Summary: Coronal line width variations
J.G. Doyle, D. Banerjee & M.E. Perez
Armagh Observatory, Armagh BT61 9DG, N. Ireland.
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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.
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