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Ann. Geophys., 24, 2997-3009, 2006
www.ann-geophys.net/24/2997/2006/
© European Geosciences Union 2006
Search for magnetically quiet CHAMP polar passes and the characteristics of ionospheric currents during the dark season
P. Ritter and H. Lühr
GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany
Abstract. The magnetic activity at auroral latitudes is strongly dependent on season.
During the dark season, when the solar zenith angle in the polar region is
larger than 100° at all local times, the ionospheric conductivity is
much reduced, and generally low activity is encountered. These time intervals
are of special interest for the main field modelling, because then the
geomagnetic field readings, in particular the field magnitude, are only
slightly affected by ionospheric currents. Based on CHAMP data, this study
examines how these quiet periods are reflected in the different magnetic
field components. The peak FAC density is used as a possible proxy for the
deviation of the total field. As a second option, the transverse field
component, which is aligned with the auroral oval, is investigated, because
it presents a measure for the FAC total current. Correlation analyses with
the scalar residuals are performed and both proxies are tested for their
suitability of predicting the intensity of the auroral electrojet during the
dark polar seasons. The indicators based on the local FAC strength or on the
amplitude of the transverse component show a reasonable correlation with the
electrojet intensity for these periods, but fail when limited to small
amplitudes. The predictability improves considerably if the time sector is
limited to dayside hours (08:00–16:00 MLT). As the activity at high latitudes is
strongly controlled by the solar wind input, we also consider IMF quantities
which may support very quiet conditions. Correlations of the magnetic field
scalar residuals with the merging electric field are strongest if only passes
in the dayside sector are considered. Best selection results for quiet passes
are obtained by combining four conditions: dark season, small average merging
electric field, Em<0.8 mV/m, absence of peak values of Em>1.2 mV/m
during a time interval of 40 min centred at the polar crossing, and
limitation to the dayside sector (08:00–16:00 MLT). The set of quiet polar passes
identified by these criteria may be used beneficially in crustal field
modelling of the polar regions.
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