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ScienceWeek
GEOPHYSICS: MAGNETIC FIELD AT THE SURFACE OF EARTH'S CORE
The following points are made by Andrew Jackson (Nature 2003 424:760):
1) The Earth's magnetic field is generated in the fluid core of the Earth (radius a = 3485 km) by a "self-exciting dynamo" mechanism. This mechanism has sustained the field from decay over most of the lifetime of the Earth, and convective motions associated with the dynamo mechanism are responsible for the "secular variation", or the slow changes in the magnetic field over timescales of decades to centuries which are seen on the Earth's surface.
2) Smooth images of the core field can be reliably constructed back to the 16th century using data collected on land and sea(3). As in virtually any inverse problem, the problem of reconstructing the core magnetic field from surface observation is formally non-unique(4,5), and therefore requires some prior information in its solution; generally a quadratic smoothness criterion is implemented on the magnetic field at the core surface, which therefore roughly measures magnetic energy. Such a methodology is termed "regularization" and is common to most inverse problems(5). There is no reason to suppose that any quadratic property of the field should remain constant. However, both theory and numerical simulations of the geodynamo support the view that on the decade-to-century timescale the approximation of the core as a perfect electrical conductor is a good one. Thus, Alfven's celebrated frozen-flux theorem will be obeyed to a high degree, and indeed numerical estimates put the level of violation at a few per cent per century, whereas observationally there is no evidence that requires violation. When faced with making images of the core field at different points in time from sometimes vastly different data sets, an appealing method would be one based on frozen-flux: such images would visually have similar complexity in time, while conserving a physically relevant quantity.
3) In summary: A large number of high-accuracy vector measurements of the Earth's magnetic field have recently become available from the satellite Oersted, complementing previous vector data from the satellite Magsat, which operated in 1979/80. These data can be used to infer the morphology of the magnetic field at the surface of the fluid core(1), approximately 2900 km below the Earth's surface. The author applies a new methodology to these data to calculate maps of the magnetic field at the core surface which show intense flux spots in equatorial regions. The intensity of these features is unusually large -- some have intensities comparable to high-latitude flux patches near the poles, previously identified as the major component of the dynamo field(2). The author suggests the tendency for pairing of some of these spots to the north and south of the geographical equator indicates they might be associated with the tops of equatorially symmetric columnar structures in the fluid, or their antisymmetric equivalents. The drift of the equatorial features may represent material flow or could represent wave motion; discrimination of these two effects based on future data could provide new information on the strength of the hidden toroidal magnetic field of the Earth.
References (abridged):
1. Backus, G. E., Parker, R. & Constable, C. Foundations of Geomagnetism (Cambridge Univ. Press, Cambridge, 1996)
2. Bloxham, J. & Gubbins, D. The secular variation of Earth's magnetic field. Nature 317, 777-781 (1985)
3. Jackson, A., Jonkers, A. R. T. & Walker, M. Four centuries of geomagnetic secular variation from historical records. Phil. Trans. R. Soc. Lond. A 358(1768), 957-990 (2000)
4. Backus, G. E. & Gilbert, F. Numerical applications of a formalism for geophysical inverse problems. Geophys. J. R. Astron. Soc. 13, 247-276 (1967)
5. Parker, R. L. Geophysical Inverse Theory (Princeton Univ. Press, Princeton, 1994)
Nature http://www.nature.com/nature
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PROPOSAL: A MISSION TO EARTH'S CORE
The following points are made by David J. Stevenson (Nature 2003 423:239)):
1) Planetary missions have enhanced our understanding of the Solar System and how planets work, but no comparable exploratory effort has been directed towards the Earth's interior, where equally fascinating scientific issues are waiting to be investigated. The author proposes a scheme for a mission to the Earth's core, a scheme in which a small communication probe would be conveyed in a huge volume of liquid-iron alloy migrating down to the core along a crack that is propagating under the action of gravity. The grapefruit-sized probe would transmit its findings back to the surface using high-frequency seismic waves sensed by a ground-coupled wave detector. The probe should take about a week to reach the core, and the minimum mass of molten iron required would be 10^(8) to 10^(10) kg -- or roughly between an hour and a week of Earth's total iron-foundry production.
2) We live on the Earth's surface, which divides what is above from what is below. The part above us (the rest of the Universe) is mostly empty, mostly unknown and about 10^(57) times larger by volume. The part below is crammed with interesting stuff and is also mostly unknown, despite its much greater proximity to us. Space probes have so far reached a distance of about 40 astronomical units (6 x 10^(9) km), but subterranean probes (drill holes) have descended only some 10 km into the Earth.
3) Travel downwards is impeded by the dense intervening matter, and the energy required to penetrate it by melting is about 10^(9) times (per unit distance travelled) the energy needed for space travel -- a fact that partly explains the large difference in distances attained. Travel downwards has also been impeded by the much more limited allocation of financial and material resources relative to those provided for space travel -- there is no underground equivalent of NASA.
4) One possible means of reaching the core appeals to the "China syndrome" idea and requires melting of the rock, but the trip times in these scenarios are thousands of years or more --geologically short but too long on a human timescale for any government to contemplate funding. However, a liquid-iron-filled crack initiated in the Earth would propagate downwards (despite very high pressures), closing up behind as it travels, and a neutrally buoyant, insoluble probe could be carried along for the ride. The author states: "This proposal is modest compared with the space program, and may seem unrealistic only because little effort has been devoted to it. The time has come for action."
Nature http://www.nature.com/nature
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CONDITIONS AT EARTH'S CORE
The following points are made by A. Jephcoat and K. Refson (Nature 2001 413:27):
1) Although in the beginning, shortly after the Earth was formed, there was apparently no inner core, this has since grown to a spherical region of dense and nearly pure solid iron approximately 2240 kilometers in diameter. The size and some of the characteristics of the internal structure of the core are known because of their effect on seismic waves produced by earthquakes, which pass through the core on their way to detector stations at the surface.
2) A longstanding puzzle is that the speed of a seismic compressional wave (an ultra-low frequency sound wave) depends on its direction across the core. Such waves travel faster along the axis of Earth's rotation (north-south) than in the equatorial plane (east-west), and this difference (called "seismic anisotropy") appears to increase with depth within the inner core and is patchy on a variety of length scales. Other vital evidence for seismic anisotropy comes from free oscillations, studies of the natural vibrational frequencies of the Earth.
3) Steinle-Neumann et al (2001) have presented calculations of the behavior of iron at high temperatures and pressures, and predicted that the crystal structure of iron will distort unexpectedly when subjected to the conditions at the center of the Earth. Together with a model of the orientation of iron crystals in the inner core by Buffet and Wenk (2001), these results apparently explain the observed seismic anisotropy, and it seems the fundamentals for understanding the phenomenon are now in place.
Nature http://www.nature.com/nature
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ON EARTH'S CORE AND THE GEODYNAMO
Seismic wave propagations are the propagated shock waves produced by earthquakes, and quantitative analysis of these waves can tell us much about the structure of the Earth and its interior physical discontinuities. Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. The mantle, the layer of dense rock and metal oxides between the molten part of the core and the surface, has plastic properties (i.e., it is a solid capable of flow under pressure). Apparently, the Earth's magnetic field is a direct result of the combination of its rapid rotation and its molten core, and the theoretical account of this is called the "dynamo effect" (the source of the effect is called Earth's "geodynamo"). The essential idea is that the liquid metallic core is stirred by convection, the rotation of the Earth couples this motion into a circulation that generates electric currents, and the electric currents in turn generate a magnetic field according to classical electromagnetic theory.
The following points are made by Bruce A. Buffett (Science 2000 288:2007):
1) Earth apparently evolved into a layered body early in its history. Molten metal (mainly iron) descended to form the present-day core, while silicates and oxides were confined to the thick shell of the mantle. The innermost part of the core is now a solid, whereas the outer portion of the core is liquid. The apparent viscosity of the liquid outer core is comparable to that of water, which permits vigorous convection as the core cools. Fluid velocities of the order of 10 kilometers per year are evidently sufficiently rapid to sustain Earth's magnetic field via the geodynamo.
2) Planetary rotation promotes the types of flows that are needed to generate the magnetic field, but the resulting magnetic field exerts a strong feedback on convection, and this complicates quantitative predictions of the field generation. An important advance in recent years is the development of numerical simulations that produce self-sustaining dynamo action. Computational limitations prevent these simulations from reaching known Earth-like conditions, but the models obtained so far have external magnetic fields that are similar to Earth's magnetic field.
3) The operation of the geodynamo depends on the internal evolution of the planet, since convection in the core is linked to the rate of cooling. (Cooling of the core causes growth of the inner core by solidification; the current rate of growth is approximately 1 millimeter per year.) The transport of heat through the mantle is crucial for powering the geodynamo, and even the existence of *plate tectonics at the surface is an important factor. Interactions between the core and the mantle are expected from theory, but it is not clear how these interactions are expressed in the magnetic field. The apparent persistence of the magnetic field over most of the history of Earth implies continual cooling and convection in the core. In contrast, the absence of magnetic fields in our nearest planetary neighbors indicates that other planetary thermal histories are possible. As we gain a better understanding of the geodynamo and the dynamics of the core, new perspectives about the processes that drive the internal evolution of Earth are expected to emerge.
Science http://www.sciencemag.org
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Notes:
plate tectonics: The term "lithosphere" refers to the outer layer of the Earth, comprising the crust and upper mantle, and extending to a depth of 50 to 70 kilometers. The traditional view of tectonics (changes in the structure of the Earth's crust) is that the lithosphere consists of a strong brittle layer overlying a weak ductile layer. "Plate tectonics" is the current consensus theory that the Earth's lithosphere is broken into fairly rigid plates, seven or eight major plates and many smaller plates, and that convection within the underlying less rigid "asthenosphere" causes the plates (and the associated continents and crust) to move relative to each other.
ScienceWeek http://scienceweek.com
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