Energetic neutral atoms from Jupiter measured with the Cassini magnetospheric imaging instrument: Time dependence and composition

Abstract

During the Cassini spacecraft flyby of Jupiter in late 2000/early 2001, the Ion and Neutral Camera (INCA, one of three sensors that make up the Cassini Magnetospheric Imaging Instrument (MIMI)) obtained the first ever images of Jupiter's magnetosphere in energetic neutral atom (ENA) flux. These emissions are dominated by hydrogen atoms in the energy range from a few to 100 keV, emitted from the planet's exosphere and neutral gas tori near the inner Galilean satellites. Heavier ions (likely a mixture of oxygen and sulfur) comprise a significant fraction of the ENA flux, as well. Neutral atom energy spectra escaping the magnetosphere are computed from measurements made at about 145 RJ from the planet. From these spectra we show that there is significant emission of oxygen and/or sulfur atoms from the Jovian magnetosphere in the 0.1–1.0 MeV total energy range. We also show that ENA flux falls off approximately with the square of radial distance from Jupiter. In three separate energy channels, we find that during a period of about 80 days (mostly before closest approach), FR2 is constant to lowest order but do observe variations close to the statistical limit of our measurement. We estimate a source rate of ∼1026/s H in the 10–100 keV range and ∼5 × 1025/s O in the 100–1000 keV energy range.