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941.
J. L. Kohl S. Fineschi R. Esser A. Ciaravella S. R. Cranmer L. D. Gardner R. Suleiman G. Noci A. Modigliani 《Space Science Reviews》1999,87(1-2):233-236
Ultraviolet emission line profiles have been measured on 15-29 September 1997 for H I 1216 Å, O VI 1032, 1037 Å and Mg X 625 Å in a polar coronal hole, at heliographic heights ? (in solar radii) between 1.34 and 2.0. Observations of H I 1216 Å and the O VI doublet from January 1997 for ? = 1.5 to 3.0 are provided for comparison. Mg X 625 Å is observed to have a narrow component at ? = 1.34 which accounts for only a small fraction of the observed spectral radiance, and a broad component that exists at all observed heights. The widths of O VI broad components are only slightly larger than those for H I at ? = 1.34, but are significantly larger at ? = 1.5 and much larger for ? > 1.75. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least ? = 2.0, but never reach the values of O VI. 相似文献
942.
Ground penetrating radar VIY-2 总被引:2,自引:0,他引:2
Prokhorenko V.P. Ivashchuk V.E. Korsun S.V. 《Aerospace and Electronic Systems Magazine, IEEE》2005,20(7):16-18
VIY-2 ground penetrating radar (GPR) with unique sounding possibilities and use simplicity is presented at this paper. VIY-2 GPR combines all units (synchronizer, transmitting and receiving modules, powering, and antenna system) into single case. The VIY-2 GPR communicates with computer via standard interface RS232 or USB1.0. Technical solutions utilized by the VIY-2 GPR reduce deployment time and simplify surveying process. The VIY-2 GPR design features and its components interaction are considered at this paper. Some field results are also presented here. The VIY-2 GPR design concept allows reducing the data acquisition time,, optimizing the time-varying gain control function, applying depth-stacking dependence, controlling the surveying window position and interference reducing by pulse repetition frequency randomizing. 相似文献
943.
Track-before-detect procedures for early detection of moving target from airborne radars 总被引:2,自引:0,他引:2
Buzzi S. Lops M. Venturino L. 《IEEE transactions on aerospace and electronic systems》2005,41(3):937-954
In this paper we present a family of track-before-detect (TBD) procedures for early detection of moving targets from airborne radars. Upon a sectorization of the coverage area, the received echoes are jointly processed in the azimuth-range-Doppler domain and in the time domain through a Viterbi-like algorithm that exploits the physically admissible target transitions between successive illuminations, in order to collect all of the energy back-scattered during the time on target (TOT). A reduced-complexity implementation is derived assuming, at the design stage, that the target does not change resolution cell during the TOT in each scan. The constant false alarm rate (CFAR) constraint is also englobed in the proposed procedures as well as the possibility of working with quantized data. Simulation results show that the proposed algorithms have good detection and tracking capabilities even for high target velocities and low quantization rates. 相似文献
944.
Until pristine samples can be returned from cometary nuclei, primitive meteorites represent our best source of information
about organic chemistry in the early solar system. However, this material has been affected by secondary processing on asteroidal
parent bodies which probably did not affect the material now present in cometary nuclei. Production of meteoritic organic
matter apparently involved the following sequence of events: Molecule formation by a variety of reaction pathways in dense
interstellar clouds; Condensation of those molecules onto refractory interstellar grains; Irradiation of organic-rich interstellar-grain
mantles producing a range of molecular fragments and free radicals; Inclusion of those interstellar grains into the protosolar
nebula with probable heating of at least some grain mantles during passage through the shock wave bounding the solar accretion
disc; Agglomeration of residual interstellar grains and locally produced nebular condensates into asteroid-sized planetesimals;
Heating of planetesimals by decay of extinct radionuclides; Melting of ice to produce liquid water within asteroidal bodies;
Reaction of interstellar molecules, fragments and radicals with each other and with the aqueous environment, possibly catalysed
by mineral grains; Loss of water and other volatiles to space yielding a partially hydrated lithology containing a complex
suite of organic molecules; Heating of some of this organic matter to generate a kerogen-like complex; Mixing of heated and
unheated material to yield the meteoritic material now observed. Properties of meteoritic organic matter believed to be consistent
with this scenario include: Systematic decrease of abundance with increasing C number in homologous series of characterisable
molecules; Complete structural diversity within homologous series; Predominance of branched-chain isomers; Considerable isotopic
variability among characterisable molecules and within kerogen-like material; Substantial deuterium enrichment in all organic
fractions; Some fractions significantly enriched in nitrogen-15; Modest excesses of L-enantiomers in some racemisation-resistant
molecules but no general enantiomeric preference. Despite much speculation about the possible role of Fischer-Tropsch catalytic
hydrogenation of CO in production of organic molecules in the solar nebula, no convincing evidence for such material has been
found in meteorites. A similarity between some meteoritic organics and those produced by Miller-Urey discharge synthesis may
reflect involvement of common intermediates rather than the operation of electric discharges in the early solar system. Meteoritic
organic matter constitutes a useful, but not exact, guide to what we shall find with in situ analytical and sample-return
missions to cometary nuclei.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
945.
946.
Streamer evaporation is the consequence of heating in ideal MHD models because plasma is weakly contained by the magnetic
field. Heating causes inflation, opening of field lines, and release of solar wind. It was discovered in simulations and,
due to the absence of loss mechanisms, the ultimate end point is the complete evaporation of the streamer. Of course streamers
do not behave in this way because of losses by thermal conduction and radiation. Heating is also expected to depend on ambient
conditions. We use a global MHD model with thermal conduction to examine the effect of changing the heating scale height.
We also extend an analytic model of streamers developed by Pneuman (1968) to show that steady streamers are unable to contain
plasma for temperatures near the cusp greater than ∼ 2 × 106 K.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
947.
2001 Mars Odyssey Mission Summary 总被引:1,自引:0,他引:1
Saunders R.S. Arvidson R.E. Badhwar G.D. Boynton W.V. Christensen P.R. Cucinotta F.A. Feldman W.C. Gibbs R.G. Kloss C. Landano M.R. Mase R.A. McSmith G.W. Meyer M.A. Mitrofanov I.G. Pace G.D. Plaut J.J. Sidney W.P. Spencer D.A. Thompson T.W. Zeitlin C.J. 《Space Science Reviews》2004,110(1-2):1-36
The 2001 Mars Odyssey spacecraft, now in orbit at Mars, will observe the Martian surface at infrared and visible wavelengths to determine surface mineralogy and morphology, acquire global gamma ray and neutron observations for a full Martian year, and study the Mars radiation environment from orbit. The science objectives of this mission are to: (1) globally map the elemental composition of the surface, (2) determine the abundance of hydrogen in the shallow subsurface, (3) acquire high spatial and spectral resolution images of the surface mineralogy, (4) provide information on the morphology of the surface, and (5) characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers. To accomplish these objectives, the 2001 Mars Odyssey science payload includes a Gamma Ray Spectrometer (GRS), a multi-spectral Thermal Emission Imaging System (THEMIS), and a radiation detector, the Martian Radiation Environment Experiment (MARIE). THEMIS and MARIE are mounted on the spacecraft with THEMIS pointed at nadir. GRS is a suite of three instruments: a Gamma Subsystem (GSS), a Neutron Spectrometer (NS) and a High-Energy Neutron Detector (HEND). The HEND and NS instruments are mounted on the spacecraft body while the GSS is on a 6-m boom. Some science data were collected during the cruise and aerobraking phases of the mission before the prime mission started. THEMIS acquired infrared and visible images of the Earth-Moon system and of the southern hemisphere of Mars. MARIE monitored the radiation environment during cruise. The GRS collected calibration data during cruise and aerobraking. Early GRS observations in Mars orbit indicated a hydrogen-rich layer in the upper meter of the subsurface in the Southern Hemisphere. Also, atmospheric densities, scale heights, temperatures, and pressures were observed by spacecraft accelerometers during aerobraking as the spacecraft skimmed the upper portions of the Martian atmosphere. This provided the first in-situ evidence of winter polar warming in the Mars upper atmosphere. The prime mission for 2001 Mars Odyssey began in February 2002 and will continue until August 2004. During this prime mission, the 2001 Mars Odyssey spacecraft will also provide radio relays for the National Aeronautics and Space Administration (NASA) and European landers in early 2004. Science data from 2001 Mars Odyssey instruments will be provided to the science community via NASA’s Planetary Data System (PDS). The first PDS release of Odyssey data was in October 2002; subsequent releases occur every 3 months. 相似文献
948.
T. Van Doorsselaere I. Arregui J. Andries M. Goossens S. Poedts 《Space Science Reviews》2005,121(1-4):79-89
We will discuss the observed, heavily damped transversal oscillations of coronal loops. These oscillations are often modeled
as transversal kink oscillations in a cylinder. Several features are added to the classical cylindrical model. In our models
we include loop curvature, longitudinal density stratification, and highly inhomogeneous radial density profiles.
In this paper, we will first give an overview of recently obtained results, both analytically and numerically. After that,
we shed a light on the computational aspects of the modeling process. In particular, we will focus on the parallellization
of the numerical codes. 相似文献
949.
虽然透博梅卡公司现在每年销售发动机超过650台,但它们却认为已经到了研制替换轴功率为450~670千瓦级发动机的时候了. 相似文献
950.
D. Ruciński A. C. Cummings G. Gloeckler A. J. Lazarus E. Möbius M. Witte 《Space Science Reviews》1996,78(1-2):73-84
The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 – 1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He+ and He++ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations. 相似文献