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| Titel |
Comparison of sporadic sodium layer characteristics observed at different time resolutions |
| VerfasserIn |
Y. J. Liu, B. R. Clemesha, J. H. Wang, X. W. Cheng |
| 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 ; 31, no. 11 ; Nr. 31, no. 11 (2013-11-04), S.1899-1912 |
| Datensatznummer |
250086139
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| Publikation (Nr.) |
 copernicus.org/angeo-31-1899-2013.pdf |
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| Zusammenfassung |
| Sporadic sodium (Nas) layers, occurring in roughly the same height
range as ionospheric sporadic-E layers, were first detected by lidar some
30 yr ago. Nas layers have a typical thickness of a few hundred meters
to a few km, with peak atom concentrations several times that of the
background layer. Despite a great deal of excellent work over the past
decades, the source of Nas layers is still not altogether clear, partly
as a result of our incomplete knowledge of Nas layer characteristics.
In this paper we concentrate on some typical case studies chosen from the
~127 h of sporadic sodium layer observations made at a
time resolution of 1.5 s at Yanqing (115.97° E,
40.47° N), Beijing, China. This is a much better time resolution
than what has been employed in most earlier measurements. The results show that
the Nas layer peak heights are dispersed at slightly different although
adjacent heights. When averaged over several minutes, as has been the case
with most earlier measurements, the height scatter results in an apparent
layer thickness of a few km. We conclude, therefore, that these dispersed peaks
at different but adjacent heights constitute the 5 min Nas layer.
Similar to the observations of sporadic-E-ion (Es) layers and meteor rate, we observe
quasi-periodic fluctuations on a timescale on the order of several minutes
in the peak height and the peak density of sporadic layers, which is a
universal feature but concealed by the lower temporal resolution previously
adopted. Spatially localized multiple scatterers and multiple thin layers
with similar apparent movement in Nas layers are also found. We discuss the
possible formation mechanism by the direct deposition of large swarms of
micrometeoroids and demonstrate a typical example of meteor trails evolving
into a Nas layer, which suggests that this mechanism might indeed
occur. |
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