Evidence of cool prominence material embedded in CMEs from polarization measurements
Abstract
In white-light coronagraph images, cool prominence material is sometimes observed as bright substructures or plasmoids in coronal mass ejections (CMEs). Based on observations of a CME with the SECCHI/STEREO coronagraphs on 31 August 2007, Mierla et al. (2011) have shown that the emission from such bright patches is characterized by very low polarization and was due to H-alpha radiation from cool prominence material. We have examined several polarization images of CMEs obtained with the LASCO-C2 coronagraph with its orange filter. This filter is approximately centered on the HeI D3 line. This low excitation line emission is superior to the H-alpha emission of HI because its FIP is much higher. In several cases, we observe very low-polarization components embedded in CMEs characterized by a much larger polarization resulting from the Thomson scattering. They most likely present evidence of cool prominence material carried by the CMEs.