The Relationship between ICMEs and Extremely Low Density Events in the Solar Wind

Periods during which the solar wind density falls from its normal range of 5 to15 particles per ccto less than unity, and the flow consequently becomes sub-Alfvenic, are rare but occur more frequently near solar maximum. The causes for this are not very well understood.In this paper we examine ten such events, and show that at least nine of them can be associated with moderately energetic ICMEs, and the other case occurs when “hot” iron ions and counterstreaming electrons are observed.Also the majority, 6/10, occur very close in time to magnetic clouds. We suggest that such rarefactions result from the passage of the observing spacecraft through low density regions resulting from ICMEs.