Charge states and abundances of heavy ions as signatures of interplanetary coronal mass ejections

Interplanetary Coronal Mass Ejections (ICMEs) are not easy to recognise. Several signatures are used to identify such events: Counterstreaming electrons, magnetic field rotation and fluctuation, low kinetic temperature, etc. However, an individual ICME rarely shows them all, so identification remains a somewhat subjective art form. Composition signatures were long known to provide a useful identifier: An alpha-to-proton ratio of >8% is a sufficient criterion for the presence of an ICME. More recently, a high average Fe charge state was established as another sufficient (but not necessary) signature, as is probably the case for an increased C and O freezing-in temperature. Such signatures are particulary attractive because, unlike kinetic and magnetic ones, they remain largely unchanged throughout the entire heliosphere. Using data obtained with Ulysses-SWICS we evaluate the occurrence rate of composition signatures in ICMEs as identified by their classical signatures. The definition of the appropriate threshold values of these signatures will receive particular attention. Finally, the distribution of ICMEs with and without a compositional signature as a function of heliographic latitude will be evaluated.