Engineering topics

Kurth, W. S.

Publications and source records attributed to Kurth, W. S..

At least 91 records · Page 5

NASA NTRS1991

Plasma wave generation near the inner heliospheric shock

There is mounting evidence that the Voyager 1 and 2 and Pioneer 11 spacecraft may approach the inner (termination) heliospheric shock near the end of this century. It is argued here, by analogy with planetary bow shocks, that energetic electrons backstreaming from the heliospheric shock along the magnetic field should be unstable to the generation of Langmuir waves by the electron beam instability. Analytic expressions for the cutoff velocity, corresponding to the beam speed of the electrons backstreaming from the shock, are derived for a standard solar wind model. At the front side of the heliosphere the maximum beam velocity is expected to be at the meridian passing through the nose of the shock, which is assumed to be aligned with the Very Local Inter-Stellar Medium flow. This foreshock region and the associated Langmuir waves are relevant to both the expected in situ observations of the heliospheric boundaries, and to the low-frequency (2-3 kHz) radio emissions observed by the Voyager spacecraft in the outer heliosphere. Provided that these radio emissions are generated by Langmuir waves, the minimum Langmuir wave electric fields at the remote source are estimated to be greater than about 3 - 30 microV/m.

Macek, W. M.↗

NASA NTRS1991

Magnetospheric radio and plasma wave research - 1987-1990

This review covers research performed in the area of magnetospheric plasma waves and wave-particle interactions as well as magnetospheric radio emissions. The report focuses on the near-completion of the discovery phase of radio and plasma wave phenomena in the planetary magnetospheres with the successful completion of the Voyager 2 encounters of Neptune and Uranus. Consideration is given to the advances made in detailed studies and theoretical investigations of radio and plasma wave phenomena in the terrestrial magnetosphere or in magnetospheric plasmas in general.

Kurth, W. S.↗

NASA NTRS1991

Voyager plasma wave observations near the outer planets

With the completion of the reconnaissance of the magnetospheres of the outer planets by Voyager, a first-order view is obtained of the existence of plasma waves in a wide variety of settings with which to begin the task of intercomparison. Voyager carried the first plasma-wave receivers to Jupiter, Saturn, Uranus, and Neptune and some very strong statements can be made about the likelihood of several types of plasma-wave modes and where in a planetary magnetosphere these emissions will occur. Further, the role of plasma waves can be understood, in general, in the physics of the magnetospheres and their effects on the energy budget, plasma energization and transport, and the generation of planetary radio emissions.

Kurth, W. S.↗

NASA NTRS1991

Wave-particle interactions in the magnetosphere of Uranus

The Voyager 2 encounter of Uranus has provided observations of plasma waves in and near the magnetosphere. These data, while the first from Uranus, will also be the only direct information on wave-particle interactions at this planet for many years to come. The observations include electrostatic waves upstream of the bow shock, turbulence in the shock Bernstein emissions and whistler mode waves in the magnetosphere, broadband electrostatic noise in the magnetotail, and a number of the other types of plasma waves which have yet to be clearly identified. Each of these types of waves exist in a plasma environment which both supports the growth of the waves and is modified by interactions with the waves. Wave-particle interactions provide the channels through which the waves can accelerate, scatter, or thermalize the plasmas. The most spectacular example in the case of Uranus is the extremely intense whistler mode activity in the inner magnetosphere which is the source of strong pitch angle diffusion. The resulting electron precipitation is sufficient to produce the auroral emissions observed by Voyager. The strong diffusion, however, presents the problem of supplying electrons in the range of 5 to 40 keV in order to support the losses to the atmosphere.

Kurth, W. S.↗

NASA NTRS1990

Whistlers in Neptune's magnetosphere - Evidence of atmospheric lightning

During the Voyager 2 flyby of Neptune, a series of 16 whistler-like events were detected by the plasma wave instrument near closest approach. These events were observed at radial distances from 1.30 to 1.99 R sub N and magnetic latitudes from -7 to 33 deg. The frequencies ranged from 6.1 to 12.0 kHz, and the dispersions fit the Eckersley law for lightning-generated whistlers. Lightning in the atmosphere of Neptune is the only known source of such signals. The frequency range of the whistlers (up to 12 kHz) indicates that the local electron densities are substantially higher (N sub e greater than 30 to 100 per cu cm) than indicated by the in situ plasma measurements. The dispersion of the whistlers is very large, typically 26,000 sec Hz(exp 0.5). Based on existing plasma density models and measurements, the dispersions are too large to be accounted for by a single direct path from the lightning source to the spacecraft. Therefore, multiple bounces from one hemisphere to the other are required. The most likely propagation path probably involves a lightning source on the dayside of the planet, with repeated bounces through the dense dayside ionosphere at low L-values.

Gurnett, D. A.↗

NASA NTRS1990

Low-frequency radio emissions at Neptune

The Voyager 2 plasma wave receiver detected weak radio emissions from Neptune's magnetosphere in the frequency range of 3 - 60 kHz. The emissions occurred in bursts lasting for typically 1.5 hours, often occurring twice per planetary rotation. Most of these radio bursts were detected within several degrees of the magnetic equatorial plane. During the passage through the magnetosphere, electrostatic upper hybrid resonance bands were observed close to the magnetic equator in conjunction with intensifications of the radio emissions at frequencies close to and above the upper hybrid bands. Further, near closest approach, the radio emissions were observed to cross the right-hand cutoff frequency with no apparent attenuation. It is concluded that the Neptunian radio emissions below about 60 kHz are produced by mode conversion from the upper hybrid waves and propagate in the ordinary mode into beams within about 12 deg of the magnetic equator. There is also evidence of an extraordinary mode emission at about 60 kHz which is apparently generated by an entirely different source from the escaping continuum radiation.

Kurth, W. S.↗

NASA NTRS1990

Comparison of plasma wave measurements in the bow shocks at Earth, Jupiter, Saturn, Uranus and Neptune

Plasma wave measurements from the Voyager 2 crossing of Neptune's bow shock are presented and compared with measurements from the bow shocks of Earth, Jupiter, Saturn, and Uranus. The wave amplitudes above 0.01fp, when normalized to the solar wind ion thermal energy density at each planet, are significantly higher at the outer planets than at Earth. Despite the differences in amplitude, the shock spectra of all the planets can be fitted to curves of similar form in this frequency range. The total normalized electric field energy densities exhibit an exponential dependence on ion thermal Mach number, Magnetosheath wave energies are comparable at all of the planets when normalized to the downstream plasma pressure.

Moses, S. L.↗

NASA NTRS1990

Electrostatic electron and ion cyclotron harmonic waves in Neptune's magnetosphere

Voyager 2 observations of electrostatic electron and ion cyclotron waves detected in Neptune's magnetosphere are presented. Both types of emission appear in a frequency band above the electron and ion (proton) cyclotron frequencies, respectively, and are tightly confined to the magnetic equator occurring within a few degrees of it. The electron cyclotron modes including an intense upper hybrid resonance emission excited by an unstable loss cone distribution of low-density superthermal electrons. The ion cyclotron waves are interpreted as hydrogen Bernstein modes including an intense lower hybrid resonance emission excited by an unstable ring distribution of low-density pickup N(+) ions deriving from the satellite Triton.

Barbosa, D. D.↗

NASA NTRS1990

Beam-generated upper hybrid noise in Jupiter's outer magnetosphere

A model for generation of upper hybrid waves in Jupiter's outer magnetosphere is presented. Energetic electrons accelerated on high-latitude auroral field lines at 1 R(J) altitudes are assumed to stream outwards to the distant magnetosphere. The large decrease in the local magnetic field strength results in a highly collimated, field-aligned electron beam due to conservation of the particle's magnetic moment. As the beam travels outward it passes through the plasma sheet boundary layer region located on the edges of the plasma sheet. Because of the large field-aligned anisotropy of the beam, the distribution is unstable to the excitation of the upper hybrid waves which amplify and then undergo mode conversion at the f(p) layer to generate the Jovian continuum radiation. A novel feature of the model is the beam-anisotropic heat flux instability which drives UH waves unstable without the need for any positive slope on the electron distribution function.

Barbosa, D. D.↗

NASA NTRS1990

Z mode radiation in Jupiter's magnetosphere - The source of Jovian continuum radiation

Observations of Z-mode waves in Jupiter's magnetosphere are analyzed. The assumption that the frequency of the intensity minimum, which isolates the signal, corresponds to the electron plasma frequency provides a consistent interpretation of all spectral features in terms of plasma resonances and cutoffs. It is shown that the continuum radiation is composed of both left-hand and right-hand polarized waves with distinct cutoffs observed at the plasma frequency and right-hand cutoff frequency, respectively. It is found that the Z-mode peak frequency lies close to the left-hand cutoff frequency, suggesting that the observed characteristics of the emission are the result of wave reflection at the cutoff layer. Another distinct emission occurring near the upper hybrid resonance frequency is detected simultaneously with the Z mode. The entire set of observations gives strong support to the linear mode theory of the conversion of upper hybrid waves to continuum radiation mediated by the Z mode via the Budden radio window mechanism.

Barbosa, D. D.↗

NASA NTRS1990

Whistlers in Neptune's magnetosphere: Evidence of atmospheric lightning

During the Voyager 2 flyby of Neptune, a series of 16 whistler-like events were detected by the plasma wave instrument near closest approach. These events were observed at radial distances from 1.30 to 1.99 R sub N and magnetic latitudes from -7 to 33 deg. The frequencies ranged from 6.1 to 12.0 kHz, and the dispersions fit the Eckersley law for lightning-generated whistlers. Lightning in the atmosphere of Neptune is the only known source of such signals. The frequency range of the whistlers (up to 12 kHz) indicates that the local electron densities are substantially higher (N sub e greater than 30 t0 100 per cu cm) than indicated by the in situ plasma measurements. The dispersion of the whistlers is very large, typically 26,000 sec Hz(exp 0.5). Based on existing plasma density models and measurements, the dispersions are too large to be accounted for by a single direct path from the lightning source to the spacecraft. Therefore, multiple bounces from one hemisphere to the other are required. The most likely propagation path probably involves a lightning source on the dayside of the planet, with repeated bounces through the dense dayside ionosphere at low L-values.

Gurnett, D. A.↗

NASA NTRS1990

Theory and observations of electrostatic ion waves in the cold Io torus

A study of the ELF plasma wave environment of the cold Io torus in Jupiter's magnetosphere is made. Voyager 1 data are presented which show three distinct types of electrostatic ion waves occurring there: the Buchsbaum ion-ion mode just below the proton cyclotron frequency f(cp), hydrogen Bernstein modes at (n + 1/2) f(cp), and lower hybrid waves near f(LHR). The presence of these waves at their characteristic frequencies is consistent with a predominantly heavy ion plasma composed of singly ionized sulfur and oxygen ions along with a small admixture of protons. The hydrogen Bernstein modes are tightly confined to the magnetic equator, occurring within + or - 4 deg of it, while the Buchsbaum mode is localized to the dense heavy ion plasma of the cold torus near the centrifugal equator. A general theory for excitation of the waves based on the ion pickup process is developed.

Barbosa, D. D.↗

NASA NTRS1990

Source location of the narrowbanded radio bursts at Uranus - Evidence of a cusp source

While Voyager 2 was inbound to Uranus, radio bursts of narrow bandwidth (less than 5 kHz) were detected between 17-116 kHz. These R-X mode bursts, designated n-bursts, were of short duration, tended to occur when the north magnetic pole tipped toward the spacecraft, and increased in occurrence with increasing solar wind density. An explicit determination of the burst source location is presented, based upon fitting the region of detection at high and low frequencies to field-aligned, symmetric cones. The region of good fits was located between the north magnetic pole and the rotational pole, corresponding approximately to the northern polar cusp.

Farrell, W. M.↗

NASA NTRS1990

Continuum radiation at Uranus

One Uranian radio emission which has thus far escaped attention is an analog of continuum radiation at earth, Jupiter, and Saturn. The emission is found to be propagating in the ordinary mode in the range of one to a few kHz on the inbound leg of the Voyager 2 encounter, shortly after the magnetopause crossing. The Uranian continuum radiation is notably weak, making it more like that detected at Saturn than the extremely intense Jovian continuum radiation. The Uranian emission shows some evidence for narrow-band components lying in the same frequency regime as the continuum, completing the analogy with the other planets, which also show narrow-band components superimposed on the continuum spectrum. It is argued that the low intensity of the Uranian continuum is most likely related to the lack of a density cavity within the Uranian magnetosphere that is deep relative to the solar wind plasma density.

Kurth, W. S.↗

NASA NTRS1990

Spacelab 2 Plasma Diagnostics Package

The Plasma Diagnostics Package is a small, deployable satellite designed to study the interaction of the Space Shuttle Orbiter with the ionospheric environment as well as to be used in joint experiments with the plasma depletion and the vehicle charging and potential investigations during the Spacelab 2 mission. This paper provides a brief description of the small spacecraft, its instrumentation and operation, and the scientific objectives of the investigations. A brief summary of the scientific results obtained thus far is also presented.

Kurth, W. S.↗

NASA NTRS1990

Radio noise in the heliospheric cavity

The LF radio emission of the heliospheric cavity is discussed, summarizing Voyager measurement data. The solar wind is considered to be the outer layer of the solar atmosphere, and its interaction with the interstellar medium is examined in detail. Typical data are presented graphically, and theoretical models proposed to explain the emission are reviewed. It is suggested that the emission may originate at the terminal shock or heliopause, thus providing a means of estimating its location.

Kurth, W. S.↗

NASA NTRS1989

First plasma wave observations at Neptune

The Voyager 2 plasma wave instrument detected many familiar plasma waves during the encounter with Neptune, including electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, and chorus, hiss, electron cyclotron waves, and upper hybrid resonance waves in the inner magnetosphere. Low-frequency radio emissions, believed to be generated by mode conversion from the upper hybrid resonance emissions, were also observed propagating outward in a disklike beam along the magnetic equatorial plane. At the two ring plane crossings many small micrometer-sized dust particles were detected striking the spacecraft. The maximum impact rates were about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about 1000 kilometers thick, centered on the equatorial plane. However, a broader more tenuous distribution also extends many tends of thousands of kilometers from the equatorial plane, including over the northern polar region.

Gurnett, D. A.↗

NASA NTRS1989

Plasma density fluctuations observed during Space Shuttle Orbiter water releases

Observations by the Langmuir probe on the Plasma Diagnostics Package flown as part of the Spacelab 2 mission in the summer of 1985 show a strong increase in the level of turbulence near the Shuttle Orbiter during operations in which liquid water is released. The spectrum of the plasma density fluctuations peaks at the lowest frequencies measured (a few Hz) and extends up to a few kHz, near the lower hybrid frequency. Two potential mechanisms for generating the plasma turbulence are suggested which are both based on the production of water ions as a result of charge exchange with the ambient oxygen ions in the ionosphere. The first mechanism proposed is the ion-plasma instability which arises from the drift of the contaminant with respect to the ambient oxygen ions. The other mechanism proposed is the Ott-Farley instability, which is a result of the ring distribution formed by the 'pick-up' water ions.

Pickett, J. S.↗