Engineering topics

Kurth, W. S.

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

At least 163 records · Page 9

NASA NTRS1982

Detection of nonthermal continuum radiation in Saturn's magnetosphere

A detailed analysis of high resolution wideband data from the Voyager 1 and 2 plasma wave receivers has revealed the presence of heretofore undiscovered nonthermal continuum radiation trapped within the Saturnian magnetosphere. The discovery of Saturnian trapped continuum radiation fills a disturbing void in the Saturnian radio spectrum. On the basis of observations at both the earth and Jupiter it was expected that continuum radiation should be a pervasive signature of planetary magnetospheres in general. Special processing of the Voyager 1 plasma wave data at Saturn has now confirmed the existence of weak emissions that have a spectrum characteristic of trapped continuum radiation. Similar radiation was also detected by Voyager 2; however, in this case it is not certain that Saturn was the only source. Considerable evidence exists which suggests that Saturn may have been immersed in the Jovian tail during Voyager 2 encounter, so that Jupiter may provide an additional source of the continuum radiation detected by Voyager 2.

Kurth, W. S.↗

NASA NTRS1982

The structure of Titan's wake from plasma wave observations

The electron density profile inferred from plasma wave emissions detected during the Voyager 1 flyby of Titan exhibits three distinct peaks with densities of about 40/cu cm, the first peak corresponding to the entry into the magnetic tail, the second corresponding to the neutral sheet crossing from the northern to the southern tail lobe, and the third corresponding to the outbound exit from the tail. Large depressions in the magnetic field strength are observed coincident with each of the density peaks, indicating that a dense plume of plasma is being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn. The 8600 K plasma temperature estimated suggests that the plasma originates from the ionosphere of Titan, probably forming a plasma plume with a theta or H cross section extending downstream from Titan.

Gurnett, D. A.↗

NASA NTRS1982

Voyager 2 plasma wave observations at Saturn

The first inbound Voyager 2 crossing of Saturn's bow shock (at 31.7 Saturn radii near local noon) and the last outbound crossing (at 87.4 Saturn radii near local dawn) had similar plasma wave signatures. However, many other aspects of the plasma wave measurements differed considerably during the inbound and outbound passes, suggesting the presence of effects associated with significant north-south or noon-dawn asymmetries, or temporal variations. Within Saturn's magnetosphere, the plasma wave instrument detected electron plasma oscillations, upper hybrid resonance emissions, half-gyrofrequency harmonics, hiss and chorus, narrowband electromagnetic emissions and broadband Saturn radio noise, and noise bursts with characteristics of static. At the ring plane crossing, the plasma wave instrument also detected a large number of intense impulses that were interpreted in terms of ring particle impacts on Voyager 2.

Scarf, F. L.↗

NASA NTRS1981

Measurements of plasma wave spectra in Jupiter's magnetosphere

Compressed plots of E field averages for all of the 16-channel spectrum analyzer data from the Voyager 1 and 2 magnetosphere traversals are presented to provide an overall framework for the discussion. The importance of considering peaks as well as averages is illustrated by using 16-channel measurements from the first inbound and last outbound bow shock for Voyager 2. Selected wideband measurements from the waveform receivers are presented to demonstrate how many important wave bursts are variable in times less than or comparable to the 4-s scan period of the 16-channel analyzer.

Scarf, F. L.↗

NASA NTRS1981

Determination of Jupiter's electron density profile from plasma wave observations

The electron density measurements obtained in the Jovian magnetosphere from the plasma wave instruments on the Voyager 1 and 2 spacecraft are summarized. Three basic techniques for determining the electron density are discussed. They are (1) local measurements from the low-frequency cutoff of continuum radiation, (2) local measurements from the frequency of upper hybrid resonance emissions, and (3) integral measurements from the dispersion of whistlers. The limitations and advantages of each technique are reviewed.

Gurnett, D. A.↗

NASA NTRS1981

Broadband electrostatic noise and field-aligned currents in Jupiter's middle magnetosphere

Voyager 1 plasma wave observations have revealed the presence of an impulsive electrostatic emission localized to the Jovian middle magnetosphere that appears on the edges of the plasma sheet. This plasma mode has the same spectral and morphological characteristics of an emission that has been extensively studied in the earth's magnetosphere and has been associated with the presence of field-aligned currents. The results of a detailed study of the properties of this Jovian emission are presented by using comparisons with terrestrial observations as a basis for mode identification. The occurrence regions of the waves are compared with the measured magnetic field configuration to establish a correspondence with the plasma sheet. It is argued that this is a quasi-permanent global system of field-aligned currents linking the ionosphere of Jupiter to the middle magnetosphere, which powers energetic plasma heating processes occurring there. On the basis of knowledge of the consequences of field-aligned currents in the terrestrial magnetosphere, a scenario for acceleration/precipitation of inverted V electrons, concomitant aurorae, and energetic (approximately 10 keV) proton deposition into the middle magnetosphere resulting from field-aligned potential drops associated with this current system is suggested.

Barbosa, D. D.↗

NASA NTRS1981

Voyager observations of Jupiter's distant magnetotail

Observations of locally trapped nonthermal continuum radiation and distant encounters with the Jovian magnetotail are reported. An attempt is made to deduce information on the morphology of the distant tail of Jupiter. It is noted that the observations of continuum radiation trapped in low-density regions of the solar wind suggest that Voyager may at times be connected to the distant tail by a low-density trough which acts as a wave guide and allows radiation from the tail to reach the spacecraft.

Kurth, W. S.↗

NASA NTRS1981

Parametric interaction and spatial collapse of beam-driven Langmuir waves in the solar wind

Observations are presented of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. Long wavelength Langmuir waves upstream of the bow shock are effectively converted into short wavelength waves no longer in resonance with the beam. The conversion is shown to be the result of a nonlinear interaction involving the beam-driven pump, a sideband emission, and a low level of ion-acoustic turbulence. The beam-driven Langmuir wave emission breaks up into a complex sideband structure with both positive and negative Doppler shifts. In some cases, the sideband emission consists of isolated wave packets with very short duration bursts, which are very intense and are thought to consist of envelope solitons which have collapsed to spatial scales of only a few Debye lengths.

Gurnett, D. A.↗

NASA NTRS1981

Narrowband electromagnetic emissions from Saturn's magnetosphere

A series of narrowband electromagnetic emissions were detected by the plasma wave instrument on board Voyager 1 coming from the inner region of Saturn's magnetosphere in the frequency range 3-30 kHz. These emissions have many similarities to continuum radiation detected in the earth's magnetosphere and narrowband kilometric radiation in the Jovian magnetosphere. The observed frequency spacing suggests that the emissions are being generated near Tethys, Dione and Rhea, probably in regions of large plasma density gradients associated with boundaries of the plasma sheet.

Gurnett, D. A.↗

NASA NTRS1981

Control of Saturn's kilometric radiation by Dione

Voyager 1 observations of Saturn's kilometric radio emissions reveal a strong but apparently transitory control by the orbital phase angle of Dione. This may be a geometrical effect and a time-variable plasma torus associated with Dione could explain most of the observed details of the Dione modulation by creating a shadow zone near the equatorial plane.

Kurth, W. S.↗

NASA NTRS1981

Plasma wave turbulence at planetary bow shocks

Voyager 1 observations of plasma wave turbulence at Saturn's bow shock are discussed and compared with corresponding data from Jupiter, earth, and Venus. The results suggest that the plasma instabilities that develop at the lower Mach number bow shocks of the terrestrial planets differ from those found at the high Mach number bow shocks of the outer planets.

Scarf, F. L.↗

NASA NTRS1981

Jupiter tail phenomena upstream from Saturn

Plasma wave and plasma probe measurements from Voyager 2 for February 1981 suggest the detection of phenomena associated with a well defined Jupiter tail, at a distance of about 6,200 Jovian radii. This is held to imply that the Saturn magnetosphere will be affected by the Jovian tail, and that insight into the physics of Saturn's magnetosphere may be obtained through comparisons of Voyager 1 and 2 data. Among the effects that can be sought in Voyager 2 data are magnetosphere size variations, bow shock location, radio emission strength and trapped radiation belt population.

Scarf, F. L.↗

NASA NTRS1981

An upper bound to the lightning flash rate in Jupiter's atmosphere

Lewis (1980) examined Voyager optical measurements and low-frequency radio-wave observations related to lightning discharges in Jupiter's atmosphere using whistler measurements from the plasma-wave system and a specific set of assumptions. Estimates of the average planetary lightning stroke rate were found to be from 0.04 to 0.0001 flashes per square kilometer per year. In the present paper, the planetary lightning rate is demonstrated to be as high as several tens of flashes per square kilometer per year. The same whistler data are used; however, different physical assumptions about the source area, including whistler paths and whistler amplitude distributions are incorporated.

Scarf, F. L.↗

NASA NTRS1981

Escaping nonthermal continuum radiation

Gurnett (1975) provided details of the continuum radiation spectrum, separating the lower frequency trapped component from an escaping component at frequencies high enough to propagate freely out of the magnetosphere and into the solar wind. The primary motivation for the present investigation is related to the new ISEE observations which show that the escaping continuum radiation differs from the trapped component in several temporal and spectral characteristics. The observations of escaping nonthermal continuum radiation are considered, taking into account spectral and temporal characteristics of escaping continuum radiation and the source region of escaping continuum radiation. Attention is given to the relationship between escaping and trapped continuum radiation, and the relationship between terrestrial escaping continuum radiation and narrowband Jovian kilometric radiation

Kurth, W. S.↗

NASA NTRS1981

Plasma waves near Saturn - Initial results from Voyager 1

The Voyager 1 plasma wave instrument detected many familiar types of plasma waves during the encounter with Saturn, including ion-acoustic waves and electron plasma oscillations upstream of the bow shock, an intense burst of electrostatic noise at the shock, and chorus, hiss, electrostatic electron cyclotron waves, and upper hybrid resonance emissions in the inner magnetosphere. A clocklike Saturn rotational control of low-frequency radio emissions was observed, and evidence was obtained of possible control by the moon Dione. Strong plasma wave emissions were detected at the Titan encounter indicating the presence of a turbulent sheath extending around Titan, and upper hybrid resonance measurements of the electron density show the existence of a dense plume of plasma being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn.

Gurnett, D. A.↗

NASA NTRS1981

Plasma waves near Saturn: Initial results from Voyager 1

The Voyager 1 encounter with Saturn provided the first opportunity to investigate plasma wave interactions in the magnetosphere of Saturn. An overview of the principal results from the Voyager 1 plasma wave instrument is presented starting with the initial detection of Saturn and ending about four weeks after closest approach. A survey plot of the electric field intensities detected during the Saturn encounter is shown starting shortly before the inbound shock crossing and ending shortly after the outbound magnetopause crossing. Many intense waves were observed in the vicinity of Saturn. To provide a framework for presenting the observations, the results are discussed more or less according to the sequence in which the data were obtained.

Gurnett, D. A.↗

NASA NTRS1980

Superthermal electrons and Bernstein waves in Jupiter's inner magnetosphere

A theoretical model for generation of banded electrostatic emissions by low density, superthermal electrons is developed for application to Jupiter's magnetosphere. The model employs a power law form for the energy dependence and a loss cone pitch angle distribution of the superthermals to drive convective instability of Bernstein modes. A direct correspondence between spectral features of the 3/2 band and resonant superthermal electrons is found. The concept of a critical flux of resonant electrons able to provide 10 e-foldings of electric field amplification yields an explicit relation in terms of the background thermal electron pressure. This result is used to construct a theoretical/empirical model of thermal electron density and temperature from 6-20 Jupiter radii in the Jovian magnetosphere which suggests that the electron temperature is less than the ion temperature which is approximately equal to 10 times the electron temperature in this region. Finally, wave ray paths are computed for propagation in the magnetic equator and in the magnetic meridional plane of a dipole magnetic field. These ray paths suggest that intense wave activity is tightly confined to a small latitudinal extent, less than + or - approximately 4 deg, about the magnetic equator.

Barbosa, D. D.↗

NASA NTRS1980

Observations of a free-energy source for intense electrostatic waves

Significant progress has been made in understanding intense electrostatic waves near the upper hybrid resonance frequency in terms of the theory of multiharmonic cyclotron emission using a classical loss-cone distribution function as a model. Recent observations by Hawkeye 1 and GEOS 1 have verified the existence of loss-cone distributions in association with the intense electrostatic wave events, however, other observations by Hawkeye and ISEE have indicated that loss cones are not always observable during the wave events, and in fact other forms of free energy may also be responsible for the instability. Now, for the first time, a positively sloped feature in the perpendicular distribution function has been uniquely identified with intense electrostatic wave activity. Correspondingly, we suggest that the theory is flexible under substantial modifications of the model distribution function.

Kurth, W. S.↗