Back reaction of the untwisting solar corona scars sunspots

The evolution of magnetic fields in the tenuous solar corona is predominantly governed by the motions of the underlying dense photosphere. Despite this, coronal magnetic restructuring driven by magnetic reconnection between interacting coronal fields can sometimes react backward to change photospheric magnetic fields. However, the mechanism of reactions remains undetermined. Here, we report the discovery of a back-reaction phenomenon: The untwisting of coronal loops that become twisted during reconnection in an eruption results in enhanced currents at the boundary of their footpoint away from the eruption, manifesting as the growth of a sunspot scar. It is revealed to arise from the Alfvénic reverse transfer of magnetic twist from the corona to the lower atmosphere, through joint space observations and a magnetohydrodynamic simulation. These findings provide a viable and quantitative interpretation for the majority of puzzling photospheric changes associated with coronal mass ejections and/or flares and warn for unexpected magnetic field evolutions in sunspots and starspots.