Lunar and luni-solar variations of the geomagnetic field in the Indian region

Summary Lunar and luni-solar geomagnetic components have been computed upto four harmonics for low latitude station Alibag, outside equatorial electrojet belt, and the equatorial electrojet stations Annamalainagar, Kodaikanal and Trivandrum in the south Indian region. The computations are confined to data of very high solar activity period 1958–61. Amplitudes of lunar semidiurnal component (L ₂), in the horizontal intensity (H), undergo an equatorial enhancement. Phase difference of 2 hrs is noticed inL ₂ (H) between nonelectrojet and electrojet stations. In the vertical intensity (Z), L ₂ is maximum ine andj-seasons at Trivandrum, close to the magnetic equator. Ind-season, however, maximumL ₂ (Z) occurs at Annamalainagar (dip 5°.4N). The phase difference between the electrojet and nonelectrojet stations observed inL ₂ (H) is not noticed inL ₂ (Z). The differential vertical upward drift motion of charged particles may explain the observed phase difference inL ₂ (H). Seasonal variations in amplitudes and times of maxima are noticed at all the stations inL ₂ (H) andL ₂ (Z). Similar variation is also noticed at Alibag inL ₂ of declination (D).     

Lunar and solar daily variations of the geomagnetic field at Toolangi

Summary Mean hourly values of magnetic declination, horizontal intensity and vertical intensity observed at Toolangi during two ten year periods (1924–1933 and 1949–1958) have been analysed to determine their solar and luni-solar diurnal components. The results, showing the variations of the first four harmonic components with season, degree of magnetic activity and annual sunspot number, are tabulated and discussed. It is shown that there are marked differences in the dependence ofS andL on the various parameters and a tentative explanation of this phenomenon is given.     

Variations in the near-surface atmospheric electric field at high latitudes and ionospheric potential during geomagnetic perturbations

We have analyzed variations in the near-surface atmospheric electric field (Ez) normalized to their daily averages that were simultaneously observed in different high-latitude regions at moderate geomagnetic activity (Kp ∼ 3). The Ez data were measured under fair weather conditions at the Vostok Antarctic research station (Φ′ = −83.5°) in the southern polar cap and at the Hornsund Arctic observatory (Φ′ = 74.0°) on Svalbard close to the polar boundary of the auroral oval in the Northern Hemisphere. It is established that variations in the atmospheric electric field in the polar cap region at the Vostok station are controlled (the correlation coefficient R ∼ 0.7–0.9) by variations in the overhead ionospheric potential. The situation at the Hornsund observatory is more complicated. During intervals when Hornsund occurred below the westward electrojet, the correlation was typically positive with R ∼ 0.60–0.85; however, while this observatory was in the region of the eastern electrojet, the correlation could be negative with R ∼ 0.7–0.8. Normally, during such periods, the westward electrojet was detected polarwards of Hornsund while, according to the SuperDARN radar data, the observatory was located below the negative vortex of the polar ionospheric convection.     

东亚地磁场的Taylor多项式模型

根据我国国家自然科学基金委员会和俄罗斯科学院的国际合作研究计划，在国际上首次同时使用中国、前苏联和蒙古的地磁复测点资料，以及东亚地区的地磁台站资料，计算出１９８０．０年东亚地磁场的Ｔａｙｌｏｒ多项式模型，并绘制出相应的东亚地磁图，给出了确定地磁场模型最大阶数的新判据。     

南极地磁场的变化特性分析

我国南极中山站、长城站地处极区或靠近极光区,地磁场的变化形态与我国所处的中低纬度地区有所不同,即使是中山站与长城站之间也有较大差异。中山站的暴时ＤＰ型变化就比长城站的明显。重构向量空间吸引子的维数也明显高于北京地区。对长城、中山地磁资料的分析可以扩大我们对地磁场全貌的认识。     

地磁场长期变化特征及机理分析

将地磁场的总变化分为三部分：偶极场自身变化,非偶极场自身变化及非偶极场磁斑区通过对核幔边界（CMB）层环形电流的调制来影响偶极场的变化. 本文利用国际地磁参考场模型IGRF）1900～2000计算分析了地球不同深度地磁场分布及长期变化特征,且讨论了变化的可能机制. 可以推论,地磁场西漂和倒转不仅是非偶极场引起,同时与偶极场有密切关系.     

地磁扰动的时空变化规律和产生原因：统计和模拟研究

地磁感应电流（Geomagnetically Induced Currents,GICs）是发生在地球表面的一种空间天气现象,对石油管道、电缆等长距离地面基础设施会产生不利影响.研究GICs的产生原因及其与太阳风驱动条件的关系对于预报灾害性空间天气具有重要意义.由于GICs的产生与地磁场扰动紧密相关,本文利用北半球100多个地磁台站的长期监测数据,统计分析了地磁扰动及其时间变化率（磁扰率）与各种太阳风参数/地磁指数之间的相关性,并以2013年3月17日的磁暴事件为例,采用全球空间天气模型框架（space weather modeling framework,SWMF）模拟了磁暴期间地磁场扰动在北半球的分布以及日地空间中各电流体系对地磁扰动的作用.研究结果表明：（1）中低纬度的地磁扰动北向分量B_x与表征环电流强度的SYM-H指数呈现较高的正相关性（相关系数CC=0.75）,高于它与亚暴AE指数以及其他太阳风参数的相关性,说明磁暴期间环电流是导致中低纬度北向磁场减弱的主因,而在平静期间东向的磁层顶电流是中低纬度北向磁场增强的原因;（2）地磁扰动率与太阳风动压、行星际磁场IMF B_z、亚暴AE指数或者磁暴SYM-H指数均没有强相关性;（3）高纬地区的磁扰率通常大于低纬地区,而较强的磁扰率倾向于出现在中等磁暴或者中等-强亚暴期间的中高纬地区;（4）SWMF模型能较好地反演地磁平静时的北向地磁扰动和磁暴时的东向地磁扰动;（5）磁暴期间,磁层电流对中低纬度北向地磁扰动的贡献最大,而电离层霍尔电流对高纬地区的北向地磁扰动有着很强的支配地位;另外,高纬地区的东向地磁扰动主要由霍尔电流控制,而中低纬地区则受制于场向电流.

     

Spatial power spectra of the crustal geomagnetic field and core geomagnetic field

Lowes (1966, 1974) has introduced the function R_n defined by $\text{R}{}^{\mathrm{n}}\text{=(n + 1)}\text{∑}\text{m=0}\text{∞}\text{[(g}{}^{\mathrm{m}}{}_{\mathrm{n}}\text{)}{}^2\text{+ (h}{}^{\mathrm{m}}{}_{\mathrm{n}}\text{)}{}^2\text{]}\text{where}\text{g}{}_{\mathrm{n}}{}^{\mathrm{m}}\text{and}\text{h}{}_{\mathrm{n}}{}^{\mathrm{m}}$ are the coefficients of a spherical harmonic expansion of the scalar potential of the geomagnetic field at the Earth's surface. The mean squared value of the magnetic field B = ??V on a sphere of radius r > α is given by $\text{〈}\text{B}\text{·〉 =}\text{∑}\text{n=1}\text{∞}\text{R}{}^{\mathrm{n}}\text{(}\text{a/}\text{r}\text{)}{}^{\mathrm{2n=4}}\text{where}\text{a}$ is the Earth's radius. We refer to R_n as the spherical harmonic spatial power spectrum of the geomagnetic field.In this paper it is shown that Rⁿ = R^M_n = R^C_n where the components R_n^M due to the main (or core) field and R_n^C due to the crustal field are given approximately by $\text{R}{}^{\mathrm{M}}{}_{\mathrm{n}}\text{= [}\text{(n =1)/}\text{(n + 2)}\text{](1.142 × 10}{}^9\text{)(0.288}{}^{\mathrm{n}}\text{Λ}{}^2\text{R}{}^{\mathrm{C}}{}_{\mathrm{n}}\text{= [}\text{(n =1){[1 — exp(-n/290)]/}\text{(n/290)} 0.52 Λ}{}^2\text{where}\text{Iγ = 1 nT}$. The two components are approximately equal for n = 15.Lowes has given equations for the core and crustal field spectra. His equation for the crustal field spectrum is significantly different from the one given here. The equation given in this paper is in better agreement with data obtained on the POGO spacecraft and with data for the crustal field given by Alldredge et al. (1963).The equations for the main and crustal geomagnetic field spectra are consistent with data for the core field given by Peddie and Fabiano (1976) and data for the crustal field given by Alldredge et al. The equations are based on a statistical model that makes use of the principle of equipartition of energy and predicts the shape of both the crustal and core spectra. The model also predicts the core radius accurately. The numerical values given by the equations are not strongly dependent on the model.Equations relating average great circle power spectra of the geomagnetic field components to R_n are derived. The three field components are in the radial direction, along the great circle track, and perpendicular to the first two. These equations can, in principle, be inverted to compute the R_n for celestial bodies from average great circle power spectra of the magnetic field components.     

地磁正常场的选取与地磁异常场的计算

根据2003年中国地磁观测数据（包括135年地磁测点和35个地磁台）以及我国邻近地区38个IGRF计算点的地磁数据,计算中国地磁异常场的分布。选取两种地磁场模型作为地磁正常场,一是国际参考地磁场的球谐模型,二是中国地磁场泰勒多项式模型。根据各个测点的地磁异常值（观测值减去模型计算值）,用球冠谐分析方法计算地磁异常场的球冠谐模型,并绘制2003年中国地磁异常（△D,△I,△F,△X,△Y,△Z）。分析和讨论了中国地磁异常场。     

北京地磁场变化特征分析

根据北京国家地球观象台北京地震台1987—2013年地磁观测资料,分析该台地磁场长期变化和短期变化,认为北京地磁场长期变化具有缓慢上升、缓慢下降和逐渐恢复上升的形态,短期变化受太阳黑子活动强度影响,对了解北京地区地磁场变化特征具有一定意义。     

Estimation of the normal geomagnetic field and geological interpretation of the normal field anomalies over middle-northern Croatia

Based on the ground survey of total-field magnetic data at 53 sites, which recorded a time-series over the interval 2003–2005, the core field + long wavelength lithospheric field over the middle-northern Croatia region was calculated. The area of the survey was 18900 km², an average distance between the neighbouring sites being 12 km. The results were reduced to 2004.5 epoch. This “normal” total-field was estimated using the firstorder Taylor polynomial as a function of geographic coordinates, and the polynomial coefficients were calculated with three methods of adjustment: simple and weighted least squares fits and adjustment according to the most frequent value. The stability of the normal field was tested using the Monte Carlo-type test, by decreasing the input data set in each adjustment (up to 90%). All obtained field residuals (measured-“normal”) were mutually compared, as well as fit coefficients of the Taylor polynomials. The residual values indicate the presence of shorter-wavelength anomalies, specifically three major residual anomalies were found (−106 nT, 74 nT and 57 nT). The geostatistical analysis of the ground survey data and the normal field residuals (respectively), using the median absolute deviation method, was further conducted in order to evaluate the calculated anomalies. The geological situation around the anomalies derived by the median absolute deviation method, and around the normal field residual anomalies, is given. The correlation was found between higher value anomalies of the normal field residuals, and shallow volcanic rocks and oil field, respectively.     

乌鲁木齐地磁台地磁场变化特征

根据乌鲁木齐地磁台有史以来的地磁观测资料,对该地区地磁场的长期变,短期变及其磁暴活动规律进行了总结分析,这对进一步认识乌鲁木齐地磁台地磁场各要素的变化规律及征,为地震震预报提供有实用价值的第一手资料都是十分有意义的。     

低纬地区地磁场谐波特征

利用我国低纬地区琼中、广州和泉州３个地磁台１９８２－１９８９年的地磁静日实测值，分析了Ｈ、Ｚ和Ｙ三个地磁分量的谐波特征。结果表明，①Ｈ、Ｚ两分量的谐波振幅一般在春秋季最大，冬季最小，Ｙ分量的谐波振幅在夏季最大，冬季最小，它们的变化形态基本上与太阳黑子数的变化形态同步；②三分量的相位都受到太阳活动和季节的影响。     

新疆地磁场变化特征的初步研究

根据新疆两个地磁台1985～2007年的地磁观测资料,对该地区地磁场的长期变,短期变活动规律进行了总结分析。这对进一步认识新疆地磁场各要素的变化规律及特征,对了解新疆地区地磁场各分量的变化规律和特征有一定的意义。     

地磁场水平梯度及高空地磁场的计算与分析

本文以2000.0年中国地区的实测数据为例,首先利用5阶Taylor多项式方法建立了各分量的地磁模型,接着对模型中各分量的纬度和经度进行微分,计算得到各分量的水平梯度值,并绘制了相应分量沿南北方向和东西方向的水平梯度分布图,最后通过Zmuda多项式方法,基于地面模型值以及水平梯度值计算了高空(100 km)的各分量磁场值,并分析了水平梯度分布规律以及各分量随垂直高度的变化.结果表明:地磁场北向分量X、垂直分量Z、总强度F和磁倾角Ⅰ分量的水平梯度主要随纬度变化,其中X、Z和F分量随纬度减少而梯度降低,东向分量y和磁偏角D分量的水平梯度不仅随纬度变化,而且随经度变化,F分量的南北向梯度值在我国中心地区最大.在垂直方向,X、Z和F分量的强度分别随高度的上升而近似线性减小,在100 km高度处,强度变化平均值分别为-4.629 nT/km、-15.368 nT/ km和-16.226 nT/km,y分量强度随高度上升而呈近似线性增加,其平均变化值为0.166 nT/km,而D和Ⅰ分量基本不发生变化.     

地球主磁场空间功率谱的变化特征

根据第8代IGRF模型，计算分析了1900－2000年地球磁场和分量X，Y，Z的空间功率谱的变化特征。结果表明1900－2000年主磁场的空间功率谱呈减小趋势，由功率谱随谐波阶次n的变化关系得出的非偶极子场的等效磁源位于核幔边界附近，而且其位置随时间变化，用n=1,2两阶谐波的谱得到的等效磁源位于地球内核边缘，1900年以来离开地心一直向外增大。     

Interplanetary structures and solar wind behaviour during major geomagnetic perturbations

We consider five different solar wind structures to study their relative geo-effectiveness in producing major geomagnetic perturbations. Geomagnetic indices and solar wind parameters during major storms have been utilized to understand the physical mechanism(s) during the passage of structures with distinct structural and dynamical properties. Attempt has been made to find distinct features of the structures responsible for large-intensity and/or long-duration geomagnetic storms. We search for precursors of major storms that may be useful for space-weather predictions. Average recovery characteristics of storms and the influence of solar wind speed on the recovery have been discussed.     

Long-term variations in geomagnetic and solar activities and secular variations of the geomagnetic field components

Summary After the removal of the eleven-year periodicity, long-term patterns of the aa indices of geomagnetic activity and of Wolf's sunspot numbers are defined. The positions of maxima and minima exhibit the same regularities as the secular variations of the geomagnetic filed components. This result is associated with the motion of the Sun round the barycentre of the solar system.Presented at symposium Planet 88, Tihany, September 1988.