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NASA NTRS1972

Universal instability associated with the plasmapause and its role in geomagnetic micropulsations.

The observed close correlation between the plasmapause and micropulsations is explained on the basis of a universal instability model. Both theoretical and experimental studies of a nonhomogeneous magnetoplasma indicate that a steep plasma density gradient at the plasmapause is likely the origin of the universal instability in the magnetosphere. Drift waves excited at the plasmapause may be unstable in the direction of the electron drift and propagate eastward nearly perpendicularly to the magnetic field. The drift waves, however, tend to convert very quickly to ion sound or Alfven waves with a much larger phase velocity parallel to the magnetic field. This may be a possible source mechanism for rather regular geomagnetic micropulsations, and specific mechanisms are identified for the long- and short-period cases.

Kikuchi, H.↗

NASA NTRS1979

Observations of ELF electromagnetic waves associated with equatorial spread F

Extreme low frequency electromagnetic waves have been observed below the F peak in the equatorial ionosphere by instruments onboard OGO-6. Electrostatic wave observations indicate that the steep gradient was unstable to the process which causes equatorial spread F above the region where the electromagnetic waves were observed. The data are very similar to observations near the polar cusp and give further evidence that ELF waves are excluded from regions of rapid and irregular density increases. Low level electromagnetic waves with similar properties were occasionally observed on the nightside by the OVI-17 electric field sensor and may be plasmaspheric hiss which has propagated to low altitude.

Kelley, M. C.↗

NASA NTRS1991

Plasma wave generation near the inner heliospheric shock

There is mounting evidence that the Voyager 1 and 2 and Pioneer 11 spacecraft may approach the inner (termination) heliospheric shock near the end of this century. It is argued here, by analogy with planetary bow shocks, that energetic electrons backstreaming from the heliospheric shock along the magnetic field should be unstable to the generation of Langmuir waves by the electron beam instability. Analytic expressions for the cutoff velocity, corresponding to the beam speed of the electrons backstreaming from the shock, are derived for a standard solar wind model. At the front side of the heliosphere the maximum beam velocity is expected to be at the meridian passing through the nose of the shock, which is assumed to be aligned with the Very Local Inter-Stellar Medium flow. This foreshock region and the associated Langmuir waves are relevant to both the expected in situ observations of the heliospheric boundaries, and to the low-frequency (2-3 kHz) radio emissions observed by the Voyager spacecraft in the outer heliosphere. Provided that these radio emissions are generated by Langmuir waves, the minimum Langmuir wave electric fields at the remote source are estimated to be greater than about 3 - 30 microV/m.

Macek, W. M.↗

NASA NTRS1988

Dynamical criterion for a marginally unstable, quasi-linear behavior in a two-layer model

A two-layer quasi-geostrophic flow forced by meridional variations in heating can be in regimes ranging from radiative equilibrium to forced geostrophic turbulence. Between these extremes is a regime where the time-mean (zonal) flow is marginally unstable. Using scaling arguments, it is concluded that such a marginally unstable state should occur when a certain parameter, measuring the strength of wave-wave interactions relative to the beta effect and advection by the thermal wind, is small. Numerical simulations support this proposal. A transition from the marginally unstable regime to a more nonlinear regime is then examined through numerical simulations with different radiative forcings. It is found that transition is not caused by secondary instability of waves in the marginally unstable regime. Instead, the time-mean flow can support a number of marginally unstable normal modes. These normal modes interact with each other, and if they are of sufficient amplitude, the flow enters a more nonlinear regime.

Ebisuzaki, W.↗

NASA NTRS1981

Energetic electrons and plasma waves associated with a solar type III radio burst

Detailed in situ observations from the ISEE 3 spacecraft of energetic electrons, plasma waves, and radio emission for the type II solar radio burst of February 17, 1979, are presented. The reduced, one-dimensional electron distribution function is constructed as a function of time. Since the faster electrons arrive before the slower ones, a bump on tail distribution forms which is unstable to the growth of Langmuir waves. The plasma wave growth computed from the distribution function agrees well with the observed onset of the Langmuir waves, and there is qualitative agreement between variations in the plasma wave levels and in the development of regions of positive slope in the function. The evolution of the function, however, predicts far higher plasma wave levels than those observed. The maximum levels observed are approximately equal to the threshold for nonlinear wave processes, such as oscillation two-stream instability and soliton collapse.

Lin, R. P.↗

NASA NTRS1983

Acceleration of hydrogen ions and conic formation along auroral field lines

Electrostatic ion cyclotron turbulence and the formation of ion conics at low altitudes (about 1500 km) along auroral field lines have been investigated analytically and by plasma numerical simulations. Ion cyclotron waves are assumed to be driven unstable by the upgoing cold ionospheric electrons associated with the downward auroral current. When the electron drift speed is comparable to the electron thermal speed, it was found that the large-amplitude (the saturation level is approximately equal to unity) coherent (omega equals the ion gyrofrequency) ion cyclotron waves should exist along auroral field lines at low altitudes extending a few hundred kilometers. Ion conics are associated with ion cyclotron turbulence, and the ion bulk temperature is found to increase by a factor of 10 from the initial ionospheric temperature, while the temperature of the high-energy tail can be as much as 100 times the ionospheric temperature. Theory and simulations are in good agreement.

Okuda, H.↗

NASA NTRS1993

Electron generation of electrostatic waves in the plasma sheet boundary layer

Broadband electrostatic noise (BEN) has been shown to occur in conjunction with ion beams; extensive investigations of possible ion beam-related instabilities that could generate the observed wave spectra have been conducted. It has also been demonstrated that unstable electron distribution functions are sometimes measured in the plasma sheet boundary layer. We present simultaneous observations of ion and electron distribution functions and electric field wave spectra measured by ISEE 1 and ISEE 2 in the Earth's magnetotail. As the spacecraft moved from the tail lobe toward the plasma sheet, the fast indication of boundary layer plasma was seen in the electron distributions, followed some minutes later by the detection of boundary layer ions. The onset of large-amplitude electrostatic waves at frequencies up to the electron plasma frequency was coincident with the onset of the boundary layer electrons, suggesting that broadband electrostatic waves may often be generated by unstable electron distributions in the plasma sheet boundary layer, particularly the higher frequency portion of the wave spectrum. The observed changes in the electron distribution functions indicate that the plasma was not heated locally by the waves.

Onsager, T. G.↗

NASA NTRS1987

A theory for low-frequency waves observed at comet Giacobini-Zinner

As the ICE spacecraft approached comet Giacobini-Zinner, low-frequency waves in two frequency ranges were observed in the magnetic field data. Under certain conditions, a gyrating beam of water group ions is capable of generating electromagnetic waves with periods near 100 s (in the spacecraft frame of reference) and shorter-period whistler branch waves with periods near 3 s. The 100-s wave can arise from either a nonresonant firehose instability or a resonant beam driven instability so long as the angle between the directions of the local magnetic field and solar wind velocity is less than 90 percent. Whenever that angle exceeds about 30 percent, the gyrating ring distribution formed by newly ionized water is also unstable to the generation of whistler waves with 3-s periods.

Goldstein, M. L.↗

NASA NTRS1980

Nonradial m-mode changes in the 53 Persei variable 22 Orionis

The paper considers nonradial m-mode changes in the 53 Persei variable 22 Orionis. The Orionis observations reveal periodic profile variations easily modeled in terms of traveling-wave nonradial pulsations; like other members of the 53 Persei class 22 Ori exhibit unstable mode amplitudes. During the 1976-1977 seasons retrograde wave oscillations of period 4.5 and 9.0 hours were observed, but during 1978-1979 larger amplitude, prograde oscillations of period 14.1 and 22.9 hours were observed. It is suggested that this retrograde to prograde change is explained by the nonlinear coupling of two sectorial modes to a third, standing wave mode.

Smith, M. A.↗

NASA NTRS1986

Simultaneous rocket and MST radar observation of an internal gravity wave breaking in the mesosphere

In June, 1983, the Structure and Atmospheric Turbulence Environment (STATE) rocket and Poker Flat Mesophere-Stratosphere-Troposphere radar campaign was conducted to measure the interaction between turbulence, electron density and electron density gradient that has produced unusually strong MST radar echoes from the summer mesosphere over Poker Flat, Alaska. Analysis or radar wind measurements and a concurrent wind and temperature profile obtained from a rocket probe carrying a three-axis accelerometer are given. The two data sets provide a fairly complete (and in some cases, redundant) picture of the breaking (or more correctly, the saturation) of a large-amplitude, low-frequency, long-wavelength internal gravity wave. The data show that small-scale turbulence and small-scale wave intensity is greatest at those altitudes where the large-scale wave-induced temperature lapse rate is most negative or most nearly unstable, but the wind shear due to the large-scale wave is a minimum. A brief review of linear gravity-wave theory is presented as an aid to the identification of the gravity-wave signature in the radar and rocket data. Analysis of the time and height cross sections of wind speed and turbulence intensity observed by the Poker Flat MST radar follows. Then, the vertical profile of temperature and winds measured by a rocket probe examined. Finally, the use of the independent data sets provided by the rocket and the radar are discussed and implications for theories of wave saturation are presented.

Smith, S. A.↗

NASA NTRS1987

Evolution of polarization in localized nonlinear Alfven waves

A polarization change of localized and finite-amplitude Alfven waves is found by use of both MHD theory and numerical simulation. One of the transverse components of the circularly polarized Alfven wave is stable as it propagates, but the other component is unstable under both self-focusing and diffraction effects. Consequently, the wave changes its polarization from circular to linear.

Hoshino, Masahiro↗

NASA NTRS1988

Evolution of nonlinear polarization in localized and finite amplitude Alfven waves

Theoretical and computational study for a polarization change of localized and finite amplitude Alfven waves propagating parallel to an applied magnetic field is presented using both reductive perturbation theory and numerical simulations. In the magnetohydrodynamic limit, where right-hand and left-hand circularly polarized waves are degenerate, one of the transverse components of the circularly polarized Alfven wave is stable as it propagates, but the other component is unstable to either self-focusing or diffraction effects. Consequently, the wave changes its polarization from circular to linear. In the high frequency regime, where two circularly polarized waves (right-hand and left-hand circularly polarized waves) are not degenerate, two transverse components of the circularly polarized Alfven wave are strongly coupled to each other, and there is almost no polarization change.

Hoshino, M.↗

DOE OSTI2020

Different responses of the Rayleigh–Taylor type and resistive drift wave instabilities to the velocity shear

The effects of velocity shear on the unstable modes driven by the effective gravity (Rayleigh–Taylor and interchange) and resistive drift wave instabilities for inhomogeneous equilibrium fluid/plasma density are analyzed for the localized eigenmode problems. It is shown that the effect of the velocity shear drastically depends on the type of instability. Whereas the velocity shear can significantly suppress both Rayleigh–Taylor and interchange instabilities, it has only a weak impact on the growth rate of the resistive drift wave. This is directly related to the physical nature of these instabilities. For the Rayleigh–Taylor and interchange instabilities, the shear flow tilts the eddies of the stream functions, while for the resistive drift wave instability the shear flow simply shifts the eddies in the radial direction with no tilting. However, for a large velocity shear, the eigenmode solutions for resistive drift waves cease to exist.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2024

Explosively driven Richtmyer–Meshkov instability jet suppression and enhancement via coupling machine learning and additive manufacturing

The ability to control the behavior of fluid instabilities at material interfaces, such as the shock-driven Richtmyer–Meshkov instability, is a grand technological challenge with a broad number of applications ranging from inertial confinement fusion experiments to explosively driven shaped charges. In this work, we use a linear-geometry shaped charge as a means of studying methods for controlling material jetting that results from the Richtmyer–Meshkov instability. A shaped charge produces a high-velocity jet by focusing the energy from the detonation of high explosives. The interaction of the resulting detonation wave with a hollowed cavity lined with a thin metal layer produces the unstable jetting effect. By modifying the characteristics of the detonation wave prior to striking the lined cavity, the kinetic energy of the jet can be enhanced or reduced. Modifying the geometry of the liner material can also be used to alter jetting properties. We apply optimization methods to investigate several design parameterizations for both enhancing or suppressing the shaped-charge jet. This is accomplished using 2D and 3D hydrodynamic simulations to investigate the design space that we consider. We also apply new additive manufacturing methods for producing the shaped-charge assemblies, which allow for the experimental testing of complicated design geometries obtained through computational optimization. We present a direct comparison of our optimized designs with experimental results carried out at the High Explosives Application Facility at Lawrence Livermore National Laboratory.

36 MATERIALS SCIENCE↗

NASA NTRS1994

On the Behavior of Three-dimensional Wave Packets in Viscously Spreading Mixing Layers

We consider analytically the evolution of a three-dimensional wave packet generated by an impulsive source in a mixing layer. The base flow is assumed to be spreading due to viscous diffusion. The analysis is restricted to small disturbances (linearized theory). A suitable high-frequency ansatz is used to describe the packet; the key elements of this description are a complex phase and a wave action density. It is found that the product of this density and an infinitesimal material volume convecting at the local group velocity is not conserved: there is a continuous interaction between the base flow and the wave action. This interaction is determined by suitable mode-weighted averages of the second and fourth derivatives of the base-flow velocity profile. Although there is some tendency for the dominant wave number in the packet to shift from the most unstable value toward the neutral value, this shift is quite moderate. In practice, wave packets do not become locally neutral in a diverging base flow (as do instability modes), therefore, they are expected to grow more suddenly than pure instability modes and do not develop critical layers. The group velocity is complex; the full significance of this is realized by analytically continuing the equations for the phase and wave action into a complex domain. The implications of this analytic continuation are discussed vis-a-vis the secondary instabilities of the packet: very small scale perturbations on the phase can grow very rapidly initially, but saturate later because most of the energy in these perturbations is convected away by the group velocity. This remark, as well as the one regarding critical layers, has consequences for the nonlinear theories.

Balsa, Thomas F.↗

NASA NTRS1991

Consequences of wave-particle interactions on chaotic acceleration

The recent model of Ashour-Abdalla et al. (1991) has proposed that the earth's plasma sheet can be formed by chaotic acceleration in a magnetotail-like field configuration. The ion velocity distributions created by chaotic acceleration have unstable features and represent robust free energy sources for kinetic plasma waves that can modify the original distributions. In the plasma sheet boundary layer, field-aligned ion beamlets are formed which drive a host of instabilities creating a broadbanded noise spectrum and cause thermal spreading of the beamlets. In addition, there is strong heating of any cold background plasma that may be present. In the central plasma sheet, ion antiloss cone distributions are created which are unstable to very low frequency waves that saturate by filling the antiloss cone.

Schriver, David↗

NASA NTRS1983

A Study of Accretion in Compact Sources: Disk Precession and Jet Helicity

The effect of adiabatic expansion on the propagation and growth of helical twisting on a supersonic jet is investigated. Cooling of the jet material increases the jet's Mach number and increases the jet's density relative to that of the external medium. This has the effect of decreasing the maximum rate of growth and increasing the maximally unstable wavelength relative to the jet's radius. Propagation effects cause the wavelength of helical waves to change at a rate different from that of the maximally unstable wavelength with the result that the characteristic wavelength of helical twisting is not equal to this wavelength. The most rapidly growing helical wave will have a wavelength as much as a factor of 2 different from the instantaneous maximally unstable wavelength.

Hardee, P. E.↗

DOE OSTI2023

Gravitational Waves from Rapid Structure Formation on Microscopic Scales before Matter-Radiation Equality

We propose a new scenario for the formation of asteroid-mass primordial black holes (PBHs). Our mechanism is based on the annihilation of the string-wall network associated with thebreaking of a U(1) global symmetry into a discrete ZN symmetry. If the potential hasmultiple local minima (N > 1) the network is stable, and the annihilation is guaranteed by a biasamong the different vacua. The collapse of the string-wall network is accompanied bycatastrogenesis, a large production of pseudo-Goldstone bosons (pGBs) — e.g. axions,ALPs, or majorons — gravitational waves, and PBHs. If pGBs rapidly decay into products thatthermalize, as predicted e.g. in the high-quality QCD axion and heavy majoron models, they do notcontribute to the dark matter population, but we show that PBHs can constitute 100% of the darkmatter. The gravitational wave background produced by catastrogenesis with heavy unstable axions,ALPs, or majorons could be visible in future interferometers.

79 ASTRONOMY AND ASTROPHYSICS↗