The Impact of Geometry on Observations of CME Brightness and Propagation

Coronal mass ejections (CMEs) have a significant impact on space weather and geomagnetic storms and so have been the subject of numerous studies. Most CME observations have been made while these events are near the Sun (e.g., SOHO/LASCO). Recent data from the Coriolis/SMEI and STEREO/SECCHI-HI instruments have imaged CMEs farther into the heliosphere. Analyses of CME observations near the Sun measure the properties of these events by assuming that the emission is in the plane of the sky and hence the speed and mass are lower limits to the true values. However, this assumption cannot be used to analyze optical observations of CMEs far from the Sun, such as observations from SMEI and SECCHI-HI, since the CME source is likely to be far from the limb. In this paper we consider the geometry of observations made by LASCO, SMEI, and SECCHI. We also present results that estimate both CME speed and trajectory by fitting the CME elongations observed by these instruments. Using a constant CME speed does not generally produce profiles that fit observations at both large and small elongation, simultaneously. We include the results of a simple empirical model that alters the CME speed to an estimated value of the solar wind speed to simulate the effect of drag on the propagating CME. This change in speed improves the fit between the model and observations over a broad range of elongations.