Behavior of the quiet day ionospheric current system in the European region

The ionospheric current system for magnetic quiet days deduced from geomagnetic field daily variations taken throughout Europe at sunspot minimum has been studied. Special attention was paid to the selection of the representative quiet days, showing the inadequacy of relying on the mean of the five international quiet days per month. We used the technique of spherical cap harmonic analysis to separate the external and internal parts of the field variation from which the derivation of the equivalent current functions is straightforward. This method provides a motion picture representation of the Sq system through the screen formed by the spherical cap, allowing the study of continuous variations with universal time. Detailed features of the current contours can be represented with such a regional analysis. We found a strong day-to-day variability of the focus position along with some complexities that differ from the standard Sq current behavior. The average monthly or seasonal patterns are therefore averages of heterogeneous behaviors in many occasions. Possible explanations to these complex structures are considered.