Engineering topics

Goldstein, M. L.

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

At least 55 records · Page 3

NASA NTRS1992

Parametric instabilities of large amplitude Alfven waves with obliquely propagating sidebands

This paper presents a brief report on properties of the parametric decay and modulational, filamentation, and magnetoacoustic instabilities of a large amplitude, circularly polarized Alfven wave. We allow the daughter and sideband waves to propagate at an arbitrary angle to the background magnetic field so that the electrostatic and electromagnetic characteristics of these waves are coupled. We investigate the dependance of these instabilities on dispersion, plasma/beta, pump wave amplitude, and propagation angle. Analytical and numerical results are compared with numerical simulations to investigate the full nonlinear evolution of these instabilities.

Vinas, A. F.↗

NASA NTRS1991

The structure of helical interplanetary magnetic fields

The interplanetary magnetic field is known to be highly helical. Although the detailed spatial structure of the fields has yet to be elucidated, the helicity spectrum has been conjectured to result from a random walk in the direction of a constant magnitude magnetic field vector. A model using three-dimensional fluctuations with variations in B is demonstrated giving a good fit to the helicity spectrum as well as to other properties of the interplanetary magnetic field.

Goldstein, M. L.↗

NASA NTRS1991

Ion cyclotron harmonic resonances driven by ion ring-beam distributions

Enhanced magnetic fluctuations with frequencies peaking at the ion cyclotron frequency and its harmonics have been observed at Comet Halley and also in the upstream regions of planetary bow shocks. It is thought that these waves are generated at comets by pickup ions that are unstable to the generation of waves at harmonics of the ion cyclotron wave number. In the spacecraft frame of reference these waves are observed as harmonics of the ion cyclotron frequency. In this report, it is shown that the ring-beam distributions of pickup ions observed in the cometary environment are capable of generating these waves if the beam speed component of the distribution (parallel to the ambient magnetic field) is much larger than the ring speed (perpendicular to the magnetic field). As the ring speed increases relative to the beam speed, other instabilities occur at the same wave number which have even larger growth rates. These additional instabilities do not lead to generation of harmonics.

Wong, H. K.↗

NASA NTRS1990

A mechanism for bursty radio emission in planetary magnetospheres

Bursty radio emissions are often observed from the polar magnetospheres of the earth, Jupiter, Saturn, and Uranus in addition to the smooth radio emissions commonly detected. It is shown that in plasma regimes in which the electron plasma frequency is less than the electron cyclotron frequency, anisotropic electron beams or gyrating electron beams can excite directly broadband electromagnetic radiation. The largest growth is for right-hand X-mode radiation with frequencies above the electron cyclotron frequency. This instability can produce bursty, broadband emission, consistent with some of the properties of the radiation observed from the magnetized planets.

Wong, H. K.↗

NASA NTRS1990

Ultra-low-frequency wave power in the magnetotail lobes. I - Relation to substorm onsets and the auroral electrojet index

Time-series observations of the magnetotail-lobe magnetic field have been Fourier analyzed to compute the frequency-weighted energy density Pfz in the range 1-30 mHz. Pfz is generally observed in the range 0.0001-0.01 gamma-squared Hz with a mean value of 0.0012 during substorm growth phases and 0.001 in the comparison intervals. No strong correlation of Pfz is found with the auroral electrojet index in either set of intervals, but during substorm growth phases Pfz may vary by an order of magnitude over time scales of 30 min, with a tendency for higher power levels to occur later in the growth phase. Increases in Pfz precede by about 10 min localized expansive phase activity observed in individual magnetograms.

Smith, R. A.↗

NASA NTRS1990

Physical consistency in modeling interplanetary magnetohydrodynamic fluctuations

The validity of the Velli, Grappin and Mangeney (1989) model is evaluated. It is argued that the model is incorrect because it mixes different dynamical models, assumes weak nonlinearities, makes predictions that vary with observations, and violates causality. It is proposed that self-similar behavior in the coronal source region of the magnetohydrodynamic fluctuations cause the Kolmogorov-like spectra.

Zhou, Y.↗

NASA NTRS1990

The amplitudes of interplanetary fluctuations - Stream structure, heliocentric distance, and frequency dependence

A study is presented of the heliocentric distance, frequency, and stream structure dependence of the amplitudes of interplanetary fluctuations in the velocity and magnetic field from 0.3 to nearly 20 AU and for spacecraft-frame periods of 10 days to a few hours. Evidence is presented that, at a given heliocentric distance, the amplitude of the magnetic field fluctuations is proportional to the magnitude of the field, nearly independently of the solar wind speed. The radial evolution of magnetic fluctuations is shown to be nearly consistent with WKB expectations except at smaller scales in the inner heliosphere and at the largest scales in the outer heliosphere. While the large-scale velocity fluctuations are kinetic energy-dominated in the inner heliosphere due to the presence of streams, the magnetic fluctuation energy eventually comes to be slightly dominant over the kinetic energy at all scales. The theoretical implications of the results are considered.

Roberts, D. A.↗

NASA NTRS1990

Generation of low-frequency waves at Comet Halley

The extent to which the properties of the waves observed at comet Halley during the Giotto encounter can be understood within the context of linearized Vlasov theory is investigated. In the region that is magnetically connected to the comet, fluctuations in the plasma frame of reference are detected near 4 and 10 mHz, close to the water cyclotron frequency, as well as at 20-60 mHz, which is well above the water cyclotron frequency. Using a variety of approximations for the ion distribution function, it is shown that waves having properties similar to those observed can then be generated with appropriate choices of plasma parameters. In the region that is magnetically disconnected from the comet, distinct peaks are observed in the magnetic power spectrum at 7, 21, 29, and 35 mHz, with a hint of a peak at 14 mHz.

Goldstein, M. L.↗

NASA NTRS1989

Time evolution from linear to nonlinear stages in magnetohydrodynamic parametric instabilities

The nonlinear evolution of the magnetohydrodynamic (MHD) parametric instability of wave fluctuations propagating along an unperturbed magnetic field is investigated. Both a magnetohydrodynamic perturbation-theoretical approach and a nonlinear MHD simulation are used. It is shown that high harmonic waves are rapidly excited by wave-wave coupling, and that the wave spectrum evolves from a state containing a small number of degrees of freedom in k space to one which contains a large number of degrees of freedom. It is found that the spectral evolution prior to nonlinear saturation is well described by the prturbation theory. During this stage, the ratio of the growth rate of the nth harmonic wave to the linear growth rate of the fundamental wave is n. The nonlinear saturation stage is characterized by a frequency shift of the fundamental wave that destroys the wave-wave resonance condition which, in turn, causes the wave amplitude to cease its growth.

Hoshino, M.↗

NASA NTRS1989

Spectral characteristics of low-frequency plasma turbulence upstream of Comet P/Halley

Two upstream regions have been identified in Giotto spacecraft magnetic field and plasma measurements subjected to cross-spectral analyses, in order to determine this cometary environment's low-frequency plasma turbulence spectral characteristics. One region's solar wind magnetic field was approximately parallel, and the other's perpendicular, to the solar wind flow velocity direction. Additional divergences relate to the regions having magnetic field lines that are either connected or disconnected to the cometary bow shock wave in either the quasi-parallel or quasi-perpendicular regions.

Glassmeier, K.-H.↗

NASA NTRS1988

Soho and Cluster - The scientific instruments

The mission goals and instrumentation of the Soho and cluster spacecraft to be launched in 1995 as part of the international Solar-Terrestrial Science Program are discussed. Instruments such as normal-incidence, grazing-incidence, and EUV coronal spectrometers aboard the Soho spacecraft will study the origin of the solar wind and the physical properties of the solar atmosphere. The four Cluster spacecraft will measure electric and magnetic fields, plasmas, and energetic particles using instruments including a wide-band receiver system, a relaxation sounder, and a search coil magnetometer.

Domingo, V.↗

NASA NTRS1988

Cluster: A fleet of four spacecraft to study plasma structures in three dimensions

The four Cluster spacecraft are spin stabilized spacecraft which are designed and built under stringent requirements as far as electromagnetic cleanliness is concerned. Conductive surfaces and low electromagnetic background noise are mandatory for accurate electric field and cold plasma measurements. The mission is implemented in collaboration between ESA and NASA. A Russian mission will be closely coordinated with Cluster.

Schmidt, R.↗

NASA NTRS1988

Proton beam generation of oblique whistler waves

It is known that ion beams are capable of generating whistler waves that propagate parallel to the mean magnetic field. Such waves may have been observed both upstream of the earth's bow shock and in the vicinity of comets. Previous analyses are extended to include propagation oblique to the mean magnetic field. The instability is generated by the perpendicular component of free energy in the ions, which can arise either via a temperature anisotropy or via a gyrating distribution. In the former case, the generation of whistler waves is confined to a fairly narrow cone of propagation directions centered about parallel propagation; in the latter case, the maximum growth of the instability can occur at fairly large obliquities (theta equal to about 50 deg).

Wong, H. K.↗

NASA NTRS1987

Origin and evolution of fluctuations in the solar wind - Helios observations and Helios-Voyager comparisons

Hour-averaged data from the Helios and Voyager spacecraft are used here to investigate the origin and evolution of low-frequency interplanetary fluctuations from 0.3 to 20 AU. The previously observed evolution toward a less purely Alfvenic state with increasing heliocentric distance is shown to occur more rapidly in the inner heliosphere and in low-speed as well as high-speed streams. It is concluded that outward-traveling flucutations are predominantly generated by the sun, but that in situ turbulence, most likely due to stream shear, generates fluctuations with both inward and outward senses of correlation.

Roberts, D. A.↗

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

The nature and evolution of magnetohydrodynamic fluctuations in the solar wind - Voyager observations

The magnetic field and plasma data acquired by Voyager between 1 and 11 AU are used to investigate the properties of interplanetary MHD fluctuations and to attempt to answer several related questions regarding the Alfvenicity of solar wind fluctuations. These questions are: (1) the extent to which the interplanetary fluctuations are Alfvenic; (2) whether there is any evolution in propagation direction of the Alfvenic fluctuations; (3) whether the degree and evolution of Alfvenicity are correlated with structure; and (4) the importance and evolution of compressive contributions to the fluctuations. The conclusions on these points are summarized, and the results are related to theoretical work.

Roberts, D. A.↗

NASA NTRS1987

Spectral signatures of jumps and turbulence in interplanetary speed and magnetic field data

It is shown here that, consistent with a suggestion of Burlaga and Mish (1987), the f exp -2 spectra in the magnitudes of the magnetic and velocity fields in the solar wind result from jumps due to various rapid changes in the time series for these quantities. If these jumps are removed from the data, the spectra of the resulting 'difference' time series have the f exp -5/3 form. It is concluded that f exp -2 spectra in these magnitudes arise from phase coherent structures that can be distinguished clearly from incoherent turbulent fluctuations.

Roberts, D. A.↗

NASA NTRS1987

Numerical simulation of the generation of turbulence from cometary ion pick-up

Observations of magnetic field fluctuations near Comet Halley have revealed a rapid development of a Kolmogoroff-like turbulence spectrum extending from below 0.01 Hz to above 0.1 Hz. Spectra obtained far from the comet have a strong peak in power near the Doppler-shifted ion-cyclotron frequency of singly ionized water. Closer to the comet, the spectrum at higher frequencies is enhanced in power level over the background solar wind spectrum by approximately an order of magnitude. The equations of incompressible MHD are solved using a two-dimensional 256 x 256 mode spectral method code to simulate this spectral evolution as an inertial range turbulent cascade. The initial conditions contained a constant magnetic field and a single coherent wave mode at a low wave number. The solar wind turbulence was modeled by a background noise spectrum having a Kolmogoroff spectral index. The coherent mode decayed into an inertial range spectrum with Kolmogoroff slope within a few eddy-turnover times. Both the time scale and the increase in power level of the turbulence seen in the simulation are in accord with the Giotto observations.

Goldstein, M. L.↗