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

In this work we analyze the statistical properties and geoeffectiveness of southward interplanetary magnetic field (IMF) B z ( B s ) events measured near the Earth from 1996 to 2015. We classify the B s events into six contrasting classes according to the duration and intensity of B s : 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 B s events is lowest for the long‐lasting major‐intensity B s classes and increases toward shorter‐lasting and weaker‐intensity B s classes. (2) Many of the B s events in nearly all classes are associated with regions other than interplanetary coronal mass ejections and coronal hole high‐speed streams. (3) Alfvén waves account for a significant fraction of the B s events (e.g., ~60% for the short‐lasting and weak‐intensity B s classes). (4) There are clear differences in solar cycle dependence of B s events among the different B s classes. (5) Even either the long‐lasting or short‐lasting weak‐intensity class B s 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 B z should be considered significant from the viewpoint of their high occurrence rate and their geoeffectiveness compared to those of long‐lasting major intensity. A much higher occurrence rate of weak B s events is found than that of stronger B s Many of the weak B s events originate from regions other than ICMEs and high‐speed streams Even weak B s events are often geoeffective and these include substorms and disturbances of the outer radiation belt