Alfvén waves in the solar polar coronal holes

Abstract

We study the variation of the line width and electron density as a function of height above two coronal holes from forbidden spectral lines of Si VIII. The spectra were obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft. The observations concentrate on the dark regions outside the plumes, which are believed to be the location, where the fast solar wind originates. The line width data show that the non-thermal line-of-sight velocity increases from 20 km s−1 at 27 arc sec above the limb to 37 km s−1 some 370 arc sec (i.e. ∼1.38 R⊙) above the limb. The electron density shows a decrease from 3.5 108 cm−3 to 1.6 107 cm−3 over the same distance. This data implies that the non-thermal velocity is inversely proportional to the quadratic root of the electron density in the range 1-1.2 R⊙, in excellent agreement with that predicted for undamped radially propagating Alfvén waves. Beyond 1.2 R⊙ our data suggests an effective breakdown of linear theory for the propagation of Alfvén waves.