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NASA NTRS1985

Estimation and comparison of quasilinear electron heating in the shock foot at Jupiter and earth

A simple quasilinear model is developed to estimate the electron heating that occurs in the foot of a supercritical, quasiperpendicular shock through interactions with electrostatic waves generated by reflected ions. At earth the increase in electron thermal energy calculated using the measured wave amplitudes is negligible, while at Jupiter it is comparable with the observed temperature gain across the shock. The anisotropic quasilinear heating should destabilize whistler mode waves in the foot. These have been detected by the plasma wave instrument on Voyager with amplitudes sufficient to isotropize the electron distribution.

Moses, S. L.↗

NASA NTRS2012

A New Paradigm for Supergranulation Derived from Large-Distance Time-Distance Helioseismology: Pancakes

With large separations (10-24 deg heliocentric), it has proven possible to cleanly separate the horizontal and vertical components of supergranular flow with time-distance helioseismology. These measurements require very broad filters in the k-$\omega$ power spectrum as apparently supergranulation scatters waves over a large area of the power spectrum. By picking locations of supergranulation as peaks in the horizontal divergence signal derived from f-mode waves, it is possible to simultaneously obtain average properties of supergranules and a high signal/noise ratio by averaging over many cells. By comparing ray-theory forward modeling with HMI measurements, an average supergranule model with a peak upflow of 240 m/s at cell center at a depth of 2.3 Mm and a peak horizontal outflow of 700 m/s at a depth of 1.6 Mm. This upflow is a factor of 20 larger than the measured photospheric upflow. These results may not be consistent with earlier measurements using much shorter separations (<5 deg heliocentric). With a 30 Mm horizontal extent and a few Mm in depth, the cells might be characterized as thick pancakes.

Duvall, Thomas L.↗

NASA NTRS1989

Excitation of kinetic Alfven waves by resonant mode conversion and longitudinal heating of magnetized plasmas

The excitation of the kinetic Alfven wave by resonant mode conversion and longitudinal heating of the plasma by the kinetic Alfven wave were demonstrated on the basis of a macroscale particle simulation. The longitudinal electron current was shown to be cancelled by the ions. The kinetic Alfven wave produced an ordered motion of the plasma particles in the wave propagation direction. The electrons were pushed forward along the ambient magnetic field by absorbing the kinetic Alfven wave through the Landau resonance.

Tanaka, Motohiko↗

NASA NTRS2017

Wave-Particle Energy Exchange Directly Observed in a Kinetic Alfven-Branch Wave

Alfven waves are fundamental plasma wave modes that permeate the universe. At small kinetic scales they provide a critical mechanism for the transfer of energy between electromagnetic fields and charged particles. These waves are important not only in planetary magnetospheres, heliospheres, and astrophysical systems, but also in laboratory plasma experiments and fusion reactors. Through measurement of charged particles and electromagnetic fields with NASAs Magnetospheric Multiscale (MMS) mission, we utilize Earths magnetosphere as a plasma physics laboratory. Here we confirm the conservative energy exchange between the electromagnetic field fluctuations and the charged particles that comprise an undamped kinetic Alfven wave. Electrons confined between adjacent wave peaks may have contributed to saturation of damping effects via non-linear particle trapping. The investigation of these detailed wave dynamics has been unexplored territory in experimental plasma physics and is only recently enabled by high-resolution MMS observations.

magnetohydrodynamic↗

NASA NTRS1984

Tropical response to lateral forcing with a latitudinally and zonally nonuniform basic state

The dynamics of lateral forcing of a stable tropical atmosphere by incident waves from the midlatitudes are examined. Attention is focused on conditions where the WKB method is invalid, and only given to barotropic flows. The tropical atmosphere is treated as a steady planetary wave and a zonal shear flow. The 200 mb level is investigated in terms of a midlatitude wave with self-compatible parametric values that is constrained to satisfy the radiation conditions at the boundary. Wave modes are found to be susceptible to trapping between two turning points in the equatorial zone in the presence of a zonal shear flow. Quasi-resonant waves can then appear over a wide range of parametric conditions of the forcing wave and the basic state. The zonal wind structure determines the resonance conditions.

Wilson, J. D.↗

NASA NTRS1995

Study of nonlinear MHD equations governing the wave propagation in twisted coronal loops

The solar corona, modelled by a low beta, resistive plasma slab, sustains MHD wave propagations due to shearing footpoint motions in the photosphere. By using a numerical algorithm the excitation and nonlinear development of MHD waves in twisted coronal loops are studied. The plasma responds to the footpoint motion by sausage waves if there is no twist. The twist in the magnetic field of the loop destroys initially developed sausage-like wave modes and they become kinks. The transition from sausage to kink modes is analyzed. The twist brings about mode degradation producing high harmonics and this generates more complex fine structures. This can be attributed to several local extrema in the perturbed velocity profiles. The Alfven wave produces remnants of the ideal 1/x singularity both for zero and non-zero twist and this pseudo-singularity becomes less pronounced for larger twist. The effect of nonlinearity is clearly observed by changing the amplitude of the driver by one order of magnitude. The magnetosonic waves also exhibit smoothed remnants of ideal logarithmic singularities when the frequency of the driver is correctly chosen. This pseudo-singularity for fast waves is absent when the coronal loop does not undergo any twist but becomes pronounced when twist is included. On the contrary, it is observed for slow waves even if there is no twist. Increasing the twist leads to a higher heating rate of the loop. The larger twist shifts somewhat uniformly distributed heating to layers inside the slab corresponding to peaks in the magnetic field strength.

Parhi, S.↗

NASA NTRS1986

On the applicability of relativistic dispersion to auroral zone electron distributions

An examination is made to identify appropriate times to include weakly relativistic wave dispersion in calculations of auroral zone electron distributions. Two dispersion relations are examined. First, solutions are found for a ring distribution of particles around a cold ambient plasma. The solutions are found only for modes near the electron gyrofrequency. A trapped mode which is decoupled from the freely propagating R-X mode of cold plasmas is found to be the most unstable mode. Wave dispersion is examined with a spherically symmetric Dory-Guest-Harris (DGH) distribution, which describes a shell distribution that has a peak momentum larger than the thermal spread. The trapped mode is again found to be the most unstable. The analysis indicates it is beneficial to include relativistic effects whenever calculating auroral plasma distributions, and that a simple model will have sufficient accuracy.

Strangeway, R. J.↗

NASA NTRS1992

A global traveling wave on Venus

The dominant large-scale pattern in the clouds of Venus has been described as a 'Y' or 'Psi' and tentatively identified by earlier workers as a Kelvin wave. A detailed calculation of linear wave modes in the Venus atmosphere verifies this identification. Cloud feedback by infrared heating fluctuations is a plausible excitation mechanism. Modulation of the large-scale pattern by the wave is a possible explanation for the Y. Momentum transfer by the wave could contribute to sustaining the general circulation.

Smith, Michael D.↗

NASA NTRS1993

A global traveling wave on Venus

Smith, Michael D.↗

NASA NTRS1997

Ultrasonic Characterization of Flaws in Composites Using Plate Waves Dispersion Data

Leaky Lamb wave (LLW) propagation in composite materials has been studied extensively since it was first observed in 1982. The wave is induced using a pitch-catch arrangement and the plate wave modes are detected by searching minima in the reflected spectra.

Composites Leaky Lamb wave composite materials↗

NASA NTRS1990

The spatial distribution of p-mode absorption in active regions

The interaction of solar p-mode waves and active regions has been the subject of recent observational and theoretical investigations. Observations show that up to one-half of the power of incident high-degree acoustic may be absorbed in and around sunspots. In this paper the horizontal spatial distribution of high-degree p-mode absorption in solar active regions is explored. An appropriate Fourier-Hankel transform can be used to detect the mean absorption of waves passing through any given point on the solar surface. By repeating the analysis at multiple positions a map of the absorption can be constructed. A technique for optimal computation of absorption maps is developed and applied to observations of several active regions and an area of quiet sun near disk center. By comparing the distribution of p-mode absorption with magnetograms and line-wing intensity images, it is directly observed that the absorption is not limited to the location of the visible sunspots but is also associated with magnetic fields in the surrounding plage. It is estimated that the absorption efficiency scales roughly with the magnetic flux density, although the absorption appears to saturate inside the strongest fields.

Braun, D. C.↗

NASA NTRS1981

Upstream hydromagnetic waves and their association with backstreaming ion populations - ISEE 1 and 2 observations

Measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock. The reflected ions, which are confined to a narrow energy and angular range, are accompanied by small amplitude (less than approximately 1/2 gamma peak to peak) left-handed waves at frequencies close to 1 Hz in the spacecraft frame. Diffuse backstreaming particles with a broad energy spectrum are associated with low frequency (approximately 30-s period), large amplitude (approximately 5 gamma peak to peak) waves. Intermediate particles are associated with a mixture of these two wave types. Often the waves associated with the diffuse beams steepen as if they were minishocks. The leading edge (trailing edge in the spacecraft frame) frequently appears to break up into a whistler mode wave packet. These discrete wave packets are right-hand polarized and have frequencies from below the proton gyrofrequency to well above it in the plasma frame and are blown back towards the earth by the solar wind.

Hoppe, M. M.↗

NASA NTRS1983

Effect of flow on the acoustic performance of extended reaction lined ducts

A model is developed for the effects of uniform and boundary-layer mean flow on the attenuation and propagation of harmonically excited sound waves in an extended reaction lined cylindrical duct. A duct geometry consisting of an annular outer region of bulk material surrounding an inner cylinder of air is utilized. A numerical solution is obtained for the coupled wave equations governing the motion of the sound in both the inner and annular regions. It is found that the numerically predicted attenuation and propagations constants are in excellent agreement with measured values using Kevlar as the liner material for plane-wave mode (O,O) excitation over a wide range of mean flows and sound frequency. The boundary-layer effects are determined to be unimportant, at least for plane-wave sound. In addition, numerical studies indicate small differences between the use of either the radial velocity or the radial displacement boundary conditions.

Hersh, A. S.↗

NASA NTRS1985

Ray tracing of Z-mode emissions from source regions in the high-altitude auroral zone

Two-dimensional ray tracing of Z-mode radiation from sources in the auroral zone has been performed. The Z-mode waves are assumed to be excited by the cyclotron maser mechanism at frequencies near the electron gyrofrequency and at wave normal angles near 90 deg. The emission is found to propagate primarily perpendicular to the magnetic field line (horizontal propagation) for all sources considered, and thus not to substantially lower altitudes. The frequency bandwidth estimated from the ray paths is much less than that observed by DE-1. It is suggested that the wave source needs to be in the region of field-aligned current in order for the cyclotron maser mechanism to explain the broad frequency bandwidth of Z-mode emissions.

Menietti, J. D.↗

NASA NTRS1987

The emission of narrow-band Jovian kilometric radiation

A model based on the nonlinear coupling of electrostatic plasma waves is proposed to explain the emission of the narrow-band Jovian kilometric radiation (nKOM) observed by the Voyager spacecraft. It is shown that upper-hybrid branch electrostatic waves propagating through the inhomogeneities in the outer periphery of the Io plasma torus can attain the proper geometry for localized upconversion interactions leading to pump depletion. Plasma waves propagating into a weak density gradient and reflected at the critical layer interact with the incident waves leading to the electromagnetic emission, which is beamed at large angles with respect to the background magnetic field. In general, both L-O and R-X mode waves can be generated. The observed power and net polarization (L-O) are consistent with pump depletion of electrostatic waves at a level of about 10 mV/m. A possible excitation mechanism for the electrostatic waves is also discussed.

Fung, S. F.↗

NASA NTRS1987

Plasma wave measurements in the magnetosphere of Uranus

As Voyager 2 traversed the magnetosphere of Uranus, the plasma-wave instrument detected very significant phenomena related to local wave-particle interactions, radio emissions, and dust impacts. Here the region between the inbound and outbound bow shock traversals (covering a distance of more than 250 Uranus radii) is considered, and it is demonstrated that intense plasma-wave activity developed only in the inner magnetosphere (r less than 12 Uranus radii); this result is similar to that found at Saturn, but it is in marked contrast with the Jupiter case, where very strong wave activity was detected out to distances of 250 Jupiter radii. The Uranus plasma-wave observations in the inner magnetosphere are compared with corresponding results from the Jupiter and Saturn encounters, and it is shown that the Uranus wave measurements are unique in several significant ways. These new aspects include: (1) the detection of a marked inbound-outbound asymmetry and (2) the detection of whistler-mode waves that yield the strongest wave-particle interactions found in outer planet magnetospheres.

Scarf, F. L.↗

NASA NTRS1989

On the role of the energy of suprathermal electrons in the generation of auroral kilometric radiation

The relativistic dispersion equation based on the cyclotron maser theory for a DGH energetic electron distribution is examined in order to identify and understand the physical conditions under which fundamental O mode and second harmonic X mode radiations can dominate over fundamental X mode emission in low density auroral kilometric radiation source regions. It is found that the energy of the auroral electrons can play a significant role in determining the dominant wave mode. The temporal and spatial growth rates of both the fundamental O mode as well as the second harmonic X mode remain high for energies as low as several hundred eV of the suprathermal electrons, while the fundamental X mode is suppressed for energies no higher than approximately 1 keV due to its relativistic resonance condition.

Wong, H. K.↗

NASA NTRS1991

ULF waves upstream of the Venus bow shock - Properties of one-hertz waves

Pioneer Venus Orbiter data are used here to study the properties of a class of ULF upstream waves with relatively high observed frequencies. These waves show significant similarity to 'one-Hz' waves identified at earth in the ISEE 1 and 2 observations and the whistler waves identified earlier by IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The wave amplitude decreases with distance from the shock measured along the magnetic field line. Group velocities calculated using the cold plasma dispersion relation indicate that the waves have sufficient upstream velocities to propagate form the shock into the solar wind. The totality of observations seem best explained by a source of right-handed whistler mode waves at the bow shock.

Orlowski, D. S.↗