Source Regions of Energetic Neutral Atoms From Jupiter

We have discovered that two discrete source regions dominate the emission fluxes of energetic neutral atoms (ENAs) from Jupiter's space environment in the 10's of keV energy range. The source regions are: 1) a ring or torus of emission just beyond the orbit of Europa, and 2) Jupiter's exosphere which glows in ENAs as a result of the near-precipitation of energetic ions onto Jupiter's atmosphere. To identify these sources we: i) deconvolved the instrument response from the ENA images obtained by the Cassini Ion and Neutral Camera during the Cassini flyby of Jupiter in January of 2001 (Krimigis et al., 2002), and ii) combined the images with in situ data obtained from the Galileo Energetic Particle Detector (EPD). The trans-Europa ENA emission source requires neutral densities greater than 10 / cc, and perhaps 10's per cc depending on the exact distribution of the ENA emissions. Thus, we hereby confirm the inference of Lagg et al. (2001), developed using Galileo EPD energetic-ion angle distributions, of the presence of a much denser population of neutral gas in the vicinity of Europa than has been predicted on the basis of the volcanic sources at Io. The Jupiter exospheric ENA source strength offers the most direct measure yet obtained of the flux of energetic ions precipitating over all onto Jupiter's atmosphere. Krimigis et al., A nebula of gasses from Io surrounding Jupiter, Nature, 415, 994, 2002. Lagg, A., N. Krupp, J. Woch, and D. J. Williams, Evidence for an extended neutral torus at Europa, Unpublished Manuscript, 2001.