Energetic neutral atoms in cometary comae

Abstract

A Monte Carlo simulation model is constructed to follow the collisional scattering of the energetic neutral atoms (ENAs) in the cometary coma. It is found that collisional thermalization in the inner coma could lead to the production of an isotropic flux of fast-moving atoms with velocities on the order of 40-60 km/s. This thermalized component may be an important source for the cometary heavy ions detected at large cometocentric distances. Collisional scattering of the ENAs with the neutral coma also results in heating of the expanding gas. The generation of hot ions in the vicinity of the magnetic field 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 here has an initial streaming velocity of 230 km/s. The theoretical prediction so obtained is not consistent with the Giotto observations that the main population of the hot ions should be confined mostly inside the ionospheric contact discontinuity. 24 refs.