CMEs and Low Density Events in the Solar Wind

From time to time the density of the Solar Wind falls below unity for a day or so. Despite studies of these events, their cause has not been established. In the solar wind there are frequent rarefactions but these are much shallower than the ones we are discussing here. In particular any role that CMEs might play in forming more extreme rarefactions is not clear . We discuss 9 cases of extreme events, leading to sub-alfvenic flow and well observed by Wind, which occurred between 1999 and 2002, and look for corresponding CME events in the LASCO cotalog. The catalog gives a fitted speed , good out to 20 Rs ,as well as a speed lower down in the corona, and this is used to distinguish between a CME and an ICME. We find one of the events ,which was simulated by Usmanov et al.,cannot be associated with an ICME. The other eight events can plausibly be associated with fast ICMEs, identified by the prescence of bi-streaming electrons also seen by Wind SWE, and by other phenomena The implications of these observations are discussed but do not seem to be conclusive as to the cause of the extreme rarefactions.