共查询到20条相似文献，搜索用时 15 毫秒
1.
Hideki Asada 《Celestial Mechanics and Dynamical Astronomy》2007,97(3):151-164
We present an exact solution of the equations for orbit determination of a two body system in a hyperbolic or parabolic motion.
In solving this problem, we extend the method employed by Asada, Akasaka and Kasai (AAK) for a binary system in an elliptic
orbit. The solutions applicable to each of elliptic, hyperbolic and parabolic orbits are obtained by the new approach, and
they are all expressed in an explicit form, remarkably, only in terms of elementary functions. We show also that the solutions
for an open orbit are recovered by making a suitable transformation of the AAK solution for an elliptic case. 相似文献
2.
M. R. Truss 《Monthly notices of the Royal Astronomical Society》2005,356(4):1471-1476
The 2001 outburst of WZ Sagittae has shown the most compelling evidence yet for an enhancement of the mass-transfer rate from the donor star during a dwarf nova outburst in the form of hotspot brightening. I show that, even in this extreme case, the brightening can be attributed to tidal heating near the interaction point of an accretion stream with the expanding edge of an eccentric accretion disc, with no need at all for an increase in the mass-transfer rate. Furthermore, I confirm previous suggestions that an increase in mass-transfer rate through the stream damps any eccentricity in an accretion disc and suppresses the appearance of superhumps, in contradiction to observations. Tidal heating is expected to be most significant in systems with small mass ratios. It follows that systems like WZ Sagittae – which has a tiny mass ratio – are those most likely to show a brightening in the hotspot region. 相似文献
3.
The influence of an interplanetary particle anisotropy on the asymmetry of solar particle entry into the magnetotail is analysed in the diffusion as well as in the reconnection model. By time dependent diffusion calculations with an asymmetric boundary condition in a cylindrical tail lobe it can be shown that a north-south interplanetary anisotropy leads in the open as well as in the closed magnetosphere to essentially the same polar cap structures when observed with a dawn-dusk polar orbiting satellite. However, depending on the satellite orbit, an east-west interplanetary anisotropy can serve to distinguish between rival magnetospheric models. Comparison of our diffusion calculations with polar cap measurements during an east-west interplanetary anisotropy, as presented in Morfill and Scholer (1972), show a large discrepancy, whereas an open tail model fits these observations best. 相似文献
4.
The equilibrium shape of a slender flux tube in the stratified solar atmosphere is studied. The path is determined by a balance between the downwards magnetic tension, which depends on the curvature of the loop, and the upwards buoyancy force. Previous results for untwisted slender tubes are extended to include twisted tubes embedded in an external magnetic field.The path of an untwisted tube in an atmosphere with an ambient magnetic field is calculated. For a given footpoint separation, the height of the tube is lowered by increasing the strength of the external magnetic field. If the footpoints are slowly moved apart, the tube rises, until a threshold separation is reached beyond which there is no possible equilibrium height. This threshold width does not depend on the strength of the external field.The effects of twisting up a curved loop are studied, using an extension of results obtained for slender curved tubes with a straight axis. It is shown that for a twisted tube of given width, there can be two possible values of the equilibrium height. If, however, the tube is twisted more than a certain amount or if the footpoints are too widely separated there is no equilibrium. The critical footpoint separation for non-equilibrium is smaller for a twisted tube that an untwisted one.Twisting a tube or moving its feet apart is thus likely to result in non-equilibrium, causing the tube to rise indefinitely under the influence of the unbalanced buoyant force. It is suggested that this phenomenon could be important in the preflare stage of a large two-ribbon solar flare, by causing the initial slow rise of an active region filament. As well as being involved in the onset of an erupting prominence, this non-equilibrium may also be relevant to the formation of coronal loop transients. 相似文献
5.
6.
V. I. Slysh M. V. Popov B. Z. Kanevskii A. I. Smirnov A. V. Kovalenko Yu. P. Ilyasov V. V. Oreshko B. A. Poperechenko H. Hirabayashi K. M. Shibata E. B. Fomalont 《Astronomy Letters》2001,27(5):277-283
The 64-m radio telescope equipped with an S-2 recording system in the town of Kalyazin was involved in an international fine-structure survey of quasars and active galactic nuclei carried out with a ground-based—space radio interferometer. The HALCA Japanese satellite in an orbit with an altitude of up to 24 000 km with an 8-m antenna was used as a space element of the interferometer. A radio image of the inner region of the CSS-type quasar 3C 147 was obtained with an angular resolution of ～0.3 mas at 6 cm. The image exhibits a core and several jet components mostly arranged in the main jet direction, but one of the components moves across the jet. No evidence was found for the superluminal separation of jet components. The estimated brightness temperature,～1011 K, is consistent with the theoretical limit imposed by synchro-Compton radiation. 相似文献
7.
Interferometry provides one of the possible routes to ultra-high angular resolution for X-ray and gamma-ray astronomy. Sub-micro-arc-second angular resolution, necessary to achieve objectives such as imaging the regions around the event horizon of a super-massive black hole at the center of an active galaxy, can be achieved if beams from parts of the incoming wavefront separated by 100s of meters can be stably and accurately brought together at small angles. One way of achieving this is by using grazing incidence mirrors. We here investigate an alternative approach in which the beams are recombined by optical elements working in transmission. It is shown that the use of diffractive elements is a particularly attractive option. We report experimental results from a simple 2-beam interferometer using a low-cost commercially available profiled film as the diffractive elements. A rotationally symmetric filled (or mostly filled) aperture variant of such an interferometer, equivalent to an X-ray axicon, is shown to offer a much wider bandpass than either a Phase Fresnel Lens (PFL) or a PFL with a refractive lens in an achromatic pair. Simulations of an example system are presented. 相似文献
8.
The instability arising in a slab model of a jet moving in an external plasma is investigated, assuming the plasmas to be governed by the Chew, Goldberger, and Low (CGL) equations. Numerical results on the growth rates of unstable modes are obtained both for symmetric and asymmetric perturbations for equal aligned and transverse fields in wide and slender jet approximations. Special cases of an incompressible jet moving in a static CGL plasma and of a CGL plasma jet moving in an incompressible environment are also considered and conditions of instability derived. 相似文献
9.
We study the parametric decays of an electromagnetic wave propagating along an external magnetic field in an electron-positron plasma. We include weakly relativistic effects on the particle motions in the wave field, and the nonlinear ponderomotive force. We find resonant and nonresonant wave couplings. These include, ordinary decay instabilities, in which the pump wave decays into an electro-acoustic mode and a sideband wave. There are also nonresonant couplings involving two sideband waves, and a nonresonant modulational instability in which the pump wave decays into two sideband modes. Depending on the parameters involved, there is a resonant modulational instability involving a forward propagating electro-acoustic mode and a sideband daughter wave. 相似文献
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11.
Contemporary surveys provide a huge number of detections of small solar system bodies, mostly asteroids. Typically, the reported astrometry is not enough to compute an orbit and/or perform an identification with an already discovered object. The classical methods for preliminary orbit determination fail in such cases: a new approach is necessary. When the observations are not enough to compute an orbit we represent the data with an attributable (two angles and their time derivatives). The undetermined variables range and range rate span an admissible region of solar system orbits, which can be sampled by a set of Virtual Asteroids (VAs) selected by an optimal triangulation. The attributable results from a fit and has an uncertainty represented by a covariance matrix, thus the predictions of future observations can be described by a quasi-product structure (admissible region times confidence ellipsoid), which can be approximated by a triangulation with each node surrounded by a confidence ellipsoid. The problem of identifying two independent short arcs of observations has been solved. For each VA in the admissible region of the first arc we consider prediction at the time of the second arc and the corresponding covariance matrix, and we compare them with the attributable of the second arc with its own covariance. By using the penalty (increase in the sum of squares, as in the algorithms for identification) we select the VAs which can fit together both arcs and compute a preliminary orbit. Even two attributables may not be enough to compute an orbit with a convergent differential corrections algorithm. The preliminary orbits are used as first guess for constrained differential corrections, providing solutions along the Line Of Variations (LOV) which can be used as second generation VAs to further predict the observations at the time of a third arc. In general the identification with a third arc will ensure a least squares orbit, with uncertainty described by the covariance matrix. 相似文献
12.
Alessandro A. Quarta Giovanni Mengali 《Celestial Mechanics and Dynamical Astronomy》2009,105(4):361-377
This paper studies the problem of a spacecraft subject to an outward radial thrust, with constant modulus, that may be switched
on or off at suitable time intervals. The problem is to find the optimal strategy to guarantee the possibility of transferring
the spacecraft from an initial to a final position in a given time interval using the least amount of thrust level. The problem
is solved in an optimal framework, using an indirect approach. A number of different mission scenarios are studied in detail:
escape missions, flyby missions and rendezvous missions. In the latter case the spacecraft uses a hybrid system comprising
an high thrust propulsion system for the final impulsive maneuver. The optimal switching strategy allows one to substantially
decrease the thrust level when compared to the continuous case (without thrust modulation). 相似文献
13.
The evolution of the spectrum of isotropic uniform radiation in an infinite space filled with a homogeneous, nonrelativistic electron gas is calculated by solving the Kompaneets equation. For an infinitely narrow initial spectrum, the time dependence of the average frequency and frequency dispersion is determined in a linear approximation of the equation. Characteristic times corresponding to changes in the character of this dependence are introduced. Two schemes are proposed for the numerical solution of the nonlinear equation: a nonconservative scheme with a grid that is uniform in frequency and a conservative scheme with automatic selection of an adaptive grid in frequency and time. For the linear equation the method yields results consistent with calculations of its solutions in terms of an eigenfunction expansion of the Kompaneets operator calculated in [D. I. Nagirner and V. M. Loskutov, Astrofizika, 40, 97 (1977)]. The influence of nonlinearity on the evolution of the spectrum of initially monochromatic radiation of various intensities is traced as an example of the application of the method. 相似文献
14.
Colin B. Roald 《Monthly notices of the Royal Astronomical Society》1998,300(2):397-410
In an attempt to produce a simple representation of an interface dynamo, I examine a dynamo model composed of two one-dimensional (radially averaged) pseudo-spherical layers, one in the convection zone and possessing an α-effect, and the other in the tachocline and possessing an ω-effect. The two layers communicate by means of an analogue of Newton's law of cooling, and a dynamical back-reaction of the magnetic field on ω is provided. Extensive bifurcation diagrams are calculated for three separate values of η, the ratio of magnetic diffusivities of the two layers. I find recognizable similarities to, but also dramatic differences from, the comparable one-layer model examined by Roald &38; Thomas. In particular, the solar-like dynamo mode found previously is no longer stable in the two-layer version; in its place there is a sequence of periodic, quasi-periodic and chaotic modes probably created in a homoclinic bifurcation. These differences are important enough to provide support for the view that the solar dynamo cannot be meaningfully modelled in one dimension. 相似文献
15.
Based on the magnetar model, we have studied in detail the processes of neutrino cooling of an electron-positron plasma generating
an SGR giant flare and the influence of the magnetar magnetic field on these processes. Electron-positron pair annihilation
and synchrotron neutrino emission are shown to make a dominant contribution to the neutrino emissivity of such a plasma. We
have calculated the neutrino energy losses from a plasma-filled region at the long tail stage of the SGR 0526-66, SGR 1806–20,
and SGR 1900+14 giant flares. This plasma can emit the energy observed in an SGR giant flare only in the presence of a strongmagnetic
field suppressing its neutrino energy losses. We have obtained a lower bound on the magnetic field strength and showed this
value to be higher than the upper limit following from an estimate of the magnetic dipole losses for the magnetars being analyzed
in a wide range of magnetar model parameters. Thus, it is problematic to explain the observed energy release at the long tail
stage of an SGR giant flare in terms of the magnetarmodel. 相似文献
16.
Takahiro Kudoh Shantanu Basu Youichi Ogata Takashi Yabe 《Monthly notices of the Royal Astronomical Society》2007,380(2):499-505
We employ the first fully three-dimensional simulation to study the role of magnetic fields and ion–neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The cores in an initially subcritical cloud develop gradually over an ambipolar diffusion time while the cores in an initially supercritical cloud develop in a dynamical time. The infalling speeds on to cores are subsonic in the case of an initially subcritical cloud, while an extended (≳0.1 pc) region of supersonic infall exists in the case of an initially supercritical cloud. These results are consistent with previous two-dimensional simulations. We also found that a snapshot of the relation between density (ρ) and the strength of the magnetic field ( B ) at different spatial points of the cloud coincides with the evolutionary track of an individual core. When the density becomes large, both the relations tend to B ∝ρ0.5 . 相似文献
17.
18.
Shinpei Shibata 《Astrophysics and Space Science》1989,161(1):145-158
A numerical method to determine the electromagnetic field of a steadily rotating magnetosphere with an inclined magnetic moment under a given boundary condition on an arbitrary shaped boundary surface is presented. The region may include the light cylinder. The present method, together with a companion method giving particle motion and creation, makes an iterative scheme to obtain a global model of the pulsar magnetosphere. A key problem for explaining the particle acceleration in pulsars is to solve field-aligned electric field in an accelerating region bounded by an ideal-MHD region. The present method is fit to connect a solution for the non-ideal-MHD region with another solution for the ideal-MHD region on a boundary surface whose location should also be solved (i.e., a floating boundary). The integration scheme is based on the boundary element method and it has great advantage as compared with other methods like the finite difference method and the Fourier transformation method. 相似文献
19.
A sounding rocket, launched into the expansive phase of an auroral substorm, measured bursts of electric field oscillations with a typical period of one second and a magnitude exceeding 20 mV/m. The oscillations appear to be due to an MHD wave propagating along the magnetic field. The bursts were observed as the sounding rocket passed over the southern border of an auroral arc. The southern border coincided with an increase in 1–5 keV electron flux and an increase in field-aligned current. 相似文献
20.
In this paper we prove the existence of ring-type bounded motion in an isolated system consisting of a massive point particle and a homogeneous cube. We study the case of planar motion where the particle moves in a symmetry plane of the cube and we use a rotating frame of reference with its center at the mass center of the cube and its axes coinciding with the symmetry axes of the cube. We prove that, for negative values of the total energy and properly chosen values of the total angular momentum, the relative distance of the bodies has an upper and a lower bound-i.e., the regions of possible motion lie inside an annulus around the cube (motion inside a ring or an island). 相似文献