Sources of variation in the steady magnetic field
Secular variation of the main field
The main magnetic field of Earth, as observed at the surface, changes continuously with time. Changes of very short duration compared with geologic processes are called secular variation. Observations of declination made in London since 1540, for example, show that the direction of the field at that site has nearly completed a full cycle with a peak-to-peak amplitude of 30°. Οther components of the field have been observed for a shorter length of time, but they also are exhibiting similar rapid change.
The characteristics of the secular variation are often represented by superimposing maps of the rate of change of a given field component on maps of the component itself. Such maps reveal that the world may be broken down into regions of continental scale in which a given component is either increasing or decreasing. Changes can be as large as 150 nanoteslas per year and persist for tens of years. If maps of secular variation from successively later times are examined, many features of the secular variation are found to be displaced westward with time.
The dominant component of the internal field is that of a centred dipole. It is useful to determine whether this component changes in the same way as the remainder of the field. Because the field of a dipole is so simple, it is more convenient to represent its change by its strength and orientation rather than by maps. Secular variation of the non-dipole components, however, are usually presented as maps. Such maps are similar to maps of secular variation of the entire field, indicating that most of the secular change is caused by the non-dipole components. On the average, the non-dipole components of the field appear to drift westward at an average rate of 0.18° per year. At this rate, drifting features circle Earth in only 2,000 years. Not all the non-dipole field exhibits drift. At least half of it appears fixed and variable only in intensity.
The dipole component also changes with time. Since 1850 its strength has decreased from about 8.5 × 1022 to about 8 × 1022 amperes per square metre. If this trend continues, the dipole component will vanish in another 2,000 years. As will be discussed in the next section, the dipole component of Earth’s field appears to be in the process of reversing.
The best estimates are that the orientation of the dipole component appears to change with time. The dominant change is a westward drift of the azimuth of the dipole but at a rate much slower (0.08° per year) than the non-dipole component. The polar angles also may be increasing but even more slowly.
The origin of the secular variation is not known. Investigators suspect that it is a secondary effect of the dynamo mechanism that generates the main field. The short timescale of the variation implies that the source is in the outer region of the liquid core. If the source was deeper, the variation would be so attenuated by the electrical conductivity of the core that it would be undetectable at the surface.
The westward drift of magnetic anomalies evident in the secular variation should provide an important clue to the origin of the main field if only it can be interpreted. One model explains the drift by postulating that the outer portion of the liquid core is rotating slower than the more rigid mantle above. As a whole, Earth rotates eastward. If features within the core rotate more slowly than surface features, they will appear to move backward relative to the general rotation—i.e., westward. In this model the secular variation is caused by portions of eddies in the internal current system that rotate more slowly than the planet as a whole.
A more recent model for the westward drift posits that it is produced by hydromagnetic waves in the core (see below Magnetohydrodynamic waves—magnetic pulsations). In this model the core rotates at the same rate as the outer mantle, but a wave propagates slowly around the outer portion of the core. Because waves in a conducting fluid distort the magnetic field frozen within it, they produce changes that can be observed at the surface. Since the characteristics of waves depend on the medium through which they propagate, it may be possible to infer properties of the outer core from surface observations.
Reversals of the main field
Earth’s internal magnetic field has not always been oriented as it is today. The direction of the dipole component reverses, on an average, about every 300,000 to 1,000,000 years. This reversal is very sudden on a geologic timescale, apparently taking about 5,000 years. The time between reversals is highly variable, sometimes occurring in less than 40,000 years and at other times remaining steady for as long as 35,000,000 years. No regularities or periodicities have yet been discovered in the pattern of reversals. A long interval of one polarity may be followed by a short interval of opposite polarity.
Available data suggest that during a reversal the strength of the dipole component shrinks to zero while maintaining its orientation. It then grows again to its former strength but with opposite orientation. During the interval in which there is no dipole component, the non-dipole part of the field appears to persist.
During field reversals the outer portion of Earth’s magnetic field is greatly altered. The absence of a dipole component would mean that the solar wind would approach much closer to Earth. Cosmic-ray particles that are normally deflected by Earth’s field or are trapped in its outer portions would reach the surface of the planet. These particles might cause genetic damage in plant or animal communities, leading to the disappearance of one species and the appearance of another. Attempts have been made to establish whether there is evidence for such changes at the time of field reversals. Thus far the results remain inconclusive.
Evidence for the occurrence of magnetic reversals is unquestionable, however. Magnetic surveys made by ship across spreading centres in the middle of the oceans provide the best evidence. These data show that strips of oppositely magnetized ocean floor appear symmetrically about such features as the Mid-Atlantic Ridge. The explanation for these strips is that molten basalt flows out of the ridge and spreads away in both directions. As the basalt cools, it captures the orientation of the prevailing magnetic field and carries it along on the spreading seafloor. Basalt emerging from the ridge and cooling at later times captures the subsequent field orientation. The seafloor thus acts like a magnetic tape, capturing the alternating sequence of field orientations.
It should be noted that more information than the sense of the dipole component is captured in cooling rocks. Rocks formed at the magnetic equator, for example, contain a horizontal magnetization. Similarly, rocks formed at higher magnetic latitudes contain a field pointing up or down at an inclination that depends on latitude. The declination of the magnetization further reveals the direction to the magnetic pole at the time of the magnetization. Together these two angles can be used to infer the location of a virtual magnetic pole relative to the location of the sample.
Such a technique has been used to study the history of Earth’s field at various locations. When virtual poles are determined from progressively older rocks, it is found that the virtual poles appear to wander with time. For many years it was thought that this “polar wandering” was a characteristic of Earth’s magnetic field. Recent studies, however, prove instead that it is a result of continental drift. Magnetic poles have not moved significantly relative to the geographic poles, but rather the continents have. Thus, progressively older rocks were formed when continents were at different locations from where they are today (see also plate tectonics: Paleomagnetism, polar wandering, and continental drift).
Reversals of the main field must be caused by the dynamo mechanism that gives rise to the field in the first place. The timescale for the reversal is so rapid that it clearly cannot be caused by geologic processes. Furthermore, reversals cannot be caused by simple decay and reappearance of a preexisting field. The electrical conductivity of the core is too high to allow the field to decay on such a short timescale. In some way minor changes in the magnetic field configuration of the core must be amplified by thermal convection, causing the field to grow rapidly in the opposite direction. Models that simulate the main field have been shown to possess this property. The solutions to equations that describe the generation of the main field are unstable, and small changes can cause solutions of opposite sign to appear.