Geomagnetic Field Variations Research Articles

Geomagnetic intensity variations for the past 8 kyr: New archaeointensity results from Eastern China

In this study, we have carried out paleointensity experiments on 918 specimens spanning the last ∼7 kyr, including pottery fragments, baked clay and slag, collected from Shandong, Liaoning, Zhejiang and Hebei Provinces in China. Approximately half of the specimens yielded results that passed strict data selection criteria and give high-fidelity paleointensities. The virtual axial dipole moments (VADMs) of our sites range from ∼2×1022 to ∼13×1022 Am2. At ∼2250 BCE our results suggest a paleointensity low of ∼2×1022 Am2, which increases to a high of ∼13×1022 Am2 by ∼1300 BCE. This rapid (less than 1000 yrs) six-fold change in the paleointensity may have important implications for the dynamics of core flow at this time. Our data from the last ∼3 kyr are generally in good agreement with the ARCH3k.1 model, but deviate significantly at certain time periods from the CALS3k.4 and CALS10k.1b model, which is likely due to differences in the data used to constrain these models. At ages older than ∼3 ka, where only the CALS10k.1b model is available for comparison, our data deviate significantly from the model. Combining our new results with the published data from China and Japan, we provide greatly improved constraints for the regional model of Eastern Asia. When comparing the variations of geomagnetic field in three global representative areas of Eastern Asia, the Middle East and Southern Europe, a common general trend of sinusoidal variations since ∼8 ka is shown, likely dominated by the dipole component. However, significant disparities are revealed as well, which we attribute to non-dipolar components caused by movement of magnetic flux patches at the core-mantle boundary.

Contribution of stratigraphic constraints of Bulgarian prehistoric multilevel tells and a comparison with archaeomagnetic observations

The general aim of archaeomagnetism is to recover the behavior of the ancient geomagnetic field for a given territory using archaeological baked clay structures that have been well-dated by other independent methods. The Sofia Palaeomagnetic laboratory possesses a large data set for the three characteristic geomagnetic field parameters obtained from one and the same material and covering the last 8000 years. The database accumulated by the authors has been updated several times and the greatest difficulty arose for the prehistoric collections, most of which had no radiocarbon or other chronometric dates. Thus as well as methodological archaeomagnetism problems, the main difficulty concerning the construction of reference curves of the variation of the geomagnetic field is related to the distribution of the experimental results over the absolute time scale. This paper examines the similarity between the archaeomagnetic profiles obtained on multilevel tells from the same prehistoric epochs. It would seem that when 14C dates for multilayer tells are missing, the comparison of such archaeomagnetic stratigraphic profiles with the corresponding chrono-stratigraphic reference profile for the same epoch can be useful in verifying their assumed chronology. However, the described peculiarities connected with the multilevel sites results still show some difficulties for the overall estimation of geomagnetic field variations for a given territory.

Characteristic ULF Band Magnetic Field Variations at MPGO, Ghuttu for the 20 June 2011 Earthquake in Garhwal Himalaya

Magnetic field variations recorded at the Multi Parameter Geophysical Observatory (MPGO), Ghuttu, using digital fluxgate magnetometer are studied in the frequency band 0.03-1 Hz and for 19-20 h UT for June 2011. Polarization analysis based on planar wave assumption for far field is applied in order to discriminate seismo-magnetic signatures. The dynamics of earthquake processes, considering them as selforganized critical systems, is also studied using fractal dimension of ultra-low frequency band geomagnetic field variations. Marginal increase in polarization ratio and fractal dimension a few days before the earthquake is significant against the background of global geomagnetic activity. At the same time, the effect of post-seismic readjustment of seismogenic processes is clearly marked by significant changes in fractal dimension and increased polarization ratio after the earthquake.

Test of the Dynamical Non-stationarity in Geomagnetic Variation Field Based on Improved Space Time-Index Method

Space Time-Index（STI）方法是一种验证时间序列中是否存在非平稳性的图示方法. 利用改进的STI方法可以定量分析外源磁场垂直分量<em>z</em>的非平稳性特征. 以不同地磁指数（<em>K</em>=0，2，4，6）、不同Lloyd季节和昼夜外源磁场<em>z</em>分量为对象进行对比分析. 结果表明，改进的STI方法能够有效检验外源磁场的非平稳特性，且<em>z</em>分量为非平稳时间序列；不同<em>K</em>指数的<em>z</em>分量分析表明，随着<em>K</em>指数的增加，<em>z</em>分量的相空间分布越来越不均匀，时间演化特征越来越复杂；不同Lloyd季节的分析表明，各季节的STI图较为相似，但随着日地距离的减小，<em>z</em>分量时间演化特征的复杂性增强，呈现出一定季节变化特征；对昼夜变化的分析可知，夜晚<em>z</em>分量STI图的波动性比白天要强，昼夜变化特征较为明显.

Исследование отклонений геомагнитной оси гироинклинометра в азимуте при построении нефтегазовых скважин

Исследование отклонений геомагнитной оси гироинклинометра в азимуте при построении нефтегазовых скважин

Day-to-Day Variability of H Component of Geomagnetic Field in Central African Sector Provided by Yaound&amp;#233;-Cameroon Amber Station

The geomagnetic data obtained from Amber Network station in Cameroon have been used for this study. The variability of H component of geomagnetic field has been examined by using geomagnetic field data of X and Y components recorded at AMBER magnetometer station hosted by the Department of Physics of University of Yaoundé (3.87°N, 11.52°E). The day-to-day variability of the horizontal intensity of the geomagnetic field was examined and shows that the scattering of H component of magnetic field variation is more on disturbed than that on quiet condition. The signatures H of geomagnetic Sq and Sd variations in intensities in the geomagnetic element, has been studied. This study shows that the daytime variations in intensities of geomagnetic elements H, Sq(H) and Sd(H) respectively are generally greater than night-time ones. This paper interests answering the two questions: 1) how can geomagnetic variations be used to study the equatorial ionosphere electrodynamics and electrojet equatorial over Africa in general and Cameroon in particular? 2) How can geomagnetic variations be used to monitor and predict space weather events in Cameroon? This paper presents and interprets the results of H component of geomagnetic field variations during magnetic storms and on quiet days.

Magnetospheric and Ionospheric Sources of Geomagnetic Field Variations

This study investigated the magnetospheric and ionospheric sources of geomagnetic field variation using the data set that consists of the hourly values of the geomagnetic element: horizontal intensity H, recorded at the geomagnetic observatory of the Department of physics, university of Ilorin, Ilorin (Long 4.670 and Lat. 8.50, dip 4.1 oS) Nigeria for the months of august, October and December in the year 2008. The study attributed the daytime variation to the ionospheric sources while various reasons were given to explain these night-time variations, which include convective drift currents in the magnetosphere and the asymmetric rings currents in the magnetospheric currents; this variation due to disturbances indicates possible non-ionospheric origin and a partial ring current in the night side magnetosphere. The mean monthly hourly variation showed seasonal variation with the Sq(H) maximizes in equinox with least variation in June solstice. Moreover, the mean ration of seasonal contributions of magnetospheric to ionospheric sources revealed that the E season has more contributions from both magnetospheric and ionospheric sources than other seasons with the least occurring during the J season.

Variability modes in core flows inverted from geomagnetic field models

The flow of liquid metal inside the Earth's core produces the geomagnetic field and its time variations. Understanding the variability of those deep currents is crucial to improve the forecast of geomagnetic field variations, which affect human spacial and aeronautic activities. Moreover, it may provide relevant information on the core dynamics. The main goal of this study is to extract and characterize the leading variability modes of core flows over centennial periods, and to assess their statistical robustness. To this end, we use flows that we invert from two geomagnetic field models (gufm1 and COV-OBS), and apply Principal Component Analysis and Singular Value Decomposition of coupled fields. The quasi geostrophic (QG) flows inverted from both geomagnetic field models show similar features. However, COV-OBS has a less energetic mean and larger time variability. The statistical significance of flow components is tested from analyses performed on subareas of the whole domain. Bootstrapping methods are also used to extract significant flow features required by both gufm1 and COV-OBS. Three main empirical circulation modes emerge, simultaneously constrained by both geomagnetic field models and expected to be robust against the particular a priori used to build them (large scale QG dynamics). Mode 1 exhibits three large vortices at medium/high latitudes, with opposite circulation under the Atlantic and the Pacific hemispheres. Mode 2 interestingly accounts for most of the variations of the Earth's core angular momentum. In this mode, the regions close to the tangent cylinder and to the equator are correlated, and oscillate with a period between 80 and 90 years. Each of these two modes is energetic enough to alter the mean flow, sometimes reinforcing the eccentric gyre, and other times breaking it up into smaller circulations. The three main circulation modes added together to the mean flow account for about 70% of the flows variability, 90% of the root mean square total velocities, and 95% of the secular variation induced by the total flows. Direct physical interpretation of the computed modes is not straightforward. Nonethe-less, similarities found between the two first modes and time/spatial features identified in different studies of core dynamics, suggest that our approach can help to pinpoint the relevant physical processes inside the core on centennial timescales.

Global distributions of storm-time ionospheric currents as seen in geomagnetic field variations

To investigate temporal and spatial evolution of global geomagnetic field variations from high-latitude to the equator during geomagnetic storms, we analyzed ground geomagnetic field disturbances from high latitudes to the magnetic equator. The daytime ionospheric equivalent current during the storm main phase showed that twin-vortex ionospheric currents driven by the Region 1 field-aligned currents (R1 FACs) are intensified significantly and expand to the low-latitude region of ~30° magnetic latitude. Centers of the currents were located around 70° and 65° in the morning and afternoon, respectively. Corresponding to intensification of the R1 FACs, an enhancement of the eastward/westward equatorial electrojet occurred at the daytime/nighttime dip equator. This signature suggests that the enhanced convection electric field penetrates to both the daytime and nighttime equator. During the recovery phase, the daytime equivalent current showed that two new pairs of twin vortices, which are different from two-cell ionospheric currents driven by the R1 FACs, appear in the polar cap and mid latitude. The former led to enhanced northward B z (NBZ) FACs driven by lobe reconnection tailward of the cusps, owing to the northward interplanetary magnetic field (IMF). The latter was generated by enhanced Region 2 field-aligned currents (R2 FACs). Associated with these magnetic field variations in the mid-latitudes and polar cap, the equatorial magnetic field variation showed a strongly negative signature, produced by the westward equatorial electrojet current caused by the dusk-to-dawn electric field.

On the anomalous changes of seismicity and geomagnetic field prior to the 2011 [formula omitted] 9.0 Tohoku earthquake

In Xu et al. (2013), it has just been reported that approximately 2months prior to the Mw 9.0 Tohoku earthquake that occurred in Japan on 11 March 2011 anomalous variations of the geomagnetic field have been observed in the vertical component at a measuring station about 135km from the epicenter for about 10days (4–14 January 2011). Here, we show that this observation is in striking agreement with independent recent results obtained from natural time analysis of seismicity in Japan. In particular, this analysis has revealed that an unprecedented minimum of the order parameter fluctuations of seismicity was observed around 5 January 2011, thus pointing to the initiation at that date of a strong precursory Seismic Electric Signals activity accompanied by the anomalous geomagnetic field variations. Starting from this date, natural time analysis of the subsequent seismicity indicates that a strong main shock was expected in a few days to one week after 08:40 LT on 10 March 2011.

A kinematic model of vertical geomagnetic field variation resulting from a steady convective flow

A kinematic hydromagnetic problem is investigated in order to follow the effects of a prescribed three-dimensional convection on an initially given magnetic field. The vertical velocity profile is -dependent, being the vertical coordinate, and consists of two parts; one appropriate to the diffusionless region of the main volume of the fluid and the other appropriate to the resistive boundary layer at the rigid wall. The induction equation for the vertical component of the magnetic field is solved analytically in Cartesian geometry in these two distinct regions. Besides the horizontal transport of the magnetic field in the main volume, there is also field distortion due to the vertical velocity gradient; the upwelling flow causes it to weaken while the downwelling causes intensification. The resulting form at the surface of the main volume is a time-varying magnetic field, which then penetrates the boundary layer. This thin region, where the velocity rapidly tends to zero, behaves like a solid conductor, thanks to the assumption of a steady flow in the main volume. It generally attenuates the magnetic field. Depending on the velocity field configuration in the main volume, it may also cause a sinusoidal variation of the magnetic field in . Under appropriate conditions, a reversed magnetic field is obtained on the surface. In the context of the geomagnetic secular variation, this may be an explanation of the westward drift of reversed magnetic flux patches.

Solar transients disturbing the terrestrial magnetic environment at higher latitudes

Geomagnetic field variations during five major Solar Energetic Particle (SEP) events of solar cycle 23 have been investigated in the present study. The SEP events of 01 oct 2001, 04 Nov 2001, 21 Apr 2002 and 14 May 2005 have been selected to study the geomagnetic field variations at two high-latitude stations, Thule and Resolute Bay of the northern polar cap. We have used the GOES protn flux in seven different energy channels. All the proton events were associated with geoeffective or Earth directed CMEs that caused intense geomagnetic storms in response to geospace. We have taken high-latitude indices, AE and PC, under consideration and found fairly good correlation of thees with the ground magnetic field records during the five proton events. The departure of H component during the events were calculated from the quietest day of the month for each event. The correspondence of spectral index, inferred from event integrated spectra, with ground magnetic signatures along with Dst and PC indices have been brought out. From the correlation analysis we found very strong correlation to exist between the geomagnetic field variations and high latitude indices AE and PC. To find the association of geomagnetic storm intensity with proton and geomagnetic field variations along with the Dst and AE index. We found a strong correlation (0.88) to exist between the spectral indices and magnetic field deviations and also between spectral indices and AE and PC.

Magnetic Field Variation Caused by Rotational Speed Change in a Magnetohydrodynamic Dynamo

We have performed numerical magnetohydrodynamic dynamo simulations in a spherical shell with rotational speed or length-of-day (LOD) variation, which is motivated by correlations between geomagnetic field and climatic variations with ice and non-ice ages. The results show that LOD variation leads to magnetic field variation whose amplitude is considerably larger than that of LOD variation. The heat flux at the outer sphere and the zonal flow also change. The mechanism of the magnetic field variation due to LOD variation is also found. The keys are changes of dynamo activity and Joule heating.

The Analysis of Marine Magnetic Field Detection Influenced by the Platform Migration

Marine magnetic field detection is influenced by the environment of marine and the platform,The influence of the platform is mainly caused by platform migration.The variation of the geomagnetic field caused by different directions of the platform migration are compared by using the World Magnetic Model WMM2010.We come to the conclusion that the geomagnetic field variation is the biggest in the direction of depth.The conclusion can be used for automatic correction of the marine magnetic field observation platform .

From solar eruption to transformer saturation: the space weather chain

AbstractCoronal mass ejections (CME) and associated interplanetary-propagated solar wind disturbances are the established causes of the geomagnetic storms which, in turn, create the most hazardous impacts on power grids. These impacts are due to the large geomagnetically induced currents (GIC) associated with variations of geomagnetic field during storms, which, flowing through the transformer windings, cause extra magnetisation. That can lead to transformer saturation and, in extreme cases, can result in power blackouts. Thus, it is of practical importance to study the solar causes of the large space weather events. This paper presents the example of the space weather chain for the event of 5-6 November 2001 and a table providing complete overview of the largest solar events during solar cycle 23 with their subsequent effects on interplanetary medium and on the ground. This compact overview can be used as guidance for investigations of the solar causes and their predictions, which has a practical importance in everyday life.

Float‐transient electromagnetic method: in‐loop transient electromagnetic measurements on Lake Holzmaar, Germany

ABSTRACTLake sediments may serve as archives on paleoclimatic fluctuations, geomagnetic field variations and volcanic activities. Lake Holzmaar in Eifel/Germany is a maar lake and its lacustrine sediments provide paleoclimatic proxy data. Therefore, knowledge about the geometry and, especially, about the thickness of the sediments is very important for determining an optimum drilling location for paleoclimatic studies.We have developed a floating in‐loop transient electromagnetic method field set up (Float‐transient electromagnetic method) with a transmitter and receiver size of 18 × 18 m2 and 6 × 6 m2 respectively. This special set up enables in‐loop transient electromagnetic method measurements on the surface of freshwater lakes that define the geometry and the thickness of sediments beneath such lakes thus helping to determine optimum drilling locations. Due to the modular design of the new Float‐transient electromagnetic method field set up, this system can be handled by two operators and can easily be transported.Sixteen in‐loop soundings were carried out on the surface of Lake Holzmaar. The transient electromagnetic method data could not be interpreted by conventional 1D inversions because of the 3D distribution of subsurface conductivity caused by the lake's geometry. Three‐dimensional finite element modelling was applied to explain the observed transients and the 3D conductivity distribution beneath the lake was recovered by taking its geometry into account. The 3D interpretation revealed approximately 55 m thick sediments beneath 20 m deep water in the central part of the lake.

Synchronism of variations in the main geomagnetic field and earth rotation velocity fluctuations

Variations in the value of the source magnetic moment, responsible for the dipole component of the main geomagnetic field, and Earth rotation velocity fluctuations have been studied comparatively. Three quasi-harmonic components with close periods, for which their own phase shifts were obtained, have been detected in time series of the analyzed processes. The mantle conductivity effect on these shifts has been studied in the scope of a one-layer model; for this purpose, the amplitude-phase characteristic of the mantle as a filter has been constructed. The mantle conductivity has been estimated (σ = 270 ± 20 S m−1) on the assumption that the phase shift mainly depends on the mantle effect. It has been assumed that the variations in the time series of the analyzed processes are almost cophased and can be caused by one external factor, e.g., tides.

Wavelet analysis of geomagnetic field data

Variations in geomagnetic field data at different spectral frequencies and with different periods are observed during increased geomagnetic activity. The formed local structures depend on the field disturbance and contain information on the magnetic storm intensity and character of development. Numerical solutions and algorithms based on wavelet transforms, which make it possible to “automatically” detect periods of increased geomagnetic activity and identify and analyze the structures forming this process, have been proposed in order to study the time characteristics of geomagnetic field variations, using the H component as an example. The separated components, characterizing disturbances, make it possible to estimate variations in the field energy characteristics. An analysis of the constructed wavelet images makes it possible to trace the dynamics of variations in the H component the day before and during a magnetic storm.

Geomagnetic field variations in Western Europe from 1500 BC to 200 AD. Part II: New intensity secular variation curve

In order to extend the secular variation curve (SVC) of archaeointensity in Western Europe to the first millennium BC, we studied 24 kilns and hearths in place, two displaced hearths and six sets of pottery sherds from French archaeological sites. Archaeological artefacts, radiocarbon and dendrochronology dated the acquisition of the thermoremanent magnetization (TRM) carried by the studied objects. Rock magnetism experiments suggest that the main carrier of the magnetization is a Ti-poor titanomagnetite. Archaeointensity was determined by the Thellier–Thellier classical protocol with pTRM-checks. A strict criteria set was applied to select only the most reliable results with linear NRM–TRM diagrams (55% of total specimens). This study demonstrates that pottery sherds with two TRMs give reliable archaeointensities in the low-temperature interval, if the NRM–TRM diagram is adequately adjusted. Eighteen new mean archaeointensities (14 corrected from the anisotropy of TRM and 16 from cooling rate) were computed. The comparison with previously published Western Europe paleointensities show a strong dispersion between data primarily due to their variable quality. Western Europe data were weighted following the archaeointensity protocol, the number of specimens per site and the type of studied materials, in order to better highlight the secular variation of archaeointensity during the first millennium BC. The SVC, built with sliding windows of 160years shifted every 50years, presents (at Paris) a maximum of 90μT around 800BC and a minimum of 60μT around 250BC. These archaeointensity maximum and minimum correspond to cusps of the geomagnetic field direction in Western Europe. This new curve is consistent with Mesopotamian and Eastern Europe data. The archaeointensity secular variation in Western Europe predicted by global geomagnetic models CALS3k.4, ARCH3k.1 and ARCH3k_cst.1 is smoother than our SVC. We used our directional dataset (Hervé et al., 2013) to build a new Western Europe VGPs and VDMs mean curves. Comparison with the predictions given by the global models points out a possible persistent non-dipole fields effect over Europe between 1000BC and 600–500BC. Finally, we note that the strong variations of intensity of the geomagnetic field (with a mean decrease rate per century close to 6μT) will be useful for archaeomagnetic dating purposes.

Geomagnetic field variations in Western Europe from 1500BC to 200AD. Part I: Directional secular variation curve

To improve the geomagnetic field secular variation curve (SVC) of Western Europe during protohistoric times, archaeomagnetic directions of 39 archaeological kilns or hearths from France were investigated. The dating of each archaeological structure was established with archaeological or chronometric methods. Thirty-seven of these structures are dated from the first millennium BC, one from the end of the second millennium BC and the last one from the fourth millennium BC. Thermomagnetic curves, unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is a Ti-poor titanomagnetite. Archaeodirections were obtained by alternating field and thermal demagnetizations on almost 900 specimens. The anisotropy tensor of thermoremanent magnetization was determined for 35 structures and 22 mean archaeodirections were corrected for anisotropy. The new archaeodirections are very consistent with previously published data. A new directional SVC was built using bivariate statistics with selected Western Europe data located within 1000km of Paris. Selection criteria include the number of samples, the dating reliability and the accuracy of the mean archaeodirection. Resulting secular variation between 1500BC and 0AD mainly shows large changes in declination, while inclinations are bracketed between ∼65° and ∼75°. The declinations show a strong maximum with values ∼30–35° around 800–750BC, followed by a sharp decrease to values around 0° at 500BC and close to −7° around 250BC. The main features of the secular variation from 1500BC to 0AD appear to be a dominant westward drift and two major changes around 800 and 250BC. Compared to the global and regional geomagnetic models, the new reference data are better fitted by ARCH3k_cst.1 and SCHA.DIF.3k than by ARCH3k.1 and CALS3k.4 models. The strong variation of the archaeodirection between 1000 and 500BC makes archaeomagnetism very useful for dating purposes.