Propagating Alfven Waves Research Articles

Large‐Amplitude Magnetohydrodynamic Waves in Nonhomogeneous Plasmas

We consider the propagation of large-amplitude magnetohydrodynamic (MHD) waves in an anisotropic nonhomogeneous plasma. A classical MHD formulation is adopted, and, by means of a sequence of transformations and use of Elsasser's variables, we derive an exact nonlinear system of equations in compact form that describes such waves. The system incorporates compressibility of the plasma and an arbitrary polarization for the waves. Accordingly, the system provides for the analysis of parallel-propagating, nonlinear Alfven waves and magnetosonic waves in a general space plasma context, including the solar wind. We obtain a new large-amplitude incompressible Alfven wave solution to the system, and also show how to construct Riemann solutions that are valid under broad assumptions. Brief consideration is given to Alfven wave propagation in a stationary plasma, and we show that the equation of propagation reduces to classical second-order partial differential equations in two special cases. For the special case in which the Alfven speed depends linearly on the spatial coordinate, a simple, exact Riemann solution is presented.

Alfven wave generation by disturbance of ionospheric conductivity in the field‐aligned current region

A fundamental problem of the Alfven wave generation from a region of disturbed ionospheric conductivity in the ambient field‐aligned current area is investigated. It is shown that the field‐aligned current of the outgoing Alfven wave appears not only at the boundary but also inside the region of enhanced (or diminished) conductivity. If the ionosphere conductivity increases, the additional field‐aligned current of the outgoing Alfven wave inside the region of disturbed conductivity is directed in the same way the background field‐aligned current. For a circular region of disturbed conductivity the electric field of the outgoing Alfven wave can be presented as the sum of electric fields of the two‐dimensional dipole, quadrupole, and screened monopole sources. For a strip‐like region of disturbed conductivity the electric field of the outgoing Alfven wave appears only inside the strip and can be presented as the sum of homogeneous and linearly varying electric fields.

FIR radiation interferometry in bismuth induced by a magnetic field

The study of FIR radiation ( λ = 337 μm) propagation through a plane-parallel plate of single crystal bismuth in the presence of a magnetic field is reported. Effects induced by electro-magnetic interference were observed and connected with the propagation of Alfven-waves. The value of the refractive index was found to vary in the range 7.3-4.6 for the magnetic field interval 3.1–5.0 T. It is shown that a single crystal of bismuth inserted in the magnetic field may be used to obtain interferograms (fast Fourier transform spectroscopy) of submillimeter radiation with a scanning duration of 10 −5 s.

Formation of Small Scales via Alfven Wave Propagation in Compressible Nonuniform Media

view Abstract Citations (50) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Formation of Small Scales via Alfven Wave Propagation in Compressible Nonuniform Media Malara, Francesco ; Primavera, Leonardo ; Veltri, Pierluigi Abstract In weakly dissipative media governed by the magnetohydrodynamics (MHD) equations, any efficient mechanism of energy dissipation requires the formation of small scales. In a previous paper (Paper I) the possibility of producing small scales has been numerically studied in the case of MHD disturbances propagating in an incompressible and inhomogeneous medium, for a strictly two-dimensional slab geometry, the ignorable coordinate being the one that is perpendicular both to the magnetic field and to the inhomogeneity direction. In this paper we extend the work of Paper I, to study the properties of Alfven waves propagating in a compressible medium, in the presence of an inhomogeneity transverse to the direction of wave propagation. The study has been performed using a 2.5-dimensional pseudospectral numerical code. The simulations show that, when an Alfven wave propagates in a compressible nonuniform medium, the two dynamical effects responsible for the formation of small scales in the incompressible case are still at work: energy pinching and phase mixing. Moreover, the interaction between the initial Alfven wave and the inhomogeneity gives rise to the formation of compressible perturbations (fast and slow waves or a static entropy wave). Some of these compressive fluctuations are subject to the steepening of the wave front and become shock waves, which are extremely efficient in dissipating their energy, their dissipation being independent of the Reynolds number. A rough estimate of the typical times which the various dynamical processes take to produce small scales and then to dissipate the energy show that for Reynolds numbers of the order of S = 108 or greater, the steepening of compressive fluctuations can represent an efficient mechanism to dissipate the wave energy and do not require large values of the wave amplitude. Publication: The Astrophysical Journal Pub Date: March 1996 DOI: 10.1086/176898 Bibcode: 1996ApJ...459..347M Keywords: MAGNETOHYDRODYNAMICS: MHD; PLASMAS; SUN: CORONA; WAVES full text sources ADS |

Ponderomotive acceleration of ions by circularly polarized electromagnetic waves

The ponderomotive acceleration of multi‐species ions due to circularly polarized electromagnetic waves propagating parallel to the magnetic field is calculated. When applied to a low frequency left‐hand circularly polarized wave, the results show differential accelerations among the different ion species. We demonstrate that differential acceleration of ion species by the ponderomotive force can occur even in the absence of a parallel electric field component. Our work extends the results previously obtained by Li and Temerin (1993) who considered the ponderomotive acceleration due to obliquely propagating Alfvén waves.

Artificial magnetic pulsation generation by powerful ground-based transmitter

The magnetic pulsation generation by powerful ground-based transmitter is considered. The magnetic pulsation generation is based on the heating of electrons by a powerful radio wave. The electron temperature increase produces a change in both the electron collision frequency and the recombination factor that produces an ionospheric conductivity variation, ionospheric currents variation and magnetic pulsation generation. Experimental investigation at the heating facility near Tromsø has shown that the magnetic pulsation amplitude differs greatly from case to case. The results of calculations of the generated magnetic pulsation amplitude as a function of initial electron density profiles and heating wave parameters (the wave frequency and effective radiated power) are presented. Anomalous absorption and a “translucence effect” due to the enhancement of electron temperature are considered. Since the anomalous absorption of the heating wave depends on the wave energy flux in unit volume it is not reasonable if the radiated beam width is too narrow. In conclusion some geophysical applications are considered: these are the magnetic pulsation polarization as a tool for electric field determination, Alfven wave generation and the Alfven resonator excitation in lower exosphere.

Nonlinear propagation of Alfvén waves in a magnetoactive compensated semiconductor plasma

The nonlinear propagation of low-frequency electromagnetic Alfven waves along an external static magnetic field in a weakly collisional compensated semiconductor plasma has been investigated. A new kind of envelope solitions in the magnetoactive semiconductor plasma are found to exist. The behaviour of the solitions is discussed.

The Excitation of Shear Alfven Wave and the Associated Modulation of Compressional Wave in the Inner Magnetosphere.

Two basic but novel equations directly describing the generation of shear Alfven and compressional waves in the inner magnetosphere filled with a cold magnetized plasma are derived. The shear Alfveen wave is characterized by the field-aligned current and the compressional wave by the compressional component of the magnetic field. Such a generation arises from the effects of inhomogeneous Alfven speed and curvilinear field line. Around the magnetic equator, if the Alfven speed is inversely proportional to a power of the geocentric distance, these effects have magnitudes of the same order and their signs are identical. Considered in the present study is a situation that the earthward propagating compressional wave is launched from a large scale oscillating current wedge centered at midnight and symmetric about the magnetic equator. Then, it is found that the field-aligned current excited around the equator by the compressional wave has opposite senses in direction in the northern and southern hemispheres, in the pre- and post-midnight sectors as well as just inside the plasmapause and in its surrounding regions. As a result of the excitation of shear Alfven wave, two types of oscillations appear on a field line: One is a forced oscillation and the other is an eigenoscillation. Although a modulation of the compressional wave may be caused locally (or microscopically) around the equator by the eigenoscillation of field line, the modulation can be globally (or macroscopically) neglected. So far as the propagation along the source longitude (source-earth line) around the equator is concerned, the coupling between compressional and shear Alfven waves can be almost neglected and so one-dimensional response of the inner magnetosphere around the equator plays a significant role in the compressional oscillation.

Generation of dynamic pressure pulses downstream of the bow shock by variations in the interplanetary magnetic field orientation

One‐dimensional resistive MHD and hybrid simulations are carried out to study the manner by which variations of the interplanetary magnetic field (IMF) direction generate dynamic pressure pulses in the magnetosheath. The reaction of the magnetosheath to the temporal IMF variation is modeled as the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD), an Alfven wave pulse (AW), or an Alfven wave train. The resistive MHD simulation indicates that the arrival of an RD produces two time‐dependent intermediate shocks (TDISs) and two slow shocks downstream of the bow shock, which propagate through the magnetosheath toward the Earth's magnetopause. An enhancement of plasma density is present throughout the TDISs and slow shocks. A plasma dynamic pressure pulse is formed in this region. In the hybrid simulation, the two TDISs are replaced by rotational discontinuities. For a bow shock with a shock normal angle θBn > 45°, the pulse in the dynamic pressure ρV2 causes the total pressure (P + B2/2μ0 + ρV2) in the magnetosheath to increases by about 0–100% of the background value. The strength of the pressure pulse increases with the field rotation angle across the incident rotational discontinuity, while it decreases with the Mach number or upstream plasma beta of the bow shock. The pressure pulse propagates toward the magnetopause with nearly a constant amplitude. On the other hand, the BS/AW interaction leads to the generation of Alfven waves downstream of the bow shock, and large‐amplitude dynamic pressure pulses are generated in the downstream Alfven wave. Pressure pulses impinging on the magnetopause may produce magnetic impulse events (MIEs) observed in the high‐latitude ionosphere.

Transport of Alfven wave beam by solar wind

Conditions of Alfven wave propagation including propagation of Gaussian wave beam from the local source in the solar wind as an inhomogeneous medium with ultra Alfven velocities are studied. By this, the special attention is concentrated on the phenomena connected with violation of geometrical optics approach. The sizeable increase of wave disturbance amplitude caused by transformation between waves was shown. The field region of heightened Alfven disturbance was estimated. The dependence of registered beam amplitude from the dimention of the solar wind exactly connected with the solar activity was numerally analysed. Amplitude increases with the growth of the solar wind speed and decreases with the growth of its temperature. The dependence of beam intensity from its primary spectrum was cleared up. When the initial beams are equal in their intensity, those beams which have wider transversal scale spectrum increase more significant. The method of determination of generating region using the analysis of observed transversal scale spectrum of Alfven waves is offered.

Nonlinear Alfven wave propagation in nonhomogeneous collisionless media

view Abstract Citations (4) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonlinear Alfven Wave Propagation in Nonhomogeneous Collisionless Media Khabibrakhmanov, Ildar K. ; Summers, Danny Abstract A collisionless model of nonlinear Alfven wave propagation in a nonhomogenous medium is formulated in terms of two amplitude equations for the slow plasma parameters, the only fast parameter being the transverse component of the unit vector along the local magnetic field. The mdoel includes plasma pressure anisotropy and takes into account the relative motion of the plasma components along the magnetic field, effects which have hitherto not been considered in a self-consistent manner. Exact one-dimensional solutions are found under the restriction that the magnetic field, bulk plasma velocity vectors, and propagation direction all remain in the same plane. It is shown that these solutions reduce exactly to the standard Wentzel-Kramer-Brillouin (WKB) approximate solution, i.e., it is shown that the WKB solutions are, in fact, exact under the above stated restriction. The restriction is not related to the limitations of the WKB approximation, but merely specifies the mode of the propagating wave. This exact solution can be used as a zero-order approximation for the investigation of the stability of finite-amplitude Alfven waves. Thermal corrections to the wave amplitude solution are obtained as well as to the bulk plasma momentum equation. The wave amplitude solution shows that direct exchange of the Alfven wave energy with the kinetic energy of the particles is possible. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175652 Bibcode: 1995ApJ...444..796K Keywords: Collisionless Plasmas; Inhomogeneity; Magnetohydrodynamic Waves; Mathematical Models; Nonlinearity; Plasma Turbulence; Wave Propagation; Anisotropy; Kinetic Energy; Momentum Transfer; Pressure Effects; Stellar Winds; Wentzel-Kramer-Brillouin Method; Astronomy; PLASMAS; TURBULENCE; WAVES full text sources ADS |

Penetration characteristics of electromagnetic emissions from an underground seismic source into the atmosphere, ionosphere, and magnetosphere

Theoretical calculations are made on electromagnetic fields in the frequency range 10−2 to 10−2; Hz on the ground surface and above the ionosphere induced by stochastic microcurrent activity inside the future seismic sources on the assumption of cylindrical symmetry of the effective current and three types of polarization. The inhomogeneity of the ground and atmosphere conductivity and anisotropy of the ionosphere are taken into consideration. The intensity of ULF magnetic and electric precursors observed on the ground, and their spatial distribution can be explained by using the results of the present computations. It is found that only the fields from a magnetic type source can penetrate into the magnetosphere and generate propagating Alfven waves. The expected values of magnetospheric electric and magnetic field are 1‐10 μV m−1 Hz−1/2 and 1–10 pT Hz−1/2 respectively, and the horizontal scale of their distribution is about 100–200 km. Finally, these theoretical predictions are compared with the corresponding results of satellite observations.

Gravitational damping of Alfven waves in stellar atmospheres and winds

We consider how gravity affects the propagation of Alfven waves in a stellar atmosphere. We show that when the ion gyrofrequency exceeds the collision rate, the waves are absorbed at a rate proportional to the gravitational acceleration g. Estimates show that this mechanism can readily account for the observed energy losses in the solar chromosphere. The mechanism predicts that the pressure at the top of the chromosphere P(sub Tc) should scale with g as P(sub Tc) proportional to g(exp delta), where delta approximately equals 2/3; this is close to empirical results which suggest delta approximately equals 0.6. Gravitational damping leads to deposition of energy at a rate proportional to the mass of the particles. Hence, heavier ion are heated more effectively than protons. This is consistent with the observed proportionality between ion temperature and mass in the solar wind. Gravitational damping causes the local g to be effectively decreased by an amount proportional to the wave energy. This feature affects the acceleration of the solar wind. Gravitational damping may also lead to self-regulation of the damping of Alfven waves in stellar winds: this is relevant in the context of slow massive winds in cool giants.

Effect of high‐latitude ionospheric convection on Sun‐aligned polar caps

A coupled magnetospheric‐ionospheric (M‐I) MHD model has been used to simulate the formation of Sun‐aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfvén shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self‐consistently to changes in the upward currents. We assume that the initial Alfvén shear wave is the result of solar wind‐magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes in conductance lead to both a modified Alfvén wave reflection at the ionosphere and the generation of secondary Alfvén waves in the ionosphere. The ensuing bouncing of the Alfvén waves between the ionosphere and magnetosphere is followed until an asymptotic solution is obtained. At the magnetosphere the Alfvén waves reflect at a fixed boundary. The coupled M‐I Sun‐aligned polar cap arc model of Zhu et al. (1993a) is used to carry out the simulations. This study focuses on the dependence of the polar cap arc formation on the background (global) convection pattern. Since the polar cap arcs occur for northward and strong By IMF conditions, a variety of background convection patterns can exist when the arcs are present. The study shows that polar cap arcs can be formed for all these convection patterns; however, the arc features are dramatically different for the different patterns. For weak sunward convection a relatively confined single pair of current sheets is associated with the imposed Alfvén shear wave structure. However, when the electric field exceeds a threshold, the arc structure intensifies, and the conductance increases as does the local Joule heating rate. These increases are faster than a linear dependence on the background electric field strength. Furthermore, above the threshold, the single current sheet pair splits into multiple current sheet pairs. For the fixed initial ionospheric and magnetospheric conditions used in this study, the separation distance between the current pairs was found to be almost independent of the background electric field strength. For either three‐cell or distorted two‐cell background convection patterns the arc formation favored the positive By case in the northern hemisphere.

Theoretical calculations of ion acceleration in the vicinity of comet Giacobini‐Zinner

Ionization of cometary neutral molecules produces ions which are picked up by the solar wind. The cometary ion pickup process for comet Giacobini‐Zinner is studied in two ways: (1) with a test particle method in which trajectories are numerically calculated for several thousand ions whose initial locations were chosen randomly with probability proportional to the neutral density and (2) with a quasi‐linear diffusion model. The cometary ion distribution function was calculated with the test particle model at several locations upstream of the bow shock and for several types of magnetic fluctuations (or waves). These waves were allowed to propagate in both directions along the magnetic field at the Alfven speed. Both pitch angle scattering and energy diffusion are evident in the derived ion distributions. The monochromatic waves result in less ion acceleration than turbulent fluctuations with about the same amplitude. The calculated ion distribution functions are in reasonable agreement with the distributions measured in the vicinity of comet Giacobini‐Zinner in 1985 by particle detectors on the ICE spacecraft when the ratio of power in sunward propagating Alfven waves to the power in antisunward propagating waves is assumed to lie between about 20% and 50%. However, the quasi‐linear diffusion model results agree best with the measured distribution functions when the power ratio is only about 3%.

Magnetoacoustic and Alfven waves excited by plasma cloud expansion in cold magnetized plasma

The propagation of Alfven and magnetoacoustic waves in a cold magnetized plasma to great distances from the expanding plasma cloud that is the source of these waves is investigated. Allowance is made for back-ground-plasma permittivity anisotropy, which leads to Alfven-and magnetoacoustic-wave interaction and channeling along the external magnetic field.

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 dissipation of surface Alfven waves in the solar corona

The propagation of small-amplitude nonlinear Alfven waves on a single magnetic interface in a cold plasma is considered. The anisotropic ion viscosity is taken into account. The governing equation is derived with the use of the reductive perturbation method. The evolution of an initially sinusoidal wave with the distance from the source is studied numerically. Calculations show the wave shape is very steep at some distance from the source if the viscosity is very small. This wave steepening leads to strong acceleration of wave damping. The dependencies of wave energy flux on the distance from the wave source is calculated for different values of the Reynolds number which determines the ratio of the nonlinearity effect to the viscosity one. This formula shows that in the case of small viscosity the wave-damping distance predicted by the nonlinear theory can be one order of magnitude or more smaller than one predicted by the linear theory. 15 refs.

Magnetohydrodynamics computations with lattice gas automata

Lattice gas automata have received considerable interest for the last several years and possibly may become a powerful numerical method for solving various partial differential equations and modeling different physical phenomena, because of their discrete and parallel nature and the capability of handling complicated boundaries. In this paper, we present recent studies on the lattice gas model for magnetohydrodynamics. The FHP-type lattice gas model has been extended to include a bidirectional random walk process, which allows well-defined statistical quantities, such as velocity and magnetic field, to be computed from the microscopic particle representation. The model incorporates a new sequential particle collision method to increase the range of useful Reynolds numbers in the model, an improvement that may also be of use in other lattice gas models. In the context of a Chapman-Enskog expansion, the model approximates the incompressible magnetic hydrodynamic equations in the limit of low Mach number and highβ. Simulation results presented here demonstrate the validity of the model for several basic problems, including sound wave and Alfven wave propagation, and diffusive Kolmogoroff-type flows.

Similarity Solution of a Nonlinear Diffusion EquationDescribing Alfvén Wave Propagation

The propagation of nonlinear Alfven waves in a compressible viscous fluid is described by a nonlinear diffusion equation (I. Nakata: J. Phys. Soc. Jpn. 60 (1991) 1952). We show that it reduces to a nonlinear ordinary differential equation of the first order by introducing an appropriate similarity variable. The numerical solutions of the reduced equation are presented together with asymptotic solutions for both small and large Reynolds numbers.