共查询到20条相似文献，搜索用时 15 毫秒
1.
The continuous spectrum of a 2D magnetostatic equilibrium with y-invariance is derived. It is shown that the continuous spectrum is given by an eigenvalue problem on each magnetic surface and is related to the different behaviour of the equilibrium quantities in different magnetic surfaces. The special case of a uniform poloidal magnetic field in a 1D equilibrium that is stratified with height, has been considered in detail and it is found that there is no continuous spectrum. It is shown that this result relies completely on the artificial property that the behaviour of the equilibrium quantities along a magnetic field line is independent of the field line considered. As a consequence the non-existence of a continuous spectrum in a 1D equilibrium with a uniform magnetic field cannot be used to argue that the continuous spectrum has no physical relevance.Research Assistant of the Belgian National Fund for Scientific Research. 相似文献
2.
John Lundberg Victor Szebehely R. Steven Nerem Byron Beal 《Celestial Mechanics and Dynamical Astronomy》1985,36(2):191-205
Recent uses of computer graphics allow the representation of the three-dimensional surfaces of zero velocity, also known as Hill's or the Jacobian surfaces. The purpose of this paper is to show the actual surfaces rather than their projections which are available in the standard literature. The analytical properties of the surfaces are also available; therefore, this paper offers the pertinent references rather than the derivations. 相似文献
3.
Scaling forces to asteroid surfaces: The role of cohesion 总被引：1，自引：0，他引：1
The scaling of physical forces to the extremely low ambient gravitational acceleration regimes found on the surfaces of small asteroids is performed. Resulting from this, it is found that van der Waals cohesive forces between regolith grains on asteroid surfaces should be a dominant force and compete with particle weights and be greater, in general, than electrostatic and solar radiation pressure forces. Based on this scaling, we interpret previous experiments performed on cohesive powders in the terrestrial environment as being relevant for the understanding of processes on asteroid surfaces. The implications of these terrestrial experiments for interpreting observations of asteroid surfaces and macro-porosity are considered, and yield interpretations that differ from previously assumed processes for these environments. Based on this understanding, we propose a new model for the end state of small, rapidly rotating asteroids which allows them to be comprised of relatively fine regolith grains held together by van der Waals cohesive forces. 相似文献
4.
L. G. Luk’yanov 《Astronomy Letters》2009,35(5):349-359
We consider the restricted circular three-body problem in which the main bodies have variable masses but the sum of their masses always remains constant. For this problem, we have obtained the possible regions of motions of the small body and the previously unknown surfaces of minimum energy that bound them using the Jacobi quasi-integral. For constant masses, these surfaces transform into the well-known surfaces of zero velocity. We consider the applications of our results to close binary star systems with conservative mass transfer. 相似文献
5.
A convenient photometric function for many particulate surfaces is the generalization of the Lommel-Seeliger law derived by Hapke (1963) and Irvine (1966). This generalization accounts for the effects of mutual shadowing among particles, but still assumes that multiple scattering within the surface layer can be neglected—an assumption which is evidently valid for dark surfaces. We describe a series of laboratory measurements which test the range of validity of this basic assumption, and the applicability of the Hapke-Irvine photometric function, for particulate surfaces whose normal reflectances ranges from 0.04 to 1.04. We find that multiple-scattering effects can be neglected, and that the Hapke-Irvine function can be used, for particulate surfaces whose normal reflectance is about 0.3, or less. The function is definitely inapplicable to surfaces whose normal reflectance exceeds 0.4. 相似文献
6.
T.J. Kalvouridis 《Celestial Mechanics and Dynamical Astronomy》2001,80(2):133-144
The zero-velocity surfaces in the three-dimensional ring problem of N+1 bodies and their parametric evolution is the subject of this paper. These surfaces, which are also known as Hill's or Jacobian surfaces, provide us with valuable information concerning the regions of the permissible particle motion and the existence of equilibrium positions. 相似文献
7.
Michael E. Hough 《Celestial Mechanics and Dynamical Astronomy》1995,62(3):263-287
A new integration theory is formulated for dynamical systems with two degrees of freedom, in the gravitational field of a rotating system. Four integrals of motion may be determined from complete solutions of a system of three first-order, partial differential equations in three independent variables. The solutions of this system define two integral surfaces with space-time coordinates. These surfaces represent two independent solutions of a second-order kinematic system to which the original fourth-order system has been reduced. An integral curve may be represented as the locus of intersection points of the integral surfaces. The new theory is the theoretical basis for a method of analytic continuation of periodic orbits of the circular restricted problem. 相似文献
8.
We have analyzed the motion of an infinitesimal mass in the restricted four-body problem with solar wind drag. It is assumed that the forces which govern the motion are mutual gravitational attractions of the primaries, radiation pressure force and solar wind drag. We have derived the equations of motion and found the Jacobi integral, zero velocity surfaces, and particular solutions of the system. It is found that three collinear points are real when the radiation factor 0<β<0.1 whereas only one real point is obtained when 0.125<β<0.2. The stability property of the system is examined with the help of Poincaré surface of section (PSS) and Lyapunov characteristic exponents (LCEs). It is found that in presence of drag forces LCE is negative for a specific initial condition, hence the corresponding trajectory is regular whereas regular islands in the PSS are expanded. 相似文献
9.
S. I. Shmatov 《Solar System Research》2018,52(7):644-648
The effect of solar radiation on two typical external surfaces of a spacecraft is studied using numerical simulation by considering real geometry (geometry deviation from the nominal shape). The results of simulation are presented in this paper. Geometry deviations are simulated by two types of regular external heterogeneities. We compare the force and torque characteristics of solar radiation on nonuniform surfaces with similar characteristics for nominal surfaces and we find that the disagreement between them can reach 90% both in absolute value and sign. This means that it is necessary to consider the deviations in geometry from nominal for proper design and operation of spacecraft. 相似文献
10.
William K. Hartmann 《Icarus》1984,60(1):56-74
Similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system. This is hypothesized to result from a steady-state “saturation equilibrium” being approached or achieved by cratering processes. This hypothesis conflicts with some recent interpretations. However, it accounts for (1) a similarity in maximum relative crater density, below certain theoretically predicted values, on all heavily cratered surfaces; (2) a leveling off at this same relative density among 100-m scale (secondary?) craters in populations on lunar maria and other sparsely cratered lunar surfaces; (3) the approximate uniformity of maximum relative densities on Saturn satellites (in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors). The lunar frontside upland crater population, sometimes described as a well-preserved production function useful for interpreting other planetary surfaces, is found not to be a production function. It was modified by intercrater plains formed (at least partly) by early upland basaltic lava flooding, recently confirmed spectrophotometrically. Consistent with this, counts in “pure uplands” (those lacking intercrater plains) match the proposed saturation equilibrium density. Variations among large (D > 64 km) crater populations are found, but these may involve several hypothesized mechanisms that rapidly obliterate large craters, especially on icy surfaces. Recent models, in which different populations of interplanetary bodies hit different planets, need further appraisal. 相似文献
11.
Donald G. Saari 《Celestial Mechanics and Dynamical Astronomy》1986,39(4):341-343
In a recent paper by Sergysels with the same title, he describes a problem concerning these surfaces. In this brief commentary, we resolve this problem. 相似文献
12.
Molecular composition of comets, planets and satellites surfaces is known to change radically after suffering impacts. New possibilities concerning the presence of volatile molecules in icy surfaces involving retaining processes are studied in this paper. To fulfill this aim we have carried out desorption experiments under high vacuum conditions based on a quadrupole mass spectrometer and a quartz crystal microbalance. From our results, the presence of certain volatiles in some frozen scenarios could be explained by several retaining mechanisms related to the structure of CO2 even when, after impact, temperatures above their characteristic sublimation ones are reached. 相似文献
13.
While it has been known for a long time that in general the albedo of a surface depends on incidence angle, this fact is commonly neglected in many calculations of planetary surface temperatures. We show that the effect is especially pronounced for bright surfaces. For objects such as Ganymede and Io, the effect produces substantially cooler temperatures near the poles and terminators that would be calculated under the assumption of a constant albedo—a factor which may be important in determining the stability of frosts on such surfaces. 相似文献
14.
In this paper we have made an attempt to define hierarchy of distribution functions for the simultaneous velocity and magnetic fields. Various properties of the constructed functions such as reduction property, separation property, coincidence properties have been discussed, and equations for the evaluation of one- and two-point bivariate distribution functions have been derived. 相似文献
15.
Topographic features affect the scattering properties of planetary surfaces by casting shadows and altering the local incidence and emission angles. Measurements of this phenomenon were obtained on the Cornell goniometer for both high and low albedo surfaces. For the low albedo surface, the decrease in reflected radiation due to topography increases sharply with increasing phase angle, whereas for the high albedo sample the effects are approximately constant between phase angles of 30 and 70°. The observations are in good agreement with a theoretical model in the case of the dark surface. However, for the high albedo surface the model overestimates the effects by about a factor of 2, since it does not include the partial illumination of shadows by multiple scattering. For both high and low albedo surfaces, the effects of topography do not become significant until a phase angle of 30–40°. 相似文献
16.
Using the famous Sundman inequality, we have constructed for the first time the surfaces for the general three-body problem that we suggest calling Sundman surfaces. These surfaces are a generalization of the widely known Hill surfaces in the restricted circular three-body problem. The Sundman surfaces are constructed in a rectangular coordinate system that uses the mutual distances between the bodies as the Cartesian rectangular coordinates. The singular points of the family of these surfaces have been determined. The possible and impossible regions of motion of the bodies have been constructed in the space of mutual distances. We have shown the existence of Hill stable motions and established sufficient criteria for Hill stability of motions. Some of the astronomical applications are considered. 相似文献
17.
Nonzero cosmological constant favours cosmological models with larger Hubble constant. The evolution of ionization during
decoupling period in a universe with nonzero cosmological constant is computed by using a corrected recombination coefficient.
Also presented in this paper is the redshift distribution of the last scattering surfaces of the cosmic background photons
while the cosmological constant is nonvanishing. Finally, we give a brief estimation about the influence of He on the last scattering surfaces.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
We have calculated the desorption rates of both physisorbed and chemisorbed ions from grain surfaces, due to the temperature increase at densities higher than 10–13 g cm–3. It has been found that physisorbed ions desorb from grain surfaces at neutral densities ofn>1.3×1011 cm–3, assuming that the desorption energyD is equal to 0.1 eV, while the desorption of chemisorbed ions from grain surface can only occur at neutral densities ofn>1015 cm–3, at which point thermal ionization becomes more dominant.The electrons are assumed to be emitted from grain surfaces in a manner similar to the thermonic emission from heated solid surfaces. It was found that the temperature at which electrons are emitted from negatively charged grains depends on the value of the work function of the material of the grain.The charge state has been calculated for two limiting cases. Neglecting the grain surface reactions in case 1, the resulting relative charge density represents an upper limit, such that the electrical conductivity remains high. In this situation the magnetic flux dissipation is mainly contributed by ambipolar diffusion. In the second case, it has been assumed that the charged particles are chemically adsorbed on grain surfaces such that their desorption is negligible. In this case the charge density decreases sharply with increase of neutral density. Therefore, the electrical conductivity decreases sufficiently and Ohmic dissipation becomes effective. 相似文献
19.
Trevor W. Marshall 《Astrophysics and Space Science》2012,342(2):329-332
The contemporary notion of black hole originates in Oppenheimer and Snyder’s 1939 article “On Continued Gravitational Contraction” (Phys. Rev. 56:455, 1939). In particular, Penrose (Phys. Rev. Lett. 14:57, 1965) showed that their metric gave rise to trapped surfaces, that is regions of space from which no light rays can escape, and proved that within such surfaces black-hole formation is inevitable. Section “No trapped surfaces” of this article shows that a simple modification of the Oppenheimer-Snyder metric, fully consistent with General Relativity, may be made, so that all radial light rays originating in the interior escape to the exterior. There is no trapped surface and no black hole; on the contrary there is a stable end state with finite density, contained within a sphere of Schwarzschild radius. Implications for the interpretation of General Relativity, and also for experimental observation of supermassive objects and the Event Horizon Telescope project, are discussed in the concluding section. 相似文献
20.
James R. Gaier 《Icarus》2012,221(1):167-173
The Thermal Degradation Sample (TDS) experiment was one of the many investigations performed on the lunar surface during Apollo 14. Remarkably, the results of this 40 year old experiment were never fully interpreted, perhaps in part because the hardware vanished after its return. Mission records, high resolution photographs returned from the mission, and recent laboratory investigations have been used to glean important results from this experiment. It is most likely that the dust adhesion to the TDS was less than anticipated because of atomic-level contamination of its surfaces. These contaminants were probably removed from most equipment surfaces on the Moon by sputter cleaning by the solar wind, but the TDS experiments were not exposed to the solar wind long enough to affect the cleaning. 相似文献