Lessons Learned from the Three-view Determination of CME Mass

With only single-viewpoint, total-brightness images, it is not possible to calculate the true mass of a coronal mass ejection (CME) because total-brightness images do not contain any depth information. If two viewpoints are available, it has been suggested that total-brightness images might be used to simultaneously derive the CME deprojected mass and propagation direction. Exploiting all available data provided by SOHO, STEREO-A, and STEREO-B supplies three combinations of spacecraft pairs for double-viewpoint analysis. We show that, if only aleatory variation is reckoned, then the CME deprojected mass can vary by as much as 100% between different pairs of spacecraft. Going beyond multiple double-viewpoint analyses, we also utilize these spacecraft to simultaneously estimate the CME deprojected mass and propagation direction with triple-viewpoint analysis. However, once again, if only aleatory variation is reckoned, then, for many events, there is no common mass value estimated by the three viewpoints. This indicates that epistemic uncertainty is significant and must be reckoned also. We consider in detail the mass in the coronagraph field of view, from one, two, and three viewpoints, for the CME observed on 2007 December 31, focusing especially on the approximations used and uncertainties in this process. Altogether, we use triple-viewpoint analysis to estimate the mass of eight CMEs. When simultaneously estimating the CME deprojected mass and propagation direction using total-brightness images, the single degree of freedom provided by a third, independent viewpoint clarifies the calculation of, and reduces ambiguity in, the deprojected mass; however, the deprojected mass should not be confused with the true mass.