Fe XII and Fe XIII Line Widths in the Polar Off-limb Solar Corona up to 1.5 Rsun

Abstract

The nonthermal broadening of spectral lines formed in the solar corona is often used to seek 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 line widths of the strong Fe \textsc{xii} 192.4, 193.5, and 195.1 \mboxÅ and Fe \textsc{xiii} 202.0 \mboxÅ in an off-limb southern coronal hole up to 1.5 $R_{\odot }$ observed by the Extreme Ultraviolet Spectrometer (EIS) on board the \textit{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 \textsc{xii} and Fe \textsc{xiii} line widths first increase monotonically below 1.1 $R_{\odot }$, and then keep fluctuating between 1.1 and 1.5 $R_{\odot }$. The synthetic line widths of Fe \textsc{xii} and Fe \textsc{xiii} below 1.3 $R_{\odot }$ 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 $R_{\odot }$ 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.

17 pages, 13 figures, accepted by ApJ

References in corpus (8)

• The NumPy array: a structure for efficient numerical computation
• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• CHIANTI - an atomic database for emission lines - Paper XV: Version 9, improvements for the X-ray satellite lines
• Data analysis recipes: Fitting a model to data
• High precision density measurements in the solar corona: I. Analysis methods and results for Fe XII and Fe XIII
• Spectroscopic signature of Alfvén waves damping in a polar coronal hole up to 0.4 solar radii
• Solar off-limb line widths: Alfven waves, ion-cyclotron waves, and preferential heating
• Revisiting the relation between nonthermal line widths and transverse MHD wave amplitudes

Cited by in corpus (1)

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