Alfvén waves as a possible source of long‐duration, large‐amplitude, and geoeffective southward IMF

Abstract

AbstractThe 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 Alfvénic wave features; therefore, those Alfvén 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 Alfvén 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.