Collisional Heating of Interstellar Helium Flux at 1 AU

Abstract

Because of relative motion of the local interstellar medium (LISM) with respect to the Sun, the interstellar wind, interstellar neutral helium atoms enter the heliosphere. Instruments on the Ulysses and Interstellar Boundary Explorer spacecraft measured directional dependencies of intensities, or sky maps, of helium atoms and determined the interstellar wind velocity vector and temperature. The article focuses on heretofore unaccounted for heating of inflowing interstellar helium by elastic collisions with the solar wind ions and its effect on the LISM temperature obtained from the Interstellar Boundary Explorer-type measurement. The calculations show that inferred LISM temperatures appear 175-270 K hotter than the actual interstellar gas due to collisions.