共查询到20条相似文献，搜索用时 500 毫秒
1.
D.P. Zidarov 《Physics of the Earth and Planetary Interiors》1985,37(1):74-86
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. 相似文献
2.
J.Peter Watt 《Physics of the Earth and Planetary Interiors》1981,25(1):57-63
Mean atomic weight profiles for the lunar mantle have been calculated from velocity-density systematic relations using lunar density and seismic velocity models. Despite large variability among the models, the calculation including Poisson's ratio yields a range of mean atomic weight values between 22 and 23 g mol?1 below 150 km. A similar calculation for the Earth's mantle produces a mean atomic weight of 21.1 ±0.4 g mol?1. This suggests that the Moon cannot be derived directly from the Earth's mantle, or that it has had a differentiation history different from the Earth's. The lunar require an Fe mole fraction between 0.25 and 0.33 for a pure olivine mantle, or between 0.33 and 0.45 for pure pyroxene.The present profiles are 0.5–3.0 g mol?1 higher than those calculated from lunar compositional models based on lunar rock compositions and petrology and assumed lunar histories, indicating inadequacies in either the seismic or compositional models, or in both. The mean atomic weight approach provides a method of comparing the consistency of seismic and compositional models of planetary interiors. 相似文献
3.
K. Ramaswamy M. Dheenathayalu S. Bharathan 《Physics of the Earth and Planetary Interiors》1985,40(1):61-64
A detailed study of the ancient intensity of the Earth's magnetic field in Tamilnadu, India is presented using pottery, bricks, tiles etc., obtained from the excavations that have been carried out at some of the important places of historical and archaeological interest. An attempt is made to reconstruct the secular variation of the geomagnetic field intensity. The results reported here reveal considerable changes in the Earth's magnetic field in this region during the past 2400 y. It is observed that the intensity of the geomagnetic field has been previously 57% greater than at present. 相似文献
4.
Jupiter's field is strongly dipolar but with relatively large high order moments compared to the Earth's. In situ magnetic field data allow us to interpret most of the Earth-based microwave observations of Jupiter, with the exception of Branson's hot spot. Decametric emissions have a complex rotational pattern which has been stable since 1950; their agreement with the spacecraft magnetic fields is much less satisfactory than that of the microwaves. We conclude that the extrapolation of magnetic fields from the spacecraft to the surface of Jupiter is in error by 40% in the Southern Hemisphere.Saturn's radio emissions show complexities similar to Jupiter's. They are strongly asymmetric about the rotational axis, although Saturn's Field is nearly axisymmetric. Their strong asymmetry suggests strong longitudinal variations in the magnetic field a few thousand kilometers from the cloud tops, in conflict with the field measured aboard Pioneer 11.The magnetic fields within a few thousand kilometers of either Jupiter's or Saturn's cloud tops are probably unknown. It is discouraging that more is not known about the fields after a total of 7 encounters. Perhaps the Galileo probe can test usefully models of the Jupiter field, even if its measurements refer to just one trajectory through the clouds. An arguable case can be made that the giant planets exhibit complexity of magnetic structure similar to the Sun. 相似文献
5.
D. Crossley 《Physics of the Earth and Planetary Interiors》1984,36(1):1-16
There has been renewed interest lately in the possibility that at least a part of the Earth's liquid core may be stably stratified. A gravitationally stable region would permit the existence of inertia-gravity or gravity-inertia waves in addition to the Rossby and Kelvin waves which exist due to rotational effects and which are well known in oceanography and atmospheric dynamics. These wave motions are of interest because their periods are dependent on the density stratification as specified by the buoyancy frequency N which in turn determines the amplitude of large-scale radial motions in the core.The waves have too high a frequency to be connected dynamically to the magnetic field in the core, but if they do exist they may be detectable by sensitive long-period gravimeters at the Earth's surface. This paper examines the available evidence for the frequency regimes, excitation and damping mechanisms of the core waves. It is concluded that although the waves may exist theoretically, their detection and interpretation as a method for determining N is a difficult proposition. 相似文献
6.
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. 相似文献
7.
Johannes Wicht 《Physics of the Earth and Planetary Interiors》2002,132(4):281-302
Speculation about its possible super-rotation has drawn the attention of many geophysical researchers to the Earth’s inner core. An issue of special interest for geodynamo modelling is the influence of the inner-core conductivity. It has been suggested that the finite magnetic diffusivity of the inner core prevents more frequent reversals of the Earth’s magnetic field. We explore the possible influence of the inner-core conductivity by comparing convection-driven 3D dynamo simulations with insulating or conducting inner cores (CIC) at various parameters. The influence on the field structure in the outer core is only marginal. The time behaviour of dipole-dominated non-reversing dynamos is also little affected. Concerning reversing dynamos, the inner-core conductivity reduces the number of short dipole-polarity intervals with a typical length of a few thousand years. Reversals are always correlated with low dipole strength and these short intervals are found in periods where the dipole moment stays low. Polarity intervals longer than about 10,000 years, where the dipole moment has time recover in strength, are equally likely in insulating and CIC models. Since these latter intervals are of more geophysical relevance, we conclude that the influence of the inner-core conductivity on Earth-like reversal sequences is insignificant for the dynamo model employed here. 相似文献
8.
R.J. Suttill 《Earth and Planetary Science Letters》1980,49(1):132-140
Palaeomagnetic measurements of 1-m cores and a 9-m Delft core of Recent tidal-flat sediments from the Wash, England have shown that these sediments possess a record of the variation of the Earth's magnetic field. The record compares well with the historic-archaeomagnetic record for the period 0–1000 years B.P. but is offset down the core due to the remanence being of post-depositional origin. A period of at least 100 years is suggested for the alignment of particles during acquisition of the post-depositional remanence. Magnetite has been identified as the major carrier of this remanence. 相似文献
9.
Edward R. Benton 《Physics of the Earth and Planetary Interiors》1981,24(4):242-244
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, Br, its horizontal gradient, and its secular variations, ?Br/?t, at the core-mantle boundary. 相似文献
10.
Cedric Wright 《Earth and Planetary Science Letters》1973,19(4):453-460
Seismological results interpreted as evidence for large inhomogeneities near the base of the Earth's mantle below Hawaii have recently been published. It is possible to place constraints on the magnitude of such heterogeneities by identifying seismic phases multiply reflected within the Earth's core. The value of such a simple technique is illustrated by using array recordings of P and S5KP waves that have traversed the bottom of the mantle beneath Hawaii to show that there is no clear evidence for the unusual physical properties attributed to this region of the Earth. Identification of the phase S7KP is also reported. 相似文献
11.
Edward Stolper 《Earth and Planetary Science Letters》1979,42(2):239-242
The average concentrations of 19 siderophile and volatile elements in shergottite meteorites differ from those in terrestrial basalts by less than a factor of ten. This observation undermines claims that the abundances of siderophile and volatile elements in the Earth's upper mantle are uniquely terrestrial. Claims that similarities in the Moon's siderophile element pattern imply a terrestrial origin for the Moon are also weakened. The implication that basalt source regions on the asteroidal parent body of the shergottites resembled the terrestrial upper mantle constrains models of planetary formation and evolution. Heterogeneous accretion models may explain many of the similarities between these planets. Alternatively, separation of sulfide from basaltic magmas or their source regions on the Earth and the shergottite parent body may explain some of these similarities. 相似文献
12.
Han-Shou Liu 《Physics of the Earth and Planetary Interiors》1983,32(2):146-159
The stress field in the Earth's crust as inferred from satellite gravity data causes crustal deformation and seismotectonic block movements in central Europe. The satellite-determined stresses in the crust of central Europe are consistent with earthquake focal mechanisms, joint-orientation and in situ stress measurements. 相似文献
13.
Steven C. Cande 《Earth and Planetary Science Letters》1978,40(2):275-286
Marine magnetic anomalies 33 and 34, corresponding to the first two reversals following the long normal polarity interval in the Cretaceous, are anomalously skewed by 30° to 40° throughout the North and South Atlantic. This phenomenon is most likely related to some aspect of the dipole paleomagnetic field. Specifically the magnetic field at the time of anomalies 33 and 34 appears to be characterized by the following: the dipole field gradually decreases in average intensity between reversals and/or there is an increase in the frequency or duration of undetected short polarity events toward the end of long periods (>106 years) of predominantly one polarity. Such long-period trends in the field are in conflict with the popular model for the generation of the earth's magnetic field that treats reversals as a Poisson process and assumes that the core has no memory greater than about 104 years. 相似文献
14.
These last 10 years, numerical models of mantle convection have emphasized the role of the 670 km endothermic phase change in generating avalanches that trigger catastrophic mass transfers between upper and lower mantle. On the other hand, scientists have emphasized the concomitance of large-scale worldwide geophysical and tectonic events, which could find their deep thermal roots in the huge mass transfers induced by the avalanches. In particular, the paleontological records show two periods of length of day (l.o.d.) shortening between 420 and 360, and 200 and 80 Myr BP. This last event is synchronous with a strong true polar wander and a global warming of the upper mantle. In order to study the potential effects of the avalanche on the main component of the Earth’s rotation, the Liouville equation has been solved and the l.o.d. evolution has been calculated from the perturbations of the inertia tensor. The results show that the inertia tensor of the Earth’s is mainly sensitive to the global transfers through the 670 km discontinuity. The l.o.d. perturbations will be synchronous with the global thermal effects of the avalanche. These theoretical results allow proposing a self-consistent physical mechanism to explain periods of the Earth’s rotation acceleration. Within this context, the l.o.d. shortening during the Cenozoic and Cretaceous brings one more clue to the possible participation of a mantle avalanche in generating the concomitant large scale events which have occurred during this very particular period of the Earth’s history. 相似文献
15.
Loren Shure Robert L. Parker George E. Backus 《Physics of the Earth and Planetary Interiors》1982,28(3):215-229
Which features of a geomagnetic field model on the surface of the core are really necessary in order to fit, within observational error, the field observations at and above the Earth's surface? To approach this question, we define ‘roughness’ in various ways as a norm on an appropriate Hilbert space of field models which is small when the field is smooth on the core surface. Then, we calculate the model with least norm (the smoothest model) which fits the data, sources outside the core being treated as noise. Sample calculations illustrate the effects of noise, of the choice of norm and of an uneven distribution of observing stations. 相似文献
16.
17.
R. Hide 《Physics of the Earth and Planetary Interiors》1985,39(4):297-300
This note summarizes recent studies of atmospheric excitation of short-term changes in the length of the day and polar motion which set useful limits on the timescales associated with angular momentum transfer between the Earth's core and mantle. It also speculates about the nature of the recently-discovered phenomenon of “impulses” or “jerks” in the geomagnetic secular variation, proposing that they might be manifestations of “loop” instability of the magnetic field within the core. Finally, it outlines novel properties of high magnetic Reynolds number flows that bear on the inverse problem of deducing core motions from geomagnetic secular variation data. 相似文献
18.
S.I. Braginsky 《Physics of the Earth and Planetary Interiors》1976,11(3):191-199
A short review of the present state of the nearly axially-symmetrical dynamo model is given. A simplified theory for hydromagnetic dynamos taking into account the forces acting in the Earth's core is considered. The role of weak core-mantle friction is discussed and a form of solution is suggested which is characterized by a large geostrophic velocity in the core and by a boundary layer of a new type. The consequences of such a model (called model Z) for the Earth's dynamo are discussed. 相似文献
19.
K.-H. Glassmeier S. Buchert U. Motschmann A. Korth A. Pedersen 《Annales Geophysicae》1999,17(3):338-350
Giant pulsations are nearly monochromatic ULF-pulsations of the Earth’s magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30–90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = −21±4, is in accord with the value m = −27±6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f/∂W ≥ 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ≥f/≥W ≥ 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed. 相似文献
20.
M.A. Khan 《Physics of the Earth and Planetary Interiors》1983,31(3):231-240
Accuracy tests on the most recent GEM (Goddard Earth Model) gravity models for the representation of the Earth's gravity field, using specially devised statistical techniques of comparative evaluation, show that there is steady improvement in these models with time. On this comparative basis, the accuracy of determination for the spherical harmonic coefficients of the Earth's gravity field is ～ 100% for n = 2–6, 90–99% for n = 7–10, 55–80% for n = 11–14 and ? 50% for n ? 15, deteriorating rapidly with increasing n. The higher degree coefficients corresponding to n ≥ 15 do not seem to be determined accurately enough to be useful from a geophysical standpoint, though their cumulative contribution is undoubtedly useful for specific orbital computations. The estimated errors are 0.3 mGal for n = 2–6, 1.5 mgal for the frequency range n = 2–10, 3 mGal for n = 2–14 and 5–6 mGal for n = 2–22. These error estimates, especially the ones for the higher frequency range, may have been affected by possible errors in the comparison standards used for this evaluation. Consequently, some of the higher degree coefficients of recent GEM models may be more accurate than predicted by these tests.Due to the inherent deficiency of the comparison standards, the errors given in this paper should be treated as error estimates. The steady and progressive improvement, shown by the various GEM gravity models when tested against comparison standards 10E and WGS 72, i.e. the more recent a gravity model, the better it tests against the comparison standards in contrast to its predecessors, is remarkable, as the comparison standards themselves are several years older than the gravity models tested here. This clearly validates our choice of comparison standards, as well as the premises and predictions of our evaluation techniques. It also demonstrates the power and potential of these techniques, which only seem to be limited by the level of accuracy of the available standard of comparison. 相似文献