Transition from ring to beam arc distributions of water ions near the Space Shuttle Orbiter

Abstract

The distribution function of water ions produced near the space shuttle by charge exchange between ionospheric oxygen ions and outgassed water molecules is investigated using solutions of Liouville's equation with a source term modeling the charge exchange process. A transition from ring distributions to beamlike distributions termed “beam arc” distributions is found with decreasing distance upstream from the orbiter. This beam arc distribution corresponds to a finite section of a ring distribution and not to a conventional beam distribution. The ratio of water ion number density to oxygen ion number density is calculated; typical values within 50 m of the shuttle are in excess of 2% with a maximum value of the order of 20% for nominal parameters, suggesting that these ions must be considered when interpreting particle data from near the space shuttle. An argument for a plasma density enhancement of the order of 10% very close to the shuttle, due to kinematic effects (corresponding to pileup of plasma) and not to plasma creation, is also presented. This kinematic density enhancement is insufficient, by an order of magnitude, to explain the plasma density enhancements inferred from Spacelab 2 data.