Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission
Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (<i>R</i><sub>e</sub>) 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....
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-03-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/33/413/2015/angeo-33-413-2015.pdf |
| Summary: | Circumterrestrial Lyman-α column brightness observations above 3
Earth radii (<i>R</i><sub>e</sub>) 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 3<i>R</i><sub>e</sub> 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 <i>R</i><sub>e</sub>, independent of solar activity, increased
H densities appear on the Earth's nightside shifted towards dawn. With
increasing distance (as measured at 8<i>R</i><sub>e</sub>) 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 8<i>R</i><sub>e</sub>. |
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| ISSN: | 0992-7689 1432-0576 |