Imaging neutral particle detector

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Abstract
With the support of NASA's Innovative Research Program (IRP), we undertook the definition and initial development of an imaging neutral particle detector, a new instrument for global imaging of planetary magnetospheres. This instrument carries out remote sensing of magnetospheric plasmas by imaging and analyzing charge exchange neutrals, which are energetic neutral atoms (ENAs) created by charge exchange reactions between ambient neutrals and energetic ions. The imaging neutral particle detector can determine the composition of the charge exchange neutrals and therefore also the composition of the original hot plasma. Such an instrument, the Ion‐Neutral Camera of the Magnetospheric Imaging Instrument (MIMI), has been selected for flight on the NASA/ESA Cassini mission to Saturn and Titan, and a similar instrument is part of the baseline payload for the proposed Inner Magnetosphere Imager (IMI) mission to carry out remote sensing of Earth's magnetosphere. Remote sensing of charge exchange neutrals is a powerful new diagnostic technique that is expected to yield qualitative advances in our understanding of magnetospheric configuration and dynamics, by providing a global perspective on hot plasma and ambient neutral populations and by separating effects of temporal and spatial variation. The IRP imaging neutral particle detector design measures the incidence direction of incoming fast neutrals with angular resolution of ∼ 2° and with total geometric factor of ∼ 2.5 cm2 sr. The composition is measured with mass resolution sufficient to separate oxygen from nitrogen, and the energy spectrum is measured above 7 keV/nuc. This instrument utilizes a charged particle deflection system, a thin foil time‐of‐flight system, imaging anode systems, and a total energy measurement by a matrix solid state detector. We describe computations of anticipated results at Saturn and show how the ring current can be identified, how Titan can serve as a monitor of magnetospheric energetic ion populations while passing through Saturn's magnetosphere, how substorms may be monitored, how exospheres of icy moons may be mapped during close encounters, and how heliospheric shock structures can be imaged from orbit apoapsis. Thus, magnetospheric imaging can provide a uniquely flexible tool for studying the large‐scale dynamics of planetary atmospheres, a task that has not been heretofore possible on a global scale.