On the Rates of Coronal Mass Ejections: Remote Solar and In Situ Observations

Wecomparetheratesofcoronalmassejections(CMEs)asinferredfromremotesolarobservationsandinterplanetary CMEs (ICMEs) as inferred from in situ observations at both 1 AU and Ulysses from 1996 through 2004. We also distinguish between those ICMEs that contain a magnetic cloud (MC) and those that do not. While the rates of CMEs andICMEstrackeachotherwellatsolarminimum,theydivergesignificantlyinearly1998,duringtheascendingphase of the solar cycle, with the remote solar observations yielding approximately 20 times more events than are seen at 1 AU. This divergence persists through 2004. A similar divergence occurs between MCs and non-MC ICMEs. We argue that these divergences are due to the birth of midlatitude active regions, which are the sites of a distinct population of CMEs, only partially intercepted by Earth, and we present a simple geometric argument showing that theCMEandICMEratesareconsistentwithoneanother.Wealsoacknowledgecontributionsfrom(1)anincreased rate of high-latitude CMEs and (2) focusing effects from the global solar field. While our analysis, coupled with numerical modeling results, generally supports the interpretation that whether one observes a MC within an ICME is sensitive to the trajectory of the spacecraft through the ICME (i.e., an observational selection effect), one result directly contradicts it. Specifically, we find no systematic offset between the latitudinal origin of ICMEs that contain MCs at 1 AU in the ecliptic plane and that of those that do not.