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1.
High-resolution radar observations of Mars at a wavelength of 3.8 cm have been carried out at the Haystack Observatory for a period of about 6 months surrounding the 1971 opposition. The relative surface height variation with longitude over a band of Martian latitudes between about ?14° and ?22° has been derived from these observations with an error of about 75m in the most favorable cases. At ?14° latitude, the dielectric constant was found to increase steadily from about 1.7 at 70°W to about 5.0 at 110°W. Over the same interval the rms surface slopes vary from 2° to 1°. The mean equatorial radius of Mars as determined from the combined radar data of 1967, 1969 and 1971 was found to be 3394 ± 2km. 相似文献
2.
《Planetary and Space Science》1999,47(8-9):971-974
Astronomical observations of size and of related outgassing rates seem not to be compatible for the nucleus of comet 46 Pz.urule;Wirtanen, the target comet of the ROSETTA mission. This possible disagreement has caused speculations about peculiar properties of this comet nucleus. It is shown by model calculations which also takes into account vertical heat fluxes into the nucleus that there is a possibility to combine the results of astronomical observations within a model of a freely sublimating ice surface of this comet with an outgassing area of about half the dayside surface. The resulting half-size parameter (i.e. the radius of an equivalent sphere) can be shown to be of about R ≈ (725±230) m, and the nucleus is shown to have an active area of about half of the dayside surface, i.e. of about 25% of the total surface. 相似文献
3.
Nalin R. Mukherjee 《Earth, Moon, and Planets》1976,15(3-4):403-413
Some of the bonded particles of lunar soil samples separated upon exposure to reactive gases such as oxygen, water vapor, their mixtures, acids and bases. The bondings between particles susceptible to gas-disruption seemed to be generally weak and appeared to have taken place via highly radiation-damaged layers at the particle surfaces. These amorphous layers with an average thickness of about 0.05µm were produced by the solar wind exposure of about 2000 years. Therefore, the solar wind was responsible for the formation of these weak bondings and also probably responsible for disruption of these bondings. Apollo 11 and 12 landed in the equatorial region and about 1500 km apart. Thus, the solar wind effects on materials at these sites should have been about the same and the proportion of bonded particles separated by reactive gas exposure should also have been about the same; but the number of separations observed was about 2.7 (average) times greater in the Apollo 11 soil sample than in the Apollo 12 soil sample. This finding suggests that the number of weakly bonded particles and probably the solar-wind damaged amorphous layer particles at these sites was about in the same proportion. It is, therefore, considered that materials from certain depth (practically not exposed to the solar wind) of another site were transported and mixed during recent years (considerably less than 2000 years) with the original materials of the Apollo 12 site. This is consistent with the conclusions made by other investigators. 相似文献
4.
A sudden commencement occurred at 2348 UT on 15 February 1967, when the ATS-1 satellite was about 2 hr past local noon at a geocentric distance of 6.6R E. Plasma was observed by the Suprathermal Ion Detector (SID) first to flow in the antisolar direction, as expected, but then to flow westward, for about 2 min, at about 50 km/sec. Analysis of ground magnetograms suggests that the surprising westward flow, which must have involved an electric field of about 10 mV/m at 6.6R E, resulted from the ionosphere's reaction to the sudden commencement. Beginning about 2 min before the start of the westward flow at ATS-1, ground magnetometers near the foot of the ATS-1 field line typically recorded magnetic-field deflections of about 70 γ, to the northeast. No attempt is made in this paper to explain these ground observations. However, taking the ground observations, assuming a height-integrated Hall conductivity of 1 mho, and a standoff distance of 7.2R E inferred from Explorer 33 solar-wind data, we derive a magnetospheric electric field that agrees in magnitude and direction with that required to produce the observed flow at ATS-1. 相似文献
5.
In most courses on continuum mechanics the law of angular momentum is studied about a fixed point (usually about the origin of the frame). In this article we clarify this law about any point in three-dimensional Euclidean space and discuss the law, about any point, for an observer in a rotating frame. 相似文献
6.
Sloan Digital Sky Survey 总被引：1，自引：0，他引：1
Stephen M. Kent 《Astrophysics and Space Science》1994,217(1-2):27-30
The Sloan Digital Sky Survey is a project which will produce a detailed digital phometric map of half the northern sky to about 23 magnitude using a special purpose wide field telescope of 2.5 meter aperture. This map will be used to select about a million galaxies and 100,000 quasars, for which high resolution spectra will be obtained using the same telescope. A catalog will be produced of all the detected objects, about 100 million galaxies and a similar number of stars, and a million quasar candidates. 相似文献
7.
Solar radio maps obtained by our group and others over a wide wavelength range (millimeter to meter) and over a considerable time span (1973–1978) have allowed us to compute the radio spectrum of an average coronal hole, i.e., the brightness temperature inside a coronal hole normalized by the brightness temperature of the quiet Sun outside the coronal hole measured at several different radio wavelengths. This radio spectrum can be used to obtain the changes of the quiet Sun atmosphere inside coronal holes and also as an additional check for coronal hole profiles obtained by other methods. Using a standard solar atmosphere and a computer program which included ray tracing, we have tried to reproduce the observed radio spectrum by computing brightness temperatures at many different wavelengths for a long series of modifications in the electron density, neutral particle density and temperature profiles of the standard solar atmosphere. This analysis indicates that inside an average coronal hole the following changes occur: the upper chromosphere expands by about 20% and its electron density and temperature decrease by about 10%. The transition zone experiences the largest change, expanding by a factor of about 6, its electron density decreases by a similar factor, and its temperature decreases by about 50%. Finally in the corona the electron density decreases by about 20% and the temperature by about 15%. 相似文献
8.
The 1998 Leonid meteor shower was observed at the Gaomeigu station of Yunnan Observatory during five successive days in November.
The visual records indicate that the number of meteors increased suddenly, from a ZHR of about 140 to over 400, in the early
morning of November 17th, Beijing time. But it decreased slowly in the following two days. During the maximum there was a
high proportion – about 10 percent – of very bright fireballs with enduring trains. The brightest one was about -10 magnitude
with a smoke train fading about three minutes after.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
Hans J. Sperling 《Celestial Mechanics and Dynamical Astronomy》1972,6(3):278-293
Let a rigid satellite move in a circular orbit about a spherically symmetric central body, taking into account only the main term of the gravitational torque. We shall investigate and find all solutions of the following problem: Let the satellite be permitted to spin about an axis that is fixed in the orbit frame; the satellite need not be symmetric, the spin not uniform, and the spin axis not a principal axis of inertia. The complete discussion of this type of spin reveals that the cases found by Lagrange and by Pringle - and the well-known spin about a principal axis of inertia orthogonal to the orbit plane — are essentially the only ones possible; the only further (degenerate) case is uniform spin of a two-dimensional, not necessarily symmetric satellite about certain axes that are orthogonal to the plane containing the body and to the orbit of the satellite around the central body. 相似文献
10.
Models of the giant planets were constructed based on the assumption that the hydrogen to helium ratio is solar in these planets. This assumption, together with arguments about the condensation sequence in the primitive solar nebula, yields models with a central core of rock and possibly ice surrounded by an envelope of hydrogen, helium, methane, ammonia, and water. These last three volatiles may be individually enhanced due to condensation at the period of core formation. Jupiter was found to have a core of about 40 earth masses and a water enhancement in the atmosphere of about 7.5 times the solar value. Saturn was found to have a core of 20 earth masses and a water enhancement in the atmosphere of about 25 times the solar value. Rock plus ice constitute 75–85% of the mass of Uranus and Neptune. Temperatures in the interiors of these planets are probably above the melting points, if there is an adiabatic relation throughout the interiors. Some aspects of the sensitivities of these results to uncertainties in rotational flattening are discussed. 相似文献
11.
日长亚季节变化的振幅调制与厄尔尼诺现象 总被引：7，自引：0，他引：7
利用Ｈｉｌｂｅｒｔ变换解算了１９６２年至１９９６年日长亚季节振荡的包络变化．结果显示,日长亚季节变化的振幅调制不仅具有一线性增长趋势,而且包含一个超前厄尔尼诺变化历程的年际变化．分别研究日长亚季节准５０天包络变化以及准１２０天包络变化后发现,这种超前性来自日长亚季节准５０天的振幅变化,这一结论与大气的研究结果一致．日长亚季节准１２０天的振幅变化在１９７２年附近发生了位相１８０°反转,因此日长亚季节准１２０天的振幅变化自１９７６年以来与厄尔尼诺的历程正好同步,１９７２年以前则截然相反．众所周知,日长变化在亚季尺度上具有显著的易变性,然而上述的非线性调制现象不仅表明了日长亚季节变化的内在规律性,而且证实了地球自转运动在不同时间尺度上存在极明显的非线性相互作用．同时也表明,日长变化作为全球风系变化的一个有价值的信息指标,将逐渐成为研究气候系统全球状态的有效手段之一． 相似文献
12.
Lowell's value for the mass of Planet X was about seven times that of the Earth. Postdiscovery determinations of the mass of Pluto from analysis of the observed motions of Uranus and Neptune reduced this value to about one Earth mass. More extended analyses in the past 10 years have lowered this value to about one-tenth of an Earth mass. The mass so derived, however, fails to agree by a factor of 50 with that determined from the motion of the newly discovered satellite Charon. The discrepancy may arise from unmodeled effects in the motions of the outer planets. 相似文献
13.
使用1994—2007年在1．56m反射望远镜CCD照相机拍摄的资料，测定了上海天文台余山工作站夜天亮度的变化。由于上海城市的发展，佘山工作站的夜天亮度在V波段从每平方角秒约19mag变到15．8mag，也就是说，夜天亮度自1994年以来变亮了约20倍。国际上优良台站的夜天亮度在V波段等于或暗于21．5mag。上述夜天亮度是优良台站的200多倍。现在在佘山工作站使用1．56m反射望远镜对暗于V=14mag的星做精确测光已经很困难了。 相似文献
14.
Eighty-seven measurements of the thermal structure in the atmosphere of Venus between the altitudes of about 40 and 85 km were derived from Pioneer Venus Orbiter radio occultation data taken during four occultation seasons from December 1978 to October 1981. These measurements cover latitudes from ?68 to 88° and solar zenith angles of 8 to 166°. The results indicate that the characteristics of the thermal structure in both the troposphere and stratosphere regions are dependent predominantly on the latitude and only weakly on solar illumination conditions. In particular, the circumpolar collar cloud region in the northern hemisphere (latitude 55 to 77°) displays the most dramatic changes in structure, including the appearance of a large inversion, having an average magnitude of about 18°K and a maximum of about 33°K. Also in this region, the tropopause altitude rises by about 4.8 km above its value at low latitudes, the tropopause temperature drops by about 60°K, and the pressure at the tropopause decreases by an average of about 240 mbar. These changes in the collar region are correlated with observations of increased turbulence and greater amplitude of thermal waves in the region, which is located where the persistent circulation pattern in the Venus atmosphere changes from zonally symmetric retrograde rotation to a hemispherical circumpolar vortex. It was shown that the large zonal winds associated with this circulation pattern are not likely to produce distortions in the atmosphere of a magnitude that could lead to temperature errors of the order of the mesosphere inversions observed in the collar region, but under certain circumstances zonal wind distortion could cause errors of 3–4°K. 相似文献
15.
A one-dimensional simulation of pure water-ice cometary nuclei is presented, and the effect of the nucleus as a heat reservoir is considered. The phase transition from amorphous to crystalline ice is studied for two cases: (1) where the released latent heat goes entirely into heating adjacent layers and (2) where the released latent heat goes entirely into sublimation. For a Halley-like orbit it was found that for case 1 the phase boundary penetrates about 15 m on the first orbit and does not advance until sublimation brings the surface to some 10 m from the phase boundary. For case 2 the phase boundary penetrates about 1 m below the surface and remains at this depth as the surface sublimates. For an orbit like that of Schwassmann-Wachmann 1 the phase boundary penetrates about 50 m initially for case 1 and about 1 m for case 2. There is no further transformation until the entire comet is heated slowly to near the transition temperature, after which the entire nucleus is converted to crystalline ice. For an Encke-type orbit case 1 gives a nearly continuous transition of the entire nucleus to crystalline ice, while for case 2 the initial penetration is about 8 m and remains at this depth relative to the surface as sublimation decreases the cometary radius. Thus the entire comet is converted to crystalline ice just before it is completely dissipated. 相似文献
16.
We study the temporal variation of subsurface flows of 788 active regions and 978 quiet regions. The vertical-velocity component
used in this study is derived from the divergence of the measured horizontal flows using mass conservation. The horizontal
flows cover a range of depths from the surface to about 16 Mm and are determined by analyzing about five years of GONG high-resolution
Doppler data with ring-diagram analysis. We determine the change in unsigned magnetic flux during the disk passage of each
active region using MDI magnetograms binned to the ring-diagram grid. We then sort the data by their flux change from decaying
to emerging flux and divide the data into five subsets of equal size. The average vertical flows of the emerging-flux subset
are systematically shifted toward upflows compared to the grand average values of the complete data set, whereas the average
flows of the decaying-flux subset show comparably more pronounced downflows especially near 8 Mm. For flux emergence, upflows
become stronger with time with increasing flux at depths greater than about 10 Mm. At layers shallower than about 4 Mm, the
flows might start to change from downflows to upflows, when flux emerges, and then back to downflows after the active regions
are established. The flows in the layers between these two depth ranges show no response to the emerging flux. In the case
of decaying flux, the flows change from strong upflows to downflows at depths greater than about 10 Mm, whereas the flows
do not change systematically at other depths. A cross-correlation analysis shows that the flows in the near-surface and the
deeper layers might change about one day before flux emerges. The flows associated with the quiet regions fluctuate with time
but do not show any systematic variation. 相似文献
17.
Analysis of the gravity gradiometer developed by R. L. Forward and C. C. Bell at the Hughes Research Laboratories suggest than an accuracy, in the range 0.1 to 0.5 EU can be expected in a lunar orbiter application. This accuracy will allow gradient anomalies associated with mascons to be mapped with 1% accuracy and should reveal a great deal of new information about the lunar gravity field.The proposed experiment calls for putting such a gradiometer into a closely circular polar orbit at an average height of about 30 km above the lunar surface. This orbit allows the entire lunar surface to be covered in fourteen days, the gradiometer to be checked twice per revolution and results in successive passes above the lunar surface being spaced at about the resolution limit of about 30 km set both by the satellite altitude and instrumental integration time. Doppler tracking will be employed and the spacecraft will carry an electromagnetic altimeter. Gradient and altitude data from the far side of the Moon can be stored for replay when communication is re-established. 相似文献
18.
For about two weeks in 1995, the balloon-borne Flare Genesis Experiment will continuously observe the Sun well above the turbulent, image-blurring layers of the Earth's atmosphere. The polarization-free 80 cm telescope will supply images to a liquid-crystal based vector magnetograph, which will measure magnetic features at a resolution of 0.2 arcsec. An electrically tunable lithium-niobate Fabry-Perot provides a spectral resolution of about 0.015 nm. In a follow-up series of Antarctic balloon flights, the Flare Genesis Experiment (FGE) will provide unprecedented details about sunspots, flares, magnetic elements, filaments, and the quiet solar atmosphere. 相似文献
19.
A new method for automated detection of polar coronal holes is presented. This method, called perimeter tracing, uses a series of 171, 195, and 304 Å full disk images from the Extreme ultraviolet Imaging Telescope (EIT) on SOHO over solar cycle 23 to measure the perimeter of polar coronal holes as they appear on the limbs. Perimeter tracing minimizes line-of-sight obscurations caused by the emitting plasma of the various wavelengths by taking measurements at the solar limb. Perimeter tracing also allows for the polar rotation period to emerge organically from the data as 33 days. We have called this the Harvey rotation rate and count Harvey rotations starting 4 January 1900. From the measured perimeter, we are then able to fit a curve to the data and derive an area within the line of best fit. We observe the area of the northern polar hole area in 1996, at the beginning of solar cycle 23, to be about 4.2% of the total solar surface area and about 3.6% in 2007. The area of the southern polar hole is observed to be about 4.0% in 1996 and about 3.4% in 2007. Thus, both the north and south polar hole areas are no more than 15% smaller now than they were at the beginning of cycle 23. This compares to the polar magnetic field measured to be about 40% less now than it was a cycle ago. 相似文献
20.
利用VLBI数据确定"探测一号"卫星的轨道 总被引：5，自引：0，他引：5
双星计划的“探测一号”是中国首颗真正严格意义上的科学实验卫星,其运行轨道为中国迄今所发射的卫星中距地球最远,远地点地心距达7．8万公里．采用射电天文的VLBI技术可以对“探测一号”以及更远的深空目标,如探月飞行器实现跟踪．为了验证VLBI技术在我国探月计划中的作用,上海天文台组织了国内目前仅有的上海、乌鲁木齐和昆明3个台站对“探测一号”进行试跟踪,利用对“探测一号”约两天的VLBI观测数据,确定“探测一号”卫星的轨道,对VLBI的定轨能力做初步的探讨．按照测控部门提供的初轨 (其精度仅保证跟踪)推算的轨道与VLBI时延的拟合误差平均约2 km,时延率的拟合误差平均约15 cm／s．而利用VLBI数据定轨后的拟合程度相对于初轨有了很大的改善,结果表明,单独利用VLBI时延定轨,时延的拟合精度约5．5 m,作为外部检核的VLBI时延率的拟合精度在2 cm／s左右．单独利用VLBI时延率定轨,时延率的拟合精度约为1．3 cm／s,作为外部检核的VLBI时延的拟合精度约为29 m．而若将时延和时延率数据联合定轨,采用其内符精度加权,VLBI时延和时延率的残差分别为5．5 m和 2 cm／s．为了合理地评估VLBI定轨的真实精度,利用模拟数据进行误差协方差分析,结果表明VLBI定轨精度受动力学模型误差的影响较大,由于"探测一号”卫星的动力学模型难以精确确定,所以利用两天弧段的VLBI数据确定“探测一号”卫星轨道的位置误差为km量级,而速度误差可达cm／s量级．模拟计算还表明, VLBI和USB数据联合定轨可以大大提高定轨精度． 相似文献