H. S. Bridge

Plasma Observations Near Uranus: Initial Results from Voyager 2

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.

The plasma experiment on the 1977 Voyager Mission

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.

Plasma Observations Near Jupiter: Initial Results from Voyager 1

Extensive measurements of low-energy positive ions and electrons were made throughout the Jupiter encounter of Voyager 1. The bow shock and magneto-pause were crossed several times at distances consistent with variations in the upstream solar wind pressure measured on Voyager 2. During the inbound pass, the number density increased by six orders of magnitude between the innermost magnetopause crossing at ∼47 Jupiter radii and near closest approach at ∼5 Jupiter radii; the plasma flow during this period was predominately in the direction of corotation. Marked increases in number density were observed twice per planetary rotation, near the magnetic equator. Jupiterward of the Io plasma torus, a cold, corotating plasma was observed and the energylcharge spectra show well-resolved, heavy-ion peaks at mass-to-charge ratios A/Z* = 8, 16, 32, and 64.

Mariner V: Plasma and Magnetic Fields Observed near Venus.

Abrupt changes in the amplitude of the magnetic fluctuations, in the field strength, and in the plasma properties, were observed with Mariner V near Venus. They provide clear evidence for the presence of a bow shock around the planet, similar to, but much smaller than, that observed at Earth. The observations appear consistent with an interaction of the solar wind with the ionosphere of Venus. No planetary field could be detected, but a steady radial field and very low plasma density were found 10,000 to 20,000 kilometers behind Venus and 8,000 to 12,000 kilometers from the Sun-Venus line. These observations may be interpreted as relating to an expansion wave tending to fill the cavity produced by Venus in the solar wind. The upper limit to the magnetic dipole moment of Venus is estimated to be within a factor of 2 of 10-3 items that of Earth.

Observations at Mercury Encounter by the Plasma Science Experiment on Mariner 10

Preliminary results from the rearward-looking electrostatic analyzer of the plasma science experiment during the Mariner 10 encounter with Venus are described. They show that the solar-wind interaction with the planet probably involves a bow shock rather than an extended exosphere, but that this is not a thin boundary at the point where it was crossed by Mariner 10. An observed reduction in the flux of electrons with energies greater than 100 electron volts is interpreted as evidence for somne direct interaction with the exosphere. Unusual intermittent features observed downstream of the planet indicate the presence of a comet-like tail hundreds of scale lengths in length.

Plasma Observations Near Neptune: Initial Results from Voyager 2

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptunes magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptunes magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Tritons atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptunes magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Evidence for heavy mesons with the decay processes Kπ2 → π + π0 and Kμ2 → μ + ν from observations with a multiplate cloud chamber→ μ + ν from observations with a multiplate cloud chamber

SummaryAnalysis of the data on S-events observed in the M.I.T. multiplate cloud chamber shows that these events represent the decay processes of two kinds of heavy mesons. The decay processes are of the type: Kμ2 → μ + ν and Kπ2 → π + π0RiassuntoL’analisi delle fotografie ottenute colla camera di Wilson a setti del Massachusetts Institute of Technology mostra che i cosidetti « eventi S » rappresentano processi di disintegrazione di due diversi tipi di mesoni pesanti, secondo gli schemi: Kμ2 → μ + ν e Kπ2 → π + π0.

The low energy plasma in the Uranian magnetosphere

Abstract The Plasma Science experiment on Voyager 2 detected a magnetosphere filled with a tenuous plasma, rotating with the planet. Temperatures of the plasma, composed of protons and electrons, ranged from 10 eV to ∼1 keV. The sources of these protons and electrons are probably the ionosphere of Uranus or the extended neutral hydrogen cloud surrounding the planet. As at Earth, Jupiter, and Saturn, there is an extended magnetotail with a central plasma sheet. Although similar in global structure to the magnetospheres of these planets, the large angle between the rotation and magnetic axes of the planet and the orientation of the rotation axis with respect to the solar wind flow make the Uranian magnetosphere unique.

Plasmas in space

About twenty years ago, the so‐called “infall” theory had been thoroughly worked out by Hoyle and by Alfven. They reasoned that, since the sun moves through clouds of interstellar hydrogen gas, the neutral hydrogen must fall in toward the sun under the influence of the gravitational field. On its way toward the surface of the sun, the hydrogen encounters a strong flux of ultraviolet radiation and, because of the small rate of recombination, it becomes completely ionized. Thus the inner solar system is filled with a dilute, highly conducting plasma. At about the same time the infall theory was worked out, it became rather generally accepted that the corona of the sun has a temperature of 1 or 2 million degrees. In fact, Hoyle tried to account for the heating of the corona in terms of the energy released by the hydrogen falling into the sun.