DDS输出频谱特点及改善方法   总被引：1，自引：0，他引：1  

刘铁新  高俊法 《时间频率学报》1999,22(1):1-5

利用非均匀采样模型得到了存在相位截断时直接数字频率合成（ＤＤＳ）的输出频谱,通过分析频谱特点,讨论了改善频谱分布的方法  相似文献   

Continuous frequency spectrum of the global hydromagnetic oscillations of a magnetically confined mountain on an accreting neutron star     

M. Vigelius  A. Melatos 《Monthly notices of the Royal Astronomical Society》2009,395(4):1963-1971

We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies, extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfvén part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum ω_low. For equilibria with accreted mass  1.2 × 10⁻⁶≲ M _a/M_⊙≲ 1.7 × 10⁻⁴  and surface magnetic fields  10¹¹≲ B _*/G ≲ 10¹³, ω_low  is approximately independent of   B _*  , and increases with M _a. The results are consistent with the Alfvén spectrum excited in numerical simulations with the zeus-mp solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than ∼100 Hz. The implications for gravitational-wave searches for low-mass X-ray binaries are considered briefly.  相似文献   

Waves and instabilities in a differentially rotating disc containing a poloidal magnetic field     

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   

On the r-mode spectrum of relativistic stars     

Horst R. Beyer  Kostas D. Kokkotas 《Monthly notices of the Royal Astronomical Society》1999,308(3):745-750

We present a mathematically rigorous proof that the r-mode spectrum of relativistic stars to the rotational lowest order has a continuous part. A rigorous definition of this spectrum is given in terms of the spectrum of a continuous linear operator. This study verifies earlier results by Kojima concerning the nature of the r-mode spectrum.  相似文献   

The last observable line in hydrogen emission spectrum     

L. N. Kurochka  L. B. Ribko 《Solar physics》1978,57(2):319-328

It is shown that outside continuous solar spectrum or diffused solar light in the Earth's atmosphere substantially reduces the contrast and quality of observable emission lines in the hydrogen spectrum. A formula is obtained which enables the electron concentration in hydrogen plasma to be determined by the last observable line with due account for outside (spurious) continuous spectrum.  相似文献   

Progress in ultraviolet stellar spectrophotometry with S59     

C. De Jager  H. J. G. L. M. Lamers  K. A. Van Der Hucht 《Astrophysics and Space Science》1975,38(2):313-325

A progress report is given of investigations related to observations of stellar spectra obtained with the ultraviolet stellar spectrophotometer S59 aboard the ESRO TD-1A satellite. We describe first the processing of the observations: intensity and wavelength calibration, identification of lines, classification of spectra. Thereafter some important groups of lines are dealt with: non-LTE computation of the Mgii lines are presented: apart from the peculiar emission line and shell stars they compare well with the observations; intensity ratios, of Feii and Feiii lines are an important temperature classification criterion. Interstellar UV lines indicate large deficiencies of some metals, as compared to solar values. A new ultraviolet continuous extinction curve is determined. We discuss the strong outstreaming motions observed in α Cygni, and the consequent mass loss of this star (< 3 × 10^?10 \(\mathfrak{M}_ \odot \) yr^?1and finally we describe the composite spectrum of the (WC8+09I) bnary γ² Velorum; the ultraviolet continuous spectrum of the WC star is about one magnitude brighter than any theory predicts.  相似文献   

Inertial waves near corotation in three-dimensional hydrodynamical discs     

Henrik N. Latter  Steven A. Balbus 《Monthly notices of the Royal Astronomical Society》2009,399(2):1058-1073

This paper concerns the interaction between non-axisymmetric inertial waves and their corotation resonances in a hydrodynamical disc. Inertial waves are of interest because they can localize in resonant cavities circumscribed by Lindblad radii and, as a consequence, can exhibit discrete oscillation frequencies that may be observed. It is often hypothesized that these trapped eigenmodes are affiliated with the poorly understood quasi-periodic oscillation phenomenon. We demonstrate that a large class of non-axisymmetric three-dimensional (3D) inertial waves cannot manifest as trapped normal modes. This class includes any inertial wave whose resonant cavity contains a corotation singularity. Instead, these 'singular' modes constitute a continuous spectrum and, as an ensemble, are convected with the flow, giving rise to shearing waves. Finally, we present a simple demonstration of how the corotation singularity stabilizes 3D perturbations in a slender torus.  相似文献   

The continuous spectrum of MHD waves in 2D solar loops and arcades. First results on poloidal mode coupling for poloidal magnetic fields     

S. Poedts  M. Goossens 《Solar physics》1987,109(2):265-286

A first attempt is made to study the continuous spectrum of linear ideal MHD for 2D solar loops and to understand how 2D effects change the continuum eigenfrequencies and continuum eigenfunctions. The continuous spectrum is computed for 2D solar loops with purely poloidal magnetic fields and it is investigated how non-circularity of the cross-sections of the poloidal magnetic surfaces and variations of density along the poloidal magnetic field lines change the continuous spectrum and induce poloidal wave number coupling in the eigenfunctions. Approximate analytical results and numerical results are obtained for the eigenfrequencies and the eigenfunctions and the poloidal wave number coupling is clearly illustrated. It is found that the continuum frequencies are substantially increased, that the ranges of the continuum frequencies are considerably enlarged and that the derivatives of the continuum frequencies normal to the magnetic surfaces are substantially increased. The eigenfunctions are strongly influenced by poloidal wave number coupling. Implications of these findings for the heating mechanisms of resonant absorption and phase mixing are briefly considered.Research Assistant of the Belgian National Fund for Scientific Research.  相似文献   

Effect of Wave Motions in the Active Region of the Solar Surface on Convection     

R. I. Kostyk 《Kinematics and Physics of Celestial Bodies》2018,34(2):82-87

The results of the observations of the active region (facula) near the center of the solar disk obtained with the German Vacuum Tower Telescope (VTT; Tenerife, Spain), are discussed. We have determined that the decrease in the contrast (brightness) of the facula with the magnetic field increasing from 130 to 160 mT is due to the fact that the V_V phase shift of waves in this range of magnetic field densities is close to zero (Φ _VV ≈ 0), i.e., the wave becomes stationary and does not transfer energy from the photosphere to the chromosphere. The sound waves that propagate from the chromosphere towards the photosphere significantly affect the temperature characteristics of turbulent vortices at the level of formation of the continuous spectrum. In particular, the contrast of granules under the influence of these waves can increase by 25%.  相似文献   

Radiation transfer in a cylinder. III. Spectrum of basic integral equation     

D. I. Nagirner 《Astrophysics》1995,38(1):39-47

We present a method for computing the spectrum of the integral equation for radiation transfer in a cylinder. This method, as in the previous articles in this series, is based on a Hankel transformation applied to the equation. Calculating the spectrum then reduces to solving the equation for the auxiliary function for each eigenvalue separately. The corresponding eigenfunction is then found by an additional integration. We find asymptotic expressions for the eigenvalues and the eigenfunctions for a cylinder with a large optical radius when there is scattering in a spectral line, with complete redistribution over frequency when the absorption coefficient obeys a power law. We also derive equations determining the quantities entering into these expressions. For the simplest kernel of the equation all quantities can be expressed in terms of Bessel functions and roots of a transcendental equation.Translated from Astrofizika, Vol. 38, No. 1, pp. 75–88, January–March, 1995.  相似文献   

The thermal continuum in coronal loops: The influence of finite resistivity on the continuous spectrum     

R. C. Ireland  R. A. M. Van Der Linden  A. W. Hood  M. Goossens 《Solar physics》1992,142(2):265-289

The normal mode spectrum for the linearized MHD equations is investigated for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfvén, slow, and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfvén continua are removed and that the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The old location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum. The quasi-continuum is investigated numerically, and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion, and may be the cause of fine structure in prominences.  相似文献   

Collisional and return current heating functions for beam-heated models of solar flares     

Brown  J. C.  McClymont  A. N. 《Astrophysics and Space Science》1987,133(2):297-306

In beam-heated models of solar flares, the bulk of the energy deposited in the flare atmosphere resides in the low-energy end of the electron spectrum. Since the shape of the spectrum at low energy is not well determined observationally, various forms of low-energy cut-off have been assumed in theoretical modelling. Certain results of such modelling may depend strongly on the assumed spectrum. We derive the heating distributions for various spectra, both for collisional energy loss and for Ohmic dissipation of the return current, and show that none of the spectra are fully satisfactory, according to the criteria that for both collisional and Ohmic heating, the heating rate should be bounded, continuous, and smooth, and have a tractable functional form. A simple form of electron spectrum is suggested, which satisfies these criteria.  相似文献   

Rotational modes in a slowly and uniformly rotating star     

P. Smeyers  D. Craeynest  L. Martens 《Astrophysics and Space Science》1981,78(2):483-501

A perturbation method is derived forr-modes in a slowly and uniformly rotating star. In contrast to previous studies, the perturbation of the gravitational potential is included in the perturbation method.On the assumption that the effects of the centrifugal force are taken into account in the equilibrium model up to the second order in the angular velocity, an eigenvalue problem of sixth-order in the radial coordinate is derived that allows one to determine the zeroth-order toroidal displacement field and the third-order term in the expansion of the eigenfrequency. Furthermore, another eigenvalue problem is derived that governs the first-order toroidal displacement field and the fourth-order term in the expansion of the eigenfrequency. This second eigenvalue problem is also of the sixth-order in the radial coordinate.It is shown that the third-order term in the expansion of the eigenfrequency is real, and that the fourth-order term is zero.  相似文献   

Alfvén polar oscillations of relativistic stars     

Hajime Sotani  Kostas D. Kokkotas 《Monthly notices of the Royal Astronomical Society》2009,395(2):1163-1172

We study polar Alfvén oscillations of relativistic stars endowed with a strong global poloidal dipole magnetic field. Here, we focus only on the axisymmetric oscillations which are studied by numerically evolving the two-dimensional perturbation equations. Our study shows that the spectrum of the polar Alfvén oscillations is discrete in contrast to the spectrum of axial Alfvén oscillations which is continuous. We also show that the typical fluid modes, such as the f and p modes, are not significantly affected by the presence of the strong magnetic field.  相似文献   

Spectral studies of comets in the ultraviolet range and prospects of the WSO-UV project in these studies     

M. E. Sachkov 《Solar System Research》2016,50(4):294-299

Many problems of determining the chemical composition of comets and studying the physical processes in cometary nuclei can only be solved by using observational data in the UV range of the electromagnetic spectrum (115–300 nm). Cometary observations have a number of features in comparison with such studies of other astronomical objects. The World Space Observatory — Ultraviolet mission, planned for launch in 2021, will overcome most of the challenges in these studies and will be able to become an essential tool of cometary UV research in the following decade.  相似文献   

The physical conditions in the shells of type I supernovae     

E. R. Mustel  N. N. Chugay 《Astrophysics and Space Science》1975,32(1):39-53

Two models of the origin of the spectrum of type I supernovae are analysed: (I) the photosphere of the ‘central remnant’ and the expanding shell are separated by a density cavity; (II) the ‘photosphere’ (the layer which produces the continuous spectrum) is the inner part of the expanding shell. The arguments are given in favour of model I. Opacity of the shell close to light maximum for λ>4000 Å is mostly due to Thomson scattering; soon after light maximum (and it may be already at light maximum) the shell becomes completely transparent in this region of the spectrum. The problem of the origin of the very large width and of the relatively high central residual intensities of the absorption lines in the spectra of type I supernovae are analysed. A very noticeable dispersion in the velocities of the radial gas motions in the shell is the principal cause of the large equivalent width of the absorption lines in the spectra of these supernovae. The role played by the inhomogeneities in the shell is also discussed. The depth of the strong absorption lines produced by a very inhomogeneous shell may be equal to the filling factor of the medium. From the analysis of the spectra of supernova 1972e a lower limit for the mass of the shell is obtained (M>10³¹ g). Then from the fact of absence of a detectable H-absorption line and a simultaneous presence of strong Siii absorption lines (6347, 6371 Å) in the spectrum of supernova 1972e it follows that the ratio Si/H is at least two or three orders greater than that for the ‘normal’ stars.  相似文献   

Unstable modes of non-axisymmetric gaseous discs     

Naser M. Asghari  Mir Abbas Jalali 《Monthly notices of the Royal Astronomical Society》2006,373(1):337-348

We present a perturbation theory for studying the instabilities of non-axisymmetric gaseous discs. We perturb the dynamical equations of self-gravitating fluids in the vicinity of a non-axisymmetric equilibrium, and expand the perturbed physical quantities in terms of a complete basis set and a small non-axisymmetry parameter ε. We then derive a linear eigenvalue problem in matrix form, and determine the pattern speed, growth rate and mode shapes of the first three unstable modes. In non-axisymmetric discs, the amplitude and the phase angle of travelling waves are functions of both the radius R and the azimuthal angle φ. This is due to the interaction of different wave components in the response spectrum. We demonstrate that wave interaction in unstable discs, with small initial asymmetries, can develop dense clumps during the phase of exponential growth. Local clumps, which occur on the major spiral arms, can constitute seeds of gas giant planets in accretion discs.  相似文献