NEUTRAL CLOUD THEORY OF THE JOVIAN NEBULA - ANOMALOUS IONIZATION EFFECT OF SUPERTHERMAL ELECTRONS

The standard model of the Jovian nebula postulates that its particle s ource is the extended cloud of neutral sulfur and oxygen atoms that es cape from the satellite Io and become ionized through electron impact from the corotating plasma. Its energy source is the gyroenergy acquir ed by newly formed pickup ions as they are swept up to corotation velo city by the planetary magnetic field. Elastic collisions between plasm a and electrons cool the ions and heat the electrons, while inelastic collisions cool the electrons and excite the ions to radiate intense l ine emission, which is the primary energy-loss mechanism for the plasm a. This neutral cloud theory of the Io plasma torus, as it has come to be known, has been the subject of recent criticism which asserts that the theory cannot account for the observed charge state of the plasma which features O+ and S2+ as the dominant ions. It is shown in this w ork that the inclusion of a small population of superthermal electrons is required to achieve the correct ion partitioning among various cha rge states. The superthermal electrons are an essential element of the theory: they are a consequence of the highly nonthermal velocity dist ribution of pickup ions generating plasma waves which dissipate by acc elerating hot electrons to kiloelectron volt energies. Coupled ionizat ion and energy balance equations are solved for a homogeneous plasma m odel featuring the addition of a superthermal-electron population. The results of the analysis show excellent agreement with the observation s. It is also argued that the anomalous ionization effect of the super thermal electrons is responsible for the overall spatial bifurcation o f the nebula into a hot multiply charged plasma region outside of 5.7 Jovian radii and a cool singly ionized plasma inside this distance.