J. A. Jacobs

Reversals of the earth's magnetic field

Abstract One of the most intriguing problems in geophysics to-day is why the Earths magnetic field reverses. This review summarizes the progress in our knowledge of the subject since Bullards [1] Bakerian Lecture to the Royal Society in 1967. Reversals have played a major role in the changed outlook in geological thinking through the development of the concept of plate tectonics. Polarity transitions also provide the geologist with horizons which may be applied to stratigraphic correlations and dating problems. The mean frequency of reversals has also shown marked changes in the past and these may well be related to major tectonic changes. In spite of all these increased benefits to the geologist, the physics of the reversal mechanism is still not well understood. The detailed behaviour of the magnetic field during a polarity transition is described as well as the more recently observed phenomenon of excursions of the field or aborted reversals. Physical and mechanical models of the geodynamo are examined in some detail in so far as they may affect reversals. The role of convection in the Earths core is also discussed.

The Earth’s Interior

A major obstacle encountered in a study of the constitution of the Earth’s interior is the difficulty of obtaining experimental data. It is impossible in the majority of cases to obtain direct measurements of physical quantities at depth. The only alternative is to simulate in the laboratory the conditions that prevail in the Earth’s interior. In this connection important pioneering work has been carried out by Bridgman whose work will be discussed in more detail later. Even so, the greatest pressure obtained by him viz. 100,000 bars, corresponds to a depth of only 300 km. in the Earth. Thus extreme extrapolation is needed to reproduce conditions deep within the Earth—extrapolations which in many cases may not be justifiable. There are, however, two means of obtaining information about the Earth’s interior. These are the study of seismic waves and the analysis of the Earth’s magnetic field and its secular variation. Both these fundamental subjects will be considered in detail later.

The stochastic excitation of reversals in simple dynamos

Abstract Several variations of the αω disc dynamo models are known to exhibit chaotic magnetic field behaviour typical of non-linear, dissipative systems. Although these models demonstrate that geomagnetic reversals can be generated by simplified dynamo equations, the behaviour of the magnetic field itself is generally too simple, showing especially an absence of long polarity epochs in most of the models. We show that the addition of three varieties of stochastic processes (Gaussian, flicker and brown noise) enriches the field evolution and can lead to realistic palaeomagnetic behaviour. We argue that noise processes must be present in the actual fluid core and suggest, from a physical point of view, a flicker noise stimulation of the dynamo. We find two features of the palaeomagnetic record that would favour the presence of noise in the dynamo process, namely the absence of a linear oscillation in field intensity between reversals or, even if present, the absence of an increase in amplitude of this oscillation prior to a reversal. We also consider the addition of a random component to the helicity driving function of the α2 dynamo process and show that various types of reversal can occur. Unfortunately, realistic field behaviour cannot be maintained over long time periods due to the tendency for the magnitudes of the poloidal and toroidal fields to equilibrate during a polarity epoch.

The Interior of the Earth

Publisher Summary This chapter discusses the composition and constitution of the Earth. The chapter presents some of the advances made in the knowledge of the interior of the Earth during the past several years. The thermal history of the Earth emphasizing the time factor is also provided. The Earth magnetic field is discussed. Theories of the main field may be classified as specific or general. A specific theory is one which attributes the magnetic properties of a rotating body to the specific properties of the matter of which it is composed. A general theory is one that attributes the field to some general property of rotating matter. Any induction theory is bound up with the details of the fluid motions within the Earths core, and there may be a considerable number of hypothetical flow patterns which could cause regeneration. Magnetohydrodynamic oscillations in the Earths core in relation to the stability of a possible dynamo action are investigated. There is a significant field in which great progress has been made during the last several years and which may well have a profound influence on geophysical thought, viz., and rock magnetism.

The Earth's deep interior

The deep interior of the Earth is inaccessible to man and in some ways we know less about it t h an we do about the distant stars. However seismology has revealed certain details of the earths structure. Below a rocky mantle some 2,900 km thick there is a core consisting mainly of nickel-iron, the outer part of which is liquid and the inner part solid. The origin and composition of the core is discussed in some detail. The core is of particular interest since it is the seat of the Earths magnetic f i e ld -mot ions of the fluid outer core providing the driving mechanism for some kind of dynamo action.

The evolution of the Earth's core and its magnetic field

Abstract The broad spectrum of variations in the Earths magnetic field is discussed with particular reference to hydromagnetic oscillations in the Earths core. Palaeo-intensity measurements are reviewed and the effects on the magnetic field of the evolution of the core and of irregular fluctuations in the rate of the Earths axial rotation are discussed.

The earth's magnetic field

Geomagnetic micropulsations are short period (usually of the order of seconds or minutes) fluctuations of the Earth’s magnetic field. They are transitory variations of small amplitude (usually less than one part in 104 of the Earth’s main field) and leave no permanent effect on the field. Like longer period disturbances such as magnetic storms they are of solar origin, in contrast to the Earth’s main field and secular variation which are of internal origin.

The origin of the Earth's core

Abstract The origin of the Earths core is bound up with the origin of the Earth itself and the solar system. The mechanism of accretion is thus examined in some detail and the origin of the Earth, the Moon and Mars discussed. The growth of the Earths core is investigated — in particular the question of when this event took place and how long it took for completion. The thermal history of the Earth, its magnetic field, tectonic events at its surface and other geophysical phenomena are reviewed in relation to the evolution of the Earths core.

Interiors of the planets

A H Cook Cambridge: Cambridge University Press 1980 ix + 348 pp price £25 This book complements Professor Cooks earlier book, The Physics of the Earth and Planets (Halstead Press 1973), which dealt essentially with the earth alone. With the recent spectacular data obtained from the Voyager encounters with Jupiter and Saturn, Professor Cooks latest book is very opportune.