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291.
D. B. Reisenfeld D. S. Burnett R. H. Becker A. G. Grimberg V. S. Heber C. M. Hohenberg A. J. G. Jurewicz A. Meshik R. O. Pepin J. M. Raines D. J. Schlutter R. Wieler R. C. Wiens T. H. Zurbuchen 《Space Science Reviews》2007,130(1-4):79-86
Analysis of the Genesis samples is underway. Preliminary elemental abundances based on Genesis sample analyses are in good
agreement with in situ-measured elemental abundances made by ACE/SWICS during the Genesis collection period. Comparison of
these abundances with those of earlier solar cycles indicates that the solar wind composition is relatively stable between
cycles for a given type of flow. ACE/SWICS measurements for the Genesis collection period also show a continuum in compositional
variation as a function of velocity for the quasi-stationary flow that defies the simple binning of samples into their sources
of coronal hole (CH) and interstream (IS). 相似文献
292.
J. L. Linsky 《Space Science Reviews》2007,130(1-4):367-375
Analysis of UV spectra obtained with the HST, FUSE and other satellites provides a new understanding of the deuterium abundance
in the local region of the galactic disk. The wide range of gas-phase D/H measurements obtained outside of the Local Bubble
can now be explained as due to different amounts of deuterium depletion on carbonaceous grains. The total D/H ratio including
deuterium in the gas and dust phases is at least 23 parts per million of hydrogen, which is providing a challenge to models
of galactic chemical evolution. Analysis of HST and ground-based spectra of many lines of sight to stars within the Local
Bubble have identified interstellar velocity components that are consistent with more than 15 velocity vectors. We have identified
the structures of 15 nearby warm interstellar clouds on the basis of these velocity vectors and common temperatures and depletions.
We estimate the distances and masses of these clouds and compare their locations with cold interstellar clouds. 相似文献
293.
It is widely accepted that diffusive shock acceleration is an important process in the heliosphere, in particular in producing
the energetic particles associated with interplanetary shocks driven by coronal mass ejections. In its simplest formulation
shock acceleration is expected to accelerate ions with higher mass to charge ratios less efficiently than those with lower
mass to charge. Thus it is anticipated that the Fe/O ratio in shock-accelerated ion populations will decrease with increasing
energy above some energy. We examine the circumstances of five interplanetary shocks that have been reported to have associated
populations in which Fe/O increases with increasing energy. In each event, the situation is complex, with particle contributions
from other sources in addition to the shock. Furthermore, we show that the Fe/O ratio in shock-accelerated ions can decrease
even when the shock is traveling through an Fe-rich ambient ion population. Thus, although shock acceleration of an Fe-rich
suprathermal population has been proposed to explain large Fe-rich solar particle events, we find no support for this proposal
in these observations. 相似文献
294.
H. J. Völk 《Space Science Reviews》2007,130(1-4):431-438
The dynamical and chemical effects of the Galactic Wind are discussed. This wind is primarily driven by the pressure gradient
of the Cosmic Rays. Assuming the latter to be accelerated in the Supernova Remnants of the disk which at the same time produce
the Hot Interstellar Medium, it is argued that the gas removed by the wind is enriched in the nucleosynthesis products of
Supernova explosions. Therefore the moderate mass loss through this wind should still be able to remove a substantial amount
of metals, opening the way for stars to produce more metals than observed in the disk, by e.g. assuming a Salpeter-type stellar
initial mass function beyond a few Solar masses. The wind also allows a global, physically appealing interpretation of Cosmic
Ray propagation and escape from the Galaxy. In addition the spiral structure of the disk induces periodic pressure waves in
the expanding wind that become a sawtooth shock wave train at large distances which can re-accelerate “knee” particles coming
from the disk sources. This new Galactic Cosmic Ray component can reach energies of a few×1018 eV and may contribute to the juncture between the particles of Galactic and extragalactic origin in the observed overall
Cosmic Ray spectrum. 相似文献
295.
R. C. Wiens D. S. Burnett C. M. Hohenberg A. Meshik V. Heber A. Grimberg R. Wieler D. B. Reisenfeld 《Space Science Reviews》2007,130(1-4):161-171
The Genesis mission returned samples of solar wind to Earth in September 2004 for ground-based analyses of solar-wind composition,
particularly for isotope ratios. Substrates, consisting mostly of high-purity semiconductor materials, were exposed to the
solar wind at L1 from December 2001 to April 2004. In addition to a bulk sample of the solar wind, separate samples of coronal
hole (CH), interstream (IS), and coronal mass ejection material were obtained. Although many substrates were broken upon landing
due to the failure to deploy the parachute, a number of results have been obtained, and most of the primary science objectives
will likely be met. These objectives include He, Ne, Ar, Kr, and Xe isotope ratios in the bulk solar wind and in different
solar-wind regimes, and 15N/14N and 18O/17O/16O to high precision. The greatest successes to date have been with the noble gases. Light noble gases from bulk solar wind
and separate solar-wind regime samples have now been analyzed. Helium results show clear evidence of isotopic fractionation
between CH and IS samples, consistent with simplistic Coulomb drag theory predictions of fractionation between the photosphere
and different solar-wind regimes, though fractionation by wave heating is also a possible explanation. Neon results from closed
system stepped etching of bulk metallic glass have revealed the nature of isotopic fractionation as a function of depth, which
in lunar samples have for years deceptively suggested the presence of an additional, energetic component in solar wind trapped
in lunar grains and meteorites. Isotope ratios of the heavy noble gases, nitrogen, and oxygen are in the process of being
measured. 相似文献
296.
J. Wicht M. Mandea F. Takahashi U. R. Christensen M. Matsushima B. Langlais 《Space Science Reviews》2007,132(2-4):261-290
Mariner 10 measurements proved the existence of a large-scale internal magnetic field on Mercury. The observed field amplitude,
however, is too weak to be compatible with typical convective planetary dynamos. The Lorentz force based on an extrapolation
of Mariner 10 data to the dynamo region is 10−4 times smaller than the Coriolis force. This is at odds with the idea that planetary dynamos are thought to work in the so-called
magnetostrophic regime, where Coriolis force and Lorentz force should be of comparable magnitude. Recent convective dynamo
simulations reviewed here seem to resolve this caveat. We show that the available convective power indeed suffices to drive
a magnetostrophic dynamo even when the heat flow though Mercury’s core–mantle boundary is subadiabatic, as suggested by thermal
evolution models. Two possible causes are analyzed that could explain why the observations do not reflect a stronger internal
field. First, toroidal magnetic fields can be strong but are confined to the conductive core, and second, the observations
do not resolve potentially strong small-scale contributions. We review different dynamo simulations that promote either or
both effects by (1) strongly driving convection, (2) assuming a particularly small inner core, or (3) assuming a very large
inner core. These models still fall somewhat short of explaining the low amplitude of Mariner 10 observations, but the incorporation
of an additional effect helps to reach this goal: The subadiabatic heat flow through Mercury’s core–mantle boundary may cause
the outer part of the core to be stably stratified, which would largely exclude convective motions in this region. The magnetic
field, which is small scale, strong, and very time dependent in the lower convective part of the core, must diffuse through
the stagnant layer. Here, the electromagnetic skin effect filters out the more rapidly varying high-order contributions and
mainly leaves behind the weaker and slower varying dipole and quadrupole components (Christensen in Nature 444:1056–1058,
2006). Messenger and BepiColombo data will allow us to discriminate between the various models in terms of the magnetic fields
spatial structure, its degree of axisymmetry, and its secular variation. 相似文献
297.
基于约简遗传规划的线参数模型及在航空发动机起动建模中的应用 总被引:4,自引:0,他引:4
提出了一种新的约简遗传规划(PGP)算法和一种新的基于约简遗传规划的航空发动机起动动态线参数模型.这种模型采用遗传规划产生航空发动机起动模型的输入输出非线性模型集,并以二叉树结构表征函数项,运用正交最小二乘算法(OLS)估计二叉树分支(基本函数项)对于模型精度的贡献并去除复杂、冗余的函数项,从而加快遗传规划的收敛速度,最后通过GP进化可获得简单、可靠、准确的线参数非线性模型.发动机起动过程试车数据建模和与支持向量机的比较证明,这种方法可以产生适用性好、解析性强的线参数非线性模型,产生的模型可获得与支持向量机相当甚至更优的结果. 相似文献
298.
LIN Jin-zhao WANG Jin-liang* ZHOU Bo WU Er-xing Department of Physics Beijing University of Aeronautics Astronautics Beijing China 《中国航空学报》2006,19(Z1)
Studies of nanoindentation were performed on nc-Si:H films to evaluate the effects of the fabrication processes on their mechani-cal properties. It is observed that with the decrease of the SiH4 contents, the grain size of the films increases gradually, and as does the crystalline volume fraction. The smaller the grains become, the more homogeneous the films, and the more even the hardness as well as the modulus will be. The hardness and the modulus will increase with the substrate’s temperature rising. The hardness and the modulus of the nc-Si:H films on the Si substrate prove to be higher than those on the glass substrate given the same technology parameters. How-ever, the films on the glass substrate appear to be more homogeneous. 相似文献
299.
某型航空发动机后支承动刚度的有限元计算 总被引:6,自引:1,他引:6
支承动刚度是影响发动机振动特性的主要因素之一,但目前航空发动机支承结构的刚度系数一般只能靠经验给出一个范围,这严重影响到转子动力特性计算结果的准确性。根据某型航空发动机的实际结构,建立发动机后支撑结构的动力学模型,应用有限元软件MSC.Nastran进行频率响应计算,再由所得位移曲线求出相应的动刚度曲线。本文所采用的计算方法和得到的结果对发动机转子动力特性的研究具有一定的参考价值。 相似文献
300.
安装误差角对陀螺加速度表的误差模型的影响 总被引:3,自引:0,他引:3
在已建立的陀螺加速度表误差模型的基础上,分析在惯性系统中安装误差角对仪表静态误差模型、动态误差模型和混合误差模型的影响。 相似文献