Comparison of the radial dependence of electron impact ionization adopted in this paper with the 1/r²-dependence for He, Ne, and O. Solid lines indicate the radial profiles calculated using from Eq. (A.9), broken lines are for electron ionization calculated from , both for solar wind proton density at 1 AU n_sw(r = 1AU) = 6.14   cm^-3, assuming the decrease in proton density like 1/r².