Geomagnetic Polarity Reversals Research Articles

A parametric study of magnetosphere–ionosphere coupling in the paleomagnetosphere

Geomagnetic polarity reversals last about 1–8 thousand years, while the intensity of the Earth’s dipole field may drop as much as 1 order of magnitude. In this paper, we aim at modeling the paleomagnetosphere, in particular, the magnetosphere–ionosphere coupling during such polarity transition epochs, using the BATS-R-US global magnetohydrodynamic (MHD) simulation code. We study the variation of the transpolar potential and the total field aligned currents with changing dipole moment, ionospheric Pedersen conductance, and interplanetary magnetic field. The simulations remarkably well reproduce the transpolar potential saturation predicted by the analytical Hill model and confirmed by observations. Estimates of transpolar potential and region 1 field aligned currents are given for paleomagnetospheres with strongly decreased dipole moments. In general, a fairly good agreement was found between the simulation results and the predictions of the Hill model, however, MHD simulations give a somewhat steeper drop of transpolar potential as the dipole moment decreases. Some corrections to the dipole scaling relation in the Hill model are suggested.

Asymmetric Polarity Reversals, Bimodal Field Distribution, and Coherence Resonance in a Spherically Symmetric Mean-Field Dynamo Model

Using a mean-field dynamo model with a spherically symmetric helical turbulence parameter alpha which is algebraically quenched and disturbed by additional noise, the basic features of geomagnetic polarity reversals are shown to be generic consequences of the dynamo action in the vicinity of exceptional points of the spectrum. This simple paradigmatic model yields long periods of constant polarity which are interrupted by self-accelerating field decays leading to asymmetric polarity reversals. It shows the recently discovered bimodal field distribution, and it gives a natural explanation of the correlation between polarity persistence time and field strength. The dependence of the persistence time on the noise shows typical features of coherence resonance.

The Mesozoic-Cenozoic boundary: Paleomagnetic characteristic

The purpose of this paper is to consider the behavior of the geomagnetic field in the vicinity of the Mesozoic-Cenozoic (Mz-Kz) boundary, in particular, of the paleointensity and polarity of the field, the frequency of geomagnetic polarity reversals, the total magnitude of the field direction variations, and the behavior of the magnetic susceptibility of oceanic sediments in the vicinity of the Mz-Kz boundary. As to these paleomagnetic events, worthy of mention is the change in the variation conditions at the Mesozoic-Cenozoic boundary. It is global growth of the average variation magnitude from 8 -8.5 in the Late Cretaceous to 11.5 in the Early Cenozoic. The global character of this event suggests a general change in the generation of the geomagnetic field direction variations in the vicinity of the Mz-Kz boundary. A change in the Earth core conditions, causing the reversals of the geomagnetic field, began at 80 Ma, that is, notably earlier than the Mz-Kz boundary, and, hence, was not associated directly with this boundary, nor with the generation of variations in the geomagnetic field direction. Some local changes in the variations of the magnetic field direction were associated with the formation of plumes near the core-mantle boundary, yet, the plumes have originated 20-40 million years prior to the Mz-Kz boundary. As follows from the examination of sequences of oceanic sediments (DSDP and ODP data), the Mz-Kz boundary is often, but not necessarily, marked by a peak of magnetic susceptibility ( -peak), i.e. that is not a characteristic feature of the Mz-Kz boundary. The accumulation of magnetic materials in the sediments lasted from a few dozens of thousand years (more common estimates) to hundreds of thousand years. And, where this interval is observed, it does include the Mz-Kz biostratigraphic boundary, except for very rare cases. The Mz-Kz biostratigraphic boundary is always restricted to the C29r chron, but its position varies as a function of its paleolatitude, amounting to about 0.7 Myr. The highest -peak values are close to the epicenters of active plumes, but high -peaks have been found far from the plumes.

地磁気逆転開始期の地球磁場変動

We report the paleomagnetic behaviors at the beginning of the latest geomagnetic polarity reversal (Brunhes-Matuyama reversal of 0.78 Ma age) recorded in Tahitian lava flows. We applied an improved paleointensity determination method (double heating technique of the Shaw method with low-temperature demagnetization, LTD-DHT Shaw method), and detected oscillation-like changes in virtual dipole moments (VDMs) with a large amplitude ranging from 1.5 × 1022 to 10.9 × 1022 AM2 just prior to the directional reversal. The VDM changes are almost linearly correlated with the virtual geomagnetic pole (VGP) latitudes (60-90°S). We call this VDM-VGP latitude trend “reversal line”. Although the Thellier data reported for the Brunhes-Matuyama reversal are more scattered than our data, they also show a weak trend in the VDM-VGP latitude diagram, supporting the reversal line from a different locality. The scatter in the Thellier data may be caused by overestimated paleointensities due to methodological problems which are indicated in the recent studies of historical lavas. Therefore it appears that the reversal line from the Tahitian lavas in this study does not show a local change but represents a global-scale phenomenon in the geomagnetic reversal process.

Flood deposits penecontemporaneous with ∼0.8 Ma tektite fall in NE Thailand: impact-induced environmental effects?

Although a crater is not yet identified, the Australasian tektite strewn field provides evidence that a major impact cratering event took place in the Southeast Asian region at ∼0.8 Ma, just prior to the Brunhes/Matuyama geomagnetic polarity reversal. Paleomagnetic evidence including reversed polarity in mud lens closely associated with in situ tektites suggests that tektite-bearing flood deposits near Ban Ta Chang and Chum Phuang in northeast Thailand are penecontemporaneous with the impact event. The deposits include abundant organic debris, including whole tree trunks and mammal bones, that preserved due to reducing conditions, which are also responsible for the presence of abundant iron sulphides. Sedimentological observations suggest a series of major flood events that are out of character with the modern meandering river system to which they are related. The deposits are consistent with the effects of regional deforestation, increased run off and erosion, and other environmental disruptions expected in the aftermath of a major impact event.

Dependence of the duration of geomagnetic polarity reversals on site latitude.

An important constraint on the processes governing the geodynamo--the flow in the outer core responsible for generating Earth's magnetic field--is the duration of geomagnetic polarity reversals; that is, how long it takes for Earth's magnetic field to reverse. It is generally accepted that Earth's magnetic field strength drops to low levels during polarity reversals, and the field direction progresses through a 180 degrees change while the field is weak. The time it takes for this process to happen, however, remains uncertain, with estimates ranging from a few thousand up to 28,000 years. Here I present an analysis of the available sediment records of the four most recent polarity reversals. These records yield an average estimate of about 7,000 years for the time it takes for the directional change to occur. The variation about this mean duration is not random, but instead varies with site latitude, with shorter durations observed at low-latitude sites, and longer durations observed at mid- to high-latitude sites. Such variation of duration with site latitude is predicted by simple geometrical reversal models, in which non-dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction, and provides a constraint on numerical dynamo models.

The C27n‐C26r geomagnetic polarity reversal recorded in the west Greenland flood basalt province: How complex is the transitional field?

We present a paleomagnetic record of the C27n‐C26r (∼60.9 Ma) geomagnetic polarity reversal (GPR) sampled in two separate lava sequences in the Vaigat Formation, west Greenland flood basalt province. The two profiles, distanced 16 km apart, consist of 45 and 35 sampled lava flows of which 31 and 23, respectively, record transitional field directions. A prominent feature of each profile is the presence of successive flows with similar paleomagnetic directions, which based on geological, geochemical, and paleomagnetic evidences are interpreted to represent flow fields (i.e., lavas erupted in a short period of time) rather than periods when the geomagnetic field stood still. Multimodel photogrammetry shows that the typical flow length of Vaigat Formation picritic lavas is <2 km, which explains why flow‐to‐flow correlation of paleomagnetic directions between the two profiles (16 km apart) is not possible. The two profiles therefore record different and independent time series of the same GPR, allowing us to cast light on some of the questions that have arisen from single records. Particularly noteworthy is the fact that the two high‐resolution paleomagnetic records share no general trends, clearly indicating that the C27n‐C26r transitional field was more complex and faster changing than can be resolved by either record. In addition, we obtained stable polarity paleomagnetic data from both pretransitional C27n and posttransitional C26r yielding a paleomagnetic pole latitude 64.8°N, longitude 141.5°E (N = 14, K = 19.8, A95 = 9.2°) that passes reversal test.

Stochastic Resonance in Geomagnetic Polarity Reversals

Among noise-induced cooperative phenomena a peculiar relevance is played by stochastic resonance. In this paper we offer evidence that geomagnetic polarity reversals may be due to a stochastic resonance process. In detail, analyzing the distribution function P(tau) of polarity residence times (chrons), we found the evidence of a stochastic synchronization process, i.e., a series of peaks in the P(tau) at T(n) approximately (2n+1)T(Omega)/2 with n=0,1,...,j and T(omega) approximately 0.1 Myr. This result is discussed in connection with both the typical time scale of Earth's orbit eccentricity variation and the recent results on the typical time scale of climatic long-term variation.

Analysis of long-term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics

SUMMARY The 1167 published cooling unit (CU) palaeointensity estimates contained in the 400‐10 Ma portion of the PINT global database were rigorously filtered according to accurate age determinations, palaeodirectional reliability, recognition of polarity and the method of palaeointensity acquisition. The remaining 865 estimates (group 1) were further filtered to ensure self-consistency, reducing the data set to 425 estimates (group 2). Group 1 and 2 data were clustered into temporally and/or spatially distinct rock suites (RS) enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation (PSV). The record was segmented according to the distribution of the data, rather than using arbitrary time windows, to ensure quasi-consistent behaviour within each segment. Differences between these segments clearly indicate that a significant long-timescale (10 7 and 10 8 yr) variation of the mean geomagnetic poloidal field intensity (GPFI) occurred during the 400‐10 Ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field. Both the mean dipole moment and its standard deviation appear to be a function of the range of values each CU may adopt at one particular time. This range is itself controlled by the variation of the maximum limit of dipole moment, while the value of the minimum limit remains relatively constant. Tentative support is provided for the recent suggestion that PSV may have been reduced during the Cretaceous normal superchron (CNS), though more data are needed in the range 120‐60 Ma to confirm this. No conclusive evidence was found to support the suggestion that the GPFI record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt pTRM checks. Indeed, offsets caused by unreliable data in well-represented parts of the record are likely to be random and cancel one another out. When GPFI variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency (RF), it is not possible to confirm whether the two parameters are anticorrelated, decoupled or related in some more complex way. However, it is clear that GPFI and RF are definitely not positively correlated as has been previously suggested. The present database documents sharp increases in GPFI around the onset times of the two recognized superchrons, itself implying an anticorrelation. The implications, for geodynamo and mantle modelling, of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly. A generic geodynamic model is proposed to explain the relationship between observed longterm changes in GPFI and global geodynamic processes. The model predicts that changes in GPFI result from a chain of geodynamic processes extending from crust to core, beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection. Supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model. Four time stages are proposed to describe the major long-term changes in GPFI since the Early Devonian: 400‐350, 350‐250, 250‐175

Oriented inclusions of magnetite in clinopyroxene: Source of stable remanent magnetization in gabbros of the Messum Complex, Namibia

Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia. Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single‐domain magnetite, consistent with the observed sizes and shape anisotropy of the magnetite inclusions. As in previous studies of similar inclusions, these are inferred to have formed by exsolution. Two arrays of inclusions are regularly present in the Messum clinopyroxenes, inclined at about 74°, consistent with formation at about 800°C deduced from optimization of phase boundary orientations. Virtual geomagnetic poles from these rocks are consistent with reference data, confirming that the magnetization is of thermoremanent origin. Bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals, suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal.

Reversed polarity patches at the CMB and geomagnetic field reversal

The International Geomagnetic Reference Field models (IGRF) for 1900—2000 are used to calculate the geomagnetic field distribution in the Earth’ interior from the ground surface to the core-mantle boundary (CMB) under the assumption of insulated mantle. Four reversed polarity patches, as one of the most important features of the CMB field, are revealed. Two patches with +Z polarity (downward) at the southern African and the southern American regions stand out against the background of -Z polarity (upward) in the southern hemisphere, and two patches of -Z polarity at the North Polar and the northern Pacific regions stand out against the +Z background in the northern hemisphere. During the 1900—2000 period the southern African (SAF) patch has quickly drifted westward at a speed of 0.2—0.3°/a; meanwhile its area has expanded 5 times, and the magnetic flux crossing the area has intensified 30 times. On the other hand, other three patches show little if any change during this 100-year period. Extending upward, each of the re-versed polarity patches at the CMB forms a chimney-shaped “reversed polarity column” in the mantle with the bottom at the CMB. The height of the SAF column has grown rapidly from 200km in 1900 to 900km in 2000. If the column grows steadily at the same rate in the future, its top will reach to the ground surface in 600—700 years. And then a reversed polarity patch will be observed at the Earth’s surface, which will be an indicator of the beginning of a magnetic field reversal. On the basis of this study, one can describe the process of a geomagnetic polarity reversal, the polarity reversal may be observed firstly in one or several local regions; then the areas of these regions expand, and at the same time, other new reversed polarity regions may appear. Thus several poles may exist during a polarity reversal.

Long-term climato-limnological cycles found in a 3.5-million-year continental record

Analysis of physical properties in long sediment cores (BDP96) from Academician Ridge in Lake Baikal indicates that major climato-limnological changes during the past 3.5 Myr occurred at about 2.5–2.8, 1.7–1.9, and 0.9–1.2 Ma, which were close to times of major geomagnetic polarity reversals (Matuyama/Gauss, Olduvai, Jaramillo + Matuyama/Brunhes). The principal climato-limnological oscillation has a long-term period of nearly 1,000 kyr, which corresponds to the periodicity of fluctuation in solar insolation. It also seems to be related to geomagnetic field intensity. Other long-term period of 400 kyr corresponds to Milankovitch parameters of eccentricity. These results suggest that changes in solar insolation were closely related to long-term environmental variations in the deep continental interior.

Dramatic variations in the Earth’s main magnetic field during the 20th century

Analysis of the International Geomagnetic Reference Field (IGRF) 1900–2000 shows that the Earth’s main magnetic field has changed dramatically during the 20th century: its dipole moment has decreased by 6.5% since 1900, the strengths of its quadrupole and octupole have increased by 95% and 74%, respectively, four major planetary-scale magnetic anomalies on the Earth’s surface have enhanced by 21%–56%, and the magnetic center has shifted 200 km towards the Pacific Ocean. These time-variation features are similar to the behavior before a geomagnetic polarity reversal.

Generic nonlinear processes in self–exciting dynamos and the long–term behaviour of the main geomagnetic field, including polarity superchrons

When the main geomagnetic field undergoes a reversal in polarity it does so in no more than a few thousand years. Many hundreds of such reversals must have occurred over geological time (the age of...

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

Assessing the fidelity of palaeomagnetic records of geomagnetic reversal

A major difficulty facing the study of geomagnetic polarity reversals lies in interpreting palaeomagnetic records of polarity transitions. These records are the sole source of information about what happens to the field as it reverses. In addition to comparison of records of the same reversal from different types of recorders to check for accuracy, we need some internal measure that can be used as a gauge of the temporal resolution provided by the data. This paper explores methods using the extent to which secular variation is recorded in the full polarity intervals bounding polarity transitions to estimate the temporal extent to which the transitional record should be interpreted. Cumulative dispersion and two autocorrelation methods are evaluated using datasets representative of different end–member palaeomagnetic records of secular variation and polarity transitions.

Possible configurations of the magnetic field in the outer magnetosphere during geomagnetic polarity reversals

Abstract. Possible configurations of the magnetic field in the outer magnetosphere during geomagnetic polarity reversals are investigated by considering the idealized problem of a magnetic multipole of order m and degree n located at the centre of a spherical cavity surrounded by a boundless perfect diamagnetic medium. In this illustrative idealization, the fixed spherical (magnetopause) boundary layer behaves as a perfectly conducting surface that shields the external diamagnetic medium from the compressed multipole magnetic field, which is therefore confined within the spherical cavity. For a general magnetic multipole of degree n, the non-radial components of magnetic induction just inside the magnetopause are increased by the factor {1 + [(n + 1)/n]} relative to their corresponding values in the absence of the perfectly conducting spherical magnetopause. An exact equation is derived for the magnetic field lines of an individual zonal (m = 0), or axisymmetric, magnetic multipole of arbitrary degree n located at the centre of the magnetospheric cavity. For such a zonal magnetic multipole, there are always two neutral points and n-1 neutral rings on the spherical magnetopause surface. The two neutral points are located at the poles of the spherical magnetopause. If n is even, one of the neutral rings is coincident with the equator; otherwise, the neutral rings are located symmetrically with respect to the equator. The actual existence of idealized higher-degree (n>1) axisymmetric magnetospheres would necessarily imply multiple (n + 1) magnetospheric cusps and multiple (n) ring currents. Exact equations are also derived for the magnetic field lines of an individual non-axisymmetric magnetic multipole, confined by a perfectly conducting spherical magnetopause, in two special cases; namely, a symmetric sectorial multipole (m = n) and an antisymmetric sectorial multipole (m = n-1). For both these non-axisymmetric magnetic multipoles, there exists on the spherical magnetopause surface a set of neutral points linked by a network of magnetic field lines. Novel magnetospheric processes are likely to arise from the existence of magnetic neutral lines that extend from the magnetopause to the surface of the Earth. Finally, magnetic field lines that are confined to, or perpendicular to, either special meridional planes or the equatorial plane, when the multipole is in free space, continue to be confined to, or perpendicular to, these same planes when the perfectly conducting magnetopause is present.Key words. Geomagnetism and paleomagnetism (reversals-process, time scale, magnetostratigraphy) · Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetospheric configuration and dynamics)

Link between the geomagnetic polarity reversal and global-geology events

The origin of the earth’s magnetic field is one of the five most important unsolved problems in physics, especially geomagnetic polarity reversal remains one of Nature’s most enigmatic phenomena. It is proposed that the geomagnetic polarity reversal links with some global-geology events, although it is difficult to clarify the mechanism of this correlation. The history of continental structures in eastern China would be also linked with some global-geology events.

Effects due to induced azimuthal eddy currents in a self-exciting Faraday disk homopolar dynamo with a nonlinear series motor. I.: Two special cases

The elucidation of the behaviour of physically realistic self-exciting Faraday-disk dynamos bears inter alia on attempts by theoretical geophysicists to interpret observations of geomagnetic polarity reversals. Hide [The nonlinear differential equations governing a hierarchy of self-exciting coupled Faraday-disk homopolar dynamos, Phys. Earth Planet. Interiors 103 (1997) 281–291; Nonlinear quenching of current fluctuations in a self-exciting homopolar dynamo, Nonlinear Processes in Geophysics 4 (1998) 201–205] has introduced a novel 4-mode set of nonlinear ordinary differential equations to describe such a dynamo in which a nonlinear electric motor is connected in series with the coil. The applied couple, α, driving the disk is steady and the Lorentz couple driving the motor is a quadratic function, x(1−ϵ)+ϵσx 2 , of the dynamo-generated current x, with 0≤ϵ≤1. When there are no additional biasing effects due to background magnetic fields etc., the behaviour of the dynamo is determined by eight independent non-negative control parameters. These include ρ, proportional to the resistance of the disk to azimuthal eddy currents, and β, an inverse measure of the moment of inertia of the armature of the motor. When β=0 (the case when the motor is absent and ϵ and σ are redundant) and ρ −1≠0 , the 4-mode dynamo equations reduce to the 3-mode Lorenz equations, which can behave chaotically [E. Knobloch, Chaos in the segmented disc dynamo, Phys. Lett. A 82 (1981) 439–440]. When β≠0 but ρ −1=0 , the 4-mode set of equations reduces to a 3-mode dynamo [R. Hide (1997), see above], which can also behave chaotically when ϵ=0 [R. Hide, A.C. Skeldon, D.J. Acheson, A study of two novel self-exciting single-disk homopolar dynamos: theory, Proc. R. Soc. Lond. A 452 (1996) 1369–1395] but not when ϵ=1 [R. Hide (1998), see above]. In the latter case, however, all persistent fluctuations are completely quenched [R. Hide (1998), see above]. In this paper we investigate two limiting cases of ϵ=0 and ϵ=1 in the 4-mode dynamo when azimuthal eddy currents are allowed to flow i.e. cases when ρ −1=0 ; in a companion paper [I.M. Moroz, R. Hide, Effects due to induced azimuthal eddy currents in the Faraday disk self-exciting homopolar dynamo with a nonlinear series motor: II The general case, 1999, submitted] we extend the present analysis to the general case of 0≤ϵ≤1. When ϵ=0, chaotic behaviour occurs even more extensively in parameter space in the presence of azimuthal eddy currents than in their absence. When ϵ=1, the quenching of chaotic and all other non-steady dynamo action is no longer complete, for aperiodic solutions are found within limited regions of parameter space where β is very small and α is very large.

Transitional geomagnetic impulse hypothesis: Geomagnetic fact or rock‐magnetic artifact?

A striking feature of the Steens Mountain (Oregon) geomagnetic polarity reversal is the two (maybe three) extremely rapid field directional changes (6 degrees per day) proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow. Each of these very fast field changes, or impulses, is associated with a large directional gap (some 90°) in the record. In order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers, we have carried out a paleomagnetic investigation of two new sections (B and F) in the Steens summit region which cover the second and the third directional gap. The main result is the description of two new directions, which are located between the pre second and post second impulse directions. These findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations, which now appears to be more ad hoc as an explanation of the paleomagnetic data. However, the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section Steens A and D 1.5 km to the north. In addition, new results for 22 transitional and normal lava flows in section B are presented that correlate well with earlier results from section A.