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161.
S. M. Krimigis D. G. Mitchell D. C. Hamilton S. Livi J. Dandouras S. Jaskulek T. P. Armstrong J. D. Boldt A. F. Cheng G. Gloeckler J. R. Hayes K. C. Hsieh W.-H. Ip E. P. Keath E. Kirsch N. Krupp L. J. Lanzerotti R. Lundgren B. H. Mauk R. W. McEntire E. C. Roelof C. E. Schlemm B. E. Tossman B. Wilken D. J. Williams 《Space Science Reviews》2004,114(1-4):233-329
The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager l spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling’’ and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R
S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge–energy–mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ∼ 2.4 cm2 sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5∘ full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (∼0.05 cm2 sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E≤ 0.884 MeV in the forward direction (G ∼ 0.02 cm2 sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ∼ 0.4 cm2 sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-α measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ∼60 R
S every 2–3 h (every ∼10 min from ∼20 R
S). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be < 0.1 s, which is more than adequate for microsignature studies. Data obtained during Venus-2 flyby and Earth swingby in June and August 1999, respectively, and Jupiter flyby in December 2000 to January 2001 show that the instrument is performing well, has made important and heretofore unobtainable measurements in interplanetary space at Jupiter, and will likely obtain high-quality data throughout each orbit of the Cassini mission at Saturn. Sample data from each of the three sensors during the August 18 Earth swingby are shown, including the first ENA image of part of the ring current obtained by an instrument specifically designed for this purpose. Similarily, measurements in cis-Jovian space include the first detailed charge state determination of Iogenic ions and several ENA images of that planet’s magnetosphere.This revised version was published online in July 2005 with a corrected cover date. 相似文献
162.
L. J. Gray S. A. Crooks M. A. Palmer C. L. Pascoe S. Sparrow 《Space Science Reviews》2006,125(1-4):357-370
Observational evidence of the 11-year solar cycle (SC) modulation of stratosphere temperatures and winds from the ERA-40 dataset
is reviewed, with emphasis on the Northern winter hemisphere. A frequency modulation of sudden warming events is noted, with
warmings occurring earlier in solar minimum periods than in solar maximum periods. The observed interaction between the influence
of the SC and the quasi biennial oscillation (QBO) on the frequency of sudden warmings is noted as a possible clue for understanding
their mechanism of influence. A possible transfer route for the 11-year solar cycle from the equatorial stratopause region
to the lowest part of the stratosphere is proposed, via an influence on sudden warming events and the associated induced meridional
circulation. SC and QBO composites of zonal wind anomalies show anomalous wind distributions in the subtropical upper stratosphere
in early winter. Mechanistic model experiments are reviewed that demonstrate a sensitivity of sudden warmings to small wind
anomalies in this region. Various diagnostics from these experiments are shown, including EP fluxes and their divergence and
also the synoptic evolution of the polar vortex, in order to understand the mechanism of the influence. Some recent GCM experiments
to investigate the SC/QBO interaction are also described. They simulate reasonably well the observed SC/QBO interaction of
sudden warming events and appear to support the hypothesis that tropical/subtropical upper stratospheric wind anomalies are
an important influence on the timing of sudden warmings. 相似文献
163.
Michael J. S. Belton Karen J. Meech Michael F. A’Hearn Olivier Groussin Lucy Mcfadden Carey Lisse Yanga R. Fernández Jana PittichovÁ Henry Hsieh Jochen Kissel Kenneth Klaasen Philippe Lamy Dina Prialnik Jessica Sunshine Peter Thomas Imre Toth 《Space Science Reviews》2005,117(1-2):137-160
In 1998, Comet 9P/Tempel 1 was chosen as the target of the Deep Impact mission (A’Hearn, M. F., Belton, M. J. S., and Delamere, A., Space Sci. Rev., 2005) even though very little was known about its physical properties. Efforts were immediately begun to improve this situation
by the Deep Impact Science Team leading to the founding of a worldwide observing campaign (Meech et al., Space Sci. Rev., 2005a). This campaign has already produced a great deal of information on the global properties of the comet’s nucleus
(summarized in Table I) that is vital to the planning and the assessment of the chances of success at the impact and encounter.
Since the mission was begun the successful encounters of the Deep Space 1 spacecraft at Comet 19P/Borrelly and the Stardust spacecraft at Comet 81P/Wild 2 have occurred yielding new information on the state of the nuclei of these two comets. This
information, together with earlier results on the nucleus of comet 1P/Halley from the European Space Agency’s Giotto, the Soviet Vega mission, and various ground-based observational and theoretical studies, is used as a basis for conjectures on the morphological,
geological, mechanical, and compositional properties of the surface and subsurface that Deep Impact may find at 9P/Tempel 1. We adopt the following working values (circa December 2004) for the nucleus parameters of prime importance to Deep Impact as follows: mean effective radius = 3.25± 0.2 km, shape – irregular triaxial ellipsoid with a/b = 3.2± 0.4 and overall dimensions of ∼14.4 × 4.4 × 4.4 km, principal axis rotation with period = 41.85± 0.1 hr, pole directions
(RA, Dec, J2000) = 46± 10, 73± 10 deg (Pole 1) or 287± 14, 16.5± 10 deg (Pole 2) (the two poles are photometrically, but not
geometrically, equivalent), Kron-Cousins (V-R) color = 0.56± 0.02, V-band geometric albedo = 0.04± 0.01, R-band geometric
albedo = 0.05± 0.01, R-band H(1,1,0) = 14.441± 0.067, and mass ∼7×1013 kg assuming a bulk density of 500 kg m−3. As these are working values, {i.e.}, based on preliminary analyses, it is expected that adjustments to their values may be made before encounter
as improved estimates become available through further analysis of the large database being made available by the Deep Impact observing campaign. Given the parameters listed above the impact will occur in an environment where the local gravity is
estimated at 0.027–0.04 cm s−2 and the escape velocity between 1.4 and 2 m s−1. For both of the rotation poles found here, the Deep Impact spacecraft on approach to encounter will find the rotation axis close to the plane of the sky (aspect angles 82.2 and 69.7
deg. for pole 1 and 2, respectively). However, until the rotation period estimate is substantially improved, it will remain
uncertain whether the impactor will collide with the broadside or the ends of the nucleus. 相似文献
164.
Detecting moving targets in SAR imagery by focusing 总被引:1,自引:0,他引:1
A new method for detecting moving targets in a synthetic aperture radar (SAR) image is presented. It involves segmenting a complex-valued SAR image into patches, focusing each patch separately, and measuring the sharpness increase in the focused patch. The algorithm is sensitive to azimuth velocities and is exquisitely sensitive to radial accelerations of the target, allowing it to detect motion in any direction. It is complementary to conventional Doppler-sensing moving target indicators, which can sense only the radial velocity of rapidly moving targets. 相似文献
165.
We present the results from a study of the variations of the cosmic-ray intensity with time, heliographic latitude, and longitude, and for varying interplanetary conditions, using our three-dimensional, time-dependent computer code for cosmic-ray transport in the heliosphere. Our code also produces a solar-wind and interplanetary magnetic field (IMF) configuration which is compared with observations. Because of the fully threedimensional nature of the model calculations, we are able to model time variations which would be expected to be observed along Ulysses's trajectory as it moves to high latitudes. In particular we can model the approximately 13-and 26-day solar-rotation induced variations in cosmic rays, solar wind and IMF, as a function of increasing heliographic latitude, as one moves poleward of the interplanetary current sheet. Our preliminary model results seem to be in general form quite similar to published data, but depend on the physical parameters used such as cosmic-ray diffusion coefficients, boundary conditions, and the nature of the solar wind and IMF and current sheet. 相似文献
166.
It is shown that the errors associated with radio elevation measurements may be investigated systematically using a variational technique. The error occurring when spaced antennas are used is compared with that for a single directional antenna. Integral expressions are obtained for the refractive errors. 相似文献
167.
The equations derived by A. J. Rainal for the probability density function of the angle error output of a monopulse radar excited by a Gaussian signal and Gaussian thermal noise are generalized to include the presence of multiple targets. The examples given demonstrate the radar's behavior for various combinations of target and noise parameters. 相似文献
168.
Geiss J. Bühler F. Cerutti H. Eberhardt P. Filleux Ch. Meister J. Signer P. 《Space Science Reviews》2004,110(3-4):307-335
Space Science Reviews - The Apollo Solar Wind Composition (SWC) experiment was designed to measure elemental and isotopic abundances of the light noble gases in the solar wind, and to investigate... 相似文献
169.
Haines K. Hipkin R. Beggan C. Bingley R. Hernandez F. Holt J. Baker T. Bingham R.J. 《Space Science Reviews》2003,108(1-2):205-216
Accurate local geoids derived from in situ gravity data will be valuable in the validation of GOCE results. In addition it will be a challenge to use GOCE data in an
optimal way, in combination with in situ gravity, to produce better local geoid solutions. This paper discusses the derivation of a new geoid over the NW European
shelf, and its comparison with both tide gauge and altimetric sea level data, and with data from ocean models. It is hoped
that over the next few years local geoid methods such as these can be extended to cover larger areas and to incorporate both
in situ and satellite measured gravity data.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
170.
Jurewicz A.J.G. Burnett D.S. Wiens R.C. Friedmann T.A. Hays C.C. Hohlfelder R.J. Nishiizumi K. Stone J.A. Woolum D.S. Becker R. Butterworth A.L. Campbell A.J. Ebihara M. Franchi I.A. Heber V. Hohenberg C.M. Humayun M. McKeegan K.D. McNamara K. Meshik A. Pepin R.O. Schlutter D. Wieler R. 《Space Science Reviews》2003,105(3-4):535-560
Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be
exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure
materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’),
with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically
placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.
Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for
solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector
arrays and elsewhere targeted for the analyses of specific solar-wind components.
Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the
ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface
and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability.
A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis
website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma
data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community
throughout the 21st Century.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献