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

摘要

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 Bs 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 Bs 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.