Search for magnetically quiet CHAMP polar passes and the characteristics of ionospheric currents during the dark season

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 degrees 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, E-m < 0.8 mV/m, absence of peak values of E-m > 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.