Properties of Quiet-Sun Coronal Plasmas at Distances of 1.03 ⩽ R☉ ⩽ 1.50 along the Solar Equatorial Plane

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Abstract
We determine the physical properties, i.e., electron temperature, density, line-of-sight emission measure, and element-abundance variation with height, in the Sun's equatorial west limb streamer region from extreme-ultraviolet (EUV) spectra recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The spectra were recorded on 1996 November 21-22 during a special "roll" maneuver in which the SUMER slit was oriented in the east-west direction. Because the SUMER spectrometer is stigmatic along the slit length, the spectra are spatially resolved in the east-west direction. The spectra we discuss cover an effective distance range outside the west limb from 1.03 to 1.50 solar radii (R☉); the maximum spatial resolution is 1''. We select a group of emission lines for analysis that under the usual ionization equilibrium assumption span the temperature range from 3 × 105 up to 2 × 106 K. However, we note that above the limb a major fraction of a line intensity may arise at a temperature that is far from the temperature of maximum emitting efficiency in ionization equilibrium. In this paper we assume ionization equilibrium in deriving plasma parameters. Readers can redo our analysis without this assumption if desired, because the line intensities we present in this paper are simply the measured photon production rates in the lines. Assuming ionization equilibrium, we determine the electron temperature, electron density, line-of-sight emission measure, and abundance variation with height above the limb from the line intensities and line intensity ratios. The spatial resolution has allowed us to detect an apparent element-abundance variation as a function of height above the west limb that is strong evidence for gravitational settling of "heavy" elements.