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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

The energy associated with MHD waves generation in the solar wind plasma

Gyrotropic symmetry is usually assumed in measurements of electron distribution functions in the heliosphere. This prevents the calculation of a net current perpendicular to the magnetic field lines. Previous theoretical results derived by one of the authors for a collisionless plasma with isotropic electrons in a strong magnetic field have shown that the excitation of MHD modes becomes possible when the external perpendicular current is non-zero. We consider then that any anisotropic electron population can be thought of as 'external', interacting with the remaining plasma through the self-consistent electromagnetic field. From this point of view any perpendicular current may be due to the anisotropic electrons, or to an external source like a stream, or to both. As perpendicular currents cannot be derived from the measured distribution functions, we resort to Ampere's law and experimental data of magnetic field fluctuations. The transfer of energy between MHD modes and external currents is then discussed.

delaTorre, A.↗
NASA NTRS1984

The adiabatic energy change of plasma electrons and the frame dependence of the cross-shock potential at collisionless magnetosonic shock waves

The adiabatic energy gain of electrons in the stationary electric and magnetic field structure of collisionless shock waves was examined analytically in reference to conditions of the earth's bow shock. The study was performed to characterize the behavior of electrons interacting with the cross-shock potential. A normal incidence frame (NIF) was adopted in order to calculate the reversible energy change across a time stationary shock, and comparisons were made with predictions made by the de Hoffman-Teller (HT) model (1950). The electron energy gain, about 20-50 eV, is demonstrated to be consistent with a 200-500 eV potential jump in the bow shock quasi-perpendicular geometry. The electrons lose energy working against the solar wind motional electric field. The reversible energy process is close to that modeled by HT, which predicts that the motional electric field vanishes and the electron energy gain from the electric potential is equated to the ion energy loss to the potential.

Goodrich, C. C.↗
NASA NTRS1986

Comparative energetics of the observed and simulated global circulation during the special observing periods of FGGE

Energetics of the observed and simulated global circulation are evaluated in the zonal spectral domain for the special observing periods of FGGE. The study utilizes GLA analyses of FGGE observational data and parallel simulation experiments. There are noticeable differences in energy transformations between the observation and simulation during SOP-1. These include the baroclinic conversion C(n) by the zonal mean motion and short-wave disturbances, and the nonlinear wave-wave interaction L(n) at the long and short waves. The energy transformations of the short-wave disturbances are much more intense in the simulated circulation than in the observation. However, good agreement is noted in the conversion and dissipation of kinetic energy in the large- and cyclone-wave range n = 1-10. Spectral distributions of global energy transformations at the long- and cyclone-wave range indicate that the SOP-2 simulation agrees more closely with the observed fields than the SOP-1 simulation. Other pertinent points of energetics diagnosis are also included in the discussion.

Kung, E. C.↗
NASA NTRS1986

Star of Lima - Overview and optical diagnostics of a barium Alfven critical velocity experiment

The Alfven critical velocity mechanism for ionization of a neutral gas streaming across the magnetic field has been demonstrated in laboratory experiments. In March 1983, two rocket-borne experiments with Ba and Sr tested the effect in the wall-less laboratory of space from Punto Lobos, Peru, near 430 km altitude. 'Star of Lima' used a conical Ba shaped charge aimed at an instrument payload about 2 km away. Because of rocket overperformance the detonation occurred in partial sunlight, so that less than 21.6 percent of the ionizing UV was present. Particle and field measurements indicate the production of hot electrons and waves in the energy and frequency range that are respectively predicted to produce a cascade of ionization by the Alfven mechanism. However, the ionization fluxes and wave energy density did not reach cascade levels, and optical observations indicate that only 2.5 to 5 x 10 to the 20th Ba ions were produced. A substantial portion and perhaps all of the ionization could have been produced by solar UV. The failure of the Alfven process in this experiment is not well understood.

Wescott, E. M.↗
NASA NTRS1974

Energy densities of Alfven waves between 0.7 and 1.6 AU

Plasma and field data from Mariner 4 and 5 between 0.7 and 1.6 AU are used to study the radial dependence of the levels of microscale fluctuation associated with interplanetary Alfven waves. The observed decrease of these levels with increasing distance from the sun is consistent with little or no local generation or damping of the ambient Alfven waves over this range of radial distance.

Belcher, J. W.↗
NASA NTRS1991

Strong plasma turbulence in the earth's electron foreshock

A quantitative model is developed to account for the distribution in magnitude and location of the intense plasma waves observed in the earth's electron foreshock given the observed rms levels of waves. In this model, nonlinear strong-turbulence effects cause solitonlike coherent wave packets to form and decouple from incoherent background beam-excited weak turbulence, after which they convect downstream with the solar wind while collapsing to scales as short as 100 m and fields as high as 2 V/m. The existence of waves with energy densities above the strong-turbulence wave-collapse threshold is inferred from observations from IMP 6 and ISEE 1 and quantitative agreement is found between the predicted distribution of fields in an ensemble of such wave packets and the actual field distribution observed in situ by IMP 6. Predictions for the polarization of plasma waves and the bandwidth of ion-sound waves are also consistent with the observations. It is shown that strong-turbulence effects must be incorporated in any comprehensive theory of the propagation and evolution of electron beams in the foreshock. Previous arguments against the existence of strong turbulence in the foreshock are refuted.

Robinson, P. A.↗
NASA NTRS1994

Discretized energy minimization in a wave guide with point sources

An anti-noise problem on a finite time interval is solved by minimization of a quadratic functional on the Hilbert space of square integrable controls. To this end, the one-dimensional wave equation with point sources and pointwise reflecting boundary conditions is decomposed into a system for the two propagating components of waves. Wellposedness of this system is proved for a class of data that includes piecewise linear initial conditions and piecewise constant forcing functions. It is shown that for such data the optimal piecewise constant control is the solution of a sparse linear system. Methods for its computational treatment are presented as well as examples of their applicability. The convergence of discrete approximations to the general optimization problem is demonstrated by finite element methods.

Propst, G.↗
NASA NTRS2020

Solar Wind Turbulence Around Mars: Relation Between The Energy Cascade Rate And The Proton Cyclotron Waves Activity

The first estimation of the incompressible energy cascade rate at magnetohydrodynamic (MHD) scales in the plasma upstream of the Martian bow shock is obtained, making use of magnetic field and plasma observations provided by Mars Atmosphere and Volatile EvolutioN (MAVEN) over 600 orbits. In particular, the energy cascade rate is computed for events with and without proton cyclotron wave (PCW) activity, for time intervals when MAVEN was in the solar wind with no magnetic connection to the bow shock. It is shown that the nonlinear cascade of energy at the MHD scales is slightly amplified when PCWs are present in the plasma, around the Martian perihelion. In addition, the analysis of the normalized cross helicity and residual energy for the turbulent fluctuations shows the presence of Alfvenic and non-Alfvenic fluctuations in a magnetic dominant regime for the majority of the cases.

solar wind↗
NASA NTRS1980

Observations of a free-energy source for intense electrostatic waves

Significant progress has been made in understanding intense electrostatic waves near the upper hybrid resonance frequency in terms of the theory of multiharmonic cyclotron emission using a classical loss-cone distribution function as a model. Recent observations by Hawkeye 1 and GEOS 1 have verified the existence of loss-cone distributions in association with the intense electrostatic wave events, however, other observations by Hawkeye and ISEE have indicated that loss cones are not always observable during the wave events, and in fact other forms of free energy may also be responsible for the instability. Now, for the first time, a positively sloped feature in the perpendicular distribution function has been uniquely identified with intense electrostatic wave activity. Correspondingly, we suggest that the theory is flexible under substantial modifications of the model distribution function.

Kurth, W. S.↗
NASA NTRS1985

Electrostatic ion-cyclotron instability caused by a nonuniform electric field perpendicular to the external magnetic field

A new mechanism that can destablize kinetic ion-cyclotron waves in the presence of a nonuniform electric field perpendicular to the uniform ambient magnetic field is given. In the absence of the electric field, the mode energy is positive, while in the presence of a uniform electric field the mode energy could be negative. However, when the electric field is nonuniform, it is possible for a finite region to be of negative wave energy surrounded by regions of positive wave energy. A nonlocal wave packet couples the two regions so that a flow of energy from the region of negative wave energy to the region of positive wave energy will cause the mode to grow. This gives rise to the instability.

Ganguli, G.↗
NASA NTRS2018

Sea State Based Estimation of White Cap Fraction: Implications for Primary Marine Aerosol Fluxes

Oceanic whitecaps (hereafter, W) or the characteristic whiteness of the sea foam is an important feature for predicting exchange of gases, sea spray aerosols (SSAs), heat and momentum transfer between the ocean and the atmosphere at the air-sea interface. Due to its increased surface emission and brightness temperature, whitecaps are critical for satellite retrievals of ocean albedo, ocean color, ocean surface wind vectors from satellite borne radiometer and microwave instruments. Most of the existing models predict W using wind speed and sea surface temperature (SST). However, numerous publications have pointed out that there are large uncertainties in the predicted W and using parameterizations based on wind-wave state can improve the precision of the predicted W. Here, we integrate the University of Miami Wave Model - 2.0 (UMWM) in Goddard Earth Observing System (GEOS) and use wave diagnostics to predict W. We choose the year 2006 for our global UMWM/GEOS runs because of the availability of W dataset from satellite observations. We run UMWM/GEOS at 0.5o x 0.5o by replaying to MERRA2 meteorology and evaluate the wave diagnostics using measurements from fixed buoys and satellite altimeters. We use three different parameterizations for W based on: 1) Reynolds number, 2) wave dissipation energy, and 3) volume of air entrained by breaking waves. We compare our results of W with previous studies and also with the satellite based observational dataset. Predicting W is important for understanding the processes at the air-sea interface. Therefore, this work is a step further in improving the uncertainties in the aerosol and atmospheric chemistry modules of the global models.

Oceanic Whitecaps↗
NASA NTRS1993

Alfven wave transport effects in the time evolution of parallel cosmic-ray-modified shocks

This paper presents a numerical study of the time evolution of plane, cosmic-ray modified shocks with magnetic field parallel to the shock normal, based on the diffusive shock acceleration formalism and including the effects from the finite propagation speed and energy of Alfven waves responsible for controlling the transport of the cosmic rays. The simulations discussed are based on a three-fluid model for the dynamics, but a more complete formalism is laid out for future work. The results of the simulations confirm earlier steady state analyses that found these Alfven transport effects to be potentially important when the upstream Alfven speed and the gas sound speed are comparable, i.e., when the plasma and magnetic pressures are similar. It is also clear, however, that the impact of Alfven transport effects, which tend to slow shock evolution and reduce the time asymptotic cosmic-ray pressure in the shock, is strongly dependent upon uncertain details in the transport models. Both cosmic-ray advection tied to streaming Alfven waves and dissipation of wave energy are important to include in the models. Further, Alfven transport properties on both sides of the shock are also influential.

Jones, T. W.↗