Propagation Of Magnetohydrodynamic Waves Research Articles

Fast surface waves in an ideal Hall-magnetohydrodynamic plasma slab

The propagation of fast sausage and kink magnetohydrodynamic (MHD) surface waves in an ideal magnetized plasma slab is studied taking into account the Hall term in the generalized Ohm’s law. It is found that the Hall effect modifies the dispersion characteristics of MHD surface modes when the Hall term scaling length is not negligible (less than, but comparable to the slab thickness). The dispersion relations for both modes have been derived for parallel propagation (along the ambient equilibrium magnetic field lines).The Hall term imposes some limits on the possible wave number range. It turns out that the space distribution of almost all perturbed quantities in sausage and kink surface waves with Hall effect is rather complicated as compared to that of usual fast MHD surface waves. The applicability to solar wind aspects of the results obtained, is briefly discussed.

Induced Deposition of Magnetic Energy in the Solar Corona

In this paper a numerical study of the propagation and dissipation properties of magnetohydro-dynamic waves in an incompressible magnetized plasma is presented. The magnetic field is assumed to be unidirectional, but its magnitude varies in a direction perpendicular to the field. The analysis concerns both linear and nonlinear waves. The main findings are the following.

Rapid reconnection in compressible plasma

A study of set-up, propagation, and interaction of non-linear and linear magnetohydrodynamic waves driven by magnetic reconnection is presented. The source term of the waves generated by magnetic reconnection is obtained explicitly in terms of the initial background conditions and the local reconnection electric field. The non-linear solution of the problem found earlier, serves as a basis for formulation and extensive investigation of the corresponding linear initial-boundary value problem of compressible magnetohydrodynamics. In plane geometry, the Green’s function of the problem is obtained and its properties are discussed. For the numerical evaluation it turns out that a specific choice of the integration contour in the complex plane of phase velocities is much more effective than the convolution with the real Green’s function. Many complex effects like intrinsic wave coupling, anisotropic propagation characteristics, generation of surface and side wave modes in a finite beta plasma are retained in this analysis.

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, β. The relevance of the obtained results to the heating problem of the upper parts of the solar atmosphere is also discussed.

U-band flares and sympathetic flaring on G1 65

We present contemporaneous observations in the U and K bands of some 26 flares, on the active flare star Gl 65. Over a period of 27 yr, the frequency of flaring seems to have changed little. We suggest that this is due to sympathetic flaring, caused by a Moreton-wave type phenomenon, the propagation of fast magnetohydrodynamic (MHD) waves from the flare site out into the corona that are reflected back to the chromosphere. We have also redetermined the time-averaged U-band flare luminosity, |$L_{\text U}^{\ast}=3.26\times10^{26}$| erg s–1, which is a factor of two less than previously reported, but is in closer agreement with the linear |$L_{\text U}^{\ast}-L_{\text X}$| correlation of Doyle & Butler. The flares fall into two main categories, with longer duration flares releasing a smaller (20 per cent) fraction of their energy over the initial rise and decay to half maximum than the shorter-lived flares ( ≥ 50 per cent). We also find that the evolution of some of the flares can be fitted by conduction and radiation from a hot 107 K thermal plasma, as modelled by Mullan. We have also been able to fit Gurzadyan’s model of a flare on the hidden hemisphere of the star, to two broad, low-amplitude continuum enhancements.

Complete integrability of a modified vector derivative nonlinear Schrödinger equation

Oblique propagation of magnetohydrodynamic waves in warm plasmas is described by a modified vector derivative nonlinear Schrödinger equation, if charge separation in Poisson's equation and the displacement current in Ampère's law are properly taken into account. This modified equation cannot be reduced to the standard derivative nonlinear Schrödinger equation and hence its possible integrability and related properties need to be established afresh. Indeed, the new equation is shown to be integrable by the existence of a bi-Hamiltonian structure, which yields the recursion operator needed to generate an infinite sequence of conserved densities. Some of these have been found explicitly by symbolic computations based on the symmetry properties of the new equation.

Normal modes of a resistive nonuniform plasma

The propagation and dissipation properties of magnetohydrodynamic waves in a nonuniform, highlyconducting plasma, is investigated with a normal mode approach. The interaction between the perturbation and the non-uniform supporting medium is analyzed as the main mechanism able to produce the small scale spatial structure necessary to dissipate efficiently the wave energy. Two fundamental classes of modes are found, characterized by their resistive or ideal asymptotic behavior; the damping rates are shown to be orders of magnitude larger than those obtained when the plasma is perfectly homogeneous, and an application to the problem of solar coronal heating is discussed.

Magnetohydrodynamic wave propagation in one-dimensional nonhomogeneous, self-gravitating clouds

We study the propagation of magnetohydrodynamic (MHD) waves through nonhomogeneous, selfgravitating, magnetic media representative of molecular cloud environments and focus on the issues of cloud support and line profiles. Since the general treatment of this topic is burdened by a complex mathematical formalism, we consider simplifying geometries which yield analytical solutions in the linear wave limit. In particular, we study both magnetoacoustic and Alfven wave propagation along the density gradient in a one-imensional slab. Of specific relevance to molecular clouds, we find that the back reaction of the Alfven waves can provide a pressure along the direction of the magnetic field lines; this pressure can help support a density enhancement against gravitational collapse. Furthermore, we find that the velocity amplitudes of these waves increase as the density decreases, in rough agreement with observational estimates of line width versus density relations.

Nonlinear oblique propagation of magnetohydrodynamic waves in the solar wind

Oblique propagating magnetohydrodynamic waves with various wave forms and amplitudes are observed both at the Earth's foreshock and at comets. The possibility of interpreting some observational results in terms of nonlinear evolution of one- and two-dimensional hydromagnetic waves is investigated. For this purpose both analytical and numerical techniques are employed. It is found that an initial monochromatic wave changes its polarization giving origin to magnetosonic shocks and rotational discontinuities; the time evolution of density-magnetic-field correlation is studied, as a function of the plasma parameters and of the propagation angle. In the two-dimensional case both a transverse instability and a self-focusing effect may take place. Moreover, a two-dimensional magnetosonic solution is found, in which the density fluctuations are driven by the total pressure fluctuation as in a one-dimensional simple wave. These theoretical predictions compare well with the features observed in the solar-wind waves.

Nonresonant resistive dissipation of compressible magnetohydrodynamic waves

view Abstract Citations (25) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonresonant Resistive Dissipation of Compressible Magnetohydrodynamic Waves Califano, Francesco ; Chiuderi, Claudio ; Einaudi, Giorgio Abstract The effect of compressibility on the propagation and resistive dissipation properties of magnetohydrodynamic waves in nonuniform media is studied by using a normal mode analysis. The paper concentrates on wave models for which the resistivity acts over the entire system, thus excluding localized resonant absorption. The wave modes in nonhomogeneous situations are identified in term of their familiar homogeneous counterparts and it is shown that only the shear Alfven and slow magnetosonic modes survive in the resistive regime. The most promising heating agents in the astrophysical context are shown to be the shear Alfven waves that behave almost incompressibly. Publication: The Astrophysical Journal Pub Date: May 1992 DOI: 10.1086/171306 Bibcode: 1992ApJ...390..560C Keywords: Compressibility; Magnetohydrodynamic Waves; Wave Attenuation; Wave Propagation; Analytic Functions; Magnetosonic Resonance; Mathematical Models; Propagation Modes; Shear; Plasma Physics; MAGNETOHYDRODYNAMICS: MHD full text sources ADS |

The theory of magnetohydrodynamic wave generation by localized sources. III - Efficiency of plasma heating by dissipation of far-field waves

The fraction of radiation emitted by Alfven waves is calculated by using two separate methods to determine whether the Alfven flux generated in the photosphere is sufficient to heat the corona. One method employs a set of scaling laws for the fluxes as functions of plasma and source parameters; the second method consist of a procedure for calculating the flux in each waveband from the interaction of vector-harmonic components of an arbitrary applied forcing. Both methods indicate that the Alfven flux accounts roughly for half of the total emission. The need to reexamine estimates of the amount of Alfven flux reaching the corona based on observations of plasma disturbances in the photosphere is emphasized.

Viscous damping of surface magnetohydrodynamic waves on magnetic interface in cold plasmas

Surface magnetohydrodynamic wave propagation on a magnetic interface in a cold plasma is studied. The anisotropic ion viscosity is taken into account. Only long waves damping weakly in a wave period are considered. The dispersion equation is obtained. This equation is shown always to have exactly one root if there is no viscosity. The dependences of phase velocity, penetration depth and damping decrement of waves on the parameters of undisturbed plasma and wave propagation direction are investigated. The resulting application for describing of surface wave damping in the solar corona is discussed.

On the generation of magnetohydrodynamic waves in a stratified and magnetized fluid. II - Magnetohydrodynamic energy fluxes for late-type stars

Magnetohydrodynamic (MHD) wave energy fluxes for late-type stars are calculated, using previously obtained formulae for the source functions for the generation of MHD waves in a stratified, but otherwise uniform, turbulent atmosphere; the magnetic fields in the wave generation region are assumed to be homogeneous. In contradiction to previous results, it is shown that in this uniform magnetic field case there is no significant increase in the efficiency of MHD wave generation, at least within the theory's limits of applicability. The major results are that the MHD energy fluxes calculated for late-type stars are less than those obtained for compressible modes in the magnetic field-free case, and that these MHD energy fluxes do not vary enough for a given spectral type to explain the observed range of UV and X-ray fluxes from such stars. It is therefore concluded that MHD waves in stellar atmospheres with homogeneous magnetic fields in the wave generation region cannot explain the observed stellar coronal emissions; if such MHD waves are responsible for a significant component of stellar coronal heating, then nonuniform fields within the generation region must be appealed to.

On the generation of magnetohydrodynamic waves in a stratified and magnetized fluid. I - Vertical propagation

view Abstract Citations (33) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Generation of Magnetohydrodynamic Waves in a Stratified and Magnetized Fluid. I. Vertical Propagation Musielak, Z. E. ; Rosner, R. Abstract The generation of MHD waves by turbulent motions in a stratified medium with an embedded uniform magnetic field, a topic which is relevant to the study of the solar atmosphere, is considered. Both compressible and incompressible MHD waves are treated in a one-dimensional approach; however, the direction of the background magnetic field is permitted to vary in an arbitrary direction. Theoretical expressions for MHD energy fluxes are obtained as a function of wave frequency and multipole coefficients. It is shown that monopole, dipole, and quadrupole emissions are responsible for the generation of the compressible components of the fast and slow modes. However, the incompressible components and the Alfven modes can be generated by the dipole emission only. Specific results obtained for special magnetic field geometries are discussed for the fast and slow modes. Publication: The Astrophysical Journal Pub Date: April 1987 DOI: 10.1086/165141 Bibcode: 1987ApJ...315..371M Keywords: Magnetohydrodynamic Waves; Solar Atmosphere; Stratified Flow; Wave Generation; Benard Cells; Solar Flux; Turbulence; Wave Propagation; Solar Physics; HYDROMAGNETICS; SUN: ATMOSPHERE; TURBULENCE full text sources ADS | Related Materials (1) Part 2: 1988ApJ...329..376M

Alfvén continuum with toroidicity

The symmetry property of the magnetohydrodynamic (MHD) wave propagation operator is utilized to express the toroidal eigenmodes as a superposition of the mutually orthogonal cylindrical modes. Because of the degeneracy among cylindrical modes with the same frequency but resonant surfaces of different helicity, the toroidal perturbation produces a zeroth-order mixing of the above modes. The toroidal eigenmodes of frequency ω20 have multiple resonant surfaces, with each surface shifted relative to its cylindrical position and carrying a multispectral content. Thus a single helicity toroidal antenna of frequency ω0 couples strongly to all different helicity resonant surfaces with matching local Alfvén frequency. Zeroth-order coupling between modes in the continuum and global Alfvén modes also results from toroidicity and degeneracy. The perturbation technique used in this study is the MHD counterpart of the quantum-mechanical methods and is applicable through the entire range of the MHD spectrum.

A leaky magnetohydrodynamic waveguide model for the acceleration of high-speed solar wind streams in coronal holes

It is now reasonably well established that there is a correlation between high-speed solar wind streams and coronal holes. It has been concluded that a significant addition of momentum and/or energy in the region of supersonic flow is needed to explain the observed particle flux and flow speed observed in the high-speed streams. The most likely source of this additional momentum appears to be magnetohydrodynamic (MHD) waves propagating up from the solar surface. The present investigation is concerned with the propagation of MHD waves in a structure of finite transverse size, taking into account the consequences for the acceleration of high-speed solar wind streams. A waveguide solution for a model coronal hole is described, giving attention to a geometric or ray analysis of the slab waveguide, a wave mode analysis, an analytic solution of the dispersion relation for high-frequency waves, and the calculation of the time-averaged wave force.

Alfvén Waves in Current-carrying Solar Magnetic Flux Tubes

In theories of the heating of the solar corona a number of authors have recently considered the propagation and damping of fast and slow magnetohydrodynamic waves, in the form of surface waves localized on the interfaces of coronal flux tubes (Ionson 1978, Wentzel 1979, Roberts 1981, Cramer and Donnelly 1983). The damping of these waves occurs, in addition to a rather weak global damping due to viscous or resistive dissipation, by means of a localized absorption at a so-called ‘resonance’ in the density or magnetic field profile forming the flux tube. At such a resonance, the wave frequency is equal to the local value of the Alfven wave frequency, in the case of Alfven resonance absorption, or at the local slow mhd wave frequency, in the case of the ‘cusp’ or ‘compressive singularity’ resonance in a finite pressure plasma.

Numerical experimentation on spherically symmetric one-dimensional magnetohydrodynamic (MHD) wave propagation

Radial propagation of one-dimensional magnetohydrodynamic (MHD) waves are analyzed numerically on the basis of the Implicit-Continuous-Fluid-Eulerian (ICE) scheme. Accuracy of the numerical method and other properties are tested through the study of MHD wave propagation. The three different modes of MHD waves (i.e. fast- slow- and Alfven (transverse) mode) are generated by applying physically consistent boundary perturbations derived from MHD compatibility relations. It is shown that the resulting flow following these waves depend upon the relative configurations of the initial magnetic field and boundary perturbations.

Stellar chromospheric and coronal heating by magnetohydrodynamic waves

An investigation is presented on the way in which the generation of magnetohydrodynamic waves by turbulent motions in stellar convection zones depends on the star's effective temperature, surface gravity, and magnetic field strength. It is shown that the emitted Alfven wave flux (and acoustic slow wave flux in a very strong magnetic field) is in reasonable agreement with the general trend of observed chromospheric radiative losses in stars, and with the observations of three stars for which magnetic field strength, surface area covered by strong fields, and radiative losses have all been measured.

Influence of cosmic rays on propagation of long magneto hydrodynamic waves

The influence of relativistic particles on the dispersion properties of a cosmic plasma are considered. Use is made of the equation of magneto-hydrodynamics for the thermal plasma and the diffusion equation for cosmic rays. The mechanism of dissipation of the waves due to the diffusion of cosmic rays is shown to be predominant for magneto acoustic waves in the interstellar medium.