Linear Inversions of ENA Images for Energetic Ion Distributions from the Ring Current and Plasma Sheet Robert Demajistre 1 (1-240-228-8756; robert.demajistre@jhuapl.edu) Edmond C. Roelof 1 (1-240-228-5411; edmond.roelof@jhuapl.edu) Pontus C. Brandt 1 (1-240-228-3837; pontus.brandt@jhuapl.edu) Donald G. Mitchell 1 (1-240-228-5981; donald.g.mitchell@jhuapl.edu) (Sponsor: Edmond C. Roelof) 1 Johns Hopkins U./Applied Physics Lab., 11100 Johns Hopkins Road, Laurel, MD 20723-6099, United States Linear, constrained inversion techniques are being applied to extracting energetic ion distributions from ENA images 10-60 keV from the IMAGE/HENA time-of-flight experiment. The interpretation of HENA image sequences during the geomagnetic storms and substorms is presented in two papers at this conference by C:son Brandt et al., while this poster describes and illustrates the details of the inversion techniques. ENA emission from high altitudes (the hydrogen geocorona) and low altitudes (primarily atomic oxygen in the exobase) are both included in the inversion kernel. The scattering of the incident ENAs in the HENA front foil produces a significant point-spread function that is energy dependent (with a standard deviation of 15 deg at 20 keV diminishing to 5 deg at 60 keV). This instrument function is also incorporated directly into the kernel, resulting in a completely self-consistent treatment of the inversion problem [as described by Roelof and Demajistre, Trans. AGU, 82, S321, 2001]. When highly reliable inversions and confidence estimates are desired, the constraints are optimized. This is done by approximating the linearly inverted ion distributions with a 38-parameter non-linear analytic model function [Roelof and Skinner, Space Sci. Rev., 91, 437, 2000] and using it to simulate the ENA image. The constraints are then fine tuned to optimize the extraction of the (known) model function from the simulated image. This optimization is particularly useful when estimating the reliability of the extracted pitch-angle dependence of the ion distribution.