Fe XII and Fe XIII Line Widths in a Southern Coronal Hole up to 1.5 Solar Radii

Abstract:

The non-thermal broadening of spectral lines formed in the solar corona are often regarded as the 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 strongest Fe XII 192.4 Å, 193.5Å, 195.1Å and Fe XIII 202.0Å line widths in an off-limb southern coronal hole up to 1.5 solar radii observed by Extreme Ultraviolet Spectrometer (EIS) on board the Hinode satellite with a total exposure time of ~30000 s. We compared our measurements to the predictions from the Alfvén Wave Solar Model (AWSoM) and the SPECTRUM module. We found the Fe XII and Fe XIII line widths first increase monotonically below 1.1 solar radii, then keep fluctuating between 1.1 - 1.5 solar radii. The synthetic line widths of Fe XII and Fe XIII below 1.3 solar radii are notably lower than the observed ones. We suggested that some other heating mechanisms besides the dissipation of the Alfvén waves are required to understand the coronal heating and solar wind acceleration of the heavy minor ions in corona holes below 1.3 solar radii.