Electron densities above a polar coronal hole based on improved SiIX density diagnostics

Using new close-coupling excitation rates for the C-like ion Si IX, density -diagnostic ratios based on Si IX lines have been re-evaluated and applied to a sequence of CDS observations taken above a polar coronal hole. The der ived electron densities are in excellent agreement with previous values of N-e estimated from the N-like ion Si VIII for another coronal hole. The con firmed trend is for a fall-off of one order of magnitude within the first 0 .3 R. above the limb. These densities are well fitted with an analytic form ula for the density profile out to at least 8 R., by which stage the electr on density has fallen to similar to 4 x 10(3) cm(-)3, from 1.5 x 10(8) cm(- )3 at 1.0 R..