HELIX: EXPLORING THE DETAILED PROPERTIES OF NEUTRAL HYDROGEN THROUGHOUT THE SOLAR SYSTEM

Atomic hydrogen is a key component of the Sun-planet interaction. As the lightest element, it becomes dominant in terrestrial atmospheres at altitudes extending from the thermosphere-ionosphere region into the exosphere and interplanetary medium. Spectro-imaging of scattered Lyman series emission is useful for mapping the spatial distribution of hydrogen over the full range of spatial scales in the solar system and provides the basis for theoretical investigation of its radiative and energetic properties. However, the spectral resolving power of the instruments used in these studies is insufficient for extracting structure in emission lineshapes. Absent this information, it is difficult to fully disentangle the effects of impact, temperature, multiple scattering, non-thermal processes, and background contamination from the observed brightness. Recent advances in far ultraviolet (FUV) spatial heterodyne spectroscopy (SHS) offer an instrumental solution providing high spectral resolving power in a compact, wide field format. The Hydrogen Emission Line Interferometric eXplorer (HELIX) is a pathfinder SHS design intended for study of hydrogen properties in terrestrial planet exospheres and the interplanetary medium. We describe here the scientific objectives and physical characteristics of a SmallSat-HELIX mission concept to study geocoronal hydrogen from observations of H Ly-a.