Fe xii and Fe xiii Line Widths in the Polar Off-limb Solar Corona up to 1.5 R ⊙

Abstract

Abstract The nonthermal broadening of spectral lines formed in the solar corona is often used to seek evidence of Alfvén waves propagating in the corona. To have a better understanding of the variation of line widths at different altitudes, we measured the line widths of the strong Fe xii 192.4, 193.5, and 195.1 Å and Fe xiii 202.0 Å in an off-limb southern coronal hole up to 1.5 R ⊙ observed by the Extreme Ultraviolet Spectrometer on board the Hinode satellite. We compared our measurements to the predictions from the Alfvén Wave Solar Model (AWSoM) and the SPECTRUM module. We found that the Fe xii and Fe xiii line widths first increase monotonically below 1.1 R ⊙ and then keep fluctuating between 1.1 and 1.5 R ⊙. The synthetic line widths of Fe xii and Fe xiii below 1.3 R ⊙ are notably lower than the observed ones. We found that the emission from a streamer in the line of sight significantly contaminates the coronal hole line profiles even up to 1.5 R ⊙ both in observations and simulations. We suggest that either the discrepancy between the observations and simulations is caused by insufficient nonthermal broadening at the streamer in the AWSoM simulation or the observations are less affected by the streamer. Our results emphasize the importance of identifying the origin of the coronal EUV emission in off-limb observations.