Earth's magnetic field modelling and earth's structure and evolution

The advantages of the approximation of the Earth's magnetic field by means of the field of the so-called natural magnetic sources are discussed. The shifting of these natural magnetic sources, determined for different epochs, is used to forecast the Earth's magnetic field and to draw conclusions about the motion of the corresponding part of the Earth. On the basis of the representation of the Earth's magnetic field from several past geological epochs as a field of one optimum dipole a new theory about the Earth's evolution is proposed.     

Large impact craters and the moon's orientation

This paper investigates the idea that large impact events have caused the moon to change its orientation in space. It is found that the very largest impact events, such as those which formed Imbrium and Orientale, probably did reorient the moon. This reorientation is primarily due to the change in the moon's moments of inertia consequent upon crater formation. The impulse delivered by the impact can at most unlock the moon's synchronous rotation for a few thousand years, and is thus not of major importance. The moon will attain its new orientation in less than a few times 10⁴ years as a result of tidal friction. Since the large craters eventually are filled by isostatic rebound and extrusive igneous activity, the moon may eventually regain its original orientation unless other phenomena cause new changes in the distribution of mass on its surface.     

Oceanic area's influence on the range of multi-annual variations of the earth's magnetic field in France

The pluri-annual variations of the earth's magnetic field in France increase their range of influence from East to West. This leads to presume a discontinuity of electric conductivities at the transition from the continental to the oceanic area.     

Evidence for long-term asymmetries in the Earth's magnetic field and possible implications for dynamo theories

Paleomagnetic data indicate that there is a north-south asymmetry in the time-averaged magnetic field and that there are small but significant differences between the normal and reverse polarity states. The geographical variation is most likely due to spatial variation in the boundary conditions at the core-mantle interface. The difference in the magnetic fields of the reverse and normal polarity states can be modeled in terms of a “standing field”. The paleomagnetic data are insufficient to determine whether or not this “standing field” is of core origin. However, consideration of mechanisms, including thermoelectric currents, indicates that there probably are important differences in core processes between the two polarity states. At first glance this interpretation is difficult to reconcile with the fact that the magnetic induction equation is antisymmetric with respect to the magnetic field. A way around this problem is the possibility that only certain transitions are allowed between acceptable eigenstates in dynamo models of the Earth's magnetic field.     

Mascons and the moon's orientation

This letter reports the discovery of a relation between the moments of inertia of the mascons (taken about the moon's center) and the moon's moments of inertia. It is found that the principal axes of the mascons alone are nearly parallel to those of the moon. Possible explanations of this parallelism are discussed. If the mascons are associated with a layer of uncompensated basalt on the moon's nearside, then the parallelism can be adequately explained on the grounds that the mascons and basalts together determined the moon's orientation. On the other hand, the third-order harmonics of the moon's gravity field indicate that the excess mass controlling the moon's orientation is on the farside. It thus appears that the mascons have been emplaced in special sites whose position was controlled by the processes which produced the farside highlands.     

On the frozen flux velocity field at the surface of earth's core necessary to account for the poloidal main magnetic field and its secular variation

An expression for the inviscid horizontal velocity field at the surface of the Earth's core necessary to account for the poloidal main magnetic field and its secular variation seen at the Earth's surface is derived for an insulating mantle in the limit of infinite core conductivity. The starting point of derivation is Ohm's law rather than the magnetohydrodynamic induction equation. Maps of the resulting motion for epoch 1965.0 at different truncation levels are presented and discussed.     

A note on convection in the earth's mantle

In the present note a boundary-layer model of thermal convection throughout the mantle is outlined. It is shown that recent criticisms of mantle-wide convection by A.E. Ringwood do not apply to this model. The phase transitions discussed by Ringwood are consistent with the model, and in fact provide an additional driving force for the convective motion. It is further noted that the model offers explanations of the core-mantle coupling hypothesized by R. Hide from consideration of correlations between the earth's magnetic and gravity fields, and of the appearance in several parts of the world of pairs of trenches separated by distances of the order of 2000 km.     

Axial depth anomalies from 10 to 50° north along the Mid-Atlantic Ridge: correlation with other mantle properties

We present an analysis of intermediate- to long-wavelength (a few hundred to a few thousand kilometers) axial depth anomalies along the Mid-Atlantic Ridge between 10 and 50°N. The maximum depth of the rift valley is chosen as the elevation datum for oceanic crust of zero-age. The large depth anomalies are correlated, for short wavelengths (less than a few hundred kilometers), with some fracture zones irrespective of their offset and, for intermediate to long wavelengths, with mantle properties expressed in (1) excess elevation near triple junctions, (2) trace element and isotope geochemistry of the basaltic liquids emplaced at the ridge axis, and (3) anomalies in the Earth's gravity field. We suggest that the correlations may be explained in two ways: the depth anomalies of intermediate to long wavelength may represent the sites of upwelling and downwelling of the deep mantle; alternatively, the depth anomalies could be due to a regionalization in temperature or mineralogy in the asthenosphere.     

Effects of an outer thin stably stratified layer on planetary dynamos

The presence of outer stably stratified layers in planetary cores has been suggested for Earth, Saturn and Mercury. In this study, we use a 3-D numerical dynamo model to investigate the effects of a thin stable layer surrounding a convecting interior on the produced magnetic field. We find that a stable layer can destabilize the field morphology through a thermal wind that produces unfavorable zonal flows throughout the core. The direction of these zonal flows is prograde in equatorial regions, unlike a model with no stable layer that has retrograde equatorial flows. Our models therefore suggest that the Earth does not have a stable layer since we observe a westward drift as opposed to an eastward drift. For Saturn, we find that due to coupling of the flows in the stable and unstable layers, the layer does not act to shear out the non-axisymmetry in the observed magnetic field, and therefore cannot explain Saturn’s axisymmetric magnetic field. For Mercury, we find that if the stable layer is thin, it can actively produce strong or weak surface fields and not necessarily attenuate smaller scale features through the skin effect.     

A simple method for determining the vertical growth rate of vertical motion at the top of earth's outer core

A simple new method is described for extracting, from magnetic observations taken at Earth's surface, the vertical growth rate of vertical motion, ?u/?r, at special isolated points on the top surface of Earth's liquid core. The technique utilizes only the radial component of the frozen-flux induction equation and it requires information only on the radial magnetic field, B_r, its horizontal gradient, and its secular variations, ?B_r/?t, at the core-mantle boundary.     

Climatic changes,magnetic intensity variations and fluctuations of the eccentricity of the earth's orbit during the past 2,000,000 years and a mechanism which may be responsible for the relationship

Our investigation of deep-sea climatic and magnetic records showing that high eccentricity of the earth's orbit, low magnetic field intensity and warm climate occur together indicates the relative importance of eccentricity as perhaps the phenomenon which has most consistently modulated both climate and magnetism for at least the past 2,000,000 years. A speculative hypothesis regarding the mechanism which may be responsible for a relationship between the eccentricity of the earth's orbit, geomagnetism, and climate is suggested.     

Electromagnetic evidence for lateral inhomogeneities within the Earth's upper mantle

The composition of the upper mantle is of great significance to our understanding of plate tectonics and global evolution. Information about the physical properties of the Earth at upper mantle depths, including lateral variations in electrical conductivity, can be deduced from measurements of the electric and magnetic fields at the Earth's surface. Electromagnetic methods appear to give poorer resolution than do some other methods, for example seismics, but as they are sensitive to quite different properties of a medium they provide a different and complementary class of information.The basic theory of electromagnetic sounding methods is briefly reviewed below, and evidence regarding lateral conductivity inhomogeneities in the Earth's upper mantle is examined. While lateral electrical conductivity inhomogeneities appear to be the rule rather than the exception, the interpretation of electromagnetic data still presents difficulties and the results from many regions are not as yet unambiguous. Where the data are of sufficient resolution, a rapid increase in electrical conductivity can usually be identified within the upper mantle. The depth to this highly conductive zone is different in different tectonic environments, but is broadly consistent between analogous but widely separated tectonic environments. A comparatively shallow conducting region is found beneath the ocean lithosphere. The depth of this region is dependent on lithospheric age. Many of the more shallow conducting regions in both continental and oceanic environments are associated with high heat flow values and seismic low velocity zones. These highly conducting regions may be zones of partial melt.     

An analogue model for studying magnetic variations induced by ocean waves

The scale factors to permit a laboratory analogue model study of the problem of magnetic fields induced by ocean waves in the earth's field are derived. An analogue model employing surface fluid waves in mercury to simulate ocean waves is described. In the analogue model, magnetic field measurements were made 1 cm above a 2 cm deep model mercury ocean for a wave period of 0.21 s. This model simulates measurements 38 m above the surface of a shallow ocean 78 m deep for a wave period of 13 s. The validity of the analogue modelling technique is established by the good agreement obtained in comparing the analogue model measurements of the induced magnetic fields with fields using Podney's expression for an ocean of finite depth.     

Techniques and instrumentation for study of natural electromagnetic induction at sea

Electromagnetic fluctuations in the ocean have external sources above (ionospheric) and below (secular variation of the earth's magnetic field), and internal, purely oceanic sources associated with interaction between water velocity fields and the earth's field. Energy diagrams indicative of the electromagnetic activity in the sea are presented. From the latter, estimates of the resolution required in electromagnetic research at sea can be made. Absolute minima of 1 γ and 0.05 μV/m are necessary for magnetic and electric fields, respectively. Because the ocean shields overhead sources at frequencies above a few hundred c/h and because motional fields have weak signatures, a resolution at least 10 times higher would considerably enhance the scope of such research.The response of electric field instruments to motionally induced fields depends upon whether they are fixed or drifting, but both types respond similarly to fields of external origin.The most stringent limitation to electric field sampling in the sea is the difficulty in achieving low-noise electrical continuity between measuring circuits and sea water. Even the best matched silver—silver chloride electrodes introduce variable electrochemical signals hard to maintain below a millivolt. These mask very low frequency signals unless sophisticated techniques such as electrode switching are used.     

Jupiter’s Great Red Spot: compactness condition and stability

Linear Rossby wave dispersion relationships suggest that Jupiter’s Great Red Spot (GRS) is a baroclinic structure embedded in a barotropic shearing zonal flow. Quasi-geostrophic (QG) two-layer simulations support the theory, as long as an infinitely deep zonal flow is assumed. However, once a finite depth of the lower layer is assumed, a self-interaction of the baroclinic eddy component produces a barotropic radiating field, so that the GRS-like eddy can no longer remain compact. Compactness is recovered by explicitly introducing a deep dynamics of the interior for the lower layer, instead of the shallow QG formulation. An implication of the result is a strong coupling of the GRS to a convectively active interior.Paper presented to the NP Symposia of the 1991 Wiesbaden EGS Assembly on “Nonlinear processes in Geophysics”     

Birch's diagram: Some new observations

Birch's diagram plotting the hydrodynamic sound velocity versus density for several metals is used in many of his publications and a number of textbooks to demonstrate the chemical changes from the earth's mantle to the core. This diagram is thoroughly discussed in this paper on the basis of theory and the periodic property for the density of elements. Birch's conclusion that “even without information concerning chemical abundances, these relations indicate that the mantle is composed principally of light elements and the core of elements of the iron group” is not convincing in the view of the present study. A more detailed velocity-density plot for all the solid elements having their densities in the range 1.5 – 8 g/cm³ excludes any elements lighter (of lower atomic weight or number) than vanadium as likely candidates for core values.     

The influence of extrinsic pressure changes on the Earth's dynamo

Reversals of the Earth's magnetic field have been claimed to correlate with ice ages, tectonic events and falls of tectites. A physical mechanism is needed to relate reversals with the other events before these correlations can be taken seriously. One possible connection lies through changes in pressure in the core. If events high up in the mantle were to lead to changes in core pressure, this would affect the rate of freezing of the liquid core and modify the power supplied to the dynamo. A sufficiently large modification could set off a reversal or perhaps change the mode of operation of the dynamo from a non-reversing to a reversing state.The model of Gubbins et al., allows a quantitative calculation to be made for the effect of a pressure change on the energy release. Any sufficiently sudden pressure change would change the power, but it seems unrealistic to consider less than a 1000 year time scale. Relaxation of shear forces in the mantle, overturning of core fluid, and changes in magnetic fields all take place on about this time scale. According to the model, a pressure change of 0.1 bar over a 1000 years could change the power supply drastically.A continuous process of mantle differentiation leading to the formation of the upper mantle from an initially homogeneous mantle can only provide 10% of the required pressure change, but the effect cannot be ruled out as a power source for the dynamo because uncertainties in the calculations can amount to at least an order of magnitude. The other effects produce changes of up to 1% in the power supply, which may be sufficient to alter the characteristics of the dynamo and produce reversals or a change in reversal behaviour. Further speculation must await a better understanding of the dynamics of reversals, and of mantle processes.     

Electromagnetic fields induced in the world's oceans and the spatial distribution of electrical conductivity functions

The results of numerical simulation of electromagnetic fields induced in the world's oceans by solar daily variations are discussed. Some examples of apparent resistivity curves simulated for continental and ocean regions are demonstrated.     

The magnetosphere of Jupiter: Coupling the equator to the poles

Jupiter is a planet of superlatives: the most massive planet in the solar system, rotates the fastest, has the strongest magnetic field, and has the most massive satellite system of any planet. These unique properties lead to volcanoes on Io and a population of energetic plasma trapped in the magnetic field that provides a physical link between the satellites, particularly Io, and the planet Jupiter. There are strong differences between the magnetospheres of Earth and Jupiter but there are also underlying basic physical principles that all magnetospheres share in common. This paper provides a rough sketch of the magnetosphere of Jupiter, briefly describes the current understanding and lists outstanding issues. As at Earth, a major issue of the jovian system is how the magnetospheric plasma is coupled to the planet's ionosphere.