Broadening of Si VIII lines observed in the Solar
polar coronal holes
D. Banerjee1 , L. Teriaca1 , J.G. Doyle1
& K. Wilhelm2
1. Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
emails: dipu@star.arm.ac.uk, lte@star.arm.ac.uk, jgd@star.arm.ac.uk
2. Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau,
Germany
email: wilhelm@linmpi.mpg.de
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 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