Earth's Magnetic Field

10 Key excerpts on "Earth's Magnetic Field"

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  Principles of the Magnetic Methods in Geophysics

  • (Author)
  • 2008(Publication Date)
  • Elsevier Science

    (Publisher)

  Chapter 4 Main Magnetic Field of the Earth

  Alex A. Kaufman, Richard O. Hansen, Robert L.K. Kleinberg

  AbstractPublisher Summary

  This chapter focuses on the main magnetic field of the earth. For several centuries, it has been known that the magnetic field is present inside and outside of the earth as well as on its surface. It may be proper to notice that while the magnetic field is almost directly observed, the presence of the gravity required a genius guess by Newton. Inasmuch as the magnetic field is vector field, it is characterized by its magnitude and direction, or its components along the coordinate axes. Inasmuch, the field on the earth’s surface, but the conduction currents are located inside the core, the distance is practically three times greater than the dimensions of this current system. A study of the magnetic field of the earth can be done in different systems of coordinates. This fact is a result of direct observations with a thin magnetic needle, called a compass. It is impossible to overestimate the importance of the invention of the magnetic compass, which explains a great interest in the origin of this amazing device. At the end of the 16th century, the magnetic compass played an extremely important role for sea navigation and was the single instrument to study the behavior of the magnetic field of the earth even though at that time the concept of the magnetic field did not exist.

  4.1 Elements of the magnetic field of the earth

  For several centuries it has been known that the magnetic field is present inside and outside of the earth as well as on its surface. This fact is a result of direct observations with a thin magnetic needle, called a compass. Let us imagine that such a needle is suspended and that it can freely rotate around its center of mass. More than thousand years ago people discovered that at any point of the earth's surface this needle tends to take a certain position around some axis of rotation but does not experience a noticeable displacement. Such a behavior indicates that there is a magnetic field, and, as was shown in Chapter 3

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  Geomagnetism, Aeronomy and Space Weather

  A Journey from the Earth's Core to the Sun

  • Mioara Mandea, Monika Korte, Andrew Yau, Eduard Petrovsky(Authors)
  • 2019(Publication Date)
  • Cambridge University Press

    (Publisher)

  PART II Geomagnetic Field Sources and Observations 4 Geomagnetic Field Sources From Earth’s Core to the Sun Daniel N. Baker The Earth’s main magnetic field originates in the molten outer core of our planet. It arises through the action of convection currents that drive the geomagnetic dynamo (see Chapter 9). Earth’s interior is modelled to have a solid inner core with a geocentric radius of ~1200 km and a liquid outer core extending from the inner core boundary out to about 3400 km geocentric radial distance. Heat flow from the inner core (~6000 K) to this core-mantle boundary maintains the outer core in its liquid state. The iron-nickel composition of the outer core allows generation of strong electrical currents due to the convective flow caused by Earth’s rotation. This flow is further shaped by viscous interaction with the inner core as well (see Figure 4.0.1). The geomagnetic field extends from the deep interior out- ward through the mantle and crust beyond the Earth’s sur- face (Gauss, 1839, in Glassmeier and Tsurutani, 2014). The geomagnetic field often is approximated as an offset, tilted dipole magnetic field with a magnetic moment of 7.94 × 10 22 A-m 2 . However, as reported in subsequent chapters, the field is immensely more complex and time-varying than this simple picture might suggest. In the dipole approximation, the mag- netic field is tilted by about 11° relative to the Earth’s rota- tional axis. At Earth’s surface (1 R E = 6372 km), the magnetic field intensity varies from about 0.25 Gauss (G) to about 0.65 G. In SI units, the field varies from 2.5 × 10 4 nano tesla (nT) in the South Atlantic region near South America to about 6.5 × 10 4 nT in the north polar regions over Siberia in the north and over Antarctica in the South (Figure 4.0.2). The geomagnetic field extending outward beyond Earth’s solid surface encounters strong, highly variable flow of hot ionised gas from the Sun.

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  Components, Laws and Concepts of Electromagnetism

  • (Author)
  • 2014(Publication Date)
  • Library Press

    (Publisher)

  ________________________ WORLD TECHNOLOGIES ________________________ Chapter- 4 Magnetic Field A magnetic field is a field of force produced by a magnetic object or particle, or by a changing electric field an d is detected by the force it exerts on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The complex mathematics underlying the magnetic field of an object is usually illustrated using magnetic field lines. These lines are strictly a mathematical concept and do not exist physically. Nonetheless, certain physical phenomena, such as the alignment of iron filings in a magneti c field, produces lines in a similar pattern to the imaginary magnetic field lines of the object. Magnets exert forces and torques on each other through the magnetic fields they create. Electric currents and moving electric charges produce magnetic fields. Even the magnetic field of a magnetic material can be modeled as being due to moving electric charges. Magnetic fields also exert forces on moving electric charges. The magnetic fields within and due to magnetic materials can be quite complicated and is d escribed using two separate fields which can be both called a magnetic field : a magnetic B field and a magnetic H field. Energy is needed to create a magnetic field. This energy can be reclaimed when the field is destroyed and, therefore, can be considered as being stored in the magnetic field. The value of this energy depends on the values of both B and H . An electric field is a field created by an electric charge and such fields are intimately related to magnetic fields; a changing magnetic field genera tes an electric field and a changing electric field produces a magnetic field. The full relationship between the electric and magnetic fields, and the currents and charges that create them, is described by the set of Maxwell's equations.

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  The Hidden Link Between Earth's Magnetic Field and Climate

  • Kilifarska N.A., Bakmutov V.G., Melnyk G.V.(Authors)
  • 2020(Publication Date)
  • Elsevier

    (Publisher)

  Hulot et al., 2002 ). Are we on the verge of changing the polarity of the magnetic field, and what is the probability that this will happen? This issue is hotly debated in the scientific literature, spreading even in mass media. Its correct answer depends on our knowledge about the evolution of the geomagnetic field throughout the geological history of our planet.

  1.4 Origin of the geomagnetic field

  The dipolar structure of Earth's Magnetic Field is currently explained by the theory of geodynamo. The ‘father’ of the recent geodynamo theory is accepted to be Walter M. Elsasser (Elsasser, 1956 ), who suggested that the geomagnetic field is induced by electric currents in Earth's melted iron outer core. The modern theory of a hydromagnetic dynamo explains many of the geomagnetic field features, such as western drift, polarity reversal, etc.

  Seismology has shown that Earth's deep environment consists of a solid inner core, with an approximate radius of 1100 km, surrounded by a fluid outer core of radius approximately 3500 km, confined in turn by the mantle (Gubbins and Herrero-Bervera, 2007 ; Core properties, physical ). The electrical conductivity of the mantle is very low compared to that of the outer core, so the liquid metal outer core is the most natural seat of the geodynamo.

  According to recent knowledge, gravitational differentiation between the inner and outer core had been completed 4.56 milliard years ago, during the solar system accretion. The growth of the proto-planetary mass is accompanied by a rise in temperature, which at some critical point leads to the melting of metallic components (e.g. iron (Fe), nickel (Ni), etc.). The melting permits the denser components to migrate towards the centre of the planet, thus achieving the process of differentiation. Note that recent compositional models of Earth's core estimate that it contains approximately 85% Fe, 5% Ni, and 10% of minor lighter components (e.g., H, C, O, Si, P, S). Partly because of the energy released during differentiation, the core is believed to be initially hot and convecting. Prior to the formation of the inner core, the terrestrial dynamo is supposedly driven by cooling of the liquid core, with possible assistance from radioactive decay. The crystallization of the inner core is suggested to have begun 2.5–1 milliard years ago (Labrosse et al., 2001 ), comprising recently ~ 5% of the core's mass. The cooling of the core depends on the rate at which the overlying colder mantle can remove its heat. The cooling forces the further solidification of the inner core, followed by a release of the lighter elements and the onset of compositional convection (i.e. upward motion of the lighter elements, due to their higher buoyancy) in the outer liquid core. Since the net effect of this process is to move heavier material towards the planetary centre, gravitational energy is liberated, supporting the thermal gradient between the core and mantle, and consequently thermal convection

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  Treatise on Geophysics, Volume 8

  Core Dynamics

  • Peter L. Olson, Peter Olson(Authors)
  • 2010(Publication Date)
  • Elsevier

    (Publisher)

  (2003) and Muller et al. (2006) using liquid sodium ( see Chapter 11). 1.4 Core Dynamics and the Geomagnetic Field The study of the geomagnetic field is a rich topic in its own right, and is the subject in this treatise. An extensive description of the geomagnetic field and paleomagnetic field can also be found in Merrill et al. (1998) and the full theory of the geomagnetic field is given in Backus et al. (1996). Here it is appropriate to briefly list some of the main features of the geomag-netic field that we seek to explain in terms of the core’s dynamics. First and foremost is the persistence of an internally generated geomagnetic field, not just the modern field, but a field sustained throughout most if not all of Earth history. The main geomag-netic field that originates in the core is at least 3.5 Ga, according to the rock record, and quite possibly is as old as the core itself. The present-day dipole moment, 7.8 10 22 A m 2 , is probably somewhat lar-ger than the long-term average value, which may be around 6.5 10 22 A m 2 . Significantly, there is little evidence for a secular trend in the dipole moment over geologic time, although there is abundant evi-dence of fluctuations, as described below. For reference, the free decay time of the dipole field in the core is only about 20 ky. The existence of an ancient field, the lack of evidence of a decreasing dipole moment, and the inadequacy of permanent magnetization as a source for the field are three facts that demand there be a regeneration mechan-ism, that is, a dynamo theory. The dominance of the dipole component is evi-dent in the present-day field, where its energy exceeds that in any other spherical harmonic, even at the core–mantle boundary. This dominance becomes even stronger if time averages of the field are considered.

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  The Earth's Magnetic Field : Its History, Origin, and Planetary Perspective

  • McElhinny(Author)
  • 1984(Publication Date)
  • Academic Press

    (Publisher)

  Origin of the Earth's Magnetic Field 1 215 (7.4) end to the colder end than vice versa. Eventually equilibrium is reached when the difference in electric field energy produced by the charge separation balances the difference in thermal energy. The magnitude of such an effect for the earth's conducting core can be estimated. The increase in temperature with depth in the core implies that there will be some charge separation. The mean radius of the positive charge is smaller than that for the negative charge. Thus there will be an external magnetic field produced on rotation (eqns 7.1 and 7.2). The external field is zero because of charge balance. If A is the Gibbs Free Energy, T the temperature, e the charge of the electron and ¢ the electric potential, then for equilibrium conditions and neglecting pressure effects, the following equality must hold: dA d4> dT = e dT (7.3) From elementary solid state theory it can be shown that dA dT = -IT where y is approximately 4 x 10-1 1 J Mol-I K-2 • Combining (7.3) with (7.4) then: d4> dr 1 --=-yT dr dT e (7.5) (7.6) For an order of magnitude calculation, assume a linear temperature gradient through the core: T= T 1 + fJ(R I -r) where R 1 = radius of the sphere with positive charge (which may be taken as zero, if desired); T, = temperature at R l ; f3 = a constant that depends on the assumed temperature gradient; r = distance from the earth's centre. Substitution into (7.5) gives: d4> 1 -d = -yfJ[T I + fJR 1 -fJrJ r e This may be substituted into Poisson's equation: V24> = ~ ~ (r 2 iJ4» = _ ~ r 2 or or I: where p is the surface charge density and I: is the dielectric constant. The result of this substitution gives: - ~ yfJ2 + ~ ~ yfJ[T 1 + fJR l -fJrJ = - ~ ere I: 216 The Earth's Magnetic Field Equation 7.6 indicates that the charge density distribution may be written simply as: where C 1 and C 2 are constants given by (7.6). This result with (7.1) and (7.2) can be used to obtain magnetic field estimates.

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  Fundamentals of Geophysics

  • William Lowrie, Andreas Fichtner(Authors)
  • 2020(Publication Date)
  • Cambridge University Press

    (Publisher)

  In 1995, G. A. Glatzmaier and P. H. Roberts presented a numerical model for the generation of a magnetic field, assuming a hot convective fluid outer core surrounding a solid inner core, with rotation rate akin to that of the Earth. The heat flow, electrical conductivity, and other material properties were made as similar as possible to those of the Earth’s core. The simulated dynamo had dominantly dipole character and intensity similar to the Earth’s, and it exhibited a westward drift of the non-dipole field comparable to that measured at the Earth’s surface. It also reversed polarity spontaneously, 11.2 GEOMAGNETISM 333 with long periods of constant polarity between short polarity transitions, as is the case for the history of geomagnetic reversals in the last 160 Myr (Section 12.3). During a polarity reversal, the field intensity decreased by an order of magni- tude, as is also observed in paleomagnetic studies (see Fig. 12.27). The simulations showed that the ability to reverse polarity was increased when the heat flow across the core–mantle boundary was non-uniform, as in the Earth, showing that thermal conditions in the lower mantle influence the forma- tion of the magnetic field in the fluid core. The solid inner core evidently plays an important role in the reversal process. Magnetic fields in the outer core can change quickly, accom- panying convection, and may act to initiate reversals. However, magnetic fields in the solid inner core change more slowly by diffusion and thus the inner core may act to stabilize the field against reversals so that they occur only occasionally. 11.3 MAGNETIC FIELDS OF THE SUN, MOON, AND PLANETS 11.3.1 Magnetic Fields of the Sun and Moon Our knowledge of the magnetic fields of the Sun and planets derives from two types of observation. Indirect observations utilize spectroscopic effects. All atoms emit energy related to the orbital and spin motions.

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  Physics of the Earth

  • Frank D. Stacey, Paul M. Davis(Authors)
  • 2008(Publication Date)
  • Cambridge University Press

    (Publisher)

  24.2 The pattern of the field As William Gilbert noticed, the magnetic field of the Earth is similar to that of a magnetized sphere, or to a small but powerful bar magnet at the centre. Such a field is termed a dipole field because it could, in principle, be produced by a pair of magnetic poles of equal strengths but opposite signs a small distance apart. The mag-netic moment or dipole moment, m , is then envisaged as the product of pole strength and separation. Since magnetic moments are pro-duced by circulating currents, the equivalent definition in terms of a current loop is the prod-uct of current i and loop area A , m ¼ iA : (24 : 1) Although the Earth’s field is predominantly dipo-lar, non-dipole components contribute about 20% of its strength at the surface. We refer to a best-fitting dipole and need to be specific about what this means. The dipole most closely fitting the observed field is slightly off centre, but if we refer to the best-fitting geocentric dipole then its moment (in 2005) is m Earth ¼ 7 : 768 10 22 A m 2 : (24 : 2) The axis of this dipole is inclined to the geo-graphic or rotational axis by 10 8 . If we discount the equatorial component of this moment and consider just the geocentric axial dipole then its moment is 7.644 10 22 A m 2 . The field of a dipole is conveniently repre-sented in terms of the scalar magnetic potential, V m , which may be differentiated to obtain any component of the field, V m ¼ m r 4 p r 3 ¼ m cos 4 p r 2 ; (24 : 3) where is the angle between the dipole axis and the radius vector r from the dipole to the point considered. These equations assume the dimen-sions of the current loop to be negligible com-pared with r . The field is 24.2 THE PATTERN OF THE FIELD 391

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  Physics Of Geomagnetic Phenomena

  • S. Matsushita(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Three methods have been applied [Kern and Vestine, 1963]. II-2. MAIN GEOMAGNETIC FIEJiD 227 The first supposes the object has a composition similar to that of the earth's crustal rocks. If the solar wind carried a large magnetic field, or if the objects were cooled in the magnetic field of another body, they might face. Surface lavas on the earth have magnetizations of about 10~ 4 cgs, the amount depending roughly on the strength of magnetic field in which the cooling takes place. The second method postulates that the planet has a liquid metallic core, as is usually assumed for the earth. The planet may then possess also a regenerative magnetic field. It may be assumed that the magnetic field of the planetary body is related to that of the earth in direct proportion to core volume and to the rate of rotation of the body. A hydromagnetic dynamo is then assumed under conditions roughly the same for all the planets. The third method is simply to suppose the field of a planet is related to the earth's in direct proportion to volume, with the surface field of the earth taken as 0.5 G. This method may be a last resource for planets devoid of other desired information. Table 9 lists tentative estimates obtained by Kern and Vestine for magnetic fields of the planets, certain satellites, and an asteroid (obtained using the numbered method listed in the right-hand column). Other estimates of the magnetic fields of planetary bodies have been made. Singer [1960] supposed planetary dipole magnetic moments were proportional to total volume and, therefore, per unit radius of the planet, obtained the same field φ at the same distance from the planetary center. Venus might have a field such as the earth's if it had a core and speed of rotation the same as that of the earth.

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  Atmosphere, Ocean and Climate Dynamics

  An Introductory Text

  • John Marshall, R. Alan Plumb(Authors)
  • 1975(Publication Date)
  • Academic Press

    (Publisher)

  4. The Earth’s magnetic field 163 considerably smaller (and of opposite sign) to those due to tidal friction (Runcorn, 1964,1970a). In both the atmosphere and the oceans the role of rotation is of funda- mental importance. This is also true-with some qualifications-for the Earth’s fluid core. With regard to rotational effects we must distinguish between the core and oceanic-atmospheric layers in the following manner. The core should only be divided up into thin layers if it is stably stratified (see Section 3.5). In such a case dynamo action, though still possible, would be significantly constrained to a predominantly two dimensional motion in concentric spherical shells. If, on the other hand, the core is not stratified, any perturbation of the otherwise steady rotation would lead to a predominantly two dimensional motion in planes perpendicular to the rotation axis as predicted by the Proudman-Taylor theorem. In both cases these statements must be modified to include the effects of the Lorentz force. In certain special cases the effects of the Lorentz force can be included without too much difficulty. For example, we can consider the possibility of interpreting the westward drift of the non-dipole field as the propagation of hydromagnetic waves in the fluid core around the axis of rotation in the presence of a dominant toroidal magnetic field (Hide, 1966). If the fluid is not stably stratified the Lorentz force enters into the equations of motion for hydromagnetic wave solutions in the same manner as the Coriolis force. In fact the flow in this case is described by the Poincare equation-the same equation that describes the flow in the absence of a magnetic field. But even in this rather simple example there is an added difficulty. We are looking for solutions to a hyperbolic differential equation with certain specified boundary conditions. This is an ill-posed mathematical problem and there are serious difficulties in attempting to find a solution.

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