Spectroscopic evidence of Alfvén wave damping in the off-limb solar corona

Abstract: We investigate off-limb active region and quiet Sun corona using spectroscopic data. Active region is clearly visible in several spectral lines formed in the temperature range of 1.1--2.8 MK. We derive electron number density using line ratio method, and non-thermal velocity in the off-limb region up to the distance of 140 Mm. We compare density scale heights derived from several spectral line pairs with expected scale heights as per hydrostatic equilibrium model. Using several isolated and unblended spectral line profiles, we estimate non-thermal velocities in active region and quiet Sun. Non-thermal velocities obtained from warm lines in active region first show increase and later show either decrease or almost constant value with height in the far off-limb region, whereas hot lines show consistent decrease. However, in the quiet Sun region, non-thermal velocities obtained from various spectral lines show either gradual decrease or remain almost constant with height. Using these obtained parameters, we further calculate Alfv\'en wave energy flux in the both active and quiet Sun regions. We find significant decrease in wave energy fluxes with height, and hence provide evidence of Alfv\'en wave damping. Furthermore, we derive damping lengths of Alfv\'en waves in the both regions and find them to be in the range of 25-170 Mm. Different damping lengths obtained at different temperatures may be explained as either possible temperature dependent damping or measurements obtained in different coronal structures formed at different temperatures along the line-of-sight. Temperature dependent damping may suggest some role of thermal conduction in the damping of Alfv\'en waves in the lower corona.