A note on the secular variation of the geomagnetic field

Geomagnetic variations with (apparent) periods from 13 to 30 y have been reported by Alldredge (1977) who has argued that the origin of these signals must be found in the core of the Earth rather than outside the Earth. It is shown in this paper that a portion of these geomagnetic variations (perhaps most of the variations) might well be due to geomagnetic signals of much longer period, originating in the core, that appear to have periods of 13–30 y because of an artifact of the data processing. Much of the remainder of these variations could well be of external origin. A method for processing these data is suggested that is superior to Alldredge's method in minimizing the generation of spurious waves by long-period secular variation.     

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.     

On the geomagnetic lunar variation at Istanbul

Summary The article describes lunar daily magnetic variation inH, D andZ components of the earth's magnetic field at Istanbul. Maximum occurs at 10.03 l.hr. (lunar hour) in theH component, 3.38 l.hr. in theD and 2.15 l.hr. in theZ component, during the period 1949 to 1968. Also, the seasonal variation of the lunar magnetic variation has been determined and it is seen that the variation of the phase inD andZ are opposite from the phase of the lunar variation inH, and the amplitudes of the lunar variation inH, D andZ are greatest during the northern solstice.     

A possible crust-mantle configuration near Trivandrum deduced from Indian geomagnetic data

Anomalies in H and Z, primarily attributable to subsurface features, were obtained from the annual mean values at five Indian magnetic stations for the period 1965–1973, after correcting for the secular and normal variations. When the average anomaly at each station over the nine years chosen, after projecting on to a reference longitude (78°E), was plotted against its geographic latitude, the resulting ΔZ and ΔH latitudinal profiles were found to reach their maximum values near Trivandrum (TRV). This rise in anomaly near TRV has been interpreted in terms of a probable plutonic dyke-like structure, superposed on the general upwarp of the mantle known to be associated with a land-to-ocean transition. A Bouguer gravity profile along the same reference longitude, supporting this inference, is also presented. A sample calculation has been done with assumed parameters for the model and the theoretical and observed anomalies are compared.     

Determination of the running quiet daily geomagnetic variation

A new automatic running method for derivation of the quiet daily geomagnetic variation—“quiet day curve” (QDC) is described. The method consists in the automatic distinction of the quietest periods using the geomagnetic variations parameterization, calculation of the proper quiet daily variation for certain days, reconstruction of QDC for each day of the elapsed period and extrapolation of QDC for the subsequent period. The method ensures statistically reliable QDCs during the epoch of the solar activity maximum if the time interval used for derivation of QDC is not less than 30 days. The method of the running QDC calculation implies the uninterrupted calculation of the QDC resulting from the continuous 1-day forward shift of the 30-day interval. The method makes it possible to derive automatically and on-line the quiet daily variation in the polar caps, where northward interplanetary magnetic field can generate large magnetic disturbances during periods of planetary magnetic quiescence. This is the main advantage of the running QDC method over other known methods. It is shown that along with the seasonal (from month to month) and the solar cycle (from year to year) changes, the QDC amplitude is modified on a time scale less then a month following solar activity flashes.     

Spherical harmonic analysis of the geomagnetic secular variation — A review of methods

Methods of producing spherical harmonic models of the secular variation of the geomagnetic field are reviewed and classified, more emphasis being given to modern methods. The relative merits of the different techniques are discussed and their relevance to possible procedures for updating the International Geomagnetic Reference Field (IGRF) are considered.     

V_r:A new index to represent the variation rate of geomagnetic activity

By calculating the hourly standard deviation of the first-order differences of the horizontal geomagnetic component minute data,a new index Vr to represent the variation rate of the geomagnetic field was introduced.Vr-indices show similar trends in the temporal change at different observatories and have simultaneous peak values at the observatories covering a large span geographically,which reveals that the source of geomagnetic disturbances represented by Vr is in the magnetosphere.Based on the comparison among Vr,Kp and ap,it is found that generally Vr changes linearly with Kp and ap,which means that the rapid changes of magnetic field usually exist together with magnetic disturbances.But there are exceptions.As Vr can be easily produced by individual observatory in quasi real time and is more sensitive to the variation rate of geomagnetic field rather than the field itself,it can be expected to serve for monitoring or predicting the geomagnetic-induced event in a quick and intuitive way.     

地磁1年周期变化的响应函数

用通日月均值减去夜均值计算的月均值资料和Ｓｏｍｐｉ谱分析法分析了１４个台站的北向分量１年周期变化的振幅向相位，时段为１９８４－１９８８年。分析结果显示用Ｐ＾０ｎ（ｄｎｄ＝１，…，６）复合模式的估算的Ｃ值较合理。各台的Ｃ值比较一致。     

Upper-mantle conductivity determined by the geomagnetic lunar daily variation

The geomagnetic lunar daily variation, and the solar daily variation, have been recently analysed by Malin. The response measurements that he obtained are used here to investigate the radial distribution of electrical conductivity in the upper mantle. It is found that these observed response measurements require low conductivity in the mantle, at least as far down as 650 km from the surface. This result is compatible with the conductivity models determined by the quiet-day daily variation, and so justifies the use of these variation fields for geomagnetic deep sounding.     

地磁Z分量日变化特征

利用昌黎、红山等台站的地磁Z分量日变资料,分析了磁静日幅度、磁扰日幅度和磁静日低点时间的年变化与长期变化,结果年变化和长期变化都非常明显。磁静日幅度的长期变化量可达14～15nT;磁静日低点时间的年变化,提前区与滞后区低点时间最大可相差42min。而低点时间的长期变幅度化幅度一般50min左右,且长期变化量由西向东有逐渐减小趋势,最大与最小两台时差可达21min。长期变幅最小的2个台为文安和昌黎,可能与海水影响有关。     

Physical constraints for the analysis of the geomagnetic secular variation

Slow changes in the magnetic field are believed to originate in the core of the Earth. Interpretation of these changes requires knowledge both of the vertical component of the field and of its rate of change at the core-mantle boundary (CMB). While various spherical harmonic models show some agreement for the field at the CMB, those for secular variation (SV) do not. SV models depend heavily on annual means at relatively few and poorly distributed magnetic observatories. In this paper, the SV at the CMB is modelled by fitting 15-year differences in the annual means of the X, Y and Z components (from 1959 to 1974). The model is made unique by imposing the constraint that $\text{?}{}_{\mathrm{CMB}}\text{B}\text{?}{}_{\mathrm{r}}{}^2\text{d}\text{S}$ be a minimum, using the method of Shure et al. (1982). If SV is attributed to motions of core fluid, then this model will yield, in some sense, the slowest core motions. The null space is determined by the distribution of observations, and therefore, to be consistent, only those observatories have been retained which recorded almost continuously throughout the interval 1959–1974.The method allows misfit between the model and the observations. The best value for the misfit can be derived from estimates of errors in the data, or alternatively, because larger misfit leads to smoother models (i.e., smaller $\text{?}\text{B}\text{?}{}_{\mathrm{r}}{}^2\text{d}\text{S}$), the best value can be estimated subjectively from the final appearance of the model. Both procedures have their counterparts in the conventional spherical harmonic expansion approach, when smoothing is achieved by lowering the truncation level. The new proposal made in this paper is to use objective criteria for determining the misfit, based on the assumption that diffusion is negligible, in which event all integrals $\text{∫}\text{B}\text{?}{}_{\mathrm{r}}{}^2\text{d}\text{S}$ will vanish when S_i is a region on the CMB bounded by a contour of zero vertical component of field. For the 1965 definitive model which is adopted here, and for most other contemporary models, there are six such areas, giving five independent integrals (the integrals over the six regions must sum to zero if ? · B = 0). Tabulating these integrals for various choices of the misfit gives minimum values near 2 nT y^?1. It is impossible to achieve this good a fit to the data using a reasonable model derived by truncating the spherical harmonic expansion. The value 2 nT y^?1 corresponds to errors of ～ 20 nT in individual annual means, which is rather larger than expected from the scatter in the data.     

Long-term variation of the semi-annual wave maxima in geomagnetic activity

Summary The time variations of the amplitudes and phases of the semi-annual variation in geomagnetic activity, characterized by the linear planetary index aa, have been analysed for the period 1868–1985. The results provide qualitative confirmation of Murayama's conclusions [13] about the systematic f phase in dependence on the changes in the level of solar activity and give support to Russel-McPherron's mechanism [16] concerning the effect of the predominant polarity of the interplanetary magnetic field. A distinctly expressed variation of the phase differences in the course of the sunspot cycle and of the 22-year cycle, and specific variations related to the sequence of four consecutive cycles have been established, as well as a well-defined 90-year period, all of them as a reflection of analogous variations in solar activity. The variations of the phase differences observed around the equinoxes can be explained by the combined effect of the mechanisms of the axial and equinoctial hypothesis. It is assumed that a displacement of the maxima of the semi-annual variation to dates after the equinoxes will be observed in the ascending parts and a reverse displacement towards the equinoxes and earlier dates in the desccending parts of the following sunspot cycles 22 nad 23.On leave from the Geophysical Institute of the Bulgarian Academy of Sciences, Akad. G. Bonchev Str. bl. 3, Sofia 1113, Bulgaria.     

Secular variation of the direction of the ancient geomagnetic field for Loyang region,China

Curves of the variation of the inclination and declination of the geomagnetic field for the Loyang region during the last 2400 years are presented. An I-D diagram of secular variation, like that for London, has been drawn for Loyang.     

Analysis of diurnal dynamics of the geomagnetic field variation spectrum

A statistical analysis of the variations of three components of the Earth’s magnetic field obtained at Almaty during 2008 is performed. It is found that in the daytime and nighttime periods, a distinct difference in the spectra of variations with periods corresponding to acoustic-gravity waves is observed. An explanation of this difference is proposed. It is based on the mechanism of change in the acoustic-gravity wave spectrum caused by the interaction of atmospheric gas with solar radiation.     

过去2000年以来地磁偶极子的长期变化

利用地磁场模型资料,计算和分析了公元0～2000年的地磁场偶极子磁矩和地磁北极位置的长期变化。结果表明:过去2000年中,地磁场的偶极子磁矩并非一直减小,有近400年的时间是持续增长的。地磁北极位置的移动并不均匀,在纬度方向的变化范围只有10°左右,经度方向变化有255°。     

Relation of PC index to the geomagnetic storm Dst variation

Relationships between the polar cap magnetic activity index PC and the magnetic storm Dst index have been studied for the magnetic storms with duration more 12 h and peak value Dst<?30 nT and, observed in 1998–2002 and 2004–2005. Along with PC index the geoeffective interplanetary electric field Em was also examined. It has been found that all examined storms, lying in range from ?30 to ?373 nT, started when the PC index and, correspondingly, the Em field firmly exceeded the threshold >2 mV/m. In particular, the “anomalous” magnetic storm on January 21–22, 2005 occurring under conditions of northward IMF B_Z (Du et al., 2008) is usual phenomena fitted well with the threshold restriction owing to the large IMF By component input. The maximal storm depression (the peak value of Dst) is linearly related to the quantities Em and PC, averaged for the time interval from the storm beginning to the storm maximum. The correlation between Dst and PC is more steady and larger than correlation between Dst and Em, the latter being dependent on Em value (effect of “Dst saturation”). The moment of the firm descent of the Em and PC quantities below the threshold level ～2 mV/m is indicative of the depression damping and transition to the recovery phase. The results are consistent with the similar peculiarities revealed for substorms development (Troshichev and Janzhura, 2009) and support the conclusion that the PC index is a reliable proxy characterizing the solar wind energy having been entered into the magnetosphere.     

管形构件结构地磁相对记录室建设

以宝坻新台为例,介绍管形构件结构地磁相对记录室的方案设计和施工要点,分析采用玻璃钢管形构件建设的地磁相对记录室的实际效果,得出玻璃钢管形构件结构方案是值得推广的一种地磁相对记录室结构新方案的结论。     

主磁场长期变化十年至百年尺度的周期

本文运用小波变换技术,通过分析历史地磁场模型gufm1（时间跨度从1590~1990年）,考察主磁场长期变化场（B场）的周期性.结果表明,B场总磁极强度存在三个主要的周期分量：稳定的30年周期,在偶极子场的赤道分量g11和非偶极子场中较常见;频散的准50年周期,主要是由轴向偶极子分量g01贡献的,此外,四极子场也有贡献;世纪尺度的110年周期,其强度会发生变化,主要来源于偶极子场的赤道分量以及八极子场.