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871.
S.J. Bauer L.M. Brace H.A. Taylor T.K. Breus A.J. Kliore W.C. Knudsen A.F. Nagy C.T. Russell N.A. Savich 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(11):233-267
Physical properties of the Venus ionosphere obtained by experiments on the US Pioneer Venus and the Soviet Venera missions are presented in the form of models suitable for inclusion in the Venus International Reference Atmosphere. The models comprise electron density (from 120 km), electron and ion temperatures, and relative ion abundance in the altitude range from 150 km to 1000 km for solar zenith angles from 0° to 180°. In addition, information on ion transport velocities, ionopause altitudes, and magnetic field characteristics of the Venus ionosphere, are presented in tabular or graphical form. Also discussed is the solar control of the physical properties of the Venus ionosphere. 相似文献
872.
Dennis W. Hewett 《Space Science Reviews》1985,42(1-2):29-40
For many astrophysical and magnetic fusion applications, the purely electromagnetic modes generated by real as well as simulation
“plasma” fluctuations are a source of high frequency radiation that is often irrelevant to the physics of interest. Unfortunately,
a numerical CFL stability limit prevents either making c infinite or Δt large while using the usual explicit Maxwell's equations
for the fields. A modification of Maxwell's equations, which provides implicitly the field components, circumvents this problem.
The solution is to neglect retardation effects so that the electromagnetic propagation speed is effectively infinite. The
purely electromagnetic modes in this limit evolve “instantly” to a time-asymptotic configuration about the macroscopic plasma
configuration at each new time level. The Darwin or magnetoinduetive approximation effectively provides infinite propagation
speeds for purely electromagnetic modes by converting Maxwell's equations from hyperbolic to elliptic in character. In practice,
this is accomplished by neglecting the solenoidal part of the displacement current. The elimination of the CFL time step constraint
more than offsets the substantially more complicated field solution that is required. The details of a numerical implementation
of this model will be presented. Numerical examples will be given and extentions of the Darwin field solution to other plasma
models also will be considered.
This work was performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory
under Contract No. W-7405-Eng-48. 相似文献
873.
Hypervelocity impacts on the retrieved Hubble Space Telescope (HST) solar array was investigated by our extended Direct Simulation Monte Carlo (DSMC) model and the result matches well with the measured data in most of the particle mass range. The revelation of the altitude dependence of particles flux onto the retrieved surfaces provides some insight in understanding the observed higher flux onto Mir space station relative to that onto Long Duration Exposure Facility (LDEF). Our analysis suggests that a slightly higher primary flux onto Mir space station would be possible even without taking into account the secondary impacts, as a result of the existence of the highly eccentric orbits of small-sized debris. It is, furthermore, predicted that 43% of the measured impact flux onto Mir station in PIE experiment may be from secondary impacts, and a corresponding 7% for the detectors in Echantillions experiment. 相似文献
874.
For update I see Energy and Environment: A Continuing Partnership, vol.3, American Nuclear Society (1991). An update of validation test results confirming the breakthrough in the low-Earth-orbit (LEO) cycle life of nickel-hydrogen cells containing 26% KOH electrolyte is presented. The results are part of an investigation of the effect of KOH concentration on life cycle. The cycle life of boiler plate cells was about 40000 LEO cycles compared to 3500 cycles for cells containing 31% KOH. The cycle regime was a stressful accelerated LEO, which consisted of a 27.5 min charge followed by a 17.5 min discharge (2×normal rate). The depth-of-discharge (DOD) was 80%. The cell temperature was maintained at 23°C. Results for six 48-Ah recirculation design IPV nickel-hydrogen flight battery cells currently being evaluated to validate the above findings are reported. Three of the cells contain 26% KOH (test cells), and three contain 31% KOH (control cells). They are undergoing real-time LEO cycle life testing. The 31% KOH cells failed at cycles 3729, 4165, and 11355. One of the 26% KOH cells failed at cycle 15314. The other two 26% KOH cells have been cycled for over 16600 cycles without failure 相似文献
875.
A strapdown mechanization and associated Kalman filter are developed to provide both ground align and airstart capabilities for inertial navigation systems (INSs) using Doppler velocity and position fixes, while not requiring an initial heading estimate. Position update during coarse mode is possible by defining sine and cosine of wander angle as filter states and modeling the position error in geographic frame while integrating velocity in the wander frame. INS Global Positioning System (GPS) differential position due to GPS antenna moment arm can aid heading convergence during hover turns in helicopter applications. Azimuth error state in the fine mode of the filter is defined as wander angle error to provide continuous estimation of navigational states, as well as inertial/aiding sensor errors, across the coarse-to-fine mode transition. Though motivated by a tactical helicopter application, the design can be applied to other vehicles. Advantages over conventional systems in addition to the airstart capability include robustness and versatility in handling many different operational conditions 相似文献
876.
L S Myers A J Carmichael T C Pellmar 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):453-456
The in vitro hippocampal brain slice is a 0.4 mm thick neural network that can be used to study brain responses to radiation and related injuries. This preparation is unique in that it responds to ionizing radiation within minutes after exposure without complications from changes in vascularity, blood flow, blood pressure, etc. Electrophysiological studies have shown that x- and gamma-rays alter synaptic transmission and spike generation, elements of normal brain activity. To evaluate the role of hydroxyl free radicals (OH) in these changes, slices were exposed to dilute H2O2 solutions. EPR spin trapping experiments verified that OH is produced. Neural responses, while similar, were not identical to those due to radiation, possibly because of a different distribution of OH. Although H2O2 is freely diffusible, it produces OH at specific sites where, e.g. iron reduces it. In contrast, x- and gamma-rays produce OH more uniformly throughout the tissue. H2O2 may provide a better model for high-LET radiation where yields of radical products of water radiolysis are decreased and peroxide reactions predominate. 相似文献
877.
G. Nagel K.W. Benz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):23-26
Single crystals of binary III-V-semiconductors, e.g. GaAs or InP, are important basic materials for optoelectronic devices, e.g. LED's and lasers. Device production needs highly perfect substrate crystals with low defect densities and homogeneous dopant distributions. In our experiment we applied the Travelling-Heater-Method to grow the III-V compound GaSb. The aim of this research project was to improve the crystal quality by investigating convective transport phenomena and the origins of dopant inhomogeneities under earth and space conditions. Earth grown crystals show strong dopant variations mainly due to convective flow phenomena. The preliminary result of our SPACELAB 1 experiment reveals an increase of dopant homogeneity in the space grown crystal because of the absence of natural convection under reduced gravity. 相似文献
878.
879.
880.
W R Schwingel J C Sager 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(1-2):293-297
An anaerobic reactor seeded with organisms from an anaerobic lagoon was used to study the degradation of inedible crop residues from potato and wheat crops grown in a closed environment. Conversion of this biomass into other products was also evaluated. Degradation of wheat volatile solids was about 25% where that of potato was about 50%. The main product of the anaerobic fermentation of both crops was acetic acid with smaller quantities of propionate and butyrate produced. Nitrate, known to be high in concentration in inedible potato and wheat biomass grown hydroponically, was converted to ammonia in the anaerobic reactor. Both volatile fatty acid and ammonia production may have implications in a crop production system. 相似文献