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At least 1,387 records · Page 77

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NASA NTRS1985

Thermalization of neutral-beam-injected ions by lower hybrid waves in Jupiter's magnetosphere

A theoretical analysis of lower-hybrid wave generation by a ring distribution of superthermal ions is presented. A kinetic instability of waves traveling across the magnetic field occurs when fast neutrals escaping from the Io plasma torus are reionized and accelerated to corotation speed in Jupiter's magnetosphere. Instability is possible for modest threshold values of V/a0 = 3 - 7 where V is the pickup gyrospeed and a0 is the background-ion thermal speed. Consequent effects include the relaxation of the pickup-ion distribution and the acceleration of superthermal electrons.

Barbosa, D. D.↗

NASA NTRS1984

Vertical distribution of disequilibrium species in Jupiter's troposphere

Sources of organic matter and inorganic tracers on Jupiter, including solar UV photolysis, lightning discharges, and convective quenching of hot gases from the lower atmosphere, are reviewed in light of earth-based and Voyager data with the purpose of predicting the tropospheric steady-state abundances and vertical distributions of HCN, CH2O, and other species. It is concluded that a steady-state mole fraction of HCN in the Jovian troposphere of only about 10 to the -12th could be maintained by vertical transport of hot gases from the deep atmosphere. The observed HCN abundance, roughly X(HCN) = 10 to the -9th, appears to be due to photochemical reactions. After HCN, the most abundant organic-disequilibrium species in the troposphere is probably C2H6, derived from direct photolysis of CH4 at high altitudes, with a mole fraction of about 10 to the -10th at the H2O cloud level. Inorganic tracers of disequilibrium processes are also briefly summarized.

Lewis, J. S.↗

NASA NTRS1985

Error analyses for the delivery of a spinning probe to Jupiter

The task of delivering the Galileo Probe to specified atmospheric entry conditions at Jupiter is especially challenging because tracking, trajectory corrections, and attitude adjustments are not possible after release of the Probe from the carrier vehicle. Statistical analysis of the spacecraft dynamics mapped into Probe dispersions in atmosphere-relative and relay-geometry parameters show that attitude stability, heating, and relay performance requirements can be satisfied. Reconstruction techniques are used to enhance estimates of the delivery parameters to permit correct interpretation of the scientific data for the Jovian atmosphere. A tradeoff which sacrifices some delivery accuracy and propellant is shown to guarantee satisfaction of a very tight reconstruction requirement for the trajectory considered in this report.

Hintz, G. R.↗

NASA NTRS1984

The ethereal rings of Jupiter and Saturn

The brighter main band of the Jovian ring, which is tenuous and exhibits little substructure, lies in the equatorial plane at 1.7-1.8 Jupiter radii, R(J); this is well within the Roche limit. Since dynamical evolution times and survival lifetimes for micron grains are of the order of less than 100-1000 years, the visible Jovian ring must be continuously replenished. It is suggested that most small grains are generated by micrometeoroids colliding into unseen parent bodies residing in the main band. The halo is composed of still smaller particles and is radially localized at about 1.3-1.7 R(J). Unlike the rings of Saturn and Uranus, the Jovian rings are distinguished by the determination of their form by single-particle dynamics rather than collective effects.

Burns, J. A.↗

NASA NTRS1985

Discovery of Jupiter's 'gossamer' ring

Enhancement of Voyager 2's parting shot of the Jupiter system reveals an additional ring. This ring is far fainter than either of the components previously identified, and extends to a radius of 210,000 km. The possible sources of this 'gossamer' ring are discussed.

Showalter, M. R.↗

NASA NTRS1985

Opening of the magnetic field lines in a fast rotating magnetosphere, with an application to Jupiter

A simple model of a magnetosphere rotating around a rotating Jupiter-like object with a spin-aligned dipolar moment mu is considered. A low-energy plasma is released by inner sources located beyond the corotation radius and is diffused outward through closed lines. The lines form a closed thin equatorial disk, maintaining a quasi-static balance between the centrifugal force and the magnetic tension. Solutions are obtained for the analytical equations characterizing the plasma in the closed region; the structure of the magnetic field outside the disk; and the self-consistent value of the critical radius r sub zero. The analytical solutions are compared to the observed properties of the Jovian magnetosphere, and the results are discussed in detail.

Aly, J. J.↗

NASA NTRS1985

Estimation and comparison of quasilinear electron heating in the shock foot at Jupiter and earth

A simple quasilinear model is developed to estimate the electron heating that occurs in the foot of a supercritical, quasiperpendicular shock through interactions with electrostatic waves generated by reflected ions. At earth the increase in electron thermal energy calculated using the measured wave amplitudes is negligible, while at Jupiter it is comparable with the observed temperature gain across the shock. The anisotropic quasilinear heating should destabilize whistler mode waves in the foot. These have been detected by the plasma wave instrument on Voyager with amplitudes sufficient to isotropize the electron distribution.

Moses, S. L.↗

NASA NTRS1985

Trapped particle absorption by the ring of Jupiter

The ring systems of Jupiter and Saturn, and their interaction with the magnetosphere were studied. Opportunities to improve the understanding of the sweeping effect of orbiting material on trapped radiation, and the use of this process to gain insight on both the trapped radiation and the target material are outlined. Within the cosmogony of Hannes Alfven, this mechanism is also the key to understanding the formation of many of the features of the Saturnian rings. A better understanding of the sweeping effect would also help to clarify this process.

Fillius, W.↗

NASA NTRS1985

A Note on Jupiter's Gossamer Ring

The newly discovered gossamer ring of Jupiter, composed largely of micron-sized grains, exhibits a significant peak very near the synchronous radius. It is believed that this peak is associated with an unobserved source consisting of large bodies that straddle the synchronous orbit. An alternative evolutionary mechanism is offered, which is the gyrophase drift towards the synchronous orbit, of fine grains, which necessarily are electrostatically charged within the Jovian magnetosphere.

Mendis, D. A.↗

NASA NTRS1984

Magnetic properties of Jupiter's tail at distances from 80-7500 Jovian radii

A detailed study of the magnetic field data from both Voyagers 1 and 2 has revealed several interesting properties of the near and distant Jovian magnetotail. During the first encounter, as Voyager 1 passed between 80 and 140 R sub J from Jupiter in the near tail, the spacecraft was almost entirely in the northerm lobe magnetic field. The frequency spectrum of magnetic fluctuation in this region cannot be characterized by a power law and does not appear to be turbulent. The distant tail spectra from Voyager 2 are compared with similar spectra obtained from Voyager 1 when it was in near radial alignment with Voyager 2. Although the gross properties of the tail and solar wind fields in most respects differ considerably, the shape and power levels of the spectra of the magnetic fluctuations are very similar, especially between .0001 and .001 Hz. At lower frequencies (.00001 to .0001 Hz) the spectra of magnetic helicity do differ.

Goldstein, M. L.↗

NASA NTRS1985

2.7- to 4.1-micron spectrophotometry of icy satellites of Saturn and Jupiter

Spectrophotometry is presented in the 2.7-4.1 micrometer spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus and Hyperion) and Jupiter (Europa, Ganymede and Callisto). The 3.6-micrometer reflectance peak characteristic of fine-grained water ice is observed prominently on the satellites of Saturn, faintly on the leading side of Europa, and not at all on Ganymede, Callisto or the dark side of Iapetus. The spectral reflectances of these icy satellites may be affected by their equilibrium surface temperatures and magnetospheric effects.

Lebofsky, L. A.↗

NASA NTRS1984

Charge-coupled device camera for the Galileo Jupiter Orbiter spacecraft

A slow-scan television camera called the solid-state imaging subsystem (SSI), built for the Galileo Jupiter Orbiter, is described. The SSI consists of a 1500-mm focal-length telescope coupled to a camera head housing a 800 x 800-element charge-coupled device (CCD) detector based on 'virtual-phase' charge transfer technology. The CCD detector provides broadband sensitivity over 100 times that of a comparable vidicon-tube camera, while also yielding improved resolution, linearity, geometric fidelity, and spectral range. The system noise floor is 30 electrons, which results in a dynamic range of about 3500. Saturation of the detector with 9000-A light, followed by a high-speed erasure cycle prior to exposing each image, stabilizes the detector quantum efficiency at its maximum level for wavelengths beyond 7000 A. An optical schematic diagram of the SSI is included.

Klaasen, K. P.↗

NASA NTRS1985

Vertical cloud structure of Jupiter's equatorial plumes

Multiple-scattering radiative transfer calculations were used to deduce the vertical cloud structure (VCS) of Jupiter's equatorial region. The VCS model of the equatorial plumes is obtained through an analysis of Voyager images of the 6190-A methane band and the 6000-A continuum, and ground-based 8900-A methane band images. The VCS of the equatorial plumes is found to be consistent with the hypothesis that the plumes are caused by upwelling at the ammonia condensation level produced by buoyancy due to latent heat release from the condensation of water clouds nearly three scale heights below the plumes.

Stoker, C. R.↗

NASA NTRS1986

Energetic neutral particles from Jupiter and Saturn

The Voyager 1 spacecraft has detected energetic neutral particles escaping from the magnetospheres of Jupiter and Saturn. These energetic neutrals are created in charge exchange reactions between radiation belt ions and ambient atoms or molecules in the magnetosphere. If the Io torus is assumed to be the dominant Jovian source region for energetic neutrals, the Voyager observations can be used to infer upper limits to the average ion intensities there below about 200 keV. No readily interpretable in-situ measurements are available in the Io torus at these energies. The middle and outer Jovian magnetospheres may also be a significant source of energetic neutrals. At Saturn, the observed neutral particle count rates are too high to be explained by charge exchange between fast protons and H atoms of the Titan torus. Most of the energetic neutrals may be produced by charge exchanges between heavy ions and a neutral cloud containing H2O in Saturn's inner magnetosphere. If so, the Voyager measurements of energetic neutral fluxes would be the first detected emissions from this region of Saturn's magnetosphere.

Cheng, A. F.↗

NASA NTRS1986

Merging of vortices in the atmosphere of Jupiter - An analysis of Voyager images

The present study of interactions between Jupiter spots, using the Voyager 2 cylindrical projection mosaics, notes that collisions between spots are irreversible, in contrast with solitary wave-type interactions. In 23 of 27 cases, interactions lead to a merging of the two original spots. Interactions of spots with filamentary regions usually lead to a disappearance of the spot; filamentary regions are noted to be the major source of spots. Stable spots do not generate other spots, instead destroying each other by merging. Most spots are anticyclonic, and lie in cyclonic shear zones.

Mac Low, M.-M.↗

NASA NTRS1986

Motions in the interiors and atmospheres of Jupiter and Saturn. II - Barotropic instabilities and normal modes of an adiabatic planet

A rotating and adiabatic inviscid fluid planet possesses low frequency motions that are barotropic, quasi-geostrophic and quasi-columnar. The limiting curvature at which flow becomes unstable upon projection onto the planetary surface is negative, with an amplitude that is 3-4 times that for thin atmospheres, in planets in which density linearly decreases to zero at the surface. This result is shown to hold for all quasi-columnar perturbations. Both the phase speed of the normal mode oscillations and the barotropic stability criterion have features in common with Saturn and Jupiter oscillations.

Ingersoll, A. P.↗

NASA NTRS1986

Medium-energy electrons and heavy ions in Jupiter's magnetosphere - Effects of lower hybrid wave-particle interactions

A theory of medium-energy (about keV) electrons and heavy ions in Jupiter's magnetosphere is presented. Lower hybrid waves are generated by the combined effects of a ring instability of neutral wind pickup ions and the modified two-stream instability associated with transport of cool Iogenic plasma. The quasi-linear energy diffusion coefficient for lower hybrid wave-particle interactions is evaluated, and several solutions to the diffusion equation are given. Calculations based on measured wave properties show that the noise substantially modifies the particle distribution functions. The effects are to accelerate superthermal ions and electrons to keV energies and to thermalize the pickup ions on time scales comparable to the particle residence time. The S(2+)/S(+) ratio at medium energies is a measure of the relative contribution from Iogenic thermal plasma and neutral wind ions, and this important quantity should be determined from future measurements. The theory also predicts a preferential acceleration of heavy ions with an accleration time that scales inversely with the root of the ion mass. Electrons accelerated by the process contribute to further reionization of the neutral wind by electron impact, thus providing a possible confirmation of Alfven's critical velocity effect in the Jovian magnetosphere.

Barbosa, D. D.↗

NASA NTRS1986

Excitation of MHD waves upstream of Jupiter by energetic sulfur or oxygen ions

Large fluxes of heavy ions have been reported upstream of Jupiter's bow shock as Voyager 1 approached the planet (Zwickl et al., 1981; Krimigis et al., 1985). Enhanced low-frequency magnetic wave activity was also observed during the particle events. The fluctuations are left-handed, elliptically polarized in the plasma frame. The spectrum of these fluctuations contains a peak close to the Doppler-shifted resonance frequency of a sulfur or oxygen beam with streaming energy of approximately 30 keV. These fluctuations are also present in the spectrum of the magnitude of the field. It is concluded that the observations result from an instability driven by an energetic beam of either sulfur or oxygen. The wave observations can be described by a heavy ion distribution with both a streaming anisotropy and a temperature anisotropy. This class of heavy ion streaming instabilities may also play a role in wave-particle interactions in the vicinity of comets.

Goldstein, M. L.↗