Broadening of Si viii lines observed in the solar polar coronal holes

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 locations, where the fast solar wind originates. The line width data show that the non-thermal line-of-sight velocity increases from 27 km s−1 at 27 arc sec above the limb to 46 km s−1 some 250 arc sec (i.e. ∼180,000 km) above the limb. The electron density shows a decrease from 1.1 108 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 excellent agreement with that predicted for undamped radially propagating Alfvén waves. We show that the energy flux associated with these hydromagnetic waves is sufficient to drive the high speed solar wind streams.