共查询到20条相似文献，搜索用时 812 毫秒
1.
The linearized theory for the parallel propagation of magnetoacoustic-gravity surface waves is developed for an interface of a horizontal magnetic field above a field-free medium. The media either side of the interface are taken to be isothermal. The dispersion relation is obtained for the case of a constant Alfvén speed. In the absence of gravity the interface may support one or two surface modes, determined by the relative temperatures and magnetism of the two media. The effect of gravity on the modes is examined and dispersion diagrams and eigenfunctions are given. In the usual - k
x diagnostic diagram, the domain of evanescence is shown to be divided into two distinct regions determining whether a given mode will have a decaying or growing vertical velocity component. In the absence of a magnetic field the transcendental dispersion relation may be rewritten as a polynomial. This polynomial possesses two acceptable solutions only one of which may exist in any given circumstances (depending on the ratio of the densities). If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate. The f-mode exists in a restricted band of horizontal wavenumber and only when the field-free medium is warmer than the magnetic atmosphere. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfvén speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths. 相似文献
2.
The occurrence of magnetoacoustic surface waves at a single magnetic interface one side of which is field-free is explored for the case of non-parallel propagation. Phase-speeds and penetration depths of the waves are investigated for various Alfvén speeds, sound speeds and angles of propagation to the applied field. Both slow and fast magnetoacoustic surface waves can exist depending on the values of sound speeds and propagation angle. The fast waves penetrate more than the slow waves.The parallel propagation of fast and slow magnetoacoustic surface waves on a magnetic-magnetic interface is investigated. The slow surface wave is unable to propagate below a critical sound speed. In a low -plasma, only the fast mode exists (0 0). 相似文献
3.
Propagation of MHD body and surface waves in magnetically structured regions of the solar atmosphere
The fact that magnetically structured regions exist in the solar atmosphere has been known for a number of years. It has been suggested that different kinds of magnetohydrodynamic (MHD) waves can be efficiently damped in these regions and that the dissipated wave energy may be responsible for the observed enhancement in radiative losses. From a theoretical point of view, an important task would be to investigate the propagation and dissipation of MHD waves in these highly structured regions of the solar atmosphere. In this paper, we study the behavior of MHD body and surface waves in a medium with either a single or double (slab) magnetic interface by use of a nonlinear, two-dimensional, time-dependent, ideal MHD numerical model constructed on the basis of a Lagrangean grid and semi-implicit scheme. The processes of wave confinement and wave energy leakage are discussed in detail. It is shown that the obtained results depend strongly on the type of perturbations imposed on the interface or slab and on the plasma parameter, . The relevance of the obtained results to the heating problem of the upper parts of the solar atmosphere is also discussed. 相似文献
4.
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
5.
Sunspots are known to have large, low-lying magnetic canopies, i.e. horizontal magnetic fields overlying a field-free medium, that cover substantial fractions of active region plage. In this paper we consider the influence of such canopies on the inclination of plage magnetic fields. We find that for observations in spectral lines like 5250.2Å the neglect of a sunspot canopy when determining magnetic inclination angles of plage fields can introduce errors exceeding 5–10°. This is particularly true if the observations do not have high spatial resolution. Thus this effect may explain some of the measurements of substantially inclined fields in solar plages. Furthermore we find that the Fe I 15648 Å line is far superior in giving correct flux-tube inclinations in the presence of a sunspot magnetic canopy. Finally, the inversion of full Stokes profiles is shown to produce more reliable results than results obtained by considering only ratios of individual Stokes profile parameters. 相似文献
6.
The effect of a non-uniform magnetic field on the gravitational instability for a non-uniformly rotating, infinitely extending
axisymmetric cylinder in a homogeneous medium has been studied. The Bel and Schatzman criterion of gravitational instability for a non-uniformly rotating medium is modified under the effect of a non-uniform/uniform
magnetic field acting along the tangential and axial directions. As a consequence the stabilizing and destabilizing effect
of the non-uniform magnetic field is obtained, a new criterion for the magneto-gravitational instability is deduced in terms
of Alfven’s wave velocity; and it is also found that the Jeans criterion determines the gravitational instability in the absence of rotation and when the non-uniform/uniform magnetic field acts
along the axis of the cylinder. 相似文献
7.
The polarization-free (POF) approximation (Trujillo Bueno and Landi Degl'Innocenti, 1996) is capable of accounting for the approximate influence of the magnetic field on the statistical equilibrium, without actually solving the full Stokes vector radiative transfer equation. The method introduces the Zeeman splitting or broadening of the line absorption profile I in the scalar radiative transfer equation, but the coupling between Stokes I and the other Stokes parameters is neglected. The expected influence of the magnetic field is largest for strongly-split strong lines and the effect is greatly enhanced by gradients in the magnetic field strength. Formally the interaction with the other Stokes parameters may not be neglected for strongly-split strong lines, but it turns out that the error in Stokes I obtained through the POF approximation to a large extent cancels the neglect of interaction with the other Stokes parameters, so that the resulting line source functions and line opacities are more accurate than those obtained with the field-free approach. Although its merits have so far only been tested for a two-level atom, we apply the POF approximation to multi-level non-LTE radiative transfer problems on the premise that there is no essential difference between these two cases. Final verification of its validity in multi-level cases still awaits the completion of a non-LTE Stokes vector transfer code.For two realistic multi-level cases (CaII and MgI in the solar atmosphere) it is demonstrated that the POF method leads to small changes, with respect to the field-free method, in the line source functions and emergent Stokes vector profiles (much smaller than for a two-level atom). Real atoms are dominated by strong ultraviolet lines (only weakly split) and continua, and most lines with large magnetic splitting (in the red and the infrared) are at higher excitation energies, i.e. they are relatively weak and unable to produce significant changes in the statistical equilibrium. We find that it is generally unpredictable by how much the POF results will differ from the field-free results, so that it is nearly always necessary to confirm predictions by actual computations.The POF approximation provides more reliable results than the field-free approximation without significantly complicating the radiative transfer problem, i.e. without solving any extra equations and without excessive computational resource requirements, so that it is to be preferred over the field-free approximation. 相似文献
8.
Richard J. Defouw 《Solar physics》1970,14(1):42-61
The convective stability of a simple model chromosphere is investigated. The model chromosphere consists of protons, electrons, and hydrogen atoms in the ground state; ionization is collisional and recombination is radiative. The analysis indicates stability when the kinetic temperature (T) is less than 17 500K (assuming T increases with height). However, for T > 17 500K, the model chromosphere is overstable in the absence of magnetic fields provided the temperature inversion is sufficiently steep. For smaller values of the temperature gradient, field-free regions are stable if the density is small and monotonically unstable if it is large. In the presence of a magnetic field, the model chromosphere is monotonically unstable for T > 17 500K, regardless of the temperature gradient.The convective instability of the model chromosphere results from the fact that the plasma is thermally unstable for T > 17 500K. Thermally unstable regions of the solar atmosphere, although not represented in detail by the model, should behave in a similar fashion.Field-free regions of the solar chromosphere are probably not monotonically unstable, but overstability is possible and may explain the origin of chromospheric oscillations with periods less than 200 sec. It is suggested that spicules result from the monotonic instability of magnetic regions. A similar instability in the corona may be responsible for the large Doppler spreading of radar echoes.Elementary considerations of thermal balance predict that the temperature gradient should diverge at levels of marginal stability. The chromospheric region of spicule formation and the corona should therefore both be bounded below by abrupt temperature jumps. 相似文献
9.
The nature of magnetoacoustic surface waves at a single magnetic interface, one side of which is field-free, is explored for the case of parallel propagation. The interface may support a slow surface wave or both slow and fast surface waves, depending upon the ordering of the sound speeds in the two media. Phase-speeds and penetration depths of the waves and the associated pressure perturbations and motions are investigated for a variety of field strengths and sound speeds. The fast wave disturbs the interface more than the slow wave. In the magnetic field region the slow wave is mainly longitudinal in nature whilst the fast surface wave is transverse for strong fields, longitudinal for weaker fields. In the field-free region both waves are longitudinal in character. The running penumbral wave phenomenon may provide an example of a magnetoacoustic surface mode, though any direct comparison requires the inclusion of gravitational effects. 相似文献
10.
The direct propagation of acoustic waves, driven harmonically at the solar photosphere, into the three-dimensional solar atmosphere
is examined numerically in the framework of ideal magnetohydrodynamics. It is of particular interest to study the leakage
of 5-minute global solar acoustic oscillations into the upper, gravitationally stratified and magnetised atmosphere, where
the modelled solar atmosphere possesses realistic temperature and density stratification. This work aims to complement and
bring further into the 3D domain our previous efforts (by Erdélyi et al., 2007, Astron. Astrophys. 467, 1299) on the leakage of photospheric motions and running magnetic-field-aligned waves excited by these global oscillations.
The constructed model atmosphere, most suitable perhaps for quiet Sun regions, is a VAL IIIC derivative in which a uniform
magnetic field is embedded. The response of the atmosphere to a range of periodic velocity drivers is numerically investigated
in the hydrodynamic and magnetohydrodynamic approximations. Among others the following results are discussed in detail: i) High-frequency waves are shown to propagate from the lower atmosphere across the transition region, experiencing relatively
low reflection, and transmitting most of their energy into the corona; ii) the thin transition region becomes a wave guide for horizontally propagating surface waves for a wide range of driver periods,
and particularly at those periods that support chromospheric standing waves; iii) the magnetic field acts as a waveguide for both high- and low-frequency waves originating from the photosphere and propagating
through the transition region into the solar corona.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
11.
G. Jovanović 《Solar physics》2014,289(11):4085-4104
We derive the dispersion equation for gravito-magnetohydrodynamical (MHD) waves in an isothermal, gravitationally stratified plasma with a horizontal inhomogeneous magnetic field. Sound and Alfvén speeds are constant. Under these conditions, it is possible to derive analytically the equations for gravito-MHD waves. The high values of the viscous and magnetic Reynolds numbers in the solar atmosphere imply that the dissipative terms in the MHD equations are negligible, except in layers around the positions where the frequency of the MHD wave equals the local Alfvén or slow wave frequency. Outside these layers the MHD waves are accurately described by the equations of ideal MHD. We consider waves that propagate energy upward in the atmosphere. For the plane boundary, z=0, between two isothermal plasma regions with horizontal but different magnetic fields, we discuss the boundary conditions and derive the equations for the reflection and transmission coefficients. In the simpler case of a gravitationally stratified plasma without magnetic field, these coefficients describe the reflection and transmission properties of gravito-acoustic waves. 相似文献
12.
The distances over which magnetohydrodynamic waves will propagate in a non-ideal, magnetic, compressible medium, representing the solar corona structured by the presence of loops of denser material, are considered. The waves are damped by ion viscosity and electron heat conduction in a radiating, optically thin atmosphere. Waves which lose their energy of propagation in distances of less than our criterion value of 4 × 109 cm are regarded as candidates for contributing towards coronal heating. Alfvénic-type waves only dissipate in this way in weak ( 15 G) magnetic fields and when they have periods of a few seconds (210 s). Acoustic-type waves can also be dissipated and we give typical values of magnetic field strength, density and temperature for which the dissipation could occur. Dissipating acoustic-type waves have periods that range from tens to hundreds of seconds (15–225 s).Calculations show that reliable measurements of velocity amplitudes will be invaluable in deciding whether these dissipating waves can contribute to heating the corona. We suggest that the waves that survive dissipation may account for some of the observed coronal oscillations. 相似文献
13.
V. V. Prudskikh 《Astronomy Letters》2012,38(1):45-50
We consider nonaxisymmetric magnetosonic oscillations of a radially stratified, weakly ionized protoplanetary disk with a
vertical magnetic field. The combined effect of the Hall electric field and the density and magnetic field inhomogeneities
present in the disk has been previously predicted to lead to an instability of its small azimuthal perturbations. We revise
the previous results and take into account the effect of inhomogeneous ionization of the protoplanetary material related to
the inhomogeneity of the disk medium. We show that the instability criterion is governed by three parameters: the magnetic
field and ionization fraction gradients and the plasma β. We have found that at high values of β typical of protoplanetary disks, the instability does not manifest itself if the gradients are directed oppositely. In the
case of codirectional gradients, the interaction of magnetosonic fluctuations with inhomogeneities of a fixed size is resonant
in character, giving rise to an instability in a narrow range of wave numbers. 相似文献
14.
Two-dimensional numerical models of the solar transition region are calculated using an inverse coordinates method which attains pressure equilibrium between the network magnetic field and the external comparatively field-free gas. If A(y, z) is the magnetic potential (a scalar in 2D), which is constant on field lines, the method involves interchanging dependent and independent variables to obtain a quasi-linear PDE for y(A, z), which is solved iteratively. The advantage of this approach is that magnetic field lines, including any magnetic interface, become coordinate lines, thereby simplifying the energy equation and free boundary problem. In order to examine the effects of self-consistent geometry on the thermal structure of the transition region network, we calculate four models. The energy balance includes the effects of radiation, conduction, and enthalpy flux. It is confirmed that the lower branch of the emission measure curve cannot be explained within the single fluxtube model if the classical Spitzer thermal conductivity is used. However, by including a turbulent thermal conductivity as proposed by Cally (1990a), transition region models are obtained for which the resulting emission measure curves exhibit the correct behaviour, including the observed turn-up below about 200 000 K. In summary, the broad conclusions of previous non-turbulent 2D models are confirmed, but most importantly, the turbulent conductivity hypothesis tested in 1D by Cally is shown to produce excellent agreement with observations in the more realistic geometry. 相似文献
15.
The magnetosonic modes of magnetic plasma structures in the solar atmosphere are considered taking into account steady flows of plasma in the internal and external media and using a slab geometry. The investigation brings nearer the theory of magnetosonic waveguides, in such structures as coronal loops and photospheric flux tubes, to realistic conditions of the solar atmosphere. The general dispersion relation for the magnetosonic modes of a magnetic slab in magnetic surroundings is derived, allowing for field-aligned steady flows in either region. It is shown that flows change both qualitatively and quantitatively the characteristics of magnetosonic modes. The flow may lead to the appearance of a new type of trapped mode, namelybackward waves. These waves are the usual slab modes propagating in the direction opposite to the internal flow, but advected with the flow. The disappearance of some modes due to the flow is also demonstrated.The results are applied to coronal and photospheric magnetic structures. In coronal loops, the appearance of backward slow body waves or the disappearance of slow body waves, depending upon the direction of propagation, is possible if the flow speed exceeds the internal sound speed ( 300 km s–1). In photospheric tubes, the disappearance of fast surface and slow body waves may be caused by an external downdraught of about 3 km s–1. 相似文献
16.
Solar p modes are one of the dominant types of coherent signals in Doppler velocity in the solar photosphere, with periods showing
a power peak at five minutes. The propagation (or leakage) of these p-mode signals into the higher solar atmosphere is one of the key drivers of oscillatory motions in the higher solar chromosphere
and corona. This paper examines numerically the direct propagation of acoustic waves driven harmonically at the photosphere,
into the nonmagnetic solar atmosphere. Erdélyi et al. (Astron. Astrophys.
467, 1299, 2007) investigated the acoustic response to a single point-source driver. In the follow-up work here we generalise this previous
study to more structured, coherent, photospheric drivers mimicking solar global oscillations. When our atmosphere is driven
with a pair of point drivers separated in space, reflection at the transition region causes cavity oscillations in the lower
chromosphere, and amplification and cavity resonance of waves at the transition region generate strong surface oscillations.
When driven with a widely horizontally coherent velocity signal, cavity modes are caused in the chromosphere, surface waves
occur at the transition region, and fine structures are generated extending from a dynamic transition region into the lower
corona, even in the absence of a magnetic field. 相似文献
17.
A. Granik 《Astrophysics and Space Science》1981,74(2):491-495
Nonlinear Alfvén waves, which in the infinitely conducting plasma are noncompressive and have a constant magnetic field strength (B
2=const), propagate in a turbulent plasma. The latter is characterized by a big (but finite) electrical conductivity eff due to micro-instabilities. The Alfvén wave in such a medium is governed by the diffusion equation. It is shown that an initial periodic perturbation (withB
2=const) while still being incompressive, decays due to dissipation. 相似文献
18.
A. G. Michalitsanos 《Solar physics》1973,30(1):47-61
The magnetohydrodynamic frequency-wavelength relation, derived by McLellan and Winterberg (1968), has been evaluated for an isothermal atmosphere. In particular, the effect which an inclined magnetic field and a finite horizontal wavelength have on the critical sonic and internal-gravity cut-off frequencies has been examined, in which it has been assumed that the magnetic field vector, wave vector, and gravity vector are coplanar. It is shown that the frequency band in which vertical wave propagation is impossible in the non-magnetic photosphere, becomes smaller when an inclined uniform magnetic field is introduced, and that low frequency magnetically coupled internal-gravity waves do not propagate vertically if the horizontal wavelengths associated with this mode are greater than a critical wavelength which decreases with field strength.It is also demonstrated that an inclined magnetic field will inhibit the resonance that occurs at the critical frequency
g in the non-magnetic atmosphere which is a result consistent with recent observations of the wiggly line structure in active regions.This work is supported by the European Space Research Organization.Presently with the Solar Astronomy Group, California Institute of Technology. 相似文献
19.
A magnetopause that separates two regimes of different flow, additional to the separation of a magnetic field from a field-free plasma, gives rise to the formation of asymmetric Stokes profiles. Using a simple two-layer model atmosphere, where one layer comprises a magnetic field, the other being field-free, it is shown by analytical derivation that a wide variety of Stokes V profiles can be produced, having amplitude asymmetries a in the range –a. These include two-humped V profiles, which have two lobes of equal sign. For the most simple models, the asymmetry depends on the ratio of continuum intensity to the Planck radiation intensity of the magnetic layer at the wavelength of the spectral line under consideration, and on the line depth. Two-humped profiles (|a|>1) require the temperature of the magnetic layer to surpass the temperature of the line-core forming region, implying a temperature inversion, so that the V profile is partially in emission. The confrontation of this formation scenario with properties of observed one-lobe profiles of quiet-Sun network regions is inconclusive due to insufficient spatial resolution and lack of a sufficient sample of simultaneously recorded Stokes spectral lines of varying line depths. It seems, however, to be in good agreement with the observed frequent occurrence of abnormal V profiles of the very strong Nai D
2 and D
2 spectral line. A possible observational verification for the present formation scenario of abnormal Stokes V profiles and a novel method of Stokes inversion are discussed. 相似文献
20.
Satoshi Hinata 《Solar physics》1988,116(2):239-258
We have investigated nonlinear equilibrium states of a microscopic current filamentation (electrothermal instability) in solar atmosphere. The microscopic filamentation instability will develop for transition zone ion temperature plasmas, provided T
e/Ti > 1, where T
e and T
i are the electron and ion temperatures, respectively. Since the temperature radio for a steady-state solar atmosphere is approximately unity, the electrothermal instability will develop only in a time-dependent solar atmosphere. Indeed, such a condition is provided by time-dependent currents, which seem to exist in many magnetic loops as recent analysis by Porter et al. (1987) indicates. When the onset condition for the electrothermal instability is satisfied, the instability drives a current filamentation to a nonlinear equilibrium state with a spatially periodic electron temperature variation with the wavelength comparable to several ion-Larmor radii. The amplitude of the periodic temperature variation may be so large that the transition layer temperature and coronal temperature plasmas may exist within several Larmor radii — coexistence of the transition zone and corona within the same macro-volume. 相似文献