J. D. Richardson

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.

Heating of the low-latitude solar wind by dissipation of turbulent magnetic fluctuations

We test a theory presented previously to account for the turbulent transport of magnetic fluctuation energy in the solar wind and the related dissipation and heating of the ambient ion population. This theory accounts for the injection of magnetic energy through the damping of large-scale flow gradients, such as wind shear and compression, and incorporates the injection of magnetic energy due to wave excitation by interstellar pickup ions. The theory assumes quasi-two-dimensional spectral transport of the fluctuation energy and subsequent dissipation that heats the thermal protons. We compare the predictions of this theory with Voyager 2 and Pioneer 11 observations of magnetic fluctuation energy, magnetic correlation lengths, and ambient proton temperatures. Near-Earth Omnitape observations are used to adjust for solar variability, and the possibility that high-latitude effects could mask possible radial dependences is considered. We find abundant evidence for in situ heating of the protons, which we quantify, and show that the observed magnetic energy is consistent with the ion temperatures.

Magnetic fields at the solar wind termination shock

A transition between the supersonic solar wind and the subsonic heliosheath was observed by Voyager 1, but the expected termination shock was not seen owing to a gap in the telemetry. Here we report observations of the magnetic field structure and dynamics of the termination shock, made by Voyager 2 on 31 August–1 September 2007 at a distance of 83.7 au from the Sun (1 au is the Earth–Sun distance). A single crossing of the shock was expected, with a boundary that was stable on a timescale of several days. But the data reveal a complex, rippled, quasi-perpendicular supercritical magnetohydrodynamic shock of moderate strength undergoing reformation on a scale of a few hours. The observed structure suggests the importance of ionized interstellar atoms (‘pickup protons’) at the shock.

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.

Saturn: Search for a missing water source

[1] The origin of the large hydroxyl radical (OH) cloud near the inner moons of Saturn, indicative of a surprisingly large water-vapor source, has represented a puzzle since its discovery in 1992. A new set of Hubble Space Telescope measurements is used to constrain the OH spatial densities and to pinpoint the source region. Our model indicates that the vast majority of the water vapor (>80%) originates from Enceladus’s orbital distance. This may indicate the presence of a dense population of small, as of yet unseen, bodies concentrated near Enceladus; collisions between these fragments are the suggested mechanism for producing the necessary amounts of water vapor. We show that collisions between plasma ions and neutral molecules substantially inflate the OH cloud, and increase the OH loss rate, requiring a water source three times larger than previous estimates. INDEX TERMS: 6275 Planetology: Solar System Objects: Saturn; 2756 Magnetospheric Physics: Planetary magnetospheres (5443, 5737, 6030); 6280 Planetology: Solar System Objects: Saturnian satellites; 6213 Planetology: Solar System Objects: Dust. Citation: Jurac, S., M. A. McGrath, R. E. Johnson, J. D. Richardson, V. M. Vasyliunas, and A. Eviatar, Saturn: Search for a missing water source, Geophys. Res. Lett. , 29(24), 2172, doi:10.1029/2002G L015855, 2002.

Plasma and magnetic field correlations in the solar wind

Data from multiple spacecraft are used to determine solar wind plasma and interplanetary magnetic field (IMF) correlation coefficients. These correlation coefficients provide information on solar wind scale lengths and the predictive capability of upstream monitors for space weather purposes. Previous work has looked at plasma and IMF correlation coefficients independently and used much smaller databases than those used in this study. We use data sets from 1977–1984 and 1994–1998 and calculate plasma and IMF correlation coefficients. The IMF correlation coefficients are, on average, slightly higher than those for plasma. Dependence of the correlation coefficients on the spatial separation of the spacecraft is important for placement of upstream monitors; we find a small dependence on the radial separation of the spacecraft but a very strong dependence on spacecraft separation in the YZ (GSE) plane. Scale lengths perpendicular to the flow are about 45 Earth radii (RE) for the IMF components, 70 RE for the speed and IMF magnitude, and over 100 RE for the density. Radial scale lengths are of the order of 400 RE. The plasma and IMF correlation coefficients are larger when values of the density standard deviation are high and when the IMF direction is perpendicular to X (GSE). Front orientations are similar for both plasma and IMF features and are more perpendicular than the average field direction.

OH in Saturn's magnetosphere: Observations and implications

The discovery of OH in Saturns inner magnetosphere changed our view of this region from one where plasma dominated the physics to one where neutrals are dominant. We revisit Hubble Space Telescope observations of OH and derive revised OH brightnesses for observations in 1992, 1994, and 1995. These OH observations as well as Voyager observations are used as constraints on a model of neutral and plasma interactions. We find that the neutral source required to produce the observed OH brightnesses is 1.4×10 27 H 2 O s -1 , with a sharp peak in the neutral source rate near 4.5 R S . A good fit to the data requires OH densities of over 700 cm -3 at 4.5 R S . Rapid diffusion times, about 5 days at 6 R S , are required to match the observed ion densities. We find that the plasma and neutral composition vary with distance from Saturn, and make predictions for the ion and neutral densities as a function of radius.

Evolution of mirror structures in the magnetosheath of Saturn from the bow shock to the magnetopause

Mirror modes have been systematically observed by Voyagers 1 and 2 in wide portions of Jupiters and Saturns magnetosheaths. In particular, in one crossing of Saturns subsolar magnetosheath, mirror waves are present almost continuously from the bow shock to the magnetopause. Therefore in this crossing, taking advantage also of relatively steady interplanetary conditions, we can track the evolution of mirror structures from a quasi-perpendicular bow shock to a low-shear magnetopause. We find that these structures evolve from quasi-sinusoidal waves to nonperiodic structures, consisting of both magnetic field enhancements and wells, and, finally, to dips in the plasma depletion layer (PDL) close to the magnetopause. Both the amplitude and wavelength of the fluctuations tend to increase with increasing distance from the bow shock, except in the PDL, where they decrease toward the magnetopause. The waves are always compressional, and the direction of maximum variance forms an angle of ∼30° with B in the outer magnetosheath and a smaller angle in the inner magnetosheath. A comparison with the predictions of a nonlinear theory of the mirror instability shows some discrepancies, indicating that further theoretical studies are necessary.