Propagating Alfven Waves Research Articles

Sunspots and the physics of magnetic flux tubes. I - The general nature of the sunspot. II - Aerodynamic drag

Analysis of the dynamical stability of a large flux tube suggests that the field of a sunspot must divide into many separate tubes within the first 1000 km below the surface. Buoyancy of the Wilson depression at the visible surface and probably also a downdraft beneath the sunspot hold the separate tubes in a loose cluster. Convective generation of Alfven waves, which are emitted preferentially downward, cools the tubes. Aerodynamic drag on a slender flux tube stretched vertically across a convective cell is also studied. Since the drag is approximately proportional to the local kinetic energy density, the density stratification weights the drag in favor of the upper layers. Horizontal motions concentrated in the bottom of the convective cell may reverse this density effect. A downdraft of about two km/sec through the flux tubes beneath the sunspot is hypothesized.

Solar wind models with Alfven waves

Solutions to the one-fluid solar-wind equations that include pressure and heating due to outwardly propagating Alfven waves are presented. The solutions are given for two forms of the heat flux: the classical Spitzer (1962) formula and a 'collisional' heat flux. Both high-speed and low-speed solar-wind streams are considered. A comparison of solutions with and without waves shows that the solutions without waves cannot come close to reproducing observations of total mass and energy flux at 1 AU, while those with waves are capable of bringing nearly all flow variables into reasonable agreement with the observations. The required wave flux is found to be about 100,000 erg/sq cm per sec for the high-speed streams and an order of magnitude less for the low-speed streams. It is concluded that the pressure and heating due to outwardly propagating Alfven waves are important elements of solar-wind dynamics that cannot be neglected and that wave heating is especially important for the protons in high-speed streams.

The reflection of Alfven waves and the cooling of sunspots

As one means of evaluating the possibility that sunspots are cooled by a flux of Alfven waves, the reflection of vertically propagating Alfven waves in a three-layer model of a sunspot unbra is studied. The results show strong downward reflection of Alfven waves in the photosphere and low chromosphere, with very little wave energy penetrating as high as the corona. This is in agreement with recent observations. The model umbra also shows very weak upward reflection of Alfven waves propagating downward into the convection zone. The results suggest that, if sunspots are indeed cooled by Alfven waves, these waves must escape downward into the solar interior.

Circular polarized nonlinear Alfvén waves—a new type of nonlinear evolution in plasma physics

A nonlinear evolution equation is derived for Alfven waves propagating along the magnetic field in a cold plasma. The equation provides new types of solitary waves. The phase of a soli­ tary wave is coupled nonlinearly with its amplitude, and the propa­ gation velocity is restricted within the range determined by the as­ ymptotic amplitude and the wave number. 1. Introduction. As early as in 1942, Alfven (1) recognized that a hydromagnetic wave propagates in an incompressible, perfectly con­ ducting fluid in the presence of a strong magnetic field. This Alfven wave is nothing but the low frequency limit of electromagnet ic waves propagating in a plasma. Generation and propagation of the Alfven waves in a gaseous plasma has been investigated experimentally (2), and has attracted renewed interest as one of the useful ways to heat a plasma (3). Alfven wave propagation in solid state plasmas provides in­ formation on the effective masses of carriers (4), Large amplitude in­ compressible magnetic field perturbation observed in the solar wind has been attributed to propagation of the Alfven wave (5). Now, turning to the studies of nonlinear wave propagation in plasmas, we have seen the remarkable success of theoretical and experi­ mental investigations of the ion acoustic solitary waves. Using reductive perturbation theory, Washimi and Taniuti (6) have predicted existence of the Korteweg-deVries type soliton for the ion acoustic mode. Their prediction has been experimentally confirmed by Ikezi et al. (7). Sys­ tematic ordering of dispersive effects and nonlinear steepening effects in the reductive perturbation theory (8) provides a rigorous procedure for reducing the hyperbolic system of nonlinear partial differential equations to a single nonlinear evolution equation; namely, the Korte­ weg-deVries equation for a weakly dispersive system and the nonlinear Schrodinger equation for a strongly dispersive system. In the case of Alfven waves, however, Kakutani and Ono (9) have noticed that it is necessary to modify the expansion scheme of the re­ ductive perturbation theory so as to be consistent with steady state soli­ tary wave solution of Kazantsev (10). Thus, they have been led to the conclusion that the Alfven wave is governed by the modified Korteweg-

Alfven wave propagation in a density gradient in sunspots

view Abstract Citations (12) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Alfvén wave propagation in a density gradient in sunspots. Geronicolas, E. A. Abstract In view of the possible generation of Alfven waves in sunspots, a calculation is presented which estimates the ratio of the upward to the downward Alfven-wave flux in sunspots. Only a slight downward preferential propagation is found, so that sufficient Alfven cooling predicts high turbulent umbral velocities, of the order of 3 km/s. Publication: The Astrophysical Journal Pub Date: February 1977 DOI: 10.1086/155008 Bibcode: 1977ApJ...211..966G Keywords: Density Distribution; Magnetohydrodynamic Waves; Sunspots; Thermomagnetic Cooling; Wave Propagation; Wave Reflection; Astronomical Models; Convective Flow; Energy Transfer; Motion Stability; Photosphere; Radiative Transfer; Solar Magnetic Field; Wave Equations; Solar Physics full text sources ADS |

Mode mixing of Alfvén waves in bismuth

We have investigated the propagation of Alfven waves in bismuth at 4.2 K using a microwave interferometer at 34.45 GHz and applying magnetic fields up to 1 Tesla. At certain angles between the external magnetic field and the direction of propagation of the Alfven waves in the crystal, we have observed intense oscillations of the amplitude and the phase of the interferometer curves. We explain these oscillations as due to a superposition of the two Alfven wave modes. The phase velocities of the two modes are calculated from the measurements. Comparing them with a general dispersion relation we find good agreement between the theoretical phase velocities and the experimental values.

Sur la magnétohydrodynamique anisotrope relativiste

This paper is concerned with the relativistic phenomenological theory of anisotropic magnetohydrodynamics. An anisotropic fluid scheme is defined and studied. The main system of anisotropic magnetohydroldynamics is deduced. This system may describe a collisionless anisotropic plasma embedded in a strong magnetic field. The main system is shown to yield to three types of waves as in isotropic (perfect) magnetohydrodynamics: the entropic waves, the magnetosonic waves and the Alfven waves. For the rays associated respectively to the magnetosonic and Alfven waves the fundamental property concerning the propagation of infinitesimal discontinuities of variables is established. The conditions under which the velocities of propagation of magnetosonic and Alfven waves are real are derived: these conditions imply as in the classical theory the absence of fire hose and mirror instabilities in the fluid. The study of wave cones allows, on the one hand to point out some particularities of the propagation of waves in anisotropic magnetohydrodynamics, and on the other hand to clear up the hyperbolicity character of differential operators associated to various waves.

Alfvén waves and the sunspot phenomenon

Parker's explanation of the sunspot phenomenon in terms of the enhanced emission of Alfven waves (solar vulcanology) is shown to be compatible with observation only if ∼ 90% of the waves propagate downwards. Further difficulties arise if the region of cooling by Alfven wave generation is restricted to a depth of 2 Mm. However, it is shown that, if Alfven wave generation is included in a recent model proposed by Meyer, Schmidt, Weiss and Wilson, these difficulties may be resolved. The problem of the sharp umbra and penumbra boundaries is discussed and it is shown that features of this combined model are relevant to the flare phenomenon.

Observation and Study of Electromagnetic Wave Generation in Metals Using Nanosecond Pulses of Laser Light: Case of Alfvén Waves in Bismuth

We have observed the generation of an electric field pulse by a nanosecond laser pulse incident on a bismuth surface in an external magnetic field. The propagation velocity and the de Haas-van Alphen-type oscillations are found to be consistent with those expected for Alfven-wave propagation. This technique may well be a general way of exciting a variety of electromagnetic waves in a plasma.

Generation of Alfvén waves in the magnetosphere by parametric interaction between whistlers

A theory is developed for the generation of Alfven waves by the mixing of whistler waves. Calculations are carried out for interaction in the magnetosphere where the whistlers fall in the VLF frequency range and the Alfven wave falls in the ULF(1-5 Hz) range. Typical amplitudes of 2-5 milligammas for the Alfven wave are calculated, and it is shown that these values might be increased by one order of magnitude through suitable variation of the experimental parameters. The study thereby develops a possible explanation for naturally occurring Pc 1 micropulsations and demonstrates the feasibility of artificial generation of such micropulsations by ground-launched whistlers.

The Propagation of Compressional Alfven Waves in Nonuniform Plasmas

A theoretical treatment is presented for the propagation of Alfven waves in a plasma. It includes the effects of resistivity, ion-neutral collisions, the ion cyclotron frequency and radial nonuniformities in ion density, neutral particle density and temperature. The theory is applied to plasmas with conducting and nonconducting walls and the results are compared with those of experiments conducted in the afterglow of a shock-produced plasma. Nonuniformity in the ion density is found to have a marked effect on the dispersion relation and wave field profiles, while non uniformities in the total particle density and temperature are less important. Excellent agreement is obtained between theory and experiment and this allows unambiguous and accurate determinations to be made of the average total particle density, which is found to be - 50 % of the initial filling density, and of the cross section for momentum transfer between protons and hydrogen atoms.

The propagation of Alfven waves along Io's flux tube

The transmission of Alfvén waves from Io to the Jovian ionosphere is discussed. Various simplified laws for the variation of plasma density are analyzed and juxtaposed to simulate a realistic density variation along the Io-Jupiter flux line. Apparently the ionosphere is previously only to frequencies in excess of 1 Hz. The ionosphere participates in tilting the flux tube trapped by Io leading to the formation of a neutral point in the vicinity of the satellite.

Non-axisymmetric Alfven wave propagation in an argon plasma

The dispersion relation and the radial distribution of the wave fields of m=+or-1 Alfven waves propagating in a cylindrical plasma have been calculated. It was assumed that a vacuum existed between the uniform plasma and the infinitely conducting boundary and consequently two boundary conditions were imposed on the wave fields; one on the radial current density and another on the radial magnetic field. The results are in better agreement with observations than some previous calculations that imposed a boundary condition on the radial magnetic field only.

Energy Release by Magnetic Tearing: The Nonlinear Limit

The nonlinear limit of the resistive tearing-mode instability is investigated by a time-dependent solution of the Fourier-transformed magnetohydrodynamic equations. The calculation is started with the fastest-growing linear mode and assumes a constant (e.g., turbulent) resistivity. Attainable magnetic Reynolds numbers are limited by the steep spatial gradients developed and by the propagation of Alfven waves in the system. At the largest values used, 10% of the background energy is released in nonthermal forms. The time and energy scales found are consistent with the results of astrophysical observations and laboratory studies of solar-flare processes.

The propagation of Alfv�n waves and their directional anisotropy in the solar wind

The propagation of solar Alfven waves in interplanetary space is studied in the approximation of geometrical optics. Ray paths and the change of wave vectors and amplitudes along the rays are determined assuming an Archimedean-spiral interplanetary magnetic field. In particular, the Alfvenic fluctuations in the 2 directions perpendicular to the magnetic field direction are calculated under the assumption that the Alfven waves are produced at the Sun and emitted with an isotropic directional distribution from a reference level close to the Sun. It turns out that due to the combined effect of spherical expansion of the solar wind flow and the spiralling of the interplanetary field the magnetic fluctuations in the direction perpendicular both to the unperturbed field and the radial direction have much more power than in the other directions (directional anisotropy).

The propagation of non-axisymmetric Alfven waves in an argon plasma

The propagation of non-axisymmetric waves in a uniform, cylindrical argon plasma was investigated; the frequency range and the plasma conditions were chosen so that both the slow and the fast waves were observed. Measurements dealt mainly with the dispersion relations and the radial variation of the wave fields which were compared with predictions from magnetohydrodynamic theory assuming that the boundary condition to be applied was that for an infinitely conducting boundary, viz., that the radial magnetic field becomes zero at the plasma boundary. The propagation of the m=-1 slow wave and the m=+1 fast wave was consistent with theory but there was a discrepancy for the m=+1 slow wave. The m=-1 fast wave was not observed to propagate.

Helicon and Alfvén wave propagation in non-magnetic semiconductors and semi-metals: Active and passive waves

Abstract It was pointed out in 1960 that metals and semiconductors can support low frequency electromagnetic excitations in the presence of a magnetic field. We now feel that it is an appropriate time to discuss some of the progress made, over the last decade, in understanding and using this novel phenomenon. Naturally the field has grown quite rapidly and it would clearly be a Herculean task to review every aspect of it in other than a superficial manner. We have therefore chosen to discuss only semiconductors and semi-metals. This choice is dictated to us partly by the fact that magneto-plasma effects in metals have been reviewed from time to time but mainly by the fact that magneto-plasma effects in semiconductors have never been previously reviewed. Of course the term ‘magneto-plasma’ covers a great deal of activity so we have decided to choose a theme which links the beginnings of the subject to the present day. This theme is that of helicon and Alfven wave propagation. We have produced a background ...

Propagation of Alfven Waves in the Magnetosphere

The purpose of this paper is to investigate the guiding mechanism of Alfven waves (Hydromagnetic whistlers) in the magnetosphere. The guiding of Alfven waves along the Earth's field aligned column in the magnetosphere is discussed by use of wave theory. The radial density distribution perpendicular to the magnetic field is assumed to be Gaussian and we use the method in which the fields are expanded into power series of the radial distance from the column axis. And the condition for trapping in the duct is discussed for enhanced-and depressed-duct. The existence of trapping frequency region which is closely related to the modulation factor, is demonstrated and the trapping duct width is also estimated.

Magnetohydrodynamic waves in second-order fluids

A class of magnetohydrodynamic waves has been investigated for second-order fluids, whose constitutive equations have been introduced by Coleman and Noll. The second-order coefficient plays an important role in the formation and propagation of waves. Waves of finite amplitude have also been discussed. In this case the second-order coefficient modifies considerably what is known as streak-effect. It is found that the introduction of elastic effects in fluids greatly influences the efficiency of the generation of Alfven waves at a solid boundary.

Propagation of compressional and torsional Alfven waves under identical plasma conditions

Though many authors have separately studied torsional and compressional Alfven waves, no measurements made on both waves for the same plasma conditions have been reported. This paper presents such measurements for several different sets of plasma conditions. For each set of conditions, velocity and attenuation measurements were made for each wave and by comparing with theoretical curves, the values of the various parameters determining the wave propagation were obtained. The compressional and torsional wave measurements gave independent estimates of the parameters. The two sets of parameters show a broad agreement which is particularly significant since de-coupling of the ions from the neutrals is taking place over the frequency range for which the measurements were made. The torsional wave measurements do, however, imply a higher total density and lower percentage ionization than the compressional wave measurements. The existence of this difference implies that non-uniformity of the radial density distribution is affecting the wave propagation.