Energetic neutral atoms in cometary comae

The collisional interaction of the energetic neutral atoms (ENAs) created via charge exchange process in the outer coma with the neutral atmosphere of comet Halley is investigated by means of a Monte Carlo model. We find that collisional thermalization effect in the inner coma could lead to the production of an isotropic flux (≈ 10²⁷-10²⁸ atoms s^-1) of fast-moving atoms with velocities on the order of 40-60 km s^-1. This thermalized component may be an important source for the distant cometary heavy ions detected at large cometocentric distances (≈ 5-8 × 10⁶ km). Collisional scattering of the ENAs with the neutral coma also results in a certain level of heating of the expanding gas. The issue concerning the generation of hot ions in the vicinity of the magnetic-field free cavity is studied by estimating the production rates of secondary ions from impact ionization of the ENAs in the inner coma. The monoenergetic beam of ENAs used in the present work has an initial streaming velocity of 230 km s^-1. The theoretical prediction, so obtained, is not consistent with the Giotto observations that the main population of the hot ions (with energies ≈100 eV) should be confined mostly inside the ionospheric contact discontinuity. In principle, the inclusion of a more suitable velocity distribution of the initial ENA population, with more 100 eV particles than keV ones, could improve the situation. Other mechanism(s) also may be involved in the production of the hot ions detected.