Impact of Planetary Gravitation on High Precision Neutral Atom Measurements

Abstract:

Measurements of energetic neutral atoms (ENAs) have been extremely successful in providing very important information on physical processes inside and outside our heliosphere. For instance, recent IBEX observations provided new insights into the local interstellar environment and improved measurements of the interstellar He temperature, velocity, and direction of the interstellar flow vector. Since particle collisions are rare and radiation pressure is negligible for these neutrals, gravitational forces mainly determine the trajectories of neutral He atoms. Depending on the distance of an ENA to the source of a gravitational field and its relative speed and direction this can result in a significant deflection and acceleration. In this presentation we study the impact of the gravitational effects of the Earth, Moon, and Jupiter on ENA measurements performed in Earth orbit. We show that planetary gravitational effects do not significantly affect the interstellar neutral gas parameters obtained from IBEX observations. We further study the possibility whether the He focusing cone of the Sun or Jupiter could be measured by IBEX, and whether these cones could be used as an independent measure of the interstellar He temperature. These topics are of particular importance for future missions such as IMAP, which will provide ENA images for a broader energy range and with better sensitivity and resolution.