Imaging Earth's magnetosphere : Measuring energy, mass, and direction of Energetic Neutral Atoms with the ISENA instrument

ISENA (Imaging Spectrometer for Energetic Neutral Atoms) was launched on board the Argentinean-US Satellite SAC-B on 4 November 1996. The 3-axis stabilized SAC-B was to orbit the Earth at 550 km with an inclination of 38°, making it a suitable platform for detecting ENAs produced by charge exchange within Earth's magnetosphere. A failure of the spacecraft to separate from the Pegasus booster has resulted in the spacecraft's assuming an attitude which does not provide sufficient illumination of the solar panels, greatly reducing the likelihood of useful data production. ISENA's sensors are based upon the GEOTAIL/HEP-LD and CLUSTER/RAPID instruments. The instrument is comprised of two identical sensors, each with a field of view of 60° x 12°. The sensors are oriented at 90° to each other, but have parallel optic axes. Each sensor contains an entrance collimator electrically biased to reject charged particles, an entrance foil to suppress H Ly α photons and provide electrons for a start signal, two microchannel plate assemblies associated with a time-of-flight measuring system, a 4-segment anode to measure ENA arrival directions with 15° angular resolution, and a solid state detector to measure the energies of 35 keV - 150 keV ENAs. ISENA has a mass of 6 kg and consumes 6W. The design and operating characteristics of ISENA's sensors and associated electronics are described. Simulation of the predicted ENA signal is discussed.