Alfven waves as a possible source of long-duration, large-amplitude, and geoeffective southward IMF

The southward component (Bs) of the interplanetary magnetic field (IMF) is a strong driver of geomagnetic activity. Well-defined solar wind structures such as magnetic clouds and corotating interaction regions are the main sources of long-duration, large-amplitude IMF Bs. Here we analyze IMF Bsevents (t> 1 h, Bz<-5nT) unrelated with any well-defined solar wind structure at 1 AU using ACE spacecraft observations from 1998 to 2004. We find that about one third of these Bs events show Alfvenic wave features; therefore, those Alfven waves in the solar wind are also an important source of long-duration, large-amplitude IMF southward component. We find that more than half of the Alfven wave (AW)-related Bs events occur in slow solar wind (Vsw < 400 km/s). One third of the AW-type Bsevents triggered geomagnetic storms, and half triggered substorms.