Investigation of Exosphere Variations by Lyman-alpha Detectors on the TWINS Mission

The dominant neutral constituent in Earth's upper exosphere, atomic hydrogen (H), resonantly scatters solar Lyman-alpha (121.567 nm) radiation, observed as the geocorona. The exosphere is also responsible for a major loss mechanism of trapped ions in the magnetosphere which leads to energetic neutral atom (ENA) emissions produced in charge exchange between energetic ions and H atoms. Knowledge of exospheric properties is consequently essential for the interpretation of ENA fluxes and to derive ion densities. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission includes Lyman-alpha Detectors (LADs) to observe the geocorona and investigate exospheric H atoms. Observations of the geocorona reveal a dynamic exosphere with complex asymmetries from day to night, dawn to dusk, and north to south. Prior models of exospheric H density distributions were usually averaged over an extended period of time, or developed under assumptions of being spherically symmetric, or representative of typical (not actual) solar conditions. In contrast, LAD/TWINS often allows obtaining global distributions with three-dimensional asymmetries on a daily basis, which opens a way to experimental probe the response of the exosphere to varying solar and geomagnetic conditions, as described by commonly used indices. Furthermore, seasonal variations, particularly at equinox and solstice, can be examined. We discuss the available data sets, their coverage, limitations, and present sets of derived exospheric H density distributions for geocentric distances above 3 Earth radii to investigate the global response of the exosphere to different seasonal, solar, and geomagnetic conditions.