First observed variations of the exosphere during geomagnetic storms

The tenuous extension of Earth's neutral atmosphere, the upper exosphere, consists predominately of atomic hydrogen (H). Exospheric H atoms resonantly scatter solar Lyman-alpha (121.567 nm) radiation, creating a phenomenon known as the geocorona. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission includes Lyman-alpha Detectors (LADs) to observe the geocorona and investigate exospheric H atoms. We use LAD measurements to reconstruct global H density distributions for geocentric distances from 3 to 8 Earth radii. We report here for the first time observed variations of the exosphere during five geomagnetic storms that occurred in the 3-month time period from 1 August 2011 to 1 November 2011. Our preliminary finding that exospheric H densities temporarily increase during geomagnetic storms may provide insight into specific mechanisms of interaction between the exosphere and magnetosphere as well as exospheric effects on ring current decay rates. In addition, the interpretation of magnetospheric images in energetic neutral atom (ENA) fluxes relies on a line-of-sight integration that directly depends on the H number density distribution.