共查询到20条相似文献,搜索用时 15 毫秒
1.
J. Flury 《Earth, Moon, and Planets》2004,94(1-2):83-91
An overview of advances in ice research which can be expected from future satellite gravity missions is given. We compare
present and expected future accuracies of the ice mass balance of Antarctica which might be constrained to 0.1–0.3 mm/year
of sea level equivalent by satellite gravity data. A key issue for the understanding of ice mass balance is the separation
of secular and interannual variations. For this aim, one would strongly benefit from longer uninterrupted time series of gravity
field variations (10 years or more). An accuracy of 0.01 mm/year for geoid time variability with a spatial resolution of 100
km would improve the separability of ice mass balance from mass change due to glacial isostatic adjustment and enable the
determination of regional variations in ice mass balance within the ice sheets. Thereby the determination of ice compaction
is critical for the exploitation of such high accuracy data. A further benefit of improved gravity field models from future
satellite missions would be the improvement of the height reference in the polar areas, which is important for the study of
coastal ice processes. Sea ice thickness determination and modelling of ice bottom topography could be improved as well. 相似文献
2.
Alberta Albertella Federica Migliaccio Fernando Sansó 《Celestial Mechanics and Dynamical Astronomy》2002,83(1-4):1-15
An artificial satellite, flying in a purely gravitational field is a natural probe, such that, by a very accurate orbit determination, would allow a perfect estimation of the field. A true satellite experiences a number of perturbational, non-gravitational forces acting on the shell of the spacecraft; these can be revealed and accurately measured by a spaceborne accelerometer. If more accelerometers are flown in the same satellite, they naturally eliminate (to some extent) the common perturbational accelerations and their differences are affected by the second derivatives of the gravity fields only (gradiometry). The mission GOCE is based on this principle. Its peculiar dynamical observation equations are reviewed. The possibility of estimating the gravity field up to some harmonic degree (200) is illustrated. 相似文献
3.
R. Rummel 《Earth, Moon, and Planets》2004,94(1-2):3-11
Precise global geoid and gravity anomaly information serves essentially three different kinds of applications in Earth sciences:
gravity and geoid anomalies reflect density anomalies in oceanic and continental lithosphere and the mantle; dynamic ocean
topography as derived from the combination of satellite altimetry and a global geoid model can be directly transformed into
a global map of ocean surface circulation; any redistribution or exchange of mass in Earth system results in temporal gravity
and geoid changes. After completion of the dedicated gravity satellite missions GRACE and GOCE a high standard of global gravity
determination, both of the static and of the time varying field will be attained. Thus, it is the right time to investigate
the future needs for improvements in the various fields of Earth sciences and to define the right strategy for future gravity
field satellite missions. 相似文献
4.
Lorenzo Iorio 《Celestial Mechanics and Dynamical Astronomy》2003,86(3):277-294
In this paper we calculate explicitly the classical secular precessions of the node and the perigee of an Earth artificial satellite induced by the even zonal harmonics of the static part of the geopotential up to degree l = 20. Subsequently, their systematic errors induced by the mismodelling in the even zonal spherical harmonics coefficients J
l
are compared to the general relativistic secular gravitomagnetic and gravitoelectric precessions of the node and the perigee of the existing laser-ranged geodetic satellites and of the proposed LARES. The impact of the future terrestrial gravity models from CHAMP and GRACE missions is discussed as well. Preliminary estimates with the recently released EIGEN-1S gravity model including the first CHAMP data are presented. 相似文献
5.
In this paper, a model is developed for the dynamics of a system of two bodies whose material points are under the influence of a central gravitational force. One of the bodies is assumed to be rigid and spherically symmetric, while the other is assumed to be deformable. To develop a tractable model for the system, the deformable body is modeled using Cohen and Muncaster's theory of a pseudo-rigid body. The resulting model of the system has several of the features, such as angular momentum conservation, exhibited by more restrictive models. We also show how the self-gravitation of the deformable body can be accommodated using appropriate constitutive equations for a force tensor. This enables our model to subsume many existing models of ellipsoidal figures of equilibrium. After the model and its conservations have been discussed, attention is restricted to steady motions of the system. Several results, which generalize recent works on rigid satellites, are established for these motions. For a specific choice of constitutive equations for the pseudo-rigid body, we determine the steady motions with the aid of a numerical continuation method. These results can also be considered as generalizations of earlier works on Roche's ellipsoids of equilibrium. 相似文献
6.
Mustafa Saltı 《Astrophysics and Space Science》2006,306(4):201-204
We use the teleparallel geometry analog of the Møller energy-momentum complex to calculate the energy distribution (due to matter plus field including gravity) of a charged black hole solution in heterotic string theory. We find the same energy distribution as obtained by Gad who investigated the same problem by using the Møller energy-momentum complex in general relativity. The total energy depends on the black hole mass M and charge Q. The energy obtained is also independent of the teleparallel dimensionless coupling constant, which means that it is valid not only in the teleparallel equivalent of general relativity, but also in any teleparallel model. Furthermore, our results also sustains (a) the importance of the energy-momentum definitions in the evaluation of the energy distribution of a given spacetime and (b) the viewpoint of Lessner that the Møller energy-momentum complex is a powerful concept of energy and momentum. 相似文献
7.
Alexander S. KonoplivJames K. Miller William M. OwenDonald K. Yeomans Jon D. GiorginiRomain Garmier Jean-Pierre Barriot 《Icarus》2002,160(2):289-299
As part of the NEAR Radio Science investigation, a global solution that includes both spherical and ellipsoidal harmonic gravity fields of Eros, Eros pole and rotation rate, Eros ephemeris, and landmark positions from the optical data was generated. This solution uses the entire one-year in orbit collection of X-band radiometric tracking (Doppler and range) from the Deep Space Network and landmark tracking observations generated from the NEAR spacecraft images of Eros. When compared to a constant density shape model, the gravity field shows a nearly homogeneous Eros. The Eros landmark solutions are in good agreement with the Eros shape model, and they reduce the center-of-mass and center-of-figure offset in the z direction to 13 m. Most of the NEAR spacecraft orbits are determined in all directions to an accuracy of several meters. The solution for the ephemeris of Eros constrains the mass of Vesta to 18.2±0.4 km3/s2 and reduces the uncertainty in the Earth-Moon mass ratio. 相似文献
8.
国家天文台分米波太阳射电频谱仪用新的观测模式获得太阳射电频谱的一些新的观测现象。新的观测模式频率在1.1—1.34GHz范围,时间分辨率是1.25ms;正常的观测模式下频率在1.1—2.06GHz范围,时间分辨率是5ms。在两种模式下频率分辨率为4MHz。发现窄带Ⅲ型爆发(“blips”)斑马纹(Zebra)和纤维结构(Fiber)中的超精细结构和一些新的精细结构。这些新的结果有助于深入理解在太阳耀斑期间低日冕中能量的释放和转移,也为拟建中的太阳射电频谱日像仪提出了新的要求。 相似文献
9.
V. R. Murthy 《Earth, Moon, and Planets》2006,99(1-4):23-32
The radioactivity of the earth is an important parameter in understanding the dynamics of the planet and the evolution of
the crust–mantle–core system but geochemical and geophysical approaches have had only a limited success in defining it. The
opportunity of a direct estimate of the radioactivity of the earth by measurement of the geoneutrino flux takes on an added
significance in this context. Such an independent new measurement will help resolve and/or clarify a number of questions about
global scale processes in the earth and will help advance earth sciences. 相似文献
10.
New Mars Reconnaissance Orbiter HiRise and CRISM imagery of polar layered terrain of Mars reveals striking similarities to icy debris fans along the base of steep escarpments in Alaska formed in high-latitude periglacial environments. Process and morphologic observations of a deglaciating site in the Wrangell Mountains reveal a complex suite of supraglacial processes involved in the construction of icy debris fans. Snow, ice, and sediment are delivered to the fans from degradation of an upper-level icecap. Alaskan icy debris fans were studied during an 8-day reconnaissance mission in July 2006. We directly observed 289 major depositional events dominated by dry snow/ice avalanches, but also including icy debris flows, rockfalls, small jokulhlaups, and glacial calving. Small fans with larger catchments receive episodic icy debris flows triggered by outburst flows that mobilize rockfall sediment temporarily stored in catchments above the fan apex. Large fans with smaller catchments have better linkage to the upper icecap, providing a direct pathway for frequent large avalanches. The large, avalanche-dominated fans thicken rapidly from an overabundance of snow/ice supply to the point where they become hybrid fan-glaciers. Surficial geology evolves rapidly in this high-latitude environment through both depositional events and solar-driven albedo changes that occur daily. Ground penetrating radar surveys show that subsurface sedimentary architecture and fan evolution is similar to the active surface processes and deposits observed on the fans. Direct field observations of active geomorphic processes provide unique insights on the pace and nature of high-latitude landscape evolution during climate changes on both planets. 相似文献
11.
据云南省气象台提供的过去10年中省内各气象站云量观测统计出的晴天日数排名第一和第二的测站,可以看出云南省内潜在着的最好的夜间光学人卫观测站址可能位于华坪、宾川、姚安、元谋、弥度5个气象站附近。文中还采用1950年至1980年的气象观测资料验证了上述结论。 相似文献
12.
High speed photometry during the lunar occultation of a stellar system provides an effective means of achieving high angular
resolution in one dimension at the sub arc second level which is well suited for resolving close binary projected separations
in the range of 10–100 milliarc seconds. An optical fast photometer designed for such a purpose is described and some results
from the initial observations taken with the system including the resolution of a projected separation of 55 milli arcsecond
in one binary system are detailed. 相似文献
13.
YANG KeJun 《时间频率学报》1996,(1)
在卫星定位与授时中,电离层折射误差的影响是十分重要的.目前采用的主要改正方法是双频法,还有电离层模型法.双频法要求卫星发射两个工作频率,而电离层模型法的偏差又较大,这对于使用单频接收机的用户来讲,电离层折射改正就成为一个严重问题,笔者提出的双极化法正好可以解决单频接收机所遇到的问题.双极化法是基于电离层的双折射特性.一个线极化波在电离层中传播时被分裂成两个圆极化波,即左旋圆极化波和右旋圆极化波传播,左旋与右旋圆极化波在电离层中传播的速度不同,则到达接收点的时间也就不同,通过测量两个圆极化波到达接收点的传播时廷差,即可确定电离层折射误差的改正量. 相似文献
14.
Cédric Langbort 《Celestial Mechanics and Dynamical Astronomy》2002,84(4):369-385
In this paper, we study circular orbits of the J
2 problem that are confined to constant-z planes. They correspond to fixed points of the dynamics in a meridian plane. It turns out that, in the case of a prolate body, such orbits can exist that are not equatorial and branch from the equatorial one through a saddle-center bifurcation. A closed-form parametrization of these branching solutions is given and the bifurcation is studied in detail. We show both theoretically and numerically that, close to the bifurcation point, quasi-periodic orbits are created, along with two families of reversible orbits that are homoclinic to each one of them. 相似文献
15.
E. Churazov S. Sazonov R. Sunyaev M. Revnivtsev 《Monthly notices of the Royal Astronomical Society》2008,385(2):719-727
We present calculations of the reflection of the cosmic X-ray background (CXB) by the Earth's atmosphere in the 1–1000 keV energy range. The calculations include Compton scattering and X-ray fluorescent emission and are based on a realistic chemical composition of the atmosphere. Such calculations are relevant for CXB studies using the Earth as an obscuring screen (as was recently done by INTEGRAL ). The Earth's reflectivity is further compared with that of the Sun and the Moon – the two other objects in the Solar system subtending a large solid angle on the sky, as needed for CXB studies. 相似文献
16.
We present an analysis of the significantly expanded HARPS 2011 radial velocity data set for GJ 581 that was presented by Forveille et al. (2011). Our analysis reaches substantially different conclusions regarding the evidence for a Super‐Earth‐mass planet in the star's Habitable Zone. We were able to reproduce their reported χ2ν and RMS values only after removing some outliers from their models and refitting the trimmed down RV set. A suite of 4000 N‐body simulations of their Keplerian model all resulted in unstable systems and revealed that their reported 3.6σ detection of e = 0.32 for the eccentricity of GJ 581e is manifestly incompatible with the system's dynamical stability. Furthermore, their Keplerian model, when integrated only over the time baseline of the observations, significantly increases the χ2ν and demonstrates the need for including non‐Keplerian orbital precession when modeling this system. We find that a four‐planet model with all of the planets on circular or nearly circular orbits provides both an excellent self‐consistent fit to their RV data and also results in a very stable configuration. The periodogram of the residuals to a 4‐planet all‐circular‐orbit model reveals significant peaks that suggest one or more additional planets in this system. We conclude that the present 240‐point HARPS data set, when analyzed in its entirety, and modeled with fully self‐consistent stable orbits, by and of itself does offer significant support for a fifth signal in the data with a period near 32 days. This signal has a false alarm probability of <4% and is consistent with a planet of minimum mass 2.2 M⊕, orbiting squarely in the star's habitable zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
Chris Flynn J. Sommer-Larsen B. Fuchs David S. Graff Samir Salim 《Monthly notices of the Royal Astronomical Society》2001,322(3):553-560
Ibata et al. have recently discovered very faint, moving objects in the Hubble Deep Field (HDF). The number, apparent magnitudes and proper motions of these objects are consistent with old white dwarfs making up part of the Galactic dark halo. We review a number of ground-based proper motion surveys in which nearby dark-halo white dwarfs might be present, if they have the colours and absolute magnitudes proposed. No such objects have been found, whereas we argue here that several times more would be expected than in the HDF. We conclude that it is unlikely that hydrogen-atmosphere white dwarfs make up a significant fraction of the Galactic dark matter. No limits can be placed as yet on helium-atmosphere dwarfs from optical searches. 相似文献
18.
Ian Smail Rob Sharp A. M. Swinbank M. Akiyama Y. Ueda S. Foucaud O. Almaini S. Croom 《Monthly notices of the Royal Astronomical Society》2008,389(1):407-414
We have undertaken a pilot survey for faint quasi-stellar objects (QSOs) in the UKIRT Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS) Field using the KX selection technique. These observations exploit the very deep near-infrared and optical imaging of this field from United Kingdom Infrared Telescope (UKIRT) and Subaru to select candidate QSOs based on their VJK colours and morphologies. We determined redshifts for 426 candidates using the AAOmega spectrograph on the Anglo-Australian Telescope in service time. We identify 17 QSOs ( M B ≲−23) in this pilot survey at z = 1.57–3.29. We combine our sample with an X-ray-selected sample of QSOs in the same field (a large fraction of which also comply with our KX selection) to constrain the surface density of QSOs with K ≤ 20, deriving limits on the likely surface density of 85–150 deg−2 . We use the good image quality available from our near-infrared imaging to detect a spatially extended component of the QSO light which probably represents the host galaxies. We also use our sample to investigate routes to improve the selection of KX QSOs at faint limits in the face of the significant contamination by compact, foreground galaxies. The brightest examples from our combined QSO sample will be used in conjunction with a large Very Large Telescope VIMOS spectroscopic survey of high-redshift galaxies in this region to study the structures inhabited by gas, galaxies and growing supermassive black holes at high redshifts in the UKIDSS UDS. 相似文献
19.
Elizabeth R. Stanway Andrew Bunker Richard G. McMahon 《Astrophysics and Space Science》2003,284(2):381-384
Photometric redshifts have proven a powerful tool in identifying galaxies over a large range of lookback times. We have been
generalising this technique to incorporate the selection of candidate high redshift QSOs. We have applied this to a large
optical/near-infrared imaging survey in 6 wavebands aiming to push farther in redshift (and fainter in luminosity) than previous
studies. We believe that study of these very faint and distant objects provides valuable insights into galaxy formation and
evolution. Here we present work in progress and preliminary results for a catalogue of objects detected as part of the Las
Campanas Infrared Survey. This is a stepping stone to the type of survey data that will become available in the next few years
from projects such as UKIDSS and VISTA.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
20.
从球对称引力场中光子在赤道面上的运动微分方程出发,采用后牛顿近似方法,讨论了Reissner-Nordstr(o|¨)m度规场中雷达回波延迟实验的后后牛顿修正.通过计算给出了后牛顿以及后后牛顿的修正结果,当荷电量Q为零时,该结果适用于电中性天体,其后牛顿部分的修正与经典文献一致.文中采用的近似展开方法同样适用于讨论更高阶的修正.雷达回波的高阶修正对目前展开的高精度空间引力试验有着重要的参考价值. 相似文献