ICMES at very large distances Original Research Article

This paper reviews recent work on the observations and evolution of interplanetary coronal mass ejections (ICMEs) in the outer heliosphere. Several case studies are shown following ICMEs or their preceding shocks from 1 AU to Voyager 2 at large (>50 AU) distances. Low temperature/speed ratios and high helium abundances have been used to create lists of ICMEs. We combine lists from the inner (0.3–5.3 AU) and outer (1–30 AU) heliosphere to perform a statistical study of ICME evolution. ICMEs expand, on average, by a factor of 5 in width between 1 and 10–15 AU, then maintain a constant width as they move beyond 15 AU. The density and magnetic field decrease more rapidly in ICMEs than the solar wind, consistent with ICME expansion. The temperature, however, decreases less rapidly in ICMEs despite their expansion, indicating that ICMEs are preferentially heated. At solar maximum, ICMEs interact with the ambient solar wind to form large pressure pulses in the outer heliosphere with correlated speed, density, and magnetic field increases.