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

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_solar<R<1.20 R_solar and 180^deg<θ<270^deg. Within the south polar coronal hole, the density varies from 2.3×10⁸ cm^-3 at 1.0 R_solar to 8.3×10⁷ cm^-3 at 1.20 R_solar, while at the equator, the density varies from 6.3×10⁸ cm^-3 at 1.0 R_solar to 1.6×10⁸ cm^-3 at 1.20 R_solar. 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.