Aharon Eviatar

Ring and plasma: The enigmae of Enceladus

Abstract The E ring associated with the Kronian moon Enceladus has a lifetime of only a few thousand years against sputteringly by slow corotating O ions. The existence of the ring implies the necessity for a continuous supply of matter. Possible particle source mechanisms on Enceladus include meteoroidal impact ejection and geysering. Estimates of ejection rates of particulate debris following small meteoroid impact are on the order of 3 × 10−18 g cm−2 sec−1, more than an order of magnitude too small to sustain the ring. A geyser source would need to generate a droplet supply at a rate of approximately 10−16 g cm−2 sec− in order to account for a stable ring. Enceladus and the ring particles also directly supply both plasma and vapor to space via sputtering. The absence of a 60 eV plasma at the Voyager 2 Enceladus L-shell crossing, such as might have been expected from sputtering, cannot be explained by absorption and moderation of plasma ions by ring particles, because the ring is too diffuse. Evidently, the effective sputtering yield in the vicinity of Enceladus is on the order of, or smaller than, 0.4, about an order of magnitude less than the calculated value. Small scale surface roughness may account for some of this discrepancy.

Effects of Io ejecta on Europa

Abstract We examine the effects of Io ejecta on the surface and environment of Europa. We find that the observed sulfur on the trailing side of Europa, when interpreted as a deposit in equilibrium between implanation of, and sputtering by, corotating Io ejecta, implies a slow loss of material from Europa by sputtering. From this we infer that the spectrum of particles sputtered from water ice is soft. The quantity of observed sulfur and its confinement to the trailing hemisphere appear to exclude significant implantation and sputtering by energetic heavy ions. We also conclude that the contribution from Europa to the magnetospheric plasma (even at Europa itself) is negligible compared to the matter ejected from Io.

Sodium in the Jovian magnetosphere

Observations of sodium D-line emission from Io and the magnetosphere of Jupiter are reported. A disk-shaped cloud of sodium is found to exist in the Jovian magnetosphere with an inner edge at about 4Rч and an outer edge at about 10Rч. The gravitational scale height above the equatorial plane is a few Jovian radii. The data are interpreted in terms of a sputtering model, in which the sodium required to maintain the cloud is sputtered off the surface of Io by trapped energetic radiation-belt protons. Conditions on the atmospheric density are obtained. The Keplerian orbits attainable by such escaping sputtered atoms can provide the observed spatial distribution. The required 500-keV proton flux required to provide the 1–10 keV protons which will sputter the sodium at the surface of Io is consistent with the limiting trapped flux determined by ion-cyclotron turbulence.

Temperature anisotropy of the Jovian sulfur nebula

Abstract We discuss the apparent paradox between the reported observation of a 3-eV gyration energy of Jupiters ionized sulfur nebula and its observed thickness. We present an observation of the thickness of the cloud taken nearly edge on and show that this implies a large bounce-averaged anisotropy of the sulfur in temperature, T ‖ ≫ T ⊥ . From these observations, we construct a self-consistent model of the sulfur nebula in which the sulfur ions are injected by Io as ions and remain sufficiently collisionless in the magnetosphere to maintain the anisotropy for a time longer than a characteristic diffusion time. We also show that the proton-electron plasma is collisionally thermalized and provides an adequate means of tapping the rotational energy of the planet to provide the power radiated in the sulfur lines.

Micrometeoroid impact on planetary satellites as a magnetospheric mass source

Abstract Planetary satellites are an important source of mass for planetary magnetospheres. Meteoroid impact vaporization is a supply mechanism which can potentially compete with charged-particle sputtering. Recent estimates of impact fluxes in the outer solar system vary by several orders of magnitude. For the larger flux values impact vaporization will play a role both at Jupiter and Saturn, although for the most part it will not dominate sputtering. At the small end of the flux range, sputtering dominates magnetospheric mass-loading everywhere.

Quasi-exospheric heat flux of solar-wind electrons

Density, bulk-velocity, and heat-flow moments are calculated for truncated maxwellian distributions representing the cool and hot populations of solar-wind electrons, as realized at the base of a hypothetical exosphere. The electrostatic potential is thus calculated by requiring charge quasineutrality and the absence of electrical current. Plasma-kinetic coupling of the cool-electron and proton bulk velocities leads to an increase in the electrostatic potential and a decrease in the heat-flow moment. If the velocities differ by the Alfvén speed along the magnetic field, for example, the potential rises to 72.6 V and the heat flux falls to 2.72×10−2 erg cm−2 s−1. In each case the heat flux is carried mainly by the quasi-exospherichot electrons.

Plasma-neutral interaction processes in the magnetosphere of Saturn

Interactions between plasma and neutral matter in the magnetosphere of Saturn can be classified by whether or not they entail reactions which change the charge or atomic state of the particle. Charge exchange and ion-atom interchange are examples of the first type while isotropizing and thermalizing collisions are examples of the second kind. A third type of interaction is that between magnetospheric charged particles and the surface layers of the icy satellites or the upper atmosphere of Titan, which results in sputtering of the surface or atmosphere and the injection of the neutrals into the magnetosphere. Interaction with ice motes appears to be less important. Dissociative recombination is, stricto sensu, an interaction between charged particles, but it is a major sink for plasma and a prime source of the neutral wind. The roles and effects of these various types of interactions as they manifest themselves in the phenomenology observed in the various regions of the magnetosphere of Saturn are surveyed in this review. Constraints supplied by known reaction rates provide a means of evaluating the abundance of the various members of the water group family. An hypothesis for resolving discrepancies in the interpretation of various Voyager data sets is proposed.