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Coronal line width variations J.G. Doyle, D. Banerjee & M.E. Perez

Summary: Coronal line width variations
J.G. Doyle, D. Banerjee & M.E. Perez
Armagh Observatory, Armagh BT61 9DG, N. Ireland.
jgd@star.arm.ac.uk, dipu@star.arm.ac.uk, epp@star.arm.ac.uk
(Received ..... ; Accepted in final form .....)
Line widths measurements of the coronal ion Si VIII confirm earlier observations
which show an increase in the non­thermal velocity above the solar limb. The present
data, taken at the equatorial limb, show an increase from 24 km s \Gamma1 at the limb to
28 km s \Gamma1 some 25,000 km above the limb. The electron density as measured from
the Si VIII line pair show a decrease from 3:5 10 8 cm \Gamma3 to 1:8 10 8 cm \Gamma3 over the same
distance. This data implies that the non­thermal velocity is inversely proportional to
the quadratic root of the electron density, in excellent agreement with that predicted
for undamped radially propagating Alfv'en waves.
1. Introduction
On­disk measurement of line widths from optically thin ultraviolet
lines formed in the transition region do not show systematic trends.
However, Roussel­Dupr'e et al. (1979) and more recently Erd'elyi et al.
(1998) have shown that chromospheric and transition region lines show
a small center­to­limb variation. Based on SKYLAB data, Doschek et


Source: Armagh Observatory


Collections: Physics