The qualitative character of the global electrical conductivity of the earth

A spectral stacking and smoothing procedure has been applied to unbroken hourly values of H and Z, for 1964 and 1965, from 17 observatories, in order to estimate the magnitude and phase of the P₁⁰ response of the Earth to long-period geomagnetic fluctuations. Exploratory techniques have been used to gauge when sufficient smoothing has been applied, and to identify the qualitative character of the global electrical conductivity of the Earth.     

A new magnetic index based on the external part of vertical geomagnetic variations

Variations of geomagnetic components X, Y, and Z recorded in 19 Intermagnet European observatories in 2004 were analysed. The original data from all observatories were preliminarily processed. In the first step, periods longer than three hours were filtered out. In the second step, variations of vertical geomagnetic component Z were separated into external and internal parts. We introduced a non-dimensional index η defined as the square root of a ratio of the energy of the external part of the vertical component to that of the horizontal components. Maps of the surface distribution of a new magnetic index η for the area of Europe at selected time periods were created, and their time changes are presented. The time changes of η for selected observatories are also shown. Moreover, we discuss a very interesting phenomenon we discovered, that has never been described in geophysical literature. Namely, in the recordings of all the observatories we noticed the presence of very regular variations, observed almost exclusively in the vertical component Z, which is quite unusual. These regular variations occur in the form of sinusoidal “wave packets”. The amplitudes of these variations do not depend on the geomagnetic latitude and appear in the records of all the observatories we analyzed. They occur in quiet days, which suggests that their source is in the ionosphere.     

Investigation of the electrical conductivity beneath China using geomagnetic spatial gradient method

Ｉｎｖｅｓｔｉｇａｔｉｏｎｏｆｔｈｅｅｌｅｃｔｒｉｃａｌｃｏｎｄｕｃｔｉｖｉｔｙｂｅ┐ｎｅａｔｈＣｈｉｎａｕｓｉｎｇｇｅｏｍａｇｎｅｔｉｃｓｐａｔｉａｌｇｒａｄｉ┐ｅｎｔｍｅｔｈｏｄＧＵＯ－ＨＵＡＦＡＮ（范国华）ＴＯＮＧ－ＱＩＹＡＯ（姚同起）ＺＵＯ...     

NUMERICAL VLF MODELING*

The VLF response of laterally inhomogeneous and anisotropic models is calculated numerically using the finite element method. Some results are presented for a slab model in terms both of the polarization parameters, i.e., the tilt angle and ellipticity of the magnetic polarization ellipse, and the amplitude ratio |H_z/H_x|. On the basis of both the ellipticity and the tilt angle, it is possible to discriminate between a poor conductor and a good one. The direction of the dip can be determined from the anomaly profiles of all diagnostic parameters. The effect of the conductive overburden is most noticeable on the ellipticity profile: one observes attenuation for a poor conductor and “negative attenuation” for a good conductor. The anomaly profiles for anisotropic cases are consistent with the ones of the isotropic cases.     

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.     

Geomagnetic results from Sabhawala,Yangi-Bazar and Alma-Ata in relation to the asthenosphere beneath the Pamir-Himalaya

An attempt is made to study the properties of the Mohorovi?i? discontinuity and the asthenosphere beneath the Pamir-Himalaya from the geomagnetic results obtained at Sabhawala (Dehra Dun), Yangi-Bazar (Tashkent) and Alma-Ata observatories, situated on its southern and northern flanks, in a narrow longitude zone.Medium-sized induction arrows (Wiese vectors) for night-time “bay” events at all the three stations are found to point northward. The northward orientation at Sabhawala indicates a subsurface concentration of induced currents in a step structure at the Moho to the south of Sabhawala and higher electrical conductivity in the Aravalli-Cambay region, where high heat flow and gravity values have also been reported, supporting an elevated asthenosphere, as against the one beneath the western Himalaya. The arrows at Yangi-Bazar and Alma-Ata also show an east-west current channelling in another step structure at the Moho to the south of these stations and north of the Pamir and the Tien Shan mountains.The Sq variations of the H, D and Z components at the three stations, lying close to the northern Sq ionospherics current focus, reveal the characteristic features, without reflecting any unusual asthenospheric conditions in the region.It is concluded that the orogenesis of the Pamir-Himalaya mountain system in terms of plate tectonics or block uplift is different from that of the Cordilleran and Andean mountains in North and South America respectively, where subsurface conductive structures have been delineated from geomagnetic observations by earlier workers.     

中国大陆地磁帕金森矢量特征及其与主要构造关系龚绍京

利用地磁差矢量法和复转换函数法计算了中国大陆36个地磁台的地磁优势面、 帕金森矢量及误差． 结果显示: 华北南部、 华中、 华南地区(包括贵阳和邵阳)及沿海台站的帕金森矢量长度随周期减小而增大, 表明浅层的电性结构横向不均匀性较大; 西部的拉萨、 通海、 西昌、 成都等台站的帕金森矢量长度, 随周期增长而增大, 表明深层的电性结构横向不均匀性较大; 诸如华北平原的稳定地块, 其内部的矢量长度很小, 表明该地区为大体水平分层的电性结构． 我国大陆存在多个较明显的帕金森矢量敛散区, 其中青藏高原和渤海湾周边台站的帕金森矢量内聚, 鄂尔多斯地块的矢量向四周发散; 沿海台站具有较明显的海岸效应． 此外, 由差矢量分布推断所得的优势面倾向和倾角与利用帕金森矢量系数公式计算所得的结果有较好的一致性, 与复转换函数方法给出的结果也比较一致．      

Absolute measurement ofD by means of a proton magnetometer

In a large coil with vertical axis the current is adjusted so thatZ=0. A smaller coil of a new type with four sets of cylindrical turns is placed with its axis horizontal. ThisD-coil is provided with a telescope pointing at a mark nearly in the magnetic meridian. A proton magnetometer sensor is placed in the common centre of the coils. Two series of readings are taken with theD-coil in the erect and inverted position respectively. Variations ofD, H, F, andD-coil current are recorded. A simple formula gives the mean value ofD.     

INDUCTIVE SOUNDING OF A STRATIFIED EARTH WITH TRANSITION LAYER RESTING ON DIPPING ANISOTROPIC BEDS *

The generalised three layer boundary value problem with a transition layer sand-witched between an isotropic overburden and dipping anisotropic substratum is discussed assuming that plane electro-magnetic waves are incident normally over the air-earth inter-face. The tangential electric (E_y) and magnetic (H_x) fields and the expression for surface-impedance (E_y/H_x) have been evaluated at the earth's surface. Through numerical analysis it is shown that changes in the values of the parameters m (coefficient of anisotropy), h (thickness of the transition layer), α (angle of inclination of the dipping beds), and b (conductivity ratio between substratum and upper layer) modify the amplitude and phase-variation curves (with skindepth) significantly.     

The planetary scale distribution of telluric currents and the effect of the equatorial electrojet: An investigation by canonical GDS

The planetary scale distribution of electrical currents in the Earth is still largely unknown. The role of the oceans for long period (hours to days) inducing electromagnetic fieldsB _e of external origin has been investigated by several authors, while the role of telluric current channelling, from the planetary viewpoint, is still far from a satisfactory understanding. Canonical geomagnetic depth sounding (GDS) analysis can yield locally a direction parallel to the strike of a telluric current density flowing in a region around the recording site and which also has the property of being the most relevant source for the internal origin fieldB _i observed at the given site at the given frequency. The use of such local information from 64 geomagnetic observatories is discussed here in a study to infer evidences of (a) the role of the polarization properties ofB _e and (b) the role of the telluric current channelling within conductivity anomalies relevant to the planetary scale circuitry. The results show clear evidence of the influence of the equatorial electrojet on the polarization ofB _e in a latitudinal band between ±(15°–20°) latitude. There is also evidence that theB _e associated with the equatorial electrojet produces telluric currents which flow at a much shallower depth than the skin depth to be expected in the case of a plane Earth. This implies that the Parkinson planes in these regions reflect the conductivity structure underground more than the polarization ofB _e due to the equatorial electrojet. Further, it clearly appears that some regular planetary scale pattern of telluric currents plays a more significant role than current channelling within some conductivity anomalies of fixed strike close to some geomagnetic observatories. Finally, the number of observatories used in this study appears to be insufficient to deduce any information concerning a seasonal evolution of the telluric current pattern on a planetary scale.     

IMPEDANCE OF A PLANE ELECTROMAGNETIC WAVE AT THE SURFACE OF A LAYERED CONDUCTING EARTH WITH DIPPING ANISOTROPY*

The problem of a plane wave incident on a non-isotropic dipping layer lying over an isotropic conducting substratum has been studied and some numerical results are presented to show the effects of

• 1) degree of anisotropy m,
• 2) conductivity contrast between the substratum to the upper layer b,
• 3) angle of inclination of the axis of anisotropy α,

on the variation of amplitude and phase difference of the wave impedance with the normalized skin-depth (d/δ₁).     

Disturbance daily variation of electrojet current at Indian longitude sector

Using the hourly mean data of the horizontal (H) and vertical (Z) components of the geomagnetic field at the set of nine observatories in India, it has been found that the Disturbance Daily Variation (SD) of H shows a prominent midday depression over the magnetic equator of the H field and a midday increase of Z field at stations near the northern fringe of the equatorial electrojet belt. The magnetic disturbance introduces a westward electric field over the equatorial region, causing a band of westward ionospheric current over the magnetic equator during the day time hours. The latitudinal extent of the disturbance time counter electrojet current seems to be larger than that of quiet time normal electrojet current. This suggests a systematic westward electric field superposed on the normal Sq field at low latitude ionosphere during disturbed periods, the source of which has to be clearly defined. Further correlative data analysis is required to isolate these sources of the disturbed equatorial electric fields.     

Portable magnetometer theodolite with fluxgate sensor for earth's magnetic field component measurements

A new portable magnetometer theodolite with fluxgate sensor has been developed for measuring the components of the earth's magnetic field. With this instrument, used as a null detector, it is possible to make absolute measurements of declination (D) and inclination (I). The compensation of the main magnetic field allows direct intensity measurements (H, X, Y, Z components); in this case the calibration is made by reference to a proton magnetometer.The observation procedure is particularly easy and quick. The accuracy is better than 5 sec of arc forD andI and better than 1 nT for direct intensity measurements.     

The digital recording system operated at the U.K. magnetic observatories

A digital recording system has been operated at the U.K. observatories since 1979. The magnetic elementsH, D, andZ are monitored continuously by a triaxial fluxgate magnetometer operating in the variometer mode. Ordinate values are recorded at 30-sec intervals. Total field and the temperature of the variometer chamber are measured at the start of each 10-min data block.Fluxgate magnetometer baseline values are derived from absolute measurements made by proton vector magnetometer and Kew declinometer in the conventional manner.In this paper the data handling procedures are described and the quality of the data over a period of two years is discussed.     

Meridional equatorial electrojet current in the American sector

Huancayo is the only equatorial electrojet station where the daytime increase of horizontal geomagnetic field (H) is associated with a simultaneous increase of eastward geomagnetic field (Y). It is shown that during the counter electrojet period when H is negative, Y also becomes negative. Thus, the diurnal variation of Y at equatorial latitudes is suggested to be a constituent part of the equatorial electrojet current system. Solar flares are known to increase the H field at an equatorial station during normal electrojet conditions (nej). At Huancayo, situated north of the magnetic equator, the solar flare effect, during nej, consists of positive impulses in H and Y and negative impulse in Z field. During counter electrojet periods (cej), a solar flare produces a negative impulse in H and Y and a positive impulse in Z at Huancayo. It is concluded that both the zonal and meridional components of the equatorial electrojet in American longitudes, as in Indian longitudes, flows in the same, E region of the ionosphere.     

河北及周边地区岩石圈磁场变化及震前异常特征

利用2019年度大华北秋季复测156个测点的流动地磁矢量资料,获得河北及周边地区岩石圈磁场半年变化图,分析归纳河北及周边地区岩石圈磁场的总体变化特征,并研究测区内2019年12月3日河北怀安M_S3.4地震和2019年12月5日河北丰南M_S4.5地震前岩石圈磁场局部变化和异常特征。结果表明:丰南M_S4.5地震发生在H矢量的转向区,总强度F负异常的高值区,D要素负异常的高梯度带和Z值变化的高值区;怀安M_S3.4地震则发生在H矢量幅值弱化且转向区,D要素零值线附近的正值区和Z值变化的低值区。     

Geomagnetic solar flare effect on February 4 and 6，1986 at the china Antarctic Great wall Station

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On the sunspot cycle and the yearly variation of solar magnetic variation at istanbul

Summary The sunspot cycle variation of the amplitude of the solar magnetic variation has been investigated for magnetically moderate, quiet and disturbed days at Istanbul for the period 1949–1968, and fairly good linear relationship has been found forZ andD components of the earth's magnetic field. In some cases, it is rather difficult to say that there is any linear relationship between sunspot number and the amplitude of theH component of the earth's magnetic field. Meanwhile,K indices has also been considered with sunspot number by means of multiple regression analysis to overcome some uncertainties in this investigation.     

Calculation of magnetic declination and horizontal intensity on the basis of the vertical intensity

If the magnetic vertical intensityZ is known over a certain area, it is possible, under more precisely defined conditions which are often complied with, at arbitrary points of the same area—though with the exception of the marginal area—to calculate the deviations of the horizontal intensityH and the magnetic declinationD, H and D, from constant values. Below some details of the calculation in practice are given, i. a. by means of an example, the mathematic basis for the calculations being outlined in the introduction (the succeeding text may, however, be read independently thereof). A graticule, as seen in Fig. 1, was used in the calculations, and the method itself is described under «2. Procedure». In Figs. 2 and 3, the figures at the crosses denote the values ofH andD computed in this way, indicated in gammas and arc-minutes, respectively; (the isomagneticH-lines in Fig. 2 are drawn on the basis ofobserved H-values; in Fig. 3 the observedD0values themselves are given in figures in small type). The accuracy of and the possibilities for application of the method are discussed in the later sections.

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Zusammenfassung Wenn man die magnetische Vertikalintensität in einem Gebiet kennt, ist es unter gewissen Bedingungen, die oft erfüllt sind, möglich in willkürlich gewählten Punkten des Gebietes — mit Ausnahme des Randgebietes — die Abweichungen H und D der HorizontalintensitätH und der DeklinationD von konstanten Werten zu berechnen. Weiter unten sind Einzelheiten der praktischen Ausführung anhand eines Beispiels mitgeteilt. Die mathematische Grundlage der Berechnungen ist in der Einleitung skizziert; doch können die folgenden Abschnitte auch unabhängig von ihn verstanden werden. Für die Berechnungen wurde ein, Integrationsgitter (Abb. 1) verwendet, und die Methode selber ist unter «2. Procedure» beschrieben. In Abb. 2 und 3 zeigen die Zahlen bei den Kreuze die in dieser Weise berechneten Werte vonH undD, angegeben in Gammawerten und Bogenminuten; (die isomagnetischen Linien in Abb. 2 sind auf Grund vonbeobachteten H-Werten gezeichnet worden; in Abb. 3 sind die beobachtetenD-Werte in kleiner Typen gedruckt). In den Abschnitten 3 und 4 werden die Genauigkeit und die Anwendungsmöglichkeiten der Methode diskutiert.

     

Short-term and imminent geomagnetic anomalies of the Wenchuan MS8.0 earthquake and exploration on earthquake forecast

The diurnal variation of the geomagnetic vertical component is exhibited mainly by changes of phase and amplitude before strong earthquakes. Based on data recorded by the network of geomagnetic observatories in China for many years, the anomalous features of the appearance time of the minima of diurnal variations (i.e, low-point time) of the geomagnetic vertical components and the variation of their spatial distribution (i.e, phenomena of low-point displacement) have been studied before the Wenchuan M_S8.0 earthquake. The strong aftershocks after two months' quiescence of M6 aftershocks of the M_S8.0 event were forecasted based on these studies. There are good correlativities between these geomagnetic anomalies and occurrences of earthquakes. It has been found that most earthquakes occur near the boundary line of sudden changes of the low-point time and generally within four days before or after the 27th or 41st day counting from the day of the appearance of the anomaly. In addition, the imminent anomalies in diurnal-variation amplitudes near the epicentral areas have also been studied before the Wenchuan earthquake.