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1.
Summary. The classical theory of multipoles is used to calculate the true quadrupole and octupole parameters for six different models of Jupiter's main magnetic field. These six magnetic-field models, which are based on measurements made by the Pioneer 10 and Pioneer 11 spacecraft, are specified in terms of the fifteen spherical harmonic coefficients required to define the Jovian dipole (3), quadrupole (5) and octupole (7). The set of five equations for the quadrupole parameters and the set of seven equations for the octupole parameters are each solved iteratively to give the corresponding true multipole moment and the directions of the associated multipole axes. It is found that the five quadrupole parameters are defined reasonably accurately by the Pioneer measurements, as are the three dipole parameters, but it is concluded that there are appreciable uncertainties in each of the seven octupole parameters. The true quadrupole and octupole magnetic moments are typically 14 and 5 per cent, respectively, of the dipole moment. These percentages are significantly different from the corresponding percentages for the pseudo-magnetic moments, namely 24 and 21 per cent, which are usually quoted in discussions of the higher multipoles of the Jovian magnetic field. Both the true and pseudo-multipole moments are larger for Jupiter than for the Earth. It is suggested finally that a proper multipole expansion of Jupiter's main magnetic field may have important applications in quantitative studies of several problems involving the Jovian magnetosphere, such as the decimetric and decametric radio emissions from Jupiter.  相似文献   

2.
The geomagnetic power spectrum   总被引：1，自引：0，他引：1  
Combining CHAMP satellite magnetic measurements with aeromagnetic and marine magnetic data, the global geomagnetic field has now been modelled to spherical harmonic degree 720. An important tool in field modelling is the geomagnetic power spectrum. It allows the comparison of field models estimated from different data sets and can be used to identify noise levels and systematic errors. A correctly defined geomagnetic power spectrum is flat (white) for an uncorrelated field, such as the Earth's crustal magnetic field at long wavelengths. It can be inferred from global spherical harmonic models as well as from regional grids. Marine and aeromagnetic grids usually represent the anomaly of the total intensity of the magnetic field. Appropriate corrections have to be applied in estimating the geomagnetic power spectrum from such data. The comparison of global and regional spectra using a consistently defined azimuthally averaged geomagnetic power spectrum facilitates quality control in field modelling and should provide new insights in magnetic anomaly interpretation.  相似文献   

3.
Summary. A conducting slab of finite thickness divided into three segments of different conductivities and overlying a perfect conductor is proposed as a suitable two-dimensional 'control' model for testing the accuracy of the various numerical modelling programs that are available for calculating the fields induced in the Earth by an external, time-varying magnetic source. An analytic solution is obtained for this control model for the case of the magnetic field everywhere parallel to the conductivity boundaries ( B -polarization). Values of the field given by this solution for a particular set of model parameters are calculated at selected points on the surface and on a horizontal plane inside the conductor, and are tabulated to three figure accuracy for reference. They are used to check the accuracy of the results given by the finite difference program of Brewitt-Taylor & Weaver and the finite element program of Kisak & Silvester for the same model. Improved formulae for calculating the derived electric field components in B -polarization are first developed for incorporation in the finite difference program, and these give surface electric fields within 1 per cent of the analytic values, while all three field components inside the conductor are calculated to better than 96 per cent accuracy by the finite difference program. The results given by the finite element program are not quite so satisfactory. Errors somewhat greater than 10 per cent are present and although the program requires much less disk space it takes rather more CPU time to complete the calculations.  相似文献   

4.
13 lava flows of known age (ages from 14C dating), which have been erupted in the last 30 000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 × 1022 A m2. Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.  相似文献   

5.
The geomagnetic field intensity during Archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 Ma Nuuk Gneisses at Nuuk (64.2°N, 51.7°W), west Greenland. Plagioclase from the dolerite dyke yields a mean K-Ar age of 2752 Ma. Palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked-contact test, indicating that the high-temperature-component magnetization of the dyke is primary. Thellier experiments on 12 dyke specimens yield a palaeointensity value of 13.5±4.4 μT. The virtual dipole moment at ca. 2.8 Ga is 1.9±0.6 × 1022 Am2, which is about one-quarter of the present value. The present study and other available data imply that the Earth's magnetic field at 2.7 ∼ 2.8 Ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca. 2.6 Ga.  相似文献   

6.
Summary. Currents in the 1.28 × 103 km Alaska oil pipeline, induced from the ionospheric, auroral electrojet, were measured at three sites, near Fairbanks, Paxson and Valdez, Alaska, using a gradient configuration of two fluxgate magnetometers. The observed pipeline current magnitudes, which reached 50 A during times of mild geomagnetic activity, displayed a linear relationship with the electric earth potential. Using the induction relationship between the electric and magnetic fields and the typical spectral composition of the geomagnetic field at high latitudes, I obtained a spectral appearance of the current that shows a maximum in the range of 4.5- to 10-min period. Near Fairbanks the pipeline current amplitudes, I (Amperes), could be represented, approximately, by I = 0.65 B x T −0.5, where B x(nT) is the north—south geomagnetic field variation amplitude and T (min) is its apparent period. There is much less pipelines current at the sites south of Fairbanks. A previously established relationship between the local electric field and the planetary geomagnetic activity index, Ap , permitted a prediction of the pipeline current surge amplitudes in the Fairbanks region as approximately I = 5.0 Ap . Current surges larger than 500 A may be expected rather often in the Alaska pipeline during large geomagnetic storms.  相似文献   

7.
Geomagnetic secular variation since 1901   总被引：3，自引：0，他引：3  
Summary. All available annual means, from the world-wide network of magnetic observatories, of north intensity ( X ), east intensity ( Y ) and vertical intensity (Z) from 1901 to 1977 are subjected to spherical harmonic analysis to obtain 38 models of the Earth's geomagnetic field at two-year intervals. These models are differenced to give 37 models of secular variation at two-year intervals from 1903.5 to 1975.5. The results show the decreasing trend of the dipole moment and are analysed for possible information on the westward drift of the magnetic field.  相似文献   

8.
Summary. Many geomagnetic variation anomalies are probably caused by the channelling, through small-scale bodies, of electric currents induced in much larger conductors elsewhere. Consequently, the direct interpretation of anomalous magnetic fields by modelling the electromagnetic response of conductive structures may give misleading results. It is suggested that, rather than attempting to proceed directly from the electromagnetic fields to conductivity models, we should instead take the intermediate step of determining the distribution of anomalous current flow.
Maps of the anomalous fields over a conductive structure can be generated from inter-station transfer functions. If it is assumed that the internal currents are concentrated in a thin sheet at a specified depth, the equivalent current system in the sheet can be computed directly from the vertical magnetic field. The most straightforward method of performing this calculation is to compute the Fast Fourier Transform of the magnetic field data, and then to apply a wavenumber filter.
The presence of any vertical currents invalidates the thin sheet model. However, if the spatial distribution of a horizontal component of the anomalous magnetic field is also known, the presence of any vertical currents can be detected directly, and their position determined. The value of the methods is illustrated by applying them to the interpretation of a Geomagnetic Deep Sounding survey of the Kenya rift valley.  相似文献   

9.
A time-varying spherical harmonic model of the palaeomagnetic field for 0–7 ka is used to investigate large-scale global geomagnetic secular variation on centennial to millennial scales. We study dipole moment evolution over the past 7 kyr, and estimate its rate of change using the Gauss coefficients of degree 1 (dipole coefficients) from the CALS7K.2 field model and by two alternative methods that confirm the robustness of the predicted variations. All methods show substantial dipole moment variation on timescales ranging from centennial to millennial. The dipole moment from CALS7K.2 has the best resolution and is able to resolve the general decrease in dipole moment seen in historical observations since about 1830. The currently observed rate of dipole decay is underestimated by CALS7K.2, but is still not extraordinarily strong in comparison to the rates of change shown by the model over the whole 7 kyr interval. Truly continuous phases of dipole decrease or increase are decadal to centennial in length rather than longer-term features. The general large-scale secular variation shows substantial changes in power in higher spherical harmonic degrees on similar timescales to the dipole. Comparisons are made between statistical variations calculated directly from CALS7K.2 and longer-term palaeosecular variation models: CALS7K.2 has lower overall variance in the dipole and quadrupole terms, but exhibits an imbalance between dispersion in   g 12  and   h 12  , suggestive of long-term non-zonal structure in the secular variations.  相似文献   

10.
Summary. An exact equation is derived for the magnetic field lines of the general axisymmetric magnetic multipole of arbitrary degree ( n ). This new result has important applications in studies of the possible nature of solarterrestrial physics during geomagnetic polarity reversals. In the limiting case of a magnetic dipole ( n=1 ), the equation for the magnetic field lines of the general axisymmetric magnetic multipole simplifies correctly to the well-known dipolar form, which is used extensively in geomagnetism, magnetospheric physics and cosmic-ray physics as a first-order approximation to the actual configuration of the geomagnetic field.
It is also shown theoretically that suites of similar magnetic field lines of the general axisymmetric multipole attain their maximum radial distances from the origin on a set of circular conical surfaces, with coincident vertices at the centre of the Earth; this set includes the equatorial plane if the degree ( n ) of the multipole is odd. The magnetic field is horizontal everywhere on all these surfaces.
Palaeomagnetic studies have suggested that during some polarity reversals the magnetic field in the inner magnetosphere can be represented approximately either by a single, non-dipolar, low-degree (2 < n < 4), axisymmetric magnetic multipole or by a linear combination of such multipoles. In this situation, the equation for the field lines of an axisymmetric magnetic multipole of low degree (2 < n < 4) would be as fundamental to a proper understanding of magnetospheric, ionospheric and cosmic-ray physics during polarity reversals as is the equation for dipolar field lines in the case of the contemporary geomagnetic field.  相似文献   

11.
景观间隙度指数在沙漠化研究中的应用   总被引：10，自引：0，他引：10  
常学礼 《中国沙漠》1997,17(4):351-354
综述了间隙度指数在景观生态学和沙漠化研究中的应用,用实例介绍了间隙度指数的定义和计算方法。详尽地分析了流动沙丘面积相同,分布形式不同而产生的间隙度指数差异。结果表明,沙丘的分布形式越集中,沙丘的间隙度指数越大;反之亦然。同时还阐述了间隙度指数在沙漠化评价和沙漠化指征研究中的作用,并对其应用范围和前景作了说明。  相似文献   

12.
Summary. The secular variation of the declination, inclination and total force of the geomagnetic field has been plotted for 74 locations in North America. A comparison of the occurrences of maxima and minima in the SV curves from different stations shows little evidence of drift in North America. Although a declination maximum exhibits westward drift up to 1915, all other extrema in declination, inclination and total intensity occur almost simultaneously over a wide area. The major feature of SV in North America appears to be a 4000 nT decrease in the total field since 1850, which may be due to a decrease in the dipole moment coupled with the decay of a large non-dipole anomaly situated under the continent. Short-period changes in the rate of decrease are possibly jerks of the magnetic field. Maximum entropy spectral analysis of all three components of the field indicates periods of 102 and 53 yr.  相似文献   

13.
A study of geomagnetic storms   总被引：4，自引：0，他引：4  
Summary. An attempt is made to find interplanetary magnetic field and solar-wind parameters which control the development of geomagnetic storms. For this purpose, the interplanetary energy flux is estimated in terms of the Poynting flux ( E × B /4π), and its time variations are compared with the rate of energy dissipation in terms of the ring-current particle injection u i( t ), Joule dissipation in the ionosphere uj ( t ) and auroral particle injection up ( t ) for 15 major geomagnetic storms.
It is shown that the growth of geomagnetic storms, namely the time variations of the rate of the total energy dissipation, u ( t ) = u i( t ) + u j( t ) + u p( t ), is closely related to the Poynting flux by the following relation:
where l 0≅ 7 R E and θ' is a measure of the angle between the interplanetary magnetic field vector and the magnetospheric field vector at the front of the magnetosphere in the equatorial plane. Further, it is shown that within a factor of 2 for each storm period.
A large increase of u ( t ) is associated with substorm activity. Thus, the energy flux ɛ( t ) entering the magnetosphere is dissipated through magneto-spheric substorm processes within the magnetosphere, and their accumulated effects can be understood as geomagnetic storm phenomena.  相似文献   

14.
Analytical solutions for the piezomagnetic potentials are derived for strike-slip, dip-slip and tensile-opening fault motions with arbitrary dip and strike angles, so as to be applicable in various types of earthquakes. These solutions are expressed as the composition of elementary functions which are identical to the magnetic potentials produced by magnetic dipoles, quadrupoles and octupoles distributed on the fault plane and other planes. Therefore, the geomagnetic field changes due to the piezomagnetic effect are expressed by the superposition of the fields produced by these equivalent sources.
Examples of calculated results show characteristic features for various types of fault motions as follows: (1) the pattern of the geomagnetic field changes becomes significantly different depending on the strike direction, although the maximum amplitude is almost the same for all directions; (2) the geomagnetic field change reaches a maximum at a dip angle of 90° for strike-slip and tensile-opening fault motions and at 45° for dip-slip fault motion.  相似文献   

15.
A physical model for palaeosecular variation   总被引：1，自引：0，他引：1  
Summary. A new model to describe the latitude dependence of the angular dispersion of the palaeomagnetic field (palaeosecular variation) is developed following previous models, but with crucial differences. It is shown that if the probability distribution of virtual geomagnetic poles (VGPs) is circularly symmétric about the rotation axis then the geométry of the distribution of field directions is latitude dependent. This has a significant effect on the latitude dependence of dispersion and is accounted for in the model. The dipole and non-dipole parts of the field are not artificially separated but are intimately linked through an observationally determined relation that the time averaged intensity of the non-dipole field is dependent upon the intensity of the dipole field. It is shown that a consequence of this relation is that no knowledge of the probability distribution of the geomagnetic dipole moment is required. This is a fundamental improvement over previous models.
The model provides excellent fits to the palaeodata and, unlike previous models, is not inconsistent with the latitude variation of the non-dipole field dispersion determined from the present field. For the past 5 Ma the point estimate of the VGP dispersion due to dipole wobble is 7.2° and of the VGP dispersion at the equator due to variation in the non-dipole field is 10.6°. This estimate of the dispersion due to variation in the non-dipole field is in excellent agreement with the value predicted from an analysis of the variation in field intensities over the same period. Fits of the model to data from earlier periods indicate that dispersion due to variation in the non-dipole field is essentially independent of the geomagnetic reversal rate while dipole wobble is positively correlated with reversal rate.  相似文献   

16.
Summary. In this investigation, we carry out a two-dimensional study of the dependence of the imaginary Parkinson arrows on the frequency of the inducing geomagnetic field. Our results demonstrate that the imaginary arrows reverse direction as the inducing period varies. Therefore, we consider that there is no way to fix a consistent sign convention for the imaginary arrows even when the time factor is taken into account. We find that in the twodimensional case there exists a characteristic period T c at which the phase difference between the vertical and horizontal magnetic components is zero. It is anticipated that T c is related to the parameters of the conductivity anomaly and the status of the half-space host.  相似文献   

17.
A region of enhanced conductivity at the base of the mantle is modelled by an infinitesimally thin sheet of uniform effective conductance adjacent to the core–mantle boundary. Currents induced in this sheet by the temporally varying magnetic field produced by the geodynamo give rise to a discontinuity in the horizontal components of the poloidal magnetic field on crossing the sheet, while the radial component is continuous across the sheet. Treating the rest of the mantle as an insulator, the horizontal components of the poloidal magnetic field and their secular variation at the top of the core are determined from geomagnetic field, secular variation and secular acceleration models. It is seen that for an assumed effective conductance of the sheet of 108  S, which may be not unrealistic, the changes produced in the horizontal components of the poloidal field at the top of the core are usually ≤10 per cent, but corrections to the secular variation in these components at the top of the core are typically 40 per cent, which is greater than the differences that exist between different secular variation models for the same epoch. Given the assumption that all the conductivity of the mantle is concentrated into a thin shell, the present method is not restricted to a weakly conducting mantle. Results obtained are compared with perturbation solutions.  相似文献   

18.
A generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0–5 Ma has been constructed for use with a relational database management system. The database includes all data whose virtual geomagnetic poles (VGP) lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling. Because many of these data were collected and processed more than 15–20 years ago, each result has been characterized according to the demagnetization procedures carried out. Analysis of these data in terms of the latitude variation of the angular dispersion of VGPs (palaeosecular variation from lavas) strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods. Differences between the angular dispersions for separate normal- and reverse-polarity data sets confirm that many older studies have not been adequately cleaned magnetically. Therefore, the use of the database for geomagnetic field modelling should be carried out with some caution. Using a VGP cut-off angle that varies with latitude, the best data set consists of 2636 results that show a smooth increase of VGP angular dispersion with latitude. Model G for palaeosecular variation, which is based on modelling of the antisymmetric (dipole) and symmetric (quadrupole) dynamo families, provides a good fit to these results.  相似文献   

19.
Summary. In 1972–78, the late N. D. Watkins and others carried out ajoint field programme of geological mapping in the Mio-Pliocene flood basalts of Iceland, including sampling for K-Ar dating and palaeomagnetic research. The major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile. This paper deals with various statistical properties of this data set.
It is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean-floor polarity time-scale. There is no evidence for significant asymmetries between normal and reverse polarity states of the field, neither as regards chron lengths, secular variation, or virtual dipole moment magnitude. Intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution. The latitude distribution of virtual magnetic poles can be fitted with a Bingham function having k' ∼ 4.5, and low-latitude poles do not occur preferentially in any particular longitude interval.  相似文献   

20.
Summary. Specimens with low and intermediate Curie points from six Hawaiian historic basalt flows were used for the determination of the intensity of the historic geomagnetic field by the Thelliers' method. The specimens were heated either in air or in a vacuum of 10−5 torr. The regional intensity of the Earth's magnetic field at the time of extrusion of these rocks is known from direct observations. The palaeointensities determined in vacuum from four low and intermediate Curie point flows are correct within the uncertainty caused by the local field anomalies, whereas those determined in air from the same four flows are of lesser quality and accuracy. Unaltered submarine basalts have similarly low Curie points and thus may also be amenable to palaeointensity determination in vacuum. The behaviour of the remaining two flows, which had higher Curie points, was more erratic, and they yielded less accurate palaeointensities regardless of whether they were determined in air or vacuum.  相似文献   

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