Search NASA⌕ Search

SEARCH · Search NASA

Results for “ALFVEN WAVE”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Hydromagnetic wave dissipation in molecular clouds

The damping of long wavelength, hydromagnetic waves in molecular cloud environments is studied with the aim of determining whether the supersonic motions observed in such clouds are likely to be due to the waves. It is found that Alfven waves propagating parallel to the average magnetic field are the longest lived wave modes. Such waves can typically survive for as long as one-million years if the wavelength is as long as a few tenths of a pc and the magnetic field is 0.1-1 milligauss. Nonlinear steepening of the waves followed by ion-neutral friction in the steepened wave profiles appears to be the most effective damping mechanism.

Zweibel, E. G.↗
NASA NTRS1991

Low-frequency waves in the solar wind near Neptune

Plasma and magnetic field observations from the Voyager 2 spacecraft when it was outbound from Neptune reveal low-frequency waves in the solar wind which are clearly associated with the planet. The waves have frequencies below the proton cyclotron frequency f(cp), which is about 0.001 Hz during the periods waves are observed. The waves are present when the interplanetary magnetic field is oriented such that the spacecraft is connected to the bow shock by the magnetic field lines. The waves are identified to be Alfvenic waves propagating at about 140 deg to the ambient magnetic field and away from the bow shock. As at the other planets, these downstream waves are thought to be generated in the upstream region, where energetic protons created near the nose of the bow shock excite waves as they stream along solar wind magnetic field lines.

Zhang, Ming↗
NASA NTRS2008

Simulations of Solar Wind Turbulence

Recently we have restructured our approach to simulating magnetohydrodynamic (MHD) turbulence in the solar wind. Previously, we had defined a 'virtual' heliosphere that contained, for example, a tilted rotating current sheet, microstreams, quasi-two-dimensional fluctuations as well as Alfven waves. In this new version of the code, we use the global, time-stationary, WKB Alfven wave-driven solar wind model developed by Usmanov and described in Usmanov and Goldstein [2003] to define the initial state of the system. Consequently, current sheets, and fast and slow streams are computed self-consistently from an inner, photospheric, boundary. To this steady-state configuration, we add fluctuations close to, but above, the surface where the flow become super-Alfvenic. The time-dependent MHD equations are then solved using a semi-discrete third-order Central Weighted Essentially Non-Oscillatory (CWENO) numerical scheme. The computational domain now includes the entire sphere; the geometrical singularity at the poles is removed using the multiple grid approach described in Usmanov [1996]. Wave packets are introduced at the inner boundary such as to satisfy Faraday's Law [Yeh and Dryer, 1985] and their nonlinear evolution are followed in time.

Goldstein, Melvyn L.↗
NASA NTRS1984

Alfvenic resonances on solar spicules

It is suggested that twisting and heating of solar spicules can be produced by Alfven waves which enter the spicule from below. The spicule is treated as a region of constant Alfven speed which is bounded above by a region of much higher Alfven speed (the corona) and below by a region of exponentially increasing Alfven speed (the photosphere and chromosphere). It is shown how the spicule can act as a resonant cavity. The transmission of the waves into the cavity is analytically determined to be enhanced at certain resonant frequencies. With reasonable spicule parameters, and assuming the spicule damping to be moderately large, it is found that twisting velocities of approximately 20-30 km/s can be induced on the spicule. It is suggested that the Alfven waves are dissipated via a turbulent cascade of their energy to higher wavenumbers. It is shown that the waves can thereby heat the spicules to the observed temperatures. It is further suggested that the continued input of energy can explain why H-alpha spicules fade, since the predicted heating rate is sufficient to heat the spicules to temperatures at which the hydrogen is fully ionized; thus H-alpha spicules may evolve into EUV spicules.

Sterling, A. C.↗
NASA NTRS1997

Ulysses Observations of Alfven and Magnetosonic Waves at High Latitude

Ulysses observations provide a unique opportunity to study diverse problems related to Alfven and magnetosonic waves. The large amplitude of the Alfven waves influences the distribution functions of the spiral angle, the azimuthal field component and, possibly, the radial component such that their averages are not equal to their most probable values.

Alfven and Magnetosonic Waves Ulysses↗
NASA NTRS1973

Solar flare produced pulsations in meter and X-ray wavelengths

The mechanism responsible for the modulation of synchrotron radiation in the meter wave range by a solar flare is examined. The proposed explanation is based on the interaction of a flare generated shock front with a magnetic flux tube extending out into the corona over the flare. As the shock wave propagates through the flux tube, Alfven waves are developing and travelling up the tube toward each other, thereby accelerating electrons and protons by the Fermi mechanism and thus filling the top of the flux tube with energetic electrons. Radial oscillations develop as the shock front becomes parallel to the magnetic field direction and the intensity of the synchrotron radiation is modulated by these oscillations. An OSO-5 experiment detected a hard X-ray burst associated with the flare that produced modulated radio emission and X-ray modulations that are attributed to density fluctuations in the X-ray bursts.

Frost, K. J.↗
NASA NTRS1979

Magnetohydroelectric waves in a fluid dielectric

An electromagnetic disturbance in a fluid dielectric substance placed in a uniform magnetic field in the positive z direction of a rectangular Cartesian coordinate system with an electric field that increases with time applied in the positive z direction is analyzed. A simultaneous solution to a set of four equations is obtained which indicates a propagating 'magnetohydroelectric' wave that corresponds to oscillations in magnetic, kinetic, and electrostatic energies. It is noted that this wave bears a superficial resemblance to Alfven waves in a magnetized conducting fluid.

De, B. R.↗
NASA NTRS1982

Surface waves on solar wind tangential discontinuities

It is demonstrated that (tangential) discontinuities in the magnetic field direction can support MHD surface waves. The surface waves are similar to the usual Alfven wave, but there are seven important differences. The first is that the surface waves exhibit a low-frequency cutoff; the second is that the velocity and magnetic field fluctuations are elliptically, and sometimes circularly, polarized. It is noted that they may account for the solar wind helicity spectrum. The third difference is that the surface waves are compressive, although there are special cases where they are noncompressive. The fourth is that the wave vector k, the local normals to the surfaces of constant phase, and the magnetic minimum variance direction do not all coincide. The fifth is that there is a tendency for the minimum variance direction to align itself with the mean magnetic field direction. The sixth difference is that the waves can be intrinsically nonplanar, and the seventh is that equipartition between magnetic and kinetic energies is not obeyed locally. These properties of the surface waves are interpreted to mean that surface waves may be common in the solar wind.

Hollweg, J. V.↗
NASA NTRS1984

The plasma wave environment of an auroral arc. II - ULF waves on an auroral arc boundary

On March 9, 1978, a sounding rocket launched from Poker Flat, Alaska, at 2200 LT, made a four-component measurement of a 0.5 Hz hydromagnetic wave as the payload crossed the poleward boundary of a quiet homogeneous auroral arc. An energy flux of about 10 to the -6th W/sq m was observed propagating upward with a left-handed polarization within the arc, and a flux 6 times greater was observed propagating downward with a right-handed polarization on the arc boundary. The waves were identified as shear mode Alfven waves. Various models for the source of the free energy are discussed with the conclusion that the most likely production mechanism was either the electromagnetic or electrostatic Kelvin-Helmholtz instability.

Gelpi, C. G.↗
NASA NTRS1970

Turbulence structure of finite-beta perpendicular fast shocks

In a finite-beta plasma ion cyclotron, radius dispersion which forms a trailing wave train for a perpendicular fast shock is examined. Collisionless dissipation is provided by the three wave decay of the wave train into very oblique fast and parallel Alfven waves. Particle thermalization results from Landau damping of oblique fast wave turbulence. The shock damping length to three wave decay is many ion cyclotron radii. Undamped Alfven turbulence should persist far downstream from the shock.

Coroniti, F. V.↗
NASA NTRS1983

Theory of hydromagnetic waves in the magnetosphere

A survey of theoretical and experimental research on the origin and characteristics of low-frequency hydromagnetic (HM) waves in the magnetosphere is presented, with a focus on advances in theory made in the last ten years. Basic wave theory and a collisionless plasma theory are applied to the magnetosphere as a HM system. Continuous energy sources are considered, such as the Kelvin-Helmholtz instability, the ring-current plasma, and drift instabilities. Other topics discussed include the theory of inhomogeneous HM waves, signal behavior in atmosphere and ionosphere, Alfven waves and ionosphere-magnetosphere coupling, Pi2 signals, damping, pulsating aurora, heavy-ion scattering, and standing waves in high-speed flows (like the wake phenomena caused on Jupiter by the passing of Io, observed by Voyager 1).

Southwood, D. J.↗
NASA NTRS1984

Resonant acceleration and heating of solar wind ions Anisotropy and dispersion

This paper is an extension of recent work by Isenberg and Hollweg dealing with the preferential acceleration and heating of solar wind heavy ions by a resonant cyclotron interaction with a turbulent spectrum of ion-cyclotron waves. Two major approximations of the previous work are eliminated: that of isotropic ion distributions, and that of nondispersive waves. The wave action flux of finite-amplitude Alfven waves in a multi-ion, differentially flowing, gyrotropic plasma is derived. This quantity is incorporated into the wave-driven solar wind model of Isenberg and Hollweg, which is then applied to gyrotropic protons and alpha particles. It is found that allowing for nonisotropic ions yields enhanced preferential acceleration of alpha particles. However, when dispersion is included in the resonant interaction, the preferential effects are again reduced. These more realistic calculations show that the observations of solar wind alpha particles cannot be explained by a resonant cyclotron interaction.

Isenberg, P. A.↗
NASA NTRS1986

Jovian magnetospheric processes

Jupiter's rotational energy (6 x 10 to the 34th J) powers a large number of processes such as auroral UV emission, radio waves, and charged particle energization. How the rotational energy may be dissipated by injection of plasma, magnetic pumping and field aligned electric fields is described. In addition, energization by radial diffusion and plasma wave absorption is described. The generation of Alfven waves by the moon Io, and their relation to the emission of the Jovian decametric (DAM) radio waves are also described.

Goertz, C. K.↗
NASA NTRS1992

Response of the dipole magnetosphere to pressure pulses

The response of the magnetosphere to pressure pulses at the magnetopause has been studied using a three-dimensional model of ULF waves in a dipole geometry. Pressure pulses at the magnetosphere directly excite compressional waves, which then convert to shear mode Alfven waves due to inhomogeneity. The behavior of the system depends on the frequency of the source at the magnetopause, with vortex structure tending to form on field lines resonant with the source frequency. The perturbations between the vortices are skewed toward noon, in agreement with observations.

Lysak, Robert L.↗
NASA NTRS1992

Stochastic gyroresonant electron acceleration on a low-beta plasma. II - Implications of thermal effects in a solar flare plasma

We consider the thermal damping of R and L waves under typical solar flare conditions. We find that the thermal particles cause rapid dissipation of the cyclotron waves, but do not affect whistlers and Alfven waves. The dissipation of electron cyclotron waves in particular leads to a threshold energy for acceleration which is about 10 times the thermal energy. Therefore, in the absence of an instability that excites these HF waves, a second mechanism is required in order to inject a sufficient number of electrons above the threshold energy and account for solar flare gamma-ray bremsstrahlung emission. We also find that the comoving gyroresonance, which occurs when the electron is in gyroresonance with an R wave whose group velocity equals the parallel electron velocity, can be realized by relativistic electrons if the plasma temperature is less than or approximately equal to 5,000,000 K.

Miller, James A.↗
NASA NTRS1993

Wave generation in a sunspot

In this paper we analyze the generation of waves in a sunspot by extending Stein's hydrodynamic approach to the turbulent medium permeated by a strong uniform magnetic field oriented parallel to the gravity. For wave sources appropriate to the sunspot, we consider magnetic perturbations and entropy changes as well as turbulent convection. To describe the anisotropy imposed by the sunspot, we use a one-dimensional correlation function relating the turbulent eddies separated along the symmetry axis of the spot. This treatment yields several interesting possibilities for wave generation in a sunspot. First, it is demonstrated that the entropy change and magnetic perturbation can lead to a relative enhancement of acoustic wave emission. Second, the energy flux of Alfven waves may be comparable to that of acoustic waves in sunspots. Third, the anisotropy of the sunspot dynamics can lead to wave energy spectrum in a form which may explain the origin of umbral atmospheric oscillations.

Lee, Jeongwoo W.↗