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Ann. Geophys., 22, 2001-2014, 2004
www.ann-geophys.net/22/2001/2004/
© European Geosciences Union 2004
High-latitude ionospheric currents during very quiet times: their characteristics and predictability
P. Ritter1, H. Lühr1, S. Maus1, and A. Viljanen2
1GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
2Finnish Meteorological Institute, Geophys. Res. Div., P.O. Box 503, FIN-00101 Helsinki, Finland
Abstract. CHAMP passes the geographic poles at a distance of 2.7°
in latitude, thus providing a large number of magnetic readings
of the dynamic auroral regions. The data of these numerous overflights
were used for a detailed statistical study on the level of
activity. A large number of tracks with very low rms of the
residuals between the scalar field measurements and a high degree
field model were singled out over both the northern and southern
polar regions, independently. Low rms values indicate best model
fits and are therefore regarded as a measure of low activity,
although we are aware that this indicator also has its limitations.
The occurrence of quiet
periods is strongly controlled by the solar zenith angle at the geomagnetic
poles, indicating the importance of the ionospheric conductivity.
During the dark polar season, about 30% of the passes can be qualified as quiet.
The commonly used magnetic activity indices turn out not to be a reliable measure
for the activity state in the polar region. Least suitable is the Dst index, followed
by the Kp. Slightly better results are obtained with the PC and the IMAGE-AE indices.
The latter is rather effective within a time sector of ±4 hours of magnetic local
time around the IMAGE array.
The orientation of the interplanetary magnetic field (IMF) is an important
controlling factor for the activity. This is also supported by the prevailing
FAC distribution during quiet times, which resembles the typical NBZ
(northward Bz) pattern. In a superposed epoch analysis we show that the
merging electric field is a suitable geoeffective solar wind parameter.
Based on the size of this electric field and the solar zenith angle at the
geomagnetic poles, a prediction method for quiet auroral region periods is proposed.
This may, among others, be useful for the data selection in main field modelling approaches.
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