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831.
Wave effects are discussed pointing to improvement of whistler propagation in the ionosphere illuminated by a powerful radio wave. The large scale irregularities (ducts) responsible for these effects are formed in the illuminated ionospheric region by the process of electron heating by the fields of the pump wave and excited plasma oscillations. These irregularities may also be created in the ionosphere and plasmasphere by fluxes of suprathermal electrons accelerated by the plasma turbulence in the reflection region of the pump wave.  相似文献   
832.
The effects 1-bit quantization of the input samples has on the direction-of-arrival (DOA) estimation accuracy are considered. The signal model assumes a single stochastic Gaussian point source that is embedded in white Gaussian noise (WGN). The inherent limitations governed by the extreme clipping of the input data are analyzed using the Cramer-Rao bound (CRB) that is derived for a two-sensor array. In addition, several estimators for the I-bit estimation are discussed. Numerical and analytical analyses of the estimation error reveal weak dependency on signal-to-noise ratio (SNR) with singular behavior of the estimation error in certain DOA angles.  相似文献   
833.
A new Optical Bidirectional Accelerometer (O.B.A.) is described in this paper.

This accelerometer gives in real time the acceleration level in two orthogonal axis.

The advantages of this instrument in microgravity environment during a parabolic flight experiment (e.g. in fluid mechanics) are explained. The capability of O.B.A. to detect variations of the gravity level is studied. Theoretical curves give the sensibility and accuracy of the accelerometer for different parameters.

We describe the prototype and the different tests realized on it. The experimental results of the ground investigations are compared with the theoretical curves described in the previous section. The second part of the tests has been realized during a ESA parabolic flight campaign with the NASA KC 135 aircraft and in collaboration with the Belgian Air Force on a Fouga Magister.  相似文献   

834.
We have restudied electron beam driven whistler waves with a 3-D electromagnetic particle code. In the initialisation of the beam-plasma system, “quiet start” conditions were approached by including the poloidal magnetic field due to the current carried by beam electrons streaming along a background magnetic field. The simulation results show electromagnetic whistler wave emissions and electrostatic beam modes like those observed in the Spacelab 2 electron beam experiment. It has been suggested in the past that the spatial bunching of beam electrons associated with the beam mode may directly generate whistler waves. However, the simulation results indicate several inconsistencies with this picture: (1) the parallel (to the background magnetic field) wavelength of the whistler wave is longer than that of the beam instability, (2) the parallel phase velocity of the whistler wave is smaller than that of the beam mode, and (3) whistler waves continue to be generated even after the beam mode space charge modulation looses its coherence. The complex structure of the whistler waves in the vicinity of the beam suggest that the transverse motion (gyration) of the beam and background electrons is involved in the generation of the whistler waves.  相似文献   
835.
Signals of VLF transmitters of the Omega navigation system located in the auroral zone (66.4°N, 13.2°E, L= 5) were recorded by the VLF receiving equipment of the Interkosmos 19 satellite. Signals at frequencies between 10.2 and 13.6 kHz were received in a region above the transmitters, frequently with whistler-type echoes. An analysis of these echoes has shown their predominating occurrence in periods of low geomagnetic activity (Kp<2+). The occurrence region of these phenomena in the outer ionosphere has the dimension of about 1000 km and its position is betweenL= 2.5 and L= 4.4. The delay of echo-signals is practically the same during one satellite pass but its values for different satellite revolutions lie between 2.5 and 3.5 s. The frequency spectrum of these signals can be broadened up to 100 Hz. On the basis of calculations made, it can be shown that the experimental results are generally in accordance with the hypothesis of nonlinear ducting of VLF waves in the magnetosphere.  相似文献   
836.
Following our previously proposed technique, we have used the recent -ray observations of Mkr421 to place theoretically significant constraints on the magnitude of the intergalactic infrared radiation field (IIRF). Our 2 upper limits are consistent with normal IR production by stars and dust in galaxies. They rule out exotic mechanisms proposed to produce a larger IIRF. Although they are still subject to revision and are unconfirmed, the data on the spectrum of Mkr421 hint at a possible absorption cutoff which could be produced by an IIRF of the magnitude expected from stellar emission and reprocessing in galaxies. Using models for the low energy intergalactic photon spectrum from microwave to ultraviolet energies, we calculate the opacity of intergalactic space to -rays as a function of energy and redshift. These calculations indicate that the GeV -ray burst recently observed by the CGRO EGRET detector originates at a redshift less than 1.5.  相似文献   
837.
We report the discovery of two hot white dwarfs which have the lowest line-of-sight neutral hydrogen column densities yet measured. The stars were found independently by the ROSAT EUV, Montreal-Cambridge-Tololo, and Edinburgh-Cape surveys. Follow-up observations made using the Voyager 2 ultraviolet spectrometer reveal strong continua shortward of the 912Å Lyman limit from which we deduce that the neutral hydrogen column densities are 1.3 × 1017 and 2.0 × 1017 atoms cm−2.  相似文献   
838.
介绍了人的可靠性评估的发展,进而提出了雷神雷达系统和欧洲猫雷达系统两种空管自动化系统下的管制员可靠性研究方法和数据采集的形式,通过对采集样本数据的分析,计算出管制员从雷神雷达系统过渡到欧洲猫雷达系统可靠性的评估结果,进一步指出提高管制员可靠性的改进措施.  相似文献   
839.
A mathematical model of the dispersed-film two-phase flow with phase slip in the vertical cylindrical channel is described.  相似文献   
840.
In this paper, the results of ultrasonic treatment (UST) influence upon the properties of epoxy compositions used in production of composite polymeric materials are presented. Also shown is the fact that the ultrasonic treatment makes it possible to improve significantly the physicomechanical properties of polymeric materials.  相似文献   
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