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| Titel |
New results on equatorial thermospheric winds and the midnight temperature maximum |
| VerfasserIn |
J. Meriwether, M. Faivre, C. Fesen, P. Sherwood, O. Veliz |
| 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 ; 26, no. 3 ; Nr. 26, no. 3 (2008-03-26), S.447-466 |
| Datensatznummer |
250016037
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| Publikation (Nr.) |
 copernicus.org/angeo-26-447-2008.pdf |
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| Zusammenfassung |
| Optical observations of thermospheric winds and temperatures determined with
high resolution measurements of Doppler shifts and Doppler widths of the OI
630-nm equatorial nightglow emission have been made with improved accuracy at
Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot
interferometer. An observing procedure previously used at Arecibo Observatory
was applied to achieve increased spatial and temporal sampling of the
thermospheric wind and temperature with the selection of eight azimuthal
directions, equally spaced from 0 to 360°, at a zenith angle of
60°. By assuming the equivalence of longitude and local time, the data
obtained using this technique is analyzed to determine the mean neutral wind
speeds and mean horizontal gradients of the wind field in the zonal and
meridional directions. The new temperature measurements obtained with the
improved instrumental accuracy clearly show the midnight temperature maximum
(MTM) peak with amplitudes of 25 to 200 K in all directions observed for most
nights. The horizontal wind field maps calculated from the mean winds and
gradients show the MTM peak is always preceded by an equatorward wind surge
lasting 1–2 h. The results also show for winter events a meridional wind
abatement seen after the MTM peak. On one occasion, near the September
equinox, a reversal was observed during the poleward transit of the MTM over
Arequipa. Analysis inferring vertical winds from the observed convergence
yielded inconsistent results, calling into question the validity of this
calculation for the MTM structure at equatorial latitudes during solar
minimum. Comparison of the observations with the predictions of the NCAR
general circulation model indicates that the model fails to reproduce the
observed amplitude by a factor of 5 or more. This is attributed in part to
the lack of adequate spatial resolution in the model as the MTM phenomenon
takes place within a scale of 300–500 km and ~45 min in local time. The
model shortcoming is also attributed in part to the need for the model to
include a hydrodynamical mechanism to describe the merging of the zonal wind
with the meridional tidal winds that converge onto the geographical equator.
Finally, a conclusion of this work is that the MTM compressional heating
takes place along the perimeter of the pressure bulge rather than within the
bulge, an issue previously not appreciated. |
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