A sensitivity study of the Enceladus torus

Abstract

We have developed a homogeneous model of physical chemistry to investigate the neutral‐dominated, water‐based Enceladus torus. Electrons are treated as the summation of two isotropic Maxwellian distributions: a thermal component and a hot component. The effects of electron impact, electron recombination, charge exchange, and photochemistry are included. The mass source is neutral H2O, and a rigidly corotating magnetosphere introduces energy via pickup of freshly ionized neutrals. A small fraction of energy is also input by Coulomb collisions with a small population (<1%) of suprathermal electrons. Mass and energy are lost due to radial diffusion, escaping fast neutrals produced by charge exchange and recombination, and a small amount of radiative cooling. We explore a constrained parameter space spanned by water source rate, ion radial diffusion, hot electron temperature, and hot electron density. The key findings are as follows: (1) radial transport must take longer than 12 days; (2) water is input at a rate of 100–180 kg s−1; (3) hot electrons have energies between 100 and 250 eV; (4) neutrals dominate ions by a ratio of 40:1 and continue to dominate even when thermal electrons have temperatures as high as ≈5 eV; (5) hot electrons do not exceed 1% of the total electron population within the torus; (6) if hot electrons alone drive the observed longitudinal variation in thermal electron density, then they also drive a significant variation in ion composition.