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At least 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfven Waves

We present the numerical simulations of winds from evolved giant stars using a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD) code. This study extends our previous fully non-linear MHD wind simulations to include a broadband frequency spectrum of Alfven waves that drive winds from red giant stars. We calculated four Alfven wind models that cover the whole range of Alfven wave frequency spectrum to characterize the role of freely propagated and reflected Alfven waves in the gravitationally stratified atmosphere of a late-type giant star. Our simulations demonstrate that, unlike linear Alfven wave-driven wind models, a stellar wind model based on plasma acceleration due to broadband non-linear Alfven waves, can consistently reproduce the wide range of observed radial velocity profiles of the winds, their terminal velocities and the observed mass loss rates. Comparison of the calculated mass loss rates with the empirically determined mass loss rate for alpha Tau suggests an anisotropic and time-dependent nature of stellar winds from evolved giants.

Airapetian, V.↗
NASA NTRS1995

Observations of micro-turbulence in the solar wind near the sun with interplanetary scintillation

Velocity and density turbulence of solar wind were inferred from interplanetary scintillation (IPS) observations at 2.3 GHz and 8.5 GHz using a single-antenna. The observations were made during September and October in 1992 - 1994. They covered the distance range between 5 and 76 solar radii (Rs). We applied the spectrum fitting method to obtain a velocity, an axial ratio, an inner scale and a power-law spectrum index. We examined the difference of the turbulence properties near the Sun between low-speed solar wind and high-speed solar wind. Both of solar winds showed acceleration at the distance range of 10 - 30 Rs. The radial dependence of anisotropy and spectrum index did not have significant difference between low-speed and high-speed solar winds. Near the sun, the radial dependence of the inner scale showed the separation from the linear relation as reported by previous works. We found that the inner scale of high-speed solar wind is larger than that of low-speed wind.

Yamauchi, Y.↗
NASA NTRS1983

Evolution of ion distributions across the nearly perpendicular bow shock - Specularly and non-specularly reflected-gyrating ions

Data from ISEE 1 and 2 spacecraft were used to study the evolution of the ion distributions in the perpendicular terrestrial bow shock. The plasma data were taken during passage of the spacecraft downstream of and through the shock. Solar wind ions had velocities ranging from Mach 2-12.4, and reflected ions featured a relative density of 1-3 percent of the solar wind density at Mach 2 to 15-25 percent at Mach 8-12. Computer simulations have indicated that the ions provide essential dissipation at the shock and gyrate about the magnetic field lines in the plasma rest frame at a speed twice that of the normal incident solar wind flow. The ion density decreases by up to two orders of magnitude at the forward end of the foot of the shock profile, suggesting that the ions are reflected by the shock specularly, and may enhance downstream ion thermalization.

Sckopke, N.↗
NASA NTRS1999

Rocket/Radar Sporadic-E Experiment Conducted during the El Coqui 2 Campaign

In order to investigate the complex electrodynamics and neutral-plasma coupling inherent to sporadic-E layers in the earth's mid-latitude ionosphere, a series of rocket/radar experiments were planned as part of the NASA El Coqui H Campaign from Tortuguero Launch Range, Puerto Rico, in March-April, 1998. The rocket experiments consisted of two pairs of "mother-daughter" payloads with limited apogees so that the payloads "hovered" in the sporadic-E region (95-125 km). Each payload pair included vector DC and AC electric field detectors, a highly accurate flux-gate DC magnetometer, an ion mass spectrometer, an ionization gauge, and spaced-electric field receivers to measure the wavelength and phase velocity of the unstable plasma waves. Separate rockets were included to simultaneously carry aloft TMA trails to measure the neutral wind and its velocity shear, believed responsible for the sporadic-E layer formation. In addition to the rocket experiments, incoherent scatter radar measurements of plasma density and drift velocity were gathered almost every night during the 3 week campaign. Continuous VHF backscatter radar operations were carried out from a site near Salinas, Puerto Rico, where 3-m backscatter echoes were observed associated with sporadic-E and other types of low altitude ionospheric layers. Other radars that operated during the campaign included an HF backscatter system near Ponce, Puerto Rico, and a second VHF backscatter radar set up near Aguadila Puerto Rico. On 24 March 1998, one of the instrumented rockets was launched, attaining an apogee of 129 km. The payloads successfully pierced an intense sporadic-E layer observed by both the Arecibo radar and the in-situ density and ion mass spectrometer probes. In-situ DC electric fields revealed very low (about 1-2 mV/m) ambient fields with small amplitude structures of the same order. No high frequency (short scale) waves were observed, consistent with the VHF backscatter observations at the time of the launch. An overview of the observations will be presented.

Pfaff, R.F.↗
NASA NTRS1993

Pioneer Venus and IMP 8 observations of the latitude dependence of the solar wind

Solar wind speeds and magnetic field data from the Pioneer Venus Orbiter (0.7 AU) and IMP 8 (1 AU) have been compared to infer the latitudinal structure of the solar wind in the inner heliosphere between 1984 and 1987. The solar wind in the inner heliosphere was found to exhibit an unusual structure during the last solar minimum (1986-1987). High-speed streams were excluded from the vicinity of the solar equator, and the solar wind at low heliographic latitudes (less than 3 deg) was characterized by low-speed solar wind with irregular fluctuations in velocity. At higher latitudes the solar wind had a conventional stream structure with two high-speed streams per solar rotation. While the velocities of these high-speed streams did not appear to vary significantly with latitude, the latitudinal gradients at the equatorward boundaries of high-speed streams were high.

Gazis, P. R.↗
DOE OSTI2012

NEW SOLAR WIND DIAGNOSTIC USING BOTH IN SITU AND SPECTROSCOPIC MEASUREMENTS

We develop a new diagnostic technique that utilizes, at the same time, two completely different types of observations-in situ determinations of solar wind charge states and high-resolution spectroscopy of the inner solar corona-in order to study the temperature, density, and velocity of the solar wind as a function of height in the inner corona below the plasma freeze-in point. This technique relies on the ability to calculate the evolution of the ion charge composition as the solar wind escapes the Sun given the wind temperature, density, and velocity profiles as a function of distance. The resulting charge state composition can be used to predict frozen-in charge states as well as spectral line intensities. The predicted spectra and ion charge compositions can be compared with observations carried out when spectrometers and in situ instruments are in quadrature configuration to quantitatively test a set of assumptions regarding density, temperature, and velocity profiles in the low corona. Such a comparison can be used in two ways. If the input profiles are predicted by a theoretical solar wind model, this technique allows the benchmarking of the model. Otherwise, an empirical determination of the velocity, temperature, and density profiles can be achieved below the plasma freeze-in point applying a trial-and-error procedure to initial, user-specified profiles. To demonstrate this methodology, we have applied this technique to a state-of-the-art coronal hole and equatorial streamer model.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1981

Studies of Jupiter's lower ionospheric layers

Fluxes of heavy ions necessary to form layers of enhanced ionization observed in the lower ionosphere of Jupiter are theoretically calculated, and possible formation mechanisms of the layers are investigated. Estimates of ion drift velocities and neutral wind speeds are made from the shape of the ionization layers, and are found to range from a few centimeters per second to meters per second. Zonal wind shear ranges from 50 m/s westward to 200 m/s eastward over a 70 km altitude range, while small meridional winds are sufficient to form the layer. Results indicate that if the layers are formed from sodium or sulfur ions from the Galilean satellite which are injected into the Jovian atmosphere, then the Na(+) flux must be 30,000 sq cm/s, and the S(+) flux must be 4000 sq cm/s in order to correlate with Pioneer 10 observations of the L(6) layer. At low altitudes of the L(6) and L(7) layers, the denser atmosphere makes diffusion very slow, and the vertical drift velocity of 1 cm/s requires a zonal wind of only several centimeters per second to drive it.

Chen, R. H.↗
NASA NTRS1992

Mass loss in main-sequence A-type stars?

We present new observations of the H-alpha line profile for five main-sequence A-type stars, where very high SNRs were achieved. A search for weak asymmetries was carried out to detect a stellar wind, but the many telluric absorption lines in this range prevent us from taking the full benefit of the high SNRs. This situation was improved by modeling the telluric absorptions to remove them from the observed spectra, but the H-alpha profiles were nevertheless found to be quite symmetric. We calculated the profile of the H-alpha line for a grid of model atmospheres of an A-type star including a weak wind, in order to assess an upper limit on the mass loss rate. The asymmetry is found to be sensitive to the velocity law and to the turbulent velocity of the wind, but its first moment depends much less on them. The upper limits deduced on the mass loss rate are between 1 and 2 x 10 exp -10 solar mass/yr. An improvement up to a factor 10 could be anticipated if new observations could be secured from a much drier location, achieving similar SNRs.

Lanz, T.↗
NASA NTRS1974

Some effects of diffusion on the flow of ions in the solar wind

The contribution of diffusive effects to the near equality of flow velocities of H+ and heavier ions of the solar wind at 1 AU is examined. Frictional drag on heavy ions from average solar wind fluxes of H+ appears to make an appreciable contribution to equalizing velocities. The frictional drag on H+ due to the presence of heavy ions may make an appreciable contribution to equalizing flow velocities, since compensating increases in coronal temperatures may be required in order to give solar wind velocities for H+. However, rather large enhancements of heavy ions near the sun appear to be required. If appreciable enhancements of heavy ions extend beyond about 2.5 solar radii, significant increases in heavy ion temperatures above H+ temperatures may be produced. This temperature difference may contribute to equalizing flow velocities and may contribute to the explanation of measured temperature differences at 1 AU between H+ and He++.

Nakada, M. P.↗
DOE OSTI2014

Radio observations reveal a smooth circumstellar environment around the extraordinary type Ib supernova 2012au

We present extensive radio and X-ray observations of SN 2012au, an energetic, radio-luminous supernova of Type Ib that exhibits multi-wavelength properties bridging subsets of hydrogen-poor superluminous supernovae, hypernovae, and normal core-collapse supernovae. The observations closely follow models of synchrotron emission from a shock-heated circumburst medium that has a wind density profile (ρ∝r {sup –2}). We infer a sub-relativistic velocity for the shock wave v ≈ 0.2 c and a radius of r ≈ 1.4 × 10{sup 16}cm at 25 days after the estimated date of explosion. For a wind velocity of 1000 km s{sup –1}, we determine the mass-loss rate of the progenitor to be M-dot =3.6×10{sup −6} M{sub ⊙} yr{sup −1}, consistent with the estimates from X-ray observations. We estimate the total internal energy of the radio-emitting material to be E ≈ 10{sup 47} erg, which is intermediate to SN 1998bw and SN 2002ap. The evolution of the radio light curve of SN 2012au is in agreement with its interaction with a smoothly distributed circumburst medium and the absence of stellar shells ejected from previous outbursts out to r ≈ 10{sup 17} cm from the supernova site. We conclude that the bright radio emission from SN 2012au was not dissimilar from other core-collapse supernovae despite its extraordinary optical properties, and that the evolution of the SN 2012au progenitor star was relatively quiet, marked with a steady mass loss, during the final years preceding explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1955

Some measurements of time and space correlation in wind tunnel

Results are presented of research obtained by means of an apparatus for measurement of time and space correlation and of a spectral analyzer in the study of the longitudinal component of turbulence velocities in a wind tunnel downstream of a grid of meshes. Application to the case of a flat-plate boundary layer is illustrated. These researches were made at the Laboratoire de Mecanique de l'Atmosphere de l'I.M.F.M. for the O.N.E.R.A.

RESEARCH TECHNIQUE, AERODYNAMICS↗
NASA NTRS1964

Turbulence in the upper atmosphere

Investigation of fluctuating turbulent velocities in upper atmosphere - ambient turbulence produced by wind shears

ATMOSPHERIC TURBULENCE↗
NASA NTRS1984

On the extended chromosphere of Alpha Orionis

A detailed examination of the extended chromosphere of Alpha Orionis was performed in an attempt to characterize the matter-emitting processes of red giants and supergiants. The problem was approached using an Alfven wave driven stellar wind model to compute the density, temperature, and velocity structure of the wind. It is shown that the mechanical energy fluxes necessary to maintain the observed chromospheric temperatures are commensurate with the momentum deposition necessary to extend the chromosphere. The chromospheric model constructed is, however, inconsistent with the kinematics of the observed symmetric profiles, which could correspond to a fountain-type model that described infalling, rather than escaping, ejected material such as He.

Hartmann, L.↗
NASA NTRS1972

Interplanetary double-shock ensembles with anomalous electrical conductivity

Similarity theory is applied to the case of constant velocity, piston-driven, shock waves. This family of solutions, incorporating the interplanetary magnetic field for the case of infinite electric conductivity, represents one class of experimentally observed, flare-generated shock waves. This paper discusses the theoretical extension to flows with finite conductivity (presumably caused by unspecified modes of wave-particle interactions). Solutions, including reverse shocks, are found for a wide range of magnetic Reynolds numbers from one to infinity. Consideration of a zero and nonzero ambient flowing solar wind (together with removal of magnetic considerations) enables the recovery of earlier similarity solutions as well as numerical simulations. A limited comparison with observations suggests that flare energetics can be reasonably estimated once the shock velocity, ambient solar wind velocity and density, and ambient azimuthal Alfven Mach number are known.

Dryer, M.↗
NASA NTRS2014

Solar Wind Sputtering of Lunar Soil Analogs: The Effect of Ionic Charge and Mass

In this contribution we report sput-tering measurements of anorthite, an analog material representative of the lunar highlands, by singly and multicharged ions representative of the solar wind. The ions investigated include protons, as well as singly and multicharged Ar ions (as proxies for the heavier solar wind constituents), in the charge state range +1 to +9, and had a fixed solar-wind-relevant impact velocity of approximately 310 km/s or 500 eV/ amu. The goal of the measurements was to determine the sputtering contribution of the heavy, multicharged minority solar wind constituents in comparison to that due to the dominant H+ fraction.

Hijazi, H.↗
NASA NTRS1995

Constraints on Solar Wind Acceleration Mechanisms from Ulysses Plasma Observations: The First Polar Pass

The mass flux density and velocity of the solar wind at polar latitudes can provide strong constraints on solar wind acceleration mechanisms. We use plasma observations from the first polar passage of the Ulysses spacecraft to investigate this question. We find that the mass flux density and velocity are too high to reconcile with acceleration of the solar wind by classical thermal conduction alone. Therefore acceleration of the high-speed must involve extended deposition of energy by some other mechanism, either as heat or as a direct effective pressure, due possibly to waves and/or turbulence, or completely non-classical heat transport.

Barnes, Aaron↗
DOE OSTI2015

Computational examination of utility scale wind turbine wake interactions

We performed numerical simulations of small, utility scale wind turbine groupings to determine how wakes generated by upstream turbines affect the performance of the small turbine group as a whole. Specifically, various wind turbine arrangements were simulated to better understand how turbine location influences small group wake interactions. The minimization of power losses due to wake interactions certainly plays a significant role in the optimization of wind farms. Since wind turbines extract kinetic energy from the wind, the air passing through a wind turbine decreases in velocity, and turbines downstream of the initial turbine experience flows of lower energy, resulting in reduced power output. Our study proposes two arrangements of turbines that could generate more power by exploiting the momentum of the wind to increase velocity at downstream turbines, while maintaining low wake interactions at the same time. Furthermore, simulations using Computational Fluid Dynamics are used to obtain results much more quickly than methods requiring wind tunnel models or a large scale experimental test.

17 WIND ENERGY↗