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1.
The effects of both elliptical shape and stage of emergence of the coronal loop on the resonant absorption of standing kink oscillations are studied. To do so, a typical coronal loop is modeled as a zero-beta longitudinally stratified cylindrical magnetic flux tube. We developed the connection formulae for the resonant absorption of standing transversal oscillations of a coronal loop with an elliptical shape, at various stages of its emergence. Using the connection formulae, the dispersion relation is derived and solved numerically to obtain the frequencies and damping rates of the fundamental and first-overtone kink modes. Our numerical results show that both the elliptical shape and stage of emergence of the loop alter the frequencies and damping rates of the tube as well as the ratio of frequencies of the fundamental and its first-overtone modes. However, the ratio of the oscillation frequency to the damping rate is not affected by the tube shape and stage of its emergence and also is independent of the density stratification parameter. 相似文献
2.
Analysis of the oscillations in HST observations of the quiescent SU UMa type dwarf nova WZ Sagittae
Warren Skidmore William F. Welsh Janet H. Wood M. S. Catalán Keith Horne 《Monthly notices of the Royal Astronomical Society》1999,310(3):750-758
An analysis of the UV oscillations in WZ Sge is presented, in which we obtain the oscillation amplitude spectra. We find a strong 27.9-s oscillation in our Hubble Space Telescope ( HST ) UV and zeroth-order light curves as well as weaker oscillations at 28.4 s in the UV and 29.1 s in the zeroth order. We find that the main oscillation amplitude spectrum can be fitted with static white dwarf spectra of about 17 000 K, an accretion hotspot of only a few 100 K hotter than the underlying white dwarf temperature or a variety of cool (<14 500 K) white dwarf pulsation amplitude spectra. A pulsating white dwarf can also explain the very blue colour of oscillations of different periods previously found in the optical. Comparing our results with those of Welsh et al., we see that the amplitude spectra of the main oscillations in WZ Sge measured with different periods in data sets from different epochs are similar to each other. Our results raise questions about using the magnetically accreting rotating white dwarf model to explain the oscillations. We suggest that the pulsating white dwarf model is still a viable explanation for the oscillations in WZ Sge. 相似文献
3.
M. S. Ruderman 《Solar physics》2011,271(1-2):41-54
In this paper we study kink oscillations of coronal loops with the density varying along the loop and also slowly changing with time. Using the Wentzel?CKramers?CBrillouin (WKB) method we obtain the adiabatic invariant that determines the time dependence of the oscillation amplitude. The obtained general results are applied to kink oscillations of cooling loops. The main conclusion of this study is that cooling causes the amplification of kink oscillations. 相似文献
4.
P. Venkatakrishnan 《Solar physics》1986,104(2):347-357
The effect of applying external pressure fluctuations on slender flux tubes is studied as a nonlinear initial value problem. Large amplitude velocity oscillations are seen to be produced when the frequency of the imposed fluctuations matches the natural frequency of the tube. Radiative cooling does not significantly damp these resonantly built-up oscillations. The absence of observational evidence for such a resonant response of the tubes is used to put a constraint on the length of tubes. 相似文献
5.
M. S. Ruderman 《Solar physics》2007,246(1):119-131
In this paper we study nonaxisymmetric oscillations of thin twisted magnetic tubes taking the density variation along the
tube into account. We use the approximation of the zero-beta plasma. The magnetic field outside the tube is straight and homogeneous;
however, it is twisted inside the tube. We assume that the azimuthal component of the magnetic field is proportional to the
distance from the tube axis and that the tube is only weakly twisted (i.e., the ratio of the azimuthal and axial components of the magnetic field is small). Using the asymptotic analysis we show that
the eigenmodes and eigenfrequencies of the kink and fluting oscillations are described by a classical Sturm – Liouville problem
for a second-order ordinary differential equation. The main result is that the twist does not affect the kink mode. 相似文献
6.
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
7.
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
8.
A. Abedini 《Solar physics》2018,293(2):22
The excitation and damping of the transversal coronal loop oscillations and quantitative relation between damping time, damping property (damping time per period), oscillation amplitude, dissipation mechanism and the wake phenomena are investigated. The observed time series data with the Atmospheric Imaging Assembly (AIA) telescope on NASA’s Solar Dynamics Observatory (SDO) satellite on 2015 March 2, consisting of 400 consecutive images with 12 s cadence in the 171 \(\mathring{\mathrm{A}}\) pass band is analyzed for evidence of transversal oscillations along the coronal loops by the Lomb–Scargle periodgram. In this analysis signatures of transversal coronal loop oscillations that are damped rapidly were found with dominant oscillation periods in the range of \(\mathrm{P}=12.25\,\text{--}\,15.80\) min. Also, damping times and damping properties of the transversal coronal loop oscillations at dominant oscillation periods are estimated in the range of \({\tau_{\mathrm{d}}=11.76}\,\text{--}\,{21.46}\) min and \({\tau_{\mathrm{d}}/\mathrm{P}=0.86}\,\text{--}\,{1.49}\), respectively. The observational results of this analysis show that damping properties decrease slowly with increasing amplitude of the oscillation, but the periods of the oscillations are not sensitive functions of the amplitude of the oscillations. The order of magnitude of the damping properties and damping times are in good agreement with previous findings and the theoretical prediction for damping of kink mode oscillations by the dissipation mechanism. Furthermore, oscillations of the loop segments attenuate with time roughly as \(t^{-\alpha}\) and the magnitude values of \(\alpha\) for 30 different segments change from 0.51 to 0.75. 相似文献
9.
A. K. Srivastava T. V. Zaqarashvili W. Uddin B. N. Dwivedi Pankaj Kumar † 《Monthly notices of the Royal Astronomical Society》2008,388(4):1899-1903
Using simultaneous high spatial (1.3 arcsec) and temporal (5 and 10 s) resolution Hα observations from the 15 cm Solar Tower Telescope at Aryabhatta Research Institute of Observational Sciences (ARIES), we study the oscillations in the relative intensity to explore the possibility of sausage oscillations in the chromospheric cool post-flare loop. We use the standard wavelet tool, and find the oscillation period of ≈587 s near the loop apex, and ≈349 s near the footpoint. We suggest that the oscillations represent the fundamental and the first harmonics of the fast-sausage waves in the cool post-flare loop. Based on the period ratio P 1 / P 2 ∼1.68 , we estimate the density scaleheight in the loop as ∼17 Mm. This value is much higher than the equilibrium scaleheight corresponding to Hα temperature, which probably indicates that the cool post-flare loop is not in hydrostatic equilibrium. Seismologically estimated Alfvén speed outside the loop is ∼300–330 km s−1 . The observation of multiple oscillations may play a crucial role in understanding the dynamics of lower solar atmosphere, complementing such oscillations already reported in the upper solar atmosphere (e.g. hot flaring loops). 相似文献
10.
We have detected coherent oscillations ('dwarf nova oscillations') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were seen towards the end of a superoutburst of OY Car. The oscillations are extraordinary compared with the many other examples in the literature for two reasons. First, their amplitude is large, with a peak-to-peak variation of 8 to 20 per cent of the total flux over the range 1100 to 2500 Å. However, most remarkably we find that there are two components present simultaneously. Both have periods close to 18 s (equivalent to 4800 cycle d−1 ) but they are separated by 57.7 ± 0.5 cycle d−1 . The lower frequency component of the pair has a strong second harmonic while its companion, which has about twice its amplitude, does not. The oscillation spectra appear hotter than the mean spectrum and approximately follow the continuum distribution of a blackbody with a temperature in the range 30 000 to 50 000 K. We tentatively suggest that the weaker non-sinusoidal component could represent the rotation of the white dwarf, although we have been unable to recover any such signal in quiescent data. 相似文献
11.
We investigate the MHD waves in a double magnetic flux tube embedded in a uniform external magnetic field. The tube consists of a dense hot cylindrical cord surrounded by a co-axial shell. The plasma and the magnetic field are taken to be uniform inside the cord and also inside the shell. Two slow and two fast magnetosonic modes can exist in the thin double tube. The first slow mode is trapped by the cord, the other is trapped by the shell. The oscillations of the second mode have opposite phases inside the cord and shell. The speeds of the slow modes propagating along the tube are close to the tube speeds inside the cord and the shell. The behavior of the fast modes depends on the magnitude of Alfvén speed inside the shell. If it is less than the Alfvén speed inside the cord and in the environment, then the fast mode is trapped by the shell and the other may be trapped under the certain conditions. In the opposite case when the Alfvén speed in the shell is greater than those inside the cord and in the environment, then the fast mode is radiated by the tube and the other may also be radiated under certain conditions. The oscillation of the cord and the shell with opposite phases is the distinctive feature of the process. The proposed model allows to explain the basic phenomena connected to the coronal oscillations: i) the damping of oscillations stipulated in the double tube model by the radiative loss, ii) the presence of two different modes of perturbations propagating along the loop with close speeds, iii) the opposite phases of oscillations of modulated radio emission, coming from the near coronal sources having sharply different densities. 相似文献
12.
Robert Popham 《Monthly notices of the Royal Astronomical Society》1999,308(4):979-983
Dwarf nova oscillations (DNOs) have been observed in a number of cataclysmic variables. I propose that these oscillations could be produced by a non-axisymmetric bulge at the transition between the optically thick disc and the optically thin boundary layer region. This would naturally explain the observed oscillation periods and the dependence of the oscillation amplitude on photon energy. The transition radius moves inward and outward with changing mass accretion rate, which explains the correlation between period and flux, and the time-scale for period variations. The underlying cause of the non-axisymmetry that produces the oscillations is not known, so it is not possible to predict the oscillation amplitude from first principles. 相似文献
13.
S. S. Hasan 《Astrophysics and Space Science》1996,243(1):155-158
The interaction of an intense flux tube, extending vertically through the photosphere, with p-modes in the ambient medium is modelled by solving the time dependent MHD equations in the thin flux tube approximation. It is found that a resonant interaction can occur, which leads to the excitation of flux tube oscillations with large amplitudes. The resonance is not as sharp as in the case of an unstratified atmosphere, but is broadened by a factor proportional toH
–2, whereH is the local pressure scale height. In addition, the inclusion of radiative transport leads to a decrease in the amplitude of the oscillations, but does not qualitatively change the nature of the interaction. 相似文献
14.
We propose three mechanisms for the generation of quasi-periodic oscillations (QPOs) in X-ray binaries. Two of them are based on an analogy with nonlinear oscillations of gaseous cavities in a fluid. The first mechanism, called magnetocavitation, implies that X-ray QPOs are produced by radial oscillations of the neutron-star magnetosphere interacting with accreted plasma. The photon-cavitation mechanism is considered when studying X-ray QPOs in neutron stars with critical (Eddington) luminosities. In this case, X-ray QPOs are generated by radial oscillations of photon cavities in the fully ionized hydrogen plasma that settles in the accretion column of a compact object. The mechanism according to which X-ray QPOs result from nonlinear oscillations of current sheets originating in accretion disks is suggested to explain QPOs in X-ray binaries with black holes and in cataclysmic variables. The calculated values of basic physical parameters of QPOs, such as the characteristic frequency, the dependences of QPO frequency and amplitude on X-ray flux, photon energy, and QPO lag time between photons at different energies are in good agreement with observational data. 相似文献
15.
We study transverse loop oscillations triggered by 17flares and filament destabilizations; only 2 such cases have been reported
in the literature until now. Oscillation periods are estimated to range over a factor of ∼15, with most values between 2 and
7 min. The oscillations are excited by filament destabilizations or flares (in 6% of the 255 flares inspected, ranging from
about C3 to X2). There is no clear dependence of oscillation amplitude on flare magnitude. Oscillations occur in loops that
close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux
in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two,
and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds
up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode
because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped
too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops
lie near magnetic separatrices that outline the large-scale topology of the field. All magnetic configurations are more complicated
than a simple bipolar region, involving mixed-polarities in the interior or vicinity of the region; this may reflect that
the exciting eruptions occur only in such environments, but this polarity mixing likely also introduces the large-scale separatrices
that are involved. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the
triggering event. We discuss the observations in the context of two models: (a) transverse waves in coronal loops that act
as wave guides and (b) strong sensitivity to changes in the field sources for field lines near separatrices. Properties that
favor model b are (1) the involvement of loops at or near separatrices that outline the large-scale topology of the field,
(2) the combined occurrence of oscillations and loop translations, (3) the small period spread and similar decay time scale
in a set of oscillating loops in one well-observed event, and (4) the existence of loops oscillating in antiphase with footpoints
close together in two cases. All other properties are compatible with either model, except the fact that almost all of the
oscillations start away from the triggering event, suggestive of an outward-pushing exciting wave more in line with model
a. The spread in periods from event to event suggests that the oscillations may reflect the properties of some driver mechanism
that is related to the flare or mass ejection.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014957715396 相似文献
16.
17.
P. S. Cally 《Solar physics》2006,233(1):79-87
Simple fluxtube models of coronal loops have previously been found to support leaky oscillations, where energy escapes from
the tube laterally, thereby damping the mode. Of particular interest is the Principal Fast Leaky Kink mode (PFLK), which may
be implicated in the decay of loop oscillations observed by TRACE. However, recently, M.S. Ruderman and B. Roberts, J. Plarma Phys. (in press), claimed that the PFLK mode, and many other leaky modes of oscillation, are unphysical, based on an initial value
calculation. In this note, their arguments are shown to be incorrect. 相似文献
18.
Hari Om Vats M. R. Deshpande O. P. N. Calla N. M. Vadher B. M. Darji V. Sukumaran 《Earth, Moon, and Planets》1996,75(2):127-133
In this article we report the peculiar oscillations in the intensity of microwave (4.15 GHz) emission seen during the impact of K fragment of comet Shoemaker-Levy 9 on July 19, 1994. The oscillations begin at 10h 13m 25s UT suddenly with a frequency of ~0.3 Hz and gradually the frequency of these oscillations increases to ~ 1 Hz. The oscillations are not due to local atmosphere or the radio interference from signals of geostationary satellite. They are intrinsic to the microwave emission from Jupiter during the impact of K fragment. Peak-to-peak amplitude of the oscillations is about 34% of the total microwave emission from Jupiter. If we assume that only 50% of the microwave emission is non-thermal and only the non-thermal emission suffered oscillations, then the oscillations are about 68% of the non-thermal emission from Jupiter. The observations also indicate that there are three continuum enhancements during this event and periodic oscillations almost all through. The third enhancement was the largest and during this enhancement there were some additional aperiodic variations. The aperiodic variations were of the order of few minutes and were possibly generated by the gravity waves. The periodic oscillations could be synchrotron emission modulated by the plasma oscillation in the outer magnetosphere. kg]Key words 相似文献
19.
Yu. D. Zhugzhda 《Astronomy Letters》2007,33(9):622-643
We show that no eigenmodes of sunspot oscillations with periods of ～ 3 min or shorter exist. A complex spectrum of the 3-min oscillations arises, because the sunspot atmosphere is a multiband filter for slow MHD waves. To ascertain why the filter transmission bands appear, we have investigated the propagation of waves through a sunspot atmosphere using both multilayered isothermal model atmospheres and various empirical model atmospheres. It turns out that there are several different mechanisms responsible for the appearance of transmission bands in the atmospheric filter for slow waves. The filter lowest-frequency transmission band arises from the effect of a Fabry-Perot interference filter at the resonance frequency of the temperature plateau. The frequency of this band is always lower than the cutoff frequency of the temperature minimum. The next (in frequency) transmission band appears at the cutoff frequency. The higher-frequency transmission bands result from the antireflection of the atmosphere, an effect well-known in optics and acoustics. The nonlinearity of the 3-min oscillations observed in the upper chromosphere and transition region has only an indirect effect on the properties of the filter, increasing its transmission in most bands due to a decrease in the amplitude of the wave reflected from the upper atmosphere caused by nonlinear wave absorption. Knowledge of the formation mechanisms for the 3-min oscillation spectrum has allowed us to suggest a technique for estimating the parameters of sunspot atmospheres from the 3-min oscillation spectrum, i.e., to lay the foundations for the seismology of sunspot atmospheres. 相似文献
20.
We consider a model of a coronal loop that is a cylindrical magnetic tube with two surface electric currents. Its principal sausage mode has no cut-off in the long-wavelength limit. For typical coronal conditions, the period of the mode is between one and a few minutes. The sausage mode of flaring loops could cause long-period pulsations observed in microwave and hard X-ray ranges. There are other examples of coronal oscillations: long-period pulsations of active-region quiet loops in the soft X-ray emission are observed. We assume that these can also be caused by sausage waves. The question arises of how the sausage waves are generated in quiet loops. We assume that they can be generated by torsional oscillations. This process can be described in the framework of the nonlinear three-wave interaction formalism. The periods of interacting torsional waves are similar to the periods of torsional oscillations observed in the solar atmosphere. The timescale of the sausage-wave excitation is not much longer than the periods of interacting waves, so that the sausage wave is excited before torsional waves are damped. 相似文献