共查询到20条相似文献，搜索用时 298 毫秒
1.
Neophytos Messios Demetrios B. Papadopoulos Nikolaos Stergioulas 《Monthly notices of the Royal Astronomical Society》2001,328(4):1161-1168
Strong magnetic fields in relativistic stars can be a cause of crust fracturing, resulting in the excitation of global torsional oscillations. Such oscillations could become observable in gravitational waves or in high-energy radiation, thus becoming a tool for probing the equation of state of relativistic stars. As the eigenfrequency of torsional oscillation modes is affected by the presence of a strong magnetic field, we study torsional modes in magnetized relativistic stars. We derive the linearized perturbation equations that govern torsional oscillations coupled to the oscillations of a magnetic field, when variations in the metric are neglected (Cowling approximation). The oscillations are described by a single two-dimensional wave equation, which can be solved as a boundary-value problem to obtain eigenfrequencies. We find that, in the non-magnetized case, typical oscillation periods of the fundamental torsional modes can be nearly a factor of 2 larger for relativistic stars than previously computed in the Newtonian limit. For magnetized stars, we show that the influence of the magnetic field is highly dependent on the assumed magnetic field configuration, and simple estimates obtained previously in the literature cannot be used for identifying normal modes observationally. 相似文献
2.
V. Landgraf 《Astronomische Nachrichten》1997,318(2):129-133
In order to search for oscillations in velocity and magnetic field strength within a sunspot umbra, a time series of spectra has been obtained through a circular analyzer and the Gregory-Coudé telescope at the Observatorio del Teide, Tenerife. The velocity oscillations clearly show peaks of power at periods between 2 and 7 minutes, with a maximum at 5 minutes. The apparent variations of the magnetic field strength, however, don't exhibit significant oscillations; these fluctuations are rather produced by the influence of parasitic stray light from the surrouding quiet sun which are also visible in the measured time variations of the umbral contrast of continuum intensity. 相似文献
3.
The magnetic field strength in sunspots was derived from time series of two-dimensional spectra taken with the Göttingen 2D-spectrometer at the Vacuum Tower Telescope on Tenerife in August 1997. For the present measurements the magnetically sensitive line Fe?i 684.3 nm was selected. The main spot of the investigated sunspot group has a maximum magnetic field strength of 2270 G. Enhanced power of the magnetic field variations was found at the boundary between umbra and penumbra for all frequency ranges. These fluctuations are not well correlated with those of intensity variations or line shifts. Other spatial power peaks occur in a dark patch inside the centreside penumbra and at the centres of some accompanying small spots. Since no clear peaks at certain frequencies are found, the variations are not harmonic oscillations. A possible relation to Hα flares is investigated. There are several cases of published observations of magnetic field variations where flares occurred soon after the measurements, but very little before. Therefore it is not very probable that flares act as exciters of magnetic field variations.
相似文献4.
W. P. Wood 《Solar physics》1990,128(2):353-364
The Uchida and Sakurai (1975), Thomas and Scheuer (1982), and Scheuer and Thomas (1981) theory of umbral oscillations as resonant modes of magneto-acoustic-gravity waves is re-examined. For an isothermal atmosphere it is found that the quasi-Alfvén approximation is not a good approximation to the complete linearized wave equations. The new results presented here show that 3 min umbral oscillation periods are fairly insensitive to magnetic field strength above some critical value. For a detailed model umbra (Thomas and Scheuer, 1982) the calculations presented here show that 3 min umbral oscillations do not depend to any great extent on the level of forcing of the oscillations for those magnetic field strengths which are observed in sunspot umbras. Modes outside the 3 min range appear, as the lowest mode, as the level of forcing is placed at deeper and deeper levels in the solar atmosphere. 相似文献
5.
The amplitudes of solar oscillations measured in Doppler velocity are modulated by the presence of a strong photospheric magnetic field. Here we show that the amount of modulation cannot be predicted solely on the local photospheric magnetic field strength. Qualitatively, magnetic fields of similar strength have similar effects on the oscillations. Quantitatively, however, we find a neighborhood effect, so that the presence of a nearby sunspot affects oscillations in the area in its vicinity that has normal quiet-Sun magnetic field strength. Thus, different types of magnetic regions alter the oscillatory power to a varying degree, and the p-mode power within regions of similar magnetic field strength is more reduced if there is a sunspot present. The neighborhood effect falls off with distance from the sunspot. We also show that our measurements of the power modulation, in which we look at the effects on oscillations pixel by pixel, can be made consistent with results of amplitude modulation of modes as obtained from ring-diagram analysis of active regions, but only if the neighborhood effect on quiet-Sun regions is taken into account. 相似文献
6.
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
7.
V. I. Efremov A. A. Solov’ev L. D. Parfinenko A. Riehokainen E. Kirichek V. V. Smirnova Y. N. Varun I. Bakunina I. Zhivanovich 《Astrophysics and Space Science》2018,363(3):61
A specific type of artifacts (named as “p2p”), that originate due to displacement of the image of a moving object along the digital (pixel) matrix of receiver are analyzed in detail. The criteria of appearance and the influence of these artifacts on the study of long-term oscillations of sunspots are deduced. The obtained criteria suggest us methods for reduction or even elimination of these artifacts. It is shown that the use of integral parameters can be very effective against the “p2p” artifact distortions. The simultaneous observations of sunspot magnetic field and ultraviolet intensity of the umbra have given the same periods for the long-term oscillations. In this way the real physical nature of the oscillatory process, which is independent of the artifacts have been confirmed again. A number of examples considered here confirm the dependence between the periods of main mode of the sunspot magnetic field long-term oscillations and its strength. The dependence was derived earlier from both the observations and the theoretical model of the shallow sunspot. The anti-phase behavior of time variations of sunspot umbra area and magnetic field of the sunspot demonstrates that the umbra of sunspot moves in long-term oscillations as a whole: all its points oscillate with the same phase. 相似文献
8.
TRACE observations from 15 April 2001 of transverse oscillations in coronal loops of a post-flare loop arcade are investigated. They are considered to be standing fast kink oscillations. Oscillation signatures such as displacement amplitude, period, phase and damping time are deduced from 9 loops as a function of distance along the loop length. Multiple oscillation modes are found with different amplitude profile along the loop length, suggesting the presence of a second harmonic. The damping times are consistent with the hypothesis of phase mixing and resonant absorption, although there is a clear bias towards longer damping times compared with previous studies. The coronal magnetic field strength and coronal shear viscosity in the loop arcade are derived. 相似文献
9.
TRACE observations from 15 April 2001 of transverse oscillations in coronal loops of a post-flare loop arcade are investigated. They are considered to be standing fast kink oscillations. Oscillation signatures such as displacement amplitude, period, phase and damping time are deduced from 9 loops as a function of distance along the loop length. Multiple oscillation modes are found with different amplitude profile along the loop length, suggesting the presence of a second harmonic. The damping times are consistent with the hypothesis of phase mixing and resonant absorption, although there is a clear bias towards longer damping times compared with previous studies. The coronal magnetic field strength and coronal shear viscosity in the loop arcade are derived. 相似文献
10.
O. K. Cheremnykh D. Yu. Klimushkin P. N. Mager 《Kinematics and Physics of Celestial Bodies》2016,32(3):120-128
A one-dimensional inhomogeneous cylindrical plasma model with the magnetic field, whose field lines are concentric circles and the equilibrium parameters of the magnetic field and a medium change across magnetic shells, has been considered. In the scope of this model, it has been indicated that Alfvén modes can have discrete spectra. Such modes originate when resonators exist across magnetic shells, which can be implemented in the ring current area or near the outer edge of the plasmapause. The characteristics of the implementation of the modes with discrete spectra have been studied. The results are compared with the satellite observations. It has been concluded that poloidallypolarized pulsations in the Earth’s magnetosphere are largely oscillations with discrete spectra. It has been shown that the proposed model, which does not consider many properties of the magnetosphere, makes it possible to explain the main features in the experimentally observed generation of azimuthal small-scale ULF oscillations in the near-Earth plasma. The results can be used to interpret the satellite and SuperDARN radar measurements. 相似文献
11.
Observations of a sunspot in the Civ line at 1548 Å formed in the transition region have been analyzed to obtain the time variations and/or mean values of the velocity, intensity, longitudinal magnetic field, and line width. Oscillations with periods between approximately 110 and 200 s are observed only over the umbra where the transition region magnetic field is highest and the line width is smallest. When periodic intensity variations occur at the same frequency as the velocity oscillations, the peak intensities occur slightly before the maximum upward motions. No periodic variations in the transition region magnetic field have been detected. Scatter diagrams are presented which show possible relationships between the flow velocity, emission line intensity, line width, and transition region magnetic field. 相似文献
12.
A. A. Ruzmaikin 《Solar physics》1985,100(1-2):125-140
The basic features of the solar activity mechanism are explained in terms of the dynamo theory of mean magnetic fields. The field generation sources are the differential rotation and the mean helicity of turbulent motions in the convective zone. A nonlinear effect of the magnetic field upon the mean helicity results in stabilizing the amplitude of the 22-year oscillations and forming a basic limiting cycle. When two magnetic modes (with dipole and quadrupole symmetry) are excited nonlinear beats appear, which may be related to the secular cycle modulation.The torsional waves observed may be explained as a result of the magnetic field effect upon rotation. The magnetic field evokes also meriodional flows.Adctual variations of the solar activity are nonperiodic since there are recurrent random periods of low activity of the Maunder minimum type. A regime of such a magnetic hydrodynamic chaos may be revealed even in rather simple nonlinear solar dynamo models.The solar dynamo gives rise also to three-dimensional, non-axisymmetric magnetic fields which may be related to a sector structure of the solar field. 相似文献
13.
Yu. D. Zhugzhda 《Astronomy Letters》2018,44(5):331-336
The conditions under which the subphotospheric slow-wave resonator can be responsible for the local oscillations in a sunspot have been determined. A rich spectrum of local 3-min oscillations can be produced by the subphotospheric resonator only if the magnetic field in the resonator magnetic flux tube is much weaker than the surrounding sunspot magnetic field. Convective upflows of hot plasma in the sunspot magnetic field satisfy this condition. Consequently, there must be a correlation between the local oscillations and umbral dots, because the latter are produced by convective flows. Various modes of operation of the subphotospheric resonator give rise to wave packets of 3-min oscillations and umbral flashes. It is shown that giant local umbral flashes can emerge under certain conditions for the excitation of oscillations in the subphotospheric resonator. 相似文献
14.
Using microwave observations made with the Nobeyama radioheliograph (=1.76 cm), we have studied temporal variations of sunspot-associated sources in the circularly polarized component. For all three cases of well-developed and rather stable sunspots we found nearly harmonic oscillations with periods in a range of 120–220 s. In one case of an unstable and quickly devolving active region, the fluctuations appear to be irregular with no dominant period. Sunspot-associated solar radio sources are known to be generated by cyclotron radiation of thermal electrons in magnetic tubes of sunspots at the level of the lower solar corona or chromosphere–corona transition region (CCTR). At the wavelength of 1.76 cm, the polarized emission arises in a layer where the magnetic field is B=2000 G (assuming the emission generated at the third harmonic of electron gyrofrequency). We suggest that the observed effect is a manifestation of the well-known 3-min oscillations observed in the chromosphere and photosphere above sunspots. The observed effects are believed to be a result of resonance oscillation of MHD waves inside a magnetic tube. Radio observations of this phenomenon open a new tool for studying regions of reflection of MHD waves near CCTR level. The method is very sensitive both to the height of the CCTR and magnetic fields above sunspots. Thus, detection of oscillations of the height of the transition region even with an amplitude of a few km are possible. The use of a spectrum of one of the observed sources obtained with the radio telescope RATAN-600 allows us to conclude that oscillations in magnetic field strength of about 4 G could be responsible for the effect and are reliably registered. The appearance of the famous 5-min oscillations in the solar atmosphere was also registered in some spectra of radio oscillations. 相似文献
15.
16.
Oscillations of magnetic flux tubes are of great importance as they contain information about the geometry and fine structure
of the flux tubes. Here we derive and analytically solve in terms of Kummer’s functions the linear governing equations of
wave propagation for sausage surface and body modes (m=0) of a magnetically twisted compressible flux tube embedded in a compressible uniformly magnetized plasma environment in cylindrical geometry. A general dispersion relation is obtained for such flux
tubes. Numerical solutions for the phase velocity are obtained for a wide range of wavenumbers and for varying magnetic twist.
The effect of magnetic twist on the period of oscillations of sausage surface modes for different values of the wavenumber
and vertical magnetic field strength is calculated for representative photospheric and coronal conditions. These results generalize
and extend previous studies of MHD waves obtained for incompressible or for compressible but nontwisted flux tubes. It is
found that magnetic twist may change the period of sausage surface waves of the order of a few percent when compared to counterparts
in straight nontwisted flux tubes. This information will be most relevant when high-resolution observations are used for diagnostic
exploration of MHD wave guides in analogy to solar-interior studies by means of global eigenoscillations in helioseismology. 相似文献
17.
M. L. Demidov 《Solar physics》1995,159(1):23-27
This paper is devoted to an analysis of some technique questions inherent in the problem of detection of weak periodic variations of the magnetograph signal in observations of large-scale magnetic fields on the Sun. Namely, using observational data obtained at the Sayan observatory STOP telescope (Grigoryevet al., 1981; Grigoryev and Demidov, 1987), we address the question of the dependence of signal strength variations in observations of the global magnetic field of the Sun on some other parameters recorded during observations: brightness, the position of the zero level, and the ratio of intensities of the splitting components. It is pointed out that, although the influence of these parameter variations can occur at several frequencies, the least reliable are long-period oscillations of the magnetic field with periodsP >L/2, whereL is the realization length. The most reliable for the data being analyzed are field variations with periods of 29 and 80 min, and the proximity of the latter to the harmonic of the well-known 160-min pulsations of the Sun suggests their possible relationship, but further investigations are needed. 相似文献
18.
M. Goossens 《Astrophysics and Space Science》1977,52(1):3-10
The frequencies of the linear and adiabatic oscillations of a gaseous polytrope with a poloidal magnetic field are determined with the aid of a perturbation method. The influence of the poloidal magnetic field on the different types of spheroidal oscillation modes is discussed. The poloidal magnetic field generally strengthens the stability of the oscillation modes and this effect is the largest in the case of the non-radialp-modes. 相似文献
19.
B. Vršnak 《Solar physics》1984,94(2):289-297
Oscillatory motions of a loop prominence observed on July 17, 1981 are analysed. The oscillations were mainly horizontal, with a period of 8 min. Restoring force was a result of magnetic tension, and assuming a simplified magnetic field configuration the expression for frequencies of oscillations is derived and compared with the observations. Taking the observed period, the strength of the magnetic field permeating the prominence is estimated as 45 G. Finally, the stability of the prominence is discussed. 相似文献
20.
G. G. Fahlman 《Astrophysics and Space Science》1971,12(2):424-455
The oscillations of a polytrope with infinite electric conductivity containing a weak internal poloidal magnetic field which is continuous with an external dipole field are examined with the aid of a variational equation. The corrections to the fundamental characteristic frequencies of the radial and non-radiall=2 pulsation modes are calculated. The magnetic field removes a degeneracy which occurs between these two modes and the resulting frequency splitting is evaluated. The relevance of the results to the known magnetic stars is briefly discussed. 相似文献