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

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.