The Relation between low density events in the Solar Wind and CMEs

This paper discusses nine periods of time observed by Wind between 1999 and 2002 when the density of the solar wind plasma fell to values below 1/cc for periods between an hour and a day. All except one of these events could be associated with CMEs, which had plasma moving at about 1400 km/s as it passed 20Rs from the Sun. We suggest that when spacecraft enter rarefactions in this plasma, they observe extreme rarefactions. Thus two coincident conditions, a normal solar wind rarefaction and the arrival of material from an ICME are required for these events to be observed