Identifying footpoints of pre-eruptive and coronal mass ejection flux ropes with sunspot scars

Context. The properties of pre-eruptive structures and coronal mass ejections (CMEs) are characterized by those of their footpoints, the latter of which attract a great deal of interest. However, the matter of how to identify the footpoints of pre-eruptive structures and how to do so with the use of ground-based instruments still remains elusive. Aims. In this work, we study an arc-shaped structure intruding in the sunspot umbra. It is located close to the (pre-)eruptive flux rope footpoint and it is expected to help identify the footpoint. Methods. We analyzed this arc-shaped structure, which we call a "sunspot scar", in a CME event on July 12, 2012, and in two CME events from observationally inspired magnetohydrodynamic simulations performed by OHM and MPI-AMRVAC. Results. The sunspot scar displays a more inclined magnetic field with a weaker vertical component and a stronger horizontal component relative to that in the surrounding umbra and is manifested as a light bridge in the white light passband. The hot field lines anchored in the sunspot scar are spatially at the transition between the flux rope and the background coronal loops and temporally in the process of the slipping reconnection which builds up the flux rope. Conclusions. The sunspot scar and its related light bridge mark the edge of the CME flux rope footpoint and particularly indicate the edge of the pre-eruptive flux rope footpoint in the framework of "pre-eruptive structures being flux ropes". Therefore, they provide a new perspective for the identification of pre-eruptive and CME flux rope footpoints, as well as new methods for studying the properties and evolution of pre-eruptive structures and CMEs with photospheric observations only.