How We are Extracting Ion Distributions from HENA Images

Roelof, E.C., R. Demajistre, P. C:son Brandt, and D. G. Mitchell

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. Inversions of HENA image sequences during the
geomagnetic storms and substorms is presented in two papers at this
conference by Mitchell et al. and C:son Brandt et al., while this poster
describes and illustrates the details of the inversion techniques. 

The observational difficulty is that the ENA emission from low altitudes can
easily be more than an order of magnitude brighter than that from high
altitudes on the same field line. The source of this ENA emission is the
charge exchange of energetic singly-charged ions with exospheric atoms at
altitudes of  ~300-400 km. It is here that the densities of atomic oxygen and
helium (and perhaps di-atomic nitrogen) overwhelm the density of the
geocoronal hydrogen that is the converter of ring current ions into ENAs at
all higher altitudes. This bright low altitude emission is from too small a
volume to be resolved by the HENA camera pixels ~6 deg x 6 deg and (to
complicate matters further) the "point spread" function of the instruments
causes the bright emission to "bloom" into adjacent pixels. 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).  However, we have been able to
incorporate this instrument function into both the high and low altitude
kernels, resulting in a completely self-consistent treatment of the inversion
problem.

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. 

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To be presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.