Global ENA IMAGE Simulations

Fok, M.-C., T. E. Moore, Y. Zheng, G. R. Wilson, D. G. Mitchell, C. J. Pollock,
J. D. Perez, and E. C. Roelof

The energetic neutral atom (ENA) images obtained in the ISEE and POLAR
satellites pointed the way toward global imaging of the magnetospheric plasmas.
IMAGE is the first mission to dedicate multiple neutral atom imagers: HENA,
MENA and LENA, to monitor the ion distributions in high-, medium- and
low-energy 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. 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 and MENA have observed
sudden appearance and disappearance of energetic ions that cannot be explained
by current understanding of ring current buildup and decay. LENA images also
showed some unexpectedly diffusive oxygen outflow in the polar region. In this
paper, we will review ENA simulations performed during several IMAGE observed
events and summarize what we have learned from these studies. We will also
discuss the discrepancies between simulations and observations, and outline
plans to understand these inconsistencies.

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