Engineering topics

Goldstein, Melvyn L.

Publications and source records attributed to Goldstein, Melvyn L..

At least 55 records · Page 3

NASA NTRS2007

A Three-Dimensional MHD Simulation of the Solar Wind for a Tilted-Dipole Magnetic Field on the Sun

Using a three-dimensional MHD model, we simulate the global steady-state structure of the solar corona and solar wind for a dipole magnetic field on the Sun inclined by 30 degrees to the solar rotation axis. This represents the solar conditions typical for a declining phase of solar cycle. The computations can extend from the coronal base out to 100-AU and at large heliospheric distances includes the effects of interstellar neutral hydrogen and their interaction with solar wind protons. The simulations can model the formation of corotating interaction regions and the heliospheric current sheet. The simulations are also capable of describing very strong rarefaction regions that include embedded sub-Alfvenic regions that form on the trailing edge of a fast flows.

Goldstein, Melvyn L.↗

NASA NTRS2007

Turbulence and Global Properties of the Solar Wind

The solar wind shows striking characteristics that suggest that it is a turbulent magnetofluid, but the picture is not altogether simple. From the earliest observations, a strong correlation between magnetic fluctuations and plasma velocity fluctuations as noted. The high corrections suggest that the fluctuations are Alven waves. In addition, the power spectrum of the magnetic fluctuation showed evidence of an inertial range that resembled that seen in fully-developed fluid turbulence. Alfven waves, however, are exact solutions of the equations of an incompressible magnetohydrodynamics. Thus, there was a puzzle: how can a magnetofluid consisting of Alfven waves be turbulent? The answer lay in the role of velocity shears in the solar wind that could drive turbulent evolution. Puzzles remain: for example, the power spectrum of the velocity fluctuations is less steep than the slope of the magnetic fluctuations. The plasma in the magnetic tail of Earth's magnetosphere also shows aspects of turbulence, as does the plasma in the dayside magnetosphere near the poles--the so-called dayside cusps.

Goldstein, Melvyn L.↗

NASA NTRS2003

Gyrophase-bunched Electrons: Cluster Observations and Simulations

The interaction of the solar wind with the Earth's bow shock is responsible for a number of phenomena both upstream and downstream. One of the least understood and studied of these is the generation of gyrophase-bunched electrons. We will describe initial work searching for and analyzing such events in (Plasma Electron And Current Experiment (PEACE) data from CLUSTER. Plasma simulations will complement the data analysis. Recent analyses suggest that gyrophase-bunched electrons are present rather frequently in the upstream region, due either to energization in the reflection off the shock front or, more likely, to phase trapping in locally produced whistler wave fields.

Gurgiolo, C.↗

NASA NTRS1995

Magnetohydrodynamic turbulence in the solar wind

The fluctuations in magnetic field and plasma velocity in solar wind, which possess many features of fully developed magnetohydrodynamic (MHD) turbulence, are discussed. Direct spacecraft observations from 0.3 to over 20 AU, remote sensing radio scintillation observations, numerical simulations, and various models provide complementary methods that show that the fluctuations in the wind parameters undergo significant dynamical evolution independent of whatever turbulence might exist in the solar photosphere and corona. The Cluster mission, with high time resolution particle and field measurements and its variable separation strategies, should be able to provide data for answering many questions on MHD turbulence.

Goldstein, Melvyn L.↗

NASA NTRS1994

Magnetic field waves at Uranus

The research efforts funded by the Uranus Data Analysis Program (UDAP) grant to the Bartol Research Institute (BRI) involved the study of magnetic field waves associated with the Uranian bow shock. Upstream wave studies are motivated as a study of the physics of collisionless shocks. Collisionless shocks in plasmas are capable of 'reflecting' a fraction of the incoming thermal particle distribution and directing the resulting energetic particle motion back into the upstream region. Once within the upstream region, the backward streaming energetic particles convey information of the approaching shock to the supersonic flow. This particle population is responsible for the generation of upstream magnetic and electrostatic fluctuations known as 'upstream waves', for slowing the incoming wind prior to the formation of the shock ramp, and for heating of the upstream plasma. The waves produced at Uranus not only differed in several regards from the observations at other planetary bow shocks, but also gave new information regarding the nature of the reflected particle populations which were largely unmeasurable by the particle instruments. Four distinct magnetic field wave types were observed upstream of the Uranian bow shock: low-frequency Alfven or fast magnetosonic waves excited by energetic protons originating at or behind the bow shock; whistler wave bursts driven by gyrating ion distributions within the shock ramp; and two whistler wave types simultaneously observed upstream of the flanks of the shock and argued to arise from resonance with energetic electrons. In addition, observations of energetic particle distributions by the LECP experiment, thermal particle populations observed by the PLS experiment, and electron plasma oscillations recorded by the PWS experiment proved instrumental to this study and are included to some degree in the papers and presentations supported by this grant.

Smith, Charles W.↗

NASA NTRS1993

Quasi-periodic transverse plasma flow associated with an evolving MHD vortex street in the outer heliosphere

We study a transverse plasma flow induced by the evolution of a Karman vortex street using a Chebyshev-Fourier spectral algorithm to solve both the compressible Navier-Stokes and MHD equations. The evolving vortex street is formed by the nonlinear interaction of two vortex sheets initially in equilibrium. We study spatial profiles of the total plasma velocity, the density, the meridional flow angle and the location of sector boundaries and find generally good agreement with Voyager 2 measurements of quasi-periodic transverse flow in the outer heliosphere. The pressure pulses associated with the meridional flows in the simulation are too small, although they are correctly located, and this may be due to the lack of any 'warp' in the current sheet in this model. A strong, flow-aligned magnetic field, such as would occur in the inner heliosphere, is shown to lead to weak effects that would be masked by the background interplanetary turbulence. We also study the plasma and magnetic transport resulting from the meridional flow and find that deficits of magnetic quantities do occur near the ecliptic. While the effect is relatively small, it is in general agreement with the most recent analysis of 'flux deficit' in the outer heliosphere.

Siregar, Edouard↗

NASA NTRS1992

Velocity shear generation of solar wind turbulence

A two-dimensional incompressible MHD spectral code is used to show that shear-driven turbulence is a possible means for producing many observed properties of the evolution of the magnetic and velocity fluctuations in the solar wind and, in particular, the evolution of the cross helicity ('Alfvenicity') at small scales. It is shown that large-scale shear can nonlinearly produce a cascade to smaller scale fluctuations even when the linear Kelvin-Helmholtz mode is stable, and that a roughly power law inertial range is established by this process. The evolution found is similar to that seen in some other simulations of MHD turbulence.

Roberts, D. A.↗

NASA NTRS1992

An evolving MHD vortex street model for quasi-periodic solar wind fluctuations

Magnetohydrodynamic (MHD) simulation are used to provide a dynamical basis for the 'vortex street' model of the quasi-periodic meridional flow observed by Voyager 2 in the outer heliosphere. Various observations suggest the existence near the current sheet at solar minimum, of a vorticity distribution of two opposite shear layers with an antisymmetric staggered velocity pattern due to structured high-speed wind surrounding low-speed equatorial flow. It is shown that this flow pattern leads to the formation of a highly stable vortex street through the nonlinear interaction of the two shear layers. Spatial profiles of various simulated parameters (velocity, density, meridional flow angle and the location of magnetic sector boundaries) and their relative locations in the quasi-steady vortex street are generally in good agreement with the observations.

Siregar, Edouard↗

NASA NTRS1991

Magnetohydrodynamic simulation of the radial evolution and stream structure of solar-wind turbulence

A unified interpretation of observations of interplanetary fluctuations is presented in terms of nearly incompressible magnetohydrodynamics. Incompressive effects explain the rapid evolution of turbulence in slow wind containing the heliospheric current sheet. The relative constancy of the spectrum of 'inward propagating' fluctuations compared to the rapid decline in 'outward' fluctuations results from incompressive spectral transfer combined with strong dissipation of the outward fluctuations. Secondary compressive effects account for nearly pressure-balanced structures and the density fluctuation levels.

Roberts, D. A.↗

NASA NTRS1991

Whistler waves associated with the Uranian bow shock - Outbound observations

High-resolution magnetic field measurements from the first outbound crossing of the Uranian bowshock by the Voyager 2 spacecraft between January 27 and 30, 1986, are examined. Evidence is found of enhanced whistler wave activity in the vicinity of three shock crossings but little or no evidence of such activity elsewhere. Two wave events display two separate and simultaneous wave enhancements each. From an investigation of these events using high-resolution field data, it is concluded that they are analogous to those whistler waves upstream of the earth's bow shock that are driven by beams of electrons. An instability analysis is presented to show that a single electron beam with reasonable parameters can penetrate both of the upstream and downstream of a shock crossing. This event displays only one relatively broad spectral enhancement in the same frequency regime and is left-hand polarized in the spacecraft frame. It is argued that this event is the result of a gyrating proton distribution associated with the oblique shock.

Smith, Charles W.↗

NASA NTRS1991

Magnetic field waves at Uranus

The proposed research efforts funded by the UDAP grant to the BRI involve the study of magnetic field waves associated with the Uranian bow shock. This is a collaborative venture bringing together investigators at the BRI, Southwest Research Institute (SwRI), and Goddard Space Flight Center (GSFC). In addition, other collaborations have been formed with investigators granted UDAP funds for similar studies and with investigators affiliated with other Voyager experiments. These investigations and the corresponding collaborations are included in the report. The proposed effort as originally conceived included an examination of waves downstream from the shock within the magnetosheath. However, the observations of unexpected complexity and diversity within the upstream region have necessitated that we confine our efforts to those observations recorded upstream of the bow shock on the inbound and outbound legs of the encounter by the Voyager 2 spacecraft.

Smith, Charles W.↗

NASA NTRS1991

Report of the theory panel

The ultimate goal of this research is to develop an understanding which is sufficiently comprehensive to allow realistic predictions of the behavior of the physical systems. Theory has a central role to play in the quest for this understanding. The level of theoretical description is dependent on three constraints: (1) the available computer hardware may limit both the number and the size of physical processes the model system can describe; (2) the fact that some natural systems may only be described in a statistical manner; and (3) the fact that some natural systems may be observable only through remote sensing which is intrinsically limited by spatial resolution and line of sight integration. From this the report discusses present accomplishments and future goals of theoretical space physics. Finally, the development and use of new supercomputer is examined.

Ashourabdalla, Maha↗

NASA NTRS1991

Anisotropy and minimum variance directions of solar wind fluctuations in the outer heliosphere

Voyager 2 magnetic field and plasma data are examined over time intervals of 1 to 12 hours in the heliospheric range of 1 to 10 AU to study the evolution of the anisotropy of solar wind fluctuations. Consistent with previous results, the directions of minimum variance vectors of magnetic fluctuations are found to be close to the mean magnetic field direction with an increasing component along the field at larger scales. At large radial distances there is more spread in the minimum variance directions than at smaller radial distances. The power in smaller-scale fluctuations in the magnetic field components perpendicular to the local mean field B(0) is in the ratio of about 5:1 near 1 AU at the scale of 1 hour but decreases to about 3:1 further out. No evidence for selective enhancement of out-of-the-ecliptic components of fluctuations is found. In contrast to results for field fluctuations, analysis of velocity fluctuations shows that the minimum variance direction systematically remains more radially oriented and becomes increasingly less oriented along B(0) with increasing heliocentric distance. The velocity fluctuations are generally more isotropic than the magnetic fluctuations. The observations cannot be explained by a superposed wave picture, and thus are consistent with the view that nonlinear turbulent evolution is responsible for the anisotropy in the fluctuations.

Klein, Larry W.↗

NASA NTRS1991

Turbulence and waves in the solar wind

Studies of turbulence and waves in the solar wind is discussed. Consideration is given to the observations and theory concerning the origin and evolution of interplanetary MHD fluctuations and to the observations, theory, and simulations of compressive fluctuations. Particular attention is given to extrapolations to near-sun and polar fields regions. Results obtained on turbulence at comets and magnetic turbulence of low-frequency waves excited by unstable distributions of ions are discussed.

Roberts, D. A.↗

NASA NTRS1990

Evidence for the presence of quasi-two-dimensional nearly incompressible fluctuations in the solar wind

Assuming that the slab and isotropic models of solar wind turbulence need modification (largely due to the observed anisotropy of the interplanetary fluctuations and the results of laboratory plasma experiments), this paper proposes a model of the solar wind. The solar wind is seen as a fluid which contains both classical transverse Alfvenic fluctuations and a population of quasi-transverse fluctuations. In quasi-two-dimensional turbulence, the pitch angle scattering by resonant wave-particle interactions is suppressed, and the direction of minimum variance of interplanetary fluctuations is parallel to the mean magnetic field. The assumed incompressibility is consistent with the fact that the density fluctuations are small and anticorrelated, and that the total pressure at small scales is nearly constant.

Matthaeus, William H.↗

NASA NTRS1990

Do interplanetary Alfven waves cause auroral activity?

A recent theory holds that high-intensity, long-duration, continuous auroral activity (HILDCAA) is caused by interplanetary Alfven waves propagating outward from the sun. A survey of Alfvenic intervals in over a year of ISEE 3 data shows that while Alfvenic intervals often accompany HILDCAAs, the reverse is often not true. There are many Alfvenic intervals during which auroral activity (measured by high values of the AE index) is very low, as well as times of high auroral activity that are not highly Alfvenic. This analysis supports the common conclusion that large AE values are associated with a southward interplanetary field of sufficient strength and duration. This field configuration is independent of the presence of Alfven waves (whether solar generated or not) and is expected to occur at random intervals in the large-amplitude stochastic fluctuations in the solar wind.

Roberts, D. Aaron↗

NASA NTRS1989

Whistler wave bursts upstream of the Uranian bow shock

Observations of magnetic field wave bursts upstream of the Uranian bow shock are reported which were recorded prior to the inbound shock crossing. Three wave types are identified. One exhibits a broad spectral enhancement from a few millihertz to about 50 mHz and is seen from 17 to 10 hr prior to the inbound shock crossing. It is argued that these waves are whistler waves that have propagated upstream from the shock. A second wave type has a spacecraft frame frequency between 20 and 40 mHz, is seen only within or immediately upstream of the shock pedestal, is right-hand polarized in the spacecraft frame, and has a typical burst duration of 90 s. The third wave type has a spacecraft frame frequency of about 0.15 Hz, is seen exclusively within the shock pedestal, is left-hand polarized in the spacecraft frame, and has a burst duration lasting up to 4 min. It is argued that the low-frequency bursts are whistler waves with phase speed comparable to, but in excess of, the solar wind speed.

Smith, Charles W.↗

NASA NTRS1989

Spectral evolution and cascade constant of solar wind Alfvenic turbulence

The theoretical model of Tu (1988) for the turbulent evolution of solar wind fluctuations assumed the constancy of the ratio alpha(1) between the energy in inward and outward propagating Alfven waves. Here, this model is extended to reflect the observed evolution in the propagation directions of the interplanetary fluctuations. The radial dependence of alpha(1) is determined by direct time domain correlations of the normalized cross helicity. The theoretical results match the observations at least as well as those of the previous, constant alpha(1), model. Measured values of alpha(1) are then used to find the value of the cascade constant that determines the overall level of the energy spectrum. The value of 1.25 for this constant is very close to the value observed in fluid turbulence if it is assumed that the correspondence between fluid and magnetofluid theories should be made for uncorrelated (zero cross helicity) MHD turbulence.

Tu, Chuan-Yi↗