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Circumterrestrial Lyman-α column brightness observations above 3\udEarth radii (Re) have been used to derive separate 3-D neutral\udhydrogen density models of the Earth's exosphere for solar minimum (2008,\ud2010) and near-solar-maximum (2012) conditions. The data\udused were measured by Lyman-α detectors (LAD1/2) onboard each of\udthe TWINS satellites from very different orbital positions with respect to\udthe exosphere. Exospheric H atoms resonantly scatter the near-line-center\udsolar Lyman-α flux at 121.6 nm. Assuming optically thin conditions\udabove 3Re along a line of sight (LOS), the scattered\udLOS-column intensity is proportional to the LOS H-column density. We found\udsignificant differences in the density distribution of the terrestrial\udexosphere under different solar conditions. Under solar maximum conditions\udwe found higher H densities and a larger spatial extension compared to solar\udminimum. After a continuous, 2-month decrease in (27 day averaged) solar activity,\udsignificantly lower densities were found. Differences in shape and\udorientation of the exosphere under different solar conditions exist.\ud Above 3 Re, independent of solar activity, increased\udH densities appear on the Earth's nightside shifted towards dawn. With\udincreasing distance (as measured at 8Re) this feature is\udshifted westward/duskward by between −4 and −5° with\udrespect to midnight. Thus, at larger geocentric distance the exosphere\udseems to be aligned with the aberrated Earth–solar-wind line, defined by the\udsolar wind velocity and the orbital velocity of the Earth. The results\udpresented in this paper are valid for geocentric distances between\ud3 and 8Re

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