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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

A Review of Nonlinear Low Frequency (LF) Wave Observations in Space Plasmas: On the Development of Plasma Turbulence

As the lead-off presentation for the topic of nonlinear waves and their evolution, we will illustrate some prominent examples of waves in space plasmas. We will describe recent observations detected within planetary foreshocks, near comets and in interplanetary space. It is believed that the nonlinear LF plasma wave features discussed here are part of and may be basic to the development of plasma turbulence. In this sense, this is one area of space plasma physics that is fundamental, with applications to fusion physics and astrophysics as well. It is hoped that the reader(s) will be stimulated to study nonlinear wave development themselves, if he/she is not already involved.

Space Plasmas↗
NASA NTRS1972

The modulation of low energy galactic cosmic rays over solar maximum (cycle 20)

A preliminary report is presented on detailed observations of low energy approximately (60 MeV/nucleon) galactic cosmic rays made during the recent period of solar maximum. For this particular cycle a significant time lag between intensity changes in the low energy and high energy components was observed. There are a number of possible explanations of this hysteresis effect. In the simplest form these include either changes in the form of the magnetic field power spectrum or changes in the size of the effective modulating region, or both.

Vanhollebeke, M. A. I.↗
NASA NTRS1990

Ultraviolet observations of NGC 205

Low-resolution IUE observations of the dwarf elliptical galaxy NGC 205 show that the UV spectral energy distribution of the galaxy is relatively flat. Spectra centered on the nucleus and on a region north of the nucleus show evidence of recent 'bursts' of star formation which contribute strongly to the UV spectral energy distribution. The UV spectra are fit with a composite spectrum based on a Miller-Scalo initial mass function, an underlying older population and an extinction based on a SMC-like extinction curve. This fit implies that the total mass of young stars in the galaxy is about 700,000 solar mass, which can be compared to the total mass of Population II stars in the galaxy of about 100 million solar mass.

Wilcots, Eric M.↗
NASA NTRS1994

Detection of EUV emission from the low activity dwarf HD 4628: Evidence for a cool corona

We present observations of low activity late-type stars obtained with the Extreme Ultraviolet Explorer (EUVE). These stars are the slowest rotators, and acoustic heating may dominate their outer atmospheric heating process. We report detection of EUV emission from the low acitivity K dwarf HD 4628 during the EUVE Deep Survey in the Lexan/boran band. This detection, in conjunction with the non-detection of this object in the ROSAT Position Sensitive Proportional Counter (PSPC) all-sky survey, suggests the existence of a cool corona with a characteristic temperature of less than 10(exp 6) K. The flux and spectral signature are consistent with current theories of acoustic heating.

Mathioudakis, M.↗
NASA NTRS1998

Observational and Theoretical Studies of Low-Mass Star Formation

Under this grant we have pursued studies of low-mass star formation with observations of candidate star-forming regions, (1) to determine the incidence of "infall asymmetry" in the spectral lines from very red young stellar objects; (2) to make detailed maps of candidate infall regions to determine the spatial extent of their infall asymmetry; (3) to compare the spatial and velocity structure of candidate infall regions with single dish and interferometer resolution; and (4) to begin a program of observations of starless dense cores to detect the presence or absence of infall motions.

Myers, Philip C.↗
NASA NTRS1993

The low-frequency interplanetary radiation

Observations of the low-frequency interplanetary radiation have continued, although no major activity has begun since mid-1991. Theoretical work has been completed which suggests that the long-speculated source at the termination shock is feasible and could at least qualitatively account for several aspects of the observations, although some difficulties exist. The Ulysses encounter with Jupiter occurred in February 1992 and has renewed discussion of a Jovian source for the emissions. We reconsider the arguments used previously to rule out Jupiter as a source and conclude that the existence of a relatively high-density boundary to the heliosphere renders most of these arguments ineffective in eliminating Jupiter as a source. We conclude, therefore, that Jupiter must be considered as likely a candidate source as the termination shock for the low-frequency emissions and, given that Jupiter is a known source of emissions in the few-kHz range, perhaps Jupiter is the more likely candidate. Even with a Jovian source, however, the low-frequency emissions continue to be useful sounders of the heliosphere which can provide information on the size of the heliospheric cavity. No conclusive evidence is available which identifies either Jupiter or the termination shock as the source.

Kurth, W. S.↗
NASA NTRS1983

Discharge characteristics of a simulated solar cell array

Measurements of the transient characteristics associated with the discharge of a simulated solar array are presented. A capacitively coupled probe is used to measure the discharge current, and antennas are used to measure the electromagnetic radiation. Discharges were observed at low surface voltages and several modes of discharge were observed. The maximum discharge current is found to be 0.2 A. This value is several orders of magnitude higher than that reported by previous measurements. Experimental evidence suggests that the inverted voltage gradient is a very likely triggering mechanism for solar array discharges.

Leung, P.↗
NASA NTRS1996

Observations of Dynamic Strain Aging in Polycrystalline NiAl

Dynamic strain aging has been investigated at temperatures between 77 and 1100 K in eight polycrystalline NiAl alloys. The 0.2% offset yield stress and work hardening rates for these alloys generally decreased with increasing temperature. However, local plateaus or maxima were observed in conventional purity and carbon doped alloys at intermediate temperatures (600-900 K). This anomalous behavior was not observed in low interstitial high-purity, nitrogen doped, or in titanium doped materials. Low or negative strain rate sensitivities (SRS) were also observed in all eight alloys in this intermediate temperature range. Coincident with the occurrence of negative SRS was the occurrence of serrated flow in conventional purity alloys containing high concentrations of Si in addition to C. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main species causing strain aging in polycrystalline NiAl is C but indicate that residual Si impurities can enhance the strain aging effect.

Weaver, M. L.↗
NASA NTRS2005

Distributed-feedback Terahertz Quantum-cascade Lasers with Laterally Corrugated Metal Waveguides

We report the demonstration of distributed-feedback terahertz quantum-cascade lasers based on a first-order grating fabricated via a lateral corrugation in a double-sided metal ridge waveguide. The phase of the facet reflection was precisely set by lithographically defined facets by dry etching. Single-mode emission was observed at low to moderate injection currents, although multimode emission was observed far beyond threshold owing to spatial hole burning. Finite-element simulations were used to calculate the modal and threshold characteristics for these devices, with results in good agreement with experiments.

Williams, Benjamin S.↗
NASA NTRS1988

Low-energy ions at low equatorial altitudes

Ion mass composition data, obtained with the ISEE-1 mass spectrometer, and other data are used to study the intense low-energy populations observed in the low-altitude equatorial magnetosphere by Williams and Frank (1984). The observed ions are identified primarily as H(+), O(+), and He(+). Observed peak energies are shown to increase with decreasing altitude and to vary by at least 30 percent over time. It is suggested that a mechanism of resonant wave-particle interaction is responsible for the energization of the observed ions, and that such a mechanism may act in a two-phase mode, resulting in the energization of minor ions up to the local Alfven energy of the ambient plasma.

Williams, D. J.↗