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71.
Robert H. Nichols Jr. 《Space Science Reviews》2000,92(1-2):113-122
Variations in the abundances of short-lived radionuclides such as 26Al (τ1/2 ≈ 0.74 Ma) and 53Mn (τ1/2 ≈ 3.7 Ma) in meteoritic solids may be used to infer relative formation intervals of these solids in the nebula at precisions
of less than 1 Ma. In a strict chronometric interpretation of the isotopic variations, whereby criteria such as spatial and
temporal isotopic homogeneity and closed system isotopic evolution are met, solid formation occurred in the nebula for at
least several million years. This is longer than some theoretical and astronomical estimates for the duration of the active
nebula. The evidence for live 41Ca (τ1/2 ≈ 0.10 Ma) in meteoritic inclusions further indicates that the onset of solid formation occurred quite early, perhaps within
a few hundred thousand years after the onset of the collapse of the sun's parent molecular cloud. Failure of the chronometric
interpretation may arise for a variety of reasons, including but not limited to, the late, inhomogeneous injection of material
from a nearby stellar source or the local production of short-lived radionuclides by an energetic particle irradiation, e. g., from T Tauri (X-wind) or galactic cosmic ray sources. Although some isotopic evidence exists that the criteria required
for a strict chronometric interpretation are not met by each of the short-lived chronometers, there is no compelling reason
to shorten the interval of solid formation in the nebula to less than 1 Ma.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
72.
73.
S. Bingert P. Zacharias H. Peter B.V. Gudiksen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We investigate a model of an active region for two sunspots surrounded by magnetic field patches comparable in magnetic flux to the sunspots. The model results in emission from the model corona being concentrated in loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV and X-ray emission outline the magnetic field, following the general paradigm. However, in our model, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field lines. This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network. 相似文献
74.
G.P. Wu G.L. Huang Y. Huang D.Y. Wang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(6):984-987
Based upon the most efficient electron acceleration near the midplane of 3D non-neutral driven reconnecting current sheet (RCS) and the electrostatic wave excitation by the drift Maxwellian distribution of electrons in Vlasov simulation, we assume that the electrostatic waves mainly propagate opposite to the reconnecting electric field and investigated how these waves affect the electron acceleration. The main results are: (1) when the electron’s velocity equals to the phase speed of the waves, they will be trapped and have the different accelerating characteristics from the untrapped electrons through solving the momentum equations of electrons analytically; (2) the test particle simulations further prove that the number of the energetic electrons decreases with the increasing intensity of unstable waves, and the distribution of the energetic electrons takes on the double power-law. 相似文献
75.
Jie Chen Henrik Lundstedt Hongqi Zhang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
In this paper, the twist values of ‘S’-shape transequatorial loops (TLs) from 1991 to 2001 are calculated, GOES soft X-ray flares dataset of the active regions connected by these TLs are investigated. The result shows the twist value of the TLs has a weak relation with the flare flux. There is no clear correlation between the twist value and the distance between the footpoint of TLs and location of flare in the corresponding active regions. 相似文献
76.
A. Grimberg D. S. Burnett P. Bochsler H. Baur R. Wieler 《Space Science Reviews》2007,130(1-4):293-300
We discuss data of light noble gases from the solar wind implanted into a metallic glass target flown on the Genesis mission.
Helium and neon isotopic compositions of the bulk solar wind trapped in this target during 887 days of exposure to the solar
wind do not deviate significantly from the values in foils of the Apollo Solar Wind Composition experiments, which have been
exposed for hours to days. In general, the depth profile of the Ne isotopic composition is similar to those often found in
lunar soils, and essentially very well reproduced by ion-implantation modelling, adopting the measured velocity distribution
of solar particles during the Genesis exposure and assuming a uniform isotopic composition of solar wind neon. The results
confirm that contributions from high-energy particles to the solar wind fluence are negligible, which is consistent with in-situ
observations. This makes the enigmatic “SEP-Ne” component, apparently present in lunar grains at relatively large depth, obsolete.
20Ne/ 22Ne ratios in gas trapped very near the metallic glass surface are up to 10% higher than predicted by ion implantation simulations.
We attribute this superficially trapped gas to very low-speed, current-sheet-related solar wind, which has been fractionated
in the corona due to inefficient Coulomb drag. 相似文献
77.
空间交会对接中太阳光照窗口的研究 总被引:1,自引:0,他引:1
在空间交会对接过程中,光学测量装置对目标航天器的观测将直接受太阳光照反射强度的影响,空间交会对接必须在满足太阳光照条件的前提下进行,从正常交会对接和异常交会对接两个方面,详细分析了满足目标航天器光照条件的光照弧段和光照窗口问题,并且简要地讨论了地影区对光照弧段和光照窗口的影响,最后通过一个仿真算例,具体说明了光照条件及地影区对交会对接的约束。 相似文献
78.
G. M. Mason 《Space Science Reviews》2007,130(1-4):231-242
3He-rich solar energetic particle (SEP) events show huge enrichments of 3He and association with kilovolt electrons and Type-III radio bursts. Observations from a new generation of high resolution
instruments launched on the Wind, ACE, Yohkoh, SOHO, TRACE, and RHESSI spacecraft have revealed many new properties of these
events: the particle energy spectra are found to be either power-law or curved in shape, with the 3He spectrum often being distinctly different from other species. Ultra-heavy nuclei up to >200 amu are found to be routinely
present at average enrichments of >200 times solar-system abundances. The high ionization states previously observed near
∼1 MeV/nucleon have been found to decrease towards normal solar coronal values in these events. The source regions have been
identified for many events, and are associated with X-ray jets and EUV flares that are associated with magnetic reconnection
sites near active regions. This paper reviews the current experimental picture and theoretical models, with emphasis on the
new insights found in the last few years. 相似文献
79.
We present the observational results on chromospheric spicules obtained at the Sayan observatory 50 cm coronograph. To investigate the evolution of chromospheric spicules, we analysed spicule spectra of strong chromospheric lines measured simultaneously at three altitudes above the solar limb during 5–60 min with a time resolution of 10 to 20 s. The spatial resolution was better than 1, and the spectral resolution was 0.03Å in 6563Å. The appearance of a spicule at a given altitude is preceded by an sharp increase in line-of-sight velocity and/or in line half-width at a lower level. Generally, the evolution has a non-monotonous impulsive character. Changes of line-of-sight velocities and other parameters of the line profile can be represented as the superposition of slow, evolutionary changes and fluctuations with periods of about 80 to 120 s. The amplitude of line-of-sight velocity fluctuations is 2–3 km/sec and tends to increase with height. By studying the phase delays of the fluctuations at different heights, we found that the propagation velocity exceeds 300 km s–1, and that the disturbances do not necessarily propagate upwards. 相似文献
80.
G. Trottet 《Space Science Reviews》1994,68(1-4):149-158
Non-thermal hard X-ray, gamma-ray and radio emission are the most direct signatures of the presence of energetic particles in the solar atmosphere. This paper lays emphasis on hard X-ray and radio imaging data, obtained during and outside flares, which reveal the sites where particles interact with the ambient medium. These observations, which provide more or less direct information on the topology and dynamics of the magnetic structures in which particles are accelerated and propagate, are discussed in the framework of the statistical flare scenario. 相似文献