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261.
W. H. Matthaeus G. P. Zank R. J. Leamon C. W. Smith D. J. Mullan S. Oughton 《Space Science Reviews》1999,87(1-2):269-275
Mechanisms for the deposition of heat in the lower coronal plasma are discussed, emphasizing recent attempts to reconcile
the fluid and kinetic perspectives. Structures at the MHD scales are believed to act as reservoirs for fluctuation energy,
which in turn drive a nonlinear cascade process. Kinetic processes act at smaller spatial scales and more rapid time scales.
Cascade-driven processes are contrasted with direct cyclotron absorption, and this distinction is echoed in the contrast between
frequency and wavenumber spectra of the fluctuations. Observational constraints are also discussed, along with estimates of
the relative efficiency of cascade and cyclotron processes.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
262.
J.L. Phillips C.T. Russell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1987,7(12):291-294
Over four Venus years of low altitude nightside PVO magnetometer observations are used to establish a new upper limit for the magnetic moment of Venus. Improvements over previous studies include data coverage and new instrument calibration information. The upper limit on an internal dipole moment is determined to be 8.4 × 1010 T m3. 相似文献
263.
The identification of extant and, in some cases, extinct bacterial life is most convincingly and efficiently performed with modern high-resolution microscopy. Epifluorescence microscopy of microbial autofluorescence or in conjunction with fluorescent dyes is among the most useful of these techniques. We explored fluorescent labeling and imaging of bacteria in rock and soil in the context of in situ life detection for planetary exploration. The goals were two-fold: to target non-Earth-centric biosignatures with the greatest possible sensitivity and to develop labeling procedures amenable to robotic implementation with technologies that are currently space qualified. A wide panel of commercially available dyes that target specific biosignature molecules was screened, and those with desirable properties (i.e., minimal binding to minerals, strong autofluorescence contrast, no need for wash steps) were identified. We also explored the potential of semiconductor quantum dots (QDs) as bacterial and space probes. A specific instrument for space implementation is suggested and discussed. 相似文献
264.
This report describes recent progress in the UN Basic Space Science Initiative (UNBSSI), which aims to facilitate space science education and research, and attendant resources in developing countries. In addition to holding workshops across the developing world, the UN Committee on the Peaceful Uses of Outer Space (COPUOS) successfully implemented the International Heliophysical Year (IHY) as a catalyst for improving understanding of the Sun and of solar-terrestrial physics. Building on this it is now preparing for the International Space Weather Initiative (ISWI). Achievements of the former are discussed, as are the goals and anticipated activities of the latter. 相似文献
265.
Stephen J. Pyne 《Space Policy》1988,4(3):187-199
The world has known three great ages of exploration-the circumnavigation of the globe, with its attendant discovery of new lands; the traversing and cataloguing of the newly-found continents; and the exploration of the uninhabited regions of Antarctica, the deep ocean basins and outer space. The author points to the culturally and historically determined nature of discovery, which has thus far been largely a Western phenomenon, but emphasizes the qualitatively different character of space which takes the Earth, rather than any particular part of it, as its starting point, and which sets forth to chart regions that are most probably abiotic. 相似文献
266.
J W Brown 《Acta Astronautica》1985,12(2):107-120
Computer graphics is being employed at the NASA Johnson Space Center as a tool to perform rapid, efficient and economical analyses for man-machine integration, flight operations development and systems engineering. The Operator Station Design System (OSDS), a computer-based facility featuring a highly flexible and versatile interactive software package, PLAID, is described. This unique evaluation tool, with its expanding data base of Space Shuttle elements, various payloads, experiments, crew equipment and man models, supports a multitude of technical evaluations, including spacecraft and workstation layout, definition of astronaut visual access, flight techniques development, cargo integration and crew training. As OSDS is being applied to the Space Shuttle, Orbiter payloads (including the European Space Agency's Spacelab) and future space vehicles and stations, astronaut and systems safety are being enhanced. Typical OSDS examples are presented. By performing physical and operational evaluations during early conceptual phases. supporting systems verification for flight readiness, and applying its capabilities to real-time mission support, the OSDS provides the wherewithal to satisfy a growing need of the current and future space programs for efficient, economical analyses. 相似文献
267.
268.
A summary of research and development work on high-power microwave filters is given here: first, design considerations in high-power microwave filters?power handling capacity, waveform distortion, multimode suppression, and stop and pass band characteristics; second, a discussion of filtering devices and techniques in two broad categories-dominant mode filters and harmonic filters (discussion covers methods of operation and significant characteristics of each filter developed to date); and third, a discussion of applications and limitations of filters in various systems and interference situations. Also included are the availability, cost, size, weight, etc. as functions of frequency for the various filter types. 相似文献
269.
A means of optimizing a moving target indicator (MTI) filter for rejecting several types of clutter, which are generated by different mechanisms such as by rain or the ground, is formulated. lt is found that the optimal performance of such a filter depends on the spectral density functions, average radar cross sections, and the relative mean Doppler frequencies of each type of clutter. lt is shown that the optimal improvement factor of such a filter is bounded by the weighted average (weighted in accordance with the radar cross sections of the clutter types) of the improvement factor for the individual clutter type. lt is also shown that the improvement factor of such a filter is a function of the relative mean Doppler frequency f0 between the clutter types. As f0 increases, the performance of the MTI system degrades. The worst improvement factor occurs when f0 is equal to half of the radar pulse-repetition frequency (PRF). 相似文献
270.
An approach to the problem of achieving frame synchronization in the presence of jamming or other large disturbances is presented. It is assumed that the disturbance can be detected at the receiver, which then erases the corresponding bits and passes this information to the synchronizer. A synchronization algorithm designed to operate in the presence of both erasures and background errors is described and evaluated for a representative set of system parameters. This algorithm is shown to have a significant advantage over an algorithm which does not use the location information inherent in the erasure model. 相似文献