Earth's Magnetic Field: Secular Variation

At every point on the Earth, its magnetic field has a direction, as indicated by a compass needle free to pivot three-dimensionally, and an intensity or strength. The direction and intensity of the magnetic field change over timescales of years to millennia, a phenomenon known as secular variation. Over longer geologic timescales of tens of thousands to millions of years, the Earth’s field reverses polarity, a phenomenon called geomagnetic reversal.

Overview

Secular variation of the Earth’s magnetic field refers to changes in the field’s direction and intensity, a phenomenon manifested everywhere on the Earth’s surface. Its most obvious effect is a gradual shift in the direction which an ordinary compass needle points. It also is seen in changes of inclination, the angle at which a magnetic needle suspended by its center tilts below the horizontal, as well as variations in the field intensity or strength. These changes appear to be noncyclic and occur over timescales of years to millennia.

The direction of the Earth’s magnetic field at any point is specified by two angles called declination and inclination. The north end of a magnetic compass needle points approximately to the north, but not exactly. The angle between geographic or true north (defined by the Earth’s north rotational pole) and magnetic north originally was called magnetic variation, but now is called declination. It was first noticed around the twelfth century, when it was thought to be caused by abnormalities in the compass needle’s magnetization or suspension. However, by the early sixteenth century, Europeans had accepted declination as a phenomenon of the Earth’s magnetism. Inclination, the downward tilt of a compass needle free to pivot three-dimensionally, was also discovered during that century. Hence William Gilbert could write in 1600 of both declination and inclination as natural features of the Earth’s magnetism.

By the early sixteenth century, Europeans had noticed that the declination varies from place to place (and this helped convince them that declination was a feature of the Earth’s field and not due to flaws in their compasses). The discovery arose in the practices of navigation, chart making, and crafting of magnetic compasses—all activities connected with exploration. Perhaps Christopher Columbus, and certainly Sebastian Cabot, noted that while compass needles pointed east of north near Europe, they pointed west of north in the New World.

All three measures of the magnetic field—declination, inclination, and intensity—vary over the entire planet. These variations are most easily depicted with maps on which curved lines connect points that have the same value of one of these three parameters. For example, maps that display curved lines of equal magnetic declination are called isogonic maps. The first such printed map was produced in about 1701 by Edmond Halley of comet fame. Initially it was hoped that isogonic maps could be used to determine longitude by compass. Maps similar to isogonic maps but displaying lines of equal inclination or equal intensity also can be drawn.

Meanwhile, Henry Gellibrand announced in 1635 that declination changes over time as well as space. He found that the magnetic declination for London had shifted from 11.3° east of north in 1580 to 4.1° east of north in 1634. Later investigators discovered that the inclination and the intensity of the magnetic field also gradually change. For example, between 1700 and 1900, the inclination at London decreased from about 75° to 67°. Currently the overall intensity of the dipole field is decreasing at the rate of about 6 percent per century. If the field were to continue to decrease at this rate, it would drop to zero in about sixteen hundred years.

The agonic line is the “line of zero declination,” along which compass needles point exactly toward geographic or true north. One aspect of secular variation has been the westward drift of the agonic line. This drift can be depicted on maps; just as one can map the magnetic parameters, one can also map how these parameters change, by drawing curved lines connecting points that change at the same rate. For example, all points where the declination is shifting westward at 10° per century would be connected together. These charts, known as isoporic charts, came into wide use in the mid-twentieth century. Areas of most rapid change are called isoporic foci. These isoporic foci are drifting westward, just as the agonic line is. While this westward drift has been a persistent feature of secular variation since Gellibrand, first pointed it out, some evidence exists for eastward drifts during prehistoric times.

Geologic evidence of ancient magnetic fields preserved in some igneous and sedimentary rocks shows that the geomagnetic field has reversed polarity many times over intervals of tens of thousands to millions of years. These geomagnetic reversals have played a major role in plate tectonics. They provide evidence of seafloor spreading and can be used to determine its rate. They indicate the locations of continents in the geologic past and thus can be used to trace continental drift.

The geomagnetic field and its secular changes traditionally have been attributed to the Earth’s interior. Thus theories of the source of the field and the causes of its secular variation are necessarily indirect and extremely diverse, given the inaccessibility of the interior for direct study. About four hundred years ago, Gilbert suggested that the Earth behaves as if it had a bar magnet or magnetic dipole of extraordinary intensity at its center. In 1674, Robert Hooke asserted that the magnetic dipole axis of the Earth is tilted about 10° from the axis of rotation and that the dipole axis rotates westward around the rotational axis every 370 years. In 1683, Halley proposed that a double dipole pattern with four magnetic poles provided a better fit to worldwide declination data than just two magnetic poles, and in 1692, he suggested that his four poles could explain secular variation. Two of these poles he assigned to the Earth’s outer crust and the other two to a central nucleus, which rotated slightly more slowly than the crust, on the same axis. The crustal magnetic poles were fixed in place, and as the nucleus, rotating a bit more slowly, drifted slowly westward relative to the crust, so did its magnetic poles. This explained, he thought, the drift of the agonic line. Theories that the core is permanently magnetized were later ruled out when models of the Earth’s interior showed that it is too hot; its temperature is above the Curie temperature of all known permanently magnetized materials. The Curie temperature (or Curie point) is the temperature above which a material is no longer permanently magnetic.

It is known that moving electrical charges generate magnetic fields. In particular, an electric current flowing around a wire loop produces a magnetic dipole field through the center of the loop. During the nineteenth and twentieth centuries, theories were developed that attributed the magnetic field and its secular variations to electric currents in the Earth’s interior. One hypothesis was that the rotation of the Earth’s iron core carried the charges with it, and this motion generated the field. This theory reached its highest state of development around 1950 in work by Patrick M. S. Blackett, but since then it has gradually lost favor. An alternative hypothesis is that the flow of molten metal in the interior carries the charge that generates the field. First introduced in rudimentary form in the nineteenth century, this theory became increasingly sophisticated with the investigations of Walter Elsasser beginning in 1939 and Sir Edward Crisp Bullard starting in 1948. Elsasser proposed that the combination of the movement of molten metal and the simultaneous flow of electricity in it produced both the Earth’s main dipole field and its secular variations. This dynamo was driven, he suggested, by the heat generated by the decay of radioactive materials in the interior. Convection of hotter, less dense materials upward and of colder, denser materials downward, he said, produced the dynamo.

There now is general agreement that the geomagnetic field and its secular variation are the result of fluid motions in the molten outer part of the Earth’s metallic core. Various models have been developed to show that convective motion in the molten outer core, modified by the Coriolis effect due to the Earth’s rotation, can produce the observed field and its secular variations. Metals are good electrical conductors because electrons can move easily through metals. The molten metal of the outer core, as it moves through the magnetic field, makes electrons in the metal move, inducing electric currents in the metal that in turn generate the magnetic field. Thus the geodynamo is self-sustaining, but it is not a perpetual motion machine; it needs an energy source to drive the motion. The geodynamo does not create its magnetic field from nothing; rather, it converts some other form of energy into magnetic energy. The two most probable energy sources to drive the convection and generate the field are heat from the decay of radioactive materials and the crystallization and settling of iron (and other dense metals) to the solid inner core.

In the end, one must remember that models of the geodynamo and its energy sources are tentative. Many debates are still waged over the details of the geodynamo and how it produces secular variation. This area of geophysical theory is a most active and challenging one, and it is in rapid flux.

Methods of Study

The simplest way to detect secular variation is to observe the changing declination of a magnetic compass over some decades; until the twentieth century, that was the only way. All the instruments employed by famous investigators of geomagnetism, from Gilbert in 1600 to Carl Friedrich Gauss in the 1830’s, used adaptations of the compass to measure the magnetic parameters and their changes. Among other goals, these scientists aimed to measure these elements more accurately, so as to reveal secular change in shorter time intervals. During the twentieth century, however, there was a sustained trend to replace traditional magnetic needle instruments with ones based on other applications of electromagnetic principles.

Around 1900, research-quality earth inductors were developed to replace the dip needle and circle in measuring inclination. The idea behind this first of the new electrically based geomagnetic instruments is simple. Rotate a coil of wire about its own diameter in a magnetic field. If the rotational axis differs from the direction of that field, an electric current is induced in the coil, but if the axis coincides with the field, the current will cease. This “null” method now is used to measure inclination more accurately and easily.

The earth inductor was followed in the 1930s by the flux-gate magnetometer. This instrument is based on a high-permeability alloy, that is, one that magnetizes readily. Around two cores of such material are wound two coils of wire, in opposite directions, that carry the same alternating current, so that the same reversing magnetic field is produced in both cores, but 180° out of phase. When placed in the Earth’s field, the changes in the magnetic fields of these two cores do not cancel out, and this changes the current flowing in the coil around each core by different amounts. The net current is related to the component of the Earth’s magnetic element in the direction the magnetometer is pointing. When, however, this magnetometer is oriented parallel to the Earth’s field, no current is produced. The flux-gate magnetometer has seen wide use in aerial geomagnetic surveys.

Other generations of magnetic instruments have appeared since the flux-gate. Some of the most useful are proton precession magnetometers, rubidium vapor magnetometers, and superconducting magnetometers. These devices take advantage of principles of quantum physics. Some of them, like the proton precession instrument, measure only the total intensity of the Earth’s field. Others, like the superconducting magnetometer, are directional. Both types are many times more sensitive than older instruments and also perform much faster.

Magnetic surveys have been an essential part of the method of studying secular variation. All over the world, teams of observers have established “repeat stations,” or places for careful observation of the magnetic field parameters at various time intervals. Magnetic surveys have been greatly facilitated not only by the new instruments mentioned above but also by the way those instruments are used. Surveys are now often conducted very quickly with instruments carried by airplanes and satellites (such as MAGSAT). Data that once took decades to gather are now collected in months. Moreover, the extensive calculations needed to analyze global data have been greatly accelerated by computers. Worldwide magnetic charts are produced much more frequently now than in 1900, and the study of secular variation is thus much more detailed.

Equally impressive changes have been wrought by the use of geomagnetic methods to study the magnetic properties of rocks. Igneous and sedimentary rocks that contain iron grains can record the Earth’s magnetic field at the time they formed. The phenomenon is called remanent magnetism or paleomagnetism. Until the development in the middle of the twentieth century of techniques for measuring remanent magnetism in rocks, secular variation studies were limited to data obtained by direct measurement of the Earth’s field during historic times. Little was known of the magnetic field before 1600. Past phenomena that have been revealed by these methods include reversals of the magnetic field polarity and geomagnetic excursions.

The study of geomagnetism has come a long way with the rapid development of new instruments and methods. No longer is the purpose restricted to just a description of the main field and its variations. With the new sensitivity and portability made possible by electronics, geomagnetic secular variation has become a useful tool in many diverse scientific endeavors, such as archaeological dating of artifacts, magnetostratigraphic dating of sediments, determining rates of seafloor spreading, and tracing continental drift, in addition to the traditional effort to understand processes occurring in the Earth’s core.

Context

Most people are familiar with the magnetic compass, and many know at least roughly how to use it. Two activities which demand close attention to magnetic declination and its secular variation are reading topographic maps and navigating at sea.

In the margin of topographic maps, there usually are arrows which point to true north and magnetic north. With this declination information, one can relate directions on the map to compass readings in the field. In areas where the secular variation of declination occurs rapidly, it is also necessary to know when declination readings were last measured and the rate of their change. For example, near Tay River in the Canadian Yukon, the declination was listed as 33° , 25 arc minutes, east of north in 1979 and decreasing at 3.3 arc minutes per year. Thus, if the secular variation there continued at that rate, in a century the declination would change by 5° , 30 arc minutes, to 27° , 55 arc minutes east of north. Secular variations cannot be predicted reliably over so long a period, however, and maps are therefore updated regularly in magnetic surveys.

Information regarding declination at sea and especially near the coast is of even greater importance. Every ship is sometimes beset by fog, and thus an essential bit of navigational data is the present declination. Up-to-date charts are, again, the best means to avoid dealing with secular variation. As the date of the magnetic declination recedes into the past, however, reliable information concerning its secular change becomes more important.

The deep interior of the Earth is inaccessible to direct study. Thus scientists must watch closely for clues received at the Earth’s surface about the conditions and processes in the interior. Magnetic secular variation is one of the ways information can be obtained about the geodynamo. The geomagnetic changes occurring over geologic time have been preserved in some igneous rocks as they cooled and some sedimentary as they settled polarity reversals of the Earth’s main magnetic field. This remanent magnetism or paleomagnetism records what the field was like in the past. Such data gathered around the world from rocks of various ages reveal that the geomagnetic field has reversed its polarity many times in the geologic past. These geomagnetic reversals typically occur at intervals of tens of thousands to millions of years; the last one happened about 700,000 years ago. The geomagnetic reversals can be tied in to the chronology of the Earth and are an important element in plate tectonics. In the years prior to 2015, evidence of the weakening of Earth's magnetic field suggested that a geomagnetic reversal might be imminent. Research from the Massachusetts Institute of Technology (MIT) and Rutgers University, however, stated that the weakening was a return to a historical average and that a geomagnetic reversal would not happen in the near future.

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