Abstract
Abstract. Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (Re) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solar-maximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3Re along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 Re, independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8Re) this feature is shifted westward/duskward by between −4 and −5° with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth–solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8Re.
Highlights
Neutral hydrogen atoms are the main component of the terrestrial exosphere
We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions
Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum
Summary
Neutral hydrogen atoms are the main component of the terrestrial exosphere. They produce the geocoronal Lyman-α glow by resonant scattering of solar Lyman-α radiation at 121.6 nm. The Lyman-α detectors (LADs) onboard the two TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) satellites (McComas et al, 2009) have provided nearly continuous, circumterrestrial Lyman-α observations of the exosphere (from ≈ 7.2 Re apogee) since 2008. Based on TWINS LAD data for the last solar minimum (2008, 2010), earlier studies (e.g., Bailey and Gruntman, 2011; Zoennchen et al, 2011, 2013) presented the first exospheric density models with angular dependence in the optically thin regime. Two typically different exospheres are fitted (separately for solar minimum and maximum) and are compared with each other This revised analysis corrects the LAD data for geocoronal multiple-scattering effects (self-absorption and re-emission) using a multiplescattering Monte Carlo model based on “coherent line-center scattering” of photons in order to calculate the locally effective solar illumination. New methods are introduced to recalibrate the TWINS LAD sensitivity based on observed stellar UV peaks and to mask UV-bright stars for exclusion from the data sets
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