Multipoint ICME encounters: Pre-STEREO and STEREO observations

摘要

The knowledge of the global properties of interplanetary coronal mass ejections (ICMEs) is of great interest for heliospheric research and space weather forecasting. Due to the large dimensions of ICMEs and the lack of systematic multipoint measurements the true three-dimensional configuration of ICMEs is still poorly understood. The launch of the STEREO twin observatory in October 2006 opened important new opportunities for ICME research. One of the scientific goals of the STEREO mission is to study the large-scale structure of ICMEs. In this paper we review the multi-spacecraft ICME observations conducted before the STEREO era and discuss the ICME properties that were identified at least by one of the STEREO spacecraft and those at the Lagrangian point L1 (Wind/ACE) from April 2007 through March 2008. The multi-spacecraft observations emphasize that ICMEs cannot be explained in terms of a simple flux rope model. The characteristics of ICMEs and the structure of the solar wind in which they were embedded varied significantly from event to event. The observations show that ICMEs can have cross-sectional shapes from almost circular to significantly distended. In the ecliptic plane ICMEs may span at least up to. 40° in longitude, consistent with the angular span of the average CME close to the Sun. However, the association between the ICME observations at different spacecraft is not straightforward as significant differences were observed even when the spacecraft were separated by only a few degrees in longitude. In addition, multipoint observations confirm that the identification of the flux rope structure is modified by the spacecraft crossing distance from the center of the ICME. We show examples of the events where one spacecraft crosses the central flux rope, but the other spacecraft traverses the ICME close to the edge where the flux rope structure is no longer obvious.