Relating white light and in situ observations of coronal mass ejections: A review

This paper provides a short review of some of the basic concepts related to the observations of coronal mass ejections (CMEs) in white light images and at large distances from the Sun. We review the various ideas which have been put forward to explain the dramatic changes in CME appearance in white light images from the Sun to 1 AU, focusing on results obtained by comparing white light observations of CMEs to the in situ measurements of Interplanetary CMEs (or ICMEs). We start with a list of definitions for the various in situ structures that form an ICME. A few representative examples of the formation of sheath regions and other interaction regions as well as the expansion of magnetic flux ropes are used to illustrate the basic phenomena which induce significant brightness variations during a CME's propagation to 1 AU and beyond. The white light signatures of a number of CMEs observed by the coronagraphs have been successfully simulated numerically by assuming that most of the coronal plasma observed in white light images is located on the surface of a croissant-shaped structure reminiscent of a magnetic flux rope. At large distances from the Sun, white light imagers show that the appearance of CMEs changes dramatically due to the changing position of the CME relative to the Thomson sphere, the expansion of the ejecta and the interaction of the ejecta with the ambient solar wind.

Research Highlights
► Review of white light observations made by STEREO.
► Direct links between white light observations of CMEs and in situ measurements of magnetic flux ropes near 1 AU.
► Discussion of the various types of ejecta observed simultaneously in white light an in situ.