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| Titel |
The theoretical impact polarization of the O I 6300 Å red line of Earth aurorae |
| VerfasserIn |
V. Bommier, S. Sahal-Bréchot, J. Dubau, M. Cornille |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 29, no. 1 ; Nr. 29, no. 1 (2011-01-10), S.71-79 |
| Datensatznummer |
250016948
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| Publikation (Nr.) |
 copernicus.org/angeo-29-71-2011.pdf |
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| Zusammenfassung |
| We are presenting a semi-classical theory of the impact polarization due to a
quadrupolar electric excitation, which is the case of this forbidden line. In
addition, this line is also radiatively forbidden being a triplet-singlet
transition. This last feature is overcome by scaling the semi-classical
result to a full quantum calculation at a single energy value. The
cross-section and impact polarization are thus obtained as a function of
energy, in agreement with the quantum calculations that exist only for
the cross-section. The behavior of the impact polarization is found to be
quite different than that of the usual dipolar electric interaction. Let us
denote as radial the polarization parallel to the incident beam or
magnetic field, and as tangential the perpendicular polarization. In
the case of the dipolar electric interaction (permitted lines), the
polarization is radial at low energy, and tangential at high energy, and it
vanishes at energy about twelve times the threshold energy. In the case of
the quadrupolar electric interaction, we observe quite different behavior, with
the polarization vanishing point much closer to the threshold energy. This
leads us to reanalyze the auroral red line polarization observation by
Lilensten et al. (2008). From polarization observations made at Svalbard,
they conclude to a rather strong tangential polarization observed during a
4-h recording including two auroral events. The existence of
tangential polarization is questioned by our new theory, which leads to
reconsidering the contribution of scattered parasitic light from a neighboring
city that was mentioned but discarded by the authors. Finally, we conclude
that the line is only weakly radially polarized by electron impact, and only
during the auroral events. The weak polarization level leads to taking the
competing depolarization by collisions with the neighboring O atoms into account, and by
the competing isotropical (thus depolarizing) processes for populating the
line upper level: the dissociative recombination of O2+ colliding with
thermal electrons, and above all the reaction N(2D)+O2. The final
diagnostic could be a density determination by depolarization, but it may be
rather complicated because it involves several species. |
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