  The aim of this work is to investigate the applications of the neutral atom
imaging to the environments of the Earth, Mars and Mercury. This innovative
technique permits the study of energetic plasma by means of analysing the
result of the interaction of this plasma with a neutral thermal population or
with a surface. The main advantage, when compared to the direct ion detection,
is that it is possible to have an instantaneous survey of the whole
magnetosphere of a planet. An example could help. Before the first ENA data,
most of the knowledge about the Earth magnetospheric plasma came from in situ
measurements of ions, electrons and electromagnetic fields. Those measurements,
of course, could not represent any real instantaneous situation, but only an
averaged picture of it, since the temporal and spatial variation cannot be
easily be distinguished. Some short time scale phenomena, such as substorms,
have been found difficult to comprehend without a global and continuous
imaging. Even if some information about the plasma may be extracted from other
sources, such as UV imaging [like aurorae, e.g. Horwitz, 1987], some
populations (for example, the ring current) remained invisible. Furthermore,
neutral atom imaging gives information not only about the energetic plasma, but
also about the thermal neutral population (in the case of charge-exchange) or
about the surface composition (in the case of sputtering). Conversely, it is
necessary to set up some dedicated unfolding techniques to recover the 3D
plasma distributions from the 2D ENA images.
