Effects of External Flow on Resonant Absorption of Coronal Loop Kink Oscillations Driven by an External Fast Wave: Selective Excitation Problem

Resonant absorption is considered as a crucial mechanism for the damping of the coronal loop oscillations and plasma heating. We study resonant absorption of the coronal loop kink oscillations excited by such external drivers as flares on the assumption that there is an intermediate shear flow region surrounding the loop. We find that for long coronal loops resonant absorption can be highly enhanced or reduced depending sensitively on the magnitude and direction of the flow and the spatial extent of the flow region when the transitional layer is thin. For short coronal loops, high flow speed and thick transitional layer are needed to have a substantial resonant absorption. We provide a potential picture to explain the results where the external Alfven speed and phase speed of the wave are important parameters. These results imply that the transport of the external wave energy into the loop is significantly changed by the shear flow region, which may cause the selective excitation of the coronal loop oscillations.