A Sensitivity Study of the Enceladus Torus

TL;DR

This study models the Enceladus torus's physical chemistry, revealing key constraints on water input, electron energies, and transport timescales, with implications for ion composition and electron populations.

Contribution

It introduces a homogeneous model incorporating detailed electron and chemical processes, providing new constraints on the torus's physical parameters and dynamics.

Findings

Radial transport exceeds 12 days.

Water input rate is 100-180 kg/s.

Hot electrons have energies of 100-250 eV.

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 H$_2$O, 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 supra-thermal 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: (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 $\approx$ 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.