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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.

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At least 379 records · Page 21
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NASA NTRS1986

Fragmented molecular complexes: The role of the magnetic field in feeding internal supersonic motions

A hierarchical structure for molecular complexes in their cold phase i.e., preceeding the formation of massive stars, was derived from extensive large scale CO(13)(J=1=0) observations: the mass is found to be distributed into virialized clouds which fill only a very low fraction approx. 01 of the volume of the complex and are supported against gravity by internal supersonic motions. An efficient mechanism was found to transfer kinetic energy from the orbital motions of the clouds to their internal random motions. The large perturbations of the magnetic field induced at the cloud boundaries by their interactions with their neighbors generate systems of hydromagnetic waves trapped inside the clouds. The magnetic field lines being closely coupled to the gas at the densities which prevail in the bulk of the clouds volume, internal velocity dispersion is thus generated. Some conclusions derived from this data are given.

Falgarone, E.↗
NASA NTRS1986

Steepened magnetosonic waves in the high beta plasma surrounding Comet Giacobini-Zinner

Studies of intense hydromagnetic waves at Giacobini-Zinner are extended to investigate the mode and direction of wave propagation. Simultaneous high-resolution measurements of electron density fluctuations demonstrate that long period waves propagate in the magnetosonic mode. Principal axis analyses of the long period waves and accompanying partial rotations show that the sum of the wave phase rotations is 360 deg, indicating that both are parts of the same wave oscillation. The time sequence of the steepened waveforms observed by ICE shows that the waves must propagate towards the Sun with Cph less than Vsw. Observations are consistent with wave generation by resonant ion ring or ion beam instability which predicts right-hand polarized waves propagating in the ion beam (solar) direction. The large amplitudes and small scale sizes of the cometary waves suggest that rapid pitch-angle scattering and energy transfer with energetic ions should occur. Since the waves are highly compressive, first-order Fermi acceleration is forecast.

Tsurutani, B. T.↗
NASA NTRS1986

Origin of large magnetic fluctuations in the magnetosheath of Venus

The origin of large-amplitude hydromagnetic waves in the Venus magnetosheath downstream of the quasi-parallel bow shock is investigated by means of numerical simulations. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O(+) ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also discussed.

Winske, D.↗
NASA NTRS1987

Viscous damping of Alfven normal modes in non-uniform plasmas

The Alfven normal mode in a nonuniform, viscous plasma is investigated. Because the ideal hydromagnetic equation is singular in a nonuniform magnetic field, viscosity is included in order to regularize the equation, analogous to the removal of the singular point by resistivity (Mok and Einaudi, 1985). The eigenvalue equation is then solved numerically for a particular magnetic configuration. The real part of the frequency of this viscous normal mode is found to be similar to the one in the resistive case, while the damping is shown to be comparable, and sometimes to exceed, the resistive effect under certain conditions in which the Lundquist number is sufficiently large. The damping rate is evaluated for various plasma conditions corresponding to different parts of the solar atmosphere. The effects of viscosity are found to dominate resistivity in the quiet sun corona and solar wind.

Mok, Y.↗
NASA NTRS1987

On the generation of magnetosheath lion roars

A theoretical model is proposed to discuss the electron dynamics associated with the mirror waves and their effects on the generation of the observed lion roars in the magnetosheath. It is pointed out that the usual double-adiabatic theory of hydromagnetics is not applicable to the electrons in mirror waves. Although the electron magnetic moment is conserved, the energy of each electron in the mirror waves is expected to be constant. Assuming an initial electron temperature anisotropy, it can be shown that in the low field region the electron temperature and thermal anisotropy are higher than the initial values, whereas in the high field region the electron temperature and anisotropy are lower. This point can lead to a theoretical explanation of the important features of the observed lion roars. Then present discussion complements the existing theories in the literature.

Lee, L. C.↗
NASA NTRS1987

Study of pickup of cometary ions in turbulent solar winds

The influence of moderately strong magnetic disturbances on the ion pickup process near a comet is studied by a test-particle method. The research is motivated by recent observations with ICE and Giotto at Giacobini-Zinner and Halley. In this numerical study, the intrinsic hydromagnetic turbulence is modelled based on the Giotto and ICE data. The time evolution of the distribution function of the newborn ions is investigated. It is found that, when the level of the intrinsic turbulence is sufficiently high, the pickup ions can form a shell distribution function rapidly. The typical time scale for such a process is of the order of a couple of ion gyroperiods. On the other hand, if the turbulence is not strong, the pickup ions usually form an incomplete shell in the initial stage. The results seem to be consistent with available observations.

Wu, C. S.↗
NASA NTRS1987

Magnetic monopole plasma oscillations and the survival of Galactic magnetic fields

This paper explores the general nature of magnetic-monopole plasma oscillations as a theoretical possibility for the observed Galactic magnetic field in the presence of a high abundance of magnetic monopoles. The modification of the hydromagnetic induction equation by the monopole oscillations produces the half-velocity effect, in which the magnetic field is transported bodily with a velocity midway between the motion of the conducting fluid and the monopole plasma. Observational studies of the magnetic field in the Galaxy, and in other galaxies, exclude the half-velocity effect, indicating that the magnetic fields is not associated with monopole oscillations. In any case the phase mixing would destroy the oscillations in less than 100 Myr. The conclusion is that magnetic monopole oscillations do not play a significant role in the galactic magnetic fields. Hence the existence of galactic magnetic fields places a low limit on the monopole flux, so that their detection - if they exist at all - requires a collecting area at least as large as a football field.

Parker, E. N.↗
NASA NTRS1987

VLA and low-frequency VLBI observations of the radio source 0503 + 467 - Austere constraints on interstellar scattering in two media

The radio source 0503 + 467 lies near the Galactic plane (l = 161.0 deg, b = 3.7 deg) and at the edge of the supernova remnant (SNR) HB 9. The VLA observations show that it has a spectrum typical of a compact extragalactic radio source. The resultant small angular size of the source makes it an excellent probe of turbulence in two media: the diffuse, or 'type A', component of interstellar turbulence and a hypothesized region of hydromagnetic turbulence upstream of the supernova remnant. An eight-station VLBI experiment at 326 MHz indicates that the source is less than about 20 milliarcseconds (mas) in angular diameter. A value of 16 mas is most appropriate as an upper limit to the interstellar scattering contribution to the measured angular size. The implications of this upper limit are twofold. First, the galactocentric radial scale to the type-A turbulence is probably less than or equal to about 6 kpc. Second, no evidence is seen for shock-associated turbulence upstream of HB 9. The measurements make it possible to constrain a parameter which is a function of the rms density fluctuation in the upstream region, the outer scale to the density turbulence, and the thickness of SNR foreshock region.

Spangler, Steven R.↗
NASA NTRS1987

Proton beam generation of whistler waves in the earth's foreshock

It is shown that proton beams, often observed upstream of the earth's bow shock and associated with the generation of low-frequency hydromagnetic fluctuations, are also capable of generating whistler waves. The waves can be excited by an instability driven by two-temperature streaming Maxwellian proton distributions which have T (perpendicular)/T(parallel) much greater than 1. It can also be excited by gyrating proton beam distributions. These distributions generate whistler waves with frequencies ranging from 10 to 100 times the proton cyclotron frequency (in the solar wind reference frame) and provide another mechanism for generating the '1-Hz' waves often seen in the earth's foreshock.

Wong, H. K.↗
NASA NTRS1987

Temperature anisotropies in the terrestrial ionosphere and plasmasphere

Theoretical work in which the solution of closed sets of transport equations has predicted the existence of temperature anisotropies in the terrestrial ionosphere-plasmasphere system is discussed, considering only thermal (less than 1 eV) particle populations. Various models used to predict ion and electron temperature anisotropies, including kinetic, semikinetic, hydromagnetic, and generalized transport models, predict temperature anisotropies in the polar wind, along plasmapause field lines, during the refilling of the outer plasmasphere after depletion by a magnetic storm, and at F region altitudes in regions of rapid plasma convection. However, only some of the theoretical predictions agree with experimental evidence. Other models predict isotropic temperature distributions in regions where observations indicated the presence of temperature anisotropies.

Demars, H. G.↗
NASA NTRS1987

Waves in the magnetic field and solar wind flow outside the bow shock at comet P/Halley

The existence of hydromagnetic waves in the mass-loaded solar wind upstream from the bow shock of a comet is well-established both for comet Giacobini-Zinner and for comet Halley. Whereas previous reports have been concerned either with the magnetic field observations or with plasma observations, here observations of the magnetic field with the solar wind proton and alpha particle distributions are combined. This allows the three possible modes of propagation for these waves to be separated. The magnetic component is predominantly transverse to the magnetic field and linearly polarized. The flow vector also has a substantial amount of power parallel to the magnetic field. An examination of the pressure variations shows that slow magnetosonic waves are more common than the fast mode.

Johnstone, A.↗
NASA NTRS1987

Cosmic ray-modified stellar winds. I - Solution topologies and singularities

In the present two-fluid hydrodynamical model for stellar wind flow modification due to its interaction with Galactic cosmic rays, these rays are coupled to the stellar wind by either hydromagnetic wave scattering or background flow irregularity propagation. The background flow is modified by the cosmic rays via their pressure gradient. The system of equations used possesses a line of singularities in (r, u, P sub c)-space, or a two-dimensional hypersurface of singularities in (r, u, P sub c, dP sub c/dr)-space, where r, u, and P sub c are respectively the radial distance from the star, the radial wind flow speed, and the cosmic ray pressure. The singular points may be nodes, foci, or saddle points.

Ko, C. M.↗
NASA NTRS1988

Temperature effects on the pickup process of water group and hydrogen ions - Extensions of 'A theory for low-frequency waves observed at Comet Giacobini-Zinner' by M. L. Goldstein and H. K. Wong

Cometary heavy ions can resonantly excite hydromagnetic wave activity with spacecraft frequency spectra strongly deviating from the ion cyclotron frequency. The influence of the newborn particle temperature on this effect is assessed, its relevance to the interpretation of the observations is discussed, and an alternative, more efficient mechanism to generate spacecraft frequencies of the order of the proton cyclotron frequency is suggested.

Brinca, Armando L.↗
NASA NTRS1988

Shock analysis - Three useful new relations

The behavior of collisionless hydromagnetic shocks in interplanetary space is considered analytically, with a focus on relations, implicit in the governing Rankine-Hugoniot equations, involving the magnetic field (B) and the plasma velocity (V). A moving reference frame aligned with the shock is employed, and expressions are derived which make it possible (1) to determine the speed of a shock of arbitrary orientation from upstream and downstream measurements of B and V; (2) to characterize the change in flow direction as the plasma crosses the shock in terms of the plasma beta, the Mach number, and the angle between the upstream field and the shock normal; and (3) to infer the third component of the upstream-downstream velocity jump from B and two-dimensional V measurements. These expressions are applied to ISEE-3 data on an interplanetary shock on April 5, 1979, and the results are presented in tables.

Smith, Edward J.↗
NASA NTRS1988

Ambipolar diffusion drifts and dynamos in turbulent gases

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

Zweibel, Ellen G.↗
NASA NTRS1988

The growth of magnetic fields prior to galaxy formation

Hydromagnetic dynamo theory is applied to the problem of magnetic field amplification in the epoch after recombination but prior to galaxy formation. Large-scale density fluctuations acquire angular momentum from tidal torques exerted by neighboring fluctuations, and small-scale subfluctuations possess helicity due to the Coriolis forces associated with the rotation of the large-scale host fluctuations. Cosmic expansion and the interaction of matter with the ambient radiation field are taken into account. Despite many uncertainties, it is concluded that significant dynamo amplification may have occurred.

Zweibel, Ellen G.↗
NASA NTRS1988

A mirror instability associated with newly created ions in a moving plasma

Using a plasma model in which the ambient magnetic field is parallel to the z axis and the wave vector, an instability that resembles the usual hydromagnetic mirror instability is demonstrated. The instability is caused by freshly created ions in a moving plasma; the source of its free energy is associated with the ring distribution of the newly created ions, resulting in a high ion kinetic temperature in the direction transverse to the ambient magnetic field. The results of a stability analysis indicate that this instability can lead to the amplification of magnetosonic waves.

Wu, C. S.↗
NASA NTRS1989

The oblique behavior of low-frequency electromagnetic waves excited by newborn cometary ions

The free energy in oxygen or hydrogen ions freshly created in the solar wind stimulates low-frequency electromagnetic waves whose growth does not always maximize at parallel propagation. Exploration of the wave vector plane discloses the frequent occurrence of islets of oblique growth unconnected to the unstable parallel modes. Contour plots of the growth rate, real frequency, polarization, and magnetic compression characterize the oblique wave behavior for large values of the initial pitch angle of the cometary particles. Although wave-particle (Landau and cyclotron) resonances feed most of the surveyed oblique instabilities, some are seemingly fluidlike. The results, obtained from the numerical solution of the kinetic dispersion and wave equations, imply that newborn ions can easily excite significant oblique hydromagnetic wave activity. Cometary environments provide the adopted plasma model, but the study is helpful in the interpretation of other low-frequency wave observations in space.

Brinca, Armando L.↗