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Ann. Geophys., 25, 1093-1101, 2007
www.ann-geophys.net/25/1093/2007/
© European Geosciences Union 2007
Average thermospheric wind patterns over the polar regions, as observed by CHAMP
H. Lühr1, S. Rentz1, P. Ritter1, H. Liu2, and K. Häusler1
1GeoForschungZentrum Potsdam, Germany
2Div. of Earth and Planet. Science, Hokkaido University, Sapporo, Japan
Abstract. Measurements of the CHAMP accelerometer are utilized to
investigate the average thermospheric wind distribution in the polar regions
at altitudes around 400 km. This study puts special emphasis on the seasonal
differences in the wind patterns. For this purpose 131 days centered on the
June solstice of 2003 are considered. Within that period CHAMP's orbit is
precessing once through all local times. The cross-track wind estimates of
all 2030 passes are used to construct mean wind vectors for 918 equal-area
cells. These bin averages are presented in corrected geomagnetic
coordinates. Both hemispheres are considered simultaneously providing summer
and winter responses for the same prevailing geophysical conditions. The
period under study is characterized by high magnetic activity (Kp=4−) but
moderate solar flux level (F10.7=124). Our analysis reveals clear wind
features in the summer (Northern) Hemisphere. Over the polar cap there is a
fast day-to-night flow with mean speeds surpassing 600 m/s in the dawn
sector. At auroral latitudes we find strong westward zonal winds on the dawn
side. On the dusk side, however, an anti-cyclonic vortex is forming. The
dawn/dusk asymmetry is attributed to the combined action of Coriolis and
centrifugal forces. Along the auroral oval the sunward streaming plasma
causes a stagnation of the day-to-night wind. This effect is particularly
clear on the dusk side. On the dawn side it is evident only from midnight to
06:00 MLT. The winter (Southern) Hemisphere reveals similar wind features, but
they are less well ordered. The mean day-to-night wind over the polar cap is
weaker by about 35%. Otherwise, the seasonal differences are mainly
confined to the dayside (06:00–18:00 MLT). In addition, the larger offset between
geographic and geomagnetic pole in the south also causes hemispheric
differences of the thermospheric wind distribution.
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