Geomagnetic Reversals: Although decreasing rapidly, the earth's magnetic field is probably not now reversing.

Variations in the lengths of geomagnetic polarity, intervals are analyzed by means of a probability model based on the theory of Bernouli trials. Polarity reversals are assumed to occur as the result of the interaction between steady oscillations of the geomagnetic dipole and secular variations of the nondipole field. The particular cycle on which a polarity inversion occurs is determined by the magnitude of the nondipole field, which is assumed to vary randomly and independently of dipole variations. The reversal properties of the geomagnetic dynamo are characterized by the single parameter p, the probability that a polarity inversion will occur during one cycle of change in the geomagnetic dipole moment. From an analysis of polarity changes during the past 10 m.y., the value of p is estimated to be 0.05. During the Permian period, it was at least two orders of magnitude smaller. The analysis suggests that within the past 10 m.y. there have occurred hitherto undiscovered short geomagnetic polarity events with durations shorter than 0.05 m.y.

Lengths of geomagnetic polarity intervals

Variations in time of the Earth's natural magnetic field have a frightfully wide spectrum, ranging over more than 20 orders of magnitude: They extend from well over 103 Hz, where they merge into the radioelectric noise spectrum, to probably more than 100 m.y. In rough summary, the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere, whereas the longer periods arise from internal sources, primarily fluid motions in the core at depths in excess of 2900 km. Methods for separating external and internal signals have been available since the time of Gauss ( 1 838), but the accuracy, the space distribution, and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations. Although periods shorter than 1 yr should clearly be assigned to external sources, the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress. Currie ( 1966, 1 968) proposed that there was little overlap and a cutoff of '" 3.7 yr. A long debated", l l -yr contribution due to the solar cycle was clearly demonstrated by Currie ( 1973a), Courtillot & Le MoueI ( 1976a,b) and Alldredge ( 1976), and the cutoff period was thus rejected to at least ",20 yr. The situation seemed to be confused again when Courtillot et al ( 1978a) provided evidence for sharp impulses of 1 yr duration i n the internal, so-called secular, variation. The debate that followed over the reality of brief internal events is one of the topics addressed in this review. We first provide a short historical account of successive discoveries

Time Variations of the Earth's Magnetic Field: From Daily to Secular

[1] The basic features of the geomagnetic reversal chronology of the last 160 million years are well established. The relationship between this history and other features of the field, however, has been elusive. The determination of past field strength (paleointensity) is especially challenging. Commonly accepted results have come from analyses of bulk samples of lava. Historic lavas have been shown to faithfully record the past field strength when analyzed using the Thellier double-heating method. Data from older lavas, however, tend to show effects of in situ and laboratory-induced alteration. Here we review an alternative approach. Single plagioclase crystals can contain minute magnetic inclusions, 50–350 nm in size, that are potential high-fidelity field recorders. Thellier experiments using plagioclase feldspars from an historic lava on Hawaii provide a benchmark for the method. Rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples. This resistance is likely related to the lack of clays. In addition, magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well-defined thermoremanent magnetization into an irresolute chemical remanent magnetization. If there is a relationship between geomagnetic reversal frequency and paleointensity, it should be best expressed during superchrons, intervals with few (or no) reversals. Thellier data sets based on single plagioclase crystals from lavas erupted during the Cretaceous Normal Polarity Superchron (∼83–120 million years ago) suggest a strong (>12 × 1022 Am2), stable field, consistent with an inverse relationship between reversal frequency and paleointensity. Superchrons may represent times when the pattern of core-mantle boundary heat flux allows the geodynamo to operate at peak efficiency, as suggested in some numerical models. Thellier data from single plagioclase crystals formed during times of moderate ( 10 reversals/million years) reversal occurrence suggest a weaker and more variable field. These paleointensity data, together with a consideration of paleomagnetic directions, suggest that geomagnetic reversals, field morphology, secular variation, and intensity are related. The linkages over tens of millions of years imply a lower mantle control on the geodynamo. On even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the Proterozoic and Archean geomagnetic field during the onset of growth of the solid inner core. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field similar to that of the modern, 2.5–2.7 billion years ago. Older Archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow. Most rocks of this age have been affected by low-grade metamorphism; investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo. Absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization, anisotropy, and slow cooling. Fortunately, prior work in rock magnetism, together with advances in analytical equipment and techniques, provides a solid foundation from which these frontier issues can be approached.

https://www.eoas.ubc.ca/~mjelline/453website/eosc453/E_prints/newfer06/2005RG000189.pdf

The paleomagnetism of single silicate crystals: recording geomagnetic field strength during mixed polarity intervals, superchrons, and inner core growth

The dipole moment of Earth's magnetic field has decreased by nearly 9% over the past 150 years and by about 30% over the past 2,000 years according to archeomagnetic measurements. Here, we explore the causes and the implications of this rapid change. Maps of the geomagnetic field on the core-mantle boundary derived from ground-based and satellite measurements reveal that most of the present episode of dipole moment decrease originates in the southern hemisphere. Weakening and equatorward advection of normal polarity magnetic field by the core flow, combined with proliferation and growth of regions where the magnetic polarity is reversed, are reducing the dipole moment on the core-mantle boundary. Growth of these reversed flux regions has occurred over the past century or longer and is associated with the expansion of the South Atlantic Anomaly, a low-intensity region in the geomagnetic field that presents a radiation hazard at satellite altitudes. We address the speculation that the present episode of dipole moment decrease is a precursor to the next geomagnetic polarity reversal. The paleomag- netic record contains a broad spectrum of dipole moment fluctuations with polarity reversals typically occurring during dipole moment lows. However, the dipole moment is stronger today than its long time average, indicating that polarity reversal is not likely unless the current episode of moment decrease continues for a thousand years or more.

Changes in earth's dipole

IN the course of an investigation into magnetic secular variation, a number of spherical harmonic analyses were performed on successive 5 year means of results from eighty magnetic observatories. From the first three spherical harmonic coefficients the moment of the equivalent dipole of the geomagnetic field (M) can be deduced in terms of the radius of the Earth (a) from the relation 



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S. R. C. Malin

Papers

Recent Changes in the Magnetic Dipole Moment of the Earth