The Radial and Angular Variation of the Electron Density in the Solar Corona

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Abstract
We derive, for the first time, electron densities as a function of both radius (R) and position angle (θ) for the southwest quadrant of the off-limb corona, using the density-sensitive Si IX λ349.9/λ341.9 and Si X λ356.0/λ347.7 extreme-ultraviolet line ratios. The observations were made with the coronal diagnostic spectrometer on board the Solar and Heliospheric Observatory over the ranges of 1.00 R☉ < R < 1.20 R☉ and 180° < θ < 270°. Within the south polar coronal hole, the density varies from 2.3 × 108 cm-3 at 1.0 R☉ to 8.3 × 107 cm-3 at 1.20 R☉, while at the equator, the density varies from 6.3 × 108 cm-3 at 1.0 R☉ to 1.6 × 108 cm-3 at 1.20 R☉. The density falloff with height is therefore faster in the equatorial region. We also find that electron densities are, on average, a factor of 2.7 larger in the equatorial regions than in the polar coronal hole at a given radial distance. Finally, we find remarkable agreement between our measured densities as a function of radius and position angle and those predicted by a recent analytic MHD model of the solar wind, strongly supporting its basic premises.