Interplanetary Coronal Mass Ejections Observed in the Heliosphere: 1. Review of Theory

With the recent advancements in interplanetary coronal mass ejection (ICME) imaging it is necessary to understand how heliospheric images may be interpreted, particularly at large elongation angles. Of crucial importance is how the current methods used for coronal mass ejection measurement in coronagraph images must be changed to account for the large elongations involved in the heliosphere. In this review of theory we build up a picture of ICME appearance and evolution at large elongations in terms of how it would appear to an observer near 1 AU from the Sun. We begin by revisiting the basics of Thomson scattering describing how ICMEs are detected, in this we attempt to clarify a number of common misconceptions. We then build up from a single electron to an integrated line of sight, consider the ICME as a collection of lines of sight and describe how a map of ICME appearance may be developed based on its appearance relative to each line of sight. Finally, we discuss how the topology of the ICME affects its observed geometry and kinematic properties, particularly at large elongations. This review is the first of a three-part series of papers, where a review of theory is presented here and a model is developed and used in subsequent papers.