The Earth's magnetic field and reversals

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<ul><li><p>The Earths magnetic field and reversals J.A. Jacobs </p><p>The problem of the origin of the Earths magnetic field is one of the oldest in science. Although much has been learned about it, particularly in the last 10 years with the advent of more powerful computers, we still do not know the details of its generation in the Earths liquid outer core. The discovery that the field reverses its polarity has heightened interest in the problem. It is suspected that the Earths solid inner core may play a much more important role than has been supposed in the dynamics of the outer core and in the maintenance of the Earths magnetic field. - </p><p>Gauss showed in 1839 that the field of a uniformly magnetized sphere, which is the same as that of a geocentric dipole oriented along the rotational axis, is an excellent first approximation to the Earths magnetic field. This was nearly two and a half cen- turies after William Gilbert had come to the same conclusion as a result of measure- ments he had made of the direction of mag- netic force over the surface of a piece of the naturally magnetized mineral lodestone which he had cut in the shape of a sphere. Apart from spatial variations, the Earths magnetic field also shows temporal changes. These range from variations on a time-scale of seconds to secular changes on a time-scale of hundreds of years. On an even longer time-scale, the Earths mag- netic field can completely reverse its polar- ity. The short-period, transient variations are due to external (solar) causes and have no lasting effect on the Earths main magnetic field, and will not be considered here. Secular changes over 10-104 years appear to be a regional rather than a planetary phe- nomenon, and can be quite large even over 20 years. Their source, like that of the main field, is believed to lie within the Earth. In addition to full reversals of polarity, the Earths magnetic field has on occasion departed for brief periods from its usual near-axial configuration without establish- ing, and not always even approaching, a reversed direction. Such excursions of the field may be aborted reversals. Before the question of the origin of the Earths magnetic field and its reversals can be dis- cussed, it is necessary to consider briefly the structure and constitution of the Earths interior. </p><p>J.A. Jacobs, MA, D.Sc., FRCS </p><p>Graduated in mathematics in the University of London and after two senior appointments in Canada was appointed Professor of Geophysics at Cambridge from 1974 to 1983. He has a particular interest in geomagnetism and since 1969 has been Honorary Professor in the Department of Earth Studies, University of Wales, Aberystwyth. </p><p>166 </p><p>The constitution of the Earths interior Most of our knowledge about the structure and constitution of the Earths interior comes from seismology and mineral physics. Apart from the outer crustal layers, which vary in thickness from about 3@60 km under the continents to not more than about 5-6 km under the oceans, the Earth consists of a semiconducting mantle extending down to a depth of about 2900 km and a metallic core. The core may be further divided into a liquid outer core (OC) and a solid inner core (IC) beginning at a depth of about 5150 km (Figure 1). The existence of a solid IC was deduced nearly 60 years ago by Inge Lehmann, the Danish seismologist who died recently at the age of 105. The OC consists mainly of Fe with about 10-15 per cent of some light alloying element, most probably S or 0. The IC is mainly Fe with perhaps a little Ni. The composition of the core has been reviewed by J.-P Poirier [I]. How this knowledge has been obtained will not be discussed in this article - a good account has been given by Jacobs [2], and R. Jeanloz and T. Lay [3]. </p><p>There is much controversy over condi- tions at the core-mantle boundary (CMB) and the structure of the 20&amp;300 km-thick layer at the bottom of the mantle (called the D layer by K.E. Bullen). One of the main unresolved questions is whether this layer is a thermal and/or chemical boundary layer. Core temperatures at the CMB are estimated to be at least 3800 K and perhaps as high as 4400 K. These high temperatures demand a temperature drop of at least 700 K across a thermal boundary layer in D. Many seis- mic models of the lower mantle indicate decreased velocity gradients as the CMB is approached and this has also been used to infer a thermal boundary layer. The CMB is also a major chemical discontinuity in the Earth where the molten Fe alloy from the OC meets solid silicate minerals from the mantle. E. Knittle and R. Jeanloz [4], on the basis of laboratory experiments, have sug- gested that chemical reactions at the CMB </p><p>create a layer of different composition and density to that of the overlying mantle. Properties such as viscosity, density and thermal diffusivity can enhance or inhibit the creation of such a chemical boundary layer which would be laterally hetero- geneous and of variable thickness. The most probable structure of the D layer seems to be a heterogeneous chemical boundary layer embedded in a thermal boundary layer. </p><p>Origin of the Earths magnetic field There have been many suggestions for the origin of the Earths magnetic field. Most of them have been shown to be inadequate and will not be reviewed here. It is now gener- ally believed that electric currents flow in the Earths OC and set up a magnetic field by induction. Palaeomagnetic measure- ments have shown that the Earths field has existed for at least 3500 Ma (mega-annum) and that its strength has never differed sig- nificantly from its present value. Since any system of electric currents will decay (in the case of the Earths OC, in about 100 ka (kilo-annum)), the geomagnetic field cannot be a relic of the past and a mechanism must be found for generating and maintaining electric currents to sustain the field. Such a mechanism is the familiar action of a dynamo and was first suggested by Sir Joseph Larmor in 1919 to explain the mag- netic field of the Sun. The Earths OC is a good conductor of electricity and a fluid in which motions can take place; that is, it per- mits both mechanical motion and the flow of electric current, the interaction of which could generate a self-sustaining magnetic field. The pioneering work on dynamo theory was carried out in the late 1940s by W.M. Elsasser and Sir Edward Bullard. Since then much work has been done on the geodynamo problem. However, it is a for- midable mathematical problem and, despite some success, no completely satisfactory model has as yet been obtained. A good account of dynamo theory has been given by R.T. Merrill and PL. McFadden [5]. </p><p>0 1995 Elsevier Science Ltd 0160-9327/95/$09.50 </p></li><li><p>Convection in the Earths outer core There have been a number of suggestions for the driving force of the geodynamo (see, for example, D. Gubbins and T.G. Masters [6]) and there is no reason to believe that only one mechanism operates. Non-linear interactions with a number of different processes giving rise to feedback are bound to lead to very complex behaviour and are typical of natural systems in a chaotic regime. It is possible that two different processes could act together to enhance the magnetic field or act in opposition to destroy the field. </p><p>There are two main contenders for the driving force - thermal convection and gravitational convection resulting from growth of the IC. Freezing of material at the inner core boundary (ICB) would separate a heavy fraction (mainly Fe). leaving behind a lighter fraction in the OC that would be buoyant, leading to compositionally driven convection. McFadden and Merrill [7] sug- gested that one of these two sources is responsible for generating the main field, the other producing instabilities that disrupt the field, leading on occasion to a reversal. It is possible that these two sources could at times change roles, depending on changing conditions at the CMB and ICB. Such dual behaviour of the two sources might explain the variable lengths of polarity intervals. </p><p>The same authors suggested two possible scenarios. In their first model, core motions, which drive the main magnetic field, result from gravitational convection associated with freezing of the OC at the ICB. In- stabilities are generated by heat loss at the CMB, cooler, more dense, cold blobs of fluid sinking and destabilizing the main convection. In their other model. core con- vection is due primarily to cooling at the CMB. The source of any instability is an occasional plume or hot blob given off at the ICB due to freezing of the OC with growth of the IC. A crucial question with either of these two models is the time-scale for a plume or blob to travel through the OC. A further question is how often are blobs/plumes released from the CMB/ICB and what is their success rate in traversing the whole OC? </p><p>H.H. Schloessin and J.A. Jacobs [8] had earlier suggested that reversals of the Earths magnetic field were the result of competing processes at the CMB and ICB. In their model of the evolution of the core, pressure freezing at the ICB and general cooling at the CMB lead to the formation of solid phases at both these boundaries, resulting in a solid IC and a lower mantle shell, which they identified with Bullens D layer. Motions in the OC are caused and sustained by currents which offset den- sity inhomogeneities at the advancing CMB and ICB. The concept of two compet- ing processes in the OC was further investi- gated by P. Olson [9] who showed, from symmetry considerations. that the effects of IC growth tend to oppose and destabilize those generated by heat loss at the CMB. </p><p>Figure 1 Cross-section of the interior of the Earth. On this scale the crust canno: be seen. </p><p>H.K. Moffatt and D.E. Loper [lo] investi- gated the dynamics of a buoyant blob of fluid released from the ICB. They showed that when Lorenz and Coriolis forces are of comparable orders of magnitude, the disturb- ance remains localized in the neighbour- hood of the blob. They calculated the veloc- ity and magnetic field associated with a given localized buoyancy distribution, and estimated that the rate of growth of the IC (assumed to be uniform) was = IO-ii m/s and, in further calculations, that the length- scale of an individual blob was in the range 0.1-100.0 km. On the assumption that the velocity is approximately uniform through- out the blob. they showed that the trajectory of the blob from the ICB to the CMB is a helix, indicating both a poleward com- ponent and a westward drift of the blob. The time rise of the blob from the ICB to the CMB was estimated to be = 100 a. Inherent in their theory is the presumption that the blobs will preserve their identity as they rise through the OC. M.G. St Pierre [ll] has carried out a numerical study of Moffatt and Lopers model and found that the blobs will be greatly distorted after rising only a few </p><p>hundred kilometres from the ICB. They will be rapidly broken up into plate-like struc- tures elongated in the direction of rotation and of the prevalent magnetic field, giving rise to highly anistropic motions as foreseen earlier by S.I. Braginsky and V.P. Meitlis [12]. If substantiated, any large coherent buoyant structure in the Earths OC may be unstable. </p><p>Thermal convection in the OC has of late been thought to play a smaller role than compositional convection because of the low efficiency of a Camot cycle at core tem- peratures. However, B.A. Buffett et al. [ 131 have shown that thermal convection can contribute significantly to the energy budget of the geodynamo. A modest heat flux from the core in excess of that conducted down the adiabatic gradient is sufficient to power the geodynamo, even in the absence of compositional convection. The relative con- tributions of thermal and compositional convection to the geodynamo are largely determined by the magnitude of the heat flux from the core and the size of the IC. For plausible present-day values, compositional convection is responsible for - two-thirds </p><p>167 </p></li><li><p>of the ohmic dissipation in the core and thermal convection for - one-third. Buffett et al. [ 131 point out that in the early Earth, when the IC was smaller and probably greater, thermal convection would have been the dominant source of energy for the geodynamo. </p><p>It is generally believed that the Earths core was formed very early in the Earths history (see, for example, C.J. All&amp;gre et al. [14]). AllBgre et al. favour a slow continu- ing growth of the core, following a rapid initial growth - they estimate that 85 per cent of the core would have formed during the first N-200 Ma, and the remaining 15 per cent over the rest of geologic time. Rocks more than 3500 Ma old have been found which possess remanent magnetiz- ation, so that it is extremely likely that the Earth then had a molten OC about the same size as that at present. When the solid IC formed is a more difficult question, but if compositional convection in the OC plays a dominant role in driving the Earths dynamo, then a key question is when did the IC begin to form and what has been its rate of growth. D.J. Stevenson et al. [ 151 suggested that the mode of powering the geodynamo may have changed over geologic time. In the Earths early history, the magnetic field was generated by thermal convection with diminishing strength. After the onset of IC nucleation, the release of gravitational energy became the dominant source. </p><p>Buffett et al. [13] have investigated the time, formation and growth of the IC. Their method is based on global heat conservation, that is, the net heat flux from the OC is equated to that lost from the liquid OC together with that produced by the growth of the IC. They assume that temperatures throughout the OC (and hence the radius of the IC) are determined by the solidification temperature alone (treated as a function of pressure and hence of depth). Their solution depends on the magnitude and time depend- ence of the heat flux Q across the CMB. They take, as examples, probable maximum and minimum values of Q and obtain the time for the IC to grow to its present size as 1800 Ma and 4200 Ma respectively (Figure 2). Whether or not growth of the IC plays a significant role in initiating reversals, it is tempting to ascribe the long-term absence of reversals which have occurred on occa- sion (see later) to a change in the IC radius. This could be brought about by changing conditions at the CMB; for example, if the OC became colder, the melting point of Fe in the OC would be encountered further out and the radius of the IC would become larger. </p><p>The magnetization of rocks Most rock-forming minerals are non- magnetic, but all rocks show some mag- netic properties due to the presence of various iron oxide and sulphide minerals making up only a few per cent of the rock. Information about the Earths magnetic field in the past comes from measurements </p><p>168 </p><p>2000 </p><p>5 1600 . </p><p>500 </p><p>0 0 2 4 6 </p><p>Time/lOga </p><p>Figure 2 Radius of the inner core (in km) measured from the...</p></li></ul>