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1.
Fluid flow below the core-mantle boundary is inferred from geomagnetic secular variation data, assuming frozen magnetic flux and a new physical assumption termed helical flow, in which the tangential divergence correlates with the radial vorticity. Helical flow introduces streamfunction diffusion and removes non-uniqueness in the inversion of the magnetic induction equation. We combine helical flow with tangential geostrophy and compare the following physical assumptions: tangential geostrophy, strong helicity, weak helicity and columnar flow, using geomagnetic field models from the 2000 Oersted and 1980 Magsat satellites. Our solutions contain some features found in previous core flow models, such as large mid-latitude vortices, westward drift in most of the southern hemisphere, and suggested polar vortices. However, our solutions contain significantly more flow along contours of the radial magnetic field than previous core flow models. 相似文献
2.
With the prospect of studying the relevance of the topographic core-mantle coupling to the variations of the Earth’s rotation and also its applicability to constraining the core surface flow, we investigate the variability of the topographic torque estimated by using core surface flow models accompanied by (a) uncertainty due to the non-uniqueness problem in the flow inversion, and (b) variance originating in that of geomagnetic secular variation models employed in the inversion. Various flow models and their variances are estimated by inverting prescribed geomagnetic models at the epoch 1980. The subsequent topographic torque is then calculated by using a core-mantle boundary topography model obtained by seismic tomography. The calculated axial and equatorial torques are found subject to the variability of order 1019 and 1020 Nm, respectively, on which (b) is more effective than (a). The variability of the torque is attributed even to (a) and (b) of the large-scale flows (degrees 2 and 3). Yet, it still seems unlikely for the decadal polar motion with the observed amplitude to be excited exclusively by the equatorial topographic torque associated with any of reasonable core surface flow models. It is also confirmed that, with the topography model adopted here, the axial topographic torque on a rigid annulus in the core (coaxial with the Earth’s rotation axis) associated with any of reasonable flow models is larger by two orders of magnitude than the plausible inertial torque on such cylinders. This implies that any core surface flow model consistent with the topographic coupling does not exist, unless the topography model is appropriately modified. Nevertheless, the topographic coupling might provide not only a weak constraint for explaining the decadal LOD variations, but also the possibility to probe the core surface flow and the core dynamics. 相似文献
3.
在国际参考地磁场（IGRF）模型中，高阶球谐系数比低阶系数小得多，所以高阶项对主磁场的贡献常被忽略不计. 但是，高阶项对主磁场的长期变化却有重大影响. 本文根据第八代IGRF模型，分析了IGRF 1945～1955中高阶球谐系数的异常特征对长期变化的影响，然后用修正IGRF模型估计并改正了这些影响. IGRF高阶项随深度增加而迅速增大，所以高阶系数的误差对深部磁场位形和磁场能量的计算有不可忽视的影响，对核幔界面流动状态的推断和地核半径的地磁估计有更为重大的影响. 相似文献
4.
C. Narteau E. Blanter J. -L. Le Mouël M. Shirnman C. J. Allgre 《Physics of the Earth and Planetary Interiors》2000,120(4):271-287
We investigate the temporal evolution of the magnetic dipole field intensity of the Earth through a multiscale dynamo mechanism. On a large range of spatio-temporal scales, the helical motions of the fluid flow are given by a schematic model of a fully developed turbulence. The system construction is symmetric with respect to left-handed and right-handed cyclones. The multiscale cyclonic turbulence coupled with a differential rotation (schematic ω dynamo) or alone (schematic 2 dynamo) is the ingredient of the loop through which poloidal and toroidal fields are built from one another. Two kinds of reaction of the magnetic field on the flow are considered: the presence of a magnetic field first favours a larger-scale organization of the flow, and, second, impedes this flow by the effect of growing Lorentz forces. We obtain the general features of the geomagnetic field intensity observed over geological times and describe a general mechanism for reversals, excursions and secular variation. The mechanism happens to keep a memory during the chrons and loose it during the events (excursions and inversions). 相似文献
5.
The geomagnetic field and secular variation exhibit asymmetrical spatial features which are possibly originating from an heterogeneous thermal control of the Earth's lower mantle on the core. The identification of this control in magnetic data is subject to several difficulties, some of which can be alleviated by the use of core surface flow models. Using numerical dynamos driven by heterogeneous boundary heat flux, we confirm that within the parameter space accessible to simulations, time average surface flows obey a simple thermal wind equilibrium between the Coriolis and buoyancy forces, the Lorentz, inertial and viscous forces playing only a secondary role, even for Elsasser numbers significantly larger than 1. Furthermore, we average the models over the duration of three vortex turnovers, and correlate them with a longer time average which fully reveals the signature of boundary heterogeneity. This allows us to quantify the possibility of observing mantle control in core surface flows averaged over a short time period. A scaling analysis is performed in order to apply the results to the Earth's core. We find that three vortex turnovers could represent between 100 and 360 years of Earth time, and that the heat flux heterogeneity at the core-mantle boundary could be large enough to yield an observable signature of thermal mantle control in a time average core surface flow within reach of the available geomagnetic data. 相似文献
6.
We study magnetic field variations in numerical models of the geodynamo, with convection driven by nonuniform heat flow imposed at the outer boundary. We concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle. At a Rayleigh number of about 100 times critical with respect to the onset of convection, the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as Earth’s historical magnetic field on the core-mantle boundary (CMB). The time scales of variation of the low-order Gauss coefficients in the model agree within a factor of two with observed values. We have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity. An average over 2000 years (the archeomagnetic time scale) is barely sufficient to reveal the long-term nondipole field. The model shows reduced scatter in virtual geomagnetic pole positions (VGPs) in the central Pacific, consistent with the weak secular variation observed in the historical field. Longitudinal drift of magnetic field structures is episodic and differs between regions. Westward magnetic drift is most pronounced beneath the Atlantic in our model. Although frozen flux advection by the large-scale flow is generally insufficient to explain the magnetic drift rates, there are some exceptions. In particular, equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds. 相似文献
7.
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
8.
地磁急变（jerk）是起源于地球外核并在导电地幔过滤效应后在地球表面观测到的一种地磁现象,其反映了地核内部某些动力学过程.Jerks在空间范围上既可以是区域性的,也可以是全球性的.中国地区地磁台能否检测到2014年jerk？针对这一问题,利用中国大陆10个地磁台的磁静日月均值和CHAOS-6全球磁场模型,分析了X、Y和Z分量2008—2018年期间的长期变化,估算了2014年前、后的长期加速度值,确定了2014年地磁jerk的时间和强度.研究表明中国地磁台Y分量的长期变化为"Λ"型,Z分量存在明显的"V"型,具有典型的jerk特点.Y分量jerk出现的时间大约在2014年6月,比非洲大陆的Algeria TAM台和南美洲French Guiana KOU台时间滞后大约4个月.这暗示着产生jerks的地核流体波动的时序特点.中国西部和东北部地磁台的长期变化形态有明显的差别,主要由非偶极磁场引起.CHAOS-6模型与地面台站的长期变化形态并非始终一致.本文结果有助于更好地理解和解释长期变化的时间演变和地理分布,并为深入探讨jerks的地核起源和驱动机制提供新的观测约束. 相似文献
9.
Michael I. Bergman 《地球物理与天体物理流体动力学》2013,107(1-4):151-176
Abstract The stratification profile of the Earth's magnetofluid outer core is unknown, but there have been suggestions that its upper part may be stably stratified. Braginsky (1984) suggested that the magnetic analog of Rossby (planetary) waves in this stable layer (the ‘H’ layer) may be responsible for a portion of the short-period secular variation. In this study, we adopt a thin shell model to examine the dynamics of the H layer. The stable stratification justifies the thin-layer approximations, which greatly simplify the analysis. The governing equations are then the Laplace's tidal equations modified by the Lorentz force terms, and the magnetic induction equation. We linearize the Lorentz force in the Laplace's tidal equations and the advection term in the magnetic induction equation, assuming a zeroth order dipole field as representative of the magnetic field near the insulating core-mantle boundary. An analytical β-plane solution shows that a magnetic field can release the equatorial trapping that non-magnetic Rossby waves exhibit. A numerical solution to the full spherical equations confirms that a sufficiently strong magnetic field can break the equatorial waveguide. Both solutions are highly dissipative, which is a consequence of our necessary neglect of the induction term in comparison with the advection and diffusion terms in the magnetic induction equation in the thin-layer limit. However, were one to relax the thin-layer approximations and allow a radial dependence of the solutions, one would find magnetic Rossby waves less damped (through the inclusion of the induction term). For the magnetic field strength appropriate for the H layer, the real parts of the eigenfrequencies do not change appreciably from their non-magnetic values. We estimate a phase velocity of the lowest modes that is rather rapid compared with the core fluid speed typically presumed from the secular variation. 相似文献
10.
We consider how to treat a finite-dimensional linear inverse problem when the form of the forward problem is known exactly, but is dependent upon some parameters whose exact value is uncertain and which enter the forward problem multiplicatively. We show one way to proceed when the uncertainty is treatable in a statistical manner. Predicting the secular variation ∂tB(t) produced by a particular fluid flow V at the core-mantle boundary (when magnetic diffusion is ignored) is one such example, because the results depend on the main magnetic field B(t) originating in the core which is improperly known because of contamination by the crustal magnetic field. This infinite-dimensional inverse problem is often solved by projection on to a finite-dimensional basis, and the resulting parameters found by a maximum likelihood technique. If the main field is contaminated with errors from a Gaussian distribution, this paper describes how the maximum likelihood solution can take this into account, and we show the probability density function that must be maximised in this case. We give an example of the effects for a simple model system, and suggest possible areas of application. 相似文献
11.
Q. Q. Shi Z. Y. Pu H. Zhang S. Y. Fu C. J. Xiao Q. -G. Zong T. A. Fritz Z. X. Liu 《Surveys in Geophysics》2005,26(1-3):369-386
Magnetic reconnection at the high-latitude magnetopause is studied using 2–1/2-dimensional (2−1/2-D) Hall-MHD simulation. Concentric flow vortices and magnetic islands appear when both Hall effect and sheared flow are considered. Plasma mixing across the magnetopause occurs in the presence of the flow vortices. Reconnected structure generated in the vicinity of the subsolar point changes its geometry with increasing flow shear while moving to high latitudes. In the presence of flow shear, with the Hall-MHD reconnection a higher reconnection rate than with the traditional MHD is obtained. The out-of-plane components of flow and magnetic field produced by the Hall current are redistributed under the action of the flow shear, which makes the plasma transport across the boundaries more complicated. The simulation results provide some help in understanding the dynamic processes at the high-latitude magnetopause. 相似文献
12.
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
13.
S. V. Starchenko 《Geomagnetism and Aeronomy》2013,53(2):243-246
The typical scales, velocities, and magnetic fields in the liquid core of the Earth are determined by using the analysis results of the magnitude of energy that is available for the geodynamo, physical regularities, and observational data. In this work, it is justified that the geomagnetic field is mainly generated in a regime where the magnetic Lorentz force is equilibrated by the Archimedean buoyancy force and by the Coriolis rotational force and the force of inertia is considerably less than these forces. The characteristic periods obtained in the course of this justification permit one to clarify not only the physical nature of secular geomagnetic variations but also that of jerks. In another regime, which is less probable for the present-day Earth, the main balance of forces is determined by inertia and buoyancy; the magnetic field has no significant effect on the typical rate and scale of convection. This regime seems to be probable in the liquid core of the Earth during inversions or digressions, as well as in depths of Mercury, Mars, Uranus, and Neptune. 相似文献
14.
根据历史地磁场模型GUFM1、第10代国际参考地磁场（IGRF10）模型和日长资料,采用小波变换方法,分析了地磁场磁矩、能量、西向漂移等参数的长期变化和日长十年尺度变化的周期分量及其时变特征.结果表明,1800～2005年期间,偶极子磁场长期变化有82年和48年准周期分量,它们与日长变化的周期没有直接关系.非偶极子磁场参数的长期变化与日长变化有66年和32年准周期分量,66年准周期比32年准周期强.在66年准周期分量,西向漂移比日长变化超前8.8年,非偶极子磁场能量比日长变化滞后15.6年.日长十年尺度波动和地磁场长期变化的起源不存在因果关系. 相似文献
15.
D. R. Barraclough T. D. G. Clark S. W. H. Cowley F. H. Hibberd R. Hide D. J. Kerridge F. J. Lowes S. R. C. Malin T. Murphy H. Rishbeth S. K. Runcorn H. C. Soffel D. N. Stewart W. F. Stuart K. A. Whaler D. E. Winch 《Surveys in Geophysics》1992,13(1):47-88
A joint Discussion Meeting of the Royal Astronomical Society and the Royal Irish Academy, held on January 11th, 1991, commemorated the establishment of some early magnetic observatories, discussed recent research using global geomagnetic data and described the present status of magnetic observatories in the United Kingdom. The observatory and instruments at the Dublin magnetic observatory; the origins of the Greenwich magnetic observatory, and why it eventually had to be resited; and the history of the Munich magnetic observatory formed the historical part of the proceedings. Current research topics discussed were the geomagnetic secular variation and deep Earth structure and dynamics; fluid flow patterns near the top of the core; the origin of the annual variation of the geomagnetic field; results of an analysis of monthly means from some British observatories; a new theory of the geomagnetic daily variation; and the interactions between ionospheric science and geomagnetism. The present-day observatory scene was described in terms of the information that can be derived from the almost 40 year series of data from Hartland magnetic observatory; of the methods used to process data from the three UK magnetic observatories, which nowadays are operated automatically and remotely; and (a look into the future) of a new project, INTERMAGNET, which aims to make available, in near real time, data from the world-wide network of magnetic observatories. 相似文献
16.
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Zmax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. 相似文献
17.
本文利用近三十年来的地面测量资料和东亚地区11个地磁台的资料,以及1965.0、1975.0国际地磁参考场等资料,对东亚大陆磁场的时、空变化特征进行了研究。内容包括:东亚大陆磁场的空间分布特征及场源深度的估算,东亚大陆磁场的中心位置及其磁场强度、磁场方向,近三十年来东亚大陆磁场的长期变化特征等。得到的主要结果是:1.东亚大陆磁场的中心位置近三十年来没有西向漂移的迹象,从而推断它的长期变化主要由场源强度的变化所引起,而不是主要由非偶场的西向漂移所引起;2.东亚大陆磁场的中心与它的长期变化的中心不相一致,同时,在青藏高原的东部还可能存在一弱中心。 相似文献
18.
为了综合反映地球表面行星尺度磁异常的展布面积、磁场极值以及磁场分布特征等多种因素及其与磁能的关系，本文用穿过各异常区的“无符号磁通量”为特征参数来表征磁异常区强度.用第八代国际参考地磁场模型(IGRF)，分析了1900年到2000年全球最大的5个磁异常区的长期变化，结果表明，在一百年中，南大西洋(SAT)、大洋洲(AUS)和非洲(AF)3个异常区的磁通量均增加了200MWb以上，欧亚异常(EA)磁通量增加幅度稍小(157MWb)，上述4个异常区磁通量增幅为30%-60%，而北美异常(NAM)的磁通量则减小了50MWb.各异常区面积虽有变化，但最大变化仅为%左右.对磁异常区的西向漂移研究表明，地球表面和核幔界面的西漂明显存在差异：地表磁场有持续而稳定的西向漂移，全球平均西漂速度为0.2°/a；但核幔界面磁场的西向漂移速度要小得多，最大不超过0.1°/a.形成这种差异的原因可能是组成地磁场的不同球谐分量有不同的漂移速度；地表磁场的西漂主要决定于占优势的低阶分量，而核幔界面的西漂则受到高阶分量的重大影响.本文指出，在把地表西漂值用作地核磁流体运动速度的典型值时必须十分谨慎. 相似文献
19.
Summary Theoretical formulae are derived for computing a variable magnetic field, excited by a harmonically oscillating radial magnetic dipole (RMD), located eccentrically at the surface of the Earth's core. By numerical computations, using a three-layered conductivity model of the Earth, it is proved that the field due to this source, computed for the surface of the Earth, is relatively weak in comparison to the field of a stationary magnetic dipole, provided the period of the changes is less than 500 years. The zone of influence of the RMD at the Earth's surface is also determined and on its basis a number of conclusions were drawn with respect to representing the non-dipole part of the geomagnetic field by means of the RMD system in the Earth's core.Dedicated to RNDr. Jan Pícha, CSc., on his 60th Birthday 相似文献
20.
M. G. Ogurtsov 《Geomagnetism and Aeronomy》2007,47(1):118-128
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. 相似文献