Global ENA IMAGE Simulations

M.-C. Fok, T.E. Moore, G.R. Wilson, J.D. Perez, X.X. Zhang, P. C:son Brandt,
D.G. Mitchell, E.C. Roelof, J.-M. Jahn, C.J. Pollock, and R.A. Wolf


The energetic neutral atom (ENA) images obtained by the ISEE and POLAR
satellites pointed the way toward global imaging of the magnetospheric
plasmas. The Imager for Magnetopause to Aurora Global Exploration (IMAGE) is
the first mission to dedicate multiple neutral atom imagers: HENA, MENA and
LENA, to monitor the ion distributions in high-, medium- and lowenergy
ranges, respectively. Since the start of science operation, HENA, MENA and
LENA have been continuously sending down images of the ring current,
ionospheric outflow, and magnetosheath enhancements from high pressure solar
wind. To unfold multiple-dimensional (equal or greater than 3) plasma
distributions from 2-dimensional images is not a trivial task. Comparison
with simulated ENA images from a modeled ion distribution provides an
important basis for interpretation of features in the observed images.
Another approach is to develop image inversion methods to extract ion
information from ENA images. Simulation studies have successfully reproduced
and explained energetic ion drift dynamics, the transition from open to
closed drift paths, and the magnetosheath response to extreme solar wind
conditions. On the other hand, HENA has observed storm-time ion enhancement
on the nightside toward dawn that differs from simple concepts but can be
explained using more sophisticated models. LENA images from perigee passes
reveal unexpected characteristics that now can be interpreted as evidence
for a transient superthermal exospheric component that is
gravitationally-influenced if not bound. In this paper, we will report ENA
simulations performed during several IMAGE observed events. These
simulations provide insight and explanations to the ENA features that were
not readily understandable previously. 

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Space Sci. Rev., 109, 77-103, 2003