Which interplanetary magnetic field condition is for the quietest direct-driven geomagnetic activity?

Geomagnetic activity of the magnetosphere is contributed by the direct-driven and loading-unloading processes. Since the loading-unloading process for the quiet IMF conditions is minimum, we are allowed to focus on the direct-driven process. Quasi-northward interplanetary magnetic field (IMF) has been well known to be a quiet IMF condition for direct-driven geomagnetic activity of the magnetosphere in contrast to that for a disturbed southward IMF condition. Quasi-radial and near-zero IMF are the other quiet IMF conditions, which were usually less studied in the field of magnetospheric physics. A comparison among the three quiet IMF conditions for which one is for the quietest direct-driven geomagnetic activity has not been performed in the past. Here we use two solar cycles of OMNI solar wind data and the three high-latitude geomagnetic indices (AE, PCN, PCS) to perform the comparison. We find that the quasi-northward IMF condition is the winner. The geomagnetic activity for quasi-radial IMF was never the quietest because of an extra dayside reconnection in addition to a lobe reconnection, contributing more energy into the magnetosphere than the other two quiet IMF conditions do.