Cassini MIMI<br>Magnetospheric IMaging

Energetic Neutral Atom Imaging

A planetary magnetosphere consists of a number of different elements, including magnetic fields (which trap and contain the charged particles), plasmas (ionized gases), magnetically trapped energetic charged particles (electrons and ions), neutral gas (from planetary and satellite atmospheres, etc.), and particulates. The coexistence of trapped energetic ions and neutral gases gives rise to an interesting interaction called "charge exchange". A very energetic charged ion can steal the electron off of a very cool neutral gas atom and become an Energetic Neutral Atom (ENA). Because the ENA is neutral it is no longer trapped by the magnetic field. In general a magnetosphere glows in ENA much like a hot oven glows with photons. Instrumentation has now been developed for the MIMI Investigation that can image these ENA's allowing us for form remotely sampled images and movies of (a) the spatial, energy and ion species distributions of the trapped ion populations, and (b) the neutral exospheric atmospheres of Titan and perhaps other satellites. Below is an actual ENA image of the Earth's magnetosphere (using instrumentation not designed to perform this task), plus a number of simulated images of what we expect Saturn's magnetosphere and Titan's exosphere to look like in the MIMI images. The images were generated by E. C. Roelof (Roelof and Williams, 1988; Roelof, 1987; Dandouras et al., 1996). The quick-time movie of Energetic Neutral Atom Images (QT) (m4v) of a Titan Flyby shows Saturn's magnetospheric ion populations in the background as the enhanced ENA emissions from Titan's exospheric regions dominate the scene with time. A merator all-sky image plane is utilized.

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