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Gary, S. P.

Publications and source records attributed to Gary, S. P..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Nonlinear and linear timescales near kinetic scales in solar wind turbulence

The application of linear kinetic treatments to plasma waves, damping, and instability requires favorable inequalities between the associated linear timescales and timescales for nonlinear (e.g., turbulence) evolution. In the solar wind these two types of timescales may be directly compared using standard Kolmogorov-style analysis and observational data. The estimated local (in scale) nonlinear magnetohydrodynamic cascade times, evaluated as relevant kinetic scales are approached, remain slower than the cyclotron period, but comparable to or faster than the typical timescales of instabilities, anisotropic waves, and wave damping. The variation with length scale of the turbulence timescales is supported by observations and simulations. On this basis the use of linear theory—which assumes constant parameters to calculate the associated kinetic rates—may be questioned. It is suggested that the product of proton gyrofrequency and nonlinear time at the ion gyroscales provides a simple measure of turbulence influence on proton kinetic behavior.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ANALYTIC MODEL OF THE IBEX RIBBON WITH NEUTRAL SOLAR WIND BASED ION PICKUP BEYOND THE HELIOPAUSE

Energetic neutral atom (ENA) full sky maps obtained with the Interstellar Boundary Explorer revealed a surprising narrow band of increased intensity, the Ribbon, which has been attributed to the ordering effect of the interstellar magnetic field immediately outside the heliosphere. Among models to explain the enhanced ENA Ribbon intensity, Heerikhuisen et al. base theirs on neutral solar wind origin. It reflects the Ribbon angular and energy distribution correctly, but experiences inherent challenges from the long-term stability of the pickup ion (PUI) ring in velocity space, required for the observed ENA fluxes, and from time variations observed after less than one year due to the long integration length. We provide a simplified analytic model of the neutral solar wind, PUI production beyond the heliopause, and subsequent ENA production. We include convection of the PUIs with the interstellar flow toward the heliopause perpendicular to the interstellar magnetic field, thus far not in any model, extinction of the outward propagating neutral solar wind, and of the PUIs and ENAs on their way inward. Based on hybrid simulations of PUI driven instabilities, with injection rates from this model, scattering and isotropization of the PUIs is noticeably weaker than previously thought, yet too fast for ENA production. Assuming a narrow PUI velocity ring, we find a strong concentration of the ENA origin just outside the heliopause and Ribbon intensities comparable with the observations. Conversely, an isotropic PUI distribution produces ENA fluxes factor of ten too low, thus reemphasizing the need of very slow scattering.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2010

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations

Observed properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T(perpindicular)/T(parallell) approximately > 2). This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch-angle scattering

Vinas, A.F.↗
NASA NTRS1998

Lower Bound for Electron Core Beta in the Solar Wind

Solar wind electrons, especially under conditions of relatively low speed flow, often can be represented as two bi-Maxwellian components, a cooler more dense core (denoted by the subscript c) and a hotter more tenuous halo.

solar↗
NASA NTRS1998

Composition Measurements at the Magnetopause and in the Plasma Mantle

This final report describes activities under NASA grant NAGW-4049 to Lockheed Missiles and Space Company. The report covers the entire period of the grant from 15 August 1994 to 31 January 1998. The original grant was for 3 years ending in August 1997; however the grant was extended 6 months to accomodate additional data analysis that added significantly to the scientific results. This is a grant under the NASA Supporting Research and Technology Program for the analysis and interpretation of the combined scientific data from the ISEE-1 Plasma Composition Experiment and the AMPTE/CCE Hot Plasma Composition Experiment. These combined data sets were used in a study of the Earth's magnetopause to develop a fundamental understanding of plasma entry and dynamics at the boundary and formation and maintenance of the low latitude boundary layer under a variety of solar wind and magnetospheric conditions and at a wide range of local times.

Gary, S. P.↗
NASA NTRS1998

Survey of Warm Pancake-Shaped Ion Distributions at Geosynchronous Orbit

It has been proposed that the electromagnetic proton cyclotron instability is the strongest source of heating for the anisotropic warm ions observed at geosynchronous orbit. We present here the results of a statistical study of warm pancake-shaped ion distributions observed with the Los Alamos magnetospheric plasma analyzer (MPA) on geosynchronous satellites. We examined the ion distributions to determine the correlation between the observed warm ion distributions and various magnetospheric parameters, and their location relative to the plasma trough, plasmasphere, plasma sheet, and local time. We find that the warm pancake-shaped ion distributions occur more frequently near noon, and during low magnetospheric activity. The implications of our observations for the proton cyclotron instability, as the source of energy for the warm ions, will be discussed.

Ober, Daniel M.↗
NASA NTRS1994

Identification of low-frequency fluctuations in the terrestrial magnetosheath

On the basis of magnetohydrodynamic (MHD) theory we develop a scheme for distinguishing among the four low-frequency modes which may propagate in a high-beta anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfven, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun-Earth line downstream of a quasi-perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvenic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvenic. The fast mode waves occur in the higher-frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.

Song, P.↗
NASA NTRS1993

Kinetic physics of the mirror instability

Kinetic mechanisms for the growth and saturation of the mirror instability are described using one-dimensional hybrid simulations. Two parameter regimes are considered. In the first regime, a relatively small ion anisotropy excites a slowly growing instability that produces small-amplitude waves; most ions respond to the waves as an adiabatic fluid. In the second regime a large anisotropy excites a rapidly growing instability that generates large-amplitude waves; the response of many ions in this case is nonadiabatic. The difference in ion response is due to the relative importance of two ion populations, resonant and nonresonant. Resonant ions, those ions with low velocities parallel to the background magnetic field, contribute to the growth of the instability as a result of their gyrointeractions with the noncoplanar component of the waves electric field and respond to the mirror waves nonadiabatically. Nonreasonant ions, those with large parallel velocities, respond as an adiabatic fluid. In both regimes, ion anisotropy is reduced by means of the magnetic mirror force; in the second regime, the anisotropy reduction reduces the free energy available for wave growth and leads to the saturation of the mirror instability.

Mckean, M. E.↗
NASA NTRS1993

Ion anisotropies in the magnetosheath

One- and two-dimensional initial value hybrid computer simulations are used with a magnetosheath parameter model to study the consequences of the growth of the mirror and ion cyclotron anisotropy instabilities. Magnetosheath observations have demonstrated inverse correlations between the proton and helium ion temperature anisotropies and the proton parallel beta. Using the maximum growth rate as a fitting pararamter, linear Vlasov instability theory for the proton cyclotron and helium cyclotron anisotropy instabilities reproduces both observed correlations. Furthermore, results from the asymptotic states of simulations of the ion cyclotron instabilities qualitatively reproduce both observations.

Gary, S. P.↗
NASA NTRS1993

Simulations of ion cyclotron anisotropy instabilities in the terrestrial magnetosheath

Enhanced transverse magnetic fluctuations observed below the proton cyclotron frequency in the terrestrial magnetosheath have been identified as due to the proton cyclotron and helium cyclotron instabilities driven by the T-perpendicular greater than T-parallel condition of the sheath ions. One-dimensional hybrid computer simulations are used here to examine the nonlinear properties of these two growing modes at relatively weak fluctuation energies and for wave vectors parallel to the background magnetic field. Second-order theory predicts fluctuating magnetic field energies at saturation of the proton cyclotron anisotropy instability in semiquantitative agreement with the simulation results. Introduction of the helium component enhances the wave-particle exchange rate for proton anisotropy reduction by that instability, thereby reducing the saturation energy of that mode. The simulations demonstrate that wave-particle interactions by the proton cyclotron and helium cyclotron instabilities lead to the anticorrelation observed by Anderson and Fuselier (1993).

Gary, S. P.↗
NASA NTRS1993

Ion cyclotron anisotropy instabilities in the magnetosheath - Theory and simulations

Second-order theory and 1D hybrid computer simulations are used to examine the nonlinear properties of proton and helium cyclotron anisotropy instabilities in the terrestrial magnetosheath at relatively weak fluctuations levels and at propagation parallel to the background magnetic field. The simulations confirm the second-order predictions that both instabilities yield efficient wave-particle interactions; the rate at which the driving species anisotropy is reduced is much greater than the rate at which that species loses kinetic energy. It is suggested that these instabilities should saturate at relatively low levels. An approximate expression is derived for the fluctuating field energy at saturation of the fastest growing modes, and it is found that it is in fair agreement with three computer simulations.

Gary, S. P.↗
NASA NTRS1993

Nonlinear evolution of electromagnetic ion beam instabilities

A comparative study of the ion/ion right-hand resonant instability and the ion/ion nonresonant instability is carried out to investigate and contrast their properties. Linear analysis demonstrates that the nonresonant instability becomes resonant if the density of the ion beam is sufficiently high. Hybrid simulations show that both the resonant and nonresonant instabilities result in the formation of nonlinear pulses, called pulsations, but with distinct features. For example, the pulsations generated by the resonant instability have a positive correlation with the ion density, while those generated by the nonresonant instability are likely to have a relatively weak negative correlation. The waves generated by the nonresonant instability are subject to a parametric decay instability and tend to form a state of condensate where the turbulence becomes nearly monochromatic.

Akimoto, K.↗
NASA NTRS1993

Ion anisotropy instabilities in the magnetosheath

Recent observations in Earth's magnetosheath have delineated several different kinds of magnetic fluctuation spectra below the proton cyclotron frequency. This paper provides a theoretical interpretation for some of these observations describing solutions of the linear Vlasov dispersion equation for fully electromagnetic instabilities for particle distributions which model those observed in the magnetosheath. This model yields three growing modes: the proton cyclotron anisotropy the helium cyclotron anisotropy, and the mirror instabilities. The results show very good agreement with the observations of mirror-like and proton-cyclotron-like events. This agreement with observations implies that the transition between cyclotron and mirror fluctuation dominance is consistent with linear theory.

Gary, S. P.↗
NASA NTRS1992

Electrostatic ion cyclotron velocity shear instability

A local electrostatic dispersion equation is derived for a shear flow perpendicular to an ambient magnetic field, which includes all kinetic effects and involves only one important parameter. The dispersion equation is cast in the form of Gordeyev integrals and is solved numerically. Numerical solutions indicate that an ion cyclotron instability is excited. The instability occurs roughly at multiples of the ion cyclotron frequency (modified by the shear), with the growth rate or the individual harmonics overlapping in the wavenumber. At large values of the shear parameter, the instability is confined to long wavelengths, but at smaller shear, a second distinct branch at shorter wavelengths also appears. The properties of the instability obtained are compared with those obtained in the nonlocal limit by Ganguli et al. (1985, 1988).

Lemons, D. S.↗
NASA NTRS1992

Mirror and ion cyclotron anisotropy instabilities in the magnetosheath

The kinetic properties of the ion cyclotron anisotropy and the mirror instabilities are investigated using one-dimensional hybrid simulations. It is found that, for moderate values of the ion beta and the proton temperature anisotropy, the two instabilities produce similar levels of turbulence. For high values of beta or temperature anisotropy, the ion cyclotron instability produces higher fluctuation levels of turbulence than does the mirror instability.

Mckean, M. E.↗
NASA NTRS1992

A second-order theory for transverse ion heating and momentum coupling due to electrostatic ion cyclotron waves

A second-order theory for electrostatic instabilities driven by counterstreaming ion beams is developed which describes momentum coupling and heating of the plasma via wave-particle interactions. Exchange rates between the waves and particles are derived, which are suitable for the fluid equations simulating microscopic effects on macroscopic scales. Using a fully kinetic simulation, the electrostatic ion cyclotron instability due to counterstreaming H(+) beams has been simulated. A power spectrum from the kinetic simulation is used to evaluate second-order exchange rates. The calculated heating and momentum loss from second-order theory is compared to the numerical simulation.

Miller, Ronald H.↗