The radial and angular variation of the electron density in the solar corona

We derive, for the first time, electron densities as a function of both radius (R) and position angle (theta) for the southwest quadrant of the off-limb corona, using the density-sensitive Si IX lambda 349.9/lambda 341.9 and Si x lambda 356.0/ lambda 347.7 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 degrees < theta < 270 degrees. Within the south polar coronal hole, the density varies from 2.3 x 10(8) cm(-3) at 1.0 R-. to 8.3 x 10(7) cm(-3) at 1.20 R-., while at the equator, the density varies from 6.3 x 10(8) cm(-3) at 1.0 R-. to 1.6 x 10(8) 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.