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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Phase-referenced VLBI observations of weak radio sources - Milliarcsecond position of Algol

The Fourier inversion of all the delay/rate referenced visibilities of Algol was used to form a 'dirty' map which has dramatically improved VLBI array sensitivity. The Algol system, in a very low state of activity with a total flux density of only 3 mJy, was observed alternatively with the reference source 0309 + 411, about 1 deg away, throughout the 7 hr VLBI experiment. By combining all the individual VLBI observations, the resulting coherent integration time for Algol is 4.2 hr and the corresponding SNR is 12. Internal consistency checks and an analysis of possible systematic astrometric errors indicate that the accuracy of the relative VLBI position of Algol is + or - 0.0005 arcsec in both coordinates and that unknown systematic effects are presently limiting this astrometric measurement.

Lestrade, J.-F.↗
NASA NTRS1992

Gamma rays from extragalactic radio sources

It is proposed that the important connection between 3C 273 and 3C 279, the first two extragalactic sources detected at greater than 100 MeV energies, is their superluminal nature. In support of this conjecture, we propose a radiation mechanism that focuses gamma rays in the superluminal direction, due to Compton scattering of accretion-disk photons by relativistic nonthermal electrons in the jet.

Dermer, Charles D.↗
NASA NTRS1992

Connection between ambient density fluctuations and clumpy Langmuir waves in type III radio sources

A recent stochastic-growth theory of clumpy Langmuir waves in type III sources is shown to imply that the clumps will have the same size distribution as the ambient low-frequency density fluctuations in the solar wind. Spectral analysis of Langmuir-wave time series from the ISEE 3 plasma wave instrument confirms this prediction to within the uncertainties in the spectra. The smallest Langmuir clump size is inferred to be in the range 0.4-30 km in general, and 2-30 km for beam-resonant waves, and it is concluded that the diffusion of waves in the source is anomalous.

Robinson, P. A.↗
NASA NTRS1993

Clumpy Langmuir waves in type III radio sources - Comparison of stochastic-growth theory with observations

Detailed comparisons are made between the Langmuir-wave properties predicted by the recently developed stochastic-growth theory of type III sources and those observed by the plasma wave experiment on ISEE 3, after correcting for the main instrumental and selection effects. Analysis of the observed field-strength distribution confirms the theoretically predicted form and implies that wave growth fluctuates both spatially and temporally in sign and magnitude, leading to an extremely clumpy distribution of fields. A cutoff in the field-strength distribution is seen at a few mV/m, corresponding to saturation via nonlinear effects. Analysis of the size distribution of Langmuir clumps yields results in accord with those obtained in earlier work and with the size distribution of ambient density fluctuations in the solar wind. This confirms that the inhomogeneities in the Langmuir growth rate are determined by the density fluctuations and that these fluctuations persist during type III events.

Robinson, P. A.↗
NASA NTRS1993

Dynamics of Langmuir and ion-sound waves in type III solar radio sources

The study traces the evolution of Langmuir and ion-sound waves in type III sources, incorporating linear growth, linear damping, and nonlinear electrostatic decay. Improved estimates are obtained for the wavenumber range of growing waves and the nonlinear coupling coefficient for the decay process. It is shown that the conditions in the solar wind do not allow a steady state to be attained; instead, bursty linear and nonlinear interactions take place, consistent with the highly inhomogeneous and impulsive waves actually observed. Nonlinear growth is found to be rapid enough to saturate the growth of the parent Langmuir waves in the available interaction time. The competing processes of nonlinear wave collapse and quasi-linear relaxation are discussed, and it is concluded that neither is responsible for the saturation of Langmuir growth.

Robinson, P. A.↗
NASA NTRS1993

Helical jets and the misalignment distribution for core-dominated radio sources

The type of a model which would explain the large population of orthogonally misaligned sources is considered. Two of the best examples of misaligned sources are investigated, namely, the quasar 3C 309.1 (Wilkinson et al., 1986; Kus et al., 1990, 1991) and the BL Lac object 1823 + 568 (Foley, 1982; O'Dea et al., 1988). It is shown that simple bend geometries cannot explain the misaligned sources, and the geometries required to give orthogonal misalignments are explained. A generally applicable model which might explain misaligned sources is proposed.

Conway, J. E.↗
NASA NTRS1992

High-energy gamma radiation from extragalactic radio sources

We propose that the important relationship between 3C 273 and 3C 279, the first two extragalactic sources detected at over 100 MeV energies, is their superluminal nature. In support of this conjecture, we propose a kinematic focusing mechanism, based on Compton scattering of accretion-disk photons by relativistic nonthermal electrons in the jet, that preferentially emits gamma rays in the superluminal direction.

Dermer, C. D.↗
NASA NTRS1994

Cosmic bullets as particle accelerators and radio sources

We have simulated in two dimensions the dynamical evolution of dense gas clouds(`cosmic bullets') moving supersonically through a uniform low-density medium. The diffusive shock acceleration of relativistic protons (cosmic rays) and their dynamical feedback on the background flow are included by the two-fluid model for this process. The acceleration of relativistic electrons is approximated by a test-particle model, and a passive magnetic field is followed by a simple advection scheme. Strong bow shocks, with Mach numbers similar to that of a bullet's motion, are the most important particle accelerators in the flow, while tail shocks and shocks inside the bullets do not play generally significant roles in this regard. For our simulation parameters, approximately greater than 10% of the initial bullet kinetic energy is converted to a combination of internal energy of gas and cosmic-ray protons by the time the bullets begin to be disrupted. Characteristically, the cosmic rays gain several percent of the available kinetic energy. Bullet destruction on timescales only a little larger than the ram pressure bullet crushing time begins in response to Kelvin-Helmholtz and especially to Rayleigh-Taylor instabilities along the forward bullet surface. For dense bullets this happens before the bullet is stopped by ram pressure. According to our simple model for synchrotron emission from relativistic electrons accelerated and transported within the flows, that emission increases rapidly as the bullet begins to fragment, when it is strongly dominated by field enhancement in sheared flows. Synchrotron emission from the acceleration region within the bow shock is, by contrast, much weaker.

Jones, T. W.↗
NASA NTRS1993

Dynamic of Langmuir and Ion-Sound Waves in Type 3 Solar Radio Sources

The evolution of Langmuir and ion-sound waves in type 3 sources is investigated, incorporating linear growth, linear damping, and nonlinear electrostatic decay. Improved estimates are obtained for the wavenumber range of growing waves and the nonlinear coupling coefficient for the decay process. The resulting prediction for the electrostatic decay threshold is consistent with the observed high-field cutoff in the Langmuir field distribution. It is shown that the conditions in the solar wind do not allow a steady state to be attained; rather, bursty linear and nonlinear interactions take place, consistent with the highly inhomogeneous and impulsive waves actually observed. Nonlinear growth is found to be fast enough to saturate the growth of the parent Langmuir waves in the available interaction time. The resulting levels of product Langmuir and ion-sound waves are estimated theoretically and shown to be consistent with in situ ISEE 3 observations of type 3 events at 1 AU. Nonlinear interactions slave the growth and decay of product sound waves to that of the product Langmuir waves. The resulting probability distribution of ion-sound field strengths is predicted to have a flat tail extending to a high-field cutoff. This prediction is consistent with statistics derived here from ISEE 3 observations. Agreement is also found between the frequencies of the observed waves and predictions for the product S waves. The competing processes of nonlinear wave collapse and quasilinear relaxation are discussed, and it is concluded that neither is responsible for the saturation of Langmuir growth. When wave and beam inhomogeneities are accounted for, arguments from quasi-linear relaxation yield an upper bound on the Langmuir fields that is too high to be relevant. Nor are the criteria for direct wave collapse of the beam-driven waves met, consistent with earlier simulation results that imply that this process is not responsible for saturation of the beam instability. Indeed, even if the highest observed Langmuir fields are assumed to he part of a long-wavelength 'condensate' produced via electrostatic decay, they still fall short of the relevant requirements for wave collapse. The most stringent requirement for collapse is that collapsing wave packets not be disrupted by ambient density fluctuations in the solar wind. Fields of several mV m(exp -1) extending over several hundred km would be needed to satisfy this requirement; at 1 AU such fields are rare at best.

Robinson, P. A.↗
NASA NTRS1996

Natural Radio Source and Spacecraft Signal Measurements at Ka-Band (32.0 GHz) and X-Band (8.4 GHz) Using a 34-Meter Beam-Waveguide Antenna

From Intro.: NASA'a Deep Space Network (DSN) Technology Program at the Jet Propulsion Laboratory (JPL) is evaluating the use of the Ka-Band frequency allocation band (31.8 GHz to 32.3 GHz) for deep space to Earth telecommunications...This paper addresses the three current Ka-Band and X-Band activities, 1)KaAp, 2)SURSAT-1, and 3)KaBLE-II's upcoming Ka-Band experiments aboard Mars Global Surveyor.

Deep Space Network DSN Ka-Band Ka frequency↗
NASA NTRS2018

Crowd-Sourced Radio Science at Marshall Space Flight Center

August 21, 2017 provided a unique opportunity to investigate the effects of the total solar eclipse on high frequency (HF) radio propagation and ionospheric variability. In Marshall Space Flight Center's partnership with the US Space and Rocket Center (USSRC) and Austin Peay State University (APSU), we engaged citizen scientists and students in an investigation of the effects of an eclipse on the mid-latitude ionosphere. Activities included fieldwork and station-based data collection of HF Amateur Radio frequency bands and VLF radio waves before, during, and after the eclipse to build a continuous record of changing propagation conditions as the moon's shadow marched across the United States. Post-eclipse radio propagation analysis provided insights into ionospheric variability due to the eclipse.

amateur radio↗