Collisional interactions of precipitating energetic neutral atoms with upper-atmospheric particles in the low-latitude region

Altitude profiles of energetic neutral atom (ENA) fluxes precipitating into the upper atmosphere are simulated numerically by using a Monte Carlo meth od with source magnetospheric protons distributed between L shells 4 and 7, taking account of collisional interactions with atmospheric particles. Num erical results are in reasonable agreement with the profile of similar to 1 0 keV neutral atom fluxes recently measured under a geomagnetically quiet c ondition. Through collisional interactions, ENAs can be converted to ions a nd then the ionized ENAs can be reneutralized and repeat this cycle. Theref ore the precipitating ENAs lose their original directionality because of gy romotions of the ionized ENAs. However, the source information is still see n in ENA data measured at low altitudes where collisions are significant, b ecause their pitch angles are almost conserved with the forward scattering approximation for each collision in the case of parallel geomagnetic fields at low altitudes. On the other hand, a part of ENAs precipitating into the atmosphere returns to the magnetosphere. These reflected ENAs act as a sec ondary source that cannot be neglected in ENA data measured in the magnetos phere.