共查询到20条相似文献，搜索用时 15 毫秒
1.
Dual-spin or gyrostat satellites subject to gravitational torques can adopt an infinite number of possible equilibria obtained by adjusting the magnitude and direction of the rotor angular momentum within the satellite. This paper seeks to answer the question, which of these equilibria is best — and best is chosen here to mean most stable in the sense that the energy required to perturb the orientation by any prescribed amount is maximized, i.e. the smallest eigenvalue of the Hessian matrix of the dynamic potential energy is maximized. Using this criterion, it is shown that the conventional configuration for dual-spin satellites with the angular momentum of the rotor, the spacecraft principal axis of maximum moment of inertia, and the perpendicular to the orbital plane coincident is not always the best orientation. The optimal configuration is shown to have the minimum moment of inertia always aligned with the local vertical, but the principal axis of maximum moment of inertia, shifts from the perpendicular to the orbital plane to lying in-plane as the angular momentum of the rotor is increased from zero (corresponding to a rigid gravity gradient satellite) to some sufficiently large value which is determined as a function of parameters. For angular momentum greater than this value, global optimality is established analytically, and otherwise local optimality is proved analytically with global optimality demonstrated numerically. 相似文献
2.
E. A. Belbruno 《Celestial Mechanics and Dynamical Astronomy》1981,25(4):397-415
A new regularizing transformation for the three-dimensional restricted three-body problem is constructed. It is explicitly derived and is equivalent to a simple rational map. Geometrically it is equivalent to a rotation of the 3-sphere. Unlike the KS map it is dimension preserving and is valid inn dimensions. This regularizing map is applied to the restricted problem in order to prove the existence of a family of periodic orbits which continue from a family of collision orbits. 相似文献
3.
Henning Knutsen 《Astrophysics and Space Science》1988,140(2):385-401
An exact solution of Einstein's field equations for an isentropic fluid sphere is examined. It turns out that the crucial factor for the physical properties and the stability of this model is the degree of incompressibility. Necessary and sufficient conditions are given for the weak and the strong energy conditions to be fulfilled and for the speed of sound to be less than the speed of light. The speed of sound always has a minimum at the center of the fluid sphere. But two possibilities exist: either the speed of sound is increasing all the way outwards to the surface of the sphere, or the speed of sound is first increasing, then reaching a maximum when still inside the fluid sphere, and thereafter decreasing outwards to the surface. The adiabatic index is investigated and is found to be increasing outwards for the actual degrees of compressibility. This adiabatic index is always greater than unity, and the temperature is thus decreasing throughout the sphere. The necessary and sufficient condition for the adiabatic index to be greater than 4/3 is also given. (This is a necessary condition for the fluid sphere to be stable.) Chandrasekhar's pulsation equation with boundary conditions is investigated, and the fluid sphere is found to bestable if and only if the degree of incompressibility is greater than a certain value.Dedicated to the memory of the late Bronislaw Kuchowicz. 相似文献
4.
Robert W. Johnson 《Astrophysics and Space Science》2012,338(1):35-48
The principle of maximum entropy is applied to the spectral analysis of a data signal with general variance matrix and containing
gaps in the record. The role of the entropic regularizer is to prevent one from overestimating structure in the spectrum when
faced with imperfect data. Several arguments are presented suggesting that the arbitrary prefactor should not be introduced
to the entropy term. The introduction of that factor is not required when a continuous Poisson distribution is used for the
amplitude coefficients. We compare the formalism for when the variance of the data is known explicitly to that for when the
variance is known only to lie in some finite range. The result of including the entropic measure factor is to suggest a spectrum
consistent with the variance of the data which has less structure than that given by the forward transform. An application
of the methodology to example data is demonstrated. 相似文献
5.
Peter Fellgett 《Astrophysics and Space Science》1972,16(3):437-444
This note is concerned with the inherent possibilities and limitations in the characterisation of irregular variables when, as is almost inevitably the case, the observations are not effectively continuous. The historical tendency to interpret almost everything in terms of strict periodicity is criticised. The basic logic of the situation is discussed, and it is emphasised that intermittency of observation inevitably blunts our knowledge of the spectrum of the variation. This can be restored to only a limited extent, depending on how reliably it is possible to presume missing observations by interpolation. Some applcation is made to a recent publication on 3C 345.A knowledge of formal Fourier theory is assumed, and the aim is to discuss critical principles rather than to recommend any fixed procedures of interpretation. 相似文献
6.
低纬子午环配备科学CCD的必要性和可行性 总被引：1，自引：1，他引：0
从基本天体测量学科的发展趋势和低纬子午环的课题目标出发，叙述了该仪器配备科学ＣＣＤ的必要性和对仪器性能的相应要求，并根据仪器的各种误差的测定方法和测定精度的分析，论述了该仪器配备科学ＣＣＤ后对天体位置作高精度绝对测定的可行性，文中还提出了对配备科学ＣＣＤ的总体要求。 相似文献
7.
Energy transport in a hot flare plasma is examined with particular reference to the influence of fluid motion. On the basis of dimensional considerations the dynamical timescale of the flare plasma is shown to be comparable to the timescale for energy loss by conduction and radiation. It is argued that mass motion is likely to have a profound influence on the evolution of the flare.The detailed response of a flare filament to a localized injection of energy is then analyzed. Radiative, conductive and all dynamical terms are included in the energy equation. Apart from greatly enhancing the rate of propagation of the thermal disturbance through space, mass motion is found to be significant in transferring energy through the moving fluid.Finally the predicted thermal structure is discussed and it is concluded that the presence of mass motions in the flare may be inferred from the form of the soft X-ray differential emission measure. 相似文献
8.
Janine Provost 《Solar physics》1975,40(2):257-273
The response of a bounded atmosphere to a non-resonant excitation applied at its basis is studied. It is shown that the essential feature related to this kind of excitation is that the distribution of the energy of the velocity field relatively to the frequency and horizontal wavelength is a function of height and merely depends on the structure of the atmosphere above the level at which it is considered. The preliminary results concerning an isothermal atmosphere are presented and their relevance to the solar case is discussed. 相似文献
9.
Sharanya Sur Axel Brandenburg 《Monthly notices of the Royal Astronomical Society》2009,399(1):273-280
The generation of mean magnetic fields is studied for a simple non-helical flow where a net cross-helicity of either sign can emerge. This flow, which is also known as the Archontis flow, is a generalization of the Arnold–Beltrami–Childress flow, but with the cosine terms omitted. The presence of cross-helicity leads to a mean-field dynamo effect that is known as the Yoshizawa effect. Direct numerical simulations of such flows demonstrate the presence of magnetic fields on scales larger than the scale of the flow. Contrary to earlier expectations, the Yoshizawa effect is found to be proportional to the mean magnetic field and can therefore lead to its exponential instead of just linear amplification for magnetic Reynolds numbers that exceed a certain critical value. Unlike α effect dynamos, it is found that the Yoshizawa effect is not notably constrained by the presence of a conservation law. It is argued that this is due to the presence of a forcing term in the momentum equation, which leads to a non-zero correlation with the magnetic field. Finally, the application to energy convergence in solar wind turbulence is discussed. 相似文献
10.
Edwin K. Schneider 《Icarus》1983,55(2):302-331
The simplified theory of steady, nearly inviscid, thermally forced axially symmetric atmospheric motions developed by Schneider (1977) is applied to the study of the problem of the Martian great dust storms. A highly idealized calculation of the atmospheric response to heating concentrated in a small latitude band is carried out. Qualitatively different local and global response regimes are identified. As the heating is increased from zero, some critical value is reached at which the response jumps from local to global. It is suggested that this transition from local to global response may be related to the observed explosive growth of great dust storms. Results from the idealized model indicate that subtropical latitudes are favored for the initiation of a dust raising global dust storm, as the meridional scale of the response to a heat source of fixed intensity is largest for the heat source located close to the equator, but the surface stress in the zonal direction produced by the response increases as the heat source is moved towards the poles. Also, the steady axially symmetric Martian response to solar forcing is examined. Modification to the solar forced response due to an added latitudinally localized heat source is briefly discussed, and it is indicated that similar transition behavior to that obtained in the more idealized model is to be expected in this case also. Implications of the dynamical model for the dependence of the occurence of great dust storms on orbital parameters are remarked on. 相似文献
11.
LP估计在星载GPS运动学定轨中的应用及精度分析 总被引：2，自引：0，他引：2
为得到高精度的星载GPS运动学定轨,必须利用观测精度高的相位观测值,但是相位观测值预处理后,仍然存在残余小周跳．在残差服从正态分布情况下LS法是最佳参数解算方法,但该方法不能解决资料的系统误差消除问题,LP估计是处理资料残差分布含有系统误差的有效方法之一．基于LS、LP方法的有效条件和GPS数据预处理的特性,将LP估计方法引入星载GPS运动学定轨数据处理中,以CHAMP卫星资料为例,研究了LP估计在星载GPS运动学定轨中的应用及其精度分析．实践表明:在处理含有残余小周跳的相位观测值时,LP估计比LS更有效,提高了星载GPS运动学定轨精度,但随着残余周跳的进一步修复,LP估计相对于LS估计的优越性越来越弱,在资料完全没有系统误差,残差服从正态分布的情况下,LP估计不能很好地体现其优越性,精度反而低于LS估计． 相似文献
12.
Pavel Voráček 《Astrophysics and Space Science》1981,74(2):463-474
In Section 1 of the paper the energy equation of the Friedmann universe, when matter dominates over radiation, is discussed. It is known that the value of the world potential is constant everywhere in the Universe, despite the pulsation motion of the Universe or a possible transformation of pulsation energy into matter or vice versa. The condition for the Universe being closed is deduced. Furthermore, the possibility to define the mass-energy of the Universe is discussed; and the conclusion is arrived at that the mass-energy of the Universe relative to an observer in the non-metric space outside the Universe is equal to zero; i.e. the Universe originated as a vacuum fluctuation. Finally, the view-point of an external observer is described. Such an observer can claim that our closed Universe is a black hole in a non-metric empty space. Besides, the differences between such a black hole and the astrophysical black holes are indicated.In Section 2 the origin of the gravitational force retarding the expansion is discussed, using the properties of the relativistic gravitational potential. In contradiction to Section 1, the view-point of an inner observer (inside the Universe) is used here. It is concluded that the boundary of the closed Universe is an unlocalizable potential barrier.In Section 3 of the paper the apparent discrepancy between Mach's principle and the general theory of relativity is resolved. The solution is based on the fact that, for the Euclidean open universe, the concept of mass is related to the potential of the background equal to –1, but the concept of the mass-energy is related to the zero-potential of the non-metric background. Because the universe is open and a potential barrier (a boundary of the universe) can be localized-i.e. is geometrically existing — by solution of the field equation, we have to refer to the background with zero-potential. The principal idea of the solution is then that the zero-density means the density of mass-energy, when simultaneously the mass density is equal to the critical value for which the Robertson-Walker metric becomes the Euclidean metric of the Minkowski (i.e., flat) space-time. Further a generalization of Newton's law of inertia is formulated, and the properties of nullgeodesics are touched upon. As a conclusion it is stated that this paper and the two previous ones (see Voráek, 1979a, b)de facto express Mach's principle. 相似文献
13.
Resonant absorption of MHD waves on a nonuniform flux tube is investigated as a driven problem for a 1D cylindrical equilibrium. The variation of the fractional absorption is studied as a function of the frequency and its relation to the eigenvalue problem of the MHD radiating eigenmodes of the nonuniform flux tube is established. The optimal frequencies producing maximal fractional absorption are determined and the condition for total absorption is obtained. This condition defines an impedance matching and is fulfilled for an equilibrium that is fine tuned with respect to the incoming wave. The variation of the spatial wave solutions with respect to the frequency is explained as due to the variation of the real and imaginary parts of the dispersion relation of the MHD radiating eigenmodes with respect to the real driving frequency. 相似文献
14.
A new version is adopted for the evaluation of the upwelling radiation from atmosphere bounded by the surface, where the surface is composed of two half semi-infinite Lambert surfaces and a stream is inserted between them. The contrast of the stream is discussed with respect to the atmospheric effect. The width of the stream is considered to be 0.5, 1, and 3km; The solar and observational direction is located in the normal plane to the stream. The observational site is located at altitude 30km. The horizontal distance of observational site to the stream is fixed to 6.28 . The atmosphere is assumed to be homogeneous, which is composed of aerosol and molecules, where the model aerosol is of the oceanic type.In the computational procedure, a probability of radiation interacting with respective half surfaces and the stream are calculated based on the assumption of single scattering in the atmosphere, where isotropic scattering is undertaken. By use of this probability, the emergent radiation at the top of the atmosphere is calculated approximately by considering the radiative interactions between atmosphere and surfaces up to twice. The numerical simulation exhibits the extraordinary effect near the stream. The contrast of the stream depends upon the albedo of the surrounding surfaces. It increases with the increase of the stream width and decreases with the optical thickness. 相似文献
15.
对中国古代彗星记录的研究状况进行了全面论述,讨论了古代彗星名称、记录和轨道的确定,以事实说明,据这些彗星记录不可能得垤精确的轨道;回顾了古代彗星证认的历史发展,着重分析进行短周期彗得证认时可能出现的问题和解决方法;总结了古代彗星证认的意义：一是有助于短周期彗星长期演化的研究,二是有助于历史年代的确定。 相似文献
16.
In the current study, a double-averaged analytical model including the action of the perturbing body’s inclination is developed
to study third-body perturbations. The disturbing function is expanded in the form of Legendre polynomials truncated up to
the second-order term, and then is averaged over the periods of the spacecraft and the perturbing body. The efficiency of
the double-averaged algorithm is verified with the full elliptic restricted three-body model. Comparisons with the previous
study for a lunar satellite perturbed by Earth are presented to measure the effect of the perturbing body’s inclination, and
illustrate that the lunar obliquity with the value 6.68∘ is important for the mean motion of a lunar satellite. The application to the Mars-Sun system is shown to prove the validity
of the double-averaged model. It can be seen that the algorithm is effective to predict the long-term behavior of a high-altitude
Martian spacecraft perturbed by Sun. The double-averaged model presented in this paper is also applicable to other celestial
systems. 相似文献
17.
Dean F. Smith 《Astrophysics and Space Science》1976,42(2):261-283
A general scheme is established to examine any magnetohydrodynamic (MHD) configuration for its acceleration potential including the effects of various types of plasma waves. The analysis is restricted to plasma waves in a magnetic field with electron cyclotron frequency less than, but comparable to, the electron plasma frequency (moderate field). The general role of electron plasma waves is examined in this paper independent of a specific MHD configuration or generating mechanism in the weak turbulence limit. The evolution of arbitrary wave spectra in a non-relativistic plasma is examined, and it is shown that the nonlinear, process of induced scattering on the polarization clouds of ions leads to the collapse of the waves to an almost one-dimensional spectrum directed along the magnetic field. The subsequent acceleration of non-relativistic and relativistic particles is considered. It is shown for non-relativistic particles that when the wave distribution has a negative slope the acceleration is retarded for lower velocities and enhanced for higher velocities compared to acceleration by an isotropic distribution of electron plasma waves in a magnetic field. This change in behavior is expected to affect the development of wave spectra and the subsequent acceleration spectrum.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
18.
19.
Incontrovertible evidence is presented that the force-free magnetic fields exhibit strong stochastic behavior. Arnold's solution is given with the associated first integral of energy. A subset of the solution is shown to be non-ergodic whereas the full solution is shown to be ergodic. The first integral of energy is applied to the study of these fields to prove that the equilibrium points of such magnetic configurations are saddle points. Finally, the potential function of the first integral of energy is shown to be a member of the Helmholtz family of solutions. Numerical results corroborate the theoretical conclusions and demonstrate the robustness of the energy integral, which remains constant for arbitrarily long computing times. 相似文献
20.
P.D. Hudson 《Planetary and Space Science》1974,22(11):1571-1577
This paper investigates the reflection and transmission of charged particles incident on a rotational discontinuity whose thickness is small compared to their Larmor radius. This is done for the full range of parameters likely to occur for rotational discontinuities in the solar wind. Usually the proportion of particles reflected is similar to that predicted by adiabatic theory, except when the magnetic field ratio is close to unity, or the bend in the fieldlines is large. 相似文献