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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
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NASA NTRS2011

The Solar Wind as a Magnetofluid Turbulence Laboratory

The solar wind is the Sun's exosphere. As the solar atmosphere expands into interplanetary space, it is accelerated and heated. Data from spacecraft located throughout the heliosphere have revealed that this exosphere has velocities of several hundred kilometers/sec, densities at Earth orbit of about 5 particles/cu cm, and an entrained magnetic field that at Earth orbit that is about 5 10-5 Gauss. A fascinating feature of the solar wind is that the magnetic field fluctuates in a way that is highly reminiscent of "Alfven waves, i.e., the fluctuating magnetic fields are more-or-less aligned with fluctuations in the velocity of the plasma and, with proper normalization, have approximately equal magnitudes. The imperfect (observed) alignment leads to a variety of complex interactions. In many respects, the flow patterns appear to be an example of fully developed magneto fluid turbulence. Recently, the dissipation range of this turbulence has been studied using search coil magnetometer data from the STAFF instrument on the four Cluster spacecraft. I will attempt to give an overview of selected properties of this large-scale and small-scale turbulence.

Goldstein, Melvyn L.↗
NASA NTRS1995

Alfven waves, alpha particles, and pickup ions in the solar wind

Past studies of the properties of Alfven waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles do not participate in the wave motions because they 'surf' on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the 'observed' wave speed, B(sub o)(delta v squared / delta B squared)exp 1/2, as determined from the ratio of the amplitudes of the velocity and magnetic fluctuations. It is proposed that the modification of Alfven wave propagation speed is due to pressure anisotropies resulting from asymmetric distributions of interstellar pickup ions. If the proposed explanation is correct, it indicates that scattering of pickup ions onto a (bi)spherical shell may not be as complete as generally supposed.

Goldstein, B. E.↗
NASA NTRS1978

Magnetic fields, plasmas, and coronal holes: The inner solar system

In situ magnetic field and plasma observations within 1 AU which describe MDH stream flows and Alfvenic fluctuations, the latest theories of those phenomena are discussed. Understanding of streams and fluctuations was enhanced by the acquisition of nearly complete sets of high resolution plasma and magnetic data simultaneously at two or more points by IMPs 6, 7, and 8, Mariner-Venus-Mercury, HELIOS 1, and HELIOS 2. Observations demonstrate that streams can have very thin boundaries in latitude and longitude near the sun. This has necessitated a revision of earlier views of stream dynamics, for it is now clear that magnetic pressure is a major factor in the dynamics of stream in the inner solar system and that nonlinear phenomena are significant much closer to the sun than previously believed. Simultaneous IMP 6, 7, and 8 observations of Alfvenic fluctuations indicate that they are probably not simply transverse Alfven waves and suggest that Alfvenic fluctuations are better described as nonplanar, large-amplitude, general Alfven waves moving through an inhomogeneous and discontinuous medium, and coupled to a compressive mode.

Burlaga, L. F.↗
NASA NTRS2016

A Nanoflare-Based Cellular Automaton Model and the Observed Properties of the Coronal Plasma

We use the cellular automaton model described in Lopez Fuentes and Klimchuk to study the evolution of coronal loop plasmas. The model, based on the idea of a critical misalignment angle in tangled magnetic fields, produces nanoflares of varying frequency with respect to the plasma cooling time. We compare the results of the model with active region (AR) observations obtained with the Hinode/XRT and SDOAIA instruments. The comparison is based on the statistical properties of synthetic and observed loop light curves. Our results show that the model reproduces the main observational characteristics of the evolution of the plasma in AR coronal loops. The typical intensity fluctuations have amplitudes of 10 percent - 15 percent both for the model and the observations. The sign of the skewness of the intensity distributions indicates the presence of cooling plasma in the loops. We also study the emission measure (EM) distribution predicted by the model and obtain slopes in log(EM) versus log(T) between 2.7 and 4.3, in agreement with published observational values.

Lopez-Fuentes, Marcelo↗
NASA NTRS1977

Injun 5 observations of vehicle potential fluctuations at 2500 km

The AFGL spherical electrostatic analyzers aboard the polar orbiting Injun 5 satellite were designed to measure the temperature and density of the plasma as well as the vehicle potential. Significant vehicle potential fluctuations were observed at altitudes near 2500 km in the nighttime, topside ionosphere. At auroral latitudes, precipitating magnetospheric electrons frequently drove the satellite to such strongly negative potentials that the ambient electrons were shielded from instruments. In such cases, simultaneous measurements can be used to calculate the vehicle potential. Potentials of up to -40 volts were observed during impulsive precipitation events. Within the plasma trough, vehicle potentials varied between -1.5 and -4 volts, as compared with the -0.5 to -1 volt observed in the polar cap. The source of this vehicle potential enhancement was ascribed to fluxes of photoelectrons that have escaped from the sunlit conjugate ionosphere.

Sagalyn, R. C.↗
NASA NTRS1985

Electrostatic instabilities in plasmas with two electron components

This paper considers the linear theory of electrostatic Vlasov instabilities driven by the relative drift between two Maxwellian electron components in an unmagnetized, homogeneous plasma bearing zero current. The dispersion properties, threshold drift speeds and growth rates of the electron beam instability, the conventional ion acoustic instability, and the ion acoustic beam instability are compared in detailed parametric studies. A new way of illustrating the parameter regimes in whih each of these three instabilities has the lowest threshold drift speed is demonstrated. This leads to clearly illustrated criteria for determining when enhanced electrostatic fluctuations between the ion and the electron plasma frequencies may be observed in plasmas with two electron components. In the case of a hot beam this criterion is a beam density n(b) which satisfies n(b)/n(e) is between 0.05 and 0.30 where n(e) is the total electron density.

Gary, S. P.↗
NASA NTRS1982

The plumes of Titan

The plasma density enhancements observed during the Voyager 1 Titan encounter are interpreted in terms of a single plume of plasma wrapped around Saturn by corotation. Consideration is given to the radial motions of the plume driven by fluctuations in solar wind pressure and the dispersal of the plasma by the centrifugal interchange instability, by heating, and by azimuthal acceleration. It is shown that Saturn cannot readily impose corotation directly on the plume and, incidentally, that the total dissipation associated with the Titan-magnetosphere interaction is insufficient to supply the power to produce the observed Titan ultraviolet airglow. A pickup velocity of 8 km/s is inferred on the basis of the observed velocity and a standing wave model. It is found that the Voyager plasma observations are consistent with the predictions of the model.

Eviatar, A.↗
NASA NTRS2018

Generation of Kappa Distributions in Solar Wind at 1 au

We examine the generation of kappa distributions in the solar wind plasma near 1 au. Several mechanisms are mentioned in the literature, each characterized by a specific relationship between the solar wind plasma features, the interplanetary magnetic field (IMF), and the kappa index-the parameter that governs the kappa distributions. This relationship serves as a signature condition that helps the identification of the mechanism in the plasma. In general, a mechanism that generates kappa distributions involves a single or a series of stochastic or physical processes that induces local correlations among particles. We identify three fundamental solar wind plasma conditions that can generate kappa distributions, noted as (i) Debye shielding, (ii) frozen IMF, and (iii) temperature fluctuations, each one prevailing in different scales of solar wind plasma and magnetic field properties. Moreover, our findings show that the kappa distributions, and thus, their generating mechanisms, vary significantly with solar wind features: (i) the kappa index has different dependence on the solar wind speed for slow and fast modes, i.e., slow wind is characterized by a quasi-constant kappa index, K (is) approx. 4.3 +/- 0.7, while fast wind exhibits kappa indices that increase with bulk speed; (ii) the dispersion of magnetosonic waves is more effective for lower kappa indices (i.e., further from thermal equilibrium); and (iii) the kappa and polytropic indices are positively correlated, as it was anticipated by the theory.

Livadiotis, G.↗
NASA NTRS1981

The electric field structure of auroral arcs as determined from barium plasma injection experiments

Barium plasma injection experiments have revealed a number of features of electric fields in and near auroral forms extending from a few hundred to many thousands of km in altitude. There is evidence for V-type potential structures over some auroras, but not in others. For some auroral arcs, large E fields are found at ionospheric altitudes outside the arc but the E field inside the arc is near zero. In a few other auroras, most recently one investigated in an experiment conducted from Poker Flat on March 22, 1980, large, rapidly fluctuating E fields were detected by barium plasma near 600 km altitude. These E fields suggest that the motion of auroral rays can be an effect of low-altitude electric fields, or that V-type potential structures may be found at low altitudes.

Wescott, E. M.↗
NASA NTRS1995

Density fluctuations in solar wind flow types at 1 AU: Comparison to Doppler scintillation measurements

Density fluctuations with periods 10 minutes to 1 hour have been investigated in ISEE 3 plasma measurements of solar wind flows at l AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results support the interpretation of the large-scale variations in density fluctuations observed by Doppler scintillation measurement techniques inside 0.2 AU. The highest absolute and relative density fluctuations occur ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi-stationary solar wind, density and relative density fluctuations are highest around the heliospheric current sheet and lowest in the high-speed coronal flow. Superposed epoch analysis shows that the region of enhanced density fluctuations and its abrupt boundaries observed in the vicinity of the heliospheric current sheet near the Sun persists to l AU, providing further support for the filamentary nature of the extensions of coronal streamers. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows with differing origins at the Sun and as a detector of propagating interplanetary disturbances.

Huddleston, D. E.↗
NASA NTRS1982

Nonlinear saturation spectra of electric fields and density fluctuations in drift wave turbulence

The detection of drift waves in the nonlinear evolution of a space plasma process driven at long wavelengths is considered, adducing measurements of the electric field and density fluctuation power spectra as evidence. Since the driving mechanism is clearly at long wavelengths, the detection of drift waves suggests that they may play an important role in the transfer of wave energy from long to short wavelengths in a low beta plasma. The saturated spectral density is compared with theoretical results in order to estimate the anomalous diffusion rate. The observed spectral form and amplitude is in excellent agreement with drift wave predictions.

Kelley, M. C.↗
NASA NTRS2010

Observations of the Magnetic Field and Plasma in the Heliosheath by Voyager 2 from 2007.7 to 2009.4

The density and temperature profiles of the plasma measured by Voyager 2 (V2) behind the termination shock changed abruptly near 2008.6 from relatively large average values and large fluctuations during 2007.7 to 2008.6 (interval A) to relatively low average values and very small-amplitude fluctuations during 2008.6 to 2009.4 (interval B). This paper shows that the change in the magnetic field strength B(t) was less abrupt than the plasma changes, and the fluctuations of the magnetic field strength in interval B were of moderate amplitude, with indications of a quasiperiodic structure in part of the interval. The magnetic field was directed away from the sun (positive polarity) approx. 78% +/- 5% of the time in both interval A and interval B, changing in an irregular way from positive to negative polarities throughout the interval. The polarity distribution indicates that the minimum latitudinal extent of the heliospheric current sheet (HCS) was near V2 throughout the interval, consistent with the extrapolated minimum latitudes of the HCS computed from solar magnetic field observations. Thus, V2 was observing magnetic fields from the southern polar coronal hole most of the time. The distribution of B was lognormal in interval A and Gaussian interval B.

Burlaga, L. F.↗
NASA NTRS1993

A new investigation of microbursts at meter-decameter wavelengths

The results of a new investigation of microbursts at meter-decameter wavelengths are reported. Several properties of microbursts reported earlier are confirmed, and some new properties, such as time profile characteristics, flux density, and energy spectra are studied in order to compare the properties of microbursts with those of normal type III bursts. Electron beams of similar characteristics are found to generate both microbursts and type III bursts. Plasma waves generated by the electron beams due to beam-plasma instability are quickly isotropized as they scatter on the density fluctuations in the corona, leading to the low brightness temperature of microbursts. The resulting low levels of plasma waves produce transverse radiation of low brightness temperature. As a result of the isotropization, the second harmonic plasma emission dominates the fundamental, so that microbursts are expected to be primarily a harmonic plasma emission.

Subramanian, K. R.↗
NASA NTRS1999

Generation of Electron Suprathermal Tails in the Upper Solar Atmosphere: Implications for Coronal Heating

We present a mechanism for the generation of non-Maxwellian electron distribution function in the upper regions of the solar atmosphere in the presence of collisional damping. It is suggested that finite amplitude, low frequency, obliquely propagating electromagnetic waves can carry a substantial electric field component parallel to the mean magnetic field that can be significantly larger than the Dreicer electric field. This long wavelength electric fluctuation is capable of generating high frequency electron plasma oscillations and low frequency ion acoustic-like waves. The analysis has been performed using 1-1/2D Vlasov and PIC numerical simulations in which both electrons and ions are treated kinetically and self consistently. The simulation results indicate that high frequency electron plasma oscillations and low frequency ion acoustic-like waves are generated. The high frequency electron plasma oscillation drives electron plasma turbulence, which subsequently is damped out by the background electrons. The turbulence damping results in electron acceleration and the generation of non-Maxwellian suprathermal tails on time scales short compared to collisional damping. Bulk heating also occurs if the fluctuating parallel electric field is strong enough. This study suggests that finite amplitude, low frequency, obliquely propagating, electromagnetic waves can play a significant role in the acceleration and heating of the solar corona electrons and in the coupling of medium and small-scale phenomena.

Vinas, Adolfo F.↗
NASA NTRS1995

Electromagnetic Radiation in the Plasma Environment Around the Shuttle

As part of the SAMPIE (The Solar Array Module Plasma Interaction Experiment) program, the Langmuir probe (LP) was employed to measure plasma characteristics during the flight STS-62. The whole set of data could be divided into two parts: (1) low frequency sweeps to determine voltage-current characteristics and to find electron temperature and number density; (2) high frequency turbulence (HFT dwells) data caused by electromagnetic noise around the shuttle. The broadband noise was observed at frequencies 250-20,000 Hz. Measurements were performed in ram conditions; thus, it seems reasonable to believe that the influence of spacecraft operations on plasma parameters was minimized. The average spectrum of fluctuations is in agreement with theoretical predictions. According to purposes of SAMPIE, the samples of solar cells were placed in the cargo bay of the shuttle, and high negative bias voltages were applied to them to initiate arcing between these cells and surrounding plasma. The arcing onset was registered by special counters, and data were obtained that included the amplitudes of current, duration of each arc, and the number of arcs per one experiment. The LP data were analyzed for two different situations: with arcing and without arcing. Electrostatic noise spectra for both situations and theoretical explanation of the observed features are presented in this report.

Vayner, Boris V.↗
NASA NTRS2019

Alfvenic Fluctuations Associated with Jupiter's Auroral Emissions

The Alfvén wave mode transmits field‐aligned currents and large‐scale turbulence throughout Jupiter's magnetosphere. Magnetometer data from the Juno spacecraft have provided the first observations of Alfvénic fluctuations along the polar magnetic flux tubes connected to Jupiter's main auroral oval and the Jovian satellites. Transverse magnetic field perturbations associated with Io are observed up to ~90° away from main Io footprint, supporting the presence of extended Alfvénic wave activity throughout the Io footprint tail. Additional broadband fluctuations measured equatorward of the statistical auroral oval are composed of incompressible magnetic turbulence that maps to Jupiter's equatorial plasma sheet at radial distances within ~20 R(sub J). These fluctuations exhibit a k(sub ||) power spectrum consistent with strong magnetohydrodynamic turbulence. This turbulence can generate up to ~100 mW/m(exp 2) of Poynting flux to power the Jovian aurora in regions connected to the inner magnetosphere's central plasma sheet.

Gershman, Daniel J.↗
NASA NTRS2011

Multifluid Simulations of the Global Solar Wind Including Pickup Ions and Turbulence Modeling

I will describe a three-dimensional magnetohydrodynamic model of the solar wind that takes into account turbulent heating of the wind by velocity and magnetic fluctuations as well as a variety of effects produced by interstellar pickup protons. The interstellar pickup protons are treated in the model as one fluid and the protons and electrons are treated together as a second fluid. The model equations include a Reynolds decomposition of the plasma velocity and magnetic field into mean and fluctuating quantities, as well as energy transfer from interstellar pickup protons to solar wind protons that results in the deceleration of the solar wind. The model is used to simulate the global steady-state structure of the solar wind in the region from 0.3 to 100 AU. The simulation assumes that the background magnetic field on the Sun is either a dipole (aligned or tilted with respect to the solar rotation axis) or one that is deduced from solar magnetograms.

Goldstein, Melvyn L.↗
NASA NTRS2011

Magnetofluid Simulations of the Global Solar Wind Including Pickup Ions and Turbulence Modeling

I will describe a three-dimensional magnetohydrodynamic model of the solar wind that takes into account turbulent heating of the wind by velocity and magnetic fluctuations as well as a variety of effects produced by interstellar pickup protons. In this report, the interstellar pickup protons are treated as one fluid and the protons and electrons are treated together as a second fluid. The model equations include a Reynolds decomposition of the plasma velocity and magnetic field into mean and fluctuating quantities, as well as energy transfer from interstellar pickup protons to solar wind protons that results in the deceleration of the solar wind. The model is used to simulate the global steady-state structure of the solar wind in the region from 0.3 to 100 AU. Where possible, the model is compared with Voyager data. Initial results from generalization to a three-fluid model is described elsewhere in this session.

Goldstein, Melvyn L.↗