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251.
P. S. Cally † A. D. Crouch D. C. Braun 《Monthly notices of the Royal Astronomical Society》2003,346(2):381-389
Long-standing observations of incoming and outgoing f- and p-modes in annuli around sunspots reveal that the spots partially absorb and substantially shift the phase of waves incident upon them. The commonly favoured absorption mechanism is partial conversion to slow magneto-acoustic waves that disappear into the solar interior channelled by the magnetic field of the sunspot. However, up until now, only f-mode absorption could be accounted for quantitatively by this means. Based on vertical magnetic field models, the absorption of p-modes was insufficient. In this paper, we use the new calculations of Crouch & Cally for inclined fields, and a simplified model of the interaction between spot interior and exterior. We find excellent agreement with phase shift data assuming field angles from the vertical in excess of 30° and Alfvén/acoustic equipartition depths of around 600–800 km. The absorption of f-modes produced by such models is considerably larger than is observed, but consistent with numerical simulations. On the other hand, p-mode absorption is generally consistent with observed values, up to some moderate frequency dependent on radial order. Thereafter, it is too large, assuming absorbing regions comparable in size to the inferred phase-shifting region. The excess absorption produced by the models is in stark contrast with previous calculations based on a vertical magnetic field, and is probably due to finite mode lifetimes and excess emission in acoustic glories. The excellent agreement of phase shift predictions with observational data allows some degree of probing of subsurface field strengths, and opens up the possibility of more accurate inversions using improved models. Most importantly, though, we have confirmed that slow mode conversion is a viable, and indeed the likely, cause of the observed absorption and phase shifts. 相似文献
252.
253.
Three-Dimensional Electromagnetic Modelling and Inversion from Theory to Application 总被引:12,自引:1,他引:11
Dmitry?B.?Avdeevdavdeev@cp.dias.ie" title="d.avdeev@mtu-net.ru davdeev@cp.dias.ie" itemprop="email" data-track="click" data-track-action="Email author" data-track-label="">Email author 《Surveys in Geophysics》2005,26(6):767-799
The whole subject of three-dimensional (3-D) electromagnetic (EM) modelling and inversion has experienced a tremendous progress
in the last decade. Accordingly there is an increased need for reviewing the recent, and not so recent, achievements in the
field. In the first part of this review paper I consider the finite-difference, finite-element and integral equation approaches
that are presently applied for the rigorous numerical solution of fully 3-D EM forward problems. I mention the merits and
drawbacks of these approaches, and focus on the most essential aspects of numerical implementations, such as preconditioning
and solving the resulting systems of linear equations. I refer to some of the most advanced, state-of-the-art, solvers that
are today available for such important geophysical applications as induction logging, airborne and controlled-source EM, magnetotellurics,
and global induction studies. Then, in the second part of the paper, I review some of the methods that are commonly used to
solve 3-D EM inverse problems and analyse current implementations of the methods available. In particular, I also address
the important aspects of nonlinear Newton-type optimisation techniques and computation of gradients and sensitivities associated
with these problems. 相似文献
254.
Having analyzed high-resolution observations of extragalactic compact radio sources with modern systems of radio telescopes, we obtained an estimate of the upper limit for the photon electric charge, eγ ? 3×10?33 of the elementary charge (assuming the photon charge to be energy independent). This is three orders of magnitude better than the limit obtained from radio pulsar timing. We also set a limit on the charge of a gamma-ray (energy ~0.1 MeV) photon. In the future, the estimate based on extragalactic sources can be improved significantly. 相似文献
255.
中国人口迁移的区域差异与流场特征 总被引:40,自引:4,他引:36
通过2000年人口普查省际迁移数据的分析,揭示了当代中国人口迁移的区域分异性,以及不同原因的流场新模式。研究发现,中国人口迁移进入高活性、高能力的新阶段;东中西部人口迁移的不平衡性不断加剧,“中部塌陷”不仅是经济意义上的,而且也是社会意义上的;人口省际迁移的辐合流场与辐散流场在进一步发展,珠江三角洲是最大的迁移辐合中心;东北与山东的对流渐趋消失,西北取代东北成为非沿海区域新的人口引力中心;以秦岭-淮河线东段和黑河-腾冲线南段为界,人口迁移分裂为东南和西北两大“流域”。就业迁移的优势程度在进一步加大,市场取代计划成为人口和人才流动的第一动力;婚姻迁移的主流方向是西南贫困山区指向华东农村,形成了西南“喀斯特新娘输出区”。 相似文献
256.
Makarov V. I. Stoyanova M. N. Sivaraman K. R. 《Journal of Astrophysics and Astronomy》1982,3(4):379-382
Ring-like filaments have been detected on the spectroheliograms in the H-alpha line. Inside these filaments the magnetic field
flux has a predominant polarity. Some of the dark filaments are connected by filament channels which can be seen at the limb
either as (a) weak prominences or (b) dense low chromospheric features or (c) multi-channel system of matter flow between
two prominences or (d) common quiescent prominences. The filament and the filament channel together form a continuous closed
contour and outline the region of thef polarity particularly at the beginning of the solar cycle. The change in sign of the polar field of the Sun is associated
with the drift of the filament band to high latitudes. 相似文献
257.
Spatial statistics of clustered data 总被引:7,自引:7,他引:0
J. L.?KovitzEmail author G.?Christakos 《Stochastic Environmental Research and Risk Assessment (SERRA)》2004,18(3):147-166
Modern spatial statistics techniques are widely used to make predictions for natural processes that are continuously distributed over some convex domain. Implementation of these techniques often relies on the adequate estimation of certain spatial correlation functions such as the covariance and the variogram from the data sets available. This work studies the practical estimation of such spatial correlation functions in the case of clustered data. The coefficient of variation of the dimensionless spatial density of the point pattern of sample locations is suggested as a useful metric for degree of clusteredness of the clustered data set. We show that the common variogram estimator becomes increasingly unreliable with increasing coefficient of variation of the dimensionless spatial density of the point pattern of sample locations. Moreover, we present a modified form of the variogram estimator that incorporates declustering weights, and propose a scheme for estimating the declustering weights based on zones of proximity. Finally, insight is gained in terms of a numerical application of the common and modified methods on piezometric head data collected over an irregular network.Acknowledgments. This work has been supported by grants from the National Institute of Environmental Health Sciences (P42 ES05948-02), the Army Research Office (DAAG55-98-1-0289), and the National Aeronautics and Space Administration (60-00RFQ041). Some of the calculations conducted in support of this work were done on the SGI Origin 2400 at the North Carolina Supercomputing Center, RTP, NC. 相似文献
258.
259.
H.M. Wiechen 《Planetary and Space Science》2006,54(1):71-77
In partially ionized dusty space plasmas collisional momentum transfer between neutral and charged components of different inertia yields significant self-induced magnetic fields on remarkable short time scales of several dust Alfven times. Considering the proto-solar accretion disk previous first self-consistent plasma-neutral gas-dust simulations have shown, that this process yields a self-magnetization of 10-5- on time scales of about 100 days. Thus, this mechanism is able to explain the remanent magnetization of chondrite type meteorite matter. New simulations show the quantitative dependence of the self-magnetization on polarity and charge numbers of the dust grains. 相似文献
260.
南北地震带北段近期地壳变形特征研究 总被引:3,自引:0,他引:3
利用1999~2007期和2007~2009期GPS速度场、应变率场、断层滑动速率和GPS速度剖面等数据,识别出了南北地震带北段主要断裂带及其邻近地区的显著变形差异。结果表明,庄浪河断裂带左旋剪切变形明显增强,祁连地块北边界左旋剪切变形明显增强;2007~2009期六盘山断裂和海原断裂带挤压变形也有所增强,但量值不大。上述特征表明,庄浪河以东地区存在较高的应变积累背景。 相似文献