Electron density and temperature of the lower solar corona

Abstract

Off limb observations of the quiet Sun corona were made with the Coronal Diagnostic Spectrometer (CDS) on SOHO during the Whole Sun Month campaign in August 1996. Selected spectral lines in the Normal Incidence range were recorded up to 1.2 solar radii above the east and west limb and above the polar coronal holes. Intensities of the coronal lines covering the temperature range from 9 × 105 to 2 × 106 K have been measured and used to derive electron temperature and electron density as a function of the radial distance above the solar limb. Results from the east and west equatorial regions and polar coronal holes are compared. The temperature and density in the coronal holes is found to be lower than in the closed field regions. A density‐sensitive line ratio of Si IX 350/342 Å is used to derive an average electron density which is found to decrease from 5 × 108 cm−3 near the limb to 1 × 108 cm−3 at 1.15RS, in the equatorial region. Over the polar coronal holes, where polar plumes dominate the emission close to the limb, the density varies from 2 × 108 cm−3 at the limb to 6 × 107 cm−3 at 1.1RS. The lowest density found inside the coronal hole on the disk is 9.9 × 107 cm−3. An increase in the quiet Sun temperature with the radial distance is found from the Si XII/Mg X and Si XII/Mg IX line ratios, and an increase in the coronal hole temperature is seen from the Mg X/Mg IX ratio. The Si XII/Mg X temperature varies from 1.1 × 106 K at r = RS to 1.4 × 106 K at r = 1.2RS in the equatorial regions. The EUV emission is compared with that of the soft X rays as measured by the Yohkoh SXT. The densities and temperatures determined from the SXT show a similar behavior to that determined from the CDS. Density and temperature, averaged over a position angle range of 20 – 54°, show very little variation over a period of 20 days.