Fast Imaging of Energetic Neutral Atoms near Mars

Holmstrom, M., S. Barabash, E. Kallio 

Energetic neutral atom (ENA) images of the Martian environment are computed
from an empirical solar wind proton model and an exosphere model. The proton
model describes the proton bulk velocity in the interaction region and the
neutral model the profiles of H, H-2 and O (hot) and O (thermal). The ENA
production model is analytical, thus an ENA image can be constructed for any
vantage point by line of sight integration and we present examples of such
images.
	
We study the influence of different parameters in the input models on the
generated images.  It is found that the exobase temperature of atomic
hydrogen is the parameter that affects the ENA production and images most.
We also investigate the backscattering of ENAs that precipitate on the
Martian atmosphere, and it is found to be an important effect for views of
Mars in the antisunward direction. The outflux of ENAs from the near Mars
region is studied, and two maxima are found. One that corresponds to ENAs
produced downstream of the bow shock, and one that corresponds to ENAs
produced upstream of the bow shock, in the solar wind.
	
For a more detailed, quantitative, analysis of observed images one can use
inversion techniques to extract parameters from a mathematical model of the
ENA production. Basically, we generate images until we find one that closely
match the observed image. The parameters that produce the best match is our
parameter estimate. The sequence of generated images is determined by the
chosen minimization algorithm. Thus, to solve the inverse problem, hundreds,
or even thousands, of images must be generated and it is crucial that we can
generate each image as fast as possible.
	
Computer imaging of volume emissions is a general problem, of which ENA
imaging is just a special case.  In computer graphics this process is
denoted volume rendering. We have developed an hierarchical, wavelet based,
method for fast imaging of volume emissions. The proposed method also
provides error estimates and error control, automatically. The approach is
to find a sparse representation of the volume emission, a three-dimensional
function, before the projection onto the image plane. The representation is
based on conservative subdivision. Numerical experiments suggest that the
proposed method can be orders of magnitude faster than traditional line of
sight integration, for similar errors in the generated images.

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