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NASA NTRS1992

Upstream waves at Mars

Weak, about 0.15 nT, narrow band emissions at the proton gyro frequency are observed by the Phobos magnetometer MAGMA, upstream from the bow shock of Mars. These waves are left-hand elliptically polarized. They may be associated with the pick up of protons from the Martian hydrogen exosphere. Strong turbulence, similar to that observed at the terrestrial bow shock, is found on occasion in the upstream region when the IMF connects to the bow shock. On two occasions this turbulence occurred when the spacecraft crossed the orbit of Phobos. This coincidence raises the possibility that material in the orbits of Phobos interacts with the solar wind in such a way to either affect the direction of the IMF or to cause instabilities in the solar wind plasma. However, since on a third occasion these waves did not occur, these waves may be shock associated rather than Phobos associated.

Russell, C. T.↗

NASA NTRS1993

Instability modes excited by natural screech tones in a supersonic rectangular jet

The evolution of hydrodynamic instability modes self-excited by harmonically related natural screech tones was experimentally investigated. A convergent rectangular nozzle with an aspect ratio of 9.63 was used to produce a supersonic shock containing jet. Measurements in the flow-field were made using standard hot-film probes positioned only in the subsonic (outer) portions of the flow. The hydrodynamic instability mode observed in the shear layer at the screech frequency was observed to be antisymmetric (sinuous) about the smaller dimension of the jet, whereas its harmonic was observed to be symmetric (varicose). In addition, the near-field noise measurements indicated that the radiated screech tone noise was out of phase on either side of the small jet dimension whereas its harmonic was in phase over the same region. To our knowledge such an observation on the nature of the harmonic has thus far gone unreported and therefore is the focus of the present work. The hydrodynamic instability modes occurring at the screech frequency and its harmonic satisfied the conditions for resonance. Detailed measurements of the coherent wave evolution in the streamwise and spanwise directions indicated that strong spanwise variations were present beyond x/h = 8. Details of the screech noise radiated by the coherent instability modes are also presented in this paper.

Raman, Ganesh↗

NASA NTRS1995

Propagation and damping of broadband upstream whistlers

Previous studies indicated that damping rates of upstream whistlers strongly depend on the details of the electron distribution function. Moreover, detailed analysis of Doppler-shift and whistler dispersion relation indicated that upstream whistlers propagate obliquely in a broad band. In this paper we present results of a kinetic calculation of damping lengths of wide-band whistlers using the sum of 7-drifting bi-Maxwellian electron distributions as a best fit to the International Sun Earth Explorer (ISEE) 1 electron data. For 2 cases, when upstream whistlers are observed, convective damping lengths derived from ISEE magnetic field and ephemeris data are compared with theoretical results. We find that the calculated convective damping lengths are consistent with the data and that upstream whistlers remain marginally stable. We also show that the slope of plasma frame spectra of upstream whistlers, obtained by direct fitting of the observed spectra is between 5 and 7 with a sharp lower frequency cutoff corresponding to a wavelength of about one ion inertial length. When the solar wind velocity is directed largely along the wave normal of the upstream whistlers the polariztion of the right hand waves becomes reversed and low frequencies are switched to high resulting in a peaked spectrum with a strong high frequency cutoff. The overall spectral, wave and particle characteristics, proximity to the shock as well as propagation and damping properties indicate that these waves cannot be generated locally. Instead the observed upstream whistlers arise in the shock ramp most likely by a variety of cross-field drift and/or anisotropy driven instabilities.

Orlowski, D. S.↗

NASA NTRS1993

Generation of waves in the Venus mantle by the ion acoustic beam instability

The ion acoustic beam instability is suggested as a mechanism to produce wave turbulence observed in the Venus mantle at frequencies 100 Hz and 730 Hz. The plasma is assumed to consist of a stationary cold O(+) ion plasma and a flowing, shocked solar wind plasma. The O(+) ions appear as a beam relative to the flowing ionosheath plasma which provides the free energy to drive the instability. The plasma is driven unstable by inverse electron Landau damping of an ion acoustic wave associated with the cold ionospheric O(+) ions. The instability can directly generate the observed 100 Hz waves in the Venus mantle as well as the observed 730 Hz waves through the Doppler shift of the frequency caused by the satellite motion.

Huba, J. D.↗

NASA NTRS1987

Initial dynamic load estimates during configuration design

This analysis includes the structural response to shock and vibration and evaluates the maximum deflections and material stresses and the potential for the occurrence of elastic instability, fatigue and fracture. The required computations are often performed by means of finite element analysis (FEA) computer programs in which the structure is simulated by a finite element model which may contain thousands of elements. The formulation of a finite element model can be time consuming, and substantial additional modeling effort may be necessary if the structure requires significant changes after initial analysis. Rapid methods for obtaining rough estimates of the structural response to shock and vibration are presented for the purpose of providing guidance during the initial mechanical design configuration stage.

Schiff, Daniel↗

NASA NTRS2010

Instability of a Supersonic Boundary-Layer with Localized Roughness

A localized 3-D roughness causes boundary-layer separation and (weak) shocks. Most importantly, streamwise vortices occur which induce streamwise (low U, high T) streaks. Immersed boundary method (volume force) suitable to represent roughness element in DNS. Favorable comparison between bi-global stability theory and DNS for a "y-mode" Outlook: Understand the flow physics (investigate "z-modes" in DNS through sinuous spanwise forcing, study origin of the beat in DNS).

Marxen, Olaf↗

NASA NTRS1984

Collisionless Shock Waves in the Solar Terrestrial Environment

Natural shocks, magnetohydrodynamics, shock extensions, global asymmetries, shock profiles, quasiperpendicular supercritical process; ion dynamics and field structure, numerical simulations, ion reflection machinery, electrons, instabilities, phenomenology, and ion acceleration were described.

Source record↗

NASA NTRS1985

Plasma waves associated with the AMPTE artificial comet

Numerous plasma wave effects were detected by the AMPTE/IRM spacecraft during the artificial comet experiment on December 27, 1984. As the barium ion cloud produced by the explosion expanded over the spacecraft, emissions at the electron plasma frequency and ion plasma frequency provided a determination of the local electron density. The electron density in the diamagnetic cavity produced by the ion cloud reached a peak of more than 5 x 10 to the 5th per cu cm, then decayed smoothly as the cloud expanded, varying approximately as t exp-2. As the cloud began to move due to interactions with the solar wind, a region of compressed plasma was encountered on the upstream side of the diamagnetic cavity. The peak electron density in the compression region was about 1.5 x 10 to the 4th per cu cm. Later, a very intense (140 mVolt/m) broadband burst of electrostatic noise was encountered on the sunward side of the compression region. This noise has characteristics very similar to noise observed in the earth's bow shock, and is believed to be a shocklike interaction produced by an ion beam-plasma instability between the nearly stationary barium ions and the streaming solar wind protons.

Gurnett, D. A.↗

NASA NTRS1986

Electromagnetic ion beam instabilities - Comparison of oneand two-dimensional simulations

Numerical simulations of electromagnetic instabilities, driven by a cool tenuous ion beam propagating along an ambient magnetic field, have been conducted in one and two spatial dimensions. The calculations employ particle ions, fluid electrons, and a predictor corrector scheme for solving the electromagnetic field in two dimensions that is described in some detail. While the principal features of the one-dimensional calculations (which reproduce previous work) are retained, the two-dimensional simulations show some reduction of the overall level of the magnetic field fluctuations. Enhancement of the heating of the beam ions at the expense of the core ions also occurs in the case where the beam density is sufficiently large that the right-hand nonresonant instability dominates. Implications of the results for modeling of the ion foreshock and quasi-parallel shocks are discussed.

Winske, D.↗

NASA NTRS1991

Gamma-rays from cosmic ray interactions in supernova shells

A model is presented for the transport and interaction of cosmic rays accelerated by a pulsar and confined inside an expanding supernova remnant. Assuming that protons are accelerated at the reverse shock in the confined pulsar wind and convected into the shell via the Rayleigh-Taylor instability, the diffusion and interaction of these protons in the expanding envelope is modeled. The resulting gamma-ray flux is lower than previous estimates due primarily to the inclusion of proton adiabatic losses in the expanding pulsar wind. Energy-dependent diffusion causes the higher energy gamma-ray light curves to decay faster than those at lower energy. The predicted flux from SN1987A, for proton luminosity less than 10 exp 40 erg/s, is below the present detector sensitivities at both GeV and TeV energies, although supernovae occurring within the Galaxy may be detectable.

Harding, A. K.↗

NASA NTRS1994

Particle acceleration, X-rays, and gamma-rays from winds

The instability of the line-driven winds of hot stars leads to the formation of strong shocks. These shocks not only emit thermal X-rays, but also accelerate a small fraction of the thermal electrons and ions to relativistic energies. Synchrotron radiation from these energetic particles can account for the non-thermal radio emission observed from some hot stars, and can also explain the hard X-rays detected in the Einstein X-ray spectra. Our calculations indicate that the gamma-ray emission from non-thermal particles should be detectable by Gamma Ray Observatory (GRO). The detection (or non-detection) of these emissions over a wide energy range, from the radio to gamma-rays, should provide a great deal of information on the structure of the unstable winds and the physics of particle acceleration by shocks.

White, Richard L.↗

NASA NTRS1994

Particle Acceleration, X-Rays, and Gamma-Rays From Winds

The instability of the line-driven winds of hot stars leads to the formation of strong shocks. These shocks not only emit thermal X-rays, but also accelerate a small fraction of the thermal electrons and ions to relativistic energies. Synchrotron radiation from these energetic particles can account for the non-thermal radio emission observed from some hot stars, and can also explain the hard X-rays detected in the Einstein X-ray spectra. Our calculations indicate that the gamma-ray emission from non-thermal particles should be detectable by GRO. The detection (or non-detection) of these emissions over a wide energy range, from the radio to gamma-rays, should provide a great deal of information on the structure of the unstable winds and the physics of particle acceleration by shocks.

White, Richard L.↗

NASA NTRS2007

Simulation Study of Magnetic Fields Generated by the Electromagnetic Filamentation Instability

We have investigated the effects of plasma instabilities driven by rapid e(sup plus or minus) pair cascades, which arise in the environment of GRB sources as a result of back-scattering of a seed fraction of the original spectrum. The injection of e(sup plus or minus) pairs induces strong streaming motions in the ambient medium. One therefore expects the pair-enriched medium ahead of the forward shock to be strongly sheared on length scales comparable to the radiation front thickness. Using three-dimensional particle-in-cell simulations, we show that plasma instabilities driven by these streaming e(sup plus or minus) pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between e(sup plus or minus) pairs and ions, and may help explain the origin of large upstream fields in GRB shocks.

Nishikawa, K.-I.↗

NASA NTRS1986

The phase relationship between gyrophase-bunched ions and MHD-like waves

Recent calculations and computer simulations of the right-hand resonant ion beam instability have indicated that gyrophase-bunched ion distributions may be generated as a first step in the disruption of field-aligned beams. Theory predicts a distinct phase angle relationship between the gyrovelocity of the phase-bunched ions and the wave magnetic field. In this paper, observations of such a relationship between gyrophase-bunched ions and MHD-like waves upstream from the earth's bow shock are presented. The observed phases are in accord with the theoretical predictions of the right-hand resonant ion beam instability.

Fuselier, S. A.↗

NASA NTRS1993

Propagation characteristics of waves upstream and downstream of quasi-parallel shocks

The propagation characteristics of waves upstream and downstream of quasi-parallel shocks are investigated by using 2D hybrid simulations. At low Alfven Mach numbers, M(A) below about 2, the shock is initially associated with upstream phase-standing whistlers. At later times, backstreaming ions excite longer-wavelength whistlers via the right-hand resonant ion/ion instability. These waves propagate along the magnetic field at a group velocity no smaller than the upstream flow speed, so that the waves remain in the upstream region. At higher MA (above about 3), these waves are convected back into the shock, causing its reformation and downstream perturbations. Shock transmitted waves mode-convert into Alfven/ion-cyclotron waves which have a wave vector along the shock normal (pointing upstream) and convect downstream. The 2D simulation results confirm our earlier suggestion that the upstream waves should be field aligned, and that their convection into the downstream is associated with linear mode conversion into the Alfven/ion-cyclotron branch.

Krauss-Varban, D.↗

NASA NTRS1987

Simulation of amplitude-modulated circularly polarized Alfven waves for beta less than one

The nonlinear properties of the amplitude-modulated circularly polarized Alfven wave are studied for beta less than one. The temporal behavior of the wave packet of the electromagnetic hybrid simulation is compared with a numerical solution of the derivative nonlinear Schroedinger (DNLS) equation. It is shown that the left-hand-polarized mode evolves into a shocklike structure due to the modulational instability. However, both cyclotron damping and a snowplow effect near the steepened wave packet suppress its further steepening, contrary to the predictions of the DNLS equation. For the right-hand mode, formation of the shock does not take place, and the initial time development is well described by the DNLS equation. The daughter Alfven wave and ion acoustic waves are excited due to the decay instability at a later time. Heating or acceleration of the particles takes place for both left- and right-hand waves. Energy transfer from the wave to the particles occurs effectively when substantial modulation in the wave amplitude is present.

Machida, S.↗

NASA NTRS1997

Foreshock Langmuir Waves for Unusually Constant Solar Wind Conditions: Data and Implications for Foreshock Structure

Plasma wave data are compared with ISEE 1's position in the electron foreshock for an interval with unusually constant (but otherwise typical) solar wind magnetic field and plasma characteristics. For this period, temporal variations in the wave characteristics can be confidently separated from sweeping of the spatially varying foreshock back and forth across the spacecraft. The spacecraft's location, particularly the coordinate D(sub f) downstream from the foreshock boundary (often termed DIFF), is calculated by using three shock models and the observed solar wind magnetometer and plasma data. Scatterplots of the wave field versus D(sub f) are used to constrain viable shock models, to investigate the observed scatter in the wave fields at constant D(sub f), and to test the theoretical predictions of linear instability theory. The scatterplots confirm the abrupt onset of the foreshock waves near the upstream boundary, the narrow width in D(sub f) of the region with high fields, and the relatively slow falloff of the fields at large D(sub f), as seen in earlier studies, but with much smaller statistical scatter. The plots also show an offset of the high-field region from the foreshock boundary. It is shown that an adaptive, time-varying shock model with no free parameters, determined by the observed solar wind data and published shock crossings, is viable but that two alternative models are not. Foreshock wave studies can therefore remotely constrain the bow shock's location. The observed scatter in wave field at constant D(sub f) is shown to be real and to correspond to real temporal variations, not to unresolved changes in D(sub f). By comparing the wave data with a linear instability theory based on a published model for the electron beam it is found that the theory can account qualitatively and semiquantitatively for the abrupt onset of the waves near D(sub f) = 0, for the narrow width and offset of the high-field region, and for the decrease in wave intensity with increasing D(sub f). Quantitative differences between observations and theory remain, including large overprediction of the wave fields and the slower than predicted falloff at large D(sub f) of the wave fields. These differences, as well as the unresolved issue of the electron beam speed in the high-field region of the foreshock, are discussed. The intrinsic temporal variability of the wave fields, as well as their overprediction based on homogeneous plasma theory, are indicative of stochastic growth physics, which causes wave growth to be random and varying in sign, rather than secular.

Cairns, Iver H.↗

NASA NTRS1993

Studies on equatorial shock formation during plasmaspheric refilling

During the grant period starting August 1, 1992, our major effort has been on examining the presence of equatorially trapped hot plasma on plasmaspheric refilling. We performed one-dimensional PIC simulations of cold plasmas expanding into a hot plasma, consisting of hot anisotropic ions and warm isotropic electrons, trapped in a region of minimum magnetic field. Simulations showed that the electric potential barrier built up by the anisotropy of the hot ion population facilitates in the formation of electrostatic shocks when the cold ion beams begin to come into contact with the hot plasma. The shock formation occurs even when the cold ion beams are highly supersonic with respect to the ion-acoustic speed. This finding is interesting because equatorial shock formation during the early stage of plasmaspheric refilling has been debated over about two decades. In the past ion-ion instability has been invoked as the main mechanism for the coupling between the cold ion beams approaching the equator from the conjugate ionspheres. This coupling occurs when the beams are sufficiently slow; the beam velocity being less than three times the ion-acoustic speed. In the presence of hot plasma, the beams slow down by the potential barrier. The slowing down and the reflection process lead to the formation of the electrostatic shock even for highly supersonic ion beams. The mixing of hot and cold plasma was also studied.

Singh, Nagendra↗