共查询到20条相似文献，搜索用时 194 毫秒
1.
W. Alpers 《Astrophysics and Space Science》1969,5(4):425-437
Plane models of the magnetopause are investigated under the assumption that ionospheric electrons are able to short-circuit electric fields (exact charge neutrality). Using the Vlasov theory a general method is presented for constructing distribution functions that lead to given magnetic field and tangential bulk velocity profiles. As an example we describe the magnetic field transition in terms of error functions and obtain particle distributions in explicit form, including bulk velocities.It is thus shown that bulk velocities in the direction of the magnetic field do not necessarily lead to a non-equilibrium magnetopause which investigations by Parker and Lerche seem to suggest.Of the European Space Research Organisation (ESRO). 相似文献
2.
A class of exact imperfect fluid solutions of the Brans-Dicke (BD) field equations, in which the geometrical part is identical to that of BD-FRW dust model withk=0 is presented. The solutions are functions of time and radial coordinates and satisfy all necessary energy and thermodynamic conditions. 相似文献
3.
We present an analysis of Chandra ACIS observations of the field of TeV J2032+4130, the first unidentified TeV source, detected serendipitously by HEGRA. This
deep (48.7 ks) observation of the field follows up on an earlier 5 ks Chandra director’s discretionary observation. Of the numerous point-like X-ray sources in the field, the brightest are shown to be
a mixture of early and late-type stars. We find that several of the X-ray sources are transients, exhibiting rapid increases
in count rates by factors 3–10, and similar in nature to the one, hard absorbed transient source located in the earlier Chandra observation of the field. None of these transient sources are likely to correspond to the TeV source. Instead, we identify
a region of diffuse X-ray emission within the error circle of the TeV source and consider its plausible association. 相似文献
4.
We have taken a plasma model of ionosphere which is quite different from the earlier model employed by several workers to study the whistler as a diagnostic technique in the ionosphere. In contrast to the earlier model, we have considered an ionosphere that includes negative ionic species and due to which the mathematical technique loses the basic assumptions chosen earlier to derive the group travel time. We have shown that the negative ion has a significant role and without its contribution, the method to diagnose the ionospheric parameters may lead to an error. We have discussed also how to uset()=
0
h
dh/v
g
as a diagnostic tool in determining the ionospheric parameters. 相似文献
5.
Gamal G. L. Nashed 《Astrophysics and Space Science》2011,335(2):593-598
We give a class of spherically symmetric-Anti de Sitter (Ads), exact solution in the teleparallel equivalent of general relativity
(TEGR). The solution depends on an arbitrary function F(R)\mathcal{F}(R) and reproduce the metric of Schwarzschild Ads space-time. In the context of the Hamiltonian formulation of the TEGR we compute
the gravitational energy of this class. The calculation is carried out by means of an expression for the energy of the gravitational
field that naturally arises from the integral form of the constraint equations of the formalism. We show that the form of
the energy depends on the arbitrary function. We make a constrain on this arbitrary function to give the correct form of energy. 相似文献
6.
The question of the influence of electronic concentration fluctuations along the line of sight on the dispersion measure (DM) and rotation measure (RM) is under consideration. it is shown that the measuring ofDM andRM enables us to determine the number of electronsN
e
=n
e
ds and the quantityM
e
=n
e
H cos ds, respectively, along the line of sight in accordance with the general assertion. Under the conditions considered, the conclusion (Lerche, 1970a, b) that the connection ofDM andRM withN
e
andM
e
greatly depends on the electronic concentration fluctuations, is found to be incorrect. 相似文献
7.
Yu. V. Glagolevskij 《Astrophysical Bulletin》2010,65(1):34-41
We model the magnetic fields of four magnetic stars using published longitudinal (Be) field measurements. The structure of
the magnetic field of each of the four stars is close to that of the central dipole. Unfortunately, the number of measurements
for each star is insufficient for accurate finding of the field parameters, and therefore we find no dipole shift exceeding
its error Δa ≈ 0.1, expressed as a fraction of the stellar radius. Our data support the opinion that the results of modeling depend most
strongly on the adopted inclination of the star’s rotation axis i. 相似文献
8.
Mukund L. Kurtadikar 《Astrophysics and Space Science》1986,124(2):305-314
In the effective straight-line trajectory (EST) approach for computation of collision-induced line widths proposed earlier, an arbitrary factorRX which has turned out to be the measure of the effectiveness of the curved trajectory for a system, was introduced. However, in the earlier paper no exact method has been suggested to determine its optimum value for a given system. In the present paper, a method has been proposed to determine its optimum value from the temperature dependence of the collision induced line widths. 相似文献
9.
We have solved the equation of radiative transfer in spherical symmetry with scattering and absorbing medium. We have set the albedo for single scattering to be equal to 0.5. We have set the Planck function constant throughout the medium in one case and in another case the Planck function has been set to vary asr
–2. The geometrical extension of the spherical shell has been taken as large as one stellar radius. Two kinds of variations of the optical depth are employed (1) that remains constant with radius and (2) that varies asr
–2. In all these cases the internal source vectors and specific intensities change depending upon the type of physics we have employed in each case. 相似文献
10.
The numerical integration of the differential equations describing dynamical systems has been shown in previous papers of this series to be most effectively accomplished by an explicit Taylor series method.In this paper we show that one explicit Taylor series method, developed earlier in this series and which appears to possess a high degree of versatility, yields considerable gains in efficiency over classical single-step and multi-step methods. (In this context efficiency is a measure of the time taken to carry out a calculation of a specific accuracy).For a given accuracy criterion governing the local truncation error (LTE) it is found that the Taylor series method is generallytwice as fast as the classical multi-step method and up totwenty times faster than the classical single-step method. 相似文献
11.
K. S. Adhav A. S. Bansod M. S. Desale R. B. Raut 《Astrophysics and Space Science》2011,331(2):689-695
We have studied Locally Rotationally Symmetric (LRS) Bianchi type-I space-time having constant deceleration parameter and
filled with perfect fluid in the Hoyle-Narlikar C-field cosmology. Here, the creation field C is a function of time t only. The geometrical and physical aspects for the models are also studied. 相似文献
12.
An exact Bianchi type-V perfect fluid cosmological model is obtained in a scalar tensor theory proposed by Sen (Z. Phys. 149:311,
1957) based on Lyra Manifold in case of β is a constant and it is shown that this cosmological model exists only in the case of Radiation Universe (ρ=3p) if β is a function of ‘t’ using negative constant deceleration parameter. Some physical and geometrical properties of these models are discussed. 相似文献
13.
J. Galindo Trejo 《Solar physics》1987,108(2):265-313
Using the MHD energy principle of Bernstein et al. (1958) we develop a formalism in order to analyze the stability properties of two-dimensional magnetostatic plasma equilibria. We apply this to four models of quiescent prominences, namely those of Menzel (1951), Dungey (1953), Kippenhahn and Schlüter (1957), and finally Lerche and Low (1980). For the observed parameter range, all models are stable and they explain reasonably well the reported flare-initiated oscillations in quiescent prominences. We also investigate other parameters regions, which may be relevant in some stellar atmospheres. It is found that, with the exception of the Kippenhahn and Schlüter model, all models become unstable. The instabilities that occur show simultaneously several features of well-known MHD-instabilities. However, an unequivocal assignment of the instabilities to specific instability prototypes is not possible. Our formalism allows one to investigate not only more realistic prominence equilibria, but also arbitrary one- and two-dimensional static ideal MHD-equilibria. 相似文献
14.
We study the static stiff-fluid model for perfect fluid distributions in the presence of incident magnetic field. The magnetic field is surrounded by static stiff fluid of infinite electric conductivity and it is due to the electric current flowing along theZ-axis. The various physical and geometrical properties together with the state of model in absence of magnetic field are also discussed. 相似文献
15.
In this review paper we discuss several aspects of magnetic reconnection theory, focusing on the field-line motions that are
associated with reconnection. A new exact solution of the nonlinear MHD equations for reconnective annihilation is presented which represents a two-fold generalization of the previous solutions. Magnetic reconnection at null points by
several mechanisms is summarized, including spine reconnection, fan reconnection and separator reconnection, where it is pointed out that two common features of separator reconnection are the rapid flipping of magnetic field lines
and the collapse of the separator to a current sheet. In addition, a formula for the rate of reconnection between two flux
tubes is derived. The magnetic field of the corona is highly complex, since the magnetic carpet consists of a multitude of
sources in the photosphere. Progress in understanding this complexity may, however, be made by constructing the skeleton of the field and developing a theory for the local and global bifurcations between the different topologies. The eruption
of flux from the Sun may even sometimes be due to a change of topology caused by emerging flux break-out. A CD-ROM attached to this paper presents the results of a toy model of vacuum reconnection, which suggests that rapid flipping
of field lines in fan and separator reconnection is an essential ingredient also in real non-vacuum conditions. In addition,
it gives an example of binary reconnection between a pair of unbalanced sources as they move around, which may contribute significantly to coronal heating. Finally,
we present examples in TRACE movies of geometrical changes of the coronal magnetic field that are a likely result of large-scale
magnetic reconnection.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005248007615 相似文献
16.
We study the mass-radius relationship for aggregates of galaxies, viz. binaries, small groups and clusters. The data are subjected
to a simple best-fit analysis similar to the one carried out earlier for individual field galaxies. The analysis shows that:
(i) The data on binary galaxies are consistent with the assumption that binaries are just two galaxies, each with an individual
isothermal (M ∫R) dark matter halo, moving under the mutual gravitational attraction, (ii) The data on the groups of galaxies are too scattered
to obey a single power-law relation of the formM = kR
n
with any degree of reliability, (iii) The data on groups and clusters fit better with a law of the formM = AR
3
+BR. This form suggests the existence of two components in dark matter—one which is clustered around the galaxies (M ∫R) and another which is distributed smoothly (M ∫R
3
). The smooth distributions becomes significant only at scales ≥ 1 Mpc and hence does not affect binaries significantly. We
briefly discuss the theoretical implications of this analysis 相似文献
17.
Werner Alpers 《Astrophysics and Space Science》1971,11(3):471-474
The way is discussed by which microinstabilities of an exact charge neutral magnetopause could lead to a trapped particle flow, the absence of which causes the non-existence of an equilibrium magnetospheric boundary layer in the Parker-Lerche model. Furthermore, it is argued that instead of the non-equilibrium effect of Parker and Lerche, microinstabilities of an exact charge neutral magnetopause might be the underlying physical process of an Axford and Hines' type viscous interaction. 相似文献
18.
M. I. Wanas 《Astrophysics and Space Science》1989,154(2):165-177
The field equations of the generalized field theory constructed by Mikhail and Wanas have been applied to a well-established geometrical structure given earlier by H. P. Robertson in connection with the cosmological problem. A unique solution, representing a specified expanding Universe (withq
0=0, 0=0.75,k=–1) has been obtained. The model obtained has been compared with cosmological observations and with FRW-models of relativistic cosmology. It has been shown that the suggested model is free of particle horizons. The existence of singularities has been discussed.The two cases, when the associated Riemannian-space has a definite or indefinite metric are considered. The case of indefinite metric with signature (+ – – –) is found to be characterized byk=–1, while the case of +ve definite metric is characterized byk=+1. Apart from that difference, the two cases give rise to the same cosmological parameters. It has been shown that energy conditions are satisfied by the material contents in both cases. 相似文献
19.
The solar magnetic field maps every point in the corona to a corresponding place on the solar surface. Identifying the magnetic
connection map is difficult at low latitudes near the heliospheric current sheet, but remarkably simple in coronal hole interiors.
We present a simple analytic magnetic model (‘pseudocurrent extrapolation’) that reproduces the global structure of the corona,
with significant physical advantages over other nearly analytic models such as source-surface potential field extrapolation.
We use the model to demonstrate that local horizontal structure is preserved across altitude in the central portions of solar
coronal holes, up to at least 30 Rs, in agreement with observations. We argue that the preserved horizontal structure may be used to track the magnetic footpoint
associated with the location of a hypothetical spacecraft traveling through the solar corona, to relate in situ measurements of the young solar wind at ∼10–30 Rs to particular source regions at the solar surface. Further, we discuss the relationship between readily observable geometrical
distortions and physical parameters of interest such as the field-aligned current density. 相似文献
20.
I. Jurkevich 《Astrophysics and Space Science》1970,6(1):4-12
Expressions are given for partial derivatives of eclipse functions with respect to geometrical depth,p, and the ratio of radii,k. The derivatives are evaluated for critical combinations ofp andk at which indeterminacies occur and the resulting expressions are listed. All expressions are given in a form suitable for numerical evaluation. Notation employed is that of Merrill. 相似文献