Statistical properties and geoeffectiveness of southward interplanetary magnetic field with emphasis on weakly southward Bz events

In this work we analyze the statistical properties and geoeffectiveness of southward IMF Bz (Bs) events measured near the Earth from 1996 to 2015. We classify the Bs events into six contrasting classes according to the duration and intensity of Bs: two subclasses of long-lasting major-intensity, two subclasses of long-lasting weak-intensity and two subclasses of short-lasting weak-intensity. We find the following results. (1) The occurrence rate of Bs events is lowest for the long-lasting major-intensity Bs classes, and increases toward shorter-lasting and weaker-intensity Bs classes. (2) Many of the Bs events in nearly all classes are associated with regions other than interplanetary coronal mass ejections and coronal-hole high-speed streams. (3) Alfven waves account for a significant fraction of the Bs events (e.g., ~60% for the short-lasting and weak-intensity Bs classes). (4) There are clear differences in solar cycle dependence of Bs events among the different Bs classes. (5) Even either the long-lasting or short-lasting weak-intensity class Bs events can often be geoeffective by triggering substorms of weak-to-medium intensity and by enhancing geosynchronous relativistic electron fluxes. In conclusion, we emphasize that even the weakly (in terms of either intensity or duration) southward IMF Bz should be considered significant from the viewpoint of their high occurrence rate and their geoeffectiveness compared to those of long-lasting major-intensity.