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

Abstract

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 derived electron densities are in excellent agreement with previous values of N_eestimated from the N-like ion Si viii for another coronal hole. The confirmed 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 formula for the density profile out to at least 8 R_⊙, by which stage the electron density has fallen to ∼4×10³ cm⁻³, from 1.5×10⁸ cm⁻³at 1.0 R_⊙.