Signatures of Alfvén waves in the polar coronal holes as seen by EIS/Hinode

Abstract

Context. We diagnose the properties of the plume and interplume regions in a polar coronal hole and the role of waves in the acceleration of the solar wind. Aims. We attempt to detect whether Alfvén waves are present in the polar coronal holes through variations in EUV line widths. Methods. Using spectral observations performed over a polar coronal hole region with the EIS spectrometer on Hinode, we study the variation in the line width and electron density as a function of height. We use the density sensitive line pairs of Fe xii 186.88 Å and 195.119 Å  and Fe xiii 203.82 Å and 202.04 Å. Results. For the polar region, the line width data show that the nonthermal line-of-sight velocity increases from at 10´´ above the limb to some 150´´  (i.e. ~110 000 km) above the limb. The electron density shows a decrease from 3.3 to 1.9 over the same distance. Conclusions. These results imply that the nonthermal velocity is inversely proportional to the quadratic root of the electron density, in excellent agreement with what is predicted for undamped radially propagating linear Alfvén waves. Our data provide signatures of Alfvén waves in the polar coronal hole regions, which could be important for the acceleration of the solar wind.