The Coronal Mass Ejection Visibility Function of Modern Coronagraphs

Abstract

We analyze the detection capability of coronal mass ejections (CMEs) for all currently operating coronagraphs in space. We define as CMEs events that propagate beyond 10 solar radii with morphologies broadly consistent with a magnetic flux-rope presence. We take advantage of multi-viewpoint observations over five month-long intervals, corresponding to special orbital configurations of the coronagraphs aboard the Solar Terrestrial Relations Observatory (STEREO) and Solar and Heliospheric Observatory missions. This allows us to sort out CMEs from other outward-propagating features (e.g., waves or outflows), and thus to identify the total number of unique CMEs ejected during those periods. We determine the CME visibility functions (VFs) of the STEREO COR2-A/B and LASCO C2/C3 coronagraphs directly as the ratio of observed to unique CMEs. The VFs range from 0.71 to 0.92 for a 95% confidence interval. By comparing detections between coronagraphs on the same spacecraft and from multiple spacecraft, we assess the influence of field of view (FOV), instrument performance, and projection effects on the CME detection ability without resorting to proxies, such as flares or radio bursts. We find that no major CMEs are missed by any of the coronagraphs, that a few slow halo-like events may be missed in synoptic cadence movies, and that narrow FOV coronagraphs have difficulties discriminating between CMEs and other ejections, leading to “false” detection rates. We conclude that CME detection can only be validated with multi-viewpoint imaging—two coronagraphs in quadrature offer adequate detection capability. Finally, we apply the VFs to observed CME rates resulting in upward corrections of 40%.