A Mechanism for the Production of a Disk-shaped Neutral Source Cloud at Io

Io adds of the order of 1000 kg s of ionized material to the jovian magnetosphere. Freshly produced molecular ions, such as SO and SO2, lead to the growth of ion cyclotron waves at frequencies characteristic of their gyro frequencies. These waves are seen over a region about 0.5 RJ radially inward and outward from Io and of similar or greater extent in the down wake direction but not upstream of Io. We examine simplified numerical simulations of the massloading process that preserve the essential physics and demonstrate how this geometry of the massloading region can be created and what we might expect to see on future Galileo Io passes. INTRODUCTION That Io plays an important role in the energization of the jovian magnetosphere has been known since the discovery of its control of jovian radio emissions [Bigg, 1964]. Voyager observations have revealed many details of the massloading process at Io [e.g. Goertz, 1980] but still it was widely believed that the primary mechanism for generating the field-aligned currents was a unipolar induction mechanism in which the electric field of the corotating jovian magnetosphere applied a potential drop to a conducting Io. In fact Galileo shows [e.g. Russell et al., 1997] that much of the flow is deflected around Io and therefore the currents in the Alfven wing must close in the massloading region. This current closure provides the Lorentz force needed to bring the added plasma up to corotational velocities. The Galileo observation also revealed that the 1000 kg s that seemed required to explain the various Voyager observations [Dessler, 1980] were not added close to Io where the torus interacted with the Io exosphere [Russell et al., 1997; Bagenal, 1997]. Rather, the evidence from the Galileo wave observations was that SO2 was being added over a very wide radial extent, perhaps a half jovian radius inward and outward from Io [Kivelson et al., 1996; Huddleston et al., 1998]. This distance is much greater than would be expected if SO2 were ejected from Io at the speed of 2.6 km s that has been taken to be characteristic of neutral sodium ejection [Smyth and Combi, 1988] because SO2 dissociates on time scales of about 0.5h near Io. This large radial extent is in contrast to the absence of any local ion production on the Io flux tubes directly beneath Io, inferred from the absence of ion cyclotron waves as Voyager I passed Io, ten Io radii beneath it, in 1979 [Russell and Huddleston, 2000]. While the initial Galileo data provided a radial range over which the massloading occurred, it was not until the availability of new Io data in the Galileo Europa Mission phase that the extent along the wake became understood. As discussed in the companion paper [Russell and Kivelson, 2001] SO2 and SO ion cyclotron waves were not seen upstream from Io but were observed downstream as far as Galileo returned data. The absence of ion cyclotron waves upstream of Io clearly indicate that the neutral cloud of SO2 and SO is not the banana-shaped cloud postulated for Na that extends upstream and down from Io but rather the neutral cloud extends only downstream. It is possible to understand the creation of such a massloading region in much the same way as has been postulated for the production of the directional feature neutral sodium cloud at Io [e.g. Wilson and Schneider, 1999] with just one more step added. This mechanism relies on the electric field of the jovian magnetosphere to accelerate ions, charge exchange to produce fast neutrals from these accelerated ions, and then transport across the magnetic field as neutrals, followed by subsequent ionization. It is the purpose of this paper to show results of a computer