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1.
This is an examination of the gravitational instability of the major large-scale perturbation modes for a fixed value of the
azimuthal wave number m = 1 in nonlinearly nonstationary disk models with isotropic and anisotropic velocity diagrams for
the purpose of explaining the displacement of the nucleus away from the geometric center (lopsidedness) in spiral galaxies.
Nonstationary analogs of the dispersion relations for these perturbation modes are obtained. Critical diagrams of the initial
virial ratio are constructed from the rotation parameters for the models in each case. A comparative analysis is made of the
instability growth rates for the major horizontal perturbation modes in terms of two models, and it is found that, on the
average, the instability growth rate for the m = 1 mode with a radial wave number N = 3 almost always has a clear advantage
relative to the other modes. An analysis of these results shows that if the initial total kinetic energy in an isotropic model
is no more than 12.4% of the initial potential energy, then, regardless of the value of the rotation parameter Ω, an instability
of the radial motions always occurs and causes the nucleus to shift away from the geometrical center. This instability is
aperiodic when Ω = 0 and is oscillatory when Ω ≠ 0 . For the anisotropic model, this kind of structure involving the nucleus
develops when the initial total kinetic energy in the model is no more than 30.6% of the initial potential energy. 相似文献
2.
Edward Walbridge 《Earth, Moon, and Planets》1975,14(1):115-121
Possible waves and oscillations in the lunar photoelectron layer (PEL) are investigated. The steady state PEL is reviewed as a basis for discussing PEL motions. Magnetic fields are neglected, so that there are four possible wave modes to consider. The propagation through the PEL of the two electromagnetic modes is discussed. Positive-ion waves, the third mode, are dismissed and plasma waves are considered at length. It is concluded that there are no propagating waves in the PEL other than electromagnetic. However, there is a type of oscillation which appears to be new and which may not be strongly damped. With these oscillations, termed flight-time oscillations, the height of the PEL fluctuates as does the electric field. These oscillations appear to be analogous to the height oscillations of the vertical jet of water in a city park water fountain. If flight-time oscillations are not much damped then it would be simplest to interpret them as plasma oscillations continually driven by the upwelling photoelectron stream. A possible laboratory investigation of these oscillations is discussed. For the surfaces of the Moon and the planet Mercury, the flight-time oscillation frequency,ω F, is found to be respectively ç 4 × 106 and ç 107 rad s?1. The PEL's of those surfaces may be in a state of continual vertical ‘quivering’ due to flight-time oscillations, or may be quiescent. 相似文献
3.
Fitting the results of linear normal-mode analysis of the solar five-minute oscillations to the observed k - ω diagram selects a class of models of the Sun's envelope. It is a property of all the models in this class that their convection zones are too deep to permit substantial transmission of internal g modes of degree 20 or more. This is in apparent conflict with Hill and Caudell's (1979) claim to have detected such modes in the photosphere. A proposal to resolve the conflict was made by Rosenwald and Hill (1980). They pointed out that despite the impressive agreement between linearized theory and observation, nonlinear phenomena in the solar atmosphere might influence the eigenfrequencies considerably. In particular, they suggested that a correct nonlinear analysis could predict a shallow convection zone. This paper is an enquiry into whether their hypothesis is plausible. We construct k - ω diagrams assuming that the modes suffer local nonlinear distortions in the atmosphere that are insensitive to the amplitude of oscillation over the range of amplitudes that are observed. The effect of the nonlinearities on the eigenfrequencies is parameterized in a simple way. Taking a class of simple analytical models of the Sun's envelope, we compute the linear eigenfrequencies of one model and show that no other model can be found whose nonlinear eigenfrequencies agree with them. We show also that the nonlinear eigenfrequencies of a particular solar model with a shallow convective zone, computed with more realistic physics, cannot be made to agree with observation. We conclude, therefore, that the hypothesis of Rosenwald and Hill is unlikely to be correct. 相似文献
4.
Local electromagnetic and hydromagnetic noise in the Jovian magnetosphere is expected to be intense due to the variety of wave-particle interactions and plasma instabilities that may be present. In order to qualitatively assess the nature of the radio noise, configuration space analogues of the well-known Clemmow-Mullaly-Allis (CMA) propagation diagrams have been prepared, based on recent models of the magnetic field and plasma density. These diagrams identify the loci of electron and ion resonances and cutoffs where absorption and reflection of wave energy occur, and specify the propagation modes and frequency bands that are anticipated in various regions. Such information may guide the selection of wave detection instruments, influence the choice of flyby trajectories, and assist in the interpretation of measurements. 相似文献
5.
A unified theory of low frequency instabilities in a two component (cold and hot) finite-β magnetospheric plasma is suggested. It is shown that the low frequency oscillations comprise two wave modes : compressional Alfvén and drift mirror mode. No significant coupling between them is found in the long-wave approximation. Instabilities due to spontaneous excitation of these oscillations are considered. It is found that the temperature anisotropy significantly influences the instability growth rate at low frequency. A new instability due to the temperature anisotropy and density gradient appears when the frequency of compressional Alfvén waves is close to the drift mirror mode frequency. The theoretical predictions are compared in detail with the Pc5 event of 27 October 1978 observed simultaneously by the GEOS 2 satellite and the STARE radar facility. It is shown that the experimental results can be interpreted in terms of a compressional Alfvén wave driven by the drift anisotropy instability. 相似文献
6.
F. J. Sánchez-Salcedo 《Monthly notices of the Royal Astronomical Society》2006,365(2):555-571
There is still no consensus as to what causes galactic discs to become warped. Successful models should account for the frequent occurrence of warps in quite isolated galaxies, their amplitude as well as the observed azimuthal and vertical distributions of the H i layer. Intergalactic accretion flows and intergalactic magnetic fields may bend the outer parts of spiral galaxies. In this paper we consider the viability of these non-gravitational torques to take the gas off the plane. We show that magnetically generated warps are clearly flawed because they would wrap up into a spiral in less than two or three galactic rotations. The inclusion of any magnetic diffusivity to dilute the wrapping effect causes the amplitude of the warp to damp. We also consider the observational consequences of the accretion of an intergalactic plane-parallel flow at infinity. We have computed the amplitude and warp asymmetry in the accretion model, for a disc embedded in a flattened dark matter halo, including self-consistently the contribution of the modes with azimuthal wavenumbers m = 0 and m = 1 . Since the m = 0 component, giving a U-shaped profile, is not negligible compared to the m = 1 component, this model predicts quite asymmetric warps, maximum gas displacements on the two sides in the ratio 3 : 2 for the preferred Galactic parameters, and the presence of a fraction ∼3.5 per cent of U-shaped warps, at least. The azimuthal dependence of the moment transfer by the ram pressure would produce a strong asymmetry in the thickness of the H i layer and asymmetric density distributions in z , in conflict with observational data for the warp in our Galaxy and in external galaxies. The amount of accretion that is required to explain the Galactic warp would give gas scaleheights in the far outer disc that are too small. We conclude that accretion of a flow with no net angular momentum cannot be the main and only cause of warps. 相似文献
7.
C. Karoff 《Monthly notices of the Royal Astronomical Society》2007,381(3):1001-1008
p-mode oscillations in solar-like stars are excited by the outer convection zone in these stars and reflected close to the surface. The p modes are trapped inside an acoustic cavity, but the modes only stay trapped up to a given frequency [known as the acoustic cut-off frequency (νac ) ] as modes with larger frequencies are generally not reflected at the surface. This means that modes with frequency larger than the acoustic cut-off frequency must be travelling waves. The high-frequency modes may provide information about the physics in the outer layers of the stars and the excitation source and are therefore highly interesting as it is the estimation of these two phenomena that cause some of the largest uncertainties when calculating stellar oscillations.
High-frequency modes have been detected in the Sun, in β Hydri and in α Cen A and α Cen B by smoothing the so-called echelle diagram and the large frequency separation as a function of frequency has been estimated. The large frequency separation has been compared with a simple model of the acoustic cavity which suggests that the reflectivity of the photosphere is larger at high frequency than predicted by standard models of the solar atmosphere and that the depth of the excitation source is larger than what has been estimated by other models and might depend on the order n and degree l of the modes. 相似文献
High-frequency modes have been detected in the Sun, in β Hydri and in α Cen A and α Cen B by smoothing the so-called echelle diagram and the large frequency separation as a function of frequency has been estimated. The large frequency separation has been compared with a simple model of the acoustic cavity which suggests that the reflectivity of the photosphere is larger at high frequency than predicted by standard models of the solar atmosphere and that the depth of the excitation source is larger than what has been estimated by other models and might depend on the order n and degree l of the modes. 相似文献
8.
9.
Umbral oscillations in sunspots are identified as a resonant response of the umbral atmosphere to forcing by oscillatory convection in the subphotosphere. The full, linearized equations for magneto-atmospheric waves are solved numerically for a detailed model of the umbral atmosphere, for both forced and free oscillations. Resonant fast modes are found, the lowest mode having a period of 153 s, typical of umbral oscillations. A comparison is made with a similar analysis by Uchida and Sakurai (1975), who calculated resonant modes using an approximate (quasi-Alfvén) form of the wave equations. Whereas both analyses give an appropriate value for the period of oscillation, several new features of the motion follow from the full equations. The resonant modes are due to upward reflection in the subphotosphere (due to increasing sound speed) and downward reflection in the photosphere and low chromosphere (due to increasing Alfvén speed); downward reflection at the chromosphere-corona transition is unimportant for these modes. 相似文献
10.
Ogilvie 《Monthly notices of the Royal Astronomical Society》1998,297(1):291-314
The theory of waves and instabilities in a differentially rotating disc containing a poloidal magnetic field is developed within the framework of ideal magnetohydrodynamics. A continuous spectrum, for which the eigenfunctions are localized on individual magnetic surfaces, is identified but is found not to contain any instabilities associated with differential rotation. The normal modes of a weakly magnetized thin disc are studied by extending the asymptotic methods used previously to describe the equilibria. Waves propagate radially in the disc according to a dispersion relation which is determined by solving an eigenvalue problem at each radius. The dispersion relation for a hydrodynamic disc is re-examined and the modes are classified according to their behaviour in the limit of large wavenumber. The addition of a magnetic field introduces new, potentially unstable, modes and also breaks up the dispersion diagram by causing avoided crossings. The stability boundary to the magnetorotational instability in the parameter space of polytropic equilibria is located by solving directly for marginally stable equilibria. For a given vertical magnetic field in the disc, bending of the field lines has a stabilizing effect and it is shown that stable equilibria exist which are capable of launching a predominantly centrifugally driven wind. 相似文献
11.
Instabilities in a nonstationary model of self-gravitating disks. I. Bar and ring perturbation modes
The instabilities of bar and ring mode perturbations against the background of a disk oscillating nonlinearly in its own plane
are examined in a disk model which is a nonstationary generalization of the well known Bisnovatyi-Kogan-Zel'dovich model.
Nonstationary analogs corresponding to a dispersion relation are found for these two oscillation modes. The results of the
calculations are presented in the form of critical dependences of the initial virial ratio on the degree of rotation. A comparative
analysis of the growth rates of the gravitational instability for these modes is also carried out. The bar mode instability
occurs if the initial total kinetic energy of the disk is no more than 10.4% of the initial potential energy. The mechanism
is associated with an instability in the radial motions which is aperiodic for small values of the rotation parameter Ω <
0.1, but is otherwise oscillatory. Calculations show that a ring structure can be formed as a result of an instability in
the radial motions if the initial total energy of the model is no more than 5.2% of the initial potential energy, regardless
of the value of Ω.
__________
Translated from Astrofizika, Vol. 51, No. 3, pp. 487–499 (August 2008). 相似文献
12.
Mininni Pablo D. López Fuentes Marcelo Mandrini Cristina H. Gómez Daniel O. 《Solar physics》2004,219(2):367-378
We present a bi-orthogonal decomposition of the temporal and latitudinal distribution of solar magnetic fields from synoptic
magnetograms. Results are compared with a similar decomposition of the distribution of sunspots since 1874. We show that the
butterfly diagrams can be interpreted as the result of approximately constant amplitudes and phases of two oscillations with
periods close to 22 years. A clear periodicity of 7 years can also be identified in the most energetic modes of both spatio-temporal
series. These results can be used to obtain relevant information concerning the physics of the solar dynamo. 相似文献
13.
The magnetoacoustic modes excited in a thermally conducting polytropic fluid layer in the presence of a vertical magnetic field are examined with a view to classify them with the help of phase diagrams. The possibility of identifying the umbral flashes with overstable magnetoacoustic modes is explored. 相似文献
14.
H. J. Muthsam B. Löw-Baselli Chr. Obertscheider M. Langer P. Lenz F. Kupka 《Monthly notices of the Royal Astronomical Society》2007,380(4):1335-1340
Using advanced numerical schemes and grid refinement, we present 2D high-resolution models of solar granulation with particular emphasis on downflowing plumes. In the high-resolution portion of our simulation, a box measuring 1.97 × 2.58 Mm2 (vertical × horizontal), the grid size is 1.82 × 2.84 km2 . Calculations at the resolution usually applied in this type of simulations amount to only a few horizontal gridpoints for a downflowing plume. Due to the increased number of gridpoints in our high-resolution domain, the simulations show the development of vigorous secondary instabilities of both the plume's head and stem. The plume's head produces counterrotating vortex patches, a topology due to the 2D nature of the simulations. Below a depth of about 1 Mm, the plume's head and stem instabilities produce, in these 2D models, patches of low density, temperature, pressure and high vorticity which may last for all of our simulation time, ∼10 min, and probably considerably longer. Centrifugal forces acting in these patches counteract the strong inward pressure. Probably most importantly, the plume's instabilities give rise to acoustic pulses created predominantly down to ∼1.5 Mm. The pulses proceed laterally as well as upwards and are ubiquitous. Ultimately, most of them emerge into the photosphere. A considerable part of the photospheric 'turbulence' in these models is due to those pulses rather than to some sort of eddies. The upflows in granules are smooth where they reach the photosphere from below even in the present calculations; however, the pulses may enter in the photosphere also in granular upflows. 相似文献
15.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
16.
Chris Koen 《Astrophysics and Space Science》1987,135(2):393-398
Possible oscillation modes for a rotating star are listed. The only assumption made is that oscillations are adiabatic and that rotation is uniform. It is found that two modes not present for non-rotating stars are possible. Oscillation frequencies of these modes are rather different from those given in the literature for so-calledr-modes. 相似文献
17.
V. I. Ferronsky S. A. Denisik S. V. Ferronsky 《Celestial Mechanics and Dynamical Astronomy》1982,27(3):285-304
A general approach to the solution of the perturbed oscillation problem for celestial bodies is considered. The solution sought describes unperturbed virial oscillations (zero approximation) affected by external perturbing effects. In the general case, these perturbations can be expressed by an arbitrary given function of time, Jacobi's function and its first derivative. Standard methods and modes of perturbation theory are used for solution of the problem.It is shown that while studying the evolution of a celestial body as a dissipative system in the framework of perturbed virial oscillations, the analytical expression for perturbing function can be derived, assuming the celestial body to be an oscillating electrical dipole emitting electromagnetic energy.The general covariant form of Jacobi's equation is derived and its spur is examined. It is shown that the scalar form of Jacobi's equation appears to be more universal than Newton's laws of motion from which it is derived. 相似文献
18.
Models of finite temperature completely degenerate stellar configurations are considered. The frequencies of fundamental radial modes of oscillations for these white dwarf models have been computed for different values of the central degeneracy parameter 1/y
0
2
and for uniform temperatures of 20×106 K and 108 K. A variational formulation as well as a direct approach is employed to calculate the temperature induced modifications in the frequencies of oscillation of these white-dwarf models. 相似文献
19.
《Planetary and Space Science》1999,47(10-11):1211-1224
The spectrum of free oscillations of Jupiter is calculated for a set of models, each of them fitting all available observational data. Diagnostic capabilities of the spectrum are studied. They could be used, as soon as relevant observations are performed, for both the identification of the observed modes and the improvement of the models themselves. The calculations were made for five-layer models. They differ in the core mass (2–10 M⊕) and in the molecular-metallic phase transition pressure of hydrogen (1.5–3 Mbar). The spectrum of Jupiter consists of gravitational modes related to density jumps in the planetary interiors and of acoustic modes. The periods of the acoustic modes are calculated for degree up to l=30 and overtone number up to n=20. The investigated models have a characteristic frequency of ≈152–155 μHz. Two outer gravitational modes related to density jumps in the molecular envelope and at the interface with the metallic envelope have nonzero displacements at the planetary surface. These modes have good diagnostic properties. The values of the kinetic energy averaged over the period of oscillation are calculated for a 1-m amplitude of the displacement at the planetary surface. The influence of all effects of rotation on the spectrum is discussed. 相似文献
20.
The stability of linear convective and acoustic modes in solar envelope models is investigated by incorporating the thermal and mechanical effects of turbulence through the eddy transport coefficients. With a reasonable value of the turbulent Prandtl number it is possible to obtain the scales of motion corresponding to granulation, supergranulation and the five-minute oscillations. Several of the acoustic modes trapped in the solar convection zone are found to be overstable and the most unstable modes, spread over a region centred predominantly around a period of 300 s with a wide range of horizontal length scales, are in reasonable accord with the observed power-spectrum of the five-minute oscillations. It is demonstrated that these oscillations are driven by a simultaneous action of the -mechanism and the radiative and turbulent conduction mechanisms operating in the strongly superadiabatic region in the hydrogen ionization zone, the turbulent transport being the dominant process in overstabilizing the acoustic modes. 相似文献