Composition and magnetic structure of interplanetary coronal mass ejections at 1 AU

Abstract

We study the magnetic structure and charge state ratio of interplanetary coronal mass ejections (ICMEs) observed by ACE and Wind spacecraft. Measurements of abundances and charge state ratio of heavy ions (e.g. O 7+/O 6+, C 6+/C 5+, and Mg 10+/O 6+) in the plasma as well as magnetic field structure are important tracers for physical conditions and processes in the source regions of ICMEs. We used ion composition (from ACE), plasma (from Wind) and magnetic field (from Wind and ACE) data from 1998 to 2002. Using the low proton temperature criterion, a common plasma signature of ICMEs, we identified 154 events which include magnetic clouds, non-cloud ejecta and complex ICMEs. The latter one refers to compound events resulting from the overtaking of successive ICMEs which can include both magnetic clouds and non-cloud ejecta. We find that there is a close relationship between the increase in the charge state ionization factor and the magnetic structure of ICMEs. Events with magnetic cloud topology show higher Q O 7 + / O 6 + and Q Mg 10 + / O 6 + charge state ratios than those with non-magnetic cloud structure. However, both magnetic cloud and non-cloud events show an increase in these ratios when compared with the ambient solar wind. In contrast, perhaps due to instrumental effects, the charge state ratio Q C 6 + / C 5 + for all events does not show a real enhancement when compared with the ambient solar wind. The difference in ionization states between non-cloud ejecta and magnetic clouds is more pronounced in fast solar wind than when events are embedded in slow wind.