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1.
We examine the excitation of oscillations in the magnetic network of the Sun through the footpoint motion of photospheric magnetic flux tubes located in intergranular lanes. The motion is derived from a time series of high-resolution G-band and continuum filtergrams using an object-tracking technique. We model the response of the flux tube to the footpoint motion in terms of the Klein-Gordon equation, which is solved analytically as an initial value problem for transverse (kink) waves. We compute the wave energy flux in upward-propagating transverse waves. In general we find that the injection of energy into the chromosphere occurs in short-duration pulses, which would lead to a time variability in chromospheric emission that is incompatible with observations. Therefore, we consider the effects of turbulent convective flows on flux tubes in intergranular lanes. The turbulent flows are simulated by adding high-frequency motions (periods 5-50 s) with an amplitude of 1 km s(-1). The latter are simulated by adding random velocity fluctuations to the observationally determined velocities. In this case, we find that the energy flux is much less intermittent and can in principle carry adequate energy for chromospheric heating. 相似文献
2.
In the current study we present ab initio numerical computations of the generation and propagation of longitudinal waves in magnetic flux tubes embedded in the atmospheres of late-type stars. The interaction between convective turbulence and the magnetic structure is computed and the obtained longitudinal wave energy flux is used in a self-consistent manner to excite the small-scale magnetic flux tubes. In the current study we reduce the number of assumptions made in our previous studies by considering the full magnetic wave energy fluxes and spectra as well as time-dependent ionization (TDI) of hydrogen, employing multi-level Ca II atomic models, and taking into account departures from local thermodynamic equilibrium. Our models employ the recently confirmed value of the mixing-length parameter \(\alpha=1.8\). Regions with strong magnetic fields (magnetic filling factors of up to 50%) are also considered in the current study.The computed Ca II emission fluxes show a strong dependence on the magnetic filling factors, and the effect of time-dependent ionization (TDI) turns out to be very important in the atmospheres of late-type stars heated by acoustic and magnetic waves. The emitted Ca II fluxes with TDI included into the model are decreased by factors that range from 1.4 to 5.5 for G0V and M0V stars, respectively, compared to models that do not consider TDI.The results of our computations are compared with observations. Excellent agreement between the observed and predicted basal flux is obtained. The predicted trend of Ca II emission flux with magnetic filling factor and stellar surface temperature also agrees well with the observations but the calculated maximum fluxes for stars of different spectral types are about two times lower than observations. Though the longitudinal MHD waves considered here are important for chromosphere heating in high activity stars, additional heating mechanism(s) are apparently present. 相似文献
3.
Diaa E. Fawzy 《New Astronomy》2010,15(8):717-725
The formation of shocks and shock heating by radiatively damped longitudinal waves in solar magnetic flux tubes of different filling factors is studied. We consider three flux tubes of filling factors: 1%, 20%, and exponentially spreading which represent normal, enhanced network regions and the interior of supergranulation cells respectively. Monochromatic waves with periods 60 s and energy fluxes of 4.0 · 108 erg cm?2 s?1 are assumed to propagate in the tubes. We find that the H?-continuum losses and the Mg II line emission are much reduced in the tube of small filling factor while the mean temperatures are roughly similar in both tubes. The exponential flux tube shows little or no shock heating and no radiation damping. Shocks form earlier in the tube of high filling factor, and have larger strength. 相似文献
4.
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
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.
The goal of this paper is a detailed statistical analysis of the low-frequency Ca II line intensity oscillations containing information about the dynamics of the lower and middle chromosphere. A pixel-by-pixel analysis of the observed parameters has been performed. The following results have been obtained. (1) The low-frequency chromospheric oscillations (periods >400 s) are seen much more frequently in networks than in chromospheric network cells. (2) The relative fraction of the low-frequency chromospheric intensity oscillations increases with height. (3) The occurrence distribution of intensity oscillations as a function of the frequency is subdivided at least into two types. (4) In contrast to the low-frequency photospheric oscillations, the phase differences between the Ca II K and 849.8 nm line intensity oscillations do not give grounds to identify the low-frequency chromospheric oscillations with internal gravity waves. (5) The spectral composition of the oscillations in the network chromosphere resembles that expected in magnetic flux tubes in the nonlinear regime of conversion of transverse MHD waves at lower levels of the atmosphere into longitudinal MHD waves in its upper layer. 相似文献
7.
Wolfgang Kalkofen 《Journal of Astrophysics and Astronomy》2008,29(1-2):163-166
The nonmagnetic interior of supergranulation cells has been thought since the 1940s to be heated by the dissipation of acoustic waves. But all attempts to measure the acoustic flux have failed to show sufficient energy for chromospheric heating. Recent space observations with TRACE, for example, have found 10% or less of the necessary flux. To explain the missing energy it has been speculated that the nonmagnetic chromosphere is heated mainly by waves related to the magnetic field. If that were correct, the whole chromosphere, magnetic as well as nonmagnetic, would be heated mainly by waves related to the magnetic field. But contrary to expectation, the radiation emerging from the nonmagnetic chromosphere shows none of the signatures of magnetic waves, only those of acoustic waves. Nearly all the heating of the nonmagnetic chromosphere must therefore be due to acoustic waves. In the magnetic network on the boundary of supergranulation cells, on the other hand, the small filling factor of the magnetic field in the photosphere implies that only a small fraction of the wave flux that travels upward to heat the chromosphere can be channeled by the magnetic field. Hence, while some of the energy that is dissipated in the magnetic network is in the form of magnetic waves, most of it must be in the form of acoustic waves. Thus, the quiet solar chromosphere, instead of being heated mainly by magnetic waves throughout, must be heated mainly by acoustic waves throughout. The full wave flux heating the quiet chromosphere must travel through the photosphere. In the nonmagnetic medium, this flux is essentially all in the form of acoustic waves; TRACE registers at most 10% of it, perhaps because of limited spatial resolution. 相似文献
8.
The aim of this paper is to model the propagation of slow magnetohydrodynamic (MHD) sausage waves in a thick expanding magnetic flux tube in the context of the quiescent (VAL-C) solar atmosphere. The propagation of these waves is found to be described by the Klein–Gordon equation. Using the governing MHD equations and the VAL-C atmosphere model we study the variation of the cut-off frequency along and across the magnetic tube guiding the waves. Due to the radial variation of the cut-off frequency the flux tubes act as low frequency filters for the waves. 相似文献
9.
在柱坐标下将黑子周围的环形区域（黑子除外）内的振荡分解为朝向黑子传播的（入射的）波和离开黑子传播的（出射的）波。对无黑子的环形区域内的振荡也进行了同样的分解。将黑子周围的入射波看成是被黑子磁流管磁化了的介质（介质内的磁场基本是水平的）中的波。而无黑子区的入射波看成是非磁化介质中的波。比较这两种波在固定波数下功率随频率的分布发现，在磁化介质中不同径向除ｎ的声波（ｐ模）频率系统降低，同时功率也降低，降低的功率最高达非磁化介质中波的功率的３０％。而比较在固定频率下功率随波数的分布发现，磁场中ｆ模及ｎ＝１，２，３的ｐ模的脊向高波数方向位移，功率的降低受频率调制，即声波在某些有限的频带中被吸收。这些观测表明，在磁场中ｐ模与磁声重力波（ＭＡＧ）产生了模式混合或耦合。模式混合的存在支持了模式转换作为ｐ模式被黑子吸收的机制的解释。此外，本文还分析了转换的ＭＡＧ波进入黑子磁流管（其中的磁场基本上是垂直的）后进一步被吸收，吸收的功率最高达ＭＡＧ波的２０％。在磁流管内没有进一步观测到模式的转换 相似文献
10.
Twisted magnetic flux tubes are of considerable interest because of their natural occurrence from the Sun’s interior, throughout
the solar atmosphere and interplanetary space up to a wide range of applicabilities to astrophysical plasmas. The aim of the
present work is to obtain analytically a dispersion equation of linear wave propagation in twisted incompressible cylindrical
magnetic waveguides and find appropriate solutions for surface, body and pseudobody sausage modes (i.e. m = 0) of a twisted magnetic flux tube embedded in an incompressible but also magnetically twisted plasma. Asymptotic solutions
are derived in long- and short-wavelength approximations. General solutions of the dispersion equation for intermediate wavelengths
are obtained numerically. We found, that in case of a constant, but non-zero azimuthal component of the equilibrium magnetic
field outside the flux tube the index ν of Bessel functions in the dispersion relation is not integer any more in general.
This gives rise to a rich mode-structure of degenerated magneto-acoustic waves in solar flux tubes. In a particular case of
a uniform magnetic twist the total pressure is found to be constant across the boundary of the flux tube. Finally, the effect
of magnetic twist on oscillation periods is estimated under solar atmospheric conditions. It was found that a magnetic twist
will increase, in general, the periods of waves approximately by a few percent when compared to their untwisted counterparts. 相似文献
11.
K. D. Cole 《Planetary and Space Science》1961,5(4):292-298
The propagation of hydromagnetic and low frequency radio waves in all directions in a fully ionized gas containing a magnetic field is examined. For longitudinal and transverse propagation the addition of one extra term in the magneto-ionic formulae (without collisions) accounts for the presence of heavy ions. where V is the Alfvén speed. Thus approximate equipartition may exist for some audio- and radio-frequencies in the Earth's exosphere.
The partition of energy of disturbance between kinetic (K) and magnetic (M) for longitudinal propagation of all frequencies is given by
Some errors in Paper I of this series are corrected. 相似文献
12.
EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are
all associated with a flare and a CME and are commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves
could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed
this alternative interpretation. Using two independent 3D MHD codes, we performed nondimensional numerical simulations of
a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast
expansion, as during a CME. We analyse the origins, the development, and the observability in EUV of the narrow electric currents
sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations
of propagating EIT waves (7 April and 12 May 1997), by scaling the vertical magnetic field components of the simulated bipole
to the line of sight magnetic field observed by SOHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids
observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This
current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields.
It intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated
over the altitude has the shape of an ellipse, which expands and rotates when viewed from above, reproducing the generic properties
of propagating EIT waves. The timing, orientation, and location of bright and faint patches observed in the two EIT waves
are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating
in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression
inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current
shells. 相似文献
13.
H. C. Spruit 《Solar physics》1982,75(1-2):3-17
Propagation speeds are derived for the wave modes of a thin magnetic tube in an otherwise homogeneous magnetized or unmagnetized fluid. These results generalize results obtained by previous authors. There are three types of wave, a (torsional) Alfvén wave and two waves which are specific for the thin tube. These are named the longitudinal and transversal tube waves, according to their polarization properties. They can be camped by radiating an MHD or acoustic wave into the surroundings of the tube. Conditions for occurrence of this acoustic damping, and the damping rates, are derived. The behavior of the waves in the solar convection zone and corona is discussed. 相似文献
14.
The propagation of linear and non-linear magnetohydrodynamic (MHD) waves in a straight homogeneous cylindrical magnetic flux tube embedded in a homogeneous magnetic environment is investigated. Both the tube and its environment are in steady state. Steady flows break the symmetry of forward (field-aligned) and backward (anti-parallel to magnetic field) propagating MHD wave modes because of the induced Doppler shifts. It is shown that strong enough flows change the sense of propagation of MHD waves. The flow also induces shifts in cut-off values and phase-speeds of the waves. Under photospheric conditions, if the flow is strong enough, the slow surface modes may disappear and the fast body modes may become present. The crossing of modes is also observed due to the presence of flows. The effect of steady-state background has to be considered particularly carefully when evaluating observation signatures of MHD waves for diagnostics in the solar atmosphere. 相似文献
15.
L.C. Lee 《Planetary and Space Science》1982,30(11):1127-1132
The reflection and refraction of MHD waves through an “open” magnetopause (rotational discontinuity) is studied. It is found that most of the incident wave energy can be transmitted through the open magnetopause. A transverse Alfvén wave (or a compressional magnetosonic wave) from the solar wind incident upon the open magnetopause would generally lead to the generation of both the transverse Alfvén and compressional magnetosonic waves in the magnetosphere. Transmission of Alfvén waves in the coplanar rotational discontinuity is studied in detail. The integral power of the Alfvén-wave transfer is found to be proportional to the open magnetic flux of the magnetosphere and is typically ～ 1% of the power of the total electromagnetic energy transfer through the open magnetopause. The transmitted wave power may contribute significantly to the geomagnetic pulsations observed on the ground, especially in the open-field-line region. 相似文献
16.
P. S. Cally 《Solar physics》1986,103(2):277-298
An extensive analysis, both analytic and numerical, of waves in flux tubes imbedded in (possibly) magnetic surroundings is given. It is shown that any wave confined to the tube and its neighbourhood can be put into one of seven categories. Simple criteria for deciding the existence of each type in any particular case are derived. Many other (leaky) modes are found which excite waves in the external medium and thereby lose energy to the surroundings. A number of asymptotic analyses allow much information to be gained about these without the need for numerical solution of the complicated equations involved. Three particular cases, pertaining to photospheric flux tubes, H fibrils, and coronal loops, are considered in detail. 相似文献
17.
P. R. Wilson 《Solar physics》1977,55(1):35-45
The configuration of a magnetic flux element in a static, compressible, gravitationally stratified plasma is considered. Under isothermal conditions an exact force-free solution is given for a two-dimensional cartesian flux sheath but for an axi-symmetric element, i.e. a flux tube, approximate solutions, applicable only to thin flux tubes, are obtained.When the isothermal condition is relaxed within the flux tube, non-force-free solutions are obtained exhibiting either a temperature excess or deficit on the axis of the flux tube. Using these solutions, the total potential energy of such a tube and its surroundings may be calculated. By comparing this with the total energy of an equivalent system ofn flux tubes the temperature excess or deficit in these tubes corresponding to the adiabatic subdivision of the single tube is determined.It is shown that under non-adiabatic conditions the subdivision process must be endothermic (i.e., external energy is required) if the temperature within the original tube is significantly less than its surroundings but exothermic if the temperatures are comparable. Thus it is conjectured that magnetic structures are less susceptible to subdivision if they are significantly cooler than their surroundings. 相似文献
18.
B. Roberts 《Solar physics》1983,87(1):77-93
The nature of non-adiabatic wave propagation in a slender magnetic flux tube is explored. The results of the theory are compared with the observations of Giovanelli et al. (1978), and found to be in general agreement. Those observations, of tubes in the photosphere-chromosphere, show outwardly propagating waves, with periods of 300 s, which take some 19 s to propagate from one level of line formation to another level higher in the atmosphere. In sharp contrast to this, is the time of 7 s for a similar disturbance outside the tube to propagate between the same two levels of line formation, estimated to be some 600 km apart in the field-free atmosphere. It is argued that the sharply contrasting propagation times for the tube and its environment is principally due to the elasticity of the tube and its subsequent propensity for propagation. A non-adiabatic disturbance may be essentially propagating within the tube but essentially non-propagating outside, with considerably slower phase speeds thus arising inside the tube. The theory suggests that the observed disturbances are non-adiabatic, acoustic-gravity waves channelled along a magnetic flux tube and modulated by external pressure variations. 相似文献
19.
程久恒 《中国天文和天体物理学报》1994,(3)
从细磁通量管运动方程组导出了特征速度、特征线及其相容关系。这些结果是用特征线法研究细磁通量管运动的基础，也是正确提出定解问题的依据。表征细磁通量管横向波传播的特征速度与管内流动的ＡｌｆｖｅｎＭａｃｈ数有关。当管内流速超过ｋｅｌｖｉｎ－Ｈｅｌｍｈｏｌｔｚ不稳定性临界值时，不存在横向波模式。 相似文献
20.
L. C. Garcia de Andrade 《Astrophysics and Space Science》2007,310(1-2):25-29
Riemannian geometrical effects on the expansion of the electron magnetohydrodynamical (EMH) superconductivity modeled twisted
nonplanar thin magnetic flux tubes are considered. A solution is found which represents almost incompressible plasma flows,
where the twist of flux tube is computed in terms of the continuous variation of its cross-section. It is shown that the twist
increases in regions where twisted flux tube expands as in Parker’s conjecture. From computation of compression along the
tube we show that when the torsion is weak a centrifugal or vorticity effect on the longitudinal direction of the tube enhances
the screening effect on the “superconductor”. Throughout the paper we consider helical flux tubes where torsion and curvature
of the tube are constants. Thus we show that the Parker’s conjecture is valid in a continuos manner for these type II superconducting
twisted flux tubes. Throughout the paper we adopt the approximation that the radial component of the magnetic field varies
so slowly along the tube axis that it can be approximated to zero along the tube. It is suggested that the models discussed
here may also be applied to DNA and nanotubes. 相似文献