TY  - JOUR
AU  - Chulliat, A.
AU  - Blanter, E.
AU  - Le Mouël, J-L
AU  - Shnirman, M.
PY  - 2005//
TI  - On the seasonal asymmetry of the diurnal and semidiurnal geomagnetic variations
T2  - J. Geophys. Res.
JO  - Journal of geophysical research-atmospheres
VL  - 110
PB  - American Geophysical Union
KW  - daily variation
KW  - lower thermospheric winds
KW  - ionospheric dynamo
KW  - current system
KW  - geomagnetic activity
KW  - equinoxes
KW  - 1555 Geomagnetism and Paleomagnetism: Time variations: diurnal to decadal
KW  - 2437 Ionosphere: Ionospheric dynamics
KW  - 3369 Atmospheric Processes: Thermospheric dynamics
KW  - 3389 Atmospheric Processes: Tides and planetary waves
N2  - 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.
SN  - 0148-0227
UR  - http://dx.doi.org/10.1029/2004JA010551
N1  - exported from refbase (http://publi.ipev.fr/polar_references/show.php?record=5485), last updated on Wed, 15 Apr 2009 14:01:54 +0200
ID  - Chulliat_etal2005
ER  -