Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

摘要

Circumterrestrial Lyman-α column brightness observations above 3Earth radii (R_e) have been used to derive separate 3-D neutralhydrogen density models of the Earth's exosphere for solar minimum (2008,2010) and near-solar-maximum (2012) conditions. The dataused were measured by Lyman-α detectors (LAD1/2) onboard each ofthe TWINS satellites from very different orbital positions with respect tothe exosphere. Exospheric H atoms resonantly scatter the near-line-centersolar Lyman-α flux at 121.6 nm. Assuming optically thin conditionsabove 3R_e along a line of sight (LOS), the scatteredLOS-column intensity is proportional to the LOS H-column density. We foundsignificant differences in the density distribution of the terrestrialexosphere under different solar conditions. Under solar maximum conditionswe found higher H densities and a larger spatial extension compared to solarminimum. After a continuous, 2-month decrease in (27 day averaged) solar activity,significantly lower densities were found. Differences in shape andorientation of the exosphere under different solar conditions exist. Above 3 R_e, independent of solar activity, increasedH densities appear on the Earth's nightside shifted towards dawn. Withincreasing distance (as measured at 8R_e) this feature isshifted westward/duskward by between ?4 and ?5° withrespect to midnight. Thus, at larger geocentric distance the exosphereseems to be aligned with the aberrated Earth–solar-wind line, defined by thesolar wind velocity and the orbital velocity of the Earth. The resultspresented in this paper are valid for geocentric distances between3 and 8R_e.