Empirical model of the Io plasma torus: Voyager measurements
TL;DR: In this paper, the authors present a description of the Io plasma torus, between 5 and 10 RJ, based on Voyager 1 observations obtained in March 1979, using a model that includes updated analyses of plasma Science (PLS) data obtained along the spacecraft trajectory as well as Ultraviolet Spectrometer (UVS) observations of composition made remotely from Jupiter.
Abstract: We present a description of plasma conditions in the Io plasma torus, between 5 and 10 RJ, based on Voyager 1 observations obtained in March 1979. The model includes updated analyses of Plasma Science (PLS) data obtained along the spacecraft trajectory as well as Ultraviolet Spectrometer (UVS) observations of composition made remotely from Jupiter. The plasma characteristics observed along the spacecraft trajectory have been extrapolated along magnetic field lines by numerically solving the equations of diffusive equilibrium to produce radial profiles of plasma properties at the centrifugal equator as well as maps of the densities of the major ionic species in a meridian plane. The diffusive equilibrium distribution of plasma along magnetic field lines depends mainly on T∥. Unfortunately, we only have measurements of T⊥ and must make assumptions about the thermal anisotropy of the plasma. We assume the thermal populations and the suprathermal electrons to be isotropic. The suprathermal ions have probably been recently picked-up and are expected to be highly anisotropic. Varying the thermal anisotropy of the hot ions between A=T⊥/T∥=1 to 5 has a minor effect on the plasma maps but makes a significant difference to the fraction of hot ions in the plasma when integrated over a complete shell of magnetic flux. We have found that the vertical extrapolation of plasma density is insensitive to the geometry of different magnetic field models except inside 5 RJ (where the plasma scale height is comparable to uncertainties in the location of the centrifugal equator) and outside 8 RJ (where the magnetospheric current sheet significantly perturbs the magnetic field). The radial profile of flux tube content (N L²) exhibits the same “precipice”, “ledge,” and “ramp” features as previous studies as well as confirming small - scale features which indicate local sources of plasma in the cold torus and near the orbit of Europa. The observations of O++ and molecular (SO2+ or S2+) ions inside 5.4 RJ, far from Io, in a region of cold dense plasma, remain difficult to explain, indicating either strong temporal variability in the Io plasma source or a strong source of plasma, possibly from the dissociation of dust, inside Io's orbit. Further evidence of a Europa source are the decrease in the ratio of sulfur to oxygen ions and the increase in plasma temperature outside 8 RJ.
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TL;DR: The dynamics of small charged dust particles can be surprisingly complex, leading to levitation, rapid transport, energization and ejection, capture, and the formation of new planetary rings.
Abstract: ▪ Abstract In most space environments, dust particles are exposed to plasmas and UV radiation and, consequently, carry electrostatic charges. Their motion is influenced by electric and magnetic fields in addition to gravity, drag, and radiation pressure. On the surface of the Moon, in planetary rings, or at comets, for example, electromagnetic forces can shape the spatial and size distribution of micron-sized charged dust particles. The dynamics of small charged dust particles can be surprisingly complex, leading to levitation, rapid transport, energization and ejection, capture, and the formation of new planetary rings. This review briefly discusses the most important processes that determine the charge state of dust particles immersed in plasmas and the resulting dynamics on exposed dusty surfaces and in planetary magnetospheres.
575 citations
TL;DR: In this article, the authors review both observational and theoretical aspects of the generation of auroral radio emissions at the outer planets, trying to organize the former in a coherent frame set by the latter.
Abstract: We review both observational and theoretical aspects of the generation of auroral radio emissions at the outer planets, trying to organize the former in a coherent frame set by the latter. Important results have been obtained in the past few years on these radio emissions at the five magnetized planets, from the observations of Ulysses at Jupiter and of Wind and other Global Geospace Science spacecraft in Earth orbit, from the reanalysis of Voyager data about Saturn, Uranus, and Neptune, from ground-based high frequency-time resolution and full polarization measurements, and from pioneering multispectral observations of the Jovian and Saturnian aurorae (radio/UV/IR). In parallel, considerable progress has been made in their generation theory (Cyclotron-Maser operating in small-scale, laminar, hot-plasma-dominated radio source structures), mostly on the basis of in situ observations of terrestrial radio sources. Particle acceleration and precipitation is also better documented, thanks to in situ measurements in the Earth auroral zones and to multispectral studies of Jupiter and Saturn. Finally, the modeling of the planetary magnetic field and magnetospheric plasma at these two planets has also been considerably improved. To organize the wealth of observational results within a coherent theoretical frame, we emphasize unresolved questions (e.g., planetary radio bursts generation) and contradictions and propose ways to answer them. Our ability, already significant, to perform remote sensing of magnetoplasmas at the giant planets and, hopefully, at other distant radio sources (solar, stellar) in the near future, depends on the good understanding of the physical processes underlying the generation of auroral electromagnetic emissions. The question of the existence of exoplanetary radio emissions and the possibility to detect and study them is briefly discussed.
490 citations
TL;DR: In this article, a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below.
427 citations
TL;DR: The detection of atomic oxygen emission from Europa is reported, which is interpreted as being produced by the simultaneous dissociation and excitation of atmospheric O2 by electrons from Jupiter's magnetosphere.
Abstract: EUROPA, the second large satellite out from Jupiter, is roughly the size of Earth's Moon, but unlike the Moon, it has water ice on its surface1. There have been suggestions that an oxygen atmosphere should accumulate around such a body, through reactions which break up the water molecules and form molecular hydrogen and oxygen2,3. The lighter H2 molecules would escape from Europa relatively easily, leaving behind an atmosphere rich in oxygen. Here we report the detection of atomic oxygen emission from Europa, which we interpret as being produced by the simultaneous dissociation and excitation of atmospheric O2 by electrons from Jupiter's magnetosphere. Europa's molecular oxygen atmosphere is very tenuous, with a surface pressure about 10−11 that of the Earth's atmosphere at sea level.
361 citations
TL;DR: In this paper, the authors present simple models of the plasma disks surrounding Jupiter and Saturn based on published measurements of plasma properties, and calculate radial profiles of the distribution of plasma mass, pressure, thermal energy density, kinetic energy density and energy density of the suprathermal ion populations.
Abstract: [1] We present simple models of the plasma disks surrounding Jupiter and Saturn based on published measurements of plasma properties. We calculate radial profiles of the distribution of plasma mass, pressure, thermal energy density, kinetic energy density, and energy density of the suprathermal ion populations. We estimate the mass outflow rate as well as the net sources and sinks of plasma. We also calculate the total energy budget of the system, estimating the total amount of energy that must be added to the systems at Jupiter and Saturn, though the causal processes are not understood. We find that the more extensive, massive disk of sulfur- and oxygen-dominated plasma requires a total input of 3–16 TW to account for the observed energy density at Jupiter. At Saturn, neutral atoms dominate over the plasma population in the inner magnetosphere, and local source/loss process dominate over radial transport out to 8 RS, but beyond 8–10 RS about 75–630 GW needs to be added to the system to heat the plasma.
292 citations
References
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Kitt Peak National Observatory1, University of Southern California2, Planetary Science Institute3, Jet Propulsion Laboratory4, University of Michigan5, Johns Hopkins University6, Centre national de la recherche scientifique7, United States Naval Research Laboratory8, York University9, Harvard University10
TL;DR: The observed resonance scattering of solar hydrogen Lyman α by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (≥ 1000 K) wvith a large atomic hydrogen abundance.
Abstract: The global hydrogen Lyman alpha, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H(2) band excitation mechanism is closely related to the solar flux. Auroras occur near 80 degrees latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N(2) is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.
755 citations
TL;DR: In situ data from the Pioneer and Voyager spacecraft, supplemented by earth-based observations and theoretical considerations, are used as the basis for the present quantitative, compact model of the 1 eV-several MeV charged particle distribution in the Jovian magnetosphere.
Abstract: In situ data from the Pioneer and Voyager spacecraft, supplemented by earth-based observations and theoretical considerations, are used as the basis for the present quantitative, compact model of the 1 eV-several MeV charged particle distribution in the Jovian magnetosphere. The thermal plasma parameters of convection speed, number density, and characteristic energy, are specified as functions of position for electrons and for the ion species H(+), O(+), O(2+), S(+), S(2+), S(3+), and Na(+). Major features of the magnetic field, thermal plasma, and trapped particle distributions, are modeled and results for each plasma region are compared with observed spectra. Comparisons show that the model represents the data to within a factor of 2 + or - 1, except where time variations are significant. Practical applications of the model to spacecraft near Jupiter are given.
362 citations
TL;DR: In this paper, the factors which govern the distribution of electrons and ions in a planet's exosphere under diffusive equilibrium are discussed, and the results predicted by the theory are compared with actual experimental observations of the electron density distribution in the earth's exospheric plasma which have been obtained in recent years from whistler data and from topside ionograms made by the Alouette satellite.
Abstract: The factors which govern the distribution of electrons and ions in a planet's exosphere under diffusive equilibrium are discussed. The theory takes into account the effect of the electric field that arises from charge separation, the centrifugal force arising from the rotation of the planet, and the effect of the planet's gravitational field. It is assumed that the charged particles are constrained to move only along the direction of the planet's magnetic lines of force. The modifications that result in the electron and ion distributions when a temperature variation is assumed along a line of force are also considered. The results predicted by the theory are compared with actual experimental observations of the electron density distribution in the earth's exospheric plasma which have been obtained in recent years from whistler data and from topside ionograms made by the Alouette satellite.
352 citations
TL;DR: In this article, the details of positive ion measurements made in the inner magnetosphere are discussed and an analysis of these measurements to obtain plasma composition, flow speeds, and temperatures is given.
Abstract: The details of positive ion measurements made in the inner magnetosphere are discussed. Attention is also given to an analysis of these measurements to obtain plasma composition, flow speeds, and temperatures and to the assumptions made in the analysis. These results for the positive ions are then combined with the direct measurements of plasma electrons between 5.7 and 9 Jupiter radii and with a theoretical distribution of plasma along dipolar magnetic field lines to build a two-dimensional model of the plasma torus.
297 citations
TL;DR: The Earth-Pointing Faraday Cup (OPEF) was used on the 1977 Voyager mission to study the properties and radial evolution of the solar wind, the interaction of solar wind with Jupiter, and the sources, properties and morphology of the Jovian magnetospheric plasma, with particular emphasis on plasma properties in the vicinity of Io, and ion of interstellar origin this paper.
Abstract: This paper contains a brief description of the plasma experiment to be flown on the 1977 Voyager Mission, its principal scientific objectives, and the expected results. The instrument consists of two Faraday cup plasma detectors: one pointed along and one at right angles to the Earth-spacecraft line. The Earth-pointing detector uses a novel geometrical arrangement: it consists of three Faraday cups, each of which views a different direction in velocity space. With this detector, accurate values of plasma parameters (velocity, density, and pressure) can be obtained for plasma conditions expected between 1 and 20 AU. The energy range for protons and for electrons is from 10 to 5950 eV. Two sequential energy per charge scans are employed with nominal values of ΔE/E equal to 29%, and 3.6%. The two scans allow the instrument to cover a broad range between subsonic (M < 1) and highly supersonic (M-100) flows; thus, significant measurements can be made in a hot planetary magnetosheath as well as in a cold solar wind. In addition, the use of two energy resolutions during the cruise phase of the mission allows simultaneously the measurement of solar wind properties and a search for interstellar ions. The Earth-pointing detector cluster has an approximately conical field of view with a half angle of 90°. The exceptionally large field of view makes this detector especially suited for use on a three-axis stabilized spacecraft. Both the solar wind direction during the cruise phase of the mission, and the deviated magnetosheath flow directions expected at Jupiter and Saturn fall within the field of view of the main detector; thus, no mechanical or electrical scanning is required. An additional sensor with a field of view perpendicular to that of the main cluster, is included to improve the spatial coverage for the drifting or corotating positive ions expected at planetary encounter. This detector is also used to make measurements of electrons in the energy range 10 to 5950 eV. The scientific goals include studies of (a) the properties and radial evolution of the solar wind, (b) the interaction of the solar wind with Jupiter, (c) the sources, properties and morphology of the Jovian magnetospheric plasma, (d) the interaction of magnetospheric plasma with the Galilean satellites with particular emphasis on plasma properties in the vicinity of Io, (e) the interaction of the solar wind with Saturn and the Saturnian satellites with particular emphasis on Titan, and (f) ions of interstellar origin.
275 citations