FUNDAMENTALS OF LOW-ENERGY NEUTRAL ATOM IMAGING

Imaging of the space plasma environment via low-energy neutral atoms ( LENAs) promises to revolutionize the way in which large-scale space pl asma phenomena are viewed and understood. LENAs are produced by charge exchange between plasma ions (less than tens of kilo-electron-volts) and cold geocoronal neutrals; these LENAs radiate outward in all direc tions from their points of origin. Previously developed methods for im aging higher energy neutrals are not suitable for observing the majori ty of the terrestrial magnetosphere, which is comprised primarily of l ower energy plasma populations. This paper briefly describes both the direct and indirect techniques that have been suggested for imaging LE NAs to date. We then examine in more detail the most advanced of these techniques appropriate for magnetospheric imaging, indirect detection based on ionization of LENAs as they transit ultrathin foils. Such a LENA imager consists of four basic components: (1) a biased collimator to remove the ambient charged particles and set the azimuthal field o f view; (2) an ultrathin foil, which ionizes a portion of the incident LENAs; (3) an electrostatic analyzer to reject UV light and set the e nergy passband; and (4) a coincidence position detector to measure con verted LENAs while rejecting noise and penetrating radiation.