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

The standard model of the Jovian nebula postulates that its particle source is the extended cloud of neutral sulfur and oxygen atoms that escape from the satellite Io and become ionized through electron impact from the corotating plasma. Its energy source is the gyroenergy acquired by newly formed pickup ions as they are swept up to corotation velocity by the planetary magnetic field. Elastic collisions between plasma and electrons cool the ions and heat the electrons, while inelastic collisions cool the electrons and excite the ions to radiate intense line emission, which is the primary energy-loss mechanism for the plasma. 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 work that the inclusion of a small population of superthermal electrons is required to achieve the correct ion partitioning among various charge states. The superthermal electrons are an essential element of the theory: they are a consequence of the highly nonthermal velocity distribution of pickup ions generating plasma waves which dissipate by accelerating hot electrons to kiloelectron volt energies. Coupled ionization and energy balance equations are solved for a homogeneous plasma model featuring the addition of a superthermal-electron population. The results of the analysis show excellent agreement with the observations. It is also argued that the anomalous ionization effect of the superthermal electrons is responsible for the overall spatial bifurcation of the nebula into a hot multiply charged plasma region outside of 5.7 Jovian radii and a cool singly ionized plasma inside this distance.