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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

A new interpretation of plasma-line overshoot phenomena

The temporal evolution of Langmuir waves excited by high-power, high-frequency (HF) radio waves in the ionosphere is studied theoretically. This study is motivated by past observations made with the 450 MHz radar at Arecibo Observatory in Puerto Rico. Two kinds of nonlinear damping to the parametric decay instability are considered in the derivation of the rate equation for the spectral intensity of enhanced Langmuir waves. They are Langmuir wave cascading caused by nonlinear Landau damping and cross-field electron diffusion. The first damping process leads to the saturation of individual unstable Langmuir wave. The second process, which results from the incoherent scattering of electron orbits by the total excited Langmuir waves, yields anomalous damping that applies to each Langmuir wave. Consequently, Langmuir waves with smaller growth rates will be suppressed by those with larger growth rates. Such a mode competition process may cause the overshoot of the HF-enhanced plasma line observed with the Arecibo 430 MHz radar. Favorable agreement is obtained between theory and the Arecibo observations.

Kuo, S. P.↗
NASA NTRS1992

The ionosphere as a gamma ray burst detector

Unlike all man made detectors, which are only sensitive to relative narrow regions of the electromagnetic spectrum, the ionosphere is practically a perfect detector for high energy radiation because it absorbs all radiation from the far-ultraviolet to the higher energy gamma-rays. Therefore, it may be possible to employ the terrestrial atmosphere as a detector of high energy celestial photons. As early as the 1940's solar flares were detected by the disturbance they caused to the ionosphere. The VLF (3 - 30 kHz) approach for detecting ionospheric disturbances is based on the following physical circumstance: celestial high energy radiation ionizes the atoms of the earth's ionosphere leading to the production of free electrons. These free electrons influence the propagation of electromagnetic waves. By studying the phase and amplitude changes of VLF radio wave propagating in the earth-ionosphere waveguide, it was hoped to ascertain the electron density in these regions and draw conclusions about the celestial radiation which caused them. To detect gamma-ray bursts, two conditions for the optimal detection are: (1) large zenith angle; and (2) alignment of burst and propagation path.

Mcgruder, Charles H., III↗
NASA NTRS1999

Radiowave Scattering in the Outer Heliosphere

The Voyager 1/2 plasma wave instruments have observed low-frequency (1.5-4kHz) radio waves apparently generated near the heliopause. The waves are found in two distinct bands.

scattering angular broadening outer heliosphere↗
NASA NTRS1991

Continuum radiation in planetary magnetospheres

With the completion of the Voyager tour of the outer planets, radio and plasma wave instruments have executed the first survey of the wave spectra of Earth, Jupiter, Saturn, Uranus, and Neptune. One of the most notable conclusions of this survey is that there is a great deal of qualitative similarity in both the plasma wave and radio wave spectra from one magnetosphere to the next. In particular, in spite of detailed differences, most of the radio emissions at each of the planets have been tentatively classified into two primary categories. First, the most intense emissions are generally associated with the cyclotron maser instability. Second, a class of weaker emissions can be found at each of the magnetospheres which appears to be the result of conversion from intense electrostatic emissions at the upper hybrid resonance frequency into (primarily) ordinary mode radio emission. It is this second category, often referred to as nonthermal continuum radiation, which we will discuss in this review. We review the characteristics of the continuum spectrum at each of the planets, discuss the source region and direct observations of the generation of the emissions where available, and briefly describe the theories for the generation of the emissions. Over the past few years evidence has increased that the linear mode conversion of electrostatic waves into the ordinary mode can account for at least some of the continuum radiation observed. There is no definitive evidence which precludes the possibility that a nonlinear mechanism may also be important.

Kurth, W. S.↗
NASA NTRS1995

Interplanetary Scintillation

Interplanetary scintillation (IPS) has been used as a diagnostic of solar wind speed and interplanetary plasma turbulence, allowing inference of speed and electron density power spectrum close to the Sun and out of the ecliptic. In that context, IPS is 'signal' and provides scientifically interesting data. IPS is also of interest because amplitude and phase perturbations imposed on radio waves are 'noise' for telemetry and precision Doppler tracking of deep space probes and for some radio astronomical observations. This paper briefly reviews the connection between scattering observables and the electron density power spectrum. Interplanetary phase scintillation on time scales of 100 to 10 000 seconds is an important noise in mass determinations of small solar system bodies during space-probe fly-bys and in searches for low-frequency gravitational radiation.

waves deep space probes electron density power spe↗
NASA NTRS1985

Dynamical roles of planetary waves on the winter anomaly in the middle and lower latitudes

The winter anomaly of radio wave absorption in the ionospheric D region (65 to 90 km), caused by a factor of about ten increase in electron density, has been considered to be due to a factor of about ten increase of the nitric oxide concentration and/or a few tens of degrees Kelvin temperature rise due to a strong temperature dependency of the conversion rate of molecular ions to cluster ions. In order to investigate the mechanism of the winter anomaly, the processes that brought about these changes are identified. It is well known that stratospheric warmings sometimes accompany the winter anomaly in middle latitudes. The present study concentrates on the winter anomaly which occurred simultaneously during a sudden warming of the 1967 to 1968 winter, from middle December 1967 to middle January 1968. Variations of geophysical and ionospheric parameters are shown.

Kawahira, K.↗
NASA NTRS1994

Asymmetries in the Io plasma torus

Using Ulysses radio wave data taken during the 1992 Jupiter encounter, we conclude that there are significant large and small spatial scale azimuthal asymmetries at high latitudes in the Io plasma torus. During a period of time near perijove when the spacecraft motion was predominantly in the azimuthal direction and was relatively fixed in both latitude and radial distance, inferred electron densities depart significantly from the common assumption of longitudinal symmetry. Specifically, electron plasma concentrations near 0 deg system III longitude (and 0400 LT) are greater than those near 180 deg (and 0000 LT). Superposed on this large-scale variation are regularly spaced density depletions, 30-50% in magnitude, and having a spatial periodicity of about 17 deg. Some of these depletions may drive various known radio and plasma wave sources by means of large B parallel electric potentials. The observations are compared with recent models and with the in-situ Voyager observations.

Desch, M. D.↗
NASA NTRS1995

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. 3: Ground-based observations in 1991 November

We present ground-based observations of the BL Lac object PKS 2155-304 during 1991 November. These data were obtained as part of a large international campaign of observations spanning the electro-magnetic spectrum from the radio waves to the X-rays. The data presented here include radio and UBVRI fluxes, as well as optical polarimetry. The U to I data show the same behavior in all bands and that only upper limits to any lag can be deduced from the cross-correlation of the light curves. The spectral slope in the U-I domain remained constant on all epochs but 2. There is no correlation between changes in the spectral slope and large variations in the total or polarized flux. The radio flux variations did not follow the same pattern of variability as the optical and infrared fluxes. The polarized flux varied by a larger factor than the total flux. The variations of the polarized flux are poorly correlated with those of the total flux in the optical (and hence UV domain; see the accompanying paper by Edelson et al.) nor with those of the soft X-rays. We conclude that the variability of PKS 2155-304 in the optical and near-infrared spectral domains are easier to understand in the context of variable geometry or bulk Lorentz factor than of variable electron acceleration and cooling rates.

Courvoisier, T. J.-L.↗
NASA NTRS1989

Shock-associated plasma density fluctuations in the interstellar medium

Theories of diffuse shock acceleration of the cosmic rays that predict that there should be regions of intense Alfven waves both upstream and downstream of the shock front are considered. Observations of similar waves near solar system shock waves show that they produce substantial density fluctuations. Such density fluctuations might produce observable scattering of radio waves. Observations which searched for angular broadening or blurring of radio sources whose lines of sight pass close to or through supernova remnants are discussed. No definite cases of remnant-associated scattering are detected. However, the source CL 4, which is viewed through the Cygnus Loop supernova remnant, may be such an object and merits further observation.

Spangler, S. R.↗
NASA NTRS1988

VLF wave emissions by pulsed and dc electron beams in space. I - Spacelab 2 observations

The properties of radio waves generated by electron beams in space were investigated using data from the wideband wave receiver on the Spacelab 2. The VLF observations were found to confirm the results of the STS 3/OSS-1 mission. It was found that a 1-keV electron beam injected from the orbiter produced copious broadband electromagnetic emissions. When the electron beam was square-wave modulated, narrow-band emissions at the pulsing frequency and harmonics of that frequency were produced along with the broadband emissions. The observations indicated that dc 50-mA electron beams and pulsed 50-percent duty-cycle 100-mA beams produce broadband radiation which is comparable in intensity and spectral shape at all points for which the wave field was sampled.

Reeves, G. D.↗
NASA NTRS2016

Radio Emission from Red-Giant Hot Jupiters

When planet-hosting stars evolve off the main sequence and go through the red-giant branch, the stars become orders of magnitudes more luminous and, at the same time, lose mass at much higher rates than their main sequence counterparts. Accordingly, if planetary companions exist around these stars at orbital distances of a few au, they will be heated up to the level of canonical hot Jupiters and also be subjected to a dense stellar wind. Given that magnetized planets interacting with stellar winds emit radio waves, such "Red-Giant Hot Jupiters" (RGHJs) may also be candidate radio emitters. We estimate the spectral auroral radio intensity of RGHJs based on the empirical relation with the stellar wind as well as a proposed scaling for planetary magnetic fields. RGHJs might be intrinsically as bright as or brighter than canonical hot Jupiters and about 100 times brighter than equivalent objects around main-sequence stars. We examine the capabilities of low-frequency radio observatories to detect this emission and find that the signal from an RGHJ may be detectable at distances up to a few hundred parsecs with the Square Kilometer Array.

stellar winds↗
NASA NTRS1984

Jovian decametric arcs - An estimate of the required wave normal angles from three-dimensional ray tracing.

Three-dimensional ray tracing is applied to an analysis of variable radio wave normal angle effects on the associated decametric (DAM) arc structures in the Jovian magnetosphere. The analysis is bed on 1-40 MHz radio signature recorded during Voyage 1 and 2 passages. The frequencies considered are above the R-X cut-off, and several ratios of the emission frequency/source frequency. The ray tracing code is based on a cold plasma formula and integration of the Hasselgrove (1955) equations. It is assumed that the emission is in the R-X mode, the source lies at the foot of an Io flux tube, and the emission cone is hollow. Attention is focused on data for two intense, vertex-late, high curvature DAM arc. A possible source for the arcs is found to be doppler-shifted gyroemission from a beam of electrons with an energy of 10 keV. A value of 1.1 is set as the limit of the doppler shift of the DAM emissions.

Menietti, J. D.↗
NASA NTRS1983

The Voyager Mission - Encounters with Saturn

The trajectories of Voyager 1 and Voyager 2 through the Saturnian system were selected in such a way that the observations would be complementary. The Voyager 1 trajectory provided a close encounter with Titan, making possible atmospheric occultations, a search for an intrinsic Titanian magnetic field, and studies of the interaction of Titan with either the solar wind or the Saturnian magnetosphere. The trajectory was also designed to provide an optimum geometry for the transmission of S and X band radio waves from the spacecraft through the rings; in this way, the attenuation and scattering of 3.6- and 13-cm waves by the ring particles could be observed. Besides this, Voyager 1 provided radio and ultraviolet occultation studies of Saturn's atmosphere as well as close approaches to three of the moderate-sized icy satellites (Mimas, Dione, and Rhea) and to a number of minor satellites. The much later arrival of Voyager 2 and the Saturn flyby distance were chosen in such a way as to make possible a gravity-assisted continuation on to Uranus and Neptune.

Stone, E. C.↗
NASA NTRS2005

Cassini Radio and Plasma Wave Observations at Saturn

Results are presented from the Cassini radio and plasma wave instrument during the approach and first few orbits around Saturn. During the approach the intensity modulation of Saturn Kilometric Radiation (SKR) showed that the radio rotation period of Saturn has increased to 10 hr 45 min plus or minus 36 sec, about 6 min longer than measured by Voyager in 1980-81. Also, many intense impulsive radio signals called Saturn Electrostatic Discharges (SEDs) were detected from saturnian lightning, starting as far as 1.08 AU from Saturn, much farther than terrestrial lightning can be detected from Earth. Some of the SED episodes have been linked to cloud systems observed in Saturn s atmosphere by the Cassini imaging system. Within the magnetosphere plasma wave emissions have been used to construct an electron density profile through the inner region of the magnetosphere. With decreasing radial distance the electron density increases gradually to a peak of about 100 per cubic centimeter near the outer edge of the A ring, and then drops precipitously to values as low as .03 per cubic centimeter over the rings. Numerous nearly monochromatic whistler-mode emissions were observed as the spacecraft passed over the rings that are believed to be produced by meteoroid impacts on the rings. Whistlermode emissions, similar to terrestrial auroral hiss were also observed over the rings, indicating that an electrodynamic interaction, similar to auroral particle acceleration, may be occurring in or near the rings. During the Titan flybys Langmuir probe and plasma wave measurements provided observations of the density and temperature in Titan's ionosphere.

Gurnett, D. A.↗
NASA NTRS1983

Application of surface acoustic wave devices to radio telemetry

Three experimental Surface Acoustic Wave Resonators (SAWR) are developed and evaluated. A desired center frequency is obtained by correct spacing of the Inter-Digital Transducers (IDT). Transmitting and receiving IDT's must be close for adequate coupling and a sufficient number of reflectors are required to create a high quality standing wave. A review of oscillator theory is given and current technology evaluated.

Strasilla, U.↗
NASA NTRS1992

Unusual radio and plasma wave phenomena observed in March 1991

During the intense solar flare activity in March 1991 a number of unusual radio emission and Langmuir wave phenomena were observed by the radio and plasma wave (URAP) experiment on the Ulysses spacecraft. These phenomena were associated with unusual conditions in the interplanetary medium (IPM) presumably resulting from intense solar activity. Some of these URAP observations cannot be explained by mechanisms usually attributed to interplanetary (IP) radio emissions and Langmuir wave activity and require other interpretations.

Reiner, M. J.↗