Evidence of Wave Damping at Low Heights in a Polar Coronal Hole

Abstract

We have measured the widths of spectral lines from a polar coronal hole using the Extreme Ultraviolet Imaging Spectrometer onboard Hinode. Polar coronal holes are regions of open magnetic field and the source of the fast solar wind. We find that the line widths decrease at relatively low heights. Previous observations have attributed such decreases to systematic effects, but we find that such effects are too small to explain our results. We conclude that the line narrowing is real.The non-thermal line widths are believed to be proportional to the amplitude of Alfvén waves propagating along these open field lines. Our results suggest that Alfvén waves are damped at unexpectedly low heights in a polar coronal hole. We derive an estimate on the upper limit for the energy dissipated between 1.1 R ⊙ and 1.3 R ⊙ and find that it is enough to account for up to 70% of that required to heat the polar coronal hole and accelerate the solar wind.

Plasma Dynamics Above Solar Flare Soft X-Ray Loop Tops

“…This is because our observations are mostly beyond peak EUV flux. Hahn et al (2012) have carefully examined the pervasive scattered light in a study of EIS spectra above the solar limb in their work on wave heating. The pervasive scattered light would tend to narrow our line profiles as the intensity of the real component decreases.…”

“…This is in the opposite direction of our results. Furthermore, Hahn et al (2012) found that a huge contribution of scattered light (greater than 45%) was necessary to begin influencing their results. Because of the Hahn et al (2012) results and the fact that scattered light produces an effect opposite to the effect we are claiming to have found, we feel that scattered light is unimportant for this work.…”

Inferring the Coronal Density Irregularity From Euv Spectra

“…We extracted line intensities by fitting Gaussian functions to the spectra (see, e.g., Hahn et al 2012). In addition to the lines given in Table 1, we also analyzed other prominent lines in the EIS spectrum, which we used for the temperature analysis.…”

“…These five pointings were summed by averaging the data radially to form a single composite pointing. The details of this averaging are described by Hahn et al (2012). Here, we have further binned the data vertically in 8 pixel bins to increase the signal to noise.…”

“…One important issue is that of the observed Alfvénic wave dissipation in the corona (Bemporad & Abbo 2012;Hahn et al 2012;Hahn & Savin 2013. These waves appear to be damped closer to the Sun than predicted by simple models (see, e.g., Cranmer 2002).…”