SPECTROSCOPIC SIGNATURE OF ALFVEN WAVES DAMPING IN A POLAR CORONAL HOLE UP TO 0.4 SOLAR RADII

Between 2009 February 24 and 25, the EUV Imaging Spectrometer (EIS) spectrometer on board the Hinode spacecraft performed special sit and stare observations above the south polar coronal hole continuously over more than 22 hr. Spectra were acquired with the 1 '' slit placed off-limb covering altitudes up to 0.48 R-circle dot (3.34 x 10(2) Mm) above the Sun surface, in order to study with EIS the non-thermal spectral line broadenings. Spectral lines such as Fe XII lambda 186.88, Fe XII lambda 193.51, Fe XII lambda 195.12, and Fe XIII lambda 202.04 are observed with good statistics up to high altitudes and they have been analyzed in this study. Results show that the FWHM of the Fe XII lambda 195.12 line increases up to similar or equal to 0.14 R-circle dot, then decreases higher up. EIS stray light has been estimated and removed. Derived electron density and non-thermal velocity profiles have been used to estimate the total energy flux transported by Alfven waves off-limb in the polar coronal hole up to similar or equal to 0.4 R-circle dot. The computed Alfven wave energy flux density f(w) progressively decays with altitude from f(w) similar or equal to 1.2 x 10(6) erg cm(-2) s(-1) at 0.03 R-circle dot down to f(w) similar or equal to 8.5 x 10(3) erg cm(-2) s(-1) at 0.4 R-circle dot, with an average energy decay rate of Delta f(w)/Delta h similar or equal to -4.5 x 10(-5) erg cm(-1). Hence, this result suggests energy deposition by Alfven waves in a polar coronal hole, thus providing a significant source for coronal heating.