Broadening of Si VIII lines observed in the Solar polar coronal holes

D. Banerjee1 , L. Teriaca1 , J.G. Doyle& K. Wilhelm2

 1. Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
2. Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany


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 at 27 arc sec above the limb to 46 km/s some 250 arc sec ( i.e. ~180,000 km) above the limb. The electron density shows a decrease from 1.1 10 8 cm-3 to 1.6 10 7 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.

Sun: Corona -Ultraviolet: SOHO--Sun: waves

To Appear in Astronomy & Astrophysics, 1998

Dipankar Banerjee

Tue Jul 21 17:14:04 BST 1998