Electron Densities in the Solar Polar Coronal Holes from Density-Sensitive Line Ratios of Si VIII and S X

We derive electron densities as a function of height in the north and south polar coronal holes from a forbidden spectral line ratio of Si VIII. Si VIII is produced at about 8 × 105 K in ionization equilibrium. We also derive densities from a similar line ratio of S X (1.3 × 106 K). The spectra were obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft. In addition to the primary mechanism of electron impact excitation, the derivation of theoretical level populations for Si VIII and S X includes both proton and resonance capture excitation. We compare the coronal hole results to quiet-Sun coronal measurements obtained outside the east and west limbs. We find for distances of a few arcseconds outside the solar limb that the average line-of-sight electron densities in the coronal holes are about a factor of 2 lower than in quiet-Sun regions. The decrease of density with height is exponential in the polar holes. We also confirm the result known from a variety of earlier observations that the temperature of most of the plasma in coronal holes does not exceed about 106 K.