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1.
The technique of bootstrapped discrete scale invariance allows multiple time-series of different observables to be normalized in terms of observed and predicted characteristic timescales. A case study is presented using the SINT2000 time-series of virtual axial dipole moment, which spans the past 2 Myr. It is shown that this sequence not only bears a clear signature of a preferred timescale of about 55.6 Ka, but additionally predicts similar features (of shorter and longer duration) that are actually observed on the timescales of historical secular variation and dipole reversals, respectively. In turn, the latter two empirical sources both predict the characteristic timescale found in the dipole intensity sequence. These communal scaling characteristics suggest that a single underlying process could be driving dynamo fluctuations across all three observed timescales, from years to millions of years.  相似文献   

2.
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   

3.
Summary. Estimates of the secular variation in the Red Sea over the period 1959 to 1972 have been obtained from an analysis of marine magnetic data. A total of 318 crossings of ships' tracks were used to determine the mean secular variation for the intervals 1959–72, 1959–65 and 1965–72. The mean secular variation 1959–72 shows a marked northward increase from approximately -10 nT/yr at 13°N to +27 nT/yr at 24°N. North of this, the data suggest a small decrease to + 25 nT/yr at 27° N. These values are consistent with the secular variation recorded at the nearby geomagnetic observatory at Helwan, Egypt, but less than those predicted by the 1965 IGRF for the same period.
Comparison of the mean secular variations for 1959–65 and 1965–72 yields a rough estimate of the secular acceleration of - 1.5 nT yr−2. Analysis of the cross-over information, corrected for the latitude dependence of the secular variation, shows a regular decrease in the secular change over the period 1959–72 at all latitudes of about -1 nTyr−2. This secular acceleration makes a substantial contribution to the overall secular change in the Red Sea and as such must be included in the correction of magnetic data covering more than a few years.  相似文献   

4.
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   

5.
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   

6.
Monopoly     
Summary. A model for the geomagnetic secular variation field is given, consisting of a series of magnetic monopoles at the surface of the Earth's core. These are distributed according to the density of the data to allow more detailed representation in areas where the density of observations is high, without introducing spurious detail where data are sparse. A monopole model is calculated from observatory secular change data for the epoch 1957.5–1962.5 and its usefulness assessed.  相似文献   

7.
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   

8.
Summary. Palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern British Columbia are reported. Radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp. No evidence for any large geomagnetic excursions (such as the so-called Mono Lake Excursion) is found, but a distinctive pattern of 'normal' secular variation is observed with declination and inclination swings of 45° and 25° peak to peak amplitude respectively. For the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude, but three relatively large perturbations occur at approximately 4000 yr intervals. These perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non-isotropic secular variation model proposed by Cox. The bias involved is about 6° in declination and 3° in inclination (overall mean D = 5.8°E, I = 64.2°, α95 = 0.9°, N = 212 horizons), which leads to a pole which is both 'far-sided' and 'right-handed'. If the horizons involved in the three major perturbations are eliminated the mean direction ( D = 1.2°E, I = 67.2°, α95 = 0.8°, N = 125 horizons) does not differ from that of a geocentric axial dipole despite the small cone of confidence.  相似文献   

9.
Summary. A precision magnetic survey for the investigation of current activity in the Earth's lithosphere has been carried out in the Urals and in the Carpathians. As a result of this research three types of time variation of the total field were discovered. These are:
(1) The normal field variation reflecting the general pattern of secular variation. The difference of initial and repeat observation where only this type of variation operates, is rather small and usually does not exceed 0.2–0.3 nT. The field changes in such regions can be used only to evaluate the observation errors and to provide the regional pattern of secular variation.
(2) The slow but localized'anomalous field'change from year to year corresponding, presumably, to anomalies of a tectonomagnetic nature. The normal pattern of the secular variation field here is disturbed by sources located in the upper part of the lithosphere.
(3) Irregular time changes of the field with rather large amplitudes (up to 10–20 nT). Repeated observations of such anomalies show that the field changes significantly here even during one day. Both in the Urals and Carpathians these anomalies form extended elongated structures with widths up to 10–30 km. These anomalies usually coincide with those deep faults where the strongest recent crustal movements have been determined by means of geodetic observations. The analysis of the results of precision geomagnetic surveys in the Urals and in the Carpathians shows that geomagnetic investigations can be used for the exploration of tectonically active zones.  相似文献   

10.
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   

11.
Summary. Using a very large body of post-1955 data, a spherical harmonic model of the geomagnetic field and its secular variation is derived for 1965.0. This model is compared with the original International Geomagnetic Reference Field (IGRF) and with individual models used, or proposed for use, in producing the IGRF. Positions of the dip-poles, the geomagnetic poles and the eccentric dipole are derived from the model, together with their rates of change, and comparisons are made with other estimates of these positions.  相似文献   

12.
We incorporate a maximum entropy image reconstruction technique into the process of modelling the time-dependent geomagnetic field at the core–mantle boundary (CMB). In order to deal with unconstrained small lengthscales in the process of inverting the data, some core field models are regularized using a priori quadratic norms in both space and time. This artificial damping leads to the underestimation of power at large wavenumbers, and to a loss of contrast in the reconstructed picture of the field at the CMB. The entropy norm, recently introduced to regularize magnetic field maps, provides models with better contrast, and involves a minimum of a priori information about the field structure. However, this technique was developed to build only snapshots of the magnetic field. Previously described in the spatial domain, we show here how to implement this technique in the spherical harmonic domain, and we extend it to the time-dependent problem where both spatial and temporal regularizations are required. We apply our method to model the field over the interval 1840–1990 from a compilation of historical observations. Applying the maximum entropy method in space—for a fit to the data similar to that obtained with a quadratic regularization—effectively reorganizes the magnetic field lines in order to have a map with better contrast. This is associated with a less rapidly decaying spectrum at large wavenumbers. Applying the maximum entropy method in time permits us to model sharper temporal changes, associated with larger spatial gradients in the secular variation, without producing spurious fluctuations on short timescales. This method avoids the smearing back in time of field features that are not constrained by the data. Perspectives concerning future applications of the method are also discussed.  相似文献   

13.
The remanent magnetic properties of an 88  m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c . 720 ± 32  yr and, after 40  mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17  Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no difference in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700  yr, the same as for unsmoothed European secular variation during the last 2000  yr. The intensity of remanence, after 40  mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127 ± 3  Ma) which shows clear regularities in behaviour, some related to changes in the Earth's orbital parameters.  相似文献   

14.
Summary. A 10 000 yr continuous secular variation record from intensively dated lake sediments in SE Australia has been subjected to periodogram and maximum entropy method analysis. Tests on synthetic data reveal some of the limitations of the latter method, particularly when applied to complex number series. Anticlockwise precession of the magnetic vector at a period of 5000 ± 1000 yr is tentatively ascribed to dipole precession, and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non-dipole field.
The effect of calibrating the radiocarbon time-scale is important and results in periodicity shifts of up to 25 per cent. Even for well-dated lacustrine sequences power spectra are poorly constrained: it is thus possible that the geomagnetic secular variation on a time-scale of thousands of years is more uniform than often supposed. Mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed.  相似文献   

15.
summary . The magnetic observatories at Vysokaya Dubrava and at Arti in Sverdlovsk region have operated in parallel since 1972 January. In spite of the rather small distance between them (160 km), it has been established that during that time, there have been differences in the geomagnetic field variations over a wide range of frequencies. The secular variation also differs appreciably. Over a period of four years the difference in the horizontal component between the observatories increased by 3 nT, the difference in the vertical component by 4 nT and the difference in the total intensity by 6 nT, while the difference in declination decreased by 0.6 min. Besides the normal geomagnetic secular variation, governed by processes in the Earth's core, secular-variation anomalies (SVA) linked with processes in the lithosphere may also take place. The intensity of the local SVAs reaches 10–2 nT/yr, whilst their size does not exceed 15 km. It is undesirable to locate an observatory in a region where an SVA exists since this will distort the normal secular-Variation pattern. It is therefore important to know to what extent observations at the observatory reflect the mean value of the secular variation for that area. For this purpose we would recommend performing a highly accurate survey, for example the total intensity in a network of radial lines in a region around the observatory of radius 50–100 km. Such investigations have been performed around the Arti observatory with an accuracy of the repeated measurements of ±0.3 nT in 1974 and ±0.2 nT in 1975. Ten local SVAs with intensities from 2–14 nT/yr were revealed around the observatory. The observatory of Arti is situated in a quiet region.  相似文献   

16.
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   

17.
Summary. A palaeomagnetic record of the geomagnetic secular variation during the last 10000 years has been obtained from 10 cores of sediment from Loch Lomond, Scotland, Lake Windermere, North England, and Llyn Geirionydd, North Wales. A time-scale is provided by 30 radiocarbon age determinations and pollen analyses on several of the cores. The main swings and much fine detail of both declination and inclination records repeat well between cores and between lakes, and the overall record is much more detailed than previous European records.
The new record shows that neither declination nor inclination swings have been periodic over the past 10000 years, but that the main swings have become progressively shorter in duration during that time. Each swing is characterized by fine detail which enables use of the record as a secondary method of dating other European sediments.
The motion of the geomagnetic vector has been predominantly clockwise throughout the time period spanned, but confirms a period of anticlockwise motion from 1100 to 600 bp first discovered by British archaeomagnetic investigations. The record agrees with British and Czechoslovakian archaeomagnetic records, but not with Japanese archaeomagnetic or North American lake sediment records. This suggests that the secular changes are controlled by local growing and decaying, drifting sources, rather than by wobbling of the main geomagnetic dipole.  相似文献   

18.
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   

19.
Summary. A spherical harmonic model of the second time-derivative of the geomagnetic field is determined, for the first time, directly from measures of the secular acceleration based on observatory annual mean data. The data span the interval 1964.5–1975.5, and 165 observatories are included. The model comprises the 32 coefficients of degree and order up to 6 that are significant at the 5 per cent level. Its primary purpose is to aid in the reduction of data to epoch for the 1980 series of navigational charts. The model is compared with earlier estimates of secular acceleration, derived by less direct methods.  相似文献   

20.
Rotation of the geomagnetic field about an optimum pole   总被引：2，自引：0，他引：2  
Since 1693, when Halley proposed that secular change was the result of the westward drift of the main field, his simple model has undergone many refinements. These include different drift rates for dipole and non-dipole parts; separation into drifting and standing parts; latitudinal dependence of drift rate; northward drift of the dipole; and non-longitudinal rotations of the individual harmonics of the geomagnetic field. Here we re-examine the model of Malin and Saunders, in which the main field is rotated about an optimum pole which does not necessarily coincide with the geographical pole. The optimum pole and rotation angle are those that bring the main field for epoch T 1 closest to that for T 2 , as indicated by the coefficients of correlation between the spherical harmonic coefficients for the two epochs, after rotation. Malin and Saunders examined the pole positions and rates of rotation using data from 1910 to 1965, and noticed a number of trends. We show that these trends are confirmed by recent IGRF models, spanning the interval 1900–2000 and to degree and order 10. We also show that the effect of the level of truncation is small.  相似文献   

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