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NASA NTRS2009

Alfven Wave - DC Dualism in Description of Stationary Field-Aligned Currents

In many cases, the field-aligned currents (FACs) in the Earth's magnetosphere and heliosphere may be described in terms of both DC currents and the currents of a propagating Alfven wave. The simplest example is when a propagating Alfven wave transports a potential hop along the magnetic fieid: between the source of the wave and its front, the problem is well stationary and includes the stationary field-aligned currents, transporting the electric charges along the magnetic field, which may be described as a DC problem, and only at the front of the wave there are the polarization (inertial) currents, closing across the magnetic field. In some cases, the Alfven wave approach brings better understanding to many problems. We will consider here the results of the applications of this approach to two long-staying problems: the effect of saturation of the transpolar voltage in the Earth's magnetosphere, and the experimentally-observed existence of the strong field-aligned currents in the subtle Mercury's magnetosphere which is not able tc close the measured field-aligned currents.

Khazanov, George V.↗
NASA NTRS1986

Alfven-wave dissipation: A support mechanism for quiescent prominences

High resolution filtergrams or spectrograms of the main body of quiescent prominences often show a very vivid dynamical picture that cannot be reconciled with static models. Even if large differences exist between individual prominences in this respect, at least parts of the prominence are usually found to be in a 'choppy', turbulent state. Evidence for systematic flows are found in local regions in the prominence and also in the transition zone in the surroundings. These two regions are probably decoupled magnetically. Alfven waves are generally believed to be responsible for the heating in the upper chromosphere and corona (Hollweg 1986). Since evidence for the presence of Alfven-waves has also been found in the solar wind field, it is highly probable that such waves are generated in the convection zone of the sun and propagated outwards in the solar atmosphere wherever a proper magnetic field is present to carry the waves. The most basic magnetic formations in the solar atmosphere are simple loops. They occur all over the solar surface and cover a large range of magnetic field strengths. Loops with the strongest magnetic fields are found in active regions. It is to be expected that the Alfven-wave flux which is channelled into the loops from below, could show considerable variation both with heliocentric latitude, with time and locally between neighbouring loops. What happens when a magnetic loop is exposed to the appropriate Alfven-wave flux required to heat the upper solar atmosphere is examined.

Jensen, Eberhart↗
NASA NTRS1988

Filamentation instability of large-amplitude Alfven waves

An instability that leads to the filamentation of large-amplitude Alfven waves and gives rise to purely growing density and magnetic field fluctuations is studied. The dispersion relation of the instability is derived, from which the threshold conditions and the growth rates of the instability are analyzed quantitatively for applications to the solar wind plasma. Their dependence on the filamentation spectrum, the plasma beta, and the pump frequency and intensity was examined for both right-hand and left-hand circularly polarized Alfven waves. The excitation of filamentation instability for certain cases of interest is discussed and compared with that of the parametric decay and modulation instability. The relevance of the proposed instability to some observations is discussed.

Kuo, S. P.↗
NASA NTRS1988

AMPTE/CCE observations of substorm-associated standing Alfven waves in the midnight sector

Magnetic-field and medium-energy particle data from the AMPTE/CCE spacecraft are used to study substorm-associated ULF pulsations in the midnight sector at a radial distance of 8 to 9 earth radii. The particle data are used to identify ion injections and to detect the electric field of ULF waves. A case study of the events on May 23, 1985 shows that the waves have the properties of a fundamental-mode standing Alfven wave. It is suggested that these observations are evidence of substorm-associated standing Alfven waves in the nightside magnetosphere.

Takahashi, K.↗
NASA NTRS1990

Interplanetary Alfven waves and auroral (substorm) activity - IMP 8

Almost 1 year of IMP 8 interplanetary magnetic field and plasma data (days 1-312, 1979) have been examined to determine the interplanetary causes of geomagnetic AE activity. The nature of the interplanetary medium (Alfvenic or non-Alfvenic) and the B(s) correlation with AE were examined over 12-hour increments throughout the study. It is found that Alfvenic wave intervals are present over 60 percent of the time, and the southward component of the Alfven waves is well correlated with AE (average peak correlation coefficient 0.62), with a median lag of 43 min. From this statistical study, no major differences in the magnetospheric response to Alfvenic and non-Alfvenic intervals were obvious. The high-intensity long-duration continuous AE activity (HILDCAA) events discussed previously by Tsurutani and Gonzales (1987) are demonstrated to be caused by the southward components of the Alfven waves, presumably through the process of magnetic reconnection.

Tsurutani, Bruce T.↗
NASA NTRS1977

Alfven wave propagation in a density gradient in sunspots

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.

Geronicolas, E. A.↗
NASA NTRS1978

An instability of finite amplitude circularly polarized Alfven waves

A demonstration is presented that a finite-amplitude circularly-polarized Alfven wave is generally unstable in a MHD fluid. The wave decays by a four-wave coupling process in which the daughter waves are forward propagating random density and magnetic fluctuations and a backward-propagating magnetic wave. For parameters typical of the solar corona and the solar wind (thermal to magnetic energy density ratios between 0.1 and 1, and values between 0.1 and 0.9 for the ratio of magnetic energy density of the initial Alfven wave to that of the background magnetic field), large decay rates are found.

Goldstein, M. L.↗
NASA NTRS1993

Influence of horizontal inhomogeneity in the ionosphere on the reflection of Alfven waves

A study of the reflection of Alfven waves at a horizontally inhomogeneous ionosphere has been carried out. In this study, the Alfven speed above the ionosphere is assumed to be uniform and the ionosphere is treated as a height-integrated conducting slab. Analytical and numerical results indicate that the horizontal nonuniformity of the ionospheric conductivity can lead to a rotation of the reflected wave fields and cause field-aligned currents that originate in the ionosphere. A strong conductivity nonuniformity on the direction perpendicular to the incident wave field, large Hall to Pedersen conductivity ratios, and low conductivity values lead to a large rotation of the reflected wave field in the range from a few to 40 deg. The implications of the theoretical results for some ionospheric phenomena are presented, including comments on the Harang discontinuity and sun-aligned arcs.

Zhu, L.↗
NASA NTRS2007

First Results of PIC Modeling of Kinetic Alfven Wave Dissipation

We present first results of an investigation of the kinetic damping of Alfven wave turbulence. The methodology is based on a fully electromagnetic, three-dimensional, particle in cell code. The calculation is initialized by an Alfven wave spectrum. Subsequently, a cascade develops, and damping by coupling to both ions and electrons is observed. We discuss results of these calculations, and present first estimates of damping rates and of the effects of energy transfer on ion and electron distributions. The results pertain to solar wind heating and acceleration.

Chulaki, Anna↗
NASA NTRS1994

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.

Khabibrakhmanov, I. K.↗
NASA NTRS1989

Interplanetary observational constraints on Alfven wave acceleration of the solar wind

Spacecraft and interplanetary remote sensing observations of wave spectra are used to constrain the possible role of Alfven waves in the acceleration of high-speed streams in the solar wind. The observations suggest a method for extrapolating the wave energy flux near 1 AU back to the sun in a way that accounts for dissipation. Applying this method both for typical parameters and in the case of a well-studied high-speed stream leads to the conclusion that the Alfven waves observed in the solar wind were not sufficiently energetic near the sun to accelerate the streams.

Roberts, D. A.↗
NASA NTRS1973

Alfven waves associated with long cylindrical satellites

The Alfven wave excited by a long cylindrical satellite moving with a constant velocity at an angle relative to a uniform magnetic field has been calculated. Assuming a plasma with infinite conductivity, the linearized momentum equation and Maxwell's equations are applied to a cylindrical satellite carrying a variable current. The induced magnetic field is determined, and it is shown that the Alfven disturbance zone is of limited extent, depending on the satellite shape. The wave drag coefficient is calculated and shown to be small compared to the induction drag coefficient at all altitudes considered.

Venkataraman, N. S.↗
NASA NTRS1973

Alfven wave refraction by interplanetary inhomogeneities

Pioneer 6 magnetic data reveals that the propagation direction of Alfven waves in the interplanetary medium is strongly oriented along the ambient field. Magnetic fluctuations of frequencies up to 1/30 sec in the spacecraft frame are shown to satisfy a necessary condition for Alfven wave normal. It appears from this analysis that geometrical hydromagnetics may satisfactorily describe deviation of the wave normal from the background field. The rotational discontinuity is likely also to propagate along the field lines.

Daily, W. D.↗
NASA NTRS1974

On the theory of large amplitude Alfven waves

Solutions of the MHD equations are derived, which describe the propagation of nonplanar large-amplitude Alfven waves in an infinite homogeneous plasma with no background flow. Some features of interplanetary Alfven waves are discussed, and a typical solution is described qualitatively.

Goldstein, M. L.↗
NASA NTRS1995

A global 3-D MHD model of the solar wind with Alfven waves

A fully three-dimensional solar wind model that incorporates momentum and heat addition from Alfven waves is developed. The proposed model upgrades the previous one by considering self-consistently the total system consisting of Alfven waves propagating outward from the Sun and the mean polytropic solar wind flow. The simulation region extends from the coronal base (1 R(sub s) out to beyond 1 AU. The fully 3-D MHD equations written in spherical coordinates are solved in the frame of reference corotating with the Sun. At the inner boundary, the photospheric magnetic field observations are taken as boundary condition and wave energy influx is prescribed to be proportional to the magnetic field strength. The results of the model application for several time intervals are presented.

Usmanov, A. V.↗
NASA NTRS1995

Alfven waves, alpha particles, and pickup ions in the solar wind

Past studies of the properties of Alfven waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles do not participate in the wave motions because they 'surf' on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the 'observed' wave speed, B(sub o)(delta v squared / delta B squared)exp 1/2, as determined from the ratio of the amplitudes of the velocity and magnetic fluctuations. It is proposed that the modification of Alfven wave propagation speed is due to pressure anisotropies resulting from asymmetric distributions of interstellar pickup ions. If the proposed explanation is correct, it indicates that scattering of pickup ions onto a (bi)spherical shell may not be as complete as generally supposed.

Goldstein, B. E.↗
NASA NTRS1987

Simulation of amplitude-modulated circularly polarized Alfven waves for beta less than one

The nonlinear properties of the amplitude-modulated circularly polarized Alfven wave are studied for beta less than one. The temporal behavior of the wave packet of the electromagnetic hybrid simulation is compared with a numerical solution of the derivative nonlinear Schroedinger (DNLS) equation. It is shown that the left-hand-polarized mode evolves into a shocklike structure due to the modulational instability. However, both cyclotron damping and a snowplow effect near the steepened wave packet suppress its further steepening, contrary to the predictions of the DNLS equation. For the right-hand mode, formation of the shock does not take place, and the initial time development is well described by the DNLS equation. The daughter Alfven wave and ion acoustic waves are excited due to the decay instability at a later time. Heating or acceleration of the particles takes place for both left- and right-hand waves. Energy transfer from the wave to the particles occurs effectively when substantial modulation in the wave amplitude is present.

Machida, S.↗