John W. Belcher

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.

Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock

The solar wind blows outward from the Sun and forms a bubble of solar material in the interstellar medium. The termination shock occurs where the solar wind changes from being supersonic (with respect to the surrounding interstellar medium) to being subsonic. The shock was crossed by Voyager 1 at a heliocentric radius of 94 au (1 au is the Earth–Sun distance) in December 2004 (refs 1–3). The Voyager 2 plasma experiment observed a decrease in solar wind speed commencing on about 9 June 2007, which culminated in several crossings of the termination shock between 30 August and 1 September 2007 (refs 4–7). Since then, Voyager 2 has remained in the heliosheath, the region of shocked solar wind. Here we report observations of plasma at and near the termination shock and in the heliosheath. The heliosphere is asymmetric, pushed inward in the Voyager 2 direction relative to the Voyager 1 direction. The termination shock is a weak, quasi-perpendicular shock that heats the thermal plasma very little. An unexpected finding is that the flow is still supersonic with respect to the thermal ions downstream of the termination shock. Most of the solar wind energy is transferred to the pickup ions or other energetic particles both upstream of and at the termination shock.

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.

Solar wind oscillations with a 1.3 year period

The IMP-8 and Voyager 2 spacecraft have recently detected a very strong modulation in the solar wind speed with an approximately 1.3 year period. Combined with evidence from long-term auroral and magnetometer studies, this suggests that fundamental changes in the Sun occur on a roughly 1.3 year time scale.

Radial evolution of the solar wind from IMP 8 to Voyager 2

Voyager 2 and IMP 8 data from 1977 through 1994 are presented and compared. Radial velocity and temperature structures remain intact over the distance from 1 to 43 AU, but density structures do not. Temperature and velocity changes are correlated and nearly in phase at 1 AU, but in the outer heliosphere temperature changes lead velocity changes by tens of days. Solar cycle variations are detected by both spacecraft, with minima in flux density and dynamic pressure near solar maxima. Differences between Voyager 2 and IMP 8 observations near the solar minimum in 1986–1987 are attributed to latitudinal gradients in solar wind properties. Solar rotation variations are often present even at 40 AU. The Voyager 2 temperature profile is best fit with a R−0.49±0.01 decrease, much less steep than an adiabatic profile.

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.

Solar wind conditions in the outer heliosphere and the distance to the termination shock

The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. A least squares fit of proton ram pressure to heliocentric distance R over this distance yields a ram pressure equal to (1.67 × 10−8 dynes cm−2) R−2.00 ± 0.02, where R is measured in astronomical units. Assuming that the interstellar pressure is due to a 5 µG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength, i.e., for a 7 µG field, the termination shock will be located on average at 64 AU. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of days), due primarily to large variations in solar wind density at a given radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. Using a simple kinematic model, we study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.

Evidence for a solar wind slowdown in the outer heliosphere

Voyager 2 and IMP 8 plasma data are used to look for the predicted slowdown of the solar wind with heliospheric distance. Decreases of roughly 7% in the radial velocity and of the same order in the flux are found if the Voyager 2 and IMP 8 velocities are normalized to agree in the inner heliosphere. This decrease is consistent with a pickup ion density equal to 8% of the total ion density, similar to predictions and other determinations of this density. Comparison with published model results allows us to infer an interstellar neutral density of 0.05 cm−3.

Pickup protons and pressure‐balanced structures: Voyager 2 observations in merged interaction regions near 35 AU

Five pressure-balanced structures, each with a scale of the order of a few hundredths of an astronomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discontinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is a transition region: the Sun is the source of the magnetic field, but the interstellar medium is the source of pickup protons. Relative to the solar wind proton gyroradius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 ± 0.14) × 10−12 erg/cm³, (5.8 ± 0.4) × 106 °K and (7 ± 2) × 10−4 cm−3, respectively.