Observations of Ray-Like Structures in Large-Scale Coronal Dimmings Produced by Limb CMEs

Observations of the off-limb corona with the SWAP wide-field telescope in the 174\,Å passband onboard the PROBA2 mission provide an opportunity to study post-eruptive processes in the dimming regions. We investigate morphology, temporal evolution, and plasma properties for four `deep' off-disk coronal dimmings associated with limb coronal mass ejections (CMEs) in 2010\,--\,2017. Using the SWAP fixed-difference images, we revealed ray-like structures that appeared in the dimming recovery phase stretching quasi-radially to distances from 1.1 to 1.6\,$\mathrm{R}_{\odot}$ and existing from tens of minutes to several hours. Similar rays were detected earlier at distances above 1.7\,$\mathrm{R}_{\odot}$ by the UVCS onboard the SOHO. These structures apparently represent the coronal roots of the flux rope trunks observed by the LASCO-C2 onboard SOHO. The EUVI data onboard the STEREO show the origins of these structures on the disc as fan rays much brighter in the 171\,Å band than in 193\,Å, which suggests their temperature being less than 2\,MK. The differential emission measure (DEM) analysis based on the AIA multi-wavelength images onboard the SDO showed that the emission measure (EM) in these rays compared to the pre-eruption plasma state increased up to 45\,\% at temperatures of 0.6\,--\,0.8\,MK, whereas EM of the ambient coronal plasma with temperatures of 1.3\,--\,3.7\,MK dropped by 19\,--\,43\,\%. For the event on 18 August 2010, the PLASTIC instrument onboard STEREO detected signatures of the cold streams in the CME tail as enriched with the ions Fe$^{8+}$\,--\,Fe$^{10+}$, which may be associated with the post-eruptive rays in the solar corona.

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