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Matthaeus, W. H.

Publications and source records attributed to Matthaeus, W. H..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Long-time states of inverse cascades in the presence of a maximum length scale

It is shown numerically, both for the two-dimensional Navier-Stokes (guiding-center plasma) equations and for two-dimensional magnetohydrodynamics, that the long-time asymptotic state in a forced inverse-cascade situation is one in which the spectrum is completely dominated by its own fundamental. The growth continues until the fundamental is dissipatively limited by its own dissipation rate. Previously announced in STAR as N83-23546

Hossain, M.↗
NASA NTRS1983

Magnetohydrodynamic turbulence in the solar wind

Recent work in describing the solar wind as an MHD turbulent fluid has shown that the magnetic fluctuations are adequately described as time stationary and to some extent as spatially homogeneous. Spectra of the three rugged invariants of incompressible MHD are the principal quantities used to characterize the velocity and magnetic field fluctuations. Unresolved issues concerning the existence of actively developing turbulence are discussed.

Matthaeus, W. H.↗
NASA NTRS1983

Turbulent generation of outward-traveling interplanetary Alfvenic fluctuations

Additional evidence is presented for the dynamic alignment of the fluctuating plasma fluid velocity and the fluctuating magnetic field in two-dimensional MHD turbulence. The evidence is seen as indicating that the interplanetary fluctuations need not represent time-asymptotic states. Rather, it is contended that interplanetary Alfvenic fluctuations are generated in situ in the early stage of dynamic alignment. In addition to providing an explanation for inertial-range alignment of the fluctuating plasma fluid velocity and the fluctuating magnetic field in the solar wind, the model presented here accounts for the fact that the sign of this correlation (relative to the direction of the mean magnetic field) corresponds to the observed 'direction of propagation', that is, away from the sun. Evidence is found to support the contention that MHD fluctuations in the solar wind resemble turbulence arising from local stirring of the medium.

Matthaeus, W. H.↗
NASA NTRS1983

Determining the alpha dynamo parameter in incompressible homogeneous magnetohydrodynamic turbulence

Alpha, an important parameter in dynamo theory, is proportional to either the kinetic, current, magnetic, or velocity helicity of the fluctuating magnetic field and fluctuating velocity field. The particular helicity to which alpha is proportional depends on the assumptions used in deriving the first order smoothed equations that describe the alpha effect. In two cases, when alpha is proportional to either the magnetic helicity or velocity helicity, alpha is determined experimentally from two point measurements of the fluctuating fields in incompressible, homogeneous turbulence having arbitrary symmetry. For the other two possibilities, alpha is determined if the turbulence is isotropic.

Matthaeus, W. H.↗
NASA NTRS1983

Turbulence analysis of the Jovian upstream 'wave' phenomenon

As Voyager 2 approached Jupiter's bow shock, large-amplitude fluctuations were seen in both the magnetic field and plasma fluid velocity. These fluctuations generally coincided with the occurrence of long-lived energetic particle events similar to the upstream waves often observed near the earth's bow shock. In this paper an analysis of the magnetic field and plasma observations using spectral methods is presented. The characteristic spectral features related to the upstream waves are generally seen near 1 mHz. The measured correlation lengths of these fluctuations suggest that they are coherent over only a few wavelengths. The analysis is consistent with the hypothesis that these fluctuations are driven by streaming ions, possibly protons. No evidence for the existence of whistler waves is found. It is argued that some of the observed spectral features suggest that dynamical turbulent processes are occurring in the uptream wave region, including a possible observation of an inverse cascade of magnetic helicity to large spatial scales.

Smith, C. W.↗
NASA NTRS1983

Anisotropy in MHD turbulence due to a mean magnetic field

The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers. Previously announced in STAR as N83-12998

Shebalin, J. V.↗
NASA NTRS1983

Reconnection rates, small scale structures and simulations

The study of reconnection in the context of one fluid, two dimensional magnetohydrodynamics (MHD), with spatially uniform constant density, viscosity and resistivity is though to retain most of the physics important in reconnection. Much of the existing reconnection literature makes use of this approach. This discussion focuses on attempts to determine the properties of reconnection solutions to MHD as precisely as possible without regard to the intrinsic limitations of the model.

Matthaeus, W. H.↗
NASA NTRS1983

Large amplitude MHD waves upstream of the Jovian bow shock

Observations of large amplitude magnetohydrodynamics (MHD) waves upstream of Jupiter's bow shock are analyzed. The waves are found to be right circularly polarized in the solar wind frame which suggests that they are propagating in the fast magnetosonic mode. A complete spectral and minimum variance eigenvalue analysis of the data was performed. The power spectrum of the magnetic fluctuations contains several peaks. The fluctuations at 2.3 mHz have a direction of minimum variance along the direction of the average magnetic field. The direction of minimum variance of these fluctuations lies at approximately 40 deg. to the magnetic field and is parallel to the radial direction. We argue that these fluctuations are waves excited by protons reflected off the Jovian bow shock. The inferred speed of the reflected protons is about two times the solar wind speed in the plasma rest frame. A linear instability analysis is presented which suggests an explanation for many of the observed features of the observations.

Goldstein, M. L.↗
NASA NTRS1983

Long-time states of inverse cascades in the presence of a maximum length scale

It is shown numerically, both for the two-dimensional Navier-Stokes (guiding-center plasma) equations and for two-dimensional magnetohydrodynamics, that the long-time asymptotic state in a forced inverse-cascade situation is one in which the spectrum is completely dominated by its own fundamental. The growth continues until the fundamental is dissipatively limited by its own dissipation rate.

Hossain, M.↗
NASA NTRS1983

Dynamic alignment and selective decay in MHD

Under some circumstances, incompressible magnetohydrodynamic turbulence will evolve toward a state in which the velocity fields and magnetic fields are aligned or anti-aligned. We propose a mechanism for this effect and illustrate with numerical computations. Under some other circumstances, the energy appears to decay selectively toward a minimum energy state in which the kinetic energy has disappeared. It has not been possible so far to identify a boundary in the phase space which divides the two regimes.

Matthaeus, W. H.↗
NASA NTRS1982

Anisotropy in MHD turbulence due to a mean magnetic field

The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers.

Shebalin, J. V.↗
NASA NTRS1982

Measurement of the rugged invariants of magnetohydrodynamic turbulence in the solar wind

Measurements of the total energy, cross helicity, and magnetic helicity of the solar wind at 1, 2.8, and 5 AU are presented. These quantities are the three rugged invariants of three-dimensional ideal incompressible MHD turbulence theory. The theoretical technique for measuring the magnetic helicity from the matrix of two-point correlations is shown. The length scales characterizing the magnetic helicity are found to be equal to or greater than those which characterize the magnetic energy. The magnetic helicity typically lies at scales larger than the magnetic correlation length, consistent with the expectations of the inverse cascade and selective decay hypotheses of three-dimensional MHD turbulence. At smaller scales, the magnetic helicity oscillates in sign. Measurements of the cross helicity are not fully consistent with the usual interpretation in terms of outward propagating Alfvenic functions. Especially during the interval at 5 AU the cross helicity is found to oscillate in sign indicating fluctuations propagating both outward and inward.

Matthaeus, W. H.↗
NASA NTRS1982

Reconnection in two dimensions - Localization of vorticity and current near magnetic X-points

Simulations of reconnection utilizing spectral methods show that the 'reconnection zone' exhibits internal structure in the form of a current filament and a 'quadrupole' of kinetic vorticity. A qualitative model of the dynamical role of these local structures is presented. It is suggested that the effect of small scale structures near the X-point is to increase the rate of reconnection.

Matthaeus, W. H.↗
NASA NTRS1982

Stationarity of magnetohydrodynamic fluctuations in the solar wind

Solar wind research and studies of charged particle propagation often assume that the interplanetary magnetic field represents a stationary random process. The extent to which ensemble averages of the solar wind magnetic fields follow the asymptotic behavior predicted by the ergodic theorem was investigated. Several time periods, including a span of nearly two years, are analyzed. Data intervals which span many solar rotations satisfy the conditions of weak stationarity if the effects of solar rotation are included in the asymptotic analysis. Shorter intervals which include a small integral number of interplanetary sectors also satisfy weak stationarity. The results are illustrated using magnetometer data from the ISEE-3, Voyager and IMP spacecraft.

Matthaeus, W. H.↗
NASA NTRS1981

Structure of correlation tensors in homogeneous anisotropic turbulence

The theory of isotropic tensors, developed by Robertson (1940), Batchelor (1946), and Chandrasekhar (1951), is extended to cover the general case of turbulence with a pseudo-vector-preferred direction, without assuming mirror-reflection invariance. Attention is focused on two-point-correlation functions, and it is shown that the form of the decomposition into proper and pseudo-tensor contributions is restricted by the homogeneity requirement. The somewhat unexpected result that the vector- and pseudo-vector-preferred-direction cases yield different results is presented: A pseudo-vector-preferred direction allows the correlation matrix one more functional degree of freedom than does the proper vector case. An explicit form of the two-point-correlation tensor in the presence of a uniform mean magnetic field is presented which may be appropriate for use in analysis of magnetic fluctuations in plasma containment devices or the interplanetary medium. A procedure for determining the magnetic helicity from experimental data is presented.

Matthaeus, W. H.↗