Summary: Electron densities above a polar coronal hole based on
improved Si ix density diagnostics
Armagh Observatory, Armagh BT61 9DG, N. Ireland.
F.P. Keenan, R.S.I. Ryans and K.M. Aggarwal
Dept. of Pure & Applied Physics, Queens University Belfast, Belfast BT7 1NN, N.
F.Keenan@qub.ac.uk; R.Ryans@qub.ac.uk; K.Aggarwal@qub.ac.uk
Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
(Received ..... ; Accepted in final form .....)
Abstract. Using new closecoupling excitation rates for the Clike ion Si ix, density
diagnostic ratios based on Si ix lines have been reevaluated 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 Ne estimated from the
N--like ion Si viii for another coronal hole. The confirmed trend is for a falloff of
one order of magnitude within the first 0:3R fi above the limb. These densities are
well fitted with an analytic formulae for the density profile out to at least 8R fi , by