VGP角离散与地磁场低阶非轴向偶极子分量

虚地磁极（VGP）的角离散携带着地磁场低阶非轴向偶极子分量空间变化的重要信息．重申地球主磁场定解问题的适定性，指出McFadden等模型G存在重复计入轴向偶极子分量的内在矛盾；作为模型G基础的VGP角离散的偶极族-四极族分解仅具有数据拟合意义．证明，如果地磁场的非轴向偶极子分量相对轴向偶极子分量为小量，那么VGP角离散的纬度变化主要由轴向偶极子Gauss系数约化的低阶Gauss系数的平方和决定．因此作出估计，在过去5Ma中，赤道偶极子和赤道四极子的Gauss系数的幅度大致相当，约为轴向偶极子Gauss系数的10％．     

用虚地磁极（VGP）方法对地磁场随时间变化的研究

首次引用古地磁学中虚地磁极（ＶＧＰ）的计算方法来研究现代地磁场的变化规律。通过对中国地区６个台站资料的分析以及对全球３９个台站资料的分析结果可以得出结论：利用地磁场角度分量来研究长期变化是可行的;ＶＧＰ的漂移情况正好表明非偶极子磁场的存在与随时间的变化;ＶＧＰ漂移方向的全球分布与地磁长期变化场的有关分量等值图之间的一致性较好。     

地球非偶极磁场对虚地磁极计算的影响

古地磁学使用的虚地磁极（VGP）是在地心偶极磁场假设下计算的,由于地球非偶极磁场的存在,VGP一般不同于真地磁极（RGP）.为了定量检验非偶极磁场对VGP的影响,本文利用国际参考地磁场模型IGRF 1900～2000,在全球5°×5°的“虚拟测点”网格上计算了VGP和RGP的位置,并求出两种磁极的经纬度偏差和二者的角距离.结果表明,南极地区VGP与GP的角距离最大,可达26°,南大西洋和欧亚大陆北部最大达到24°和18°,其余地区一般小于15°.VGP对RGP的偏差与地磁场分布有关：在非偶极磁场较弱的地区（如太平洋半球）,纬度偏差一般不大（≤10°）,但是在主要地磁异常区（如南大西洋和南极地区）,VGP对RGP的纬度偏差可达25°.VGP对RGP的经度偏差要比纬度偏差大得多,例如在欧亚大陆北部地区,经度偏差分布在-180°到180°的大范围内.     

地磁场长期变化和日长十年尺度变化的周期特征

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.     

CHAMP卫星主磁场长期变化和长期加速度的分布特征

本文利用CHAMP卫星磁测资料建立的新一代地磁场模型POMME-4.2S,分析了全球和中国地区地磁长期变化X、Y、Z、H、F、J、D 7个分量以及长期加速度X、Y、Z3个分量的分布特征,比较了POMME-4.2S长期变化模型与中国长期变模型CGRF-SV的磁异常分布差异.主要结果表明,地磁长期变化磁四极子起主要贡献,长期加速度变化磁八极子贡献最大.2004.0年全球长期变化的减小要比增加大得多,地磁场总体在减弱.长期加速度在中低纬度地区变化较大,高纬度地区变化小.Z和Y分量磁异常沿经度方向正、负交错形成几个大片区,X分量主要正负磁异常成对分布于赤道两侧.长期加速度X、Y、Z3个分量的磁异常分布存在明显的关系:在Z异常焦点的南北两边出现X的异常焦点,东西两边出现Y的异常焦点.中国地区的长期变化和长期加速度变化量小,分布相对均匀.     

2000年中国地磁场及其长期变化冠谐分析

根据1998～2000年完成的118个地磁测点 和39个地磁台的三分量绝对测量资料以及IGRF2000，计算2000年中国地磁场冠谐模型(截断 阶数为8)，以及2000～2005年中国地磁长期变化冠谐模型(截断阶数为6). 球冠极位于36 °N，104°E，球冠半角为30°. 中国地磁场冠谐模型能更好地表示我国地磁场的时空变化 ，地磁场模型的均方偏差为：104.4 nT(X分量)，103.3 nT(Y分量)，123.9 nT(Z分量). 依据地磁场及其长期变化的冠谐模型，分别绘制2000年中国地磁图(F，X，Y，Z)和异常磁场图(ΔF，ΔX，ΔY，ΔZ)，以及2000～2005年地磁长期变化图(F，X，Y，Z). 指出改善地磁场模型边界效应 的途径，并对如何布设地磁复测点提出了建议.     

两种地磁场模型与地磁台站年均值一致性分析

研究了地磁场综合模型CM4和国际参考地磁场模型IGRF12与中国部分地磁台站年均值的一致性,并分析CM4与IGRF12模型之间的差异,结果表明,模型值与台站观测值的差值不随台站经纬度的改变而呈现规律性变化,两种模型值与台站观测值差值的两种变化形态相差较大;同一台站,同一地磁分量,两种地磁场模型的误差水平基本相当;同一地...     

中国及邻近地区地磁长期变化分析

利用2010.0—2016.0年中国国际地磁交换台站和中国邻近地区共14个地磁台站的X,Y,Z分量观测数据,结合世界资料中心发布的静扰日数据,计算了台站十日静日午夜均值,并进行三次多项式拟合,得到X,Y,Z分量的长期变化和年变率。同时,利用CHAOS-6和IGRF-12模型计算了2010.0—2016.0年F分量的长期变化,分别得到了CHAOS-6及IGRF-12模型计算值与台站观测值的差值。结果表明:同一地区的台站X,Y,Z分量变化趋势基本一致;Z分量的平均年变率与X,Y分量相比偏大;CHAOS-6和IGRF-12模型描述的F分量的长期变化与台站观测值之间的差值的绝对值分别为4.8～253.7 nT和0.9～420.0 nT。产生上述误差的原因主要是CHAOS-6和IGRF-12模型是描述全球地磁场的模型,而地磁场及其长期变化具有显著的区域特征和局部特征。     

蒙城地震台地磁偏角分量趋势性变化分析研究

蒙城地震台地磁偏角日均值自1999年7月起出现了趋势性变化,台站观测人员搜集整理了周边泰安、郑州、佘山、武汉地磁台的三分量数据,应用对比、相关分析等方法,研究了此变化,认为是地磁场的长期变,非地震趋势异常。     

2006～2010年云南东部地区磁偏角变化特征

利用2006～2010年云南东部地区9期地磁三分量观测资料,经过日变改正、长期变化改正消除地磁场的日变化及长期变化影响,计算各期地磁内源场的曲面样条模型并剥离出地磁异常场分布,求解相邻两期地磁异常场差异获取地磁异常场的变化,并对磁异常场中磁偏角分布的变化进行了描述.     

Palaeomagnetic Reversals in Mississippian Sediments of Missouri

--Normal to reversed polarity transitions have been found in Mississippian limestones of Missouri at longitude 269.7 and latitude 38.6 in an area of approximately 2,000 sq.km centered on the city of St. Louis. The transitions suggest a reversal of the geomagnetic field during the time these limestones were deposited. The mean normal NRM direction of declination 323.2, inclination 14.8, alpha-95 10.0 and corresponding reversed direction declination 144.8, inclination of 11.7 with alpha-95 of 5.2, are each tightly grouped, while in between these two groups the circles of confidence reveal a large scatter. Six normally magnetized groups of sites and corresponding VGP positions were found while seven groups are reversed. The mean normal VGP was at longitude 146.2 and latitude 44.6 and the reversed one at longitude 313.9 and latitude ⪶.8. The large scatter in between these two groups is interpreted as being due to rapid changes in the direction of the geomagnetic field when compared with the rate of deposition of the sediments. Alternating field (AF) and thermal demagnetization (TH) techniques were used to remove secondary components of the NRM.     

磁极性转换与地磁漂移

地磁极性转换期间地球磁场变化特征是地磁场最基本最重要的特征之一,是现代地磁学的研究热点之一。本文简要地论述了地磁场署期期间虚地磁极和地球磁场强度变化特征的研究现状。     

Geomagnetic Field Variations as Determined from Bulgarian Archaeomagnetic Data. Part II: The Last 8000 Years

The knowledge about past secular variations of the geomagnetic field is achieved on the basis of archaeomagnetic researches of which the Bulgarian studies form an extended data set. In Part I (Kovacheva and Toshkov, 1994), the methodology used in the Sofia palaeomagnetic laboratory was described and the secular variation curves for the last 2000 years were shown. In Part II (this paper), the basic characteristics of the prehistoric materials used in the archaeomagnetic studies are emphasised, particularly in the context of the rock magnetic studies used in connection with palaeointensity determinations. The results of magnetic anisotropy studies of the prehistoric ovens and other fired structures are summarised, including the anisotropy correction of the palaeointensity results for prehistoric materials, different from bricks and pottery. Curves of the direction and intensity of the geomagnetic field during the last 8000 years in Bulgaria are given. The available directional and intensity values have been used to calculate the variation curve of the virtual dipole moment (VDM) for the last 8000 years based on different time interval averages. The path of virtual geomagnetic pole (VGP) positions is discussed.     

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.     

The tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central Afar, Africa

Deformation throughout Afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region. In eastern Afar, this situation has evolved to localized extension associated with the westward propagation of the Gulf of Aden/Gulf of Tadjurah seafloor spreading system into the Asal–Ghoubbet Rift. During the gradual process of rift propagation and localization, crustal blocks in eastern Afar sustained clockwise rotations of 11°. To better understand the processes of rift propagation and localization and how they affect the rest of Afar, we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern Afar. Unlike eastern Afar, the mean paleomagnetic direction from central Afar indicates that vertical-axis rotations are statistically insignificant (3.6°±4.4°), though small clockwise rotations (<8°) are permitted. Thus, propagation and localization in central Afar have not had the same influence in causing crustal block rotations or, perhaps more likely, have not reached the same stage of evolution as seen in eastern Afar. In addition, several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions, excursions, or large secular variation events. Interestingly, the transitional or anomalous virtual geomagnetic poles (VGPs) tend to cluster in two nearly antipodal regions, one in the northern Pacific Ocean and the other in the southwest Indian Ocean. One lava flow has recorded both of the antipodal transitional components, with the two components residing in magnetic minerals with unblocking temperatures above and below 500°C, respectively. Reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow. The high-temperature component gives a VGP in the northern Pacific, whereas the lower-temperature component gives a nearly antipodal VGP south of Cape Town, South Africa. Hence, the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern-hemisphere transitional state to a southern-hemisphere one during this normal-to-reverse polarity transition.     

Blake亚时及其形态学研究

本文对西宁黄土剖面古土壤层S₁的721块定向样品进行了详细的岩石磁学和古地磁学研究,获得了Blake亚时转换过程中地球磁场变化特征较精确的记录。主要结果为:1.Blake亚时位于S₁的中下部;与地球磁场方向和强度变化相联系的持续时间分别为3900a和5000a。2.Blake亚时转换过程中地球磁场主体变化特征是由6次快速倒转构成,每次快速倒转所经历的时间为百年的量级,虚地磁极（VGP）极移曲线沿美洲大陆移动,这意味着在此期间地球磁场可能仍以偶极子场为主。3.Blake亚时是一次不成功的极性转换。4.黄土高原西部黄土-古土壤序列的主要磁性矿物可能仍是磁铁矿,次生剩磁主要是粘滞剩磁,当加热温度达到300℃之后即可获得单一组分的特征剩磁。     

Paleointensity in the Quaternary West Eifel volcanic field, Germany: preliminary results

Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.     

1985-1997年中国地磁场长期变化的正交模型

选取中国地区20个地磁台,日本地区3个地磁台的基本数据,使用自然正交分量方法(简称NOC方法)求得1985-199年中国地区地磁场三分量(X ,Y,Z)的自然正交分量的时间函数,即地磁场时间变化的正交模型. 对阶数从1-9的正交分量进行了试验计算,并计算出这23个地磁台分量的均方根偏差(RMS)值. 以1995年为例 ,取kNOC=1-3,将23个地磁台的正交模型值绘出了平面等值线图. 结果显示X , Y,Z长期变化速度的等值线非常平滑,分布均匀,且均具有明显的分区特征,这反映了主磁场的长期变化规律,而其中长期变化速率比较大的地区,反映地磁场的局部特征和有关的地球物理现象.     

IGRF-11描述的2005-2010年中国地区地磁长期变化及其误差分析

第11代国际地磁参考场(IGRF-11)是国际地磁学与高空物理学联合会(IAGA)于2009年12月提出的最新与比较准确的IGRF.根据IGRF-11模型,计算了2005-2010年中国地区地磁长期变化(SVC).IGRF-11所描述的2005-2010年中国地区地磁长期变化与实际观测的地磁长期变化(SVO)是基本一致的,但亦有明显差异.分析比较了在中国地区34个台站上的SVO与SVC之间的差异,并得到了它们之间的差值及其均方误差σ,对于地磁偏角和倾角,σ分别为0.35′/a与0.53′/a;对于地磁总强度、水平分量、北向分量、东向分量与垂直分量,σ分别为5.12nT/a,8.91nT/a,8.89nT/a,3.27nT/a与3.59nT/a.引起IGRF-11所描述的2005-2010年中国地区地磁长期变化的误差原因是:中国地区的区域性与局部性的磁异常、IGRF忽略了外源场与IGRF模型的截断阶数、全球台站与测点的分布不均匀、地磁观测误差等因素.由于中国地磁模型(CGM)优于IGRF模型,并能比较准确地描述中国地区地磁场及其长期变化,故在实际应用中应选用CGM.     

Analyses of primary remanence vector data from a large collection of lava flows: towards improved methodology in paleo-geomagnetism

Research into the properties of past geomagnetic fields (paleo-geomagnetism) has been carried out worldwide for over half a century. This research utilizes rocks which are to a varying degree inadequate for that purpose, and evaluation of the results is therefore not always on firm ground. One resource which can be utilized to constrain several aspects of the interpretation of other paleo-geomagnetic data, consists of stable primary remanence vectors in large homogeneous collections of dated fresh lava flows. We outline how the available remanence data from several thousand lavas in Iceland may be analysed in a semi-quantitative way to test some of current results, methodology and concepts of paleo-geomagnetism. Among topics where such analysis provides new insights, are the geomagnetic polarity time scale, the frequency distribution of virtual geomagnetic poles in latitude and longitude, and the relative intensity of the magnetic field as a function of virtual pole latitude. Comparison of the scatter in remanence intensities and in absolute paleointensity determinations on Icelandic lavas indicates, along with other evidence, that quality criteria for the latter are in need of revising. It is also confirmed here that long-term changes have occurred in the amplitude of the geomagnetic secular variation; they should be taken into account in studies on other properties of the paleo-geomagnetic field.