共查询到20条相似文献，搜索用时 31 毫秒
1.
R. Bourret 《Astrophysics and Space Science》1973,24(2):407-419
The application of stochastic perturbation theory to the propagation of electromagnetic radiation in a cold plasma with random magnetic inhomogeneities leads to a field equation for the propagation of the coherence matrix of the field. This has nine independent components since the longitudinal mode must be included. An exact solution is obtained leading to simple expressions for the Stokes parameters as a function of distance and one physical parameter characterizing the interstellar plasma. While the total polarization decays exponentially, it is found that the ratio of elliptical to linear polarization increases with propagation distance in an exponential fashion. Using current estimates of the properties of the interstellar plasma it is expected that this effect whereby the polarization ellipse opens into a circle should be clearly evident for sources at distances of the order of 10. parsec It is suggested that this phenomenon might be used as a distance scale for certain objects. 相似文献
2.
J. Katz 《Astrophysics and Space Science》1980,72(1):117-125
The coherence of the radiation from a cubic lattice ofQ charges is evaluated at all wavelengths. The novelty of a second characteristic length, the distance between charges, adds a domain of wavelength in which enormous amplifications of power may exist (Q
1/3 times that of incoherent radiation). This amplification may be significant even if the charges are only poorly ordered becauseQ is extremely high in astrophysically relevant conditions. 相似文献
3.
A continuous transition from early Friedmann-like radiation era through to late time cosmic acceleration passing through a long Friedmann-like matter dominated era followed by a second phase of radiation era has been realized in modified theory of gravity containing a combination of curvature squared term, a linear term, a three-half term and an ideal fluid. Thus the history of cosmic evolution is explained by modified theory of gravity singlehandedly. The second phase of radiation-like era might provide an explanation to the hydrogen and helium reionization at low redshift. 相似文献
4.
Transfer of resonance radiation in an infinite medium is considered as a process of random walks of photons. Close relation is shown to exist between the problems of transfer of line radiation and the stable distributions of the probability theory. This relation is used as a basis of a new method for the investigation of the asymptotic properties of the radiation field far from the sources. 相似文献
5.
Lee W. Casperson 《Astrophysics and Space Science》1977,48(2):389-399
A modification of the rotating-star model has been developed to interpret the periodic energy bursts from pulsars. This new configuration involves -directed oscilltion modes in the stellar atmosphere or magnetosphere, and most aspects of the typical pulse characteristics are well accounted for. Gain is provided by resonant interactions with particles trapped in the stellar magnetic field. The most significant feature is the fact that highly directional beaming of the output energy results as a natural consequence of coherence between the radiation fields emerging from various locations about the pulsar; and a localized radiation origin is not required. 相似文献
6.
Pulsars are fast rotating neutron stars with a strong magnetic field, that emit over a wide frequency range. In spite of all efforts during the 40 years after the discovery of pulsars, the mechanism of their radio emission so far remains unknown. We propose a new approach to solving this problem for a subset of pulsars with a high-frequency cutoff of the spectrum from the Pushchino catalogue (the “Pushchino” sample). We provide a theoretical explanation of the observed dependence of the high-frequency cutoff on the pulsar period, and we predict the dependence of the cutoff position from the magnetic field. This explanation is based on a new mechanism for electron radio emission in pulsars. Namely, radiation occurs in the inner (polar) gap, when electrons are accelerated in the electric field that is increasing from zero level at the star surface. In this case the acceleration of electrons passes through a maximum and goes to zero when the electron velocity approaches the speed of light. All the radiated power is located within the radio frequency band. The averaging of radiation intensity over the polar cap, with some natural assumptions of the coherence of the radiation, leads to the observed spectra. It also leads to an acceptable estimate of the power of radio emission. 相似文献
7.
Christopher Essex 《Planetary and Space Science》1984,32(8):1035-1043
Even though irreversible thermodynamics is meant to be a universal theory for nonequilibrium processes, that theory has not taken nonequilibrium radiation processes, which can normally be ignored, into account; thus it is primarily a theory for matter. When the simple bilinear entropy production rate of irreversible thermodynamics is modified, to account for the entropy production due to radiation while preserving local equilibrium for matter, the simple bilinear form of that rate, on which the current theory depends, is lost. Despite substantial compromises, generalized definitions for forces and fluxes cannot restore that simple homogeneous bilinear form, nor do they succeed in generating even generalized bilinearity in the entropy production rates for some problems concerning radiation. The possibility of a broader nonequilibrium theory is suggested by radiation problems, characterized by an example due to Planck, where those generalizations fail and the entropy production rate is a minimum in the steady state. 相似文献
8.
A previous study of electromagnetic radiation from a finite train of electron pulses is extended to an infinite train of such pulses. The electrons are assumed to follow an idealized helical path through a space plasma in such a manner as to retain their respective position within the beam. This leads to radiation by coherent spontaneous emission. The waves of interest in this region are the whistler slow (compressional) and fast (torsional) Alfvén waves. Although a general theory is developed, analysis is then restricted to two approximations, the short and long electron beam. Formulas for the radiation per unit solid angle from the short beam are presented as a function of both propagation and ray angles, electron beam pulse width and separation and beam current, voltage, and pitch angle. Similar formulas for the total power radiated from the long beam are derived as a function of frequency, propagation angle, and ray angle. Predictions of the power radiated are presented for representative examples as determined by the long beam theory. 相似文献
9.
C.E. KenKnight 《Icarus》1977,30(2):422-433
It is proposed that the presence or absence of Jupiter-like planets, and perhaps even Venus-like planets, around nearby stars be studied with a 2-m telescope in Earth orbit. According to the Abbe theory of imaging, the coherence of the light from an unresolvable star can be used to discriminate between planet light and scattered light from the star. Most scattered light is shown to arise from the imperfect figure of the telescope surface, but an analog of a phase contrast trast microscope can be used to control the figure automatically. A number of arrangements are possible for using the interference properties of light to cancel the residual scattered starlight by two to three orders of magnitude without affecting the planet light. 相似文献
10.
T. X. Zhang 《Astrophysics and Space Science》2010,330(1):157-165
Modifying slightly the big bang theory, the author has recently developed a new cosmological model called black hole universe.
This new cosmological model is consistent with the Mach principle, Einsteinian general theory of relativity, and observations
of the universe. The origin, structure, evolution, and expansion of the black hole universe have been presented in the recent
sequence of American Astronomical Society (AAS) meetings and published recently in a scientific journal: Progress in Physics.
This paper explains the observed 2.725 K cosmic microwave background radiation of the black hole universe, which grew from
a star-like black hole with several solar masses through a supermassive black hole with billions of solar masses to the present
universe with hundred billion-trillions of solar masses. According to the black hole universe model, the observed cosmic microwave
background radiation can be explained as the black body radiation of the black hole universe, which can be considered as an
ideal black body. When a hot and dense star-like black hole accretes its ambient materials and merges with other black holes,
it expands and cools down. A governing equation that expresses the possible thermal history of the black hole universe is
derived from the Planck law of black body radiation and radiation energy conservation. The result obtained by solving the
governing equation indicates that the radiation temperature of the present universe can be ∼2.725 K if the universe originated
from a hot star-like black hole, and is therefore consistent with the observation of the cosmic microwave background radiation.
A smaller or younger black hole universe usually cools down faster. The characteristics of the original star-like or supermassive
black hole are not critical to the physical properties of the black hole universe at present, because matter and radiation
are mainly from the outside space, i.e., the mother universe. 相似文献
11.
The investigated model supposes Bernstein waves emerging from Harris instabilities at a definite coronal level. The nonlinear process is considered for a higher region, where the quasimonochromatic waves forming the primary spectrum are reflected. Spatial dispersion takes place corresponding to the decreasing magnetic field. Thus each quasimonochromatic wave can be treated separately. According to the nonlinear resonance condition there result electromagnetic waves of twice the primary frequency, the power density of which is calculated. Assuming a coherence time of 480 periods and an oscillation velocity of the electrons of 10-3 times the thermal velocity the effective radiation temperature 1011 K of a type IVmA-burst is obtained at about 180 MHz, if the range of the nonlinear interaction is about 3.9 km long. In the discussion the interpretation of occurring zebra patterns is treated. 相似文献
12.
T. T. Chia 《Astrophysics and Space Science》1977,46(1):239-246
A definite integral which occurs in radiation theory is shown to be equal in value to another definite integral by evaluating the flux from a spherically symmetrical radiating sphere in two ways. As a corollary, an alternate proof of the invariance of the specific intensity of a ray in empty space along its path is presented.Furthermore, the equality of these two indefinite integrals leads to the conversion of members of a class of indefinite and definite integrals involving arbitrary functions of angle into other integrals. These transformations facilitate the calculation of some of these integrals which arise not only in the theory of radiation, but in other physical situations with spherical or axial symmetry — especially those in which inverse-square laws are involved. 相似文献
13.
A. I. Shapiro 《Astrophysics》2002,45(2):215-222
A model problem in the theory of line formation in an optically thick, purely scattering, stellar atmosphere is considered. The integral equation of radiation transfer at line frequencies is solved numerically for a two-level atom in the approximation of complete frequency redistribution in scattering. The numerical results are compared with those calculated from equations of the asymptotic theory. On the basis of the asymptotic theory, the positions of intensity maxima in a line are found for different absorption profiles. 相似文献
14.
J. Gil 《Astrophysics and Space Science》1985,110(2):293-296
A model of pulsar emission fluctuations down to sub-microsecond time-scales is presented. The model is consistent with the empirical amplitude modulated noise model. It is proposed that the time-scale of the shortest observed micropulses is determined by the coherence of single-particle curvature radiation.The National Radio Astronomy Observatory is operated by the Associated Universities, Inc., under contract with the National Science Foundation. 相似文献
15.
Rodolpho Vilhena De Moraes 《Celestial Mechanics and Dynamical Astronomy》1981,25(3):281-292
A semi analytical theory is proposed to study the joint effects of direct solar radiation pressure and atmospheric drag on the orbit of an artificial Earth satellite. Making the solar radiation pressure equal to zero the problem is reduced to one already solved by Brouwer and Hori. The solutions are not equivalent, however, since in the Brouwer and Hori theory one has spurious Poisson terms. 相似文献
16.
Based upon a novel paradigm of galaxy formation under the influence of an ultraviolet background, the evolutionary bifurcation of pre-galactic clouds is compared with observations of elliptical and spiral galaxies. The theory predicts that the dichotomy between dissipational and dissipationless galaxy formation stems from the degree of self-shielding from the ultraviolet background radiation. This is demonstrated on a bifurcation diagram of collapse epochs versus masses of pre-galactic clouds. Using the observed properties, the collapse epochs and mass are assessed for each type of galaxy . By direct comparison of the theory with observations, it turns out that the theoretical bifurcation branch successfully discriminates between elliptical and spiral galaxies. This suggests that the ultraviolet background radiation could play a profound role in the differentiation of galactic morphology into the Hubble sequence. 相似文献
17.
By now there is no doubt that the gamma-ray bursts (GRB) have a cosmological origin. This allows to regard GRB as the most
powerful known energy sources, ε∼ 1054 erg (with a total number of gamma quanta N_γ∼ 1060). A plausible mechanism of coherent synchrotron radiation (CSR) of relativistic electrons driven by a local magnetic field
is studied in this paper. We consider relativistic electrons arising in the Compton scattering of a GRB in directions close
to that of the ray from the source to a ground-based observer. The synchrotron pulses from Compton electrons located at different
points on the line between the GRB source and the observer arrive at the observation point simultaneously. This simultaneity
ensures the coherence of the detected radiation. Both molecular clouds in the host galaxy of the GRB and our own Galaxy, as
well as the Earth atmosphere are assumed to be scatterers of the GRB radiation. Signals of each scatterer reach the Earth
surface, and can be detected at radio wavelengths. We estimate the characteristics of this radiation. The comparison of GRB
data with the corresponding information on CSR pulses offers a way to determine some global characteristics of the medium
between the Earth and the GRB source. 相似文献
18.
We survey the observational data on infrared, optical and X-radiation sources associated with energetic cosmic events, and note the occurrence of an apparently preferred value of the spectral index,n=1, for the radiation continua. We review the essentials of standard synchrotron radiation theory; the conventional interpretation of the observational data in terms of an energy distribution of electrons injected into a constant, low valued magnetic field; and the somewhat unsatisfactory attempts that can be made to explain this electron energy distribution in terms of the Fermi acceleration mechanism. We examine the evidence for the presence in the radiation sources of high magnetic fields, which cause evolution of the synchrotron radiation power spectrum to occur. We work out the consequences of this evolution, and obtain a new form of synchrotron radiation theory, which we describe astime-integrated synchrotron radiation theory, the particular advantage of which is that it is able to give a unique value (n=1/2 of the spectral index for radiation produced by a single high energy electron, independently of the initial electron energy. We consider the consequences of there being a distribution of magnetic field values in a radiation source; and in particular we consider a uniform distribution (in which all values are equally probable), which is capable of producing the required spectral indexn=1. We show that this uniform distribution can be explained in terms of a model in which there exist condensations of material containing high magnetic fields and within which electrons can be generatedin situ, through the familiar pion production and decay processes. We also consider systems in which electrons in a radiation source have injection patterns that enable the radiation continua to be interpreted in terms of time-integrated synchrotron radiation theory, originally devised for a single electron. We apply these considerations to sources of optical and higher frequency radiation; we also show that they have limited application to certain types of radio source. We suggest in conclusion that the condensations that feature in our model could act as basic units of structure for complex radiation sources associated with different types of energetic cosmic event, and that therein could lie the clue to the evident similarity of their radiation continua. 相似文献
19.
We study the distortion of the gravitational force field of an N-body ring-typecluster due to the presence of many radiation sources. We basically apply theRadzievskii's theory in this general model, considering that each member of thecluster is an independent radiation source. 相似文献
20.
J. B. Bhowmick 《Astrophysics and Space Science》1981,74(2):481-485
The Brinkley-Kirkwood theory, as modified by Bhatnagar and Kushwaha for the inclusion of radiation pressure, is applied to obtain an exact analytical solution for radiation pressure, shock velocity, etc., when a strong explosion takes place in a cold undisturbed gas obeying an exponential density distribution. Cases involving spherical symmetry, axial symmetry or spheroidal symmetry are also considered. 相似文献