共查询到20条相似文献，搜索用时 62 毫秒
1.
Bibhas R. De 《Solar physics》1973,31(2):437-447
A mechanism is suggested for the formation of loop-type prominences in solar-active regions following flare events. The mechanism is based on the already existing idea of compression of a coronal plasma element resulting in enhanced radiation and consequent cooling of the element. A model is suggested for such a compression based on the concept of a contracting, force-free filamentary structure. If the current in a filament increases with time, then there is a radial contraction of the filament. Since the coronal plasma is frozen into the magnetic field lines of the filament, a contraction of the filament causes a compression of the filamentary plasma. This model of compression is shown to be in approximate qualitative and quantitative agreement with observations. 相似文献
2.
J. C. Sherman 《Astrophysics and Space Science》1973,24(2):459-485
A review is given of the experimental investigations concerning the critical velocity of the interaction between a neutral
gas and a plasma in relative motion. In most of the experiments this critical velocity is equivalent to a voltage limitation
of a discharge through a partially ionized magnetized plasma. The critical velocity phenomenon can have been of importance
in a large number of experiments but it has been observed in rather few cases and studied in detail in less than half a dozen
plasma machines. The major investigations were made in rotating plasma devices like the Homopolar, plasma guns and a plasma-neutral
gas impact experiment. The emphasis of this paper is concentrated, though not limited, to collision-free plasmas. Thus the
(MPD-)arc experiments are not extensively treated. It is concluded that the existence, under certain conditions, of a critical
velocity, critical voltage or criticalE/B (depending on the particular observation) is proved by sufficient experimental evidence.
In a following article in this issue by J. Sherman the theoretical work in the same field is discussed. 相似文献
3.
T. G. Vozmischeva 《Celestial Mechanics and Dynamical Astronomy》2000,77(1):37-48
The generalization of a test particle motion in a central field of two immovable point-like centers to the case of a constant
curvature space, on a three-dimensional sphere, is investigated in the paper. The bifurcation set in the plane of integrals
of motion is constructed and the classification of the domains of possible motion is carried out on a two-dimensional sphere.
The regularization of the Kepler’s problem on a two-dimensional sphere is carried out.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Bruno Bertotti 《Celestial Mechanics and Dynamical Astronomy》2001,80(1):21-38
We show that, when a natural satellite like Titan is invisible (e.g., due to an opaque atmosphere) its planetary orbit and its mass can be determined by tracking a spacecraft in close flybys. This is an important problem in the Cassini mission to the Saturnian system, which will be greatly improved by a good astrometric model for all its main components; in particular, an accuracy of a few hundred meters for the orbit of Titan is necessary to allow a measurement of its moment of inertia. The orbit of the spacecraft is the union of elliptical arcs, joined by short hyperbolic transitions: a problem of singular perturbation theory, whose solution leads to a matching condition between the inner hyperbolic orbit and the elliptical orbital elements. Since the inner elements are given in terms of the relative position and velocity of the spacecraft, accurate Doppler measurements in both regions can provide a satisfactory determination of Titan's position and velocity, hence of its Keplerian elements. The errors in this determination are discussed on the basis of the expected Allan deviation of the Doppler method; it is found that the driving errors are those in the elliptical arcs; the fractional errors in Titan's orbital elements are expected to be 10–7. It is also possible to measure the mass of the satellite; however, when the eccentricity e of the flybys is large, the mass and a scaling transformation are highly correlated and the fractional error in the mass is expected to be e times worse. 相似文献
5.
The surroundings of 75 active SB galaxies are investigated. It is shown with a fairly high degree of certainty that more galaxies are observed in the direction of the bar than in other directions. It is concluded that this may be a consequence of the galaxy-forming activity of these galaxies. It is noted that the influence of the surroundings on a galaxy is traditionally discussed, although there is much evidence that the surroundings of a galaxy are shaped due to its activity. 相似文献
6.
Resistance to shearing stress is introduced into a model of a homogeneous, rotating, gravitating medium. It is shown that this eliminates gyroscopic stabilization, both in the presence and in the absence of viscosity. A model of gaseous and rigid media in contact is also considered. Contact with a rigid support also partially eliminates gyroscopic stabilization. This fact may result in some compromise between the theories of density waves and of a self-propagating excitation for explaining the spiral structure of galaxies. 相似文献
7.
Jacques Henrard 《Celestial Mechanics and Dynamical Astronomy》2005,91(1-2):131-149
We present a semi-analytical theory of the rotation of Europa the Galilean satellite of Jupiter. The theory is semi-analytical
in the sense that it is based on a synthetic theory of the orbit of Europa developed by Lainey. The theory is developed in
the framework of Hamiltonian mechanics, using Andoyer variables and assumes that Europa is a rigid body. We consider this
theory as a first step toward the modelization of a non rigid Europa covered by an ocean. 相似文献
8.
Robert A. Greenkorn 《Solar physics》2009,255(2):301-323
A nonlinear analysis of the daily sunspot number for each of cycles 10 to 23 is used to indicate whether the convective turbulence
is stochastic or chaotic. There is a short review of recent papers considering sunspot statistics and solar activity cycles.
The differences in the three possible regimes – deterministic laminar flow, chaotic flow, and stochastic flow – are discussed.
The length of data sets necessary to analyze the regimes is investigated. Chaos is described and a chronology of recent results
that utilize chaos and fractals to analyze sunspot numbers follows. The parameters necessary to describe chaos – time lag,
phase space, embedding dimension, local dimension, correlation dimension, and the Lyapunov exponents – are determined for
the attractor for each cycle. Assuming the laminar regime is unlikely if chaos is not indicated in a cycle by the calculations,
the regime must be stochastic. The sunspot numbers in each of cycles 10 to 19 indicate stochastic behavior. There is a transition
from stochastic to chaotic behavior of the sunspot numbers in cycles 20, 21, 22, and 23. These changes in cycles 20 – 23 may
indicate a change in the scale of turbulence in the convection zone that could result in a change in the convective heat transfer
and a change in the size of the convection region for these four cycles. 相似文献
9.
Jacques Henrard 《Celestial Mechanics and Dynamical Astronomy》2005,93(1-4):101-112
In a previous paper (The Rotation of Europa, Henrard, Celest. Mech. Dyn. Astr., 91, 131–149, 2005) we have developed a semi-analytical theory of Europa, one of the Galilean satellites of Jupiter. It is based
on a synthetic theory of the orbit of Europa and is developed in the framework of Hamiltonian formalism. It was assumed that
Europa is a rigid body and Jupiter a point mass. Several additional effects should be investigated in order to complete the
theory. The present contribution considers the effect of the shape of Jupiter and of the gravitational pull of Io. The sensitivity
of the main theory to a change in the values of the moments of inertia of Europa is also considered. 相似文献
10.
Ni Chen-ping 《Chinese Astronomy and Astrophysics》1990,14(4):422-428
The supernova exploded at the boundary of a dense molecular cloud in a diffuse gas. The eastern half of the shock wave entered the diffuse gas and is now in the stage of adiabatic expansion, forming the observed semicircular remnant; the western half entered the dense cloud, moved at a much slower speed and is now in the pressure-driven snowplow phase, the radiation it emits is in the uv, and is completely absorbed by the interstellar gas along the line of sight. 相似文献
11.
A method of investigation of the magnetic field structure in subphotospheric layers of the Sun has been developed. The method is based on observations of the torisonal oscillations of single sunspots. Characteristics of the torsional oscillations have been obtained from observations of the longitudinal magnetic field and radial velocities of seven single sunspots in the photospheric line Fe I λ5253 Å. The parameters of the torsional oscillations and magnetic tubes in the deep layers have been determined. The radius of the cross section of a magnetic flux tube forming a sunspot is greatest near the Sun’s surface and is approximately equal to the radius of a sunspot umbra. Down to the deeper layers, it decreases quite quickly. The longitudinal electric current appearing in the magnetic tube changes direction. The typical time of the current changes is determined by the period of the torsional oscillations. The intensity of the longitudinal magnetic field in the tube increases with depth. The Alfven wave velocity averaged over the length of a magnetic tube is tens or hundreds of times less than this velocity in a sunspot umbra. It decreases with an increase in the period of oscillations. A decrease in the Alfven wave velocity leads to an increase in the twisting angle of magnetic field lines. 相似文献
12.
Equilibrium configuration of the magnetosphere of a star loaded by the gravitationally accreted plasma having its own magnetic
field is investigated. Axisymmetry around the star’s magnetic axis is assumed for simplicity. It is seen that two distinct
configurations appear for the cases of parallel and antiparallel magnetic field of the accreted plasma with respect to the
star’s magnetic moment. If the external field is antiparallel to the star’s magnetic moment, the stellar magnetosphere is
confined within a spherical region surrounded by the external field with a separatric surface between them. This is an extension
of the case of the spherical accretion of non-magnetic plasma dealt with thus far in connection with the mass accretion by
the degenerate stars in X-ray binaries. It is noticed that the mass slides down along the field lines to the point closest
to the star and is stratified hydrostatically in equilibrium to form a disk in the equatorial plane. The mass loading compresses
the sphere as a whole in this case. If, on the other hand, the external field is parallel to the star’s magnetic moment, there
appears a ring of magnetic neutral point in the equatorial plane. Polar field is open and extends to infinity while the low-latitude
field is closed and faces the external field of opposite polarity across the neutral point. The increase of the loaded mass
in this case causes a shrink of the closed field region, and the open polar flux is increased. Therefore, the transition between
equilibria with small and large amount of the loaded mass requires the reconnection of magnetic lines of force, and the reconnection
of the flux through the magnetic neutral ring is proposed as the mechanism of the steady or the intermittent mass leakage
like the ones postulated for some X-ray bursters.
Visiting Scientist supported by the Japan Society for the Promotion of Sciences. 相似文献
13.
B. P. Kondratyev 《Solar System Research》2013,47(1):1-10
A two-component theoretical model of the physical libration of the Moon in longitude is constructed with account taken of the viscosity of the core. In the new version, a hydrodynamic problem of motion of a fluid filling a solid rotating shell is solved. It is found that surfaces of equal angular velocity are spherical, and a velocity field of the fluid core of the Moon is described by elementary functions. A distribution of the internal pressure in the core is found. An angular momentum exchange between the fluid core and solid mantle is described by a third-order differential equation with a right-hand side. The roots of a characteristic equation are studied and the stability of rotation is proved. A libration angle as a function of time is found using the derived solution of the differential equation. Limiting cases of infinitely large and infinitely small viscosity are considered and an effect of lag of a libration phase from a phase of action of an external moment of forces is ascertained. This makes it possible to estimate the viscosity and sizes of the lunar fluid core from data of observations. 相似文献
14.
Secular perturbations of fictitious satellites that are initially circular and in the equatorial plane of Uranus are discussed. Satellites located in the region where the solar perturbation is dominant become highly eccentric and inclined with respect to the equator, and have a possibility to collide with Uranus. Satellites located in the region where the oblateness perturbation is dominant keep the original eccentricity and the inclination. A scenario of a possible extinction of outer satellites of Uranus is also discussed. 相似文献
15.
R. A. Angiras † C. J. Jog † A. Omar † K. S. Dwarakanath † 《Monthly notices of the Royal Astronomical Society》2006,369(4):1849-1857
The H i surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the H i spatial distribution. The lopsidedness is quantified by the Fourier amplitude A 1 of the m = 1 component normalized to the average value. It is also shown that in the radial region where the stellar disc and H i overlap, their A 1 coefficients are comparable. All the galaxies studied show significant lopsidedness in H i . The mean value of A 1 in the inner regions of the galaxies (1.5–2.5 scalelengths) is ≥ 0.2. This value of A 1 is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies; this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction (∼30 per cent) shows a higher degree of lopsidedness ( A 1 ≥ 0.3). It is also seen that the disc lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two times larger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be ∼10 per cent, assuming that the lopsidedness in H i disc is due to its response to the halo asymmetry. 相似文献
16.
The recently discovered accelerated expansion of the universe is of current interest in theoretical research on the evolution
of the universe. The cause of this behavior is presumably the presence of dark energy, which has been estimated to form up
to 70% of the universe and generates a “repulsive force.” In this paper a cosmological model is constructed which takes the
dark energy into account in a Jordan-Brans-Dicke tensor-scalar model with a dominant, nonminimally coupled scalar field in
the presence of a cosmological scalar. The radiation dominant epoch is discussed.
__________
Translated from Astrofizika, Vol. 51, No. 1, pp. 151–159 (February 2008). 相似文献
17.
We propose a new method based on Lie transformations for simplifying perturbed Hamiltonians in one degree of freedom. The method is most useful when the unperturbed part has solutions in non-elementary functions. A non-canonical Lie transformation is used to eliminate terms from the perturbation that are not of the same form as those in the main part. The system is thus transformed into a modified version of the principal part. In conjunction with a time transformation, the procedure synchronizes the motions of the perturbed system onto those of the unperturbed part.A specific algorithm is given for systems whose principal part consists of a kinetic energy plus an arbitrary potential which is polynomial in the coordinate; the perturbation applied to the principal part is a polynomial in the coordinate and possibly the momentum.We demonstrate the strategy by applying it in detail to a perturbed Duffing system. Our procedure allow us to avoid treating the system as a perturbed harmonic oscillator. In contrast to a canonical simplification, our method involves only polynomial manipulations in two variables. Only after the change of time do we start manipulating elliptic functions in an exhaustive discussion of the flows. 相似文献
18.
V. Krishan 《Journal of Astrophysics and Astronomy》1988,9(4):231-236
The occurrence of superluminal motion in extragalactic radio sources is believed to be quite common. Among others, the geometrical
scattering of radio radiation can also cause superluminal expansion and or motion and halo formation, In this paper, the effectiveness
of the stimulated Raman scattering in producing these features is investigated. The scattering medium is a plasma whose position,
density and temperature decide the rate and angle of scattering. When the radiation from a stationary and constant source
gets scattered from a stationary plasma, a halo is formed around the source. However, the scattering of a rotating radiation
beam does produce superluminal motion of the virtual source. It is found that the plasma should have the characteristics of
the emission-line regions and the intercloud medium in order to Raman scatter the radiation. Since the scattering is polarization
dependent, it is possible to estimate the rotation of the electric vector along the direction of the apparent motion of a
radio source. 相似文献
19.
H. A. Harutyunian 《Astrophysics》2011,54(2):290-299
An attempt is made to study the concept of “black holes” from the standpoint of the axioms of modern physics. It is found
that matter which lies inside a Schwarzschild sphere must disappear, both as a source of electromagnetic waves and as a source
of a gravitational field. To resolve this paradox a hypothesis is proposed according to which the accelerated expansion of
the universe interacts with atomic nuclei in such a way as to transfer a positive energy to every nucleus in accordance with
its volume. The influx of energy into a nucleus gradually neutralizes its binding energy, so that there is an increase in
the mass of the nucleus, as well as of its component nucleons. This mechanism suggests that during the inverse process, when
matter is compressed, the opposite phenomenon should be observed with a release of binding energy, and the average mass of
the nucleons involved in this process should decrease; that is, part of the mass of the material is simply converted into
energy. 相似文献
20.
The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant
HI deficiency up to a factor of 2–3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies
is observed to be directly correlated to the local projected galaxy density, and inversely correlated to the line-of-sight
radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions.
It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions
are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the
hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could
have originated in groups. The co-existence of S0s and severely HI deficient galaxies in the Eridanus group suggests that
tidal interaction is likely to be an effective mechanism for transforming spirals to S0s. 相似文献