Geomagnetic field intensity between 70 000 and 130 000 years B.P. from a volcanic sequence on La Réunion, Indian Ocean

A detailed paleointensity study was made of a sequence of 70 successive lava flows of the Piton des Neiges volcano on the island of La Réunion (Indian Ocean). Radiometric dating brackets the age of this sequence between 130 ± 3 ka and 72 ± 3 ka. Rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the Natural Remanent Magnetisation (NRM). Over 350 samples were used for paleointensity determinations carried out with the Thellier method in vacuum or in an argon atmosphere. Of these, 89 samples yielded reliable results, with within-flow scatter often lower than 20%. These results indicate that the geomagnetic field intensity has varied at La Réunion between 13 and 65 μT during the period of time explored. The average value, 42 μT, is higher than the present field at La Réunion (35 μT). The results from the upper part of the section are consistent with previous results obtained for the 82–98 ka period also at La Réunion [1] and document a broad low around 95 ka. not associated with large directional changes. On a larger geographic scale, the paleointensity values from La Réunion are significantly higher than those obtained from Mount Etna [2]. Precise comparison is, however, difficult because of the lack of detail in the Etna results. In the lower part of the section, a marked intensity low, coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the Blake event.     

澳门路环地区地磁场的环境及近期变化

2002年5月,对澳门路环岛地区及相邻珠海横琴岛的地磁流动观测网的25个测点进行了地磁场总强度(F)的观测。同时还进行了地磁环境噪音的测量及十字剖面测量。2003年1月,对此流动观测网进行了复测。尽管该地区四周邻海,存在相当数量的干扰因素。但两次观测结果经通化处理后表明大多数测点的地磁场总强度的变化不大,变化量在1～2nT左右。两次观测结果的总体均方差为2．8nT。     

Long period variations of the geomagnetic field and their spatial distribution in Europe

a 27 nu amu ama m¶rt;au nma aaua ¶rt; nu nma anum¶rt; 27-¶rt; auauu u ÿeau nu¶rt;au 13–14 u 9 ¶rt;. mam an¶rt;u nma anum¶rt; Z u H mau a n nuuuu auauuu uuP ₁ (cos ) u P ₁ /, mmm. mau mau a um auumu mau auau ¶rt; mmu mu Z/H ¶rt; a¶rt; amuu. au ¶rt; mmuuu u umu a¶rt;au muu ¶rt; ¶rt;am uau mu ¶rt;¶rt;mmu mu u. a aaua au mu Z/H ¶rt;m u uam n u m aa amu.     

Geomagnetic field variations induced by internal and surface waves in the four-layer model of the marine environment

The layered model of the marine environment, including the atmosphere, two seawater layers with different conductivity and density, and the bottom rock layer, has been considered. The geomagnetic field variations, generated by internal and surface waves with different frequency and propagation direction, have been found in the scope of this model. The effect of magnetic permeability and electric conductivity of bottom rocks on induced magnetic field has been taken into account. The transfer functions and spectral densities of these variations have been analytically determined and numerically estimated.     

Total ozone and temperature field at 100 mb over Europe 1967–1968

Pure and Applied Geophysics -     

Geomagnetic field variations between chrons 33r and 19r (83–41 Ma) from sea-surface magnetic anomaly profiles

Sea-surface magnetic profiles exhibit coherent short wavelength “micro-anomalies” (or “tiny wiggles”) superimposed to the main anomalies due to reversals. In this study, we investigate the nature and distribution of these tiny wiggles on oceanic crust formed during the  42 Myr-long period following the Cretaceous Normal Superchron. To this end, we compute stacks of anomaly profiles from different areas in the Indian and the Pacific oceans. Using a simple method based on upward continuation, we demonstrate that, the tiny wiggles are consistent worldwide although their patterns exhibit different resolutions at different spreading rates. They are therefore confidently ascribed to past fluctuations of the geomagnetic dipole moment. A high resolution record of these fluctuations is obtained by selecting and stacking profiles from areas with the highest spreading rates. Modeling the micro-anomalies as short magnetic polarity intervals yields durations for these intervals generally shorter than 10 kyr, likely too short to be indeed “true” subchrons. Moreover, the number of detected tiny wiggles clearly depends on the spreading rate. These results support geomagnetic intensity fluctuations as being the cause of most tiny wiggles, as also suggested by recent magnetostratigraphic data. The tiny wiggles are uniformly distributed within chrons, indicating that paleointensity fluctuations are neither inhibited after, nor enhanced before, a reversal beyond a “blind” zone of about 10 km (corresponding to 80 to 250 kyr depending on the spreading rate) for which the anomalies due to reversals prevent the detection of tiny wiggles. Most tiny wiggles probably represent a filtered record of a uniform secular variation regime, as suggested by their uniform spatial distribution over the whole investigated period.     

南极中山站与北极地磁活动相关性研究

本文利用南极中山站和新奥尔松（NAL）等5个北极站2000年9月到2003年3月共719天的地磁观测数据,以20 min间隔内地磁北向水平分量的互相关系数大于0.9为判据,对南北极地磁活动相关的发生率进行了统计,并与T89地磁模型得到的中山站共轭位置的分布进行了比较. 结果表明,南北极地磁活动相关的发生率与台站位置、季节、地方时和地磁活动性有着密切的关系. 在北极的5个台站中朗伊尔站（LYR）与中山站地磁相关的时间最多（占17%）;在两分季发生相关的情况比冬季和夏季多;在傍晚发生相关的情况最多,中午发生相关的情况最少;地磁活动较强时,相关的情况比地磁宁静时更多一些.     

Geomagnetic secular variation from recent lake sediments,ancient fireplaces and historical measurements in southeastern Australia

Compilations of historical observations, archaeomagnetic data from ancient fireplaces and palaemagetic results from short cores of sediment from lakes in southeastern Australia, particularly Lake Keilambete, provide a detailed record of the geomagnetic secular variation during the last 3000 years. The independent sets of data are in good agreement if the radiocarbon time scale for the lacustrine record is about 450 years too old. The error is attributed to systematic incorporation of ancient carbon into the lake floor sediments, mainly through erosion of sediment on the crater walls at times of low water level. A significant lag between deposition and the acquisition of stable magnetic remanence is ruled out. Inclination has been abnormally steep during the last 500 years but remained fairly close to the axial dipole field value prior to that. During the last 1000 years the predominant sense of looping of the magnetic vector corresponds to westward drift of the nondipole field. Secular variations on a time scale of ～ 100 years can be resolved by the lacustrine record.     

Phase velocities of Rayleigh waves in southeastern Europe and Eastern Mediterranean Sea

Summary Measurements of phase velocities along several paths in southeastern Europe and along one path in the Eastern Mediterranean have been made in the period range 15 to 60 sec for the fundamental mode of Rayleigh waves. The data are fitted by layered models of the crust and upper mantle. The crust in the Balkan peninsula is thicker than the normal continental crust by about 10 km while the crust in the northern Aegean Sea has the normal continental thickness. The existence of a low shear velocity layer of thickness equal to 140 km with its top in a depth of 90 km interpretes the data well for the longer periods. The data for the eastern Mediterranean show that this region has an oceanic character. The mean crustal thickness in this area is of the order of 20 km.     

Nature of the sunrise effect in daily electric field variations at Kamchatka: 2. Electric field frequency variations

The power spectra of time variations in the electric field strength in the near-Earth’s atmosphere and in the geomagnetic field horizontal component, which were simultaneously observed at the Paratunka observatory (φ = 52°58.3′ N; λ = 158°14.9′ E) in September 1999, have been studied. The periods of the day (including sunrise, sunset, and night) have been considered. It has been indicated that oscillations with periods T ～ 2.0–2.5 h are present in the power spectra of these parameters during the day. The intensity of these oscillations increases noticeably and the oscillations in the band of periods T < 1 h increase simultaneously in the field strength power spectra at sunrise. The variations in the argument of the cross-spectrum of these parameters indicated that oscillations in the 2.0–2.5 h period band are caused by sources that are located above the ionospheric dynamo region; at the same time, oscillations in the 0.5–1 h period band are caused by sources in the lower atmosphere. A possible mechanism by which these oscillations are generated, related to the vortex motion of convective cells that originate at sunrise in the boundary atmospheric layer, is proposed.     

Seasonal variations and trophic ecology of microzooplankton in the southeastern Arabian Sea

The seasonal ecological response of microzooplankton in the southeastern Arabian Sea is presented. During the spring intermonsoon period, stratification and depletion of nitrate in the surface waters (nitracline was at 60 m depth) cause low integrated chlorophyll a (av. 19±11.3 mg m⁻²) and primary production (av. 164±91 mgC m⁻² d⁻¹). On the other hand, nutrient enrichment associated with coastal upwelling and river influx during the onset and peak summer monsoon resulted in high integrated chlorophyll a (av. 21±6 mg m⁻² and av. 29±21 mg m⁻², respectively) and primary production (av. 255±94 mgC m⁻² d⁻¹ and av. 335±278 mgC m⁻² d⁻¹, respectively). During all three periods, diazotropic cyanobacterium Trichodesmium erythraeum dominated in the nutrient depleted surface waters. A general increase in abundance of larger diatoms was evident in the surface waters of the inshore region during monsoon periods. The microzooplankton abundance was found to be significantly higher during the spring intermonsoon (av.241±113×10³ ind m⁻²) as compared to onset of summer monsoon (av. 105±89×10³ ind m⁻²) and peak summer monsoon (av.185±175×10³ ind m⁻²). Microzooplankton community during the spring intermonsoon was numerically dominated by ciliates while heterotrophic dinoflagellate was the dominant ones during the monsoon periods. The high abundance of ciliates during the spring intermonsoon could be attributed to the stratified environmental condition prevailed in the study area which favors high abundance of smaller phytoplankton and cyanobacteria, the most preferred food of ciliates. On the other hand, the dominance of heterotrophic dinoflagellates during the monsoon periods could be linked to their ability to graze larger diatoms which were abundant during the monsoon periods. The overall results show low abundance of microzooplankton in the eastern Arabian Sea during the monsoon periods mainly due to a decline in ciliates abundance. This decline during the monsoon periods could be the result of (a) low abundance of smaller phytoplankton and (b) high stock of mesozooplankton predators (av. 245 ml 100 m⁻³).     

Relationship between auroral oval poleward boundary intensifications and magnetic field variations in the solar wind

As a rule, bright auroral arcs evolve near the poleward boundary of the auroral oval at the growth phase of a substorm, a phenomenon that is known to occur near the poleward edge of the auroral oval. The closeness of these arcs to the projection of the magnetic separatrix on the night side suggests that their generation is related to magnetic reconnection in the magnetospheric tail in a particular way. In this study this suggestion is confirmed by the fact that integral brightness of the auroral oval at the poleward edge correlates with magnetic field structures in the solar wind that are observed by ACE and Wind satellites at distances of 50–300 R_E upstream and are shifted towards the magnetospheric tail with time delays of ~ 10–80 min, consistent with measurements of the solar wind velocity. About 50 examples of this correlation have been found. The possible physical mechanisms of the effect observed are discussed.     

广西地区地磁异常与地震关系的研究

采用空间相关法计算了1987~2013年广西邕宁地震台和河池地震台的地磁Z分量日变化幅度相对海南琼中地震台的空间相关系数,分析了研究时段内空间相关曲线异常变化特征与广西及北部湾海域中强地震的关系。结果表明,广西及北部湾海域中强地震通常发生在邕宁地震台和河池地震台的空间相关曲线同步出现低值异常持续过程中或异常结束后3个月内,且低值异常持续时间超过1个月、异常最小幅值≤0.9。预报效能检验结果显示,该方法对广西地区M≥4.4地震具有较好的映震效果,可为该地区短临地震趋势分析提供定量判据。     

满洲里地磁台墩差与仪器差检验

讨论了满洲里地磁台墩差与仪器差检验的必要性,实施的方法步骤及所取得的成果,并给出了D,I,F,H,Z等要素的仪器差及墩差的校正公式.文章明确指出,凡需要更换仪器或墩位的台站,在此前后均须进行仪器对比观测和墩差测定,以保证观测数据的长期衔接.文章首次指出,为监视记录仪、记录墩的长期稳定性,一个标准地磁台不仅要测定绝对观测室内各墩位间的墩差,还须测定记录墩和观测墩之间的墩差.     

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.     

Problem of the nature of the sunrise effect in diurnal variations in the electric field in Kamchatka: 1. Time variations in the electric field

The effect of sunrise in time variations in the electric field in the near-Earth atmosphere at the Kamchatka Paratunka observatory has been studied. Twenty-nine records under fair-weather conditions have been selected. It has been indicated that the estimated effect parameters—the times of the effect’s onset and field strength maximum relative to the sunrise time, as well as the ratio of the strength maximum to its value before sunrise and the effect duration—coincide with the previously published data. Thereby, the conclusion is confirmed that the sunrise effect in diurnal variations in the electric field in the near-Earth atmosphere is related to the turbulence and convection processes in the atmospheric boundary layer at a change in atmospheric temperature.     

Geomagnetic variations for the interval 7000–25,000 yr B.P. as recorded in a core of sediment from station 1474 of the Black Sea cruise of “Atlantis II”

Palaeomagnetic measurements have been carried out on 474 specimens taken at about 2.5-cm intervals along an 11 m long piston core.The magnetic inclination exhibits variations with a period of approximately 2800 yr deduced from five radiocarbon age determinations along the core. The assumption that this period was regular allows “magnetic” ages to be assigned to the core and when these are plotted against depth, two straight lines are obtained indicative of steady rates of deposition of 0.4 mm yr^?1 since about 15,000 yr B.P. and 0.8 mm yr^?1 before then. The radiocarbon ages, except that at 16,800 yr, fit these lines suggesting that the assumption that the magnetic inclination fluctuated with regular period is justified. These swings correlate well, but on a two to one basis, with swings in inclination described for core V10-58 from the Aegean Sea (N.D. Opdyke, D. Ninkovich, W. Lowrie and J.D. Hays, 1972). The extra detail carried by the Black Sea core is possibly explained by the faster rate of deposition.The close correspondence of the periodity of these inclination swings with that observed in declination at Lake Windermere (F.H. Mackereth, 1971) is noted and the possibility that they are both caused by oscillations in intensity of the same geomagnetically active region in the core is discussed.The ratio of NRM intensity to susceptibility varies between about 0.4 and 25 within the banded lutites. This range of values may reflect different magnetic mineralogy in the two types of band, of which there are typically 3–7 per specimen, as well as variations in geomagnetic field intensity.