Geomagnetic secular variation and secular acceleration in the Red Sea area

Summary. Estimates of the secular variation in the Red Sea over the period 1959 to 1972 have been obtained from an analysis of marine magnetic data. A total of 318 crossings of ships' tracks were used to determine the mean secular variation for the intervals 1959–72, 1959–65 and 1965–72. The mean secular variation 1959–72 shows a marked northward increase from approximately -10 nT/yr at 13°N to +27 nT/yr at 24°N. North of this, the data suggest a small decrease to + 25 nT/yr at 27° N. These values are consistent with the secular variation recorded at the nearby geomagnetic observatory at Helwan, Egypt, but less than those predicted by the 1965 IGRF for the same period.
Comparison of the mean secular variations for 1959–65 and 1965–72 yields a rough estimate of the secular acceleration of - 1.5 nT yr⁻². Analysis of the cross-over information, corrected for the latitude dependence of the secular variation, shows a regular decrease in the secular change over the period 1959–72 at all latitudes of about -1 nTyr⁻². This secular acceleration makes a substantial contribution to the overall secular change in the Red Sea and as such must be included in the correction of magnetic data covering more than a few years.     

Is the Earth contracting?

The fossil record of the variation of the solar day and the synodic month with geological time is examined for evidence of the steady contraction of the Earth postulated by Lyttleton to explain a discrepancy between the apparent secular accelerations of the Sun and Moon. Data for the Phanerozoic and the Precambrian agree in showing that a change in the Earth's moment of inertia as large as that suggested by Lyttleton is only consistent with the fossil record if the secular change in the gravitational constant Ġ/ G ≥+ 4 × 10^–11/yr. A variation of G of this magnitude appears to be ruled out by a recent analysis of lunar occultation observations utilizing Atomic Time.     

A geomagnetic secular variation study (31000–19 500 bp) in Western Canada

Summary. Palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern British Columbia are reported. Radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp. No evidence for any large geomagnetic excursions (such as the so-called Mono Lake Excursion) is found, but a distinctive pattern of 'normal' secular variation is observed with declination and inclination swings of 45° and 25° peak to peak amplitude respectively. For the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude, but three relatively large perturbations occur at approximately 4000 yr intervals. These perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non-isotropic secular variation model proposed by Cox. The bias involved is about 6° in declination and 3° in inclination (overall mean D = 5.8°E, I = 64.2°, α95 = 0.9°, N = 212 horizons), which leads to a pole which is both 'far-sided' and 'right-handed'. If the horizons involved in the three major perturbations are eliminated the mean direction ( D = 1.2°E, I = 67.2°, α95 = 0.8°, N = 125 horizons) does not differ from that of a geocentric axial dipole despite the small cone of confidence.     

Monopoly

Summary. A model for the geomagnetic secular variation field is given, consisting of a series of magnetic monopoles at the surface of the Earth's core. These are distributed according to the density of the data to allow more detailed representation in areas where the density of observations is high, without introducing spurious detail where data are sparse. A monopole model is calculated from observatory secular change data for the epoch 1957.5–1962.5 and its usefulness assessed.     

Investigation of geomagnetic-field secular variation around the Sverdlovsk magnetic observatory

summary . The magnetic observatories at Vysokaya Dubrava and at Arti in Sverdlovsk region have operated in parallel since 1972 January. In spite of the rather small distance between them (160 km), it has been established that during that time, there have been differences in the geomagnetic field variations over a wide range of frequencies. The secular variation also differs appreciably. Over a period of four years the difference in the horizontal component between the observatories increased by 3 nT, the difference in the vertical component by 4 nT and the difference in the total intensity by 6 nT, while the difference in declination decreased by 0.6 min. Besides the normal geomagnetic secular variation, governed by processes in the Earth's core, secular-variation anomalies (SVA) linked with processes in the lithosphere may also take place. The intensity of the local SVAs reaches 10–2 nT/yr, whilst their size does not exceed 15 km. It is undesirable to locate an observatory in a region where an SVA exists since this will distort the normal secular-Variation pattern. It is therefore important to know to what extent observations at the observatory reflect the mean value of the secular variation for that area. For this purpose we would recommend performing a highly accurate survey, for example the total intensity in a network of radial lines in a region around the observatory of radius 50–100 km. Such investigations have been performed around the Arti observatory with an accuracy of the repeated measurements of ±0.3 nT in 1974 and ±0.2 nT in 1975. Ten local SVAs with intensities from 2–14 nT/yr were revealed around the observatory. The observatory of Arti is situated in a quiet region.     

Time series analysis of the 10 000 yr geomagnetic secular variation record from SE Australia

Summary. A 10 000 yr continuous secular variation record from intensively dated lake sediments in SE Australia has been subjected to periodogram and maximum entropy method analysis. Tests on synthetic data reveal some of the limitations of the latter method, particularly when applied to complex number series. Anticlockwise precession of the magnetic vector at a period of 5000 ± 1000 yr is tentatively ascribed to dipole precession, and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non-dipole field.
The effect of calibrating the radiocarbon time-scale is important and results in periodicity shifts of up to 25 per cent. Even for well-dated lacustrine sequences power spectra are poorly constrained: it is thus possible that the geomagnetic secular variation on a time-scale of thousands of years is more uniform than often supposed. Mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed.     

Palaeosecular Variation of the Geomagnetic Field in the Aleutian Islands, Alaska

Sequences of late Tertiary to Quaternary lava flows at six Aleutian Island sites were sampled for palaeosecular variation (PSV) measurement using palaeomagnetic methods. Due to limitations in probable time span represented by each sequence of flows and statistical constraints, data from only two sites are considered in themselves to be reliable indicators of PSV. When all the data are combined to give greater statistical significance, an angular standard deviation of palaeofield directions of = 10.8° is obtained. This is compatible with other Pacific Ocean PSV data and points to the persistence of the present secular variation low in this region at least into Quaternary time.     

Geomagnetic field analysis - I. Stochastic inversion

Summary. Most of the Earth's magnetic field and its secular change originate in the core. Provided the mantle can be treated as an electrical insulator, stochastic inversion enables surface observations to be analysed for the core field. A priori information about the variation of the field at the core boundary leads to very stringent conditions at the Earth's surface. The field models are identical with those derived from the method of harmonic splines (Shure, Parker & Backus) provided the a priori information is specified appropriately.
The method is applied to secular variation data from 106 magnetic observatories. Model predictions for fields at the Earth's surface have error estimates associated with them that appear realistic. For plausible choices of a priori information the error of the field at the core is unbounded, but integrals over patches of the core surface can have finite errors. The hypothesis that magnetic fields are frozen to the core fluid implies that certain integrals of the secular variation vanish. This idea is tested by computing the integrals and their standard and maximum errors. Most of the integrals are within one standard deviation of zero, but those over the large patches to the north and south of the magnetic equator are many times their standard error, because of the dominating influence of the decaying dipole. All integrals are well within their maximum error, indicating that it will be possible to construct core fields, consistent with frozen flux, that satisfy the observations.     

Geospatial Analysis of Space–Time Patterning of ENSO Forced Daily Precipitation Distributions in the Gulf of Mexico

The Gulf of Mexico experiences significant changes in the distribution of daily precipitation totals that are linked to the El Niño–Southern Oscillation (ENSO). This research uses geospatial techniques to examine distribution patterns of ENSO-related precipitation. Kolmogorov–Smirnov test results comparing daily rainfall distributions for El Niño and La Niña are mapped at a 1° × 1° latitude/longitude resolution, and hotspot analysis using local Moran's I is performed to identify spatial clustering. Results indicate that ENSO-forced spatial and temporal variation in daily precipitation distributions influence large areas of the Gulf of Mexico region from August through January.     

Slow-ridge/hotspot interactions from global gravity, seismic tomography and 87Sr/86Sr isotope data

Among the mantle hotspots present under oceanic areas, a large number are located on—or close to—active oceanic ridges. This is especially true in the slow-spreading Atlantic and Indian oceans. The recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models, and allow one to study interactions between mantle plumes and active slow-spreading ridges. The observed correlations allow us to draw preliminary conclusions on the general links between surficial processes, which shape the detailed morphology of the ridge axes, and deeper processes, active in the upper mantle below the ridge axial domains as a whole. The interactions are first studied at the scale of the Atlantic (the Mid-Atlantic Ridge from Iceland to Bouvet Island) from the correlation between the zero-age free-air gravity anomaly, which reflects the zero-age depth of the ridge axis, and Sr isotopic ratios of ridge axis basalts. The study is then extended to a more global scale (the slow ridges from Iceland to the Gulf of Aden) by including geoid and upper-mantle tomography models. The interactions appear complex, ranging from the effect of large and very productive plumes, almost totally overprinting the long-wavelength segmentation pattern of the ridge, to that of weaker hotspots, barely marking some of the observables in the ridge axial domain. Intermediate cases are observed, in which hotspots of medium activity (or whose activity has gradually decreased) located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle. Such observations tend to preclude the use of a single hotspot/ridge interaction model and stress the need for additional observations in various plume/ridge configurations.     

Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

The remanent magnetic properties of an 88  m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c . 720 ± 32  yr and, after 40  mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17  Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no difference in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700  yr, the same as for unsmoothed European secular variation during the last 2000  yr. The intensity of remanence, after 40  mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127 ± 3  Ma) which shows clear regularities in behaviour, some related to changes in the Earth's orbital parameters.     

Deglaciation effects on the rotation of the Earth

Summary. The viscoelastic response of the Earth to the mass displacements caused by late Pleistocene deglaciation and concomitant sea level changes is shown to be capable of producing the secular motion of the Earth's rotation pole as deduced from astronomical observations. The calculations for a viscoelastic Earth yield a secular motion in the direction of 72° W meridian which is in excellent agreement with observed values. The average Newtonian viscosity and the relaxation time obtained from polar motion data are about (1.1 ± 0.6)10²³ poise (P) and 10⁴ (1 ± 0.5) yr. The non-tidal secular acceleration of the Earth can also be attributed to the viscoelastic response to deglaciation and results in an independent viscosity estimate of 1.6 × 10²³ P with upper and lower limits of 1.1 × 10²³ and 2.8 × 10²³ P. These values are in agreement with those based on the polar drift analysis and indicate an average mantle viscosity of 1–2 × 10²³ P.     

Testing a palaeomagnetic study for the averaging of secular variation

Summary. It is shown that the interval estimate for K given by Cox (1969) is inappropriate as a test for determining whether secular variation has been averaged out in a particular study. The appropriate test is presented in two formulations, one for use with χ² tables and the other for use with F tables.
The well known secular variation models are investigated and it is shown that Model A due to Irving & Ward (1964) is, statistically, the most efficient predictor. Finally, for the 83 palaeomagnetic results selected by Brock (1971), it is shown that the most efficient predictor for the secular variation K is k '= 18.1 (1 + 3 sin²λ), where λ is the palaeolatitude, and the uncertainty in this predicted value is discussed.     

Anomalous variations of the geomagnetic field in East Fergana — magnetic precursor of the May earthquake with M= 7.0 (1978 November 2)

Summary. A secular variation anomaly has been discovered at the north-east part of the Fergana vdey by repeated measurements every year or less. The change of total field Δ F at the 'magnetic epicentre' was 9 nT in 1977 and 16 nT in 1978 relative to the level of 1973. In 1977 an anomalous region was recognized, where according to the data from 25 observation points Δ F increased in the northern part up to 5.2 nT, and decreased by 4.7nTin the southern part according to a further 22 points. Permanent observations were begun at the epicentre in 1978 October. We normally observed variations of Δ F differences with magnitude ± 2–3 nT, which were not correlated with worldwide magnetic activity. Anomalous variations appeared on October 26 and rose to a maximum value of + 23 nT on October 30. The decrease of this anomalous field began on October 31. This made it possible to predict a potential earthquake. The Alay earthquake with M = 7.0 occurred on November 2 six hours after the prediction was issued; Δ F then returned to the initial level. Thus, using the geomagnetic field variations in the Fergana region, geophysicists were able to predict the moment of a strong earthquake.     

Low-velocity zone parameters as functions of focal depth

Summary. Explicit inversion for the velocity-depth structure within a low-velocity zone (LVZ) can be made from surface observations of the variations of the travel-time intercept () and the epicentral distance extent (γ) of the shadow zone as functions of focal depth. Prior knowledge of the focal depths is not required. The observed -γ curve can be converted into the corresponding travel-time curve that crosses the shadow zone in the surface focus travel-time curve. The inversion of this travel-time curve requires the construction of the LVZ ray parameter—epicentral distance ( p- Δ) curve by the relation p-p _L= d / d γ where p _L is the p at which the LVZ occurs. Inversion is then performed by treating each layer in the LVZ as a separate LVZ.     

塔克拉玛干沙漠腹地近地面臭氧浓度变化特征及影响因素分析

利用塔克拉玛干沙漠腹地塔中地区2010年6月10日-2012年3月20日地表臭氧浓度连续自动观测数据,结合相应气象要素资料,对地表臭氧质量浓度的日、周、月、季节变化与不同天气条件下日变化特征进行了分析,同时探讨了影响臭氧浓度变化的主要因素。结果表明：①臭氧浓度日变化具有明显的单峰型日变化规律,夜间变化平缓,白天变化剧烈。09:00前后达到最低值,18:00前后达到最高值,出现时间稍迟于其他城市地区。②臭氧浓度变化具有“周末效应”现象。最高值出现在星期日,最低值出现在星期三;星期一至星期三浓度逐渐降低,星期四又逐渐上升。③最高月平均浓度出现在2010年6月,其浓度为89.6 μg·m-3,最低月平均浓度出现在2012年1月,其浓度为32.0 μg·m-3,2010年6-12月,浓度逐月降低。④春、夏季臭氧浓度较高,秋季和冬季明显低于春季和夏季,与大中型城市变化特征基本一致。⑤臭氧浓度日变化最剧烈的是晴天,其次为小雨天气,阴天日变化平缓。沙尘暴出现前,臭氧小时平均浓度变化较小,沙尘暴开始时浓度下降,且下降速度较快。⑥辐射变化也具有单峰型日变化规律,臭氧浓度变化明显晚于辐射变化,太阳辐射的强弱直接影响光化学反应速度,从而导致臭氧浓度的变化。⑦沙尘天气臭氧日平均浓度高于有间隙小雨天气和晴天。相对湿度、风速、风向、日照日数同时影响近地面臭氧浓度的变化,臭氧污染的发生是多种因素共同作用的结果。     

Geomagnetic secular variations in Greece through the last 6000 years obtained from lake sediment studies

Summary. Geomagnetic declination and inclination secular variation curves have been constructed back to 6000 yr bp from palaeomagnetic measurements of sediments cored in Lakes Volvi, Trikhonis and Begoritis, Greece. They have been dated by palynological studies and by comparison with observatory and archaeomagnetic records. Radiocarbon ages are all affected by the'old carbon'factor. Cores from all three lakes contain a thin ash layer which we associate with the Pompeii eruption of Vesuvius in ad 79. Averaged curves of secular variations of declination and inclination have been computed for the whole study. The broader scale pattern of secular variations is basically similar to lake sediment records from the UK, Poland and Switzerland, though finer detail in the structure of some of the principal features is resolved.     

Long-term features of drifting and standing non-dipole fields as determined from Holocene palaeomagnetic secular variations

Geomagnetic field motions of Holocene secular variations are investigated using a separation method. The palaeomagnetic secular variations from Britain, North America and Australia have been subjected to maximum-entropy method analyses. Based on the results of spectral analyses, the secular variations are separated by band-pass filters into low-frequency components, generally including the period band 1800-3600 yr, and high-frequency components, generally including the period band 1000-1200 yr. There is an interval, from 4200 to 1700 yr BP, which shows clockwise rotational motions in the low-frequency components of all three sites. Westward drifting of geomagnetic fields may be globally dominant. Swinging or elliptical looping motions constrained to a certain direction were observed in the low-frequency components of the British data. The time duration for the persistence of the swinging motion constrained to a certain direction was 3500 years or so, which could be the lifetime of an oscillating stationary field. The duration of the transitional motion was 1000-1300 years, which may indicate the recurrence time of a stationary field.     

The palaeomagnetism and magnetic fabric of sediments from Peak Cavern, Derbyshire

Summary. This paper describes the magnetic remanence and susceptibility anisotropy of 42 samples of laminated clay from Peak Cavern. The sediments are shown to have acquired a stable remanent magnetization due to the depositional alignment of magnetite grains. The magnetic fabric data confirm the existence of alteration within a reddened surface layer. A record of geomagnetic secular variation in the underlying clay is compared to a dated lake sediment magnetostratigraphy to suggest that deposition occurred around 7000 yr bp . A method for estimating water current directions based on the remanence and magnetic fabric data is critically assessed before deducing the sense of palaeoflow in the cave.     

中国中东部秋季PM10时空变化及其与日气温的关系

利用2000-2010年秋季中国中东部83个重点城市的PM10浓度数据以及其中63个城市的逐日气象资料,分析了PM10浓度的时空变化以及晴空条件下PM10浓度与日气温之间的关系,讨论了不同云量条件下二者关系的稳定性以及辐射的相应变化.结果表明:(1)近11年来,秋季PM10浓度呈现下降趋势,全部天气条件下和晴空条件下的线性趋势值分别为-2.87 μg·m-3/年、-4.92 μg·m-3/年;空间分布上,中国中东部重点城市的秋季PM10浓度普遍下降,其中华北地区的下降最快最显著.(2)秋季PM10浓度与日气温的波动之间存在显著相关,定量统计表明:当PM10浓度偏高10 μg·m-3时,日最高气温、日最低气温和日平均气温分别偏低0.15℃、0.14℃和0.16℃,同时气温日较差减小0.01 ℃.(3)秋季日气温的上述变化可能主要与气溶胶的直接效应有关.PM10增多会造成地面总辐射和地表净辐射的显著减少,进而造成日最高气温、日平均气温的显著下降;同时,PM10增多对近地面的影响总体上是致冷效果.