Experimental study of exospheric hydrogen atom distributions by Lyman‐alpha detectors on the TWINS mission

[1] Exospheric atomic hydrogen (H) resonantly scatters solar Lyman‐a (121.567 nm) radiation, observed as the geocorona. Measurements of scattered solar photons allow one to probe time‐varying three‐dimensional distributions of exospheric H atoms. The Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission images the magnetosphere in energetic neutral atom (ENA) fluxes and additionally carries Lyman‐a detectors (LADs) to investigate exospheric atomic hydrogen. Knowledge of exospheric properties is essential for the interpretation of magnetospheric images in ENA fluxes produced in charge exchange between energetic ions and H atoms. We describe the process of obtaining exospheric distributions and present, as an example, a global H number density distribution for 11 June 2008. The TWINS LAD experimental data are especially sensitive to atomic hydrogen at geocentric distances from 3 to 8 Earth radii. The distribution reveals asymmetries from day to night, dawn to dusk, and north to south. We discuss the available data sets and their coverage, limitations, and promise for a study of exospheric response to seasonal, solar, and geomagnetic variations.