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°。     

Secular variation of the geomagnetic virtual dipole field during the last 2000 years as inferred from the spherical harmonic analysis of the available archeomagnetic data

Summary Recently excellent archeomagnetic data sequences have been bublished from several parts of the world. Using these sequences, an attempt is made to trace the secular variation of the virtual geomagnetic dipole field characterized by the three first order spherical harmonic coefficients . The archeomagnetic data (declination, inclination and total intensity) are transformed into the first order coefficients mentioned by a simple mathematical method. The secular variations of these coefficients, however, contain both dipole and non-dipole components. The separation of these is also attempted.Paper presented at the IAGA Symposium, Madrid, September 1969.     

Rate of seismicity of the dead sea region over the past 4000 years

The results of two millennia of earthquake documentation, a few decades of macroseismic and instrumental routine seismological observations and five months of microearthquake monitoring, are used to estimate the rate of seismic activity of the Dead Sea fault. It is found that these vastly diverse data which combine long- and short-term tectonic processes, are in good accord with the formula:

$\text{log}{}_{10}\text{N=2.54 ? 0.86M}{}_{\mathrm{<mtext>L</mtext>}}$

where N is the annual number of events of local magnitude M_L or greater. If this equation is extrapolated to ca. 2000 B.C., it yields a Richter magnitude M_s = 7 for the event of Sodom and Gomorrah which is believed to be associated with the strongest earthquake in the region during historical times.Comparing our findings with the results of other investigators in Turkey, Greece, Aegean Sea and Iran, we note that the b values along the Syrian-African rift zone (0.78–0.86) are smaller than those in Greece and its surrounding seas (0.94–1.16).     

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.     

近年地磁场的长期变特征

利用红山、后土桥等台近30年的的地磁绝对观测资料,对F、H、Z各分量进行了分析,对Z分量的年速率进行了研究。结果:1999年和2000年F总场和Z分量长期变相继出现了转折,结束了逐年上升的变化趋势,但局部地区近两年有所回升。而H分量趋势没有变化,依照原来的趋势在下降;Z分量年速率的减缓变化往往与华北地区较强地震有关。     

Tropical Indian Ocean Basin Mode recorded in coral oxygen isotope data from the Seychelles over the past 148 years

The tropical Indian Ocean(TIO) displays a uniform basin-wide warming or cooling in sea surface temperature(SST) during the decay year of El Niδo-Southern Oscillation(ENSO) events. This warming or cooling is called the tropical Indian Ocean Basin Mode(IOBM). Recent studies showed that the IOBM dominates the interannual variability of the TIO SST and has impacts on the tropical climate from the TIO to the western Pacific. Analyses on a 148-year-long monthly coral δ 18 O record from the Seychelles Islands demonstrate that the Seychelles coral δ 18 O not only is associated with the local SST but also indicates the interannul variability of the basin-wide SST in the TIO. Moreover, the Seychelles coral δ 18 O shows a dominant period of 3–7 years that well represents the variability of the IOBM, which in return is modulated by the inter-decadal climate variability. The correlation between the Seychelles coral δ 18 O and the SST reveals that the coral δ 18 O lags the SST in the eastern equatorial Pacific by five months and reaches its peak in the spring following the mature phase of ENSO. The spatial pattern of the first EOF mode indicates that the Seychelles Islands are located at the crucial place of the IOBM. Thus, the Seychelles coral δ 18 O could be used as a proxy of the IOBM to investigate the ENSO teleconnection on the TIO in terms of long-time climate variability.     

Archaeomagnetic secular variation in the UK during the past 4000 years and its application to archaeomagnetic dating

This paper examines the limitations and deficiencies of the current British archaeomagnetic calibration curve and applies several mathematical approaches in an attempt to produce an improved secular variation curve for the UK for use in archaeomagnetic dating. The dataset compiled is the most complete available in the UK, incorporating published results, PhD theses and unpublished laboratory reports. It comprises 620 archaeomagnetic (directional) data and 238 direct observations of the geomagnetic field, and includes all relevant information available about the site, the archaeomagnetic direction and the archaeological age. A thorough examination of the data was performed to assess their quality and reliability. Various techniques were employed in order to use the data to construct a secular variation (SV) record: moving window with averaging and median, as well as Bayesian statistical modelling. The SV reference curve obtained for the past 4000 years is very similar to that from France, most differences occurring during the early medieval period (or Dark Ages). Two examples of dating of archaeological structures, medieval and pre-Roman, are presented based on the new SV curve for the UK and the implications for archaeomagnetic dating are discussed.     

The secular variation of the geomagnetic field in Egypt in the last 5000 years

The palaeo-intensities (F _a) of the geomagnetic field in Egypt at some ages are determined by archaeomagnetic measurements and found to be:F _a=36.2 T at 3100 B.C., F_a=46.8 T at 3000 B.C.,F _a=36.5 T at 2780 B.C., 49.0 T at 2500 B.C., 36.4 T at 2200 B.C., 57.5 T at 1990 B.C., 62.1 T atca 1400 B.C., 61.5 T at 1400 B.C., 69.9 T at 600 B.C., 59.3 T at 550 B.C., 79.9 T at 460 B.C., 73.7 T at 450 B.C., 69.7 T at 320 B.C., 56.2 T at A.D. 50, 64.9 T, at A.D. 400, 54.4 T at A.D. 300, 57.5 T at A.D. 700 and 43.0 T at A.D. 1975.The palaeo-inclinations (I _a) at some ages are found to be:I _a=24.2° at 420 B.C., 44° at A.D. 50, 60.7° at A.D. 703 and 42° at A.D. 1795.The measured values ofF _a are affected by the anisotropy of magnetic susceptibility of the samples by 13% to 20% of the expected correct value. The suitable correction of this effect is by multiplyingF by 1/((1+0.2(/90)) andF by 1/((1–0.13 (/90)), whereF andF are the resultant values ofF _a if the laboratory field is perpendicular or parallel to the wall of the sample during the Thelliers' experiments, respectively, and is the angle between the direction of natural remnant magnetization of the sample and the direction of the laboratory field.The results of this paper, together with the previous results for Egypt and the neighbourhoods, lead to the production of the secular variation curve of the geomagnetic field in Egypt for the last 5000 years. The intensity of the field shows a periodicity of about 400 years with multiples.     

河北省近年内地磁场变化规律的初步分析

利用河北省境内的9个地磁台自1986—2007年的地磁绝对观测资料,对该地区地磁场的变化特征进行了总结分析。进一步认识河北省地磁场变化规律及其特征,为地震预报提供有实用价值的数据很有意义。     

Results of the monitoring of geomagnetic field variations in the Kamchatka region

The transfer function between the vertical and horizontal components of geomagnetic field variations is studied and the frequency responses of its parameters are presented. The relation to geoelectric heterogeneities of the medium is analyzed. The coast effect is considered and a deep curve of the apparent electrical resistivity constructed on its basis is used for estimating the depth to the asthenospheric conducting layer. The behavior of the induction vectors in the frequency-time domain is studied. Specific features of the behavior of the real and imaginary parts of the induction vector related to geoelectric heterogeneities of the medium are determined. Monitoring results are compared with time moments of earthquakes of K = 13?14 at epicentral distances of up to 150 km.     

Wavelet analysis of paleomagnetic data: 3. Wavelet analysis of the basic series of archaeomagnetic data on the geomagnetic field intensity for the past 7500 years

We performed wavelet analysis of series of the magnetic field intensity obtained through archaeomagnetic studies in Bulgaria, Georgia, Central Asia, China, and Japan. The duration of the studied intervals is 2000, 4000, and more than 7000 yrs. It has been shown that periodic oscillations as well as those with varying characteristic times are present in this series. The distribution of oscillations with characteristic times up to 800–1000 yrs by periods, characteristic times, time, and place of their manifestation is random.     

Directional secular variation of the geomagnetic field in Late Devonian volcanics of the Northern Tien Shan

We have paleomagnetically studied sixty andesite-basalt lava flows of the Late Devonian (Frasnian) in the Northern Tien Shan. Stepwise thermal demagnetization revealed a high-temperature dualpolarity magnetization in most of the samples. The primary origin of this component was established by the positive reversal and conglomerate tests. We found a clear difference in angular dispersion of this component between the lower and upper portions of the section. In the former, the dispersion is close to the predictions of various models of geomagnetic secular variation, while the scatter in the top part of the sequence is smaller by a factor of four. We conclude that this distinction is most likely due to the geomagnetic field behavior and hypothesize that the magnitude of secular variation could vary severalfold over time intervals of 10⁵−10⁶ years. This is in sharp contrast with other models of secular variation, where this magnitude has been assumed to be rather time-independent (for a given latitude). We discuss probable implications and alternative interpretations of the observed data.     

On the intensity of the geomagnetic field in the geological past

A detailed analysis of the data on the intensity of the geomagnetic dipole and frequency of its reversals presented in the world’s paleointensity databases provided the arguments in favor of the hypothesis of the negative correlation between the average virtual dipole moment (VDM) and the frequency of the reversals on the interval from 5 Ma to 100 Ma ago. However, the statistical confidence level of this hypothesis is only 60–70%, which is far below 95%, the standard required confidence level of a hypothesis to be considered statistically reliable. At a high level of confidence (above 99%), the presence of a positive correlation between the mean value and variance of VDM for a number of intervals of stable polarity in the Cenozoic and Mesozoic is confirmed. This finding means that the distribution of VDM on these time intervals is certainly non-Gaussian and is rather described by the gamma- or lognormal law. At the same time, in contrast to the earlier intervals, the histogram of VDM for the Brunhes epoch is closer to the normal distribution. Compared our conclusions with the published results on the numerical modeling of the geodynamo, we found that they are consistent in terms of a probable negative correlation between the average VDM and reversal frequency, as well as the lack of correlation between the average VDM and the length of the interval of stable polarity.     

Excursions of the Pleistocene geomagnetic field recorded in Gulf of Mexico sediments

The detrital remanent magnetism of a series of deep-sea sediment cores from the Gulf of Mexico has been measured. Together with microfaunal analysis, the data show that excursions of the geomagnetic field occurred at 17,000 ± 1500years B.P. and32,000 ± 1500 years B.P. It is suggested that the former may be the Laschamp excursion and that the latter may be the Lake Mungo excursion. No similar geomagnetic behavior is detected for the past 50,000 years. Sedimentation rates as high as 19 cm per 1000 years are indicated.Susceptibility (χ) maxima in the cores are due to tephra layers. Correlation between the intensity of magnetization (J) and χ shows that variations of intensity are more a function of ferrimagnetic mineral concentrations than geomagnetic field intensity variations.     

Short-Period Secular Variations (SPSV) of the geomagnetic field recorded in highly scattered palaeomagnetic records of Holocene lake sediments from North Poland

A simple method of calculating running means over different intervals within palaeomagnetic records has been used to identify short-period secular variations of the geomagnetic field. The records from Polish lake sediments reveal variations in declination with a period between 290 and 372 years. The periodicity of variations in the inclination has been estimated to be between 522 and 670 years.     

Secular variations in the main geomagnetic field within the scope of the dynamic model of field sources

The effect of different-level sources on the spatial structure of the secular variations has been considered based on the dynamic model of sources of the main geomagnetic field developed by us. It has been obtained that the development of 13 most powerful dipoles only roughly characterizes global anomalies of the secular variations, and each anomaly results from the superposition of the dynamics of several sources. The model secular variations have been compared with the data from the observatories. It has been obtained that it is impossible to describe local anomalies of the observed secular variations ignoring sources of the third order of smallness as compared to the main dipole. It has been assumed that topographic vortices, originating around inhomogeneities of the core-mantle boundary, can be physical sources responsible for dipoles of the third order.     

Changes in geomagnetic activity and global temperature during the past 40 years

The purpose of this study is to investigate the effect of geomagnetic activity (used as a measure of solar wind parameters) on the variability of large-scale climate patterns and on changes in the global temperature. We show that positive statistically significant correlations between global temperature and the distribution of surface temperature over Eurasia, the East and Equatorial Pacific and over the North Atlantic for the period 1966?C2009 correspond to large-scale climate patterns defined by climate indices. We found very similar positive correlations between geomagnetic activity and the distribution of surface temperature in the mentioned regions. As an effect of geomagnetic storms, energetic particles penetrate from the magnetosphere into the region of the stratospheric polar vortex. The increase of temperature and pressure can be observed over northern Canada. The vortex shifts towards Europe, rotates counter-clockwise and the wind blows from the polar region over Greenland southwards. It diverts the warm flow proceeding northward over the Atlantic, eastward along the deep Icelandic low extending as far as the Barents Sea and takes part in warming Eurasia. The strengthened zonal flow from Siberia cools the western Pacific with the impact on the warming of the equatorial and eastern Pacific when also a distinct 1976?C78 climate shift occurred. Processes in the Atlantic and Pacific play a significant role and a time delay (wind forcing over the previous 1?C4 yr) appears to be the most important for the relocation of the oceanic gyres. Results showing statistically significant relations between time series for geomagnetic activity, for the sum of climate indices and for the global temperature help to verify findings concerning the chain of processes from the magnetosphere to the troposphere.