Anomalies in the secular variations in the main geomagnetic field in the context of the hierarchic dipole model

The developed dynamic model of the main geomagnetic field (MGF) includes the sources of three levels, which have existed and continuously changed during the 100-year period. The spatial-temporal dynamics of the secular variations in MGF has been considered based on this model. It has been indicated that, at different times, the formation of anomalies (located in similar regions) depended on the total dynamics of different combinations of sources, including a change in the parameters of the dipoles of the secondorder smallness. Anomalies generated by the dynamics of the dipoles of the third-order smallness do not evidently manifest themselves in the total secular variations but are responsible for the specific shapes of isodynamic lines. Based on the stability and continuity of the constructed model, the conclusion is made that long-living turbulent flows of different scales can exist in the liquid core.     

Global anomalies of the main geomagnetic field and the dynamic model of their sources

The model of the main geomagnetic field sources, including 12 more dipoles in addition to the main dipole, is presented. The source parameters have been determined for the 100-year-long period at an interval of 5 years. The problem is solved using the iteration method proposed and developed by the authors. Six more sources have been included in the iteration process at its current step. This made it possible to substantially smooth the time variations in the dipole parameters obtained previously and, on the other hand, to describe all global anomalies of the main geomagnetic field using this model. A change in the parameter values can be approximated by rather smooth functions describing specific features of development of the sources operating for 20–30 years. However, the short duration of the obtained series (21 points) makes the problem of prediction substantially more difficult. The parameters of 13 dipoles, extrapolated for 2000 using the linear prediction filter, are in good agreement only with the general development of the sources.     

Construction of the spatial-temporal model of the main geomagnetic field using satellite data

The spatial-temporal model of the geomagnetic field has been constructed using the data of the high-accuracy survey of the CHAMP German satellite, obtained during its operation from May 2001 to September 2007. The daily average spherical harmonic models calculated at an interval of four days are used as initial data in order to expand these models by the method of natural orthogonal components (NOCs). It has been indicated that the obtained NOC series rapidly converges. The secular variations, secular acceleration, and Dst variation are distinguished as individual NOC components, which makes it possible to construct the spatial-temporal field model. In addition, the models-predictions have been constructed for the year 2008 based on the candidate models of the main field and the secular variation for the year 2005, which were used to obtain the IGRF2005 international model. A comparison of the models-predictions with the model constructed for the year 2008 using our method indicated that the accuracy of our model is not lower than that of the models, obtained by other scientific groups using the traditional method.     

The geomagnetic field in the 20th century

The continuous spatial-temporal model of variations in the main geomagnetic field in the 20th century has been elaborated. All available data—from the navigation to the satellite surveys at the beginning and end of the century, respectively—have been used to construct the model. Since the accuracy of measurements was different during the century and the data are nonuniformly distributed over the Earth’s surface, the methods for regulating solutions based on global parameters of the magnetic field and its secular variation, invariant on the simulation interval, have been used to correct the model. The secular variation model has been represented as the sum of the models obtained by means of expansion in terms of natural orthogonal components. The conclusions that the character of field variations is complex have been made and the spatial and temporal characteristics of the secular variations of different origin have been estimated based on the simulation results.     

Spherical capacitor hypothesis of the earth's electric field

Summary Stemming from the suggestion that the earth-ionospheric fair weather field arises from the lowest order resonant excitation of the earth-ionosphere cavity by lightning, it is shown that the same field is generated through electrostatic induction by equivalent charge dipoles from thunderclouds in the earth's atmosphere. An electrostatic induction model was used to derive from known worldwide thunderstorm data the magnitude of the expected earth-ionosphere fair weather potential. A value of only approximately one third of the observed potential was obtained. Possible sources of the discrepancy are suggested.     

Motion of the north magnetic pole within the scope of the dynamic model of the main geomagnetic field sources

The dipole model of the main geomagnetic field sources has been developed by the authors for several years. At present, the model includes 13 sources that existed and continuously developed during the 20th century. It has been assumed that the main dipole motion can be related to the motion of the Earth’s axis of inertia. At the same time, the known sharp changes in the direction of this motion, the so-called “wanderings” of the axis of inertia coincide in time with a change in the coordinates of the exit point of the main dipole magnetic moment vector on the Earth’s surface, dependent mostly on changes in the vector inclination. The motion of the north magnetic pole has been studied based on the model. It has been obtained that the dynamics of the main dipole parameters and, mainly, a stable variation in the inclination of the magnetic moment vector are responsible for the westward pole motion. At the same time, the observed rapid northward motion of the pole is related to the time variations in the parameters of 12 sources approximating the so-called nondipole part of the main field.     

东亚地区地磁非偶极场长期变化的分析

本文利用东亚及其邻近地区50个地磁台站1955-1977年地磁三要素（D、H、Z）的实测年平均值算出的非偶极场值X、Y、Z,选用“按距离加权最小二乘曲面拟合”方法,计算、绘制、分析了东亚地区地磁非偶极场及其长期变化的时空分布特征,并与国际参考场（IGRF）模式进行了粗略的对比。所得的主要结果是:1.1955.0-1965.0年代,东亚地区Z分量的增长速度在迅速减小;1965.0年代以后,Z值在逐年下降。2.从整体上看,东亚地区地磁非偶极场虽然形态与IGRF给出的结果基本吻合,但长期变化有明显差异。1965.0年代的IGRF长期变化模式与实测结果偏差甚大。为了使IGRF模式更好地拟合地磁长期变化处于转折时期的复杂地区的情况,看来时间间隔应更短些,例如5年为好,采用现在通用的方法是不适宜的。     

Interpretations of self potential anomalies due to localised causative bodies

Summary Some direct methods of interpretation of SP anomalies that may arise from localised causative bodies have been developed in this paper. The models of the current sources that have been considered here are the cases of 1. single vertical dipole, 2. a pair of similar and similarly situated vertical dipoles and 3. an inclined dipole. A set of three master curves has been prepared in terms of which the interpretation in the first and the third case is complete. With an additional assumption that the distance between the dipoles is small, the second case also is completely solved with the help of the same set of master curves.Published under the kind permission of the Director General, Geological Survey of India, Calcutta.     

非洲磁异常对地磁场结构及其长期变化的控制作用

地球非偶极磁场在主磁场结构及其长期变化中起着重要作用.非偶极磁场主要表现为行星尺度磁异常,它们是南大西洋磁异常、非洲磁异常、欧亚大陆磁异常、澳洲磁异常和北美磁异常.在这5块磁异常中,非洲磁异常对磁赤道的形状和位置以及全球长期变化特征有极大的影响.非洲磁异常的重要性主要表现在3方面：第一,由于异常区位于赤道这一特殊的地理位置,所以它极大地影响磁赤道的形状和位置.相对于偶极场的地磁赤道而言,异常区所在的中北非洲和中大西洋地区的磁赤道向北移动,最大移动量可达约15°.第二,非洲磁异常的快速西漂对全球长期变化的分布起着决定性作用,它在该异常区西边的中美洲形成了全球最主要的长期变化区,在1900~2005年期间,最大年变率Z_max超过200 nT/a.第三,非洲负磁异常区与其南面的南大西洋正磁异常区相结合,〖HJ〗它们的变化使西半球地磁场强度大大减弱,也使全球磁场发生显著畸变.这两块磁异常与深部的反极性斑区有着成因联系.     

Depth and structural index from normalized local wavenumber of 2D magnetic anomalies

Recent improvements in the local wavenumber approach have made it possible to estimate both the depth and model type of buried bodies from magnetic data. However, these improvements require calculation of third‐order derivatives of the magnetic field, which greatly enhances noise. As a result, the improvements are restricted to data of high quality. We present an alternative method to estimate both the depth and model type using the first‐order local wavenumber approach without the need for third‐order derivatives of the field. Our method is based on normalization of the first‐order local wavenumber anomalies and provides a generalized equation to estimate the depth of some 2D magnetic sources regardless of the source structure. Information about the nature of the sources is obtained after the source location has been estimated. The method was tested using synthetic magnetic anomaly data with random noise and using three field examples.     

Behaviour of the earth's magnetic field as recorded in the sediment of Loch Lomond

A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from¹⁴C analyses on the Lomond sediment and comparison with other¹⁴C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations.The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced.The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole.     

Use of non-linear filtering for the regional–residual separation of potential field data

Regional–residual separation is essential in gravity and magnetic data interpretation and a variety of techniques have been proposed. Graphical determination of the regional allows geological information to be taken into account. Upward continuation can be used to obtain the regional field either empirically or using some hypothesis about the geology. In some cases, a matched filter can be designed and used to separate deep and shallow sources. Simple low pass filtering has also been used but without much success. Here we propose to use a non-linear filter approach to remove gravity and magnetic anomalies smaller than a given width. This technique attempts to mimic the graphical separation method. The results from synthetic models are presented as well as the results from a case study in eastern Canada and compared to regional gravity and magnetic anomalies obtained by other techniques. Contrary to the regional fields obtained by upward continuation, non-linear filtering does not have any physical meaning. However, its main advantage is that it gives a regional component of the gravity or magnetic field similar to the one obtained from a graphical separation.     

Secular variation in aerosol transparency of the atmosphere as the possible link between long-term variations in solar activity and climate

Information about variations in solar activity and climate on the time intervals from 130 years to four–five last centuries, including results of instrumental measurements (Wolf numbers, actinometry, thermometry) and indirect indicators (ice core acidity, NO ₃ ^? ion concentration in polar ice, temperature tree-ring reconstructions), has been analyzed for the Northern Hemisphere and its high-latitude part. It has been obtained that the observed relation between secular variations in solar activity and near-Earth temperature resulted from the effect of the corresponding variation in aerosol transparency of the stratosphere on terrestrial climate. It has been also indicated that long-term variations in the aerosol content of the stratosphere can, in turn, be related to secular cycles in atmospheric ionization caused by variations in fluxes of ionizing cosmic particles.     

Solar activity secular cycles

Long-term variations in solar activity secular cycles have been studied using a method for the expansion of reconstructed sunspot number series Sn(t) for 11400 years in terms of natural orthogonal functions. It has been established that three expansion components describe more than 98% of all Sn(t) variations. In this case, the contribution of the first expansion component is about 92%. The averaged form of the 88year secular cycle has been determined based on the form of the first expansion coordinate function. The quasi-periodicities modulating the secular cycle have been revealed based on the time function conjugate to the first function. The quasi-periodicities modulating the secular cycle coincide with those observed in the Sn(t) series spectrum. A change in the secular cycle form and the time variations in this form are described by the second and third expansion components, the contributions of which are about 4 and 2%, respectively. The variations in the steepness of the secular cycle branches are more pronounced in the 200-year cycle, and the secular cycle amplitude varies more evidently in the 2300-year cycle.     

Space-time analysis of the relative local secular variations of the geomagnetic field (1966–2005) along the Zgorzelec-Wiżajny profile

The Zgorzelec-Wiżajny profile cuts through the main European geotectonic units of the Palaeozoic Western and Central European Platform and the East European Craton, including the connecting Trans-European Suture Zone. The paper summarises 40 years of research on geomagnetic secular variations along the Zgorzelec-Wiżajny profile since 1966. It discusses methods of processing measurement records and presenting their results. The latter are presented against the background of the local geology and the crustal deep magnetic structure data. The hitherto method of variation analysis was modified to address a new trend in the geomagnetic secular variations that has emerged in the East European Craton during the last five years (2001–2005).     

A study of alternative schemes for extrapolation of secular variation at observatories

The geomagnetic secular variation is not well known. This limits the useful life of geomagnetic models. The secular variation is usually assumed to be linear with time. It is found that attenative schemes that employ quasiperiodic variations from internal and external sources can improve the extrapolation of secular variation at high-quality observatories. Although the schemes discussed are not yet fully applicable in worldwide model making, they do suggest some basic ideas that may be developed into useful tools in future model work.     

1900-2000年地球主磁场的本征模分析

利用1900-2000年国际参考地磁场模型（IGRF）,对地球主磁场的空间结构和长期变化进行了自然正交分量分析,得到了组成地磁场的主要成分（本征模）及其随时间的长期变化（SV）．第一本征模描述主磁场稳定结构的总体特征,其强度随时间单调减小;第二本征模主要描述印度洋和南大西洋地区正异常特征,其强度也随时间单调变化;第三本征模对应于加勒比地区正异常,其强度呈100a周期的变化,目前正处于下降阶段;第四本征模的主要特点是北美正异常和南亚负异常,其强度呈80a周期的变化;第五本征模对应尺度较小的局部磁异常,其强度的变化周期约60a;第六及其以后的本征模则对应空间尺度更小和时间变化更快的磁异常．分析表明,太平洋地区的磁场经历着缓慢的单调减小变化,其结构比较稳定,几乎完全包括在第一本征模中．但是,印度洋磁场结构较复杂,它是由空间结构不同和变化周期各异的多个本征模组成的．大西洋的北部、中部和南部分属不同本征模,它们有完全不同的时间变化特性．     

Detection of fast secular variations based on the data of satellite magnetic surveys

The usage of only the vector satellite survey data made it possible to detect rapidly developing anomalies of the secular variations with the foci in central Asia and Indian Ocean. It has been indicated that the field at the foci increased by several hundred nanoteslas for eight years. It is assumed that the origination of the foci was accompanied by jerks, which could be observed in this region in the mid-1990s. It is assumed that the increasing foci are the initial stage of the 60-year variation.     

Correlation of the anomalies in the electric field and electric conductivity of the lithosphere to earthquakes in Kamchatka

The data of long-term electromagnetic monitoring are used for studying the dynamics of electric conductivity of the medium and the electric field of the terrestrial sources. The electric conductivity of the medium is estimated from the magnetotelluric transfer functions (impedance tensor and telluric tensor). The electric field of terrestrial sources is identified by filtering the variations of the observed electric field of the Earth. The magnetotelluric parameters and the electric field of terrestrial sources feature anomalous changes of supposedly earthquake-related origin. The anomalies associated with the same earthquake are not simultaneous. It is shown that these anomalies are generated by processes occurring at different depths. The strong earthquake is preceded by the appearance of surface anomalies several months before the event and accompanied by a deep coseismic anomaly. The probable nature of the recognized anomalies is discussed.     

Modeling of the geomagnetic field secular variation based on the satellite surveys

The accuracy of the geomagnetic field secular variation (SV), obtained using different methods of analysis based on different data sets, has been estimated. A comparison of different SV models (developed by different groups of researchers) as candidates for compiling the IGRF 10th generation is the example of such an estimation. These models were obtained based on the Oersted and CHAMP satellite data. In the present work these models are compared in order to estimate the modeling algorithms and the effect of initial data selection on modeling results. To solve this problem, several SV models have been constructed based on the independent algorithms and data sets. The differences between the candidate models using different data proved to be of the same order. A comparison of the spatial power spectra indicated that all models constructed using data subjected to the frequency selection considerably differ from the models the data of which were not selected.