We present a nonlinear analytical model of the Alfv6n current tubes continuing the currents through Io (or rather its ionosphere) generated by the unipolar inductor effect due to Io's motion relative to the magnetospheric plasma. We thereby extend the linear work by Drell et al. (1965) to the fully nonlinear, sub-Alfv6nic situation also including flow which is not perpendicular to the background magnetic field. The following principal results have been obtained: (1) The portion of the currents feeding Io is aligned with the Affv•n characteristics at an angle 0A --tan -z MA to the magnetic field for the special case of perpendicular flow where M• is the Affv•n Mach number. (2) The Alfv6n tubes act like an external conductance • --l/(goV•(l + M• 2 + 2M• sin 8) z/2) where VA is the Affv•n speed and 0 the angular deviation from perpendicular flow towards the direction of Alfv•n wave propagation. Hence the Jovian ionospheric conductivity is not necessary for current closure. (3) In addition, the Alfv6n tubes may be reflected from either the torus boundary or 'the Jovian ionosphere. The efficiency of the resulting interaction with these boundaries varies with Io position. The interaction is particularly strong at extreme magnetic latitudes, thereby suggesting a mechanism for the Io control of decametric emissions. (4) The reflected Alfv6n waves may heat both the torus plasma and the Jovian ionosphere as well as produce increased diffusion of high-energy particles in the torus. (5) From the point of view of the electrodynamic interaction, Io is unique among the Jovian satellites for several reasons: these include its ionosphere arising from ionized volcanic gases, a high external Alfv6nic conductance •, and a high corotational voltage in addition to the interaction phenomenon with a boundary. (6) We find that Amalthea is probably strongly coupled to Juoiter's ionosphere while me outer Galilean satellites may occasio•y experience super-Alfv6nic conditions.

A survey of solar wind three‐dimensional proton velocity distributions as measured by the Helios solar probes between 0.3 and 1 AU is presented. A variety of nonthermal features like temperature anisotropies, heat fluxes, or proton double streams has been observed. The relative speed of the second proton component increases on the average with increasing wind speed and decreasing heliocentric radial distance and shows a correlation with the local Alfvén speed. A marked anisotropy in the core of proton distributions with a temperature larger perpendicular than parallel to the magnetic field (T∥c < T∥c) is a persistent feature of high speed streams and becomes most pronounced in the perihelion (≈0.3 AU). Fairly isotropic distributions have only been measured very close to and directly at magnetic sector boundaries. Low and intermediate speed distributions usually show a total temperature anisotropy T∥p/T⊥p > 1 frequently caused by ‘high‐energy shoulders’ or a resolved second proton component. No clear radial gradient of the temperature anisotropy could be established in these cases. The average dependence of the proton temperature on heliocentric radial distance is given by a power law R−α, where α ≈ 1 for T⊥p and 0.7 < α < 1 for T⊥p are compatible neither with isothermal nor adiabatic expansion. Flattest radial temperature profiles are obtained in high‐speed streams. These observations indicate that local heating or considerable proton heat conduction occurs in the solar wind. Some consequences of nonthermal features of proton distributions for plasma instabilities are discussed as well as kinetic processes that may shape the observed distributions.

On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 +/- 0.25 K, and the pressure was 1,467 +/- 1 hPa.

Abstract.Recent observations by the Galileo spacecraft and Earth-based techniques have motivated us to reconsider the sub-Alfv•nic interaction between the Galilean satellites of Jupiter and the magnetosphere. (1) We show that the atomic processes causing the interaction between the magnetoplasma and a neutral atmosphere can be described by generalized collision frequencies with contributions from elastic collisions, ion pickup, etc. Thus there is no fundamental difference in the effect of these processes on the plasma dynamics claimed in the recent literature. For a magnetic field configuration including possible internal fields, we show that the sub-Alfv•nic, low-beta interaction can be described by an anisotropically conducting atmosphere joined to an Alfv•n wing as one extreme case and the Jovian ionosphere as the other extreme case.

A survey of solar wind helium ion velocity distributions and derived parameters as measured by the Helios solar probes between 0.3 and 1 AU is presented. Nonthermal features like heat fluxes or He2+ double streams and temperature anisotropies have been frequently observed. Fairly isotropic distributions have only been measured close to sector boundaries of the interplanetary magnetic field. At times in slow solar wind, persistent double‐humped helium ion distributions constituting a temperature anisotropy T∥α/T⊥α > 1 have been reliably identified. Distributions in high‐speed wind generally have small total anisotropies (T∥α/T⊥α ≳ 1) with a slight indication that in the core part the temperatures are larger parallel than perpendicular to the magnetic field, in contrast to simultaneous proton observations. The anisotropy tends to increase with increasing heliocentric radial distance. The average dependence of helium ion temperatures on radial distance from the sun is described by a power law ∼ R−β with 0.7 ≲ β ≲ 1.2 for T∥α and 0.87 ≲ β ≲ 1.4 for T⊥α. In fast solar wind the T⊥α profile is compatible with nearly adiabatic cooling. Pronounced differential ion speeds Δvαp have been observed with values of more than 150 km/s near perihelion (0.3 AU). In fast streams Δvαp tends to approach the local Alfvén velocity vA, whereas in slow plasma values around zero are obtained. Generally, the differential speed increases with increasing proton bulk speed and (with the exception of slow plasma) with increasing heliocentric radial distance. The role of Coulomb collisions in limiting Δvαp and the ion temperature ratio Tα/Tp is investigated. Collisions are shown to play a negligible role in fast solar wind, possibly a minor role in intermediate speed solar wind and a distinct role in low‐speed wind in limiting the differential ion velocity and temperature.

Context. Electromagnetic coupling of planetary moons with their host planets is well observed in our solar system. Similar couplings of extrasolar planets with their central stars have been studied observationally on an individual as well as on a statistical basis. Aims. We aim to model and to better understand the energetics of planet star and moon planet interactions on an individual and as well as on a statistical basis. Methods. We derived analytic expressions for the Poynting flux communicating magnetic field energy from the planetary obstacle to the central body for sub-Alfvénic interaction. We additionally present simplified, readily useable approximations for the total Poynting flux for small Alfvén Mach numbers. These energy fluxes were calculated near the obstacles and thus likely present upper limits for the fluxes arriving at the central body. We applied these expressions to satellites of our solar system and to HD 179949 b. We also performed a statistical analysis for 850 extrasolar planets. Results. Our derived Poynting fluxes compare well with the energetics and luminosities of the satellites' footprints observed at Jupiter and Saturn. We find that 295 of 850 extrasolar planets are possibly subject to sub-Alfvénic plasma interactions with their stellar winds, but only 258 can magnetically connect to their central stars due to the orientations of the associated Alfvén wings. The total energy fluxes in the magnetic coupling of extrasolar planets vary by many orders of magnitude and can reach values larger than 10 19 W. Our calculated energy fluxes generated at HD 179949 b can only explain the observed energy fluxes for exotic planetary and stellar magnetic field properties. In this case, additional energy sources triggered by the Alfvén wave energy launched at the extrasolar planet might be necessary. We provide a list of extrasolar planets where we expect planet star coupling to exhibit the largest energy fluxes. As supplementary information we also attach a table of the modeled stellar wind plasma properties and possible Poynting fluxes near all 850 extrasolar planets included in our study. Conclusions. The orders of magnitude variations in the values for the total Poynting fluxes even for close-in extrasolar planets provide a natural explanation why planet star coupling might have been only observable on an individual basis but not on a statistical basis.

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Abstract. The dual technique magnetometer system onboard the Cassini orbiter is described. This instrument consists of vector helium and fluxgate magnetometers with the capability to operate the helium device in a scalar mode. This special mode is used near the planet in order to determine with very high accuracy the interior field of the planet. The orbital mission will lead to a detailed understanding of the Saturn/Titan system including measurements of the planetary magnetosphere, and the interactions of Saturn with the solar wind, of Titan with its environments, and of the icy satellites within the magnetosphere.