Determination of the neutral number density in the Io torus from Galileo-EPD measurements

Abstract

During the Galileo spacecraft encounter with Io in December 1995, the Energetic Particles Detector (EPD) [Williams et al., 1992] measured pitch angle distributions with high spatial and temporal resolution. All ion and electron channels show a depletion of field-aligned particles due to the interaction of trapped particles with the ionosphere / atmosphere of Jupiter. The medium nuclei channels, containing mainly sulphur and oxygen ions, show a second minimum for particles moving perpendicular to the magnetic field direction. This observation indicates that charge exchange between the singly-charged energetic ions and neutral atoms in the Io torus represents an important loss mechanism for energetic ions within the Io torus regions. Additionally, knowing the cross-sections of the involved charge exchange reactions it is possible to calculate the average neutral number density in the Io torus. The obtained result of ???35 cm-3 is in good agreement with density estimations using ground based measurements.