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	Author	Chulliat, A.; Blanter, E.; Le Mouël, J.-L.; Shnirman, M.
	Title	On the seasonal asymmetry of the diurnal and semidiurnal geomagnetic variations			Type	Journal Article
	Year	2005	Publication	Journal of geophysical research-atmospheres	Abbreviated Journal	J. Geophys. Res.
	Volume	110	Issue		Pages
	Keywords	daily variation; lower thermospheric winds; ionospheric dynamo; current system; geomagnetic activity; equinoxes; 1555 Geomagnetism and Paleomagnetism: Time variations: diurnal to decadal; 2437 Ionosphere: Ionospheric dynamics; 3369 Atmospheric Processes: Thermospheric dynamics; 3389 Atmospheric Processes: Tides and planetary waves
	Abstract	The diurnal and semidiurnal variations of the geomagnetic field are investigated at 18 observatories using long series of hourly values (up to 97 years at Sitka). The seasonal variations of amplitude of the 12-hour and 24-hour lines are obtained for the H and Z components using a 28-day sliding window. The Fourier analysis is performed using either all days within the window or only the five quietest days. At midlatitudes a strong lack of symmetry about the summer solstice is observed for both lines and both components. This effect is enhanced when selecting quiet days. When averaged over the entire series, the sign of this seasonal asymmetry is the same at 9 out of 10 midlatitude observatories for both lines and for a given component; it is opposite for the H and Z components. Such a coherent seasonal asymmetry is not found at low and high latitudes. At high latitudes a strong annual variation is found inside the polar caps, while a strong semiannual variation is found in the auroral zones. When selecting quiet days, these two effects are weaker, and the seasonal asymmetry becomes comparable to that at midlatitudes. At all latitudes the year-to-year variations of the seasonal asymmetry are uncorrelated with solar activity. It is suggested that lower thermospheric winds may have a similar seasonal asymmetry between spring and autumn equinoxes. Such an asymmetry is present in the published literature but has been overlooked. Possible causes for this wind asymmetry are reviewed.
	Programme	139
	Campaign
	Address
	Corporate Author				Thesis
	Publisher	American Geophysical Union	Place of Publication		Editor
	Language		Summary Language		Original Title
	Series Editor		Series Title		Abbreviated Series Title
	Series Volume		Series Issue		Edition
	ISSN	0148-0227	ISBN		Medium
	Area		Expedition		Conference
	Notes				Approved	yes
	Call Number	IPEV @ Thierry.Lemaire @			Serial	5485
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