共查询到20条相似文献，搜索用时 31 毫秒
1.
K. Karami S. Nasiri S. Amiri 《Monthly notices of the Royal Astronomical Society》2009,394(4):1973-1977
The standing quasi-modes of the ideal magnetohydrodynamics (MHD) in a zero-β cylindrical magnetic flux tube that undergoes a longitudinal density stratification and radial density structuring are considered. The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae of the resonant absorption, the dispersion relation for the fast MHD body waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone, k = 1, 2 , modes of both the kink ( m = 1) and fluting ( m = 2) waves, where k and m are the longitudinal and azimuthal mode numbers, respectively. 相似文献
2.
The standing quasi-modes in a cylindrical incompressible flux tube with magnetic twist that undergoes a radial density structuring is considered in ideal magnetohydrodynamics (MHD). The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae, the dispersion relation for the MHD waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone modes of both the kink (m=1) and fluting (m=2,3) waves. It was found that a magnetic twist will increase the frequencies, damping rates and the ratio of the oscillation frequency to the damping rate of these modes. The period ratio P 1/P 2 of the fundamental and its first-overtone surface waves for kink (m=1) and fluting (m=2,3) modes is lower than two (the value for an untwisted loop) in the presence of twisted magnetic field. For the kink modes, particularly, the magnetic twists B φ /B z =0.0065 and 0.0255 can achieve deviations from two of the same order of magnitude as in the observations. Furthermore, for the fundamental kink body waves, the frequency band width increases with increasing magnetic twist. 相似文献
3.
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
4.
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
5.
The standing magnetohydrodynamic (MHD) quasi-linear modes in a zero-β cylindrical magnetic flux tube that undergoes a longitudinal density stratification and radial density structuring are considered. The radial structuring is assumed to have a step-like density profile. The dispersion relation for the fast MHD body waves is derived and solved numerically to obtain the frequencies of the fundamental, first-overtone and second-overtone k = 1, 2, 3 modes of both kink ( m = 1) and fluting ( m = 2) waves, where k and m are the longitudinal and azimuthal mode numbers, respectively. Damping rates due to both viscous and resistive dissipations in the presence of the density stratification are derived and solved numerically for the first three modes of both kink and fluting waves. 相似文献
6.
We investigate the effect of a variable, i.e. time-dependent, background on the standing acoustic (i.e. longitudinal) modes generated in a hot coronal loop. A theoretical model of 1D geometry describing the coronal loop is applied. The background temperature is allowed to change as a function of time and undergoes an exponential decay with characteristic cooling times typical for coronal loops. The magnetic field is assumed to be uniform. Thermal conduction is assumed to be the dominant mechanism for damping hot coronal oscillations in the presence of a physically unspecified thermodynamic source that maintains the initial equilibrium. The influence of the rapidly cooling background plasma on the behaviour of standing acoustic (longitudinal) waves is investigated analytically. The temporally evolving dispersion relation and wave amplitude are derived by using the Wenzel–Kramers–Brillouin theory. An analytic solution for the time-dependent amplitude that describes the influence of thermal conduction on the standing longitudinal (acoustic) wave is obtained by exploiting the properties of Sturm–Liouville problems. Next, numerical evaluations further illustrate the behaviour of the standing acoustic waves in a system with a variable, time-dependent background. The results are applied to a number of detected loop oscillations. We find a remarkable agreement between the theoretical predictions and the observations. Despite the emergence of the cooling background plasma in the medium, thermal conduction is found to cause a strong damping for the slow standing magneto–acoustic waves in hot coronal loops in general. In addition to this, the increase in the value of thermal conductivity leads to a strong decay in the amplitude of the longitudinal standing slow MHD waves. 相似文献
7.
We studied nonlinear kink oscillations of a thin magnetic tube using the cold-plasma approximation. We assumed that the plasma density varies along the tube but does not vary in the radial direction. Using the regular perturbation method, we show that the nonlinearity does not affect the oscillation amplitude. We also calculated the nonlinear correction to the oscillation frequency, which is proportional to the oscillation amplitude squared. As an example, we considered nonlinear oscillations of a coronal magnetic loop of half-circle shape in an isothermal atmosphere with equal plasma temperatures inside and outside the loop. 相似文献
8.
The influence of a constant coronal magnetic field on solar global oscillations is investigated for a simple planar equilibrium model. The model consists of an atmosphere with a constant horizontal magnetic field and a constant sound speed, on top of an adiabatic interior having a linear temperature profile. The focus is on the possible resonant coupling of global solar oscillation modes to local slow continuum modes of the atmosphere and the consequent damping of the global oscillations. In order to avoid Alfvén resonances, the analysis is restricted to propagation parallel to the coronal magnetic field. Parallel propagating oscillation modes in this equilibrium model have already been studied by Evans and Roberts (1990). However, they avoided the resonant coupling to slow continuum modes by a special choice of the temperature profile. The physical process of resonant absorption of the acoustic modes with frequency in the cusp continuum is mathematically completely described by the ideal MHD differential equations which for this particular equilibrium model reduce to the hypergeometric differential equation. The resonant layer is correctly dealt with in ideal MHD by a proper treatment of the logarithmical branch cut of the hypergeometric function. The result of the resonant coupling with cusp waves is twofold. The eigenfrequencies become complex and the real part of the frequency is shifted. The shift of the real part of the frequency is not negligible and within the limit of observational accuracy. This indicates that resonant interactions should definitely be taken into account when calculating the frequencies of the global solar oscillations. 相似文献
9.
The role of leaky waves in the coronal loop oscillations observed by TRACE is not yet clearly understood. In this work, the
excitation of fast waves in solar coronal loops modelled as dense plasma cylindrical tubes in a uniform straight magnetic
field is investigated. We study the trapped and especially leaky modes (whose energy escapes from the tube) that result from
an initial disturbance by solving the time-dependent problem numerically. We find that the stationary state of the tube motion
is given by the trapped normal modes. By contrast, the transient behaviour between the initial and the stationary phase is
dominated by wave leakage. The so-called trig leaky modes are clearly identified since the transient behaviour shows periods
and damping times that are in agreement with the values calculated from the normal-mode analysis. Consequently, these radiating
modes have physical significance. However, we have not found any evidence for the excitation of other types of modes, such
as the principal leaky kink mode.
J. Andries is postdoctoral Fellow of the National Fund for Scientific Research – Flanders (Belgium) (F.W.O.-Vlaanderen). 相似文献
10.
We investigate the damping of longitudinal (i.e., slow or acoustic) waves in nonisothermal, hot (T≥ 5.0 MK), gravitationally stratified coronal loops. Motivated by SOHO/SUMER and Yohkoh/SXT observations, and by taking into account a range of dissipative mechanisms such as thermal conduction, compressive viscosity, radiative cooling, and heating, the nonlinear governing equations of one-dimensional hydrodynamics are solved numerically for standing-wave oscillations along a magnetic field line. A semicircular shape is chosen to represent the geometry of the coronal loop. It was found that the decay time of standing waves decreases with the increase of the initial temperature, and the periods of oscillations are affected by the different initial footpoint temperatures and loop lengths studied by the numerical experiments. In general, the period of oscillation of standing waves increases and the damping time decreases when the parameter that characterises the temperature at the apex of the loop increases for a fixed footpoint temperature and loop length. A relatively simple second-order scaling polynomial between the damping time and the parameter determining the apex temperature is found. This scaling relation is proposed to be tested observationally. Because of the lack of a larger, statistically relevant number of observational studies of the damping of longitudinal (slow) standing oscillations, it can only be concluded that the numerically predicted decay times are well within the range of values inferred from Doppler shifts observed by SUMER in hot coronal loops. 相似文献
11.
Karam Bahari 《Solar physics》2017,292(8):110
Transverse oscillations of a thin coronal loop in a zero-beta plasma in the presence of a twisted magnetic field and flow are investigated. The dispersion relation is obtained in the limit of weak twist. The twisted magnetic field modifies the phase difference and asymmetry of standing kink oscillations caused by the flow. Using data from observations the kink speed and flow speed have been determined. The presence of the twisted magnetic field can cause underestimation or overestimation of the flow speed in coronal loops depending on the direction of the flow and twisted magnetic field, but a twisted magnetic field has little effect on the estimated value of the kink speed. 相似文献
12.
We consider a pressureless plasma in a thin magnetic-flux tube with a twisted magnetic field. We study the effect of twisted magnetic field on the nature of propagating kink waves. To do this, the restoring forces of oscillations in the linear ideal magnetohydrodynamics (MHD) were obtained. In the presence of a twisted magnetic field, the ratio of the magnetic-tension force to the gradient of the magnetic pressure increases for the mode with negative azimuthal wave number, but it decreases for the mode with positive azimuthal wave number. For the kink mode with positive azimuthal mode number, the ratio of the forces is more affected by the twisted magnetic field in dense loops. For the kink mode with negative azimuthal mode number, the perturbed magnetic pressure is negligible under some conditions. The magnetic twist increases (diminishes) the damping of the kink waves with positive (negative) azimuthal mode number due to resonant absorption. Our conclusion is that introducing a twisted magnetic field breaks the symmetry between the nature of the kink waves with positive and negative azimuthal wave number, and the wave can be a purely Alfvénic wave in the entire loop. 相似文献
13.
《天文和天体物理学研究(英文版)》2016,(6)
Standing oscillations with multiple periods have been found in a number of atmospheric structures on the Sun.The ratio of the period of the fundamental to twice the one of its first overtone,P_1/2P_2,is important in applications of solar magneto-seismology.We examine how field-aligned flows impact P_1 /2P_2 of standing modes in solar magnetic cylinders.For coronal loops,the flow effects are significant for both fast kink and sausage modes.For kink modes,they reduce P_1 /2P_2 by up to 17% relative to the static case even when the density contrast between the loop and its surroundings approaches infinity.For sausage modes,the reduction in P_1 /2P_2 due to flow is typically 5.5% compared with the static case.However,the threshold aspect ratio,only above which can trapped sausage modes be supported,may increase dramatically with the flow magnitude.For photospheric tubes,the flow effect on P_1/2P_2 is not as strong.However,when applied to sausage modes,introducing field-aligned flows offers more possibilities in interpreting the multiple periods that have recently been measured.We conclude that field-aligned flows should be taken into account to help better understand what causes the departure of P_1/2P_2 from unity. 相似文献
14.
The damping of standing slow waves in hot (T>6 MK) coronal loops of semicircular shape is revisited in both the linear and nonlinear regimes. Dissipation by thermal conduction,
compressive viscosity, radiative cooling, and heating are examined for nonstratified and stratified loops. We find that for
typical conditions of hot SUMER loops, thermal conduction increases the period of damped oscillations over the sound-crossing
time, whereas the decay times are mostly shaped by compressive viscosity. Damping from optically thin radiation is negligible.
We also find that thermal conduction alone results in slower damping of the density and velocity waves compared to the observations.
Only when compressive viscosity is added do these waves damp out at the same rate as the observed rapidly decaying modes of
hot SUMER loop oscillations, in contrast to most current work, which has pointed to thermal conduction as the dominant mechanism.
We compare the linear predictions with numerical hydrodynamic calculations. Under the effects of gravity, nonlinear viscous
dissipation leads to a reduction of the decay time compared to the homogeneous case. In contrast, the linear results predict
that the damping rates are barely affected by gravity. 相似文献
15.
M. S. Ruderman 《Solar physics》2010,267(2):377-391
In this paper we study kink oscillations of coronal loops in the presence of flows. Using the thin-tube approximation we derive
the general governing equation for kink oscillations of a loop with the density varying along the loop in the presence of
flows. This equation remains valid even when the density and flow are time dependent. The derived equation is then used to
study the effect of flows on eigenfrequencies of kink oscillations of coronal loops. The implication of the obtained results
on coronal seismology is discussed. 相似文献
16.
The damping of fast kink oscillations of solar coronal loops attributable to the radiation of MHD waves into the surroundings is considered in the thin-tube approximation. The oscillation damping decrement is calculated both by using a new energy method and by solving the dispersion equation for magnetic-tube eigenmodes. The two approaches are in good agreement under appropriate assumptions. The damping is negligible if MHD waves are radiated perpendicular to the magnetic field. The low Q factor of the loop oscillations in active regions found with the TRACE space telescope is associated with the generation of running waves that propagate along magnetic field lines. 相似文献
17.
N. S. Petrukhin 《Astronomy Letters》2014,40(6):372-381
The free oscillations of coronal loops with a constant density and a variable magnetic field changing according to parabolic laws are investigated. Using our developed method, we derive the wave equations with constant coefficients that describe the kink oscillations of symmetric and asymmetric magnetic flux tubes. For such models, we obtain analytical expressions for the oscillation spectra and amplitudes as well as the magnitudes and directions of the displacements of the extrema of the fundamental and first modes relative to their values for homogeneous tubes. For the first mode of an asymmetric loop, we have determined the dependence of the coordinate displacement for the internal node on the ratios of the magnetic field strengths in its asymmetric parts and the ratio of the amplitudes at the extremum points. 相似文献
18.
The effect of radiation losses on the dispersion and damping of magnetohydrodynamic waves in the solar corona is studied. The conditions are determined under which radiation losses are most appreciable. A damping of kink modes of coronal loops with plasma temperatures within 106–106.3 K and 106.3–107 K are compared. It is concluded that the radiation damping dominates in the temperature range 106–106.3 K, which can cause the observed fast damping of kink oscillations of coronal loops. Radiation losses should be taken into account in full magnetohydrodynamic equations with radiative transfer. 相似文献
19.
Yu. G. Kopylova A. V. Melnikov A. V. Stepanov Yu. T. Tsap T. B. Goldvarg 《Astronomy Letters》2007,33(10):706-713
The dispersion properties of the sausage eigenmodes of oscillations in a thin magnetic flux tube are numerically analyzed in terms of ideal magnetohydrodynamics (MHD). The period of the modes accompanied by the emission of MHD waves into the surrounding medium, which leads to acoustic damping of oscillations, is determined by the radius of the tube, not by its length. The dissipation of the sausage oscillations in comparatively high (?0.7R ⊙) and tenuous (?6 × 108 cm?3) coronal loops is considered. Their Q factor has bound found to be determined by the acoustic damping mechanism. The ratio of the plasma densities outside and inside the loop and the characteristic height of the emission source have been estimated by assuming the quasi-periodic pulsations of meter-wavelength radio emission to be related to the sausage oscillations. 相似文献
20.
Huang Guangs LI 《Solar physics》1987,114(2):363-373
A theoretical analysis of electron-cyclotron maser instabilities indicates that the distribution function of non-thermal electrons influences millisecond radio spikes in solar flares, and that a hollow beam distribution is more likely than a loss-cone distribution. The restrictions of classical theories of cyclotron resonant absorption are discussed and a formula is derived for the absorption coefficient near the resonant frequency. Finally, the computations show that for typical coronal parameters, the growth rates of the fundamental of fast extraordinary modes are much faster than those of their second harmonics; and because the directional angle of the fundamental is smaller, its resonant absorption may be neglected. Moreover, the band-width of the fundamental is consistent with observation of radio spikes; therefore, we claim that the millisecond radio spikes in the decimetric range are composed mainly of fundamentals of the fast extraordinary modes. The second harmonics of fast extraordinary modes may be generated for directions near to the vertical to the magnetic field, but it is impossible to observe both fundamental and second harmonics in the same direction. 相似文献