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241.
G. Stratta A. Pozanenko J.L. Atteia A. Klotz S. Basa B. Gendre F. Verrecchia M. Boër S. Cutini M. Henze S. Holland M. Ibrahimov F. Ienna I. Khamitov S. Klose V. Rumyantsev V. Biryukov F. Vachier S. Arnouts D. Perley 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The detection of a bright optical emission measured with good temporal resolution during the prompt phase makes GRB 060111B a rare event that is especially useful for constraining theories of the prompt optical emission. Comparing this burst with other GRBs with evidence of optical peaks, we find that the optical peak epoch (tp) is anti-correlated with the high energy burst energetic assuming an isotropic energy release (Eiso) in agreement with Liang et al. (2009), and that the steeper is the post-peak afterglow decay, the less is the agreement with the correlation. GRB 060111B is among the latters and it does not match the correlation. The Cannonball scenario is also discussed and we find that this model cannot be excluded for GRB 060111B. 相似文献
242.
L. Zampieri C. Germanà C. Barbieri G. Naletto A. Čadež I. Capraro A. Di Paola C. Facchinetti T. Occhipinti D. Ponikvar E. Verroi P. Zoccarato 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
We are developing fast photon-counter instruments to study the rapid variability of astrophysical sources by time tagging photon arrival times with unprecedented accuracy, making use of a Rubidium clock and GPS receiver. The first realization of such optical photon-counters, dubbed AquEYE (the Asiago Quantum Eye), was mounted in 2008 at the 182 cm Copernicus Observatory in Asiago. AquEYE observed the Crab pulsar several times and collected data of extraordinary quality that allowed us to perform accurate optical timing of the Crab pulsar and to study the pulse shape stability on a timescale from days to years with an excellent definition. Our results reinforce the evidence for decadal stability of the inclination angle between the spin and magnetic axis of the Crab pulsar. Future realizations of our instrument will make use of the Galileo Global Navigation Satellite System (GNSS) time signal. 相似文献
243.
应用边界元法,并结合一种特殊的复变函数基本解,研究了压电介质的断裂问题。由于该基本解完全遵循本构方程没有对电位移边界作假设,所以结论同非穿透性裂纹假设的结果有较大差异。通过计算表明,外加力载荷不但会引起裂纹尖端应力奇异性,也会引起电场奇异性,同样,外加电压也会引起裂纹尖端应力奇异性。 相似文献
244.
245.
246.
Optimal speckle reduction in polarimetric SAR imagery 总被引:9,自引:0,他引:9
Speckle is a major cause of degradation in synthetic aperture radar (SAR) imagery. With the availability of fully polarimetric SAR data, it is possible to use the three complex elements (HH , HV , VV ) of the polarimetric scattering matrix to reduce speckle. The optimal method for combining the elements of the scattering matrix to minimize image speckle is derived, and the solution is shown to be a polarimetric whitening filter (PWF). A simulation of spatially correlated, K -distributed, fully polarimetric clutter is then used to compare the PWF with other, suboptimal speckle-reduction methods. Target detection performance of the PWF, span, and single-channel |HH |2 detectors is compared with that of the optimal polarimetric detector (OPD). A novel, constant-false-alarm-rate (CFAR) detector (the adaptive PWF) is as a simple alternative to the OPD for detecting targets in clutter. This algorithm estimates the polarization covariance of the clutter, uses the covariance to construct the minimum-speckle image, and then tests for the presence of a target. An exact theoretical analysis of the adaptive PWF is presented; the algorithm is shown to have detection performance comparable with that of the OPD 相似文献
247.
MESSENGER: Exploring Mercury’s Magnetosphere 总被引:1,自引:0,他引:1
James A. Slavin Stamatios M. Krimigis Mario H. Acuña Brian J. Anderson Daniel N. Baker Patrick L. Koehn Haje Korth Stefano Livi Barry H. Mauk Sean C. Solomon Thomas H. Zurbuchen 《Space Science Reviews》2007,131(1-4):133-160
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity
to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in
many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands
off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic
particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere,
allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar
wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects
may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the
only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive
ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived,
∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic
tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces
in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling
of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are
expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions
all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close
in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the
solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents
is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field.
MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin
of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review
what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the
outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere. 相似文献
248.
J. L. Linsky 《Space Science Reviews》2007,130(1-4):367-375
Analysis of UV spectra obtained with the HST, FUSE and other satellites provides a new understanding of the deuterium abundance
in the local region of the galactic disk. The wide range of gas-phase D/H measurements obtained outside of the Local Bubble
can now be explained as due to different amounts of deuterium depletion on carbonaceous grains. The total D/H ratio including
deuterium in the gas and dust phases is at least 23 parts per million of hydrogen, which is providing a challenge to models
of galactic chemical evolution. Analysis of HST and ground-based spectra of many lines of sight to stars within the Local
Bubble have identified interstellar velocity components that are consistent with more than 15 velocity vectors. We have identified
the structures of 15 nearby warm interstellar clouds on the basis of these velocity vectors and common temperatures and depletions.
We estimate the distances and masses of these clouds and compare their locations with cold interstellar clouds. 相似文献
249.
L. Colangeli J. J. Lopez-Moreno P. Palumbo J. Rodriguez M. Cosi V. Della Corte F. Esposito M. Fulle M. Herranz J. M. Jeronimo A. Lopez-Jimenez E. Mazzotta Epifani R. Morales F. Moreno E. Palomba A. Rotundi 《Space Science Reviews》2007,128(1-4):803-821
The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted
to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the
dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and
time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation
or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The
coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma
evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure
mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10−10 ÷ 4.0 × 10−4 kg m s−1 for velocities up to about 300 m s−1. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10−10 g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties
in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts
demonstrate that GIADA is behaving as expected according to the design specifications.
The International GIADA Consortium (I, E, UK, F, D, USA). 相似文献
250.
Sean C. Solomon Ralph L. McNutt Jr. Robert E. Gold Deborah L. Domingue 《Space Science Reviews》2007,131(1-4):3-39
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, is nearing the halfway point on its voyage to become the first probe to orbit the planet Mercury. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet: (1) What planetary formational processes led to Mercury’s high ratio of metal to silicate? (2) What is the geological history of Mercury? (3) What are the nature and origin of Mercury’s magnetic field? (4) What are the structure and state of Mercury’s core? (5) What are the radar-reflective materials at Mercury’s poles? (6) What are the important volatile species and their sources and sinks near Mercury? The mission has focused to date on commissioning the spacecraft and science payload as well as planning for flyby and orbital operations. The second Venus flyby (June 2007) will complete final rehearsals for the Mercury flyby operations in January and October 2008 and September 2009. Those flybys will provide opportunities to image the hemisphere of the planet not seen by Mariner 10, obtain high-resolution spectral observations with which to map surface mineralogy and assay the exosphere, and carry out an exploration of the magnetic field and energetic particle distribution in the near-Mercury environment. The orbital phase, beginning on March 18, 2011, is a one-year-long, near-polar-orbital observational campaign that will address all mission goals. The orbital phase will complete global imaging, yield detailed surface compositional and topographic data over the northern hemisphere, determine the geometry of Mercury’s internal magnetic field and magnetosphere, ascertain the radius and physical state of Mercury’s outer core, assess the nature of Mercury’s polar deposits, and inventory exospheric neutrals and magnetospheric charged particle species over a range of dynamic conditions. Answering the questions that have guided the MESSENGER mission will expand our understanding of the formation and evolution of the terrestrial planets as a family. 相似文献