共查询到20条相似文献,搜索用时 0 毫秒
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Clinton J. Giardini D. Böse M. Ceylan S. van Driel M. Euchner F. Garcia R. F. Kedar S. Khan A. Stähler S. C. Banerdt B. Lognonne P. Beucler E. Daubar I. Drilleau M. Golombek M. Kawamura T. Knapmeyer M. Knapmeyer-Endrun B. Mimoun D. Mocquet A. Panning M. Perrin C. Teanby N. A. 《Space Science Reviews》2018,214(8):1-51
Space Science Reviews - In recent decades, volcanic and cryovolcanic activity on moons within the Solar System has been recognised as an important source of cosmic dust. Two moons, Jupiter’s... 相似文献
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Efficient coding of continuous speech signals for digital representation has attracted much interest in recent years. The underlying aim of efficient coding methods is to reduce the channel capacity required to represent a signal to meet a specific reconstruction fidelity criterion. To achieve this objective, modern speech data compression techniques rely on two very similar procedures. One procedure uses predictive deconvolution which subtracts from the current signal value that portion which can be predicted from its past and thus removes redundancy in the speech by removing sequential correlation. The signal thus requires fewer bits for equivalent quantization error. The second procedure involves identification of a complete mathematical model of the speech producing mechanism. This involves determination of the characteristics of the source that drives this transfer function. Data reduction is again achieved since the rate of change of the parameters of the speech model is much smaller than the rate of change of the speech waveform. This paper develops these data reduction procedures in terms of modern estimation theory, specifically a Kalman filter model, and illustrates the utility of this model as an analysis tool by means of an example based on a uniform tube which provides a qualitative assessment of the potential of the technique for application to real speech signals. 相似文献
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曹婧 《郑州航空工业管理学院学报(管理科学版)》2011,29(2)
人民币汇率制度选择问题一直是我国进行人民币制度建设的核心问题,其关乎着我国货币的主权和金融经济的独立和稳定,更关乎着我国内外经济的均衡和经济的增长。关于人民币汇率制度选择这一命题,国内外学者作了大量的研究,对这些学者的研究成果进行梳理,有助于我们更好地研究人民币汇率制度选择问题,从而整体把握人民币汇率制度。 相似文献
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The study of the electrical environment of the Earth's atmosphere has rapidly advanced during the past century. Great strides have been made towards the understanding of lightning and thunderstorms and in relating them to the global electric circuit. The electromagnetic fields and currents connect different parts of the Earth's environment, and any type of perturbation in one region affects another region. Starting from the traditional views in which the electrodynamics of one region has been studied in isolation from the neighboring regions, the modern theory of the global electrical circuit has been discussed briefly. Interconnection and electrodynamic coupling of various regions of the Earth's environment can be easily studied by using the global electric circuit model. Deficiencies in the model and the possibility of improvement in it have been suggested. Application of the global electric circuit model to the understanding of the Earth's changes of climate has been indicated. 相似文献
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A coupled climate model is used to explore the regional response of the climate system to solar forcing, with emphasis on
the role of the ocean. It is shown that both the transient and the equilibrium response of surface temperature to changes
in total solar irradiation is smaller over ocean than over land because of the ocean’s large heat capacity and the feedback
involving evaporation. Furthermore, the advection of temperature anomalies and changes in ocean currents have an impact on
the timing and the geographical distribution of the response. Nevertheless, at regional scales, the response to the forcing
is embedded within the large internal variability of the system making the detection and analysis of the forced response difficult.
Furthermore, this forced response could imply both changes in the mean state of the system as well as in its variability. 相似文献
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Gerard J. M. Versteegh 《Space Science Reviews》2005,120(3-4):243-286
The nature of the climatic response to solar forcing and its geographical coherence is reviewed. This information is of direct
relevance for evaluating solar forcing mechanisms and validating climate models.
Interpretation of Sun-climate relationships is hampered by difficulties in (1) translating proxy records into quantitative
climate parameters (2) obtaining accurate age assessments (3) elucidating spatial patterns and relationships (4) separating
solar forcing from other forcing mechanisms (5) lacking physical understanding of the solar forcing mechanisms. This often
limits assessment of past solar forcing of climate to identification of correlations between environmental change and solar
variability.
The noisy character and often insufficient temporal resolution of proxy records often exclude the detection of high frequency
decadal and bi-decadal cycles. However, on multi-decadal and longer time scales, notably the ∼90 years Gleisberg, and ∼200
years Suess cycles in the 10Be and 14C proxy records of solar activity are also well presented in the environmental proxy records. The additional ∼1500 years Bond
cycle may result from interference between centennial-band solar cycles.
Proxy evidence for Sun-climate relations is hardly present for Africa, South America and the marine realm; probably more due
to a lack of information than a lack of response to solar forcing. At low latitudes, equatorward movement of the ITCZ (upward
component of the Hadley cell) occurs upon a decrease in solar activity, explaining humidity changes for (1) Mesoamerica and
adjacent North and South American regions and (2) East Africa and the Indian and Chinese Monsoon systems. At middle latitudes
equatorward movement of the zonal circulation during solar minima probably (co-)induces wet and cool episodes in Western Europe,
and Terra del Fuego as well as humidity changes in Southern Africa, Australia, New Zealand and the Mediterranean. The polar
regions seem to expand during solar minima which, at least for the northern hemisphere is evident in southward extension of
the Atlantic ice cover. The forcing-induced migration of climate regimes implies that solar forcing induces a non linear response
at a given location. This complicates the assessment of Sun-climate relations and calls for nonlinear analysis of multiple
long and high resolution records at regional scale. Unfortunately nonlinear Sun-climate analysis is still a largely barren
field, despite the fact that major global climate configurations (e.g. the ENSO and AO) follow nonlinear dynamics.
The strength of solar forcing relative to other forcings (e.g. volcanism, ocean circulation patterns, tides, and geomagnetism)
is another source of dynamic responses. Notably the climatic effects of tides and geomagnetism are hitherto largely enigmatic.
Few but well-dated studies suggest almost instantaneous, climatic deteriorations in response to rapid decreases in solar activity.
Such early responses put severe limits to the solar forcing mechanisms and the extent of this phenomenon should be a key issue
for future Sun-climate studies. 相似文献
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Øieroset Marit Mitchell David L. Phan Tai D. Lin Robert P. Crider Dana H. Acuña Mario H. 《Space Science Reviews》2004,111(1-2):185-202
Using magnetometer and electron observations from the Mars Global Surveyor (MGS) and the Wind spacecraft we show that the region of magnetic field pile-up and density decrease located between the Martian ionosphere and bow shock exhibit strong similarities with the plasma depletion layer (PDL) observed upstream of the Earth's magnetopause in the absence of magnetic reconnection when the magnetopause is a solid obstacle in the solar wind. A PDL is formed upstream of the terrestrial magnetopause when the magnetic field piles up against the obstacle and particles in the pile-up region are squeezed away from the high magnetic pressure region along the field lines as the flux tubes convect toward the magnetopause. We here discuss the possibility that at least part of the region of magnetic field pile-up and density depletion upstream of Mars may be formed by the same physical processes which generate the PDL upstream of the Earth's magnetopause. More complete ion, electron, and neutral measurements are needed to conclusively determine the relative importance of the plasma depletion process versus exospheric processes. 相似文献
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Y. Futaana S. Barabash A. Grigoriev D. Winningham R. Frahm M. Yamauchi R. Lundin 《Space Science Reviews》2006,126(1-4):315-332
As a part of the global plasma environment study of Mars and its response to the solar wind, we have analyzed a peculiar case
of the subsolar energetic neutral atom (ENA) jet observed on June 7, 2004 by the Neutral Particle Detector (NPD) on board
the Mars Express satellite. The “subsolar ENA jet” is generated by the interaction between the solar wind and the Martian
exosphere, and is one of the most intense sources of ENA flux observed in the vicinity of Mars. On June 7, 2004 (orbit 485
of Mars Express), the NPD observed a very intense subsolar ENA jet, which then abruptly decreased within ∼10 sec followed
by quasi-periodic (∼1 min) flux variations. Simultaneously, the plasma sensors detected a solar wind structure, which was
most likely an interplanetary shock surface. The abrupt decrease of the ENA flux and the quasi-periodic flux variations can
be understood in the framework of the global response of the Martian plasma obstacle to the interplanetary shock. The generation
region of the subsolar ENA jet was pushed towards the planet by the interplanetary shock; and therefore, Mars Express went
out of the ENA jet region. Associated global vibrations of the Martian plasma obstacle may have been the cause of the quasi-periodic
flux variations of the ENA flux at the spacecraft location. 相似文献
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A review of the basic tools of kinematics widely used by aerospace engineers is presented. Two different approaches are discussed. One is the familiar "vectorial mechanics," while the other is called the "variational method." It is indicated that either approach should produce the same set of motion equations for the dynamical system considered. 相似文献
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Syun-Ichi Akasofu 《Space Science Reviews》2011,164(1-4):85-132
The concept of geomagnetic storm-producing solar plasma flows has evolved and advanced considerably over the last 100 years or so. This particular field of study began in an effort to understand geomagnetic disturbances and the aurora. The purpose of this paper is try to follow the ways in which early concepts evolved to later ones, not to review each concept in detail. It is fascinating to see a step-by-step buildup of these concepts, from the earliest idea of flow of solar electrons to coronal mass ejections (CMEs). The time line, though tentative, of the studies of geomagnetic storm-producing plasma flows is presented. The author hopes that this paper will serve young researchers in particular to consider how they plan to advance further this scientific field. There is still much uncertainty about geomagnetic storm-producing solar plasma flows. Some of the major questions are listed from the point of view of a geophysicist in the summary sections by grouping them in terms of the quiet-time solar wind, solar streams from corona holes and CMEs associated with solar flares. 相似文献
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Thomas H. Zurbuchen Rudolf von Steiger Jacob Gruesbeck Enrico Landi Susan T. Lepri Liang Zhao Viggo Hansteen 《Space Science Reviews》2012,172(1-4):41-55
In this discussion of observational constraints on the source regions and acceleration processes of solar wind, we will focus on the ionic composition of the solar wind and the distribution of charge states of heavy elements such as oxygen and iron. We first focus on the now well-known bi-modal nature of solar wind, which dominates the heliosphere at solar minimum: Compositionally cool solar wind from polar coronal holes over-expands, filling a much larger solid angle than the coronal holes on the Sun. We use a series of remote and in-situ characteristics to derive a global geometric expansion factor of?~5. Slower, streamer-associated wind is located near the heliospheric current sheet with a width of 10–20°, but in a well-defined band with a geometrically small transition width. We then compute charge states under the assumption of thermal electron distributions and temperature, velocity, and density profiles predicted by a recent solar wind model, and conclude that the solar wind originates from a hot source at around 1 million?K, characteristic of the closed corona. 相似文献
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When the problem of predicting the past, present, or future value of a polynomial signal or any of its derivatives is considered, where the signal is in white Gaussian noise, the standard approach has been to minimize mean-square-error with constraints by use of Lagrange multipliers. In this paper an alternate approach is described, using results of Rao and Bhattacharyya from the statistical literature, which reduce the specified prediction problem to a simple one requiring no formal minimizations and no use of Lagrange multipliers. It further has the advantage of yielding the covariances between estimates of the polynomial and its derivates. Useful engineering formulas for smoothing and prediction are developed in the main part of the paper. These include both filter and covariance expressions. A tutorial discussion of the theory is given in two appendixes. 相似文献
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文章论述了在客户使用端开发工具开发数据库软件的过程中,使用ODBC数据接口对各种常用数据源进行操作的问题,并对此提出了两种有效的解决方法,讨论了具体的实施方案。 相似文献
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Fernando Sim?es Robert Pfaff Jean-Jacques Berthelier Jeffrey Klenzing 《Space Science Reviews》2012,168(1-4):551-593
Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave observations in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfvén waves and particle precipitation related to solar and magnetospheric processes. We review ionospheric processes as well as surface and space weather phenomena that drive the coupling between the troposphere and the ionosphere. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface perturbations and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and to solve inverse problems and outline in a final section a few challenging subjects that are important to advance our understanding of tropospheric-ionospheric coupling. 相似文献