low temporal frequencies (because of the slightly conducting Mantle) and thelarge spatial scales (because of the magnetised Crust) of the Core Field can potentially be identified atthe Earths surface where it makes the Main Field. However, the screening of the Core Field by thequasi-permanent Crustal Field will not affect the first (and higher) time derivative of the small scalesof the Core Field, which can unambiguously be identified to the observed Secular Variation up to thehighest recoverable degrees, even beyond degree 13 (which can now be achieved, as will be shown insection 4.1).*June 2005) | doi:10.1038/nature03674; Received 23 July 2004; Accepted 13 April 2005Geomagnetic dipole strength and reversal rate over the past two million yearsJean-Pierre Valet1, Laure Meynadier2 and Yohan Guyodo3 Gomagntisme et Palomagntisme (UMR CNRS 7577), Gochimie et Cosmochimie (UMR CNRS 7579), Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France Laboratoire des Sciences du Climat et de l'Environnement, Avenue de la Terrasse, 91190 Gif-sur-Yvette, FranceCorrespondence to: Jean-Pierre Valet1 Correspondence and requests for materials should be addressed to J.-P.V. (Email:[email protected]).Top of page AbstractIndependent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1, 2 and thus provide information regarding the processes governing the geodynamo. So far, this procedure has been limited to the past 800,000years (800kyr; ref. 3), which does not include any geomagnetic reversals. Here we present a composite curve that shows the evolution of the dipole moment during the past two million years. This reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas, which were used for calibration. We show that, at least during this period, the time-averaged field was higher during periods without reversals but the amplitude of the short-term oscillations remained the same. As a consequence, few intervals of very low intensity, and thus fewer instabilities, are expected during periods with a strong average dipole moment, whereas more excursions and reversals are expected during periods of weak field intensity.

We also observe that the axial dipole begins to decay 6080kyr before reversals, but rebuilds itself in the opposite direction in only a few thousand years.

*An aborted reversal during the case c simulation of Glatzmaier et al. (1999) that occurs around 88000 years in the record of Figure 14(c). Left: True dipole path and VGP paths for five locations around the globe. Right: Longitudinally averaged poloidal flux in the outer core at the midpoint of the directional excursion shows that while the field reversed in most of the outer core and above, it retained the original normal polarity close to the inner core and within it. Note that each plotted Time Step represents about 100 years and 3500 numerical time steps.

*A snapshot of the simulated geomagnetic field produced by Glatzmaier and Roberts (1995). A set of magnetic lines of force illustrated the 3D structure of the field, which is intense and complicated inside the fluid core and smooth and dipole-dominated outside the core. The rotation axis of the model Earth is vertical in the illustration and yellow lines represent outward directed field and blue line represent inward directed field. The field is sheared around the tangent cylinder to the inner-core equator.

*Eight dynamo simulations with different imposed patterns of radial heat flux at the CMB. The top row shows the patterns of CMB heat flux. Solid contours represent greater heat flux out of the core relative to the mean; broken contours represent less. Case g has a uniform CMB heat flux and case h has a pattern based on seismic tomography, assuming lower sound speed corresponds to warmer mantle and therefore smaller heat flux out of the core. The second row shows the trajectory of the south magnetic pole of the dipole part of the field outside the core, spanning the times indicated in the plots below; the marker dots are about 100 years apart. The plots in the third and fourth rows show the south magnetic pole latitude and the magnitude of the dipole moment (in units of 1022 A m2) vs time (in units of 1000 years). Reproduced from Glatzmaier GA, Coe RS, Hongre L, and Roberts PH (1999) The role of the Earths mantle in controlling the frequency of geomagnetic reversals. Nature 401: 885890 with permission from Nature.

*A sequence of snapshots of the longitudinally averaged magnetic field through the interior of the core and of the radial component of the field at the CMB and at what would be the surface of the Earth, displayed at roughly 3000-year intervals, spanning the first dipole reversal of case h in Figure 14. In the plots of the average field, the small circle represents the inner-core boundary and the large circle is the CMB. The poloidal field is shown as magnetic field lines on the left-hand sides of these plots (blue is clockwise and red is counter-clockwise). The toroidal field direction and intensity are represented as contours (not magnetic field lines) on the right-hand sides (red is eastward and blue is westward). Hammer (equal area) projections of the entire CMB and surface are used to display the radial field (with the two different surfaces displayed as the same size). Reds represent outward-directed field and blues inward field. The surface field, which is typically an order of magnitude weaker, was multiplied by 10 to enhance the color contrast. Adapted from Glatzmaier GA, Coe RS, Hongre L, and Roberts PH (1999) The role of the Earths mantle in controlling the frequency of geomagnetic reversals. Nature 401: 885890.

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