# Seismological Research Letters

- © 2004 by the Seismological Society of America

Any of us who have ever taught magnetics in an undergraduate geophysical exploration course are familiar with the common curly questions regularly asked by students about the Earth's magnetic field. *What* is the source of the Earth's magnetic field? *Why* do the magnetic poles not align with the rotational poles? *How* does an optically pumped magnetometer work? *Why* does the magnetic field reverse polarity at irregular intervals? *How long* does a field reversal take? Will we all die if a reversal happens tomorrow? The answers to these and myriad similar questions lie outside the normal range of experience for a large majority of geophysicists, who generally concentrate on measuring the field and interpreting the geological implications of the measurements. It is, therefore, a great relief to be able to refer students to a book such as Campbell's. In five easy-to-read, logically organized chapters, Campbell fills us in on the current theories and beliefs regarding the Earth's magnetic field: the origins of the major component, the causes of periodic fluctuations, the effect the sun has on the Earth's field, measuring methods, and the practical applications of those measurements.

Chapter 1 is a necessary evil. It describes the current thoughts on the outer core origin of the major component of the Earth's magnetic field and explains how “geomagneticians” mathematically model the distribution of the field at the Earth's surface. Although Campbell sticks reasonably well to his stated aim to “emphasize the meanings of important equations, rather than obscure the relationships with complex formulas”, there is a practical limit to how far he is able to go because the concepts he describes are, primarily, mathematical in nature. I, personally, was floundering a bit by the time we reached Gauss coefficients of twelfth degree, sixth-order Legendre polynomial solutions to Maxwell's equations. Although the actual mathematics may have been beyond my meager abilities, the *concepts* were carefully, slowly, and clearly developed so at least I understood *why* the math is important.

Chapter 2 looks at the currents of electrically charged particles caused by tidal forces and solar radiation in the atmosphere and oceans, and describes the effect these have on the Earth's magnetic field. These currents are the primary cause of the “diurnal” drift that must be monitored during ground- and aeromagnetic area surveys. As with Chapter 1, spherical harmonic mathematical techniques are called upon to describe the effects of these periodic phenomena upon the magnetic field at the Earth's surface. The mathematics are, again, onerous, but the descriptive text accompanying the equations is sufficient to explain their relevance and importance.

Chapter 3 is the longest chapter in the book and the most in-depth. In it, Campbell betrays his most recent personal interests. He takes us out beyond the atmosphere to investigate the flows of charged particles originating from the sun and deep space, and examines their impact on the Earth's magnetic field. A significant portion of the chapter describes the fascinating (but arguably irrelevant) processes observed on the surface of the sun. Granted, it is these processes that give rise to the particles that ultimately affect the Earth's magnetic field, but for the practicing exploration geophysicist, the amount of space devoted to this discussion is out of proportion to the importance of “magnetic storms”... especially in an “introductory” text. I personally became quite lost in the myriad of concepts, coordinate systems, graphs, jargon, three-dimensional diagrams, and TLA's. But I did learn how auroras form, which is a good thing.

Chapter 4, from an exploration point of view, is perhaps the most useful chapter in the book. After a brief explanation of the physics behind the making of a bar magnet, the chapter clearly and comprehensively describes a wide variety of magnetometers. Each magnetometer is discussed in terms of its operating principles, sensitivity, accuracy, and dynamic range. Campbell concludes the chapter with a look at the current global network of geomagnetic observatories and a discussion of the inherent spatial bias in their distribution.

Chapter 5 tells us *why* we are interested in studying the geomagnetic field. Titled “Applications”, it provides a comprehensive list of the purposes for which we make magnetic field measurements. Some of these were new to me, such as strategies to combat pipeline corrosion at high latitudes, while others, such as aeromagnetic geological surveys and magnetotelluric conductivity sounding, will be familiar to most exploration geophysicists.

Summaries and exercises included at the end of each chapter are excellent for consolidating the important points contained in the text. Also, at the end of the book, Campbell includes an appendix on “Mathematical Topics.” This appendix provides handy little refresher courses on a range of mathematical themes, such as logarithms, scientific notation, calculus, vectors, trigonometry, and complex numbers. This section greatly extends the accessibility of the text for those who may not have a strong mathematical background. Campbell also provides Internet links to software applications referred to in the text.

The book is not without some faults (nothing in this life is perfect, after all). One that particularly struck me was the number of unreferenced assertions it contains. For instance, Campbell presents a “possible” temperature profile for the Earth (Figure 1.30) with no source acknowledged for the data. I know from personal research that the error margins on such a profile are large and the profile presented by Campbell is just one of many possibilities. The meanings of some of the figures in the book are also a bit obscure. Are we really meant to believe from Figure 1.29, for example, that electrical currents in the magnetosphere flow forward and backward through time?

On the whole, though, Campbell has produced an excellent reference and teaching book that answers many of the common questions raised by students of geomagnetism. As an added bonus, the snippets of interesting scientific trivia included throughout the book add to the overall enjoyment of the read. Processes only marginally related to the geomagnetic field (*e.g.*, the formation of ozone in the atmosphere, the dimensions of the galaxy, the surface processes of the sun, the quantum behavior of Rubidium) are placed in context so that one gains a feel for the complex interrelationships that make our universe such a fascinating place.