A statistical study of the properties of interplanetary coronal mass ejections from 0.3 to 5.4 AU

Abstract

We make a comprehensive survey of interplanetary coronal mass ejections (ICMEs) observed by the Helios 1 and 2, Ulysses, WIND and ACE spacecraft, which together cover heliocentric distances from 0.3 to 5.4 AU. The signatures used to identify ICMEs are enhanced helium abundances and depressed proton temperatures. We use the ICME list to study the radial evolution of ICMEs in a statistical sense. We find that ICMEs expand during propagation in the solar wind; the radial width increases as R 0.92 . Most ICMEs (10 out of 13) observed at Ulysses from 5.15 to 5.4 AU seem to be co-moving with the solar wind. The density and magnetic field magnitude decrease faster with distance than the ambient solar wind, as expected for an expanding feature. The temperature, in contrast, decreases more slowly inside ICMEs than in the ambient solar wind, so the plasma in the ICMEs must be heated. The expansion behaves more like an isothermal than adiabatic process, with a polytropic index of γ ∼ 1.15 . This index is constant over distance and solar cycle, and is unchanged when the electron pressure and the magnetic pressure are included. The polytropic index for ICME electrons is less than unity ( γ e ∼ 0.73 ), inconsistent with their decreasing temperature observed by Ulysses. The occurrence rate of ICMEs at 1 AU approximately tracks the sunspot numbers and the CMEs observed by LASCO, with a temporary reduction between 1998 and 1999. The radial width of near-Earth ICMEs has a solar cycle dependence, with a mean radial width of 0.34 AU.