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[3]  ICMEs have been studied by in situ measurements of their plasma and magnetic field properties when they encounter spacecraft. Signatures of ICMEs identified in interplanetary space include anomalously low proton and electron temperatures, bidirectional streaming of superthermal electrons and energetic ions, helium abundance enhancements, unusual ionization states of heavy ions, strong magnetic fields with low field variance and smooth field rotations [ Neugebauer and Goldstein , 1997 , and references therein]. Previous work show that at least one third of all ICMEs appear to be nearly force-free magnetic flux ropes [ Gosling , 1990 ; Marubashi , 1997 ]. When the field strength is high and the plasma β is low, such ICMEs are defined as magnetic cloud (MC)[ Burlaga et al. ,1981 ], which are perhaps the most frequently referred to examples of ICMEs. ICMEs with speeds high enough relative to the ambient solar wind may generate shocks ahead of them [ Sheeley et al. , 1985 ; Bothmer and Schwenn , 1996 ]. However, individual signatures may not be detected in all ICMEs [ Cane and Richardson , 2003 ]. Even if several signatures are present in an ICME, they are not necessarily coincident. Some signatures have been reported relatively infrequently, while other signatures, such as proton temperature depressions, are generally present [ Richardson and Cane , 1995 ; Cane and Richardson , 2003 ].


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