Role of projection effects on solar coronal mass ejection properties: 1. A study of CMEs associated with limb activity

Many properties of Coronal Mass Ejections (CMEs), such as size, location and brightness, have been determined from measurements of white light coronal observations. We expect the average properties derived from these measurements contain systematic inaccuracies due to projection effects and suggest that CME properties are most accurately determined for those events occurring near the plane-of-the-sky (i.e., over the solar limb as observed from Earth), where projection effects are minimized. A set of 111 such "limb" events have been identified in Solar Maximum Mission (SMM) white light observations through associations with Erupting Prominences at the Limb (EPLs), and X-ray and limb optical flares. These "limb" CMEs have greater average speeds (519 ±46 km/sec) and masses (4.5 ±.5 × 10¹⁵ grams) than the average values obtained from all SMM CMEs, consistent with the expected behavior of projection effects. Only a very small percentage of "limb" CMEs are centered at high latitudes, suggesting there are many fewer "true" high latitude CMEs than has previously been reported. No "limb" CMEs have widths greater than 110°, consistent with the interpretation that very wide CMEs (i.e., halos) are actually events of more typical widths originating away from the solar limb and viewed in projection. Only a small percentage of "limb" CMEs have measured speeds below 200 km/sec, indicating there may be fewer "true" subsonic SMM CMEs than previously reported. The correlation detected between the kinetic energy of the "limb" CMEs and the peak intensity of the associated GOES X-ray flares, is stronger than was previously found using a set of CMEs of undetermined limb distances. All these results provide strong evidence that projection effects systematically influence the deduced properties of CME events.