Tidal currents and residual currents at Norderney,Elbe 1, and Aussen-Eider lightships and their variation under the influence of prevailing winds

Summary Four hourly current-and wind observations during the years 1924–1927 at the German lightvessels Norderney, Elbe 1, and Aussen-Eider were subjected to harmonic analysis with emphasis on the influence of the wind on the residual as well as on the tidal current. The tidal current is strongest at Elbe 1 and weakest at Aussen-Eider. The half-monthly inequality of the current is strongly influenced by a ₂ tidal component. Wind influences the velocity, phase and duration of ebb-and flow current in a systematic way at Norderney and Elbe 1. Deviations from the mean tidal current are caused mainly by the change in wind direction rather than by wind velocity. The mean residual current is weak at the three stations. But wind driven currents have a velocity up to 5 times as great as the mean residual current and reverse their direction with the wind. The annual variation of the mean residual current, however, is caused only to a small part by the annual wind variation.Abbreviations used in this paper Gr. M. Tr. Greenwich moon transit, i.e. Greenwich civil time of the upper or lower transit of the moon through the meridian of Greenwich - C _n computed tidal current at M1/2Hn - C _n ^m computed mean tidal current at M1/2Hn - M _n Moon-half hour mean, i.e. mean of all current velocities observed during M1/2Hn - M.A. Moon age of an observation, true Greenwich time of Gr.M.Tr. directly preceeding the time of observation, expressed in 12 integral numbers, each representing M.A. falling in 12 different hourly intervals - M1/2H Moon-half hour, 1/2 of the interval between one moon transit and the next, i.e. 1/24 of 12h25m - R _n ^o ,R _n ^' ,R _n ^" residual current computed by harmonic analysis ofn M1/2H means of the mean current, the current at weak winds, and the current at strong winds respectively - d.o.f. degrees of freedom - standard deviation ofC _n fromM _n - ^* mean standard deviation ofC _n fromM _n for analysis with weighted means - A _o Standard error of the residual currentA _o - AB standard error of the harmonic coefficientsA ₁,B ₁,A ₂,B ₂,... - S ₂ Phase of the current componentS ₂     

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g^0₁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.     

Secular variations ing 1 0 component of geomagnetic field and its origin

The secular variations (SV) ing ₁ ⁰ of geomagnetic field, caused by the interaction between the geomagnetic field and the radial fluid-flow of the outer-core, are discussed with both statistical and analytic methods. When the value ofg ₁ ⁰ s ₂ ⁰ is relatively high compared with other terms, SV ing ₁ ⁰ is characterized by exponential change. When the effect of non-dipole field is notable, SV ing ₁ ⁰ shows complex features. Especially when the value of g ₁ ⁰ is close to zero, SV ing ₁ ⁰ is completely determined by the random process of non-dipole fields. Project supported by the National Natural Science Foundation of China (Grant No. 49429405) and by the Chinese Academy of Sciences.     

1994年台湾海峡7.3级地震与仑坪台、泉州台地磁变化的关系

地磁月均值的长期变化结果表明，1994年台湾海峡7．3级地震前4年，泉州台与仑坪台的X和Z分量均出现异常，X分量呈现负异常，Z分量呈现正异常，异常幅度达20－30nT.另外，地震前一年半，泉州台与通海台的视深比或仑坪台与通海台的视深比（周期为24h)都出现过先降后升的异常变化。     

Sq指数的周期变化分析

利用1960-1980年中国北京和广州的地磁场X分量小时值数据,根据徐文耀(1992)提出的用来描述每日Sq变化幅度的地磁活动指数的方法,计算并分析了Asq指数的周期变化特征.结果表明:Asq指数具有11年、年和半年变化等主要周期成分,与F107指数傅氏谱主要周期成分存在对应关系,表明Asq指数的周期变化与太阳辐射密切相关.然而互相关分析表明,日变幅dSq与F107的相关关系略强,这是由于Asq指数计算中每月平均ΔS_q(t)携带了部分Sq场的周期变化信息.     

Uncertainties in field-line tracing in the magnetosphere. Part II: the complete internal geomagnetic field

The discussion in the preceding paper is restricted to the uncertainties in magnetic-field-iine tracing in the magnetosphere resulting from published standard errors in the spherical harmonic coefficients that define the axisymmetric part of the internal geomagnetic field (i.e. g_n⁰ ± g_n⁰). Numerical estimates of these uncertainties based on an analytic equation for axisymmetric field lines are in excellent agreement with independent computational estimates based on stepwise numerical integration along magnetic field lines. This comparison confirms the accuracy of the computer program used in the present paper to estimate the uncertainties in magnetic-field-line tracing that arise from published standard errors in the full set of spherical harmonic coefficients, which define the complete (non-axisymmetric) internal geomagnetic field (i.e. g_n^m ± g_n^m and h_n^m ± h_n^m). An algorithm is formulated that greatly reduces the computing time required to estimate these uncertainties in magnetic-field-line tracing. The validity of this algorithm is checked numerically for both the axisymmetric part of the internal geomagnetic field in the general case (1 n 10) and the complete internal geomagnetic field in a restrictive case (0 m n, 1 n 3). On this basis it is assumed that the algorithm can be used with confidence in those cases for which the computing time would otherwise be prohibitively long. For the complete internal geomagnetic field, the maximum characteristic uncertainty in the geocentric distance of a field line that crosses the geomagnetic equator at a nominal dipolar distance of 2 R_E is typically 100 km. The corresponding characteristic uncertainty for a field line that crosses the geomagnetic equator at a nominal dipolar distance of 6 R_E is typically 500 km. Histograms and scatter plots showing the characteristic uncertainties associated with magnetic-field-line tracing in the magnetosphere are presented for a range of illustrative examples. Finally, estimates are given for the maximum uncertainties in the locations of the conjugate points of selected geophysical observatories. Numerical estimates of the uncertainties in magnetic-field-line tracing in the magnetosphere, including the associated uncertainties in thelocations of the conjugate points of geophysical observatories, should be regarded as first approximations in the sense that these estimates are only as accurate as the published standard errors in the full set of spherical harmomic coefficients. As in the preceding paper, howerver, all computational techniques developed in this paper can be used to derive more realistic estimates of the uncertainties in magnetic-field-line tracing in the magnetosphere, following further progress in the determination of more accurate standard errors in the spherical harmonic coefficients.Also Visiting Reader in Physics, University of Sussex, Palmer, Brighton, BN1 9QH, UK     

The upper mantle conductivity analysis method using observatory records of the geomagnetic field

The electrical conductivity of the Earth's upper mantle can be inferred from geomagnetic quiet-day,Sq, variations recorded at the world's observatories using the, coefficients of a spherical harmonic analysis (SHA) that separate the external (source) and internal (induced) parts of the surface field. The conductivity profile determined from such an analysis can be sensitive to special characteristics of the quiet field itself as well as the separation techniques employed. This review of the Sq-analysis features critical to a conductivity derivation is pictorially presented along with the equations for application of theSchmucker (1970) technique to theSHA coefficients for a conductivity determination. Three examples illustrate the use of these equations with differentSq models.     

On the variation of temperature structure parameter in cloud- and clear-air during the summer monsoon season

High resolution aicraft observations of temperature were made in cloud-air and clear-air at different heights in the lower atmosphere over the Deccan Plateu, India, during the summer monsoon season (June–September) of 1976. Temperature fluctuations in the horizontal at each flight level were utilised for computing the temperature structure parameter (C _T ² ) at that level for studying the hieght dependence ofC _T ² . The results were found to be in agreement with those obtained by other investigators. Also, the height variations ofC _T ² in cloud-air and clear-air during active and weak monsoon conditions are studied. Marked differences were noticed in the behaviour ofC _T ² during active and weak monsoon conditions.     

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.     

Studying the dynamics of the lunar daily geomagnetic variations

Results of studying the lunar daily geomagnetic variations in the spectral and time regions at the network of observatories are presented. The seasonal variations in the amplitudes of the fundamental harmonic constituents of three lunar variation components have been revealed. The seasonal time variations have been analyzed using the digital bandpass filtering and harmonic synthesis based on the data of the Kakioka and Memambetsu geomagnetic observatories. The 11-year solar cycle and annual and semiannual periods have been distinguished in the seasonal variation spectrum. Studying the spectral singularities of the lunar daily variation at these observatories and the sea level variations in daytime and nighttime hours has made it possible to identify the contribution of the oceanic dynamo to the lunar variation vertical component.     

Attenuation and dispersion ofSH waves due to scattering by randomly distributed cracks

The effect of randomly distributed cracks on the attenuation and dispersion ofSH waves is theoretically studied. If earthquake ruptures are caused by sudden coalescence of preexisting cracks, it will be crucial for earthquake prediction to monitor the temporal variation of the crack distribution. Our aim is to investigate how the property of crack distribution is reflected in the attenuation and dispersion of elastic waves.We introduce the stochastic property, in the mathematical analysis, for the distributions of crack location, crack size and crack orientation. The crack size distribution is assumed to be described by a power law probability density (p(a) a ^– fora _minaa _max according to recent seismological and experimental knowledge, wherea is a half crack length and the range 13 is assumed. The distribution of crack location is assumed to be homogeneous for the sake of mathematical simplicity, and a low crack density is assumed. The stochastic property of each crack is assumed to be independent of that of the other cracks. We assume two models, that is, the aligned crack model and the randomly oriented crack model, for the distribution of crack orientation. All cracks are assumed to be aligned in the former model. The orientation of each crack is assumed to be random in the latter model, and the homogeneous distribution is assumed for the crack orientation. The idea of the mean wave formalism is employed in the analysis, and Foldy's approximation is assumed.We observe the following features common to both the aligned crack model and the randomly oriented crack model. The attenuation coefficientQ ^–1 decays in proportion tok ^–1 in the high frequency range and its growth is proportional tok ² in the low frequency range, wherek is the intrinsic wave number. This asymptotic behavior is parameter-independent, too. The attenuation coefficientQ ^–1 has a broader peak as increases and/ora _min/a _max decreases. The nondimensional peak wave numberk _p a _max at whichQ ^–1 takes the peak value is almost independent ofa _min/a _max for =1 and 2 while it considerably depends ona _min/a _max for =3. The phase velocity is almost independent ofk in the rangeka _max<1 and increases monotonically ask increases in the rangeka _max>1. While the magnitude ofQ ^–1 and the phase velocity considerably depend on the orientation of the crack in the aligned crack model, the above feature does not depend on the crack orientation.The accumulation of seismological measurements suggests thatQ ^–1 ofS waves has a peak at around 0.5 Hz. If this observation is combined with our theoretical results onk _p a _max, the probable range ofa _max of the crack distribution in the earth can be estimated for =1 or 2. If we assume 4 km/sec as theS wave velocity of the matrix medium,a _max is estimated to range from 2 to 5 km. We cannot estimatea _max in a narrow range for =3.     

A preliminary study on variations in the Gauss coefficients of the geomagnetic potential over several hundred years

Geomagnetic secular variations are examined in terms of time variation in the Gauss coefficients. Major parts of the variations over several hundred years can be represented by a two mode model which consists of a standing and a drifting field. When the Gauss coefficients are plotted on a diagram with g_n^m in the abscissa and h_n^m in the ordinate, the drifting component describes a circle. However, some of the observed coefficients depict an elliptical trajectory rather than a circular one. Improvement of the model is attempted in two different ways. One is to assume time variability of the amplitude of the drifting component. The other is to introduce another drifting mode. Selecting a few spherical harmonic terms, variations in the Gauss coefficients since A.D. 1600 are analysed.When the amplitude of the drifting field is assumed to vary, the observed nature of the elliptical trajectory is well represented. In this case, phase velocity also changes with time. It is large while the amplitude is small, and it is small while the amplitude is large. Three mode models, in which an eastward drifting mode is incorporated, approximate the observed variations as well, not only for the period over several hundred years but also for the last several decades. In this model the westward drifting mode dominates the eastward mode.     

Geomagnetic diurnal-variation anomalies and their relation to strong earthquakes

The diurnal-variation anomalies of the vertical-component in geomagnetic field are mainly the changes of phase and amplitude before strong earthquakes. On the basis of data recorded by the network of geomagnetic observatories in China for many years, the anomalous features of 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 over 30 strong earthquakes with M _S≥6.6 such as Kunlunshan M _S=8.1 earthquake on November 14, 2001; Bachu-Jashi M _S=6.8 earthquake on February 24, 2003; Xiaojin M _S=6.6 earthquake on September 22, 1989, etc. There are good relations between such rare phenomena of geomagnetic anomalies and the occurrence of earthquakes. It has been found that most earthquakes occur in the vicinity of the boundary line of sudden change of the low-point displacement and generally within four days before and after the 27th or 41st day counting from the day of appearance of the anomaly. In addition, the anomalies of diurnal-variation amplitude near the epicentral area have been also studied before Kunlunshan M _S=8.1 earthquake and Bachu-Jiashi M _S=6.8 earthquake. Foundation item: National Science Technology Tackle Key Project during the Tenth Five-year Plan (2001BA601B01-05-04)     

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.     

Variations in the schumann resonance polarization ellipse in the horizontal and vertical planes according to observations at the Barentsburg and Lovozero observatories

The diurnal variations in the amplitude and parameters of polarization ellipses of the first Schumann resonance according to three magnetic field components, observed on December 1–10, 2007, at the Barentsburg and Lovozero observatories, have been studied. Ellipses have been constructed in the (H, D) and (H, Z) planes. The value of the minor axis, inclination of the major axis, ellipticity, and the rotation direction have been estimated. The vertical magnetic component of the Schumann resonance is three to four times as small as the horizontal component. The difference in the diurnal variations in the ellipse parameters between both observatories has been found. The effect can be caused by a difference in the Earth’s conductivity in the vicinity of the observatories and by conductivity anisotropy. The major axis inclination and ellipticity have semidiurnal components. The polarization vector mostly rotates from D to H in the horizontal plane and from Z to H in the vertical plane at both stations.     

The diurnal and semidiurnal barometric oscillations global distribution and annual variation

Summary The global distributions of the annual and seasonal means of the diurnal (S ₁) and semidiurnal (S ₂) surface pressure oscillations are investigated by spherical harmonic analysis. The main waves are,S ₁ ¹ (with wave number 1) forS ₁ andS ₂ ² forS ₂.S ₁ ¹ is much less predominant among the waves ofS ₁ thanS ₂ ² among those ofS ₂. As in the case of the lunar semidiurnal barometric tideL ₂ the pressure maxima occur earlier in the Southern than in the Northern Hemisphere. In the case ofS ₂ the standing waveS ₂ ⁰ and the waveS ₂ ³ are also of interest besidesS ₂ ². Although the present analysis extends only from 60°N to 60°S, whileS ₂ ⁰ is largest at polar latitudes, its results show thatS ₂ ⁰ should be smaller at high southerly than at high northerly latitudes, as has been observed. Thus this observed asymmetrical distribution ofS ₂ ⁰ may be due to causes outside the polar regions rather than to their geographical differences. The best approximation to the observed distribution ofS ₂ ⁰ is obtained by including a mode representing an oscillation independent of longitude and latitude indicating a small semidiurnal variation of the mean global surface presure, which is an unlikely result on physical grounds.The seasonal variation ofS ₁ ¹ expressed in percent of the annual mean is smaller than that ofS ₂ ², and both are less than the unexplained seasonal variation ofL ₂ ².The main wavesS ₁ ¹ andS ₂ ² are expressed not only by associated Legendre functions, but also by Hough functions.National Center for Atmospheric Research, Boulder, Colorado, U.S.A., sponsored by the National Science Foundation.     

On the periodic variations of geomagnetic activity indices Ap and ap

Yearly averages of geomagnetic activity indices Ap for the years 1967–1984 are compared to the respective averages of v² · B_s, where v is the solar wind velocity and B_s is the southward interplanetary magnetic field (IMF) component. The correlation of both quantities is known to be rather good. Comparing the averages of Ap with v² and B_s separately we find that, during the declining phase of the solar cycle, v² and during the ascending phase B_s have more influence on Ap. According to this observation (using Fourier spectral analysis) the semiannual and 27 days, Ap variations for the years 1932–1993 were analysed separately for years before and after sunspot minima. Only those time-intervals before sunspot minima with a significant 27-day recurrent period of the IMF sector structure and those intervals after sunspot minima with a significant 28–28.5-day recurrent period of the sector structure were used. The averaged spectra of the two Ap data sets clearly show a period of 27 days before and a period of 28–29 days after sunspot minimum. Moreover, the phase of the average semiannual wave of Ap is significantly different for the two groups of data: the Ap variation maximizes near the equinoxes during the declining phase of the sunspot cycle and near the beginning of April and October during the ascending phase of the sunspot cycle, as predicted by the Russell-McPherron (R-M) mechanism. Analysing the daily variation of ap in an analogue manner, the same equinoctial and R-M mechanisms are seen, suggesting that during phases of the solar cycle, when ap depends more on the IMF-B_s component, the R-M mechanism is predominant, whereas during phases when ap increases as v increases the equinoctial mechanism is more likely to be effective.     

甘肃嘉峪关、瓜州地电场日变化与地电暴差异机理分析

通过甘肃省嘉峪关台地磁场观测资料,研究嘉峪关台、瓜州台磁静日地电场日变化的时频特征波;由地电场分钟值观测数据的时序叠加残差方法,研究嘉峪关、瓜州山的地电暴变化。结果表明:(1)两台地电场静日变化以两次起伏变化为主,无相位差,但两台之间日变幅差异较大;(2)地电场分量变化与地磁场正交分量变化显著相关;地电场与地磁场日变波形不同,极值时间有差异。2个台存在很明显的高频成分,在去除了高频变化后,其优势周期也相同,从大到小依次为12 h、8 h、24 h。地磁场H分量因存在磁暴影响,故高频变化较多,在去除了磁暴影响后,其优势周期从大到小依次为24 h、12 h、8 h;(3)当电磁暴扰动剧烈时,两台可以较清晰地记录到地电暴的完整变化。在发生电磁暴时,地电场与地磁场的相关性明显降低,且不同台、不同测向之间的变化幅度也不尽相同。两台东分量E_Y暴日的日变幅较静日明显增大,磁暴期间Y分量变化率与地电场东分量E_Y观测数据显著相关,由此说明:两台日变幅的不同与台站台址电导率有关,太阳风引起的电离层活动是引起了地电场日变化主因。引起电暴的原因可能不同于引起日变化的原因,主要是两台之间及不同测向之间的浅、深层电阻率和地质构造等诸多因素的结果。     

Day-time variations of the equivalent electron concentration in the non-sounded lower ionosphere

Summary Based on data on the lowest reflected frequencyf _min and on information on the lower and upper boundaries of the non-sounded lower ionosphere, an equivalent electron concentration for all concentrations below the correspondingf _min was determined. Day-time variations of the equivalent concentration are investigated, confirming that there is a cosine relation to the solar zenith angle. The power index of that relation has an outlined seasonal course with a maximum in April and October, while the absolute seasonal minimum is during the winter (the summer minimum is slightly outlined). The mean yearly values of the index are almost constant:n _N 0.5 for solaractivity,I ₁₅₀₀ to 115.10^–22 W Hz^–1 m². During higher activityn _N changes correspondingly toI ₁₅₀₀ according to relation (12). The variations ofn _N during high solar activity show that the altitude gradient and temperature gradient in the low ionosphere are becoming proportional toI ₁₅₀₀ when the solar x-ray radiation exceeds a certain level. The results obtained confirm the reliability of the method developed for employingf _min in aeronomic investigations.     

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.