On the Extraction of Ring Current Ion Pitch-Angle Distributions from ENA Images

E. C. Roelof and R. Demajistre

There is growing evidence that pitch-angle distributions (PADs) of ring
current ions (10-200 keV) are strongly affecting the brightness variations
in energetic neutral atom (ENA) images being obtained with the IMAGE/HENA
camera [C:son Brandt et al., this Conference].  Because the evolution
of these PADs is a critical diagnostic of the injection and transport of
ring current ions, the extraction of the global behavior of PADs from the
ENA images is a priority task.  The mathematical difficulty is that a single
ENA image is 2-dimensional, whereas the energetic ion distribution is
three-dimensional (L, MLT, alpha).  The extraction process therefore
requires the assumption of the invariance of the ion magnetic moment
sin^2 alpha/B, and it depends on comparing lines of sight from different
pixels that view high altitudes and low altitudes along the same field lines
(similar ranges of L and MLT).  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 degrees x 6 degrees), and (to complicate matters further)
the "point spread" function of the instruments causes the bright emission to
"bloom" into adjacent pixels. However, the basic formulas for calculating
exospheric ENA emission [Roelof, Adv. Space Res., 20(3), 361-366, 1997] 
have now been combined with the HENA point-spread function in computer
for comparison with IMAGE/HENA images of 10-100 keV ENA hydrogen.  Examples
will be presented of how these simulations are used to constrain the global
spatial dependence of PADs in the IMAGE/HENA images of the storm-time ring
current.

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Submitted to the Spring 2001 AGU Meeting, Boston, Massachusetts