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NASA NTRS1977

Lower hybrid emission diagnostics on the NASA Lewis bumpy torus

The feasibility of using RF emission near the lower hybrid frequency of the NASA Lewis Bumpy Torus plasma for diagnostic purposes is examined. The emission is detected using a spectrum analyzer and a 50 omega miniature coaxial antenna that is sensitive to the polarization of the incoming signal. The frequency shift of the lower hybrid emission peak is monitored as a function of the background pressure, electrode voltage, electrode ring configuration and the strength of the toroidal dc magnetic field. Simultaneous measurements of the average plasma density are made with a polarization diplexing microwave interferometer. Data derived from the experiment are discussed with reference to the following: (1) the strength of the dc magnetic field in the emitting region; (2) comparison of the lower hybrid plasma density with the average plasma density; and (3) validity of the cold plasma lower hybrid resonance formula in the high density operating regime of the bumpy torus plasma.

Mallavarpu, R.↗

NASA NTRS1977

Lower hybrid emission diagnostics on the NASA Lewis Bumpy Torus

The feasibility of using RF emission near the lower-hybrid frequency of the NASA Lewis Bumpy Torus plasma for diagnostic purposes is examined. The emission is detected using a spectrum analyzer and a 50-ohm miniature coaxial antenna that is sensitive to the polarization of the incoming signal. The frequency shift of the lower-hybrid emission peak is monitored as a function of the background pressure, electrode voltage, electrode ring configuration, and the strength of the toroidal dc magnetic field. Simultaneous measurements of the average plasma density are made with a polarization diplexing microwave interferometer. The experimental results extend previous work to include negative electrode voltages and plasma densities up to 1 trillion per cu cm. The information derived from the experiment is discussed with reference to: (1) the strength of the dc magnetic field in the emitting region, (2) a comparison of the lower-hybrid plasma density with the average plasma density, and (3) the validity of the cold-plasma lower-hybrid resonance formula in the high-density operating regime of the bumpy-torus plasma.

Mallavarpu, R.↗

NASA NTRS1979

Auroral hiss observed near the Io plasma torus

Characteristics of auroral hiss emissions detected near the Io plasma torus by Voyager 1 are described. These characteristics are compared with those of similar emissions detected in earth's magnetosphere. Implications are discussed regarding the interaction of the Io plasma torus with the Jovian magnetosphere.

Gurnett, D. A.↗

NASA NTRS1979

Pitch-angle diffusion by whistler mode waves near the Io plasma torus

As Voyager 1 traversed the inner radiation belt of Jupiter, wave-particle interactions involving energetic electrons and whistler mode turbulence were strongly affected by the presence of the Io plasma torus. Within the high density torus the resonant electron energy was low and the associated high index of refraction yielded high B-to-E ratios for the wave fields, leading to very strong pitch-angle scattering. It is shown that significant spatial and temporal variations in plasma conditions produced large fluctuations in local scattering times, and the problems associated with the evaluation of precipitation lifetime are discussed.

Scarf, F. L.↗

NASA NTRS1980

Spatial distribution of plasma in the Io torus

In situ measurements of ion densities and temperatures have been analyzed to produce profiles of these plasma parameters along the Voyager 1 inbound trajectory between 7 and 5 Jupiter radii. The temperature profile shows a sharp decrease by a factor of 50 between 5.8 and 5.2 Jupiter radii corresponding to a temperature gradient of 7 x 10 to the 5th per Jupiter radius. The electron density profile, inferred from the ion density measurements, has two maxima at 5.7 and 5.3 Jupiter radii. A two-dimensional model of the spatial distribution of various ionic species in the Io plasma torus has been constructed. Using this model a contour map of electron density in a meridional plane has been made, it exhibits a well-defined inner edge to the torus at 5.6 Jupiter radii. The contour map of S(+) ion density indicates that most of the S(+) ions are concentrated close to the centrifugal symmetry surface and radially inward of the larger electron density maximum near 5.7 Jupiter radii.

Bagenal, F.↗

NASA NTRS1980

The Jovian SII torus - Its longitudinal asymmetry

The variations with Jovian magnetic longitude observed in the intensity of the S II emission from the plasma torus surrounding Jupiter near the orbit of Io are investigated. Spectrographic observations of S II emission from 2.1 to 8.3 Jupiter radii from the planet obtained from 1976 to 1979 were examined to determine emission intensities in relation to position. The detected emission is found to be sharply confined to a magnetic latitude within 12 deg of the equator and a region inside 6.7 Jupiter radii, just outside of Io's orbit, with maximum emission at a magnetic longitude of 60 deg. The region of maximum brightness is observed to include the active centers for the currents expected in the flux tubes connected with Io, support of the magnetic anomaly models of Dressler and Hill (1979). Electron temperatures between 10 to the 3.92 and 10 to the 4.80, and densities of 10 to the 4.2/cu cm and a S II column abundance of 1.5 x 10 to the 13th/sq cm are estimated for the brightness region near the ansa of the emission torus. An order of magnitude difference between S II column abundances obtained in the present investigation and found by Brown (1978) is attributed to a change in sulfur content, possibly related to Ionian volcanic activity, while electron temperatures and densities are found to remain fairly constant.

Trafton, L.↗

NASA NTRS1981

Observations of the Io plasma torus

The short wavelength spectrography on the IUE satellite was used to obtain spectra of the plasma torus near the orbit of Io about Jupiter. Three exposures of about 8 hours each taken in March and May 1979 show emission features due to SII, SIII, and OIII. The absence of features at other wavelengths permits upper limits to be other species in the torus.

Lane, A. L.↗

NASA NTRS1981

Atomic clouds as distributed sources for the Io plasma torus

Several recent developments have implications for the neutral particle environment of Jupiter. Very hot sulfur ions have been detected in the Io torus with gyrospeeds comparable to the corotation speed, a phenomenon that would result from a neutral sulfur cloud. Current evidence supports the hypothesis that extensive neutral clouds of oxygen and sulfur exist in the Jupiter magnetosphere and that they are important sources of ions and energy for the Io torus.

Brown, R. A.↗

NASA NTRS1981

Dispersive Doppler measurement of the electron content of the torus of Io

As Voyager 1 made its swing-by of Io, it passed through and behind the satellite's plasma torus. The phase paths of the coherent 13 cm and 3.6-cm wavelength signals transmitted from the spacecraft were shortened differentially by the plasma, resulting in the observation of a dispersive Doppler signature in the signals received at the NASA/Jet Propulsion Laboratory (deep space network) stations. Ray path integration through three different models of the electron distribution of the torus of Io (Warwick et al. 1979; Birmingham et al. 1981; Bagenal et al., 1981) have been performed. The results of the integrations are compared with the dispersive Doppler data.

Levy, G. S.↗

NASA NTRS1981

Observations of electron gyroharmonic waves and the structure of the Io torus

Narrow-banded emissions observed by the planetary radio astronomy experiment on the Voyager 1 spacecraft as it traversed the Io plasma torus are discussed. It is found that the waves occur between harmonics of the electron gyrofrequency; they are Jovian analogue of electrostatic emissions observed and theoretically studied for the terrestrial magnetosphere. It is noted that the observed frequencies always include the component near the upper hybrid resonant frequency but that the distribution of the other observed emissions varies in a systematic way with position in the torus. A detailed discussion of the observations is given. Also included is a refined model of the electron density variation, based on identification of the upper hybrid resonant frequency line.

Birmingham, T. J.↗

NASA NTRS1981

Observations of the plasma torus of Jupiter with a Fabry-Perot/charge-coupled device /CCD/ imaging spectrometer

A description is presented of the use of a CCD imaging spectrometer which has been employed at a 2.1 m telescope to obtain monochromatic images in the red and near infrared. The system studied was Jupiter's plasma torus which circles the planet with radial extent about 5 RJ and 7 RJ (RJ is the radius of Jupiter). In ground based measurements the torus has been observed in the forbidden emission lines of S(plus) at 6716 A and 6731 A and S(plus plus) at 9531 A. Attention is given to aspects of instrumentation, observations, and performance. It is felt that the particular significance of the obtained results from the instrumental point of view is the demonstration that the CCD is an excellent detector for monochromatic imaging in the near infrared out to at least 10830 A and that pixel binning before readout can produce significantly improved S/N ratios for the study of faint, diffuse sources in cases where readout noise is dominant.

Roesler, F. L.↗

NASA NTRS1983

Analytical and experimental study of a standing torus with normal loads

Finite element analysis is applied to study the large deflection and stress of a thin-walled pressurized torus loaded by normal contact with a plane. Experimentally determined stresses are compared to those resulting from finite element analysis at various meridional and circumferential coordinates of the torus, including the footprint area. Circumferential strains compare favorably while meridional strains are higher in the finite element analysis, probably due to slippage of the boundary at the rim. The STAGS finite element computer program was used. The purpose of the study was to evaluate the various program options for structural loading, for material modeling, for stress analysis, and for grid refinement. The experimental model, a thin-walled rubber tube mounted on a steel cylindrical rim, provided measured results to compare with various analytical trade-offs. It was found that there was almost no difference in predicted deflections or stress distributions between linear and nonlinear material description. However, the difference between linear analysis and that of geometric nonlinearities utilizing incremental loading was marked.

Mack, M. J., Jr.↗

NASA NTRS1983

Observed departure of the Io plasma torus from rigid corotation with Jupiter

A study of forbidden S II red doublet spectra indicates that the Io plasma torus at 5.9 Jupiter radii does not corotate rigidly with Jupiter. The lag is found to be 6 percent + or 4 percent, where the variability range, not the uncertainty, is indicated. Comparison with existing models indicates the lag may be due primarily to ion creation in the Io torus.

Brown, R. A.↗

NASA NTRS1983

Analysis of a thin-walled pressurized torus in contact with a plane

Finite element analysis is applied to study the large deflection of a standing torus loaded by a plane. The internally pressurized thin-walled structure is found to have an elliptical footprint area. Considerable bulge occurs in the sidewall in the region of the load plane. Stress distributions throughout the torus are shown for various load levels and for various modeling strategies at a given load level. In large load ranges finite element calculations show compressive circumferential stress and negative curvature in the footprint region. Results are compared with inelastic wall analysis.

Mack, M. J., Jr.↗

NASA NTRS1983

Ion cyclotron waves in the Io plasma torus - Polarization reversal of whistler mode noise

Because of the presence of multiple ion species in the Io plasma torus, whistler mode noise can be converted to ion cyclotron waves via a polarization reversal process at the local crossover frequency. Using whistler mode intensity measurements in the Jovian magnetosphere from Voyager 1, the pitch-angle diffusion rates that would occur if the noise is converted to ion cyclotron waves is estimated. Typical pitch-angle diffusion coefficients range from 0.000001/sec for protons resonating near the equator to 0.0001/sec for 10-keV O(+) ions resonating at high latitudes. Although complete bounce averaged diffusion coefficients have not yet been computed, preliminary estimates indicate that the energetic ion precipitation caused by these waves may be able to account for the EUV auroral emissions at the foot of the torus field lines.

Gurnett, D. A.↗

NASA NTRS1983

Longitudinal asymmetry of the Io plasma torus

Remote observations of the Io plasma torus have revealed a persistent longitudinal asymmetry in the brightness of optical S(+) emission lines but not of ultraviolet S(2+) emission lines. The S(+) asymmetry (with respect to the corotating System III coordinates) has been attributed to an asymmetric plasma source; the question then arises, why do the higher ionization states of sulfur not reflect a similar asymmetry? The explanation proposed here is based on the fact that torus ions do not quite corotate at the System III rate because of mass loading. If the resulting System III drift period of the ions is intermediate between the ionization lifetimes of S(+) and S(2+), then the asymmetry of the source will be obscured by longitudinal drift for the relatively long-lived multiply-ionized sulfur ions, but not for short-lived S(+).

Hill, T. W.↗

NASA NTRS1983

Coulomb thermal properties and stability of the Io plasma torus

Coulomb collisional energy exchange rates are computed for a model of the Io plasma torus consisting of newly created pickup ions, a background of thermally degraded intermediary ions, and a population of cooler electrons. The electrons are collisionally heated by both the pickup ions and background ions and are cooled by electron impact excitation of plasma ions which radiate in the EUV. It is found that a relative concentration of S III pickup ions forbidden S III/electrons = 0.1 with a temperature of 340 eV can deliver energy to the electrons at a rate of 3 x 10 to the -13th erg/cu cm per sec, sufficient to power the EUV emissions in the Io torus. The model predicts a background ion temperature Ti of about 53 eV and an electron temperature Te of about 5.5 eV on the basis of steady-state energy balance relations at Coulomb rates. The model also predicts electron temperature fluctuations at the 30 percent level on a time scale of less than 11 hours, consistent with recent observations of this phenomenon.

Barbosa, D. D.↗

NASA NTRS1984

Voyager 1 evidence for ion-cyclotron instability in the vicinity of the Io plasma Torus

The Voyager 1 traversal of the Io plasma torus was marked by pronounced enhancement in low frequency wave activity with especially strong wave intensification below the proton gyrofrequency, suggesting the excitation of ion-cyclotron waves over the same radial range in which the energetic ring current ions exhibit phase space density depletion due to strong pitch angle scattering loss to the atmosphere. If the more intense emissions observed intermittently by Voyager 1 are representative of waves in the high latitude source region, the resulting pitch angle scattering would be sufficiently rapid to explain the observed energetic ion losses and the excitation of intense Jovian auroral emissions of field lines passing through the torus.

Thorne, R. M.↗