Geomagnetic field from the Earth's core: Its westward drift rate and some probable global-tectonics implications (original) (raw)
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Eastward and westward drift of the Earth's magnetic field for the last three millennia
We analyse the secular variation captured by the archaeomagnetic field model CALS7K.2 in an effort to determine episodes of eastward and westward motions of Earth's magnetic field at the core-mantle boundary (CMB) over the past 3000 yr. The direction, amplitude and geographical distribution of these motions are described. We find that the clearest azimuthal motions are observed at mid-to high latitudes in the Northern hemisphere, where both eastward and westward motions occur. These azimuthal motions correspond to displacements and distortions of the two main, quasi-stationary, high-latitude magnetic flux patches. Similar motions are not observed in the Southern hemisphere, although this may be a consequence of the poorer data coverage there. The globally averaged drift for the past 1000 yr has been westward since 1400 AD, but eastward between 1000 AD and 1400 AD. In the broad region of the CMB under Europe, the times of transition in the direction of the mean azimuthal motion coincide with the times at which "archaeomagnetic jerks" have been reported. Our results suggest that these are caused by a relatively rapid (< 100 yr) change in the direction of the underlying azimuthal flow near the core surface. We find indications that equatorial westward motions of field features at the CMB, similar to those observed during historical times, may have been present for much of the past 3000 yr. When observed, these low-latitude motions are most prominent in the Atlantic hemisphere, which we interpret as a signature of core-mantle thermal coupling.
Inner core translation and the hemispheric balance of the geomagnetic field
Earth and Planetary Science Letters, 2015
Bulk translation of the Earth's inner core has been proposed as an explanation of observed quasi-hemispheric seismic structure. An important consequence of inner core translation would be the generation of a spherical harmonic degree one heat flow anomaly at the inner core boundary (ICB) that would provide an inhomogeneous forcing for outer core convection. We use geodynamo simulations to investigate the geomagnetic signature of such heterogeneity. Strong hemispheric heterogeneity at the ICB is found to produce a hemispheric signature in both the morphology of the magnetic field and its secular variation; in particular, we note the formation of high-intensity flux patches at high-latitudes and American longitudes in our model with strong ICB heterogeneity. In our simulations, this model provides the best match to the Earth's field over the past 400 years according to previously proposed measures of field structure. However, these criteria do not include the hemispheric balance of the field. We propose new criteria to measure this balance and find that our model with strong ICB heterogeneity produces the poorest match to the hemispheric balance of the historical geomagnetic field.
The Earth’s Magnetosphere and Geomagnetic Polar Transitions
2007
Though there is little scientific or academic disagreement as to the reality of the existence of geomagnetic polar shifts and reversals, there is much speculation and theorizing as to what the hypothetical impacts and effects of such an event would be; and still more controversial is just how hypothetical such an eventuality actually is. The magnetic field waxes and wanes, poles drift and, occasionally, they flip. The magnetic field has exhibited frequent but dramatic variation at irregular times in the geologic past: It has completely changed direction. Our planet’s magnetic field varies with time, indicating it is not a static or fixed feature. Instead, some active process works to maintain the field. However, we still don’t know for certain how the Earth’s magnetic field is generated and maintained. It is known, however, that the mechanism of magnetic field generation is related to Earth’s rotation. The origin of the magnetic field and its reversals is one of the oldest problems in physics, and one of the most active areas of research in geophysics today. The study of Earth’s past magnetism is called paleomagnetism. The rotation of planets may be among the necessary conditions for the formation of their magnetic fields. However, rotation alone is insufficient for the creation of a planetary magnetic field. Most scientists believe Earth’s magnetic field is sustained by a complex self-sustaining interaction known as the “geomagnetic dynamo”. According to general accepted theory—the dynamo theory—interactions between the churning convecting flow of molten iron in the Earth’s outer core and the magnetic field generate electrical current that, in turn, creates new magnetic energy that sustains the field. This idea that turbulent activity at the outer core of the planet generates its magnetic field currently dominates scientific thinking. It is generally accepted that during a reversal during a reversal, the geomagnetic field decreases to about 10 percent of its full polarity value. While nobody quite knows why this is occurring, the weakening of Earth’s magnetism is believed by many of the most respected scientists in the field of geomagnetism to be one of the factors predictive of a pole realignment, a precursor, and perhaps even a forecaster, of magnetic polar reversal sometime in the near future.
Simulated geomagnetic reversals and preferred virtual geomagnetic pole paths
Geophysical Journal International, 2004
The question of whether virtual geomagnetic poles (VGPs) recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists. One possible mechanism for such VGP clustering is the heterogeneity of heat flux at the core-mantle boundary (CMB). We use 3-D convection-driven numerical dynamo models with imposed non-uniform CMB heat flow that show stochastic reversals of the dipole field. We calculate transitional VGPs for a large number of token sites at the Earth's surface. In a model with a simple heat flux variation given by a Y22 harmonic, the VGP density maps for individual reversals differ substantially from each other, but the VGPs have a tendency to fall around a longitude of high heat flow. The mean VGP density for many reversals and excursions shows a statistically significant correlation with the heat flow. In a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean VGP density at American and East Asian longitudes, roughly consistent with the VGP paths seen in several palaeomagnetic studies. We find that low-latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated. They contribute to the equatorial dipole component and bias its orientation in longitude. During reversals the equatorial dipole part is not necessarily dominant at the Earth's surface, but is strong enough to explain the longitudinal preference of VGPs as seen from different sites.
Proceedings of the National Academy of Sciences, 2012
To understand the dynamics of the Earth's fluid, iron-rich outer core, only indirect observations are available. The Earth's magnetic field, originating mainly within the core, and its temporal variations can be used to infer the fluid motion at the top of the core, on a decadal and subdecadal time-scale. Gravity variations resulting from changes in the mass distribution within the Earth may also occur on the same time-scales. Such variations include the signature of the flow inside the core, though they are largely dominated by the water cycle contributions. Our study is based on 8 y of highresolution, high-accuracy magnetic and gravity satellite data, provided by the CHAMP and GRACE missions. From the newly derived geomagnetic models we have computed the core magnetic field, its temporal variations, and the core flow evolution. From the GRACE CNES/GRGS series of time variable geoid models, we have obtained interannual gravity models by using specifically designed postprocessing techniques. A correlation analysis between the magnetic and gravity series has demonstrated that the interannual changes in the second time derivative of the core magnetic field under a region from the Atlantic to Indian Ocean coincide in phase with changes in the gravity field. The order of magnitude of these changes and proposed correlation are plausible, compatible with a core origin; however, a complete theoretical model remains to be built. Our new results and their broad geophysical significance could be considered when planning new Earth observation space missions and devising more sophisticated Earth's interior models.
Outer Core Stratification From the High Latitude Structure of the Geomagnetic Field
Frontiers in Earth Science
The presence of stable stratification has broad implications for the thermal and compositional state of the outer core, the evolution of Earth's deep interior, and the energetics of the geodynamo. Yet the origin, strength, and depth extent of stratification in the region below the core-mantle boundary remain open questions. Here we compare magnetic fields produced by numerical dynamos that include heterogeneous stable thermal stratification below their outer boundary with models of the geomagnetic field on the core-mantle boundary, focusing on high latitude structures. We demonstrate that the combination of high magnetic field intensity regions and reversed magnetic flux spots, especially at high latitudes, constrains outer core stratification below the core-mantle boundary. In particular, we find that the negative contribution to the axial dipole from reversed flux spots is a strong inverse function of the stratification. Comparison of our numerical dynamo results to the structure of the historical geomagnetic field suggests up to 400 km of permeable, laterally heterogeneous thermal stratification below the core-mantle boundary.
Geomagnetic jerks from the Earth’s surface to the top of the core
Earth, Planets and Space, 2007
Rapid changes in the magnetic field characterised by an abrupt change in the secular variation have been named "secular variation impulses" or "geomagnetic jerks". Three of these events, around 1968, 1978 and 1990, occurred during the time-span covered by the comprehensive model CM4 (Sabaka et al., 2002, 2004). This model, providing the best temporal resolution between 1960 and 2002 as well as a fine separation of the different magnetic sources, can be used to study rapid phenomena of internal origin. In order to analyse these events all over the globe, synthetic time series were obtained from the CM4 model between 1960-2002. Geomagnetic jerks are detected here as a rapid movement of the zero isoline of the second field derivative. Analysis of the area swept out by this isoline as a function of time allows us to map the spatial extent of jerks though time, and to identify an event around 1985 that is localized in the Pacific area. At the core surface, we compute the fluid flows under the frozen-flux and tangentially geostrophic assumptions. The flows do not exhibit any special pattern at jerk times, but instead show a smooth temporal evolution over the whole time period. However, the mean amplitude of the dynamical pressure associated with these flows present maxima at each jerk occurrence and helps to confirm the identification of a jerk in 1985.